Jonathan Heckman, University of Pennsylvania
Why reconciling quantum mechanics with everyday phenomena leads to a puzzle about the size of the Universe.
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If you roll a fair six-sided die, there are six equally likely outcomes. We don’t know exactly what will happen when you throw the die, but we know the probability of each given outcome is one out of six.
It is natural to ask whether this uncertainty is really due to chance, or just a convenient computational shortcut. Said differently, if we knew enough about the position of the die, and the exact way it was thrown, could we actually predict with complete precision what would happen on any given roll?
In a more complicated scenario—say, predicting the behavior of the atoms in the air around you right now—perhaps we could just track the position and speed of every single particle and completely forecast the future. From this perspective, could it be that probability is really more of a practical tool in dealing with a hugely complicated problem which an extremely powerful computer could, in principle, solve?
The short answer is no. Quantum physics tells us probability is not a shortcut, and that it is really hardwired into the story right from the start. Everything in the universe is, at small distances, governed by quantum particles and their associated wave functions. The odds of finding a specific outcome (like the location of a particular atom ten seconds from now) are derived from the famous wave function—symbolized by Ψ (the Greek letter psi).
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The name “wave function” comes from the fact that much like there can be ripples and waves on the surface of a pond with high and low points, the odds of finding an atom at any given point are best described in terms of the crests and troughs of such a wave. These waves can add up to produce an even bigger effect, leading to higher probability events, and they can also interfere, destroying the odds of seeing any event at all. This leads to some of the less intuitive features of quantum physics. For example, quantum particles can pass through solid walls, along with other seemingly “miraculous feats” that confound both our intuitive expectations and those of classical physics.
From quantum to classical
Given the rules of quantum physics, how do we ever get back to the relatively predictable world of everyday experience?
Everything is quantum mechanical. Each particle or system has a degree of freedom—the number of independent parameters by which its movement or change can be measured. The number of individual degrees of freedom in a quantum mechanical system dictates how likely it is to have behavior that clashes with our classical intuitions. When you have a huge set or collection of these degrees of freedom all together, they interfere with each other and the odds of a highly quantum state tend to get averaged out and cancelled. If you have a single electron you can have a quantum effect—as captured by the wave function—but a room full of air has a huge number of such particles, and maintaining the coherency of the wave is much more difficult. So the more quantum mechanical objects (or particles) that we have, the more classically the system generally behaves.
The decoherence in these different particles produces a macroscopic world, one which can, to good approximation, ignore the subtleties of microscopic quantum physics. The passage from the very small and quantum to the very big and classical is one of the central concepts of physics, and continues to play an important role in our understanding of Nature.
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A very big question
Sometimes, though, these quantum effects do not cancel out. This leads to one of the biggest puzzles in physics today: Why is the Universe so big in the first place? The size of the Universe is, to good approximation, controlled by a parameter known as the cosmological constant. Roughly speaking, the smaller this parameter, the bigger the size of the Universe. The puzzle is that contributions from all the particles in the Universe add up to typically quite large numbers, leading to a surprising (and wrong) prediction that the Universe should actually be small and quantum mechanical, rather than big and classical!
A central aim in contemporary research is to better understand how these quantum effects average out to a cosmological constant which is really quite small. There are many theoretical proposals currently being debated, and some will likely be tested in the next few years.
Some possible explanations include the use of supersymmetry, extra dimensions, dynamical dark energy models, and the string landscape—as well as combinations of many of these ideas all at the same time. We do not yet know which elements of these proposals are correct, but the answer hinges on figuring out how quantum physics works at the biggest imaginable distance scales—the scale of the universe itself.
Jonathan Heckman
is a theoretical physicist working at the University of Pennsylvania. He likes to spend his days thinking about the physics of the very big and very small.