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Reality is quantum through and through

There are, I believe, two main reasons why physics seems stuck at present. The last revolution was quantum mechanics and it began with Heisenberg’s famous paper exactly 100 years ago. And since then, not a single experiment has challenged the quantum description of reality. Not one. The first reason for this century-long absence of a new fundamental theory is that we simply haven’t had the appropriate experimental technology to probe regions where something could go wrong. This has now changed rapidly with the ongoing worldwide race to build a universal quantum computer. The technologies that go into this enterprise and that are being pursued by all the major industrial players are becoming sophisticated enough to test fundamental physics in a non-trivial way. However, there is a second reason for being stuck. It is the fact that we still haven’t agreed on the way to understand quantum mechanics. It is for this reason that I’d like to offer my own interpretation. Some might see it as radical, but it seems to me that it is the only one that fits all the experimental evidence and theoretical constraints so far.

So let me start at the beginning. Quantum interpretations resemble the First World War battlefield. All warring sides are deeply entrenched, and there is remarkably little movement and activity to succeed in changing the frontlines between different territories. The dominant view is still the Copenhagen Interpretation (originally promoted by Niels Bohr), which probably occupies half of the landscape of all interpretations. According to Copenhagen, quantum systems need classical observers to interact with them and reduce quantum superpositions (“being in many different states at the same time”) to one well-defined “classical” state.

The other half of the landscape is then the battleground for the Many Worlds Interpretation, the Hidden Variables Interpretation, and a number of other smaller competitors, such as Quantum Bayesianism (affectionately known as QBism). Without going into too much detail, the proponents of Hidden Variables (originators: Louis de Broglie and David Bohm) try to hold onto a classical reality, but at a price of introducing action at a distance (things that go faster than light, but cannot be detected); the QBists (originating in the work of Carlton Caves, Christopher Fuchs and Ruediger Schack) claim that quantum physics exists only in the heads of observers; finally, the Many Worlders (starting with Erwin Schrödinger and Hugh Everett) take the idea of quantum superpositions to be universal and to apply not just to quantum systems but to everything else in the universe.

Now, when I told my editor at Allen Lane about my own interpretation, he immediately said “It’s Many Worlds on steroids!” There is a grain of truth in that, but I prefer to call it “Everything is a Quantum Wave Interpretation” instead. It’s best to tell you what it is straight away and demark it against the other views as we go along.

The whole point of the quantum revolution was that it unified two previously very different kinds of objects of classical physics, particles and waves. Particles in classical physics obey Newton’s laws of motion, and waves obey some kind of wave equation, depending on whether they are water, sound, or waves of the electromagnetic field (aka light). Their behaviors are completely different: classical waves interfere, meaning, for example, that waves can bend around corners; particles cannot do that. Waves can also go through two slits at the same time and produce interference fringes based on the combined effect of doing so, while particles only go through one slit at a time.

Quantum physics predicted that particles can also interfere, and experiments quickly backed this idea up. A single electron can indeed go through two slits at the same time and produce interference. In that way, quantum physics got rid of the classical dichotomy between particles and waves and stipulated that everything in quantum physics is made of the same kind of stuff. Great!

But – and this is the key “but” in our story – quantum objects don’t just behave like the classical waves. Both classical particles and classical waves now become quantum waves in quantum mechanics. Quantum waves are the ultimate unification. What is a quantum wave? A classical wave is characterized by two numbers: its amplitude and its phase, and both are given at every point in space and they change in time. The amplitude is the height of the wave at a given point, and the phase tells us how far the wave is along the “cycle of waving” at that same point.

Now, the ingenious idea of Heisenberg’s in 1925 was to replace these ordinary c(lassical)-numbers by what Dirac later called q(uantum)-numbers. Q-numbers are tables of numbers, mathematically represented by objects called matrices. The physical motivation for introducing them was the fact that when we probe atoms by shining light on them, they shine light back and we measure the intensities and frequencies of this light. And these intensities (which are amplitudes squared) and frequencies (which determine the phase of light) only come in discrete sets when an electron makes a jump from one orbital to another. So, Heisenberg, who was very much influenced by positivist philosophy, thought that we should get rid of things that we do not measure directly, such as the trajectory of the electron inside an atom, and keep only the observable entities, which happen to be collections of numbers characterising all possible transitions inside the atom.

The picture I am advocating is a reality underpinned by all these q-numbers dynamically interacting as waves.

This ultimately turned out to be the right intuition. The procedure we call quantization simply means upgrading all the relevant c-numbers (i.e., ordinary real numbers) pertaining to position, momentum, amplitude, phase, and so on into q-numbers. The most amazing consequence of this, and it took Heisenberg another two years to fully appreciate it, is that the order in which quantities are now multiplied matters: position times momentum is not the same as momentum times position in the quantum world. So one can still talk about the position of an electron and its motion, but we cannot think of electrons as moving on well-defined, continuous classical trajectories. Physically, this is the basis of the Heisenberg Uncertainty Principle, and it implies that the position and momentum cannot be specified to an arbitrary accuracy. The same goes for the phase and amplitude of a wave, so that the better we know the height of a wave, the less we know how far it’s advanced.

Back to my interpretation, the picture I am advocating is a reality underpinned by all these q-numbers dynamically behaving as waves. This simply means that they oscillate in time and space as waves do, formally meaning that they satisfy a wave equation. They also interact with one another, which is how we get any complex phenomenon such as light emission from an atom, formation of molecules, repulsion of like charges, existence of solids, liquids and gases, or anything else you care to ask – it’s all (so far) part of this picture. Its key feature is that it assumes that everything in the universe is made up of the same interacting q-waves. My interpretation of quantum mechanics is now complete.

However, I’d like to talk about some of its rewarding consequences, which are what, I think, make it superior to most other interpretations. First, there is no measurement problem. If everything is made up of q-numbers, then a quantum measurement is just a formation of entanglement between two quantum systems. Entanglement is a correlation between q-numbers of two systems in such a way that both would give the same results if the same respective q-numbers were measured. This quantum correlation is simply established through an interaction between the two, such as electrical charges interacting through the electromagnetic force (each being a collection of q-numbered waves).

We ourselves are also a collection of q-waves. It is when our q-waves correlate with the q-waves of the system we observe that the results emerge as the ordinary c-numbers. Since quantum correlations between q-waves give rise to classical properties, the classical world actually owes its own existence to quantum entanglement. In quantum physics, even a collision between two particles is actually described as an interaction between two q-waves. This kind of description constitutes our most accurate account of all natural phenomena, called quantum field theory. A particle in this theory is not a fundamental entity, but it is just one stable configuration of the underlying q-wave (or a single excitation of the quantum field, in a more formal language of quantum field theory).

Since everything is quantum, there is no need or special place for observers, in contrast with the Copenhagen Interpretation and QBism. Entanglement is a completely symmetric property of entangled systems and the observers and the observed can swap places without any consequences: an atom is as much observing us as we are observing the atom. Both Hidden Variable and Many-Worlds interpretations would agree with the lack of need for observers. An immediate consequence is that there are no discontinuities or quantum jumps that are caused by measurements either. The dynamical evolution is always smooth as there is no artificial boundary between the quantum and the classical world.

If there is a radical component to my view, it is this: I am suggesting that we purge physics of all classical notions.

Second, we can retain the notion of locality which has served us so well in classical physics (classical field theory, to be more specific). Q-numbers in one location cannot be changed by changing q-numbers in another location. There is no spooky action at a distance in a universe in which everything is made of quantum waves. This even applies to entanglement, where particles become quantum supercorrelated; even entangled particles cannot affect each other instantaneously. The account of entanglement according to q-waves is that the q-numbers of particles become entangled through their interaction and then, when we interact with one of the particles, our own q-numbers become correlated to the q-numbers of that particle. But, crucially for the concept of locality, there is no change in the q-numbers of the other particle until we also interact directly with it. In contrast, the Hidden Variable interpretation, which holds onto the c-number-based reality, can only do so by introducing spooky action to explain quantum entanglement and our observations of entangled particles. QBists presumably don’t worry about this issue, since everything takes place inside the heads of observers (and is therefore always local). The Many Worlds interpretation should also be seen as local, although some of its proponents claim otherwise.

Third, all the symmetries that we think are important, such as the laws of energy and momentum conservation, as well as the equivalence principle of General Relativity, can be phrased within the fully q-number-based universe without any inconsistencies. In the Copenhagen interpretation, where observers are classical, energy and momentum are only conserved “on average.” Sometimes a measurement can give us more energy, sometimes less, and it is only in the long run, when we average over all the outcomes of our measurements, that we will get the same amount as we started with. I suspect that QBists and Hidden Variable interpretations must have the same problem of not being able to conserve things exactly. So far, no experiment has detected any violation of conservation laws, which is another reason for advocating quantum waves.

In fact, I would say that all apparent paradoxes in quantum physics (apparent because there are no real paradoxes), be they related to the measurement problem or entanglement or Heisenberg’s uncertainty, appear only because we do not treat everything on an equal q-number footing. If there is a radical component to my view, it is this: I am suggesting that we purge physics of all classical notions. This is why I think that the Many Worlds Interpretation is insufficient, as the notion of a world itself is also a relic of classical physics. It’s more appropriate to say that there is only one world, but that it is quantum through and through. Quantum universe is not just a superposition of classical ones (though that’s a possible quantum state too) – it contains infinitely many more possibilities. It even allows us to recombine (quantum interfere) some worlds and merge them back into a single one (this manoeuvre is key to quantum computation since there is no point in getting the correct result in another universe).

In addition to the arguments above, what adds to my confidence regarding the q-wave picture is the last 30 years of experimental progress. We have taken quantum physics from the original microscopic domain of atoms and subatomic particles and into the domain of large objects, such as electrical circuits, and we have shown that quantum features persist there too. This year’s physics Nobel Prize is another testament to that, having been given to three researchers who showed that electrical currents can also exist in superpositions, thereby violating the classical laws of Kirchhoff. I am confident that the q-number-based reality will continue to inspire us to test further regions of the universe (gravity being the last remaining bastion of classicality, but I am predicting not for long), as well as to understand processes in chemistry and biology in greater detail. And, who knows, people are already speculating that consciousness may also owe itself to quantum mechanics…

I think the philosophical implications of this perspective are also noteworthy. We see that some interpretations, like QBism or even Copenhagen, have an idealistic flavour in the sense that all important actions take place in the heads of observers. Sometimes we hear statements claiming that the act of observation creates reality. This would make quantum physics lend support to the philosophy of idealism, according to which a tree falling in a forest does not make a sound if no one is around to hear it. Contrary to this, I believe that my view, like Hidden Variables and Many Worlds, is actually a realist interpretation of quantum physics. Q-numbers are always around. They change in time, entangle with one another, but they could never be created or destroyed; they simply are. To me, this is a beautiful unification of all the physical phenomena so far which at the same time does not ask us to give up the idea of an independent reality out there. Whether the q-wave view of reality I have been promoting continues to hold, only future experiments can tell us.