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

by Philip Ball

For quantum physics is not replaced by another sort of physics at large scales. It actually gives rise to classical physics. Our everyday, commonsense reality is, in this view, simply what quantum mechanics looks like when you’re six feet tall. You might say that it is quantum all the way up. The question, then, is not why the quantum world is ‘weird’, but why ours doesn’t look like that too.

The quantum–classical transition is then like an ocean crossing between two continents: drawing a border somewhere in the open sea is an arbitrary exercise, but the continents are undeniably distinct.

It is this waviness that gives rise to distinctly quantum phenomena like interference, superposition and entanglement. These behaviours become possible when there is a well-defined relationship between the quantum ‘waves’: in effect, when they are in step. This co-ordination is called coherence. It’s a concept that comes from the science of ordinary waves. Here too, orderly wave interference (like that from double slits) happens only if there’s coherence in the oscillations of the interfering waves. If there is not, there can be no systematic coincidence of peaks and troughs and no regular interference pattern, but just random, featureless variations in the resulting wave amplitude. Likewise, if the quantum wavefunctions of two states are not coherent, they cannot interfere, nor can they maintain a superposition. A loss of coherence (decoherence) therefore destroys these fundamentally quantum properties, and the states behave more like distinct classical systems. Macroscopic, classical objects don’t display quantum interference or exist in superpositions of states because their wavefunctions are not coherent.

There is no reason (that we yet know of) why in principle objects cannot remain in coherent quantum states no matter how big they are – provided that no measurement is made on them. But it seems that measurement somehow does destroy quantum coherence, forcing us to speak of the wavefunction as having ‘collapsed’. If we can understand how measurement unravels coherence then we would be able to bring measurement itself within the scope of quantum theory, rather than making it a boundary where the theory stops. We might even be able to figure out what happens to Schrödinger’s cat. (But I make no promises about that.)

Because the superposition is now a shared property of the system and its environment – because the quantum system has lost its integrity and exists in a shared state with all the other particles – we can’t any longer ‘see’ the superposition just by looking at the little part of it. We can’t see the wood for the trees. What we understand to be decoherence is not actually a loss of superposition but a loss of our ability to detect it in the original system. Only by looking closely at the states of all these entangled particles in the system and its surroundings can we deduce that they’re in a coherent superposition.

In other words, what we previously called measurement can, at least in large part (not completely, as we’ll see), be instead called decoherence. We obtain classical uniqueness from quantum multiplicity when decoherence has taken its toll.

The universe is always looking.

All the same, the mere existence of even the most rudimentary of quantum computers demonstrates that quantum mechanics has moved far beyond being a language to describe an esoteric world that most people never encounter. The use of quantum mechanics to improve information technology supplies one of the most compelling demonstrations that it really does describe something real about the world. The significance goes deeper, however. The very idea of treating quantum systems as repositories of information, which can be stored, manipulated and read out just as it can using the digital circuitry of conventional computers, reinforces the view that information lies at the heart of the theory.

Understanding exactly how quantum mechanics can improve computing may turn out to provide insights into one of the deepest questions of the field: what quantum information really is and how it can be transmitted and altered.

The MWI is surely the most polarizing of interpretations. Some physicists consider it almost self-evidently absurd; ‘Everettians’, meanwhile, are often unshakeable in their conviction that this is the most logical, consistent way to think about quantum mechanics. Some of them insist that it is the only plausible interpretation of quantum mechanics – for the arch-Everettian David Deutsch, it is not in fact an ‘interpretation’ of quantum theory at all, any more than dinosaurs are an ‘interpretation’ of the fossil record. It is simply what quantum mechanics is. ‘The only astonishing thing is that that’s still controversial’, Deutsch says.

The issue is whether, like Bohr, you believe that quantum mechanics provides a prescription for evaluating the possible outcomes we might observe when we look at the quantum world, or whether you regard the Schrödinger equation as an inviolable and universal law that describes – in some sense is – reality.

At the outset I mentioned John Wheeler’s proposition that if we really understood the central point of quantum theory, we ought to be able to state it in one simple sentence. That is a matter of faith: there’s no guarantee that the world’s innermost workings will fit a language developed mostly to conduct trade, courtship and banter.

Wheeler’s conviction is shared by Fuchs, who believes that we will one day tell a story about quantum mechanics – ‘literally a story, all in plain words’ – that is ‘so compelling and so masterful in its imagery that the mathematics of quantum mechanics in all its exact technical detail will fall out as a matter of course’. That story, he says, should not only be crisp and compelling. It should also ‘stir the soul’.

This is really a question about the logic that applies to quantum mechanics. It comes down to this. If you describe a system using a kind of algebra in which the various terms in the equations commute – crudely meaning (page 151) that the answers you get don’t depend on the order in which you perform the calculations – then what you see is classical behaviour. But if the algebra of your equations doesn’t commute – if the order matters – then you get a quantum-type theory. Remember that this is where the uncertainty principle comes from: the fact that in quantum mechanics some quantities do not commute. Bub believes that non-commutativity is what distinguishes quantum from classical mechanics. This property, he says, is a feature of the way information is fundamentally structured in our universe.

As a candidate for a simple unifying principle of quantum mechanics, Časlav Brukner and Anton Zeilinger have offered the notion that every fundamental constituent of a system*4 can only encode one bit of information: it can be this, or it can be that, and nothing else. After all, if it were any more complex than that, would it really be so fundamental? Quantum mechanics then emerges from a mismatch between the actual information-carrying capacity of the basic units of stuff and our beliefs about what they ought to be able to encode. If all the information-bearing potential is used up in answering particular questions, anything else we might try to measure is simply random. If the information content of a particle is all devoted to maintaining a correlation of some property with another particle, say, then the rest is random – and we are forced to make do with probabilities and with statements that are only statistically true.

Certainly, philosophers have a thing or two to tell physicists about how delicate and slippery a term ‘the nature of reality’ is.

In that regime, says Omnès, we can’t any longer talk about a ‘reality’. For him, reality must be a space in which facts are unique: in which, you might say, there are events.

We’re used to the notion of things that in some sense contain information: books, computer memories, messages left on an answerphone. And we’re used to the idea that we can possess information: I can know your email address, say. And these seem distinct: one is potential knowledge, the other actual knowledge, culled from potential knowledge according to our individual capacity. But quantum mechanics seems to make the interaction two-way: knowledge we possess affects what is knowable (and to others, or just to us?). Yes, it’s confusing. But that is surely the right confusion to embrace, if we want to grapple with what this wonderful theory means. I like to think of this informational perspective in terms of a distinction between a theory of Isness and a theory of Ifness. Quantum mechanics doesn’t tell us how a thing is, but what (with calculable probability) it could be, along with – and this is crucial – a logic of the relationships between those ‘coulds’. If this, then that.

This Ifness is perplexing, because it is not what we’ve come to associate with science. We’re used to science telling us how things are, and if ‘Ifs’ arise, that’s just because of our partial ignorance. But in quantum mechanics, Ifs are fundamental. Is there an Isness beneath the Ifness? That’s possible – and simply admitting as much takes us beyond the simplistic view of the Copenhagen Interpretation according to which there is nothing meaningful to be said beyond the results of observation. But even if there is, it will not be like the Isness of everyday life, in which objects have intrinsic, non-contextual, localized properties. It will not be a ‘common sense’ Isness. Where we still struggle is in deciding what degree of reality, if any, to attribute to the Ifs of quantum mechanics.