Quantum common sense (1)
Despite its confounding reputation, quantum mechanics both guides and helps explain human intuition
Do we need to teach a new kind of intuition to dispel the seeming weirdness of modern physics?
Quantum theory contradicts common sense. Everyone who has even a modest interest in physics quickly gets this message. The quantum view of reality, we’re often told, is as a madhouse of particles that become waves (and vice versa), and that speak to one another through spooky messages that defy normal conceptions of time and space. We think the world is made from solid, discrete objects – trees and dogs and tables – things that have objective properties that we can all agree on; but in quantum mechanics the whole concept of classical objects with well-defined identities seems not to exist. Sounds ridiculous? The much-lauded physicist Richard Feynman thought so, yet he implored us to learn to live with it. ‘I hope you can accept Nature as She is – absurd,’ he said in 1985.
Except that much of the popular picture is wrong. Quantum theory doesn’t actually say that particles can become waves or communicate in spooky ways, and it certainly does not say that classical objects don’t exist. Not only does it not deny the existence of classical objects, it gives a meaningful account of why they do exist. In some important respects, the modern formulation of the theory reveals why common sense looks the way it does. You could say that the classical world is simply what quantum mechanics looks like if you are six feet tall. Our world, and our intuition, are quantum all the way up.
Why, then, is it still so common to find talk of quantum mechanics defying logic and generally messing with reality? We might have to put some of the blame on the Danish physicist Niels Bohr. He was probably the deepest thinker about the meaning of quantum theory among its founding pioneers, and his intuitions were usually right. But during the 1920s and ’30s, Bohr drove a lasting wedge between the quantum and classical worlds. They operate according to quite different principles, he said, and we simply have to accept that.
According to Bohr, what quantum mechanics tells us is not how the world is, but what we’ll find when we make measurements. The mathematical machinery of the theory gives us the probabilities of the various possible outcomes. When we make a measurement, we get just one of those possibilities, but there’s no telling which; nature’s selection is random. The quantum world is probabilistic, whereas the classical world (which is where all of our measurements happen) contains only unique outcomes. Why? That’s just how things are, Bohr answered, and it is fruitless to expect quantum mechanics to supply deeper answers. It tells us (with unflagging reliability) what to expect. What more do you want?
Bohr’s ‘Copenhagen interpretation’ – named after the location of the physics institute he founded in 1921 – didn’t exactly declare a contradiction between classical and quantum physics, but it implied an incompatibility that Bohr patched over with a formula of what he called ‘complementarity’. The classical and quantum worlds are complementary aspects of reality, he said: there’s common sense and there’s quantum sense, but you can’t have both – at least, not at the same time.
The principle of complementarity seemed a deeply unsatisfying compromise to many physicists, since it not only evaded difficult questions about the nature of reality but essentially forbade them. Still, complementarity had at least the virtue of pinpointing where the problems lay: in understanding what we mean by measurement. It is through measurement that objects become things rather than possibilities – and furthermore, they become things with definite states, positions, velocities and other properties. In other words, that’s how the counterintuitive quantum world gives way to common-sense experience. What we needed to unite the quantum and classical views, then, was a proper theory of measurement. There things languished for a long time.
Now we have that theory. Not a complete one, mind you, and the partial version still doesn’t make the apparent strangeness of quantum rules go away. But it does enable us to see why those rules lead to the world we experience; it allows us to move past the confounding either/or choice of Bohr’s complementarity. The boundary between quantum and classical turns out not to be a chasm after all, but a sensible, traceable path.
It’s a strange idea that measurement needs explaining at all. Usually what we mean by a measurement seems so trivial that we don’t even ask the question. A ball has a position, or a speed, or a mass. I can measure those things, and the things I measure are the properties of the ball. What more is there to say?
But in the quantum world things aren’t so obvious. There, the position of a particle is nothing more than a whole set of possible positions until the moment when it is observed. The same holds true for any other aspect of the particle. How does the multitude of potential properties in a quantum object turn into one specific reading on a measuring device? What is it about the object that caused the device to point to that precise answer? The modern answer is surprising: the act of measurement doesn’t entail a collapse of quantum-ness and a shift to classical-ness after all.
Quantum objects have a wave nature – which is to say, the theory tells us that they can be described as if they were waves, albeit waves of a peculiar sort. The waves do not move through any physical substance, as do waves in air or water, but are encoded in a purely mathematical object called a wave function that can be converted to probabilities of values of observable quantities.
As a result, quantum particles (such as photons of light, electrons, atoms, or even entire molecules) can exhibit interference, a classical property of waves in which two peaks reinforce each other when they overlap, whereas when a peak coincides with a trough the two can cancel each other out. It’s hard to talk about this phenomenon without giving the impression that the particles themselves are somehow wavy, and the unfortunate expression ‘wave-particle duality’ only compounds the confusion. But all we’re really seeing here is a feature of the particles’ wave functions, for want of a better term. Asking if these quantum objects really are particles or waves misses the point, because both of those are classical concepts. The reason we ask anyway is that we’re trying instinctively to recover some common-sense picture of the quantum world. But what we call ‘common sense’ is a feature of the classical world, and we can’t expect to use it for quantum things.
The environment is what conjures classical physics – and ‘common-sense’ behaviour – out of the quantum soup
Quantum effects such as interference rely on the wave functions of different entities being coordinated (the technical term is coherent) with one another. If they’re not, the effects are averaged away. That sort of coherence is what permits the quantum property of superposition, in which particles are said to be in two or more states at once. Again, they’re not really in two states at once – we don’t know how best to describe what they really are in a classical sense. But if the wave functions of those states are coherent, then both states remain possible outcomes of a measurement.
If their wave functions are not coherent, two states cannot interfere, nor maintain a superposition. The process called decoherence therefore destroys these fundamentally quantum properties, and the states behave more like distinct classical systems. Macroscopic objects don’t display quantum interference or exist as superpositions because they can’t be described by coherent wave functions. This – and not sheer size per se – is the fundamental dividing line between what we think of as quantum versus classical (familiar) behaviour. Quantum coherence is essentially what defines ‘quantum-ness’.
What, though, causes decoherence? This arises because of a long-neglected aspect of quantum entities: their environment. The way a quantum system behaves and evolves can depend crucially on the fact that it doesn’t exist in isolation. The environment is what conjures classical physics – and ‘common-sense’ behaviour – out of the quantum soup.
There’s no obvious reason why decoherence couldn’t have been understood by Bohr and his peers in the early days of quantum mechanics, because it involves nothing but the basic principles of quantum theory. The reason it was neglected might have been largely because that’s what usually happens in science. Researchers figure that they can focus in on the system they’re interested in, and either ignore its surroundings totally or relegate them to a minor background perturbation. Usually that works fine. But not if we want to observe anything about the quantum world.
The foundations of decoherence theory were laid in the 1970s by the German physicist H Dieter Zeh. Even then it was largely ignored until two papers on the ‘decoherence programme’ the following decade, by Wojciech Zurek at the Los Alamos National Laboratory in New Mexico, brought it to a wide audience. Polish by birth and exuberantly curly haired, Zurek displays a laconic calm in the face of the mind-boggling aspects of quantum mechanics that he has uncovered. That composure makes sense once you appreciate that he studied under John Wheeler, the near-legendary American physicist who himself worked with Bohr and had a rare talent for the wry epigram. (He coined the term wormhole and popularised the concept of black holes.)
Zurek has become one of the key architects and advocates of decoherence theory, helping to establish it as the central concept connecting the quantum and classical worlds. This connection comes from the fact that quantum coherence is contagious. If one quantum object interacts with another, they become linked into a composite superposition: in some sense, they become a single system. This is, in fact, the only thing that can happen in such an interaction, according to quantum mechanics. The two objects are then said to be entangled. It might sound spooky, but this is merely what happens when a quantum system interacts with its environment – as a photon of light or an air molecule bounces off it, say. As a result, coherence spreads into the environment.
In theory, there is no end to this process. An entangled air molecule hits another, and the second molecule gets drawn into the entangled state. Meanwhile, other particles hit the initial quantum system, too. As time passes, the system becomes more and more entangled with its environment, which means that it can’t be broken down into separate entities any more.
This spreading of entanglement is the thing that destroys the manifestation of coherence in the original quantum system. Because superposition becomes a shared property of the system and its environment, we can’t any longer see the superposition just by looking at the little part of that shared state corresponding to the original system. We can’t see the wood for the trees, you might say. Decoherence is not actually a loss of superposition and coherence, but rather a loss of our ability to detect these things in the original system.
We don’t need a conscious mind to measure or look. With or without us, the Universe is always looking
Only by looking closely at the states of all the entangled particles can we deduce that they’re in a superposition. And how can we possibly hope to do that – to monitor every photon that bounces off the original system, every air molecule that collided with it and then subsequently with others? The pieces of the puzzle have been scattered so widely that they are lost, for all practical purposes, even though in principle they are still out there, and remain so (as far as quantum mechanics tells us) indefinitely. That’s the essence of what decoherence is: a loss of (personally) meaningful coherence. It is a gradual and real process that occurs at a particular rate.
Quantum mechanics allows us to calculate that rate, so that we can put the theory of decoherence to the test. Serge Haroche and colleagues at the École Normale Supérieure in Paris first did that in 1996 by measuring decoherence of an atom held in a device called a ‘light trap’ and interacting with photons. The loss of interference between states of the atom owing to decoherence, as calculated from quantum theory, matched the experimental observations perfectly. And in 2003 a team at the University of Vienna led by Anton Zeilinger and Markus Arndt watched interference vanish between the quantum waves of large molecules, as they altered the rate of decoherence by gradually admitting a background gas into the chamber where the interference took place, so that the gas molecules would collide with those in the matter waves. Again, theory and experiment tallied well.
Decoherence is a phenomenally efficient process, probably the most efficient one known to science. For a dust grain 100th of a millimetre across floating in air, it takes about 10-31 seconds: a million times faster than the passage of a photon of light across a single proton! Even in the near-isolation of interstellar space, the ubiquitous photons of the cosmic microwave background – the Big Bang’s afterglow – will decohere such a grain in about one second.
So, for objects approaching the macroscopic scale under ordinary conditions, decoherence is, to all practical purposes, inevitable and instantaneous: you can’t keep them looking ‘quantum’. It’s almost as if the laws of quantum physics that make the world are contrived to hide those very laws from anything much bigger than atom-sized, tricking us into thinking that things just have to be the way we experience them. But if we watch nature carefully enough, we can see how the trick is done.
Notice that this effect of decoherence has nothing to do with observation in the normal sense. To turn quantum to classical, we don’t need a conscious mind to measure or look; we just need an environment full of stuff. With or without us, the Universe is always looking.
The decay of quantum superposition and interference by decoherence is only the first element in a quantum theory of measurement, however. We also have to explain why classical measuring instruments register the values they do. Exactly how we define a superposition state depends on how we choose to write the maths. From the quantum perspective, all states are equally valid solutions to the equations. So why do some of these states survive decoherence and get translated into those unambiguous readouts, or ‘pointer states’, in a measuring device, while others don’t?
Read the second part of the article.
October 26, 2018