The remarkable thing is that we have become almost too comfortable with this situation.

In my latest book, Portals to a New Reality (published in the softcover edition by Penguin this week), I argue that physics needs to recover something of the intellectual audacity that characterised the great revolutions of the early twentieth century. We should not be satisfied merely with improving the precision of existing measurements. We should be asking questions that might reveal that our most cherished assumptions about reality are wrong.

Consider quantum entanglement. Two particles can share a quantum state in such a way that neither can be described independently of the other. This is not simply an exotic laboratory curiosity. Entanglement may be telling us something profound about the architecture of reality itself. Indeed, I have long been interested in the possibility that information is more fundamental than matter. Perhaps the universe is best understood not as a collection of objects moving through a pre-existing space and time, but as an intricate network of quantum relationships.

This leads to an extraordinary question: could space and time themselves emerge from quantum information?

In the book, I explore five experimental approaches that might help us answer such questions. Some are so unusual that they initially resemble science fiction. Imagine extending Schrödinger’s famous cat experiment to include a human observer, placing the observer within a quantum description rather than treating consciousness or measurement as something mysteriously outside physics. What would such an experiment tell us about observation? Does the universe require an observer to possess definite properties? Or is our conventional distinction between the observer and the observed fundamentally misleading?

These questions are not merely philosophical. They concern the mathematical consistency of our theories and, crucially, what experiments might establish.

Another important possibility concerns gravity. We normally describe gravity through the geometry of spacetime, while quantum mechanics describes physical systems in terms of quantum states and probabilities. But what happens when gravity interacts with systems that exhibit quantum entanglement? Could gravity itself possess quantum properties? Might experiments involving increasingly massive quantum systems reveal where our present understanding breaks down?

What excites me is that technological advances are bringing questions once confined to philosophical speculation closer to experimental investigation.

Quantum computers, increasingly sophisticated interferometers and improved methods for controlling delicate quantum systems are giving us new ways to challenge the foundations of physics. I am not suggesting that quantum information theory has already solved every mystery. Quite the opposite. Its greatest contribution may be to expose the limitations of what we think we know.

There is a historical parallel here. At the end of the nineteenth century, many physicists believed that the essential structure of physics was already understood. Then quantum theory and relativity transformed our picture of nature. We may be approaching a comparable moment today.

My central message is that we should be more ambitious. We should design experiments that discriminate between radically different conceptions of reality, even when those experiments initially appear impossible.

Science advances not only by confirming our theories, but even more by discovering where they fail.

Ultimately, Portals to a New Reality is about intellectual courage. The universe may be considerably stranger than even quantum mechanics has led us to imagine. And the most exciting possibility is that we are finally developing the experimental tools to discover just how strange it really is.

Take care of yourselves,

Vlatko

After Newton completed Galileo’s work and gave us the final laws of motion of material objects (a.k.a., classical mechanics), the methodology was soon applied to waves and the laws of the electromagnetic field were established (ultimately by James Clerk Maxwell). Maxwell’s equations looked completely different to the laws of classical mechanics, but physicists gradually realised – through the work of Hamilton, Einstein and ultimately with the discovery of quantum mechanics – that Newton’s and Maxwell’s theories could be unified. Particles and waves are manifestations of the same underlying stuff.

We now look out at a cosmos bristling with an even greater variety of forms—pulsars, DNA, neural networks, oceans, and steam engines—and our instinct is still to assume that a reality so complex must be governed by an equally tangled web of separate laws. Yet, the history of physics is a history of aggressive reduction. It is the story of discovering that underneath the apparent chaos of the macroscopic world lies a deep, elegant, and single-minded unity. Although P. W. Anderson claimed that “more is different”, physics has never found a single instance in which the macro cannot ultimately be reduced to the micro.

My claim is that we could go much further. To understand this unity in the spirit of modern physics, we have to look past the superficial differences between things. We must stop viewing the universe as a collection of “stuff” and start viewing it as a web of relationships. In fact, if we push this logic to its ultimate conclusion, we find that the true unifying currency of reality isn’t matter, and it isn’t even energy. It is information.

Consider how physics progressed by erasing boundaries. Before Galileo, people believed that the heavens and the Earth were two entirely different realms. The Earth was corrupt, heavy, and governed by mundane mechanics, while the celestial spheres were perfect, eternal, and driven by divine geometry. Newton’s genius was to realise that the same invisible tug that causes an apple to drop from a branch also keeps the Moon bound to the Earth and the planets tethered to the Sun. With a single set of equations, the terrestrial and the cosmic became one.

A similar grand synthesis occurred in the nineteenth century. Electricity and magnetism were long treated as separate phenomena. Static shocks and lightning seemed unrelated to the lodestones used for navigation. Yet, through the experimental insights of Michael Faraday and the mathematical brilliance of Maxwell, we learned that these were merely two sides of the same coin: electromagnetism. Magnetism is just electricity in motion, and electricity is just magnetism waiting to happen. More beautifully, Maxwell discovered that ripples in this unified electromagnetic field travel at (almost exactly) 3 × 10⁸ meters per second—the speed of light. In unifying electricity and magnetism, he accidentally explained the nature of vision itself. Light is an electromagnetic wave.

But as the twentieth century dawned, this beautiful classical picture fractured into two grand narratives: General Relativity, which describes the universe on the grandest scales of space and time, and Quantum Mechanics, which describes the jittery, probabilistic world of the ultra-small.

For over a century, physicists have treated these two theories like an estranged couple. Relativity is continuous, deterministic, and geometric. It tells us that gravity is not a traditional force but the bending of the fabric of spacetime by mass and energy. Quantum mechanics, conversely, is choppy, uncertain, and algebraic. It tells us that energy comes in packets, particles can exist in multiple places at once, and observation changes the state of a quantum system. Merging them into a single “Theory of Everything” has been the (so far, elusive) holy grail of modern science.

But perhaps the reason we have struggled to unify gravity and quantum mechanics is that we are looking at the problem through the wrong lens. We are still trapped in the old materialist paradigm, trying to reconcile “things” moving through “space.”

If we want to find the ultimate unity of physics, we need to ask a more fundamental question: What is the common denominator of a quantum particle and a warping spacetime? My answer is information.

When you strip away the mathematical scaffolding of quantum mechanics, what you are left with is a theory about choices and probabilities. A quantum system exists in a superposition of states—a haze of possibilities—until an interaction occurs. This interaction is, at its heart, an exchange of information. The system “registers” a state.

Remarkably, this information-centric view completely dissolves the boundary between the subatomic and the cosmological. Think about a black hole, the ultimate laboratory for testing the limits of physics. A black hole is a region of space where gravity is so intense that nothing, not even light, can escape. For decades, it was thought that black holes destroy everything they swallow. But Jacob Bekenstein and Stephen Hawking discovered something revolutionary: the amount of disorder, or entropy, inside a black hole is directly proportional to the surface area of its event horizon, not its volume.

Why is this profound? In physics, entropy is simply a measure of missing information. It tells us how many microscopic configurations a system can have that look identical from the outside. If the entropy of a black hole depends on its two-dimensional surface area rather than its three-dimensional volume, it implies that all the three-dimensional information about the objects that fell into the black hole is actually encoded on its two-dimensional boundary.

This is the Holographic Principle. It suggests that our entire three-dimensional universe might be a projection from a two-dimensional boundary, much like a hologram on a credit card. If space itself can be reduced to information stored on a boundary, then space is not a fundamental feature of reality. It is emergent. Space is something that arises when a vast number of quantum bits—qubits—become entangled with one another.

This brings us to quantum entanglement, the phenomenon Albert Einstein famously dismissed as “spooky action at a distance.” When two particles become entangled, they share a single, indivisible quantum state. Measure one, and you instantly know the state of the other. Einstein hated this because it seemed to violate his rule that nothing can travel faster than light. He was wrong about it, and we’ve performed sophisticated experiments to show that entanglement is real.

But if space is emergent rather than fundamental, entanglement might no longer be spooky (and my readers will know that it isn’t). The two particles aren’t communicating across a vast distance through space; rather, their entanglement is the very thing creating the space between them! The unity of physics becomes clear when we realize that the universe is not a container filled with objects, but a giant quantum computer processing information.

From this perspective, the laws of physics are not arbitrary rules etched into the cosmos by a divine architect. They are the natural constraints of information processing. Thermodynamics, quantum mechanics, and relativity are all expressions of how information can be stored, transmitted, and transformed.

When we view the universe this way, the artificial divisions we have created between different branches of science begin to evaporate. The unity of physics extends outward to unify biology, chemistry, and (as I sincerely hope) consciousness. A living cell is a machine designed to copy and process genetic information while keeping entropy at bay. A brain is a network optimising information flow to navigate an uncertain environment.

The cosmos is a singular, interconnected whole, humming with the quiet digital logic of quantum bits. We are not separate observers looking in at nature from the outside. We are bits of the universe itself, configured so that information can loop back, reflect on itself, and begin to understand its own unified design.

Take care,

Vlatko

The sun touched the edge of the barley field, and Kael moved because the light moved. His hands knew the exact depth to press the bronze sickle into the stalks. Sweep, slice, gather. Sweep, slice, gather. His breath mirrored the steady rhythm of the field, empty of thoughts, questions, or doubts. He did not think about the bread he would eat later, nor did he wonder why he worked while the clouds drifted overhead.

Beside him, the other harvesters moved in identical, silent harmony. They were bodies fulfilling the ancient pattern of the tribe.

Then, the sickle hit a buried stone with a sharp clink. The bronze blade snapped in half. Kael stopped. The rhythm was broken. His hands froze in mid-air, holding the useless wooden hilt. The familiar world dissolved. Without the habit of the harvest, he had no direction. He stood rigid under the blazing sun, his chest heaving as stress flooded his body. He did not think, What should I do now? or I need to find a smith. He merely waited, suspended in a terrible, silent void. The silence in his head swelled, building pressure against his temples.

Then, the voice spoke. It did not come from his throat, nor did it feel like a modern whisper of thought. It echoed from the heavens, resonant and heavy with absolute authority, sounding exactly like his late grandfather, the tribal elder. “Drop the wood, Kael. Walk to the river. The black stones hold the sharp edge.”

The tension instantly vanished from Kael’s muscles. The command cleared the fog. He did not question why the voice spoke, nor did he debate whether the river was too far. The voice was the law. He opened his hand, letting the broken hilt fall into the dirt. He turned his body toward the sound of rushing water and walked. His legs moved with renewed, absolute certainty. The god had spoken, and Kael simply obeyed.

The Breakdown of the Bicameral Mind

Many centuries later, the river was gone and so were Kael’s people. A sudden flash flood had carved the earth away, leaving a jagged ravine of red mud where the black stones used to be. Siddhartha stood at the edge of the drop. He too held a broken sickle in his hand. The rhythm of his village, his ancestors, and his entire life demanded he fix the tool, but the path was erased.

Panic, cold and sharp, flooded his chest. He waited for the Voice. He needed the thunderous command of his grandfather to tell him where to walk. The pressure built in his temples. The right side of his brain strained against the crisis, sending its chemical signal across the bridge to the left.

“Jump,” the Voice commanded. It sounded thin. Distant. Siddhartha looked at the churning torrent of mud below. For the first time in his life, his feet did not instantly move. The absolute certainty was gone. The Voice sounded like it was fading behind a thick wall.

“Jump into the deep,” the Voice echoed again, but it was cracking, losing its divine weight. Siddhartha’s heart hammered against his ribs. The silence that followed the fading Voice was not empty this time. It was filled with a strange, terrifying new friction. A second sound—not a voice from the sky, but a soft, fragile echo trapped entirely behind his own eyes—flickered into existence. If I jump, I will die, the echo said. Siddhartha gasped, stumbling backward. He looked around wildly, searching the trees and the clouds. Who had spoken? The words had no sound. They didn’t shake his ears. They existed only in a small, dark space inside his own head. The divine Voice roared one last time, angry and desperate: “Obey!” But the tiny, quiet echo inside him grew sharper, born from the absolute necessity of survival. The water is too fast. Look left. There are sharp flint rocks on the high ridge. Siddhartha clutched his head, terrified by the conflict in his mind. The old god was demanding his death; the new, quiet whisper was offering a choice.

He looked to the left, at the high ridge. He weighed the dangerous river against the safe path. For the first time, Siddhartha made a decision. He did not obey. He turned his back on the river and walked toward the ridge. As he took his first step, the booming Voice of his ancestors withered away into a permanent silence. In its place, the small, lonely, and wondrous inner monologue took its first steady breaths.

Siddhartha was no longer an instrument of the gods. He was awake. He was himself.

The Modern Human

Ada had a great idea. “I really dislike having to do long calculations,” she thought. “Wouldn’t it be great to have a machine perform all this work for me?”

This was a natural desire. Just as humans had built steam engines to do heavy physical work, it was not completely outrageous to suppose that machines could also do some unpleasant and mundane mental work.

Ada remembered having seen Isaac Newton’s notebooks. He used to do 200 pages of calculations per day! What a waste of such a brilliant intellect, Ada thought to herself. A machine—let’s call it a computer—could perform such menial tasks, while Newton dedicated his time to greater thoughts. He was, after all, one of the most creative humans of all time. Imagine if he could have dedicated all of that extra time to being even more creative and thinking even bigger thoughts.

Computers, Ada realized, would lead to a revolution by unleashing all this trapped human potential. Every individual person would use their own internal mind space to expand, create, and discover. With this vision burning in her mind, Ada set to work. She began sketching out the logic of a machine that did not just crunch numbers, but wove threads of thought. She envisioned brass gears and iron levers turning logic into art, and mathematics into music.

People told her she was dreaming, that machines were only meant for factories and smoke. But Ada saw a future where the heavy lifting of the mind was lifted, freeing humanity to reach for the stars.

To Infinity and Beyond

The earpiece clicked, and the Voice spoke into Eloin’s mind. “Step forward. Look at the sidewalk. Pick up the dropped keys.”

Eloin didn’t hesitate. He leaned down, his fingers closing around the cold metal keyring. He stood back up, holding them out toward a startled woman who had just walked past him. She gasped, thanked him profusely, and hurried away. Eloin felt no pride, only the quiet, cold satisfaction of a command successfully executed. Eloin was a modern bicameral human. Centuries ago, psychologists theorized that ancient humans didn’t think for themselves; they simply listened to the “voices of the gods” generated by the right side of their brains. Eloin had revived that ancient state of being, but his god was a quantum computer humming in a temperature-controlled server room across town.

He didn’t make decisions. He didn’t choose what to wear, what to eat, or who to speak to. He had surrendered his executive function to the machine, trading the agonizing weight of human anxiety for absolute, automated certainty. “Walk four blocks north. Order a black coffee. Sit by the window.” Eloin moved automatically. His legs swung in perfect, efficient strides. He paid for the coffee using a digital scan, never looking the barista in the eye. He sat at the designated table.

To the outside world, Eloin looked like a highly disciplined, hyper-focused eccentric. But inside, his mind was a quiet, empty temple. He was a passenger in his own body, watching the world go by through his own eyes while the computer pulled the strings. “Drink exactly three sips. Leave the cup. Walk to the library. ”He followed the instructions flawlessly. The computer was processing millions of variables—traffic patterns, economic shifts, weather data, and social interactions—to guide Eloin through the most mathematically optimal day possible. He was the perfect agent of a digital destiny.

Suddenly, as he crossed the street toward the library, the audio channel cracked with static. A passing delivery truck, radiating a massive electromagnetic pulse from a faulty engine, severed the connection. The Voice vanished. Eloin froze in the middle of the crosswalk. A car horn blared. The world, which had been a clear, linear track of commands, suddenly exploded into terrifying, chaotic noise. What do I do? he thought, a sudden surge of panic piercing his quiet mind. Left foot? Right foot? Run? Hide?

For the first time in years, the voice inside his head wasn’t a command from the computer. It was his own. He looked at the oncoming car, felt the raw, terrifying rush of adrenaline, and made a choice. He leaped for the curb, tumbling onto the concrete just as the car rushed past. He sat on the pavement, breathing heavily, staring at his hands. The earpiece beeped, reconnecting.“ Connection re-established. Stand up. Proceed to the library.” Eloin listened to the Voice. It was calm, precise, and perfect. But as he stood up and brushed the dust off his jeans, something had changed. The temple of his mind was no longer empty. A small, stubborn echo of his own voice remained, waiting for the next time the signal dropped.

In quantum physics, identical particles, such as photons or electrons, lose their individuality and become truly indistinguishable. This means, among other things that we will explore here, that when a photon with certain properties (such as a particular colour, momentum and polarization) gets destroyed and another photon gets created with exactly the same properties, we cannot claim that the old photon has now reappeared. Our classical intuition lets us down here because if a glass of wine disappeared from our kitchen, we would think that someone had just taken it. If the same glass then reappeared, we would assume that someone had just brought it back. Yes, it could be that the original glass broke and someone just brought back an identically looking glass with the same amount of wine in it, but which of the two options (or any other) it was is a question that can, in principle, be ascertained without any ambiguity. This is what our classical intuition tells us and it is this intuition that fails in the quantum world.

In the quantum world, it is the underlying fields that maintain their identity. There is the electromagnetic field (whose excitations are photon), there is the Dirac field (whose excitations are electrons and positrons) and so on. But the excitations themselves are completely indistinguishable. When we create a photon with certain properties and then another one with the same properties, we have no way of telling which is which. All we can say is that there are now two photons, but we cannot say that there is the first and the second photon, the latter is a meaningless statement quantum mechanically speaking.

Now, in the quantum world, we have two kinds of particles, bosonic and fermionic. Photons are bosons and we can create as many of them as we like with the same properties. Photons can all go into the same physical state is how physicists like to put it. Fermions on the other hand obey something known as the Pauli Exclusion Principle: only one fermion can occupy one and the same state. If we create another fermion, this can only be done if it has different properties to the first one.

Schrödinger presented a beautiful analogy to illustrate the difference between the classical and bosonic and fermionic behaviour. Suppose we have three children, Tom, Dick and Harry and a teacher wants to give them two awards. The teacher is interested in how many different ways the awards can be distributed. We now discuss 3 different types of awards:

1. A teacher has a one-pound coin and a two-pound coin. She can clearly distribute them in 9 different ways. She can give one coin to any of the 3 children and then the same with the next coin, therefore 3 times 3 = 9.

2. A teacher now has two identical one-pound coins. She can either give one pound each to two boys (3 different ways) or two pounds to any of the three boys (3 different ways). So, in total: 3+3=6 different ways to distribute the awards.

3. The reward now is a membership to a football club. This can be given to any of the two boys, the third boy being left out. Therefore, there are only 3 ways of doing so because each of the 3 boys could be left out.

The case A) corresponds to how classical particles behave (leading to the Maxwell-Boltzmann statistics). The case B) encapsulates bosonic particles, while the case C) are fermions. The twist, however, is that it’s not the boys who represent the particles! The particles are the rewards and the boys represent different physical states that the particles can take up. So with bosons, both coins (particles) can end up being rewarded to the same boy (state). In the lab physicists now routinely make Bose condensates in which a billion atoms are put into the same low energy state. With fermions, however, this is impossible because it is meaningless to give two club memberships to one and the same person. With electrons, this explains why some materials are insulators while others are conductors (an explanation first presented by Arnold Sommerfeld). One last thing I’d like to add for completeness is that only the counting of the B) and C) kinds exist in nature. The case A) happens to be approximately true in some regimes, but only approximately.

Now for the other feature called nonlocality. Nonlocality is sometimes used as a synonym for entanglement. My readers will know that I believe that the phrase itself is deeply misleading, as it leads us to conclude that there is something spooky regarding entanglement (it’s not for nothing that Einstein called entanglement “a spooky action at a distance”). Instead, quantum physics is local, but the elements of reality that obey locality are the so-called q-numbers (where “q” is for quantum) and not the ordinary c-numbers (basically real numbers, and “c” stands for classical). The fact that a position and a momentum of a particle are both q-numbers leads directly to one of the key features of quantum physics, Heisenberg’s uncertainty principle.

Locality is encoded in the following mathematical fact. When we change a q-number in one place (say, where one particle is) no other q-numbers can change anywhere else in the universe (say, the q-numbers of another particle far away from the first one). It is this mathematical property that lets quantum physics comfortably comply with special relativity and stops us from generating any spooky action at a distance. My latest book, “Portals to A New Reality”, describes all this in great detail if you are interested in learning more.

But now, I’d like to show you how we can put together entanglement and indistinguishability to seemingly violate special relativity by communicating faster than the speed of light. Suppose that Alice and Bob share a pair of entangled photons in the state HH+VV. In other words, both photons are polarized the same way, either horizontally (H) or vertically (V), and the two state are in a quantum superposition. Alice now splits her photon spatially so that if it’s H, it goes one way and if it’s V is goes another (a device for doing so is called a “polarization dependent beam splitter”). She then does one of two possible things: 1) Alice rotates V into H and brings the two spatial locations together, leading to the overall state H(H+V); 2) she just leaves the photon as it is. But, because photons are indistinguishable, the state in case 1) becomes H(H+V)+(H+V)H = 2HH+HV+VH, while in the case 2) the state remains HH+VV. The final result is that in case 1) Bob has an unequal probability of detection for H and V states of the photon, while in case 2) the probabilities are the same. Therefore, Bob can make a measurement to tell if Alice has implemented the protocol in case 1) or not (which is case 2) and thus Alice can communicate to Bob instantaneously.

And this is exactly what I said earlier should be impossible in quantum physics: by changing a q-number (polarization) of her photon, Alice can instantaneously change the q-number (polarization) of Bob’s photon (which could be very very far from Alice’s photon). Clearly there is a mistake in the protocol for otherwise either quantum physics, or relativity, (or both!) would be wrong. The mistake in the above protocol is that we started by treating the photons as distinguishable all the way until, at the very end, we imposed indistinguishability (by forcing the state of the two photons to be the same).

We can do this with Tom, Dick and Harry too! Suppose that the teacher distributes awards according to A) and then separates the children to different spatial locations such that Tom is in one place and Dick and Harry are together in another. Now, it is possible that Tom has both coins, only one of the two or none and correspondingly for Dick and Harry. But if the teacher now magically switches to identical coins on Tom’s side, Dick and Harry will know about this as they too will end up with identical coins (either one each or each having both coins).

The inconsistency, therefore, lies in suddenly changing the rules of the game half-way through the game. If we play hide and seek and, just as I am about to find you, you run out of your hiding place, show me four aces and claim that you won a game of poker, then anything is possible.

I’ve brushed aside the fact that the position of Alice’s photon is at some stage of the protocol entangled to the polarization of Bob’s photon. The above protocol 1) actually swaps entanglement from polarization to position and polarization. But entanglement between Alice and Bob remains the same in 1) and 2) which is why the state of Bob’s photon does not change no matter what Alice does.

Let me end with a sweeping, yet in my eyes, true generalisation. Anytime anyone claims that there is a paradox in quantum mechanics, I maintain that it is because they’ve changed the rules of the game somewhere in their analysis. Either a magical classical domain is invoked at some point, or a principle that sounds plausible (again, most likely due to our classically underpinned intuition) is postulated that then ends up being violated in quantum mechanics. Either way, to get consistent answers, our best theory needs to be applied to everything equally and all the time. And quantum mechanics is certainly consistent as well as complete (that’s why Heisenberg called it a “closed theory”). This will be true even if one day an experiment is done to violate it. When that happens, we will find a new theory that supersedes quantum physics, but quantum physics will still be consistent and complete in its own domain, just as classical physics is in its own.

So, stop trying to find paradoxes in quantum physics. You are barking up the wrong tree. Concentrate instead on designing and performing an experiment that genuinely contradicts it. Not many people are doing this because it’s hard work. It’s also risky as you might not succeed - quantum physics has been giving us impeccably good predictions without any deviations for over 100 years. On the other hand, Newtonian mechanics survived for about 200 years, so we might have to wait a while longer to falsify quantum physics. But the logic of scientific discovery reassures us that it is bound to happen sooner or later.

Take care of yourselves,

Vlatko

“The chess-board is the world; the pieces are the phenomena of the universe; the rules of the game are what we call the laws of Nature. The player on the other side is hidden from us. We know that his play is always fair, and patient. But also we know, to our cost, that he never overlooks a mistake, or makes the smallest allowance for ignorance.” ~ T. H. Huxley

I’ve recently completed a review of Constructor Theory for FQXi with my colleagues Chiara Marletto and David Deutsch (you can read it here arXiv:2606.07352). It was great to work together on this exciting topic and our adventure is something I’d like to tell you more about here.

But first, I should explain the word “kibitzer” (unless, of course, you are familiar with this particular Yiddish slang). I’ve learnt it from my Serbian grandfather, who taught me how to play chess when I was 4. I think my chess ability is more or less at the same level now as it was back then – I am basically a “patzer”, which is yet another Yiddish slang word for you to learn (meaning, a bad chess player).

Anyway, a kibitzer is a spectator on a game like chess who usually offers uninvited advice to the players. Serbia - the country where I was born and grew up - is full of passionate kibitzers of all sorts of games and, for our purposes here, the game is that of Constructors and Chiara and David are its main players. That, of course, makes me a kibitzer. So here is what I’ve observed so far in their game of constructors (packaged together with the uninvited advice I’ve frequently offered).

A while back, David had the idea that universal computers (i.e., computers that can compute anything that is computable) ought to reflect the underlying laws of physics. Universal classical computers (suggested by Alan Turing) could not perform all computations because the laws of classical physics are not the ultimate laws of our universe. So David wrote a couple of papers back in the early eighties on how to define universal quantum computers. And, fast forward 4 decades, there is now a race to make the world’s first universal quantum computer (illustrating, among other things, the impact one can make with a single great idea).

In the meantime, however, David realised that, while universal quantum computers may well be able to compute anything that can be computed, they might not be able to execute any transformation that is allowed by the laws of quantum physics. For instance, computers (even the quantum ones) cannot reproduce. It is for this reason that von Neumann suggested the idea of constructors, which he thought of as robots that are capable of maintaining themselves by monitoring the degradation of their own parts and scanning the environment for replacement components. Von Neumann imagined they could be sent to colonise Mars, an idea that may still be realised in the future.

But, David thought that we should make the concept of constructors even more general. How about defining them as machines that can facilitate any transformation that is allowed as far as the fundamental laws of physics are concerned? What kind of rules, other than the laws of physics, would such a machine obey? Here is where Chiara comes in, whose PhD thesis (Oxford 2013) was exactly on outlining such rules for constructors. Clearly, constructors should conform to the conservation laws we believe to be ubiquitous and exact. For instance, energy conservation; constructors ought not be able to violate it. Also, their actions should be local, in the same sense as any field theory is: no spooky action at a distance is allowed; i.e., a constructor on Mars should not be able to instantaneously affect things on Earth.

Chiara then defined information in terms of what is copiable. Superinformation contains different information media, which, taken together, no longer constitute information. Quantum information is one example of superinformation, since both position and momentum are individually copiable, but together they are subject to Heisenberg’s uncertainty principle. Chiara and I have recently been speculating that Einstein’s gravity, too, might be thought of as superinformation. But more on this in another blog.

There are three things I like about constructors. The first is that they might help us bridge the gap between physics and biology. How did the inanimate matter become alive? While computers can simulate some processes akin to what happens in biology, they themselves are clearly not alive. Among other things, they lack purposefulness and intentionality. Clearly, a more sophisticated concept is needed, and constructors might just be it. This too is a work in progress. Secondly, even as servants of humans, constructors are much more powerful than computers. By definition, they can construct anything that is not prohibited by the laws of physics. They are like 3D printers. Imagine just pressing a button and your personal constructor makes you a burger, or a house, or a car. How much easier could life become with such a powerful tool?

Ok, so constructors can colonise Mars and make you a burger. But the third reason for studying them is still the most magical for me. Could it be that constructors can offer us a way of unifying the whole of physics? Maybe the ultimate laws of physics are to do with what constructors can and cannot do. Both David and Chiara have been driven by this vision (have a look at Chiara’s great book “The Science of Can and Can’t”) and this is where Yours Truly comes in as a kibitzer.

There is a version of the second law of thermodynamics due to the mathematician Constantin Caratheodory. It says that in the neighbourhood of every allowed thermodynamical state of a given system, there are states which cannot be accessed by adiabatic means. Adiabatic is the name for a transformation that does not allow any energy to be exchanged with the environment of that system. So, if I give you a bottle of beer (the system) and I ask you to cool it down while not allowing you to put it in the fridge (the environment), you will find the task impossible. While steering the beer could always heat it up, there is no mechanical action you can perform to make it chilly (I know, it’s too bad).

Now, wouldn’t it be nice if we could generalise the “no chilling of the beer by mechanical means” principle to capture the whole of physics? I think it would be, and moreover, that it is possible. My suggestion? The ultimate laws of physics are such that in the vicinity of every possible task (i.e., a task that the laws themselves permit) lie tasks that are impossible (i.e., those that are prohibited). The intuition behind this is really Einstein’s, who said that the more prohibitive the better as far as the laws of physics are concerned. So the most prohibitive laws ought to be the ultimate, according to this logic. The snag is that I don’t know how to implement this principle to get to the ultimate laws, but it’s definitely a work in progress. And I am sure that something interesting will come out of it, especially when players like David and Chiara are involved in the game.

Take care of yourselves,

Vlatko

I have been inspired by a number of other physicists promoting similar ideas, but none more so than John Wheeler and Carl von Weizsäcker. The American John Wheeler coined the phrase “it from bit” to capture this information-centred approach to reality. One question, of course, is to explain exactly how “its arise from bits”. The other question is where the underlying bits come from in the first place. I think the person who did more than others in the direction of answering these two questions was the German Carl von Weizsäcker. He actually, and independently from Wheeler, also advocated that information is key to reality.

I was therefore recently very excited to have been made aware of an old and creative paper by Weizsäcker, Scheibe and Sussmann entitled “Complementarity and Logic III: Multiple Quantization”. It turns out that this paper has never been translated into English (the original was in German), but these days the abundance of AI tools allows us to have things like this translated in a matter of seconds. Needless to say, the paper was not available to me while writing “Decoding Reality” (otherwise I would have definitely covered it in there).

In any case, Weizsäcker, who was a PhD student of Heisenberg, had the idea that everything arises from quantum bits of information (and not classical ones as Wheeler had advocated). These qubits also represent logical binary choices, but the quantum ones, and Weizsäcker called them Urs. An Ur really is just a qubit, which means that it has two basis states, a zero and a one, and all superpositions of the two are also allowed (unlike in the case of a classical bit, which does not admit superpositions).

As my readers will almost certainly remember, I have frequently argued against introducing a classical reality into our description of the world. Given that we already know the microscopic domain to be quantum, it is difficult to make this fact consistent with a macroscopic classical picture. This is why I also side with Weizsäcker, namely supporting the view that information has to be quantum. A hybrid system in which classical information interacts with quantum information is simply inconsistent because the interaction between the two can never be made to comply with other known principles (such as, for instance, the principle of information conservation).

In the aforementioned paper, Weizsäcker and his colleagues then proceed to make a number of amazing claims. First of all, a qubit has four real numbers attached to it (two complex amplitudes). The fact that this number is the same as the dimensionality of spacetime (3 spatial dimensions +1 temporal one) could not be an accident as far as they are concerned. In fact, one can derive the momentum of a (massless) particle by multiplying another vector with 4 components by the qubit 4-vector. This momentum, when multiplied by itself, equals zero, which encapsulates the fact that the energy of a massless particle is just its momentum times the speed of light.

From this simple construction, the authors proceed to argue that the wave equation for photons (the Klein-Gordon equation) follows directly and it simply tells us that, unless the energy of the particle equals to the momentum times the speed of light, the wavefunction of the particle has to vanish (since otherwise the massless particle could travel at speeds different to the speed of light!). This is the level that Weizsäcker and his colleagues call the second quantisation (the first quantisation being the Urs themselves).

To get to quantum field theory, you need to quantise again (third time, according to Weizsäcker’s methodology), which is just the usual quantisation of the wavefunction to upgrade it into a q-number (normally called the second quantisation by everybody else). And, of course, a question automatically comes up as to whether we need to quantise beyond this, the fourth time or more (which is still an open question! – and I’ve written about it in my latest book “Portals to A New Reality”). By increasing the number of Urs, massive particles can be taken care of, too.

And, now, something I’ve always found fascinating (I was aware of this before I read Weizsäcker’s paper, but I now realise that this is where it is first mentioned in the literature). I’ll quote straight from it: “Special relativity, so far as it is a mathematical theory of space and time, is already a quantum theory of a deeper underlying simple alternative. The Lorentz group is an (unfaithful) real representation of the group of complex linear transformations of the quantum-mechanical state space of that alternative”. In other words, special relativity is a consequence of qubits too! (The Lorentz group is a mathematical description of transformations that allow us to move between observers travelling at different speeds, a key technique in special relativity.)

But, if special relativity is a consequence of quantum logic, what about general relativity? General relativity is our best theory of gravity, so could gravity also be a consequence of quantum information? I think so myself, but I’d better leave the reasons behind this to another blog post.

Till then, take care of yourselves,

Vlatko

I didn’t like chemistry in my high school because it was taught through a bunch of seemingly ad-hoc rules. For example, you have electronic orbitals represented as bubbles and, if the bubbles of one atom overlap with the bubbles of another, then these two atoms form a molecule. None of that made sense to me. First of all, why are electronic orbitals suddenly bubbles (and not circles or ellipses)? Secondly, what does it mean for them to overlap? And finally, why does that represent molecular bonding?

Then I learnt quantum physics (for the first time, properly, as an undergraduate at Imperial College in London). Chemistry suddenly started to make sense and, in fact, became an amazing subject. What I realised a while later (after my PhD, also at Imperial) is that atomic and molecular physics could be taught (and phrased) much better than how it was done in my high school textbooks.

In fact, I’ve since realised that the classification of molecular bonds in chemistry (covalent, ionic and metallic) could be done more appropriately using quantum entanglement. In the final analysis, all molecular bonding is based on quantum entanglement, and what distinguishes different molecules is the degree of entanglement between their shared electrons. When two atoms get close to one another, then electrons from each of them can hop (or “tunnel” through, in the quantum parlance) to the other atom. The new states of the electrons in which they are delocalised between the atoms are energetically more favourable, which is what leads to the creation of molecules. So the bubbles and overlaps of bubbles I was complaining about in my high school are all actually different quantum entanglements.

Back to Andrew Cochran. He was intrigued (as all of us are) by the fact that living systems are composed of only a handful of elements. Carbon and hydrogen account for about 80 per cent of all organic molecules, and if one throws in nitrogen, oxygen, phosphorus, and sulphur, we basically have 99 per cent of the structure of all proteins that provide the scaffolding for all living systems. Why only these 6 elements? What’s so special about them?

And here is where Cochran’s amazing hypothesis comes in. He claims that these 6 elements have the lowest heat capacities among all elements. This means that the structures made of them are more robust to environmental perturbations. Having a low heat capacity implies that if the system is heated, its temperature does not increase much. This makes sense, as one would expect nature to seek stability in living systems. But, this is only one half of Cochran’s hypothesis. The other half says that systems with low heat capacity are more quantum-mechanical.

Now, Cochran had a simple logic as to why this is so (simple to the point of being wrong), but I will give you my slightly more sophisticated version (which is, at least, less wrong). This takes me back to some work I did, a long time ago, with Časlav Brukner. The idea was to use macroscopic quantities, such as the heat capacity, to witness quantum entanglement in complex systems. Why is the value of heat capacity related to entanglement? The reason is that the heat capacity tells us about the correlations between the constituents (usually atoms) of complex systems. And the interesting point is that low heat capacities can only be achieved if the constituents are entangled. This is essentially, in my own interpretation, the reason behind Cochran’s insight: low heat capacity means the system is largely in a highly quantum-mechanical state. In fact, there is a third law of thermodynamics telling us that as the temperature approaches absolute zero, the heat capacity should approach zero. If quantum entanglement were not present at low temperatures, then the third law of thermodynamics would have to be violated.

Now, Cochran indeed does not stop there. Unfortunately (from my personal perspective), he is a panpsychist and uses the connection between quantumness and heat capacity to argue that (and I quote): “…atoms and the fundamental particles have a rudimentary degree of consciousness, volition, or self-activity; the basic features of quantum mechanics are a result of this fact; the quantum mechanical wave properties of matter are actually the conscious properties of matter; and living organisms are a direct result of these properties of matter.” In other words, he views quantum wave properties of matter (which, for us, is entanglement) as a consequence of the fact that fundamental particles possess a “rudimentary degree of consciousness”. I believe this conclusion is unwarranted and – at least as far as I am concerned – back to front.

It might be more useful to think of things the other way round. Specifically, elements forming living systems require low heat capacity because this signifies greater resilience to external noise. Simultaneously, low heat capacity indicates a higher degree of quantumness, meaning that the constituents are more entangled. If one were to speculate, perhaps somewhat wildly, one might suggest that consciousness results from the concentration of entanglement in living matter. I personally do not share this view; however, it does lead to an intriguing hypothesis. If low heat capacity is essential for consciousness, then our current computers (which are essentially classical) can never become conscious (although Silicon itself has low heat capacity!). So, then, here is a reason to quantise current Artificial Intelligence Machines—an issue that has already been a subject of much discussion and research—and one I plan to explore further in my future blogs.

Take care of yourselves,

Vlatko

There is, of course, a psychological dimension to this question and, as a physicist, I would not be able to tell you much about it (no surprises here). I, too, feel that time is real and that it even flows (“one second per second” as in the sarcastic remark by the physicist David Park); however, I’ve always found it interesting how in physics, we do not need to use any of these psychological notions. In fact, and this is the point I want to make here, time does not exist at the fundamental level anywhere in any of our theories so far. The important words are “so far” since this might all change when we come up with the full theory of quantum gravity.

Let me explain what I have in mind. First of all, we never measure time directly (whatever this might even mean, to measure time directly). Instead, we use another physical system, called a clock, whose positions actually designate different times. For early humans, the sun and the moon provided such natural clocks and it was their positions in the sky that told them the time. Astronomers still use distant stars for the same purpose (“distant” so that the Earth’s motion does not affect the time-keeping, as we know from the mess we got ourselves into with the Julian and Gregorian calendars). But we have come up with even more sophisticated methods, such as the atomic clocks. Here, we measure how many times the electron changes its position within an atom. Typically, this happens a hundred million billion times in a second, which gives us a reliable tick of the duration of about 10 to the power of -16 seconds. And there are even more accurate ones now.

All of the above illustrate the point that we always use the dynamics and the change of state of one system and call this a time measurement. In this way, we can, in fact, eliminate time from all our fundamental dynamical equations in physics. Think about a particle whose positions are given by x₁, x₂,…,x_N while being “timed” by a clock whose hand positions are respectively given by c₁,c_2,…c_N. All we need is a correlated table of positions of the particle against another such particle that we happen to call a clock. This clearly shows that we do not need the variable “t” anywhere in the equations. There is no time, nor is there any flow of it. Everything can be encapsulated in the questions of the form: when the clock is in the position c_100, where is the particle? And the table will tell us that the particle has reached the location designated by x₁₀₀.

The same “trick” works in quantum mechanics. We need to give the correlated state (the classical correlation now becomes an entangled state in quantum physics) between the states of the system and the corresponding states of the clock. This kind of formulation goes under the name of Page-Wootters, after Don Page and Bill Wootters, who wrote a paper titled “Evolution without evolution” in 1983 and postulated this entangled state. You can see from the title of the paper that they wanted to describe how dynamics arises out of the entangled state between the system and the clock, which itself (the entangled state) does not change in time. Exactly the same as our correlated table of positions in the previous paragraph, since the table itself does not “change in time”.

The magical property of the quantum timeless formulation of dynamics is that different instances of time now become different universes! Time emerges out of entanglement in the same way that the dead and alive cat emerge through entanglement with the decaying atom and the poison in the Schrödinger cat experiment. This is fascinating because the property of being in another universe (say, seeing a living cat instead of a dead one) now becomes equivalent to existing at another time (which, incidentally, we do routinely by just waiting a bit).

Ok. So why would quantum gravity potentially change all this? The reason is that we still don’t know how we should think of Einstein’s theory of general relativity. Some people say (and the “young” Einstein agreed) that gravity is all about the bending of spacetime. If this is true, then when we quantise gravity, we need to quantise space and time. Time, therefore, becomes a fundamental, albeit quantum, element of reality. However, there is another school of thought (and the “older” Einstein agrees with them) which says that general relativity is all about different gravitational fields (there are 10 of those in GR) and when we quantise gravity, we actually need to quantise all these fields. The spacetime would simply emerge out of this in the same way that time emerges from the entangled state of Page and Wootters. In other words, if gravity is ultimately just another field theory, time remains unnecessary, while if gravity is the curvature of spacetime, time (and space) now become fundamental and unavoidable.

So, the fate of time is in the hands of the next revolution in physics. But one thing is for sure. Even the new revolution in physics won’t explain the psychological component of time. For that, we need to understand what consciousness is and how it works.

Take care,

Vlatko

Our result is surprising for two reasons. First, we are only aware of gravity leading to attraction and there is a fundamental relativistic reason for this (I will explain shortly). Secondly, there is no classical way to make gravity behave in this bizarre way. Our idea relies on the quantum superposition principle, a.k.a. being in two places at the same time.

OK. So why does relativity stipulate that gravity must always be attractive? Well, in order to comply with Einstein’s special relativity, all quantities in physics have to be either 4-component vectors (such as 3 components of space and one time taken together into spacetime) or 4-by-4 matrices or higher by always in multiples of 4. The reason for this weird-looking rule is that the relativistic transformations (which we call Lorentz) can only consistently act on such things.

Let’s take the electromagnetic field. The key thing is that it is described by the four-component vector called (not surprisingly) the electromagnetic vector potential. The component responsible for the repulsion of the like charges (such as two electrons) always comes with the imaginary number “i” equal to the square root of -1. This again is the requirement of relativity and I have written extensively about it in the past (and in my latest book “Portals to A New Reality”). Now when we have two charges, each brings one “i” to the interaction and the product of these two is -1. This is the minus one that leads to the repulsion between two electrons!

In gravity the fundamental quantity is not a vectorial one, but a 4-by-4 matrix. Mathematically speaking this is two electromagnetisms put together. So now we have double the number of imaginary “i”s that each gravitating object brings to the table when interacting with another gravitating objects. So with two objects, this is (-1) times (-1) = 1. It is for this reason that the gravitational forces are always attractive. In other words, if gravity were not attractive, this would violate Einstein’s special relativity. So how did Pablo, Chiara and I manage to pull this off?

Enter quantum superpositions. Imagine that one of the two gravitating masses (the “source”) is in a superposition of states while the other one (the “probe”) is localised. In the branch of the superposition where the source is closer to the probe, the gravitational attraction is stronger, while in the branch in which the source is further away from the probe, the gravitational attraction is weaker. Ok, but in both branches the force of gravity is attractive, so how do we make this into repulsion?

There are three parts of every quantum experiment. Prepare a superposition, let it evolve in time, and finally measure in any other superposition. It is this last part that gives us the repulsion. But, it only does so for one of the outcomes of the final measurement. So, what is important is the post-selection: we only have anti-gravity conditionally on observing the right outcome. On average, if both outcomes are included, gravity is always attractive and this is why there is no violation of relativity.

“Oh, well, anything can happen if we post-select”, I hear you say. Yes and no. Indeed, throwing away “bad” outcomes (like the ones where the particles attract) leads us to observe what we want (i.e., repulsion), however, classically, this is not possible, no matter how much we post-select. Our experiment, if confirmed experimentally, would therefore show that gravity can act from two different places at the same time. In other words, gravity is quantum.

How difficult is this experiment to perform? Pretty difficult. It is in the ballpark of other experiments I’ve covered in my blogs (like the B-M-V one). But, luckily, my Italian buddies Marco Genovese, Fabrizio Piacentini and Ettore Bernardi are wizards in the lab and my bet is on them to do it by 2030. So watch this space!

Take care of yourselves,

Vlatko

There are two things I like about emptiness. One is that Buddhists use this logic to try to avoid the need for postulating an “uncaused cause”. If everything needs a cause, then we quickly run into infinite regression since the chain can never be terminated. Even if, as some physicists might say, the cause for everything we see at present in our universe lies in the Big Bang, a question immediately arises as to what caused the Big Bang itself. Note that postulating God does not necessarily solve the problem, since this then begs the question of what caused God to exist before the Big Bang. Realising this problem, Aristotle postulated that God is the cause of everything, but whose cause for existence also lies within God. That’s why he called God the unmoved mover.

But, as I said, Buddhism found another way to get out of the uncomfortably infinite chain of causes, and it is simply to say that everything is mutually interdependent. It seems to be a more elegant solution than Aristotle’s because it does not need to postulate an exception to the principle that “everything happens for a reason” (aka Leibniz’s principle of sufficient reason). Everything happens for an (to it external) reason and even the totality does not exist independently of something else outside of it (if it did, we would be back to Aristotle, and nothing would be resolved) – hence we have an overall emptiness.

The second reason I like the Buddhist relativity is that it goes far beyond the relativity we encounter in physics. In physics, some things are relative, but others are absolute (meaning that they don’t depend on our vantage point). For instance, time is relative to motion, but the time you measure on your own wristwatch is an absolute. Thus, if I am moving with respect to you, my time will pass more slowly than yours (so the passage of time is relative), but everyone in the universe must agree on the rate of my time flow. “Our own time in our own reference frame” is an absolute.

It’s not just relativity that’s relative, as many of my readers will know. Quantum physics is relative too. The states we ascribe to various systems are relative to us. I may think that the state of an atom is excited (one of the electrons has moved from the lowest energy state to one of the higher states), but you may see the state of the atom entangled with me, which means that it has a probability to be excited, but also a probability not to be excited. This is behind the famous Schrödinger’s cat experiment, in which the person inside the box with the cat sees it definitely either dead or alive, while, as far as an outside observer is concerned, the state of the cat is entangled with the internal observer (hence, abusing the language just a bit, the cat is dead and alive). I’ve talked extensively about this in my papers “Observing the observer I and II” as well as in my latest book “Portals to a New Reality”.

Now, the physical relativity, both Einsteinian and quantum, is completely quantitative and therefore has huge predictive power. Consider this: you can never tell whether you are moving at a constant speed, no matter what you do within your own reference (you are not allowed to look outside of the window). This means that, even though your wristwatch ticks more slowly than someone else’s who is stationary, all other processes in your frame have to slow down at exactly the same rate. Prediction: your thought processes are also slower, all your biological processes are slower and your toothache lasts longer, too. Why? Because otherwise you’d be able to tell that you are moving, which the principle of relativity says is prohibited!

Buddhist relativity, on the other hand, is much more radical since it says that there are no absolutes. Both you and your wristwatch lack any essence and only become “real” through your mutual relationship. In fact, even that relationship only acquires meaning through something else and so on. But, the price to pay for this generality is that the Buddhist relativity does not have any quantitative predictions to make. It may be psychologically comforting (though even that is dubious, to say the least), but it lacks the explanatory power of scientific principles.

However, and this is the main point of my blog, could we use the Buddhist logic to relativise more things in physics, but in a quantitative manner? A legendary (now late) physicist, David Finkelstein, definitely thought so. One of his mind-blowing suggestions was to relativise the laws of physics. At present we think of the laws of physics as absolute (to the point that we believe even aliens must be aware of Newton’s laws), but could it be that the laws too are relative to something else? Finkelstein sadly didn’t get a chance to make progress in this direction…but it is a fascinating question to contemplate.

I wouldn’t be me if I didn’t finish with something quantum. Entanglement is a concept that comes very close to emptiness. When two systems are entangled, then their individual properties lack essence (which you can think of as the definiteness of classical physics). The cat is neither dead nor alive and you have neither seen it nor not seen it, but the joint state of you and the cat is still definite (and could be said to possess essence). In the joint state, both options are present simultaneously, the one where the cat is dead and you see that the cat is dead and the other one where the cat is alive and you see it so. One might thus be tempted to say that nothing but entanglement is real.

However, and this twist is worthy of the Buddhist emptiness, even entanglement can lack essence. This is because there are states in quantum physics in which entangled states get themselves entangled. This is known as (no surprises here) entangled entanglement. So, even entanglement can lose its essence because the entangled systems could get entangled with some other systems. We can push this to the edges of the universe and create higher and higher level entanglements; however, surely the ultimate entangled state of everything in the universe would still be real in the sense that there is nothing else it could be entangled to. Could relativising even this state lead us to a generalisation of quantum physics of the kind that Finkelstein was suggesting? I don’t know but I sure feel lucky to be able to earn a living by thinking about problems like this.

Take care,

Vlatko

There are books and books written on the topic of time and, if you read even a handful of them, you will quickly realise that there are as many views regarding the nature of time as there are philosophers out there (maybe even more). The two extreme points are the view that time is the most fundamental entity in the universe and the view that there is no such thing as time at all but everything in between is represented too.

For Newton, time was fundamental, and it provided the background arena (together with space) in which the stuff of physics unfolds. For Leibniz, on the other hand, time was simply a way of parametrising changes that happen around us. Both Newton and Leibniz have been just two in the long line of thinkers stretching back to the Ancient Greeks (not surprisingly, I admit. In fact, I challenge you to think of a single contemporary idea whose germ was not already present in the Ancient Greece).

Psychologically, we all side with Newton, in a way. We “feel” the passage of time, so it’s difficult not to think of time as real and flowing like a river, independently of us, from its spring to the final destination, whatever that might be. But, and this is where Leibnizian thoughts come in, time is something we never measure directly (unless you think that our feelings regarding the passage of time represent such measurements). We always compare the rate of change in one system using another system as a reference. And actually, we almost invariably look at the changes in spatial configurations. For example, we think about how much the world’s population has changed since the Earth has gone 50 times around the Sun. In other words, our clock here is the Earth revolving around the Sun and the system we observe is the number of people living on it.

And it’s not just our astronomical clocks that work like this. Our atomic clocks do too. For instance, we would ask how many times the electron in the atomic clock has changed its energy and this is our most accurate way of measuring time. But, actually, we are just counting the number of times the electron has moved further away from the nucleus, i.e. how many times it changed its position within the atomic clock. Time is always “relational”, meaning that it is measured with respect to some standard (like the Sun and an atomic clock) and it is the changes in the state of this standard that we use as a proxy for time.

In other words, and this is perhaps what Aristotle would have agreed with, stuff happens, and we conveniently use this fact (that stuff happens) to distil time out of it. But time, according to this logic, would not be fundamental. It’s a convenient way of accounting for the fact that changes are fundamental.

Now, believe it or not, in the whole of physics, including all of classical physics, quantum physics and quantum field theory, time is always relational. We can, in fact, do away with it by removing it from the equations just as we have described above: whenever you want to talk about the state of a physical system (say, the speed of a car) at a time t, you can substitute the position of the Sun for t and get exactly the same result.

But, and this is the point of my writing this blog, the situation seems to change dramatically if gravity is quantum.

My readers will be aware that physics has two fundamental theories, quantum physics and general relativity, which have not been unified to date. Many credit this lack of unification to the incompatibility between the two theories; however, I think it’s really all down to the total lack of experiments in the domain where both quantum physics and general relativity matter. Be that as it may, why would quantum gravity impact the discussion whether time is fundamental or not?

The reason is that one way of thinking about gravity is as a twisting of space and time. When there is no gravity, space and time (in fact, united into spacetime) are flat, but, with gravity, they become curved. As an example, a clock closer to the Earth ticks more slowly than the clock further away. Putting this in our relational language, the electron in the atomic clock changes its position fewer times inside the atomic clock closer to the Earth than the electron further away. The dramatic shift of perspective here is to say that it’s not that gravity affects behaviour of clocks, but that gravity IS the change in the behaviour of clocks in different spatial locations (in other words, there is no such thing as gravity).

Doesn’t this perspective make time fundamental? Not yet. The reason is that there is no backaction on time! What I mean here is that in classical general relativity, the time may affect clocks, but clocks do not affect (the supposed underlying) time. In this sense, the time (and space too) in classical general relativity would be the unmoved mover (just like Aristotle’s God). The gravitational field is affected, but that doesn’t necessarily mean that time is (it could still be just a label for the field). However, this would all change if time were quantised because then the interactions of the quantum time would have to include the backaction from the matter (like an atomic clock) that time couples to. Quantum time would affect clocks, but clocks would also affect quantum time. In fact, if you put a quantum clock into a superposition of being closer and further away from Earth, then – if time is really quantum – this clock would get entangled to that quantum time. It’s Schrödinger’s cat all over again.

There are two questions arising from all this. One is whether we could ever detect this supposed backaction on the quantum time. We don’t know, and it also depends on how time ends up being quantised. The second question is how we could discriminate the effect on the time itself from the effect on the gravitational field (which is an entity that exists in time, like all other fields). It’s fair to say that we have no answer to this question either and that’s because we don’t really know what it means for gravity to be quantum.

It is possible that all quantising gravity may entail is basically quantising the gravitational field while time still remained a fictitious parameter with which to express the fact that (quantum) stuff happens (and one can still express this without time and by using the number of “jumps” of an atomic clock)? In that case, even quantising gravity would not mean that time is a real fundamental thing in the universe.

My two cents on this? Quantum gravity will turn out to be “timeless” and time will therefore remain a convenient fiction (as it has been in other parts of physics). But it will take much longer to explain why, if the universe is mathematically just so, we feel that there is a temporal quality to everything. The future and the past don’t exist as far as we are concerned; only the “moving” present does. Of course, this wouldn’t be the first (or the last) time that our intuitions are out of sync with the physical reality. Surprising ourselves with the counterintuitive discoveries regarding the nature of reality is, after all, what makes doing science the most intellectually stimulating endeavour of all time.

Take care,

Vlatko

This account is misleading for a number of reasons. For starters, it gives us the wrong impression that we, as observers, make a crucial difference to how the particle behaves. Secondly, it seems to suggest that looking at superpositions collapses them and, at the same time, it hints at the fact that it is our knowledge of the way the particle behaves that matters. All of this is, to put it bluntly, wrong.

First of all, observers are not needed to remove quantum interference (or to create it for that matter). In addition, there is no such thing as a collapse of quantum superpositions. Lastly, it does not matter one iota what we know or do not know about the quantum system. Quantum physics is as objective as the classical physics of Newton (warning: not every quantum physicist agrees with this).

You might say, “Oh, well, that’s just your opinion. It’s just your own interpretation of quantum mechanics.” That statement too would be wrong and now we have a beautiful set of experiments to demonstrate why. I am talking about two recent Physical Review Letters where the results of Jian-Wei Pan’s (one of the leading quantum computational experts) and Wolfgang Ketterle’s (Nobel Prize in physics for achieving the first Bose Condensation of atoms) groups were announced.

The experiment they performed (independently, but the setups were similar) was a variant of the one endlessly discussed by Bohr and Einstein. It was one of Einstein’s many attempts to contradict quantum mechanics. He kept insisting so much that he “didn’t believe God played dice with the universe”, that at some point Bohr lost patience and told him to: “Please stop telling God what to do”.

Einstein’s idea was this: when a particle goes through the upper slit, it gives an upward momentum kick to the screen containing the slits, while when the particle goes through the lower slit, it gives a momentum kick in the opposite direction (this is just a consequence of momentum conservation). So, Einstein says, if you measure the momentum of the screen, it tells you about the momentum of the particle, and then you can measure the position of the particle directly. This looks like you can know a particle’s position and momentum at the same time. Ergo, Heisenberg’s uncertainty principle is violated and, since Einstein used quantum physics to prove that quantum physics is wrong, it must be that quantum physics is inconsistent. In other words, Einstein claimed that you can have interference while (seemingly at the same time) know which slit the particle goes through and this is a clear violation of quantum physics.

It’s an ingenious argument, like all of Einstein’s arguments, but (unlike most of Einstein’s arguments) it is wrong. The way that Bohr replied to these kinds of arguments, however, was always convoluted and in terms of complementarity. The commonest view is that Bohr won the debates; however, it is not always entirely clear how and why. Bohr would say something like this. The wave and particle aspects of a quantum system are complementary and can never be confirmed simultaneously in one and the same experimental setup. So, if we want to see interference, this requires an experiment to be set up that lets the system behave like a wave. If we want to confirm that it’s a particle, then we must set up a detector, but this then prohibits any wavelike behaviour. As I said at the beginning, though, this narrative places too much emphasis on our choices of what to do and whether to measure or not.

The experiments by Pan and Ketterle showed exactly what’s at stake (hint: think Schrödinger). They used a single atom that could be prepared in different states of momentum uncertainty: its momentum is in a superposition of different values. Now, a photon can be emitted by this atom, and we would like to see if this photon can interfere. The photon can be emitted in one direction, in which case the atom (like the screen) recoils in the opposite direction. But if the photon is emitted in another direction, then the atom recoils opposite to that direction. This way, the state of the atom becomes entangled to the state of the photon. More precisely, the photon being emitted in two different directions is entangled with two different corresponding recoils of the atom.

Now we are in the position to talk about when the photon interference takes place and we don’t need the wave-particle duality or any other complementarity-related language! If the photon is maximally entangled with the atom, it loses the ability to interfere. This happens when the kick that the photon gives to the atom when emitted is much larger than its initial momentum uncertainty. Then the kicks in the opposite directions become perfectly distinguishable states which maximizes the amount of entanglement between the two. If, on the other hand, the kick is small compared to the uncertainty, the two atomic states after the emission remain virtually one and the same state, in which case there is little entanglement between the atom and the photon – and this leads to the possibility of photonic interference. One can say that in the former case, the atom has measured the path of the photon while in the latter in has not.

Note, however, that all other states between the “no” and “maximum” entanglement are possible. In this case, the photon can interfere, and the degree of interference is reduced by the amount of entanglement. This “in-between scenario” is sometimes referred to as a weak measurement.

In general, the more entangled two systems are, the less quantumness (interference) each can exhibit individually. So the quantumness (entanglement) at the level of the total system is what prevents quantumness at the lower level of subsystems. This is an aspect of a more general principle, namely that quantum dynamics preserves quantumness. That’s all there is to it. No observers, no complementarity, no wave-particle duality. Or, as the philosopher A. J. Ayer might have put it, “boo Copenhagen and hurrah entanglement”.

Take care of yourselves,

Vlatko

There is one question I get asked frequently as a quantum physicist. It’s to do with my preferences regarding the interpretations of quantum mechanics. I am a realist in the sense that I think there is a world out there independent of us. I also do not like the idea that observers play an important role (some would say crucial) in quantum physics. I don’t think observers are needed at all, and, moreover, I am convinced that the only consistent way to account for everything (at present) is by treating the whole universe as a quantum system.

Interestingly enough, there is one interpretation that ticks all of the above requirements. This is why the question I referred to earlier happens to be: “Why are you not a Bohmian?” A Bohmian is a person who subscribes to the de Broglie-Bohm, or hidden variable, interpretation of quantum mechanics. The interpretation is certainly realistic as its ontology (i.e., what it considers to be real) is basically that of classical Newtonian physics: particles have well-defined positions and momenta at the same time. Hence, it has the alternative name (hidden variable interpretation) since in quantum physics proper, positions and momenta are subject to the uncertainty principle and their values cannot be specified at the same time. The de Broglie-Bohm interpretation also does not require us to postulate a special status for any observers. In fact, observers and everything else in the universe are subject to the very same universal ontology. So far so good.

To make things even more interesting, this interpretation agrees with all the results of the standard quantum mechanics (which is why we call it an interpretation, for otherwise it would be a different theory). So, we can safely say that it has been “confirmed” by all the experiments done so far (just like the standard quantum mechanics).

Given all this good stuff, what’s my problem with it? I have basically two issues with Bohmian mechanics. One is that it is non-local. In other words, it admits spooky action at a distance: you do something over here and instantaneously something over there changes. Bohm himself emphasised this issue and actually thought of it as a welcome feature rather than a bug. His book on this topic, co-authored with Basil Hiley, is titled “The Undivided Universe” and you can already see from the title that they regard non-locality as the key aspect of the universe. Everything is non-locally connected to everything else.

The non-locality is needed because all the properties in this interpretation are described by c-numbers (i.e., the ordinary real numbers). And, if you want to reproduce quantum results – entanglement in particular, you then need action at a distance to correlate these c-numbers in such a way that they behave like quantum numbers (i.e., like matrices). After all, as John Bell told us, you have to give up either c-number-based reality or locality if you want to account for the correlations between quantum entangled states. I choose to give up the c-number-based reality, but Bohm chose to give up locality because he believed that giving up the c-number-based reality is a price too high to pay. Note that Bohmian non-locality does not imply signalling, so that the Bohmian mechanics fully complies with relativity (as I said, it really is just another interpretation).

But, if all Bohmian properties are c-numbers, where do we get the uncertainty principle from? Well, we assume that there is a (classical) uncertainty in the initial position and momentum of the particles and, since Bohmian mechanics is basically Hamiltonian, it preserves this uncertainty in time. So every particle is basically an ensemble of randomly distributed particles. This is another reason why I don’t like this interpretation, because it postulates an unavoidable initial uncertainty which, unlike with q-numbers, has no explanation for it (yes, many papers have been written on this very topic and some creative ideas have been suggested, but still, there is nothing convincing to put forward here as to the origin of this uncertainty).

Now, for my main point of discomfort with the hidden variable interpretation. It is extremely inelegant and cumbersome to use when calculating anything. Take the standard single-qubit interference experiment. We first put a particle in a superposition of being in two locations at the same time (I am using the language I like here), and then we bring these two back together to the same location to interfere them. All quantum experiments are of this kind. But, because Bohm does not want to accept that a particle can be in two places at once, his interpretation says that the particle is only in one of the two places. So how does it then interfere at the end? The trick is done by the so-called pilot wave (de Broglie’s terminology). The particle is guided by the pilot wave to interfere just as if it was in a superposition of two places in the first place. So, this pilot wave is the entity that has to exist in those two locations at the same time, even though it only acts on the particle in the (single) position where the particle is. This is where the non-locality of the interpretation creeps in.

Ok, you might say, tomayto, tomahto. Either the particle is in two places or the pilot wave is, I don’t see the big difference.

The difference is that we’ve (Bohm has) bent over backwards to avoid using q-numbers and give a c-number account of quantum physics, but – in the end – we still need the pilot wave (aka the quantum wavefunction) to lead us to the same conclusions (that are supported by all experiments). This is because the pilot wave ultimately obeys the same Schrödinger equation as the quantum wavefunction.

So, to many practising physicists, the Bohmian interpretation sounds the same as the standard quantum mechanics, but with just an awful lot of extra unnecessary classical baggage (such as particles being localised and having a well defined classical momentum).

There is, of course, another interpretation that says that observers are not needed and that the wave function is real and applies to the whole universe, and it goes under the name of Many Worlds. This is why my colleague David Deutsch says that “Bohmians are Many Worlders in denial”. Funny, but it is certainly true that these two interpretations are the only two realist ones.

Finally, how do Bohmians view quantum fields? You might not be surprised to hear that there is no consensus here. Some supporters say (Bohm was of this opinion) that the fields are classical but non-local and there are therefore no particles corresponding to them (so photons do not exist as excitations of the quantum electromagnetic field in Bohmian mechanics). Others believe that only particles should exist, even when it comes to fields (in other words, there are no fields according to this version of Bohmian mechanics). Each approach has advantages and disadvantages.

I find the Bohmian account of quantum fields interesting because, as some of my readers may know, I firmly believe that gravity will also turn out to be quantum mechanical. The experiments I have in mind to test this, however, won’t tell us if gravity is classical and non-local in the Bohmian sense or really underpinned by q-numbered gravitational potentials (and local, as I’d like to believe). Maybe, just maybe, the Bohmian interpretation will thus always remain an interpretation, and therefore choosing it over another interpretation will only be a matter of taste. I personally don’t like it, but I also don’t like Marmite, though I cannot give you any rational reasons for that...

Take care, Vlatko

It’s true that one need not use q-numbers to account for all the quantum experiments so far – but, then we would need to admit signals that propagate faster than the speed of light. In other words, we would need to introduce a “spooky action at a distance”, as Einstein called such superluminal effects, which clearly would violate the theory of relativity (according to which no information can travel faster than light). I myself think there is actually nothing wrong with relativity and, if we stick to q-numbers, quantum physics is perfectly fine with relativity too (needless to say there is no violation of relativity to date either).

Now I’d like to tell you about a thought experiment that probes exactly the domain where both quantum physics and relativity matter. And, initially, this thought experiment was a puzzle that came up during a discussion between Anton Zeilinger (Nobel Prize for physics, 2022) and Roy Glauber (Nobel Prize for physics, 2005). Actually, to tell you the truth, I am not sure what the exact history of this puzzle is, but you’ll hopefully excuse me because I am not a historian.

Suppose you have an excited atom inside an optical cavity (two parallel mirrors, one on each side of the atom). Now, if the length of the cavity (i.e., the distance between the mirrors) is suitably adjusted, the atom will not be able to emit a photon and get de-excited. The reason is that no frequency in the cavity matches the energy of the would-be-emitted photon and so this process cannot happen. The effect is hence called the “inhibited spontaneous emission”. It is of great practical importance for quantum technologies when we want to have stable storage of quantum information.

Now, the thought experiment that was discussed involves a sudden removal of one of the mirrors and its replacement by a photo-detector. Question: When will this detector be able to click? Zeilinger argued that as soon as the mirror is moved, the atom will emit a photon since it is no longer inhibited, and this photon will travel to the detector and thus be detected. So, the time of detection is the time it takes the photon to go from the atom to the detector. Glauber, on the other hand, argued that, first, the atom needs to “learn” that the mirror is removed. So, a signal travels from the location of the mirror to the atom and, only then, the atom emits a photon which propagates to the detector. So Glauber’s time for the detector to click is twice as long because light needs to make a round trip mirror-atom-detector.

To make things more complicated, Heidi Fearn, Richard Cook and Peter Milonni wrote a formal paper about this thought experiment in 1995, arguing that the detector could detect a photon immediately! At first sight, this sounds like spooky action at a distance. Surely the emission from the atom cannot reach the detector instantaneously. But, Fearn et al. argued differently. The state of the atom in the cavity is not just the “atom excited and no photons in the cavity”. It is instead quantum entangled. There is a probability for the atom to be excited, but there is also a probability for the atom to be already de-excited and for one photon to be in the cavity. This is a stable (entangled) state and remains so until one of the mirrors is removed. The reason for this entanglement is that the atom interacts with the light in the cavity and it is this interaction that generates entanglement. And it is the photon in this entangled state that could be detected immediately when the mirror is removed.

We therefore have three possible answers: a) the photon can be detected after light gets from the atom to the detector, b) after twice that time and c) immediately . Luckily, the experiment did not remain just a thought experiment. David Branning , Alan L. Migdall and Paul G. Kwiat performed it in 2007. So who was right in the end? Well, as fate would have it, neither of the Nobel Laureates got it right. That honour went instead to Fearn and her colleagues, Cook and Milonni.

Amazing stuff, but I can tell you one thing for sure: Schrödinger (of the cat fame) would not be surprised. Unlike Feynman, who thought that superpositions were the only mystery, Schrödinger maintained that entanglement was the characteristic trait of quantum physics. And it is Schrödinger’s entanglement that allowed the photon to be detected instantaneously by Branning, Migdall and Kwiat. More precisely, it is because the atom and the cavity light were entangled to start with, before the mirror was removed.

But, and here you have to trust your humble narrator (or, even better, go and read his latest book), this does not mean that entanglement is spooky. Seeing it through q-numbers makes all the difference.

Take care of yourselves and stay tuned (for next time, I will get even more physical),

Vlatko

We have been stuck with the same two physics theories for over one century and it’s getting pretty boring. We’d love to be in what we think was the position of physicists in 1900, having to resolve two major inconsistencies that led to two massive scientific upheavals through the discoveries of general relativity and quantum physics. However, even at that time, most leading physicists didn’t see it coming. Kelvin gave a famous Royal Society address in 1900 – at the dawn of the new century - at which he was announcing the end of physics, in the sense that everything there was to understand had already more or less been understood. Kelvin admitted that there were only two clouds on the horizon, which were the inability to detect aether and the inability to explain the black body radiation. The clouds were clearly not small, since the former led to the theory of relativity and the later to quantum physics.

I think that we are now in a similar position, namely on the brink of a new revolution. But this is not necessarily because of any existing experimental inconsistencies. Instead, quantum information has opened up new avenues for experiments which we haven’t really capitalised on yet. So, in my book, I use all these things we’ve learnt to argue that the next step might be closer than we think. It’s just that we need to look for the right things and do it with an emphasis on the fundamental physics.

And the right things to look for are 5 key experiments that probe the features of our current theories that are most questionable and problematic. In my view, they are not just small clouds on the horizon, but are actually portals into the future of physics. The interesting revelation that I have come upon during my latest 10 years of work, which is why I think the word portal is apt, is that most of these experiments are not actually that far from being technologically feasible. At present, they are just still at the level of ideas - but nothing actual is in the way of realising them, other than a huge inertia that makes us less likely to attempt ambitious new experiments. In my book, I have explained why these experiments are essential to our progress in physics, and at the same time that they have now become accessible with our current technology. It is this fact that makes the book timely and should be an exciting read for anyone curious about the current state of physics and about how we can reach the physics of the future.

The main questions are “what’s next for physics?” and “how do we get there?”. Along the way, the introduction and the first few chapters offer a fresh perspective on both quantum physics and the theory of gravity, which are the two current theories on which our entire understanding of the universe is based. I believe that my perspective is novel and different from the prevailing views that are represented in the popular press. One of the dominant ones is that quantum physics is weird and cannot possibly be true at the macroscopic scales (e.g. Penrose, Smolin). Another one is that the fundamental theory that unifies quantum physics and gravity is super esoteric (estring theory, e.g. Greene) and even not accessible to experiments (but that’s ok, a la Tegmark, who thinks that every possible mathematics is represented in some universe and we just live in one of those – clearly a view that cannot in any way be tested). There are also modern-day Lord Kelvin’s out there (Wilczek is perhaps the most prominent example) who think that the present Standard Model of physics is more or less it. Instead, I will explain how quantum theory is in fact the rock-solid basis on which these windows of the future theory can be built, and that quantum gravity and theories beyond are a natural expansion of the ideas of quantum theory to gravity, with no special mystery involved - just fascinating problems that the experiments in each chapter will uncover.

The main unique feature of this book is that it offers a new perspective on the quantum universe by presenting the unexplored territory where quantum physics gives us insights going well-beyond what is currently understood. It presents a report from the battle-front by someone who is engaged with testing quantum theory in completely new domains, such as at the interface with living systems, artificial intelligence, as well as the macroscopic objects such as gravitating masses, black holes and, ultimately, the whole universe. It is exactly from these kinds of ideas that I claim a new theory of physics is expected to emerge. Each chapter centres around the description of one experiment, each of which is seen as a portal to the new reality. I have done this in the “style of Feynman” that emphasises, in an honest way, the true spirit of the physical sciences. I’ve done my best to project the marvel and excitement regarding the open questions we are facing and the mysteries we need to grapple with in order to understand the deeper nature of reality.

I hope that my book is an exciting exposition of our intellectual adventures as we explore new physics frontiers. I would like the wider public to know that we have some major roads leading us to the future reality and that we are actually very close to exploring them. I hope that I have also communicated that what’s ahead of these roads is something that will be as exciting as quantum theory, if not more, but is not incomprehensible or esoteric. It’s going to make perfect sense and it is for this reason that it will be beautiful and fascinating.

When I write, I always have my 16-year old self in mind (though I think anyone curious about science will find the ideas exposed exciting). Indeed, I would also like to inspire the next generation of physicists to be more adventurous than the current and previous ones, and to try and realise these experiments. I would like my book to become a manifesto to champion transformative and fundamental research, advocating that we are at a fertile point in the history of physics and that we must seize this moment, both from the theoretical and the experimental points of view.

I frequently hear it said that the last century was the century of physics (as far as the fundamental discoveries are concerned) but that this century belongs to biology. I profoundly disagree with that and believe that this century too belongs to physics. I hope that this book will present a convincing argument of why this is the case.

The physics community is always split when it comes to the fundamental questions (even when it comes to what the fundamental questions are). In particular as far as quantum physics is concerned, there are many views present in the literature. One is that quantum physics will someday collapse, with a number of culprits offered by various researchers (living systems, gravity, complexity, scalability and so on). I will be arguing against all of these. While I will be saying that from the current perspective, it seems perfectly consistent and reasonable to view the whole universe as one big quantum system, it also seems likely that there will be a superseding theory to quantum physics. But this theory will be more general and will have a similar relationship to quantum physics as quantum physics has to the classical one. I have presented my logic for this point of view and offered these five doors which we have to try to open in order to take a peek at the next theory.

I sincerely hope you enjoy the ride.

Take care of yourselves,

Vlatko

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Here, I want to counter such sentiment and instead discuss one crucial similarity between the two theories. In fact, if it were not for this similarity, probably the theory of relativity would not make sense in the first place! The similarity I have in mind is that things in quantum physics are observer-dependent too.

A few comments are in order. If you’ve read my previous blogs, you will know that I don’t think that observers are needed anywhere in physics. I haven’t changed my mind; it’s just that I am using the concept of “observers” simply as an educational tool in this particular blog. Everything could be said (but in a less transparent manner) without observers. Secondly, everyone knows that in Einstein’s relativity, it is space and time that are relative to the observer (namely, they depend on the speed of the observer). The key point here is that the concept of simultaneity is relative to the motion of the observer. In other words, two events that appear to happen at the same time as far as I am concerned will not be happening at the same time for another person traveling at some velocity with respect to me.

So far so good, but what is the relativity in quantum physics? The answer is that the state that we ascribe to a physical system is relative to the observer in quantum physics. For instance, I may see an atom here or there, but for you, the atom may still be in a superposition of the two states or even a mixture of the two states of here and there. Just think about Schrödinger's cat experiment. As far as the cat is concerned, she may well be either dead or alive (i.e., in a definitive state), but a person who has not interacted with her will have to accept that the state is a mixture of one or the other.

Now, the crux of what I’d like to communicate and discuss is that if things in quantum physics were not relative in this way, we would have a problem with relativity in the theory of relativity too. The reason is that we should be able to think of getting a definitive outcome in quantum physics as a relativistic event. Something happens when we make a measurement and obtain an outcome. The outcome itself constitutes a relativistic event, and we can attribute a space and a time coordinate to it (dependent on the observer's state of motion as before).

So, if in quantum states of physical systems were absolute (a dead cat is dead as far as everyone in the universe was concerned), this would be a problem for relativity because what’s simultaneous for one observer is not so for other observers. Take the famous example in which two observers are moving away from each other at a constant speed. Each observer thinks that the other one ages less! Pushing this to its most dramatic conclusion, in each reference frame, the observer in that frame will die before the other observer has died. If quantum physics were not relative in the same way as relativity, we could never have a unified theory of quantum physics and special relativity (called quantum field theory, a topic I covered at great length in the past).

So, we may well think that both quantum physics and relativity are counterintuitive (and they certainly are) but their main counterintuitive features are exactly the ones that make them compatible. Whether the same will turn out to be true for general relativity is an open question; however, as far as I am concerned, this is a real possibility.

In my conclusions, I'd like to get more philosophical. We have two main interpretations of quantum physics and two main interpretations of the special theory of relativity. The quantum interpretations I have in mind are the Copenhagen interpretation and the Many Worlds interpretation (or what I would more generally like to call “Everything is a Quantum Wave” interpretation). The interpretations of relativity are the Einsteinian and the Lorentzian ones.

The remarkable thing (at least to me) is the similarity between Einsteinian relativity and the Many Worlds interpretation on the one hand and between the Lorentzian relativity and the Copenhagen interpretation on the other hand.

Here is what I have in mind. The issue in quantum physics is whether we think of it as applicable at all scales. The same is true for relativity. If you think the answer is yes to both, you subscribe to the Many Worlds interpretation and Einsteinian relativity. In this case, you think that there is no place for an absolute classical world outside of the quantum domain, and you also think that all motion at constant speed is relative, in the sense that there is no absolute frame of reference anywhere in the universe with respect to which to measure your own speed. In either case, there is no place for absolute observers (the ones whose outcomes are true everywhere and everywhen).

On the other hand, if you believe that an outside classical context is needed to make sense of quantum measurements, you are a Copenhagen supporter (or you believe in the collapse of the quantum superpositions, but this is then not any interpretation, it’s a different theory to quantum physics altogether). As far as relativity is concerned, if you think that the universe contains an absolute frame (what used to be called aether, but many things could play this role such as the cosmic Microwave background), then you are a Lorentzian as far as the interpretation of relativity.

Of course, your beliefs could also be crisscrossed. You could be a Lorentzian Many Worlder or an Einsteinian Copenhagen supporter. These are interesting alternatives as the choice seems bipolar: you think that quantum physics applies to everything, but the principle of relativity stops at some point where the absolute frame starts; or, you think that quantum physics needs a classical boundary, but the fact that everything is relative is universal. Both are logically possible.

These questions are not just interpretational. If we establish experimentally that some objects are only classical and that some frames of reference are absolute (establishing such things could be challenging, but – at least in principle – doable), then it is clear that Copenhagen and Lorentz would be vindicated.

It is also possible, as I have been advocating, that the two issues in quantum physics and relativity are connected. Namely, the objects that we prove behave only classically could also provide the absolute frame in relativity! This option is interesting to contemplate, as it would reduce two mysteries to one; however, it would force us to look for new theories that upgrade both quantum physics and relativity. Win-win, I’d say.

Take care of yourselves,

Vlatko

The basis for the editorial was an extensive survey of more than a thousand quantum physicists (including yours truly) about the interpretations of quantum physics. Have a look at the Nature article for yourself if you feel like it, but the gist is this: we are all still in disagreement about what quantum physics is trying to tell us, but – or so do the editors of Nature conclude – this is for the better (why? Because there is strength in diversity, or something along those lines).

Let me make a sharp turn momentarily before I return to the main topic, which will be to try to explain why researchers who have spent their lives working with quantum physics still end up having completely diverging views about its status and meaning.

Enter: Thomas Sowell, an American economist. Sowell wrote an influential book in the eighties, asking a similar question, but about politics. How can people who all genuinely want to make the world a better place (fairer, more just, more equal, etc) end up taking diametrically opposing views as to the policies required to do so? Crudely speaking, why do some people end up on the left of the political spectrum, while others populate the political right (or, quantum mechanically, why do some people end up supporting the Copenhagen Interpretation while others are Many Worlders?).

Sowell’s insight was this: people fall into two categories as far as their beliefs in human nature are concerned. One bunch, which he called the “unconstrained”, believes that there is no limit to human improvement. The other bunch, the “constrained”, on the other hand, maintains that human nature will never change and that it will always remain fault-prone.

Those who have the optimistic, unconstrained view of human nature also believe that society can nudge people to keep improving ad infinitum. More significantly, they naturally think that some people are already further down this road of improvement and that it would be great to entrust them with the key decision-making facilities (you can see how easily this logic could lead us into a totalitarian society).

The pessimistic view of human nature, in contrast, impels people to believe in the need for strong institutions, the rule of law, traditional values, and so on (all as part of the error-correcting machinery which is necessary for a stable society if human nature is unchanging and faulty). And this, to state the obvious, is the basis of modern liberal societies.

Now, ever since I leant about Sowell’s idea – it’s possible that I read about it in another book – I wanted to adapt it to quantum physics to explain the diversity of its interpretations. Can quantum physicists be classified – again, crudely speaking – into two categories according to some fundamental issue they might have regarding a more basic worldview? What are quantum physicists constrained and unconstrained about?

My feeling is that the relevant question is (and here I am really speculating): will physics be able to explain everything in the Universe? If so, a view some might call “scientism”, this leads us to one interpretation of quantum physics; otherwise, we have non-scientism, and those with such intuitions will see quantum physics very differently.

The researchers in the first category are normally Many Worlders; they are comfortable with determinism, absence of free will, and most of them hold strong AI views (the position maintaining that consciousness can be simulated by computers). The researchers in the second category are either Copenhagen, collapse or Qbist supporters. In a nutshell, they believe that quantum physics needs to be supplemented with something else (something intrinsically classical) to make sense of it. They are almost always anti-determinists; they invariably believe in free will and think that human consciousness transcends not only quantum physics, but possibly also any conceivable computation. Not surprisingly perhaps, members of the second group are more likely to be religious.

Like I said, the above is just my guess based on anecdotal evidence and conversations with my colleagues (Nature magazine can do another survey to try to test my speculation). However, and this is the key difference between science and politics, all quantum physicists agree on all the experiments so far – the mathematics underpinning quantum physics, unlike the logic underpinning a perfect society, has never led anyone astray! But – and an important but at that – different interpretations will stimulate us to ask different questions. Some of these questions will be better than others, meaning that they will more likely take us to a new physical theory (while others are basically just dead ends).

My upcoming book, “Portals to a New Reality”, is all about the kinds of questions we should be asking, experimentally speaking, to finally break through to new physics. And, if you read it, you will most certainly find out which group of physicists I myself belong to.

Take care,

Vlatko

The reader who is familiar with how we teach physics, and indeed with how some of the pioneering papers in quantum physics and relativity were written, will be baffled by my first claim. Didn’t Einstein talk about an observer on the train and another one outside on the platform, and how they perceive different events as being simultaneous? Also, isn’t the whole point of Heisenberg’s uncertainty principle in quantum physics to put a limitation on what observers can observe and how well they can measure positions and momenta of particles?

Yes, this is indeed how many relativistic and quantum concepts were introduced. But observers are ultimately not needed. Every physical object we study is already a sufficient “observer”, and we need not introduce any extra ones on top. So, any object (say, an atom) undergoing dynamics on a moving train will have its time slowed down with respect to the objects stationary on the platform. These different times need not be recorded by observation or written into the computer memory; it simply suffices to compare the moving and the stationary particles and no other physical objects need to be present (let alone conscious human observers). As a side remark, quantum physics makes this scenario even simpler because a quantum particle could be in a superposition of being on the train and being on the platform! In this case, the difference between times could be inferred by interfering the two branches and, again, no observers are needed for this.

A modern way of talking about special relativity is simply to acknowledge that all our dynamical laws are invariant under the relativistic transformations. Relativity is already encoded into the basic structure of everything and does not require observers to verify it. Every bit of the universe, the micro and the macro, obeys relativity so far as we have tested.

OK. You might grant me that observers are not needed is true for relativity. But how about quantum physics? Surely here, observers are necessary. Aren’t things always in superpositions of different states until observed? That seems to be the key aspect of quantum physics.

Not so. The key aspect of quantum physics is entanglement (Schrödinger called it “the characteristic trait”). When two systems are maximally entangled, neither of them contains superpositions and can therefore exhibit no quantum interference. Let me give you an example. A photon going through two slits will interfere at the detection screen behind the slits. If we observe which slit the photon goes through than no interference takes place. Before you conclude that our observation therefore affects reality (it does, but not in the way most people would like to think), I’d like to say that anything else getting entangled to the photon in a way that reveals which slit the photon goes through will have the same effect.

For instance, if we make the photon going through one slit polarized in a way that is distinguishable to how the photon going to the other slit is polarized, then interference also disappears. So, in this case, one degree of freedom of the photon (its polarization) entangles itself to the path of the photon (i.e., which slit the photon goes through) and this is enough to affect its quantum behaviour and make it behave classically as far as the two slit interference is concerned. In this sense, we should stop saying that observation affects quantum superpositions because it is in fact the right kind of entanglement that does that (and our observations are only one of infinitely many ways of generating that entanglement, but they are nothing exceptional).

Right. I hope that you get a sense of what I am trying to communicate. In fact, it is precisely because we think that physics applies to everything that observers have no special status. Observers are physical systems like any other physical system, no more and no less, as far as physics is concerned. What one must not do is to assume that observers are classical when coupling to quantum systems. If this is done, then all sorts of “paradoxes” follow as I will discuss below.

OK, so it might be that the fact that many people insist on observers is just an innocent interpretational way of talking about things, but surely it can do no harm. Well, it is here that I’d like to make my second point. If one assumes that observers are necessary, one is actually not unlikely to get the basic physics wrong.

Example number one. Quantum physics tells us that when we observe a superposition of a particle in two places, we will detect the particle in one of the two places only. However, and so the complaint goes, the equations of quantum physics do not tell us that there is only one outcome. Therefore (or so the mistaken logic goes), we need to change the dynamical equations of quantum physics to ensure one and only one outcome. This is the usual logic behind the so-called measurement problem.

The fault in this logic is two-fold. Firstly, there is an assumption that only one outcome exists after the observation. Secondly, there is an assumption that only classical observers can guarantee the existence of that one outcome. Both of these are incorrect. After the system under observation and the system doing the observing have interacted, which implements a quantum measurement, they become entangled, and their joint superpositions are still present. There is no collapse into a single world. However, in each branch of the superposition, a single outcome of the observation is “recorded” by the system doing the observing. This is the logic of Schrödinger's cat experiment. In one branch, the poison kills the cat while in the other it doesn’t, but both exist in a superposition. Within each branch and relative to any system in that branch, to any observers, call-them-what-you-want, in that branch, a definitive outcome (either dead or alive) exists.

Bottom line: quantum mechanics does not need to be augmented by classical observers (or observers of any other kind) in order for us to make sense of all the quantum experiments. Quantum physics already does so by its very logic of entanglement. There is no measurement problem (and the problem only appears when ad hoc entities get added to quantum mechanics).

Example number two. The equivalence principle says that, locally, gravity is indistinguishable from acceleration. An observer (here we go again) in a box without windows cannot tell if he is glued to the bottom of the box because the box is sitting on Earth or because someone is accelerating the box upwards with the acceleration that matches Earth’s gravity! We all agree with this (other than the fact that the said observer in the box could be an apple sitting on a scale which records its weight – and in both scenarios, the weight would be the same).

Now, some people think that there is a problem with this principle in quantum physics. Why? Because, what happens if an object exists in a superposition of two accelerations? How does that feel to an observer? Is it like the average of the two accelerations, and is that the strength of the gravitational field that would be indistinguishable from the average acceleration? I think not and this is where people who invoke observers miss the point badly.

What happens is this. Anyone getting entangled to the particle would experience the acceleration corresponding to each branch. It’s Schrödinger’s cat all over again (it always is). But, and this is an important but, for the two branches to interfere at the end, we need to interfere everything including the observers. Which begs the question: what’s the point of the observer here? By now you know the answer: there isn’t any. So a particle being in a superposition of two accelerations simply obeys the (standard classical) equivalence principle in each branch. And this is the quantum version of the equivalence principle (a formulation I’ve been advocating for a while with my colleague Chiara Marletto).

Of course, there is a lot more to say about all this (there always is). But a blog is a blog. However, if you are curious to read more, I recommend my upcoming book “Portals to a New Reality” (published by Basic Books and Penguin in the autumn this year). It’s written playfully, but it does go in-depth into these very questions.

Conclusion: Observers are physical systems and there is no need to use observers anywhere in physics. Plus, they are a dangerous fiction that frequently leads us astray. In fact, let me finish by saying something even stronger: if observers prove to be fundamental in physics at any point in the future, I’d much rather be a plumber than a physicist.

Take care, Vlatko

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Wheeler was seeking to understand the mind of God. Could it be that even God was constrained when choosing how to make the world and that he/she/it could not have made the universe any other way? In other words, is there a principle, or a set of them, that ultimately leads us invariably only to quantum physics? Einstein famously captured this sentiment by saying, “I want to know how God created this world. I'm not interested in this or that phenomenon, in the spectrum of this or that element. I want to know His thoughts; the rest are just details.”

When looking for deeper principles, there is no better place to start than with the idea of symmetry. If we were to displace the universe 5 meters to the left, would anything change? Could we even tell if such a displacement took place? It seems natural to answer this in the negative. Likewise, if we change the clock time and add 5 hours (or 5 minutes or anything) to it, would this lead to an observable effect? Again, our assignment of a time label to the present instant seems arbitrary, and how we decide to do so should not affect the resulting physics.

As it happens, there are 10 things we can do to the universe and not change the underlying physics: three spatial shifts (in the x, y, and z directions), three rotations (around the x, y, and z directions), three changes of velocity (in the x, y, and z directions) and one shift in time (because, presumably, time is one dimensional).

Curiously, there is now an interesting further fact that almost automatically leads us to the key quantum principle, that of Heisenberg’s uncertainty. Namely, a shift in space is generated by velocity, whereas, symmetrically, a shift in velocity is generated by space. What I mean by this is: if I want to change my spatial location, I can simply do that by traveling at a certain velocity to another location; likewise, if I want to change my velocity (say from zero to some other value), I just need to move from one spatial location to another!

Once we accept this seemingly obvious statement, a mind-blowingly profound realisation follows. In mathematics, the object that tells us how shifts are generated is called a commutator. The commutator of the position and velocity therefore encapsulates the fact that each generates a change in the other. And since they are symmetrical, the resulting value must be a constant. This constant happens to be equal to the square root of minus one, multiplied by Planck’s constant and divided by the mass of the particle whose motion and displacement we are considering. However, the exact value is not the point I am trying to make. Instead, I am saying that the fact that the commutator equals a constant implies that moving in space and then changing velocity is, in quantum physics, not the same as first changing the velocity and then displacing in space. And this is the good old Heisenberg Uncertainty Principle.

So we used 3 positions and 3 velocities, and what remains are 3 rotations and 1 time displacement. The three rotations lead us to the angular momentum and the time displacement leads us to the energy (aka the Hamiltonian) of the particle. Bottom line: all relationships between observable entities in quantum physics are fixed by the 10 symmetries of the universe.

Yes, I have concealed a few details here and there (for the experts: the above describes a unitary representation of the Galilean group), but it seems to me that this road to quantum physics is the closest to what Wheeler would have been looking for.

Now I’d like to conclude with why I don’t think this is the best way to learn quantum physics (even though it might be logically the tightest one). The reason is that humans learn in a particular way by repeatedly encountering instances in which a new concept is used. Learning is all about practice and not about logical or mathematical coherence (there is a funny story about the Hungarian physicist Edward Teller according to which he learnt how to swim by first studying carefully a swimming manual and only then jumping into a pool and swimming; however, I don’t think I am aware of any other such examples where theory comes before practice).

In quantum physics, this means learning about all the situations in which quantum phenomena play a role, such as the double slit experiment, electron diffraction, quantum tunneling, and so on, before describing the formal structure of quantum theory (most textbooks do it this way too). The latter (the formal structure) is of course crucial for a physicist, for else the physical knowledge would merely be a taxonomy of different phenomena not connected by any underlying logic (it would be akin to stamp-collecting according to the memorable words of Ernest Rutherford). But, in terms of learning, the connecting logic comes a posteriori. And it (the logic) gets more and more refined with time, which is how we ended up with the understanding I briefly outlined above.

Will there be an even better way to understand quantum physics? Perhaps. More likely, in my view, is that we will have a theory that supersedes quantum physics, and then all our efforts will be directed toward deepening our understanding of that new theory. And we would have all but forgotten about how much we worried about quantum physics and its meaning in the past…

Take care,

Vlatko

The title of this blog, on the other hand, is not a Zen koan even though it might well resemble one. It is a topic of a legitimate scientific inquiry, and it’s meant to encourage thinking (at least of the creative kind).

Galileo said: “mathematics is the language of the universe”. One of the (one might say, unintended) benefits of using mathematics to understand nature is that phenomena which, at first sight, appear to be different to one another, are actually seen to be related when expressed in terms of maths.

My regular readers will know that the best description we have of all natural phenomena is in terms of waves (of the quantum kind, but this will not be directly relevant here). And the key piece of mathematics required to handle waves is the (no surprises here) “wave equation”. This equation relates the spatial variation of a wave to its temporal variation via the speed of the wave. All the wave phenomena (interference, diffraction, reflection, refraction and so on) are fully captured by the solutions of the wave equation.

So, water waves, sunlight and the sound all appear to be very different physical phenomena. Ultimately, however, they all obey one and the same wave equation (the only difference being the speed of respective propagations, which for water waves is different to light which is yet different to the speed of sound in the air).

What this means is that for every phenomenon we observe with light waves, there must also be an equivalent phenomenon involving water and sound waves. For instance, sound bends around corners and so this must mean that light does that too. And, yes, light indeed does that. In fact, it seems to me that we have extensive experimental evidence for all such correspondences bar one.

Enter rainbow. It has taken us a long time to understand the formation of a rainbow. It was clear to the Ancient Greeks that the rainbow has something to do with sunlight and raindrops, but – unfortunately - they didn’t have the wave equation to figure out the details of the mechanism. The first proper explanation was due to Newton, and his iconic prism dispersing white light into the rainbow colours was a beautiful demonstration of the basic physics behind the phenomenon.

But even Newton thought of light in terms of particles and therefore missed a few important details that rely on its wave nature. The fully modern explanation is due to Fresnel, a French physicist who wrote down the wave equation for light, and developed the full theory to understand light’s behaviour in different settings and scenarios. Back to the rainbow, and in a nutshell, when sunlight diffracts through a raindrop, different colours do so differently so that they end up exiting in different directions. The same story as with Newton’s prism.

And, the same thing ought to be true for sound. It is indeed. Surprisingly, the artificial “acoustic prism” was first made only as late as 2016. The honour goes to Hussein Esfahlani, Sami Karkar, Herve Lissek and Juan R. Mosig of Ecole Polytechnique Fédérale de Lausanne who did just that using a carefully designed metamaterial to (acoustically) mimic the optical prism.

This brings us to the main question of this blog: is there a naturally occurring acoustic rainbow? As far as I know no one has ever reported hearing anything like that. But, what should one be hearing when exposed to the sounds of an acoustic rainbow?

Here I can only speculate. The frequencies we see in a rainbow are about 12 orders of magnitude higher than the frequencies we can hear. So, if we take the colours of a rainbow and scale their frequencies down by 12 orders of magnitude, what would the tones be that correspond to these frequencies? It seems to me they would closely resemble the tones comprising the A-major cord!

One can’t help but wonder if nature is trying to tell us something with this. Of course, it’s most likely just an accident; however, a far more interesting option would be to entertain a wild conspiracy.

Everyone agrees that rock music started with Chuck Berry’s legendary song “Johnny B. Goode”.

Like most rock songs this one begins with the A cord, followed by (what else?) D and E cords. Is it just a coincidence that the sound of a naturally occurring rainbow would resemble the beginning of Johnny B. Goode? It probably is, but I prefer to think that God, just like me, is particularly fond of rock’n’roll.

Take care of yourselves,

Vlatko