Sunday, November 22, 2015

Why Does Time Always Run Forwards And Never Backwards? (3/3)


Continued from Part 2

The importance of being clumpy


The answer is that gravity affects entropy, in a way that physicists still don't fully understand. With truly massive objects, being clumpy is higher entropy than being dense and uniform. So a universe with galaxies, stars and planets actually has a higher entropy than a universe filled with hot, dense gas.


This means we have a new problem. The sort of universe that emerged immediately after the Big Bang, one that is hot and dense, is low-entropy and therefore unlikely. "It's not what you would randomly expect out of a bag of universes," says Carroll.

So how did our universe start in such an unlikely state? It's not even clear what kind of answer to that question would be a satisfying one. "What would count as a scientific explanation of the initial state [of the universe]?" asks Tim Maudlin, a philosopher of physics at New York University.
Perhaps our universe is one of many (Credit: Detlev van Ravenswaay/Science Photo Library)

One idea is that there was something before the Big Bang. Could that account for the low entropy of the early universe?

Carroll and one of his former students proposed a model in which "baby" universes are constantly popping into existence, calving off from their parent universe and expanding to become universes like our own. These baby universes could start out with low entropy, but the entropy of the "multiverse" as a whole would always be high.

If that's true, the early universe only looks like it has low entropy because we can't see the bigger picture. The same would be true for the arrow of time. "That kind of idea implies that the far past of our big-picture universe looks the same as the far future," says Carroll.

But there's no wide agreement on Carroll's explanation of the past hypothesis, or any other explanation. "There are proposals, but nothing is even promising, much less settled," says Carroll.

Part of the trouble is that our best theories of physics can't actually handle the Big Bang. Without a way to describe what happened at the universe's birth, we can't explain why it had low entropy.
Physics still can't explain everything (Credit: Markus Schieder/Alamy)

Modern physics relies on two major theories. Quantum mechanics explains the behaviour of small things like atoms, while general relativity describes heavy things like stars. But the two can't be made to combine.

So if something is both very small and very heavy, like the universe during the Big Bang, physicists get a bit stuck. To describe the early universe, they need to combine the two theories into a "theory of everything".

This ultimate theory will be the key to understanding the arrow of time. "Finding that theory will ultimately let us know how nature builds space and builds time," says Marina Cortês, a physicist at the University of Edinburgh in the UK.

Unfortunately, despite decades of trying, nobody has managed to come up with a theory of everything. But there are some candidates.
Maybe all matter is made of tiny strings (Credit: Equinox Graphics/Science Photo Library)

The most promising theory of everything is string theory, which says that all subatomic particles are actually made of tiny strings. String theory also says that space has extra dimensions, beyond the familiar three, that are curled up to microscopic size, and that we live in a kind of multiverse where the laws of physics are different in different universes.

This all sounds quite outlandish. Nevertheless, most particle physicists see string theory as our best hope for a theory of everything.

But that doesn't help us explain why time moves forwards. Like almost every other fundamental physical theory, the equations of string theory don't draw a strong distinction between the past and the future.

String theory, if it turns out to be correct, might not help explain the arrow of time. So Cortês is trying to come up with something better.
Time only ever goes forwards, but no one knows why (Credit: dbimages/Alamy)

Working with Lee Smolin of the Perimeter Institute in Waterloo, Canada, Cortês has been working on alternatives to string theory that incorporate the arrow of time at a fundamental level.

Cortês and Smolin suggest that the universe is made up of a series of entirely unique events, never repeating itself. Each set of events can only influence events in the next set, so the arrow of time is built in. "We are hoping that if we can use these types of equations to do cosmology, we can then arrive at the problem of the initial conditions [of the universe] and find they're not as special," says Cortês.

This is completely unlike Boltzmann's explanation, in which the arrow of time emerges as a kind of accident from the laws of probability. "Time isn't really an illusion," says Cortês. "It exists and it's really moving forward."

But most physicists don't see a problem with Boltzmann's explanation. "Boltzmann pointed the correct direction to the solution here, a long time ago," says David Albert, a philosopher of physics at Columbia University in New York. "There's a real hope that if you dig carefully enough, the whole story is in what Boltzmann said."

Carroll agrees. "If you have that low-entropy Big Bang, then we're done," he says. "We can explain all the differences between the past and the future."
Inside the Large Hadron Collider (Credit: Julian Herzog, CC by 3.0)

One way or another, to explain the arrow of time we need to explain that low-entropy state at the beginning of the universe. That will take a theory of everything, be it string theory, Cortês and Smolin's causal sets, or something else. But people have been searching for a theory of everything for 90 years. How do we find one? And how do we know we have the right one once we've got it?

We could test it using something very small and very dense. But we can't go back in time to the Big Bang, and regardless of what a recent blockbuster movie suggested, we also can't dive into a black hole and send information back about it. So what can we do, if we really want to explain why eggs don't un-break?

For now, our best hope lies with the largest machine in human history. The Large Hadron Collider (LHC) is a particle accelerator that runs in a 27 km circle under the border of France and Switzerland. It smashes protons together at nearly the speed of light. The phenomenal energy of these collisions creates new particles.

The LHC has been closed for repairs for the last two years, but in the spring of 2015 it will turn back on — and for the first time, it will be operating at full power. At half-strength in 2012, it found the long-sought-after Higgs boson, the particle that gives all the others mass. That discovery led to a Nobel Prize, but the LHC could now top it. With any luck, the LHC will catch a glimpse of new and unexpected fundamental particles that will point the way to a theory of everything.

It will take several years for the LHC to collect the necessary data, and for that data to be processed and interpreted. But once it's in, we may finally understand why you can't get that stupid egg off your face.

Click here for Part 1

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