Born in the midst of quantum uncertainty, how did the Universe become so very classical?
The Universe, it seems, was born in a quantum blip. So why does it now seem so solid and so sensible? Recall how the quantum computer manages to hang onto its internal quantum state while working on some calculation, only yielding a definite answer in response to a correctly designed measurement. Every physical process in the world works in broadly the same way: elementary objects interact according the rules of quantum theory, becoming recognisable only when we make a measurement or an observation. Any classical phenomenon in the world around us is in effect the result of a quantum computation answering the question: "What classical thing am I?".
The same principle applies to billiard balls colliding, ocean waves rolling in and out, continental plates moving across the Earth's surface, even the sunlight that struck the planet 4 billion years ago and nudged life into existence. At a fundamental level everything is made of quantum components, and at the same time everything that sticks around long enough for us to notice is classical. This also holds true for the Universe as a whole. Without a quantum theory of gravity, we don't know exactly how the Universe that we see around us emerged from the big bang. But emerge it evidently did, so who or what brought forth classical meaning from quantum origins?
Traditionally, following Niels Bohr's lead, arguments about the nature of measurement in quantum theory have assumed a clear distinction between the quantum system that's being measured and a separate classical system that's doing the measuring. At one extreme is the belief that measurements become real only when there's a conscious human observer around to notice them. That would mean a mechanical robot arm opening the box containing Schroedinger's "half-dead, half-alive" cat couldn't resolve the hapless creature's dilemma. Instead the robot would have to drag the cat in front of a human observer before the state could be resolved.
This sort of philosophy causes trouble when applied to the Universe as a whole. Stars, planets and galaxies are quantum systems like everything else. But are we to imagine that the whole Universe remained in a state of cosmic quantum indeterminacy until human beings evolved consciousness? And when during the dawning of human consciousness was the Universe forced to drop its cloak of quantum indeterminacy and take on solid form? Put this way, the argument seems absurd, but on the other hand if the Universe congealed into a classical solidity before we came upon the scene, what sort of measurements or observations accomplished the transformation?
Could decoherence be the answer to this conundrum? If classical properties can emerge from quantum systems simply because random and uncontrollable interactions sabotage the coherence necessary for true quantum behaviour, could classical behaviour also emerge inexorably as the whole Universe evolves. This idea seems to make sense. Think how impossible it would be to keep something as huge as our Universe in a true quantum state for anything more than a tiny fraction of a second. Is decoherence then what makes the cosmos and everything in it seem solid and definite to us?
That picture may be appealing, but it can't be the whole story. When two billiard balls collide, the maths of decoherence can only work if each ball is a separate and independent quantum system. If there is instead some pre-existing quantum connection between them - a quantum collusion, if you like - then decoherence fails.
Born of a single quantum event, the Universe is at some basic level a single interconnected quantum system. There are no truly independent pieces of this system and therefore no random and uncontrollable interactions to impose a classical solidity on the quantum maelstrom. Once a quantum system, always a quantum system.
If decoherence alone doesn't explain the appearance of our Universe, what else is needed? There is one possible, if incomplete, solution. The idea is delightfully simple, hinging on the notion that stable collective properties tend to emerge naturally from any complex system. Think, for example, of a river flowing through a winding channel. Water is fundamentally a large collection of molecules, but trying to analyse its motion in those terms will get you nowhere fast. Instead, you think of pressures and stream velocities, of turbulence and eddies. Although these collective properties spring from the behaviour of all the molecules and atoms in a river, it's more helpful to picture them as basic features of fluid flow in their own right.
Similarly, you might expect that certain collective properties would naturally emerge from a complex quantum system. Could this then explain the order of our Universe? The idea is that collective characteristics can emerge with enough independence for decoherence arguments to apply to them--which then justifies their being treated as independent in the first place.
In the circumstances, this is an appealingly circular definition, allowing stable classical properties to emerge from a quantum system in a way that does not demand the introduction of any new physical principles. It's not hard to decide what sort of collective characteristics would qualify as classical attributes. For example, they would have to have a stable meaning even while the underlying quantum system was in an unceasing riot of imperceptible change. And they would need to obey the rules of conventional logic, of classical cause and effect, at least to the extent that any departures from those rules would be enormously improbable.
A successful description of the evolution of the Universe in terms of a set of properties defined in this way has been labelled a "consistent history". Despite a great deal of subtle thinking and powerful mathematical analysis, little can be said for now beyond the fact that consistent histories exist, in principle. At least we know that it's possible to describe the classical Universe without contradicting quantum theory or needing any new physics. That's no mean achievement, when you remember that the debate about whether a cat is dead or alive has lasted for decades.
While these ideas may be comforting, they don't come close to a specific explanation of why our Universe looks the way it does. But that problem is not unique to quantum theory. As imperfect theories go, the notion of self-consistent histories is in distinguished company. Newton recognised that his brand-new theory of gravity had something to tell us about the form of the Universe, but he also realised it didn't contain any specification for how the whole thing began. Einstein, on the other hand, wondered fondly if the laws of physics allowed the Lord any choice in the creation of the Universe. So far, it seems, the answer is plenty. The uncertainty principle, which Einstein never liked, says that you can't always get what you want. And in the broadest possible terms, perhaps it applies to the limits of our knowledge about the Universe we find ourselves in. We can always ask questions, but we may not always be entitled to answers.