A Debate Over the Physics of Time
Quanta Magazine said:...
There are a few things that everyone agrees on. The directionality that we observe in the macroscopic world is very real: Teacups shatter but do not spontaneously reassemble; eggs can be scrambled but not unscrambled. Entropy — a measure of the disorder in a system — always increases, a fact encoded in the second law of thermodynamics. As the Austrian physicist Ludwig Boltzmann understood in the 19th century, the second law explains why events are more likely to evolve in one direction rather than another. It accounts for the arrow of time.
But things get trickier when we step back and ask why we happen to live in a universe where such a law holds. “What Boltzmann truly explained is why the entropy of the universe will be larger tomorrow than it is today,” said Sean Carroll, a physicist at the California Institute of Technology, as we sat in a hotel bar after the second day of presentations. “But if that was all you knew, you’d also say that the entropy of the universe was probably larger yesterday than today — because all the underlying dynamics are completely symmetric with respect to time.” That is, if entropy is ultimately based on the underlying laws of the universe, and those laws are the same going forward and backward, then entropy is just as likely to increase going backward in time. But no one believes that entropy actually works that way. Scrambled eggs always come after whole eggs, never the other way around.
To make sense of this, physicists have proposed that the universe began in a very special low-entropy state. In this view, which the Columbia University philosopher of physics David Albert named the “past hypothesis,” entropy increases because the Big Bang happened to produce an exceptionally low-entropy universe. There was nowhere to go but up. The past hypothesis implies that every time we cook an egg, we’re taking advantage of events that happened nearly 14 billion years ago. “What you need the Big Bang to explain is: ‘Why were there ever unbroken eggs?’” Carroll said.
Some physicists are more troubled than others by the past hypothesis. Taking things we don’t understand about the physics of today’s universe and saying the answer can be found in the Big Bang could be seen, perhaps, as passing the buck — or as sweeping our problems under the carpet. Every time we invoke initial conditions, “the pile of things under the rug gets bigger,” said Marina Cortes, a cosmologist at the Royal Observatory in Edinburgh and a co-organizer of the conference.
To Smolin, the past hypothesis feels more like an admission of failure than a useful step forward. As he puts it in The Singular Universe: “The fact to be explained is why the universe, even 13.8 billion years after the Big Bang, has not reached equilibrium, which is by definition the most probable state, and it hardly suffices to explain this by asserting that the universe started in an even less probable state than the present one.”
Other physicists, however, point out that it’s normal to develop theories that can describe a system given certain initial conditions. A theory needn’t strive to explain those conditions.
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