Beyond Weird: Why Everything You Thought You Knew about Quantum Physics Is Different

by Philip Ball

No, the equations aren’t why quantum mechanics is perceived to be so hard. It’s the ideas. We just can’t get our heads around them. Neither could Richard Feynman. His failure, Feynman admitted, was to understand what the math was saying. It provided numbers: predictions of quantities that could be tested against experiments, and which invariably survived those tests. But Feynman couldn’t figure out what these numbers and equations were really about: what they said about the ‘real world’.

Or maybe, it’s sometimes said, the math tells us that ‘everything that can happen does happen’ – whatever that means.

If anything deserves to be called weird, it is us.

It’s very peculiar that a scientific theory should demand interpretation at all. Usually in science, theory and interpretation go together in a relatively transparent way. Certainly a theory might have implications that are not obvious and need spelling out, but the basic meaning is apparent at once.

Talking about the interpretation of quantum mechanics was, on the other hand, a parlour game suitable only for grandees in the twilight of their career, or idle discussion over a beer. Or worse: only a few decades ago, professing a serious interest in the topic could be tantamount to career suicide for a young physicist. Only a handful of scientists and philosophers, idiosyncratically if not plain crankily, insisted on caring about the answer. Many researchers would shrug or roll their eyes when the ‘meaning’ of quantum mechanics came up; some still do. ‘Ah, nobody understands it anyway!’ How different this is from the attitude of Albert Einstein, Niels Bohr and their contemporaries, for whom grappling with the apparent oddness of the theory became almost an obsession. For them, the meaning mattered intensely.

Such work is already winning Nobel Prizes, and will win more. What it tells us above all else is very clear: the apparent oddness, the paradoxes and puzzles of quantum mechanics, are real. We cannot hope to understand how the world is made up unless we grapple with them.

What has emerged most strongly from this work on the fundamental aspects of quantum theory over the past decade or two is that it is not a theory about particles and waves, discreteness or uncertainty or fuzziness. It is a theory about information. This new perspective gives the theory a far more profound prospect than do pictures of ‘things behaving weirdly’. Quantum mechanics seems to be about what we can reasonably call a view of reality.

So we don’t have all the answers. But we do have better questions, and that’s some kind of progress.

When words come too easily, it’s because we haven’t delved deeply enough

‘If a man does not feel dizzy when he first learns about the quantum of action [that is, quantum theory],’ said Bohr, ‘he has not understood a word.’

Quantum physics implies that the world comes from a quite different place than the conventional notion of particles becoming atoms becoming stars and planets. All that happens, surely: but the fundamental fabric from which it sprang is governed by rules that defy traditional narratives. It is another quantum cliché to imply that those rules undermine our ideas of ‘what is real’ – but this, at least, is a cliché that we might usefully revisit with fresh eyes. The physicist Leonard Susskind is not exaggerating when he says that ‘in accepting quantum mechanics, we are buying into a view of reality that is radically different from the classical view’. Note that: a different view of reality, not a different kind of physics. If different physics is ‘all’ you want, you can look (say) to Einstein’s theories of special and general relativity, in which motion and gravity slow time and bend space. That’s not easy to imagine, but I reckon you can do it. You just need to imagine time passing more slowly, distances contracting: distortions of your grid references. You can put those ideas into words. In quantum theory, words are blunt tools. We give names to things and processes, but those are just labels for concepts that cannot be properly, accurately expressed in any terms but their own. A different view of reality, then: if we’re serious about that, we’re going to need some philosophy.

David Mermin expressed this adeptly when describing how many quantum physicists feel about Niels Bohr himself, who acquired the reputation of a guru with a quasi-mystical understanding that leaves physicists even now poring over his maddeningly cryptic words.

Perhaps quantum mechanics pushes us to the limits of what we can know and comprehend. Well then, let’s see if we can push back a little.

It’s not easy to understand what the words mean, but they reflect the fact that the most fundamental message of quantum theory isn’t a purely mathematical one.

But there was already a mature mathematical theory of classical waves; maybe we could use that to describe the alleged waviness of particles? That’s just what Erwin Schrödinger, a professor of physics at Zurich, did. After being given de Broglie’s thesis and challenged to describe wave-like particles in formal terms, he wrote down an expression for how they might behave. It was not quite like an ordinary wave equation of the sort used to describe water waves or sound waves. But it was mathematically very similar. Why wasn’t it identical? Schrödinger didn’t explain his reason, and it now seems clear that he didn’t exactly have one. He simply wrote down what he thought a wave equation for a particle such as an electron ought to look like. That he seems to have made such a good guess is even now rather extraordinary and mysterious. Or to put it another way: Schrödinger’s wave equation, which is now a part of the core conceptual machinery of quantum mechanics, was built partly by intuition and imagination, albeit combined with a deeply informed sense of which parts of classical physics it was appropriate to commandeer. It can’t be proved, but only inferred by analogy and good instinct. This doesn’t mean that the equation is wrong or untrustworthy; but its genesis shows how creativity in science depends on more than cold reason.

So quantum particles can tunnel through barriers: well, why not? The feat is not possible within a classical picture, but it’s imaginable if we don’t worry too much about how it was achieved. This doesn’t mean, however, that we should picture the electron wriggling its way through the barrier. We can predict, using the Schrödinger equation, what we will measure in a tunnelling process, but we can’t relate that to an underlying picture of an electron ‘doing’ anything. It’s better to see this effect as a manifestation of the randomness that sits at the core of quantum mechanics. The wavefunction tells us where we might potentially find an electron when we look; but what we do find in any given experiment is random, and we can’t meaningfully say why we find it here rather than there.

Does the wavefunction express a limitation on what can be known about reality, or is it the only meaningful definition of reality at all?

This possibility seemed to Einstein to be a profoundly anti-scientific idea, because it meant relinquishing not just a complete description of reality but the notion of causality itself. Things happen, and we can say how likely they are to happen, but we cannot say why they happened just as or when they did. Take radioactive decay. Some radioactive atoms will decay by emitting an electron from inside the nucleus: this electron is, for historical reasons, called a beta particle, but it’s just a common-or-garden electron. Atomic nuclei don’t exactly contain electrons – we saw that these orbit outside the nucleus. But they do contain particles called neutrons, which may spontaneously decay into an electron, which gets spat out, and a proton, which stays in the nucleus.*1 Beta decay of carbon-14, one of the natural forms of carbon atoms, is the process used for radiocarbon dating, and it transforms the carbon atom to a nitrogen atom. Beta decay is a quantum process, so the probability of the neutron decaying is described by a wavefunction. (It’s actually a kind of quantum tunnelling process: the electron tunnels out of the nucleus, where it would otherwise be bound by electrical attraction.) All the wavefunction can tell you is the probability that the decay will occur – not when that will happen. Take any specific atom of carbon-14, and its decay could happen tomorrow, or in 1,000 years. And there’s nothing, nothing, you can do to figure out which it will be, for all the carb...

We are suspended in language. From the human perspective, it certainly looks as though quantum objects can possess two different, even contradictory, values of some property at once. But the human perspective is the wrong one for understanding quantum mechanics. Nonetheless, it’s all we have.

The relationship between what we observe and what is has long preoccupied philosophers.

Stephen Hawking has written that ‘mental concepts are the only reality we can know. There is no model-independent test of reality.’

All the same, such ‘paradoxes’ have an important role in illustrating why quantum mechanics confounds intuition. They generally arrange quantum outcomes in such a way as to apparently permit the answers Yes and No simultaneously. Whatever we are to make of that, we must surely aspire to do better than shrug and call it ‘weird’.

This is one of the most challenging, if not infuriating, aspects of quantum theory: it seems to demand that we regularly disregard its insistence on what can and cannot be said.

Nature seems to ‘know’ if we’re making a measurement or not – of the path of a photon or electron through the double slits, say – and changes its behaviour accordingly. It’s as if nature can sense whether or not we are trying to spy on the photon’s path.

Yet in the view of Bohr and Wheeler, there are no fundamental quantum phenomena about which we have any right to speak until we measure them. To the question ‘What was happening to the photon between its emission from the laser and its detection?’, we can’t simply reply ‘I don’t know, I wasn’t looking.’ We have to say ‘Because I wasn’t looking, that question has no meaning.’

Nature always seems to ‘know’ our intentions.

From our perspective the world is made up of phenomena – things happen – and a phenomenon only exists when it has been measured. Wavefunction collapse is simply a name we give to the process by which we turn quantum states into observed phenomena.

Wavefunction collapse is then a generator of knowledge: it is not so much a process that gives us the answers, but is the process by which answers are created.

Should we consider the wavefunction to have collapsed within the measuring device, or within the brain of the human experimenter? At what point in the chain from quantum event to macroscopic measuring device to observer reading the result and noting it in a lab book do we consider collapse to have occurred? Werner Heisenberg pondered this problem, and the point at which we separate the quantum from the classical world became known as the ‘Heisenberg cut’.

Measurement is precisely how we acquire knowledge – so if measurement wasn’t classical, we wouldn’t be able to get any knowledge about a quantum system by experiment.

The problem is that in quantum mechanics it is almost impossible to be unambiguous and consistent, to say what you mean, or perhaps even to know what you mean, because you are dealing with concepts that defy language.

We know that measurements of a quantum system seem to collapse the wavefunction. We most certainly don’t know how, or why, or indeed if that actually happens.

It implies that there is no obvious way to deal with what strikes us as strange about quantum mechanics. Nothing we try will make it go away. It’s for this reason that the proliferation of interpretations is not a failing of quantum mechanics, but a necessity. We need different perspectives, just as we need to look at a sculpture from many different angles to appreciate it fully.

He felt that the ‘active information’ that it could transmit to a particle had parallels with the activity of the mind, turning the entire universe into something resembling a conscious organism. This confers a unity that Bohm called the ‘implicate order’, which underpins the ‘explicate order’ accessible to the senses. Thought exists in the cosmos as a holistic entity akin to the quantum potential, which it would, he said, be ‘wrong and misleading to break . . . up into my thought, your thought’.

Another physical collapse model was devised in the 1980s and 90s by the British mathematical physicist Roger Penrose, and independently by the Hungarian physicist Lajos Diósi. They suggested that collapse might be induced by the disrupting influence of gravity.

It’s not that ‘because the particle went through both slits, there’s an interference pattern’, but rather, ‘the outcome in which there’s an interference pattern contains no meaningful definition of particle trajectories’.

We don’t ban some questions simply because we don’t know what to say about them, but instead recognize that quantum mechanics has no math that can provide an answer: it’s rather like expecting simple arithmetic to tell us what an apple tastes like.

What is impressive is that what Bohr and Einstein (more than any of their contemporaries) had to say remains relevant.

Quantum mechanics might seem ‘weird’, but it is not illogical. It’s just that it employs a new and unfamiliar logic. If you can grasp it – if you can accept that this is just how quantum mechanics works – then the quantum world may stop seeming weird and become just another place, with different customs and traditions and with its own beautiful internal consistency.

Heisenberg’s Uncertainty Principle is not exactly a constraint on how precisely we can make a measurement of some quantum property. Rather, it constrains how precisely the property we want to know about exists at all. It might have been better christened the Unknowability Principle – better still, the Unbeability Principle – although doubtless that would have spawned a mysticism of its own.

But actually Einstein was ‘wrong’ about many things. He made a few trivial lapses in his calculations. He famously fudged his theory of general relativity to avoid its prediction of an expanding universe, just a few years before astronomers found that to be precisely the state of the cosmos. Even his many proofs of the celebrated E = mc2 contained little gaffes. Heck, there’s an entire book enumerating Einstein’s mistakes.*1 None of this has the slightest bearing on Einstein’s status as the greatest scientist of the twentieth century. To imagine that genius implies freedom from error is to misunderstand the nature of creativity and insight. Arguably, geniuses (whatever that means) must incur an above-average chance of being wrong.

Perhaps too it might seem reassuring to imagine that even Einstein was unable to take an imaginative leap beyond the preconceptions of his early training. The notion that randomness, indeed absence of causality, lies at the heart of things is unsettling in the extreme, and there’s some comfort in seeing that Einstein shared our instinctive reluctance to countenance it.

The emerging appreciation of entanglement’s role in quantum mechanics over the past several decades has shifted the emphasis of the whole field. Entanglement is indeed a real attribute of quantum objects, as numerous careful experiments since the 1970s have demonstrated.

The main thing you need to know about entanglement is this: it tells us that a quantum object may have properties that are not entirely located on that object.

What can it mean to say that an object’s properties are not located solely on that object? My pen is black – the blackness doesn’t have any existence beyond the pen. But what if I were to say that the blackness of my pen is also partly associated with my pencil? I don’t mean that the pencil is black too. I mean that the blackness of the pen is partly in the pencil.

Quantum events don’t appear to have an explanation as such – one in which definable causes lead to specific effects – but only a probability of occurrence. This is what Einstein found unreasonable. Who can pretend that it isn’t?

We can’t regard particle A and particle B in the EPR experiment as separate entities, even though they are separated in space. As far as quantum mechanics is concerned, entanglement makes them both parts of a single object.

What in fact we’re dealing with here is another kind of quantum superposition. We’ve seen that superposition refers to a situation in which a measurement on a quantum object could produce two or more possible outcomes, but we don’t know which it will be, only their relative probabilities. Entanglement is that same idea applied to two or more particles: a superposition of the state in which particle A has spin up and B spin down, say, and the state with the opposite configuration. Although the particles are separated, they must be described by a single wavefunction. We can’t untangle that wavefunction into some combination of two single-particle wavefunctions. Quantum mechanics is able to embrace such a notion without batting an eyelid; we can simply write down the math. The problem is in visualizing what it means.

It has been proposed, albeit in a highly speculative theoretical scenario, that the interdependence across space that manifests as quantum entanglement is what stitches together the very fabric of space and time, creating the web that allows us to speak of one part of spacetime in relation to another. Spacetime is the four-dimensional fabric described by Einstein’s theory of general relativity, in which it is revealed to have a particular shape. It’s this shape that defines the force of gravity: mass makes spacetime curve, and the resulting motions of objects in that curved arena make manifest the force of gravity. In other words, entanglement could be the key to the long-standing mystery of how to reconcile quantum mechanics with the theory of gravitation supplied by general relativity. In some simple models of a quantum universe, a phenomenon that looks like gravity emerges spontaneously from the mere existence of entanglement.

But many researchers suspect that this deep connection between entanglement and spacetime is telling us something about how quantum mechanics and general relativity are related: about what needs to change in our view of spacetime if quantum theory is to be made consistent with general relativity.