The universe follows the arrow of time
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Where does time really come from? I am often asked this question after acquaintances or friends of friends learn that I am a physics reporter. There is not a set answer – but to better understand it, it helps to look at the arrow of time.
Dating back to the 1920s, this concept stems from the laws of physics that describe energy, heat and entropy. Entropy is the big one, as time seems to move from low- to high-entropy states – this is the direction in which the “arrow of time” flies. Entropy gets a bad reputation for being about disorder, but the more precise way to think about it is to count how many ways something large – a macrostate – can be assembled from smaller constituent parts, or microstates.
A macrostate that corresponds to many microstates, like a cutlery drawer where spoons and forks are mixed, has a higher entropy than a macrostate where the microstates are more constrained, like the same drawer with all the forks neatly piled on the left and all the spoons on the right. If you arrange the drawer in this way, but the next time you open it the spoons and forks are intermixed, that suggests entropy has increased, and time’s arrow has pushed the drawer from the past to the future.
There is, however, one problem with extrapolating from cutlery to the cosmos. Why would there have ever been a starting state where everything was neat and entropy was low?
Physicists call this the “past hypothesis”, and they aren’t fans of it. When they do the mental exercise of rewinding the arrow of time, they end up in a state where the entropy of the universe was exceedingly low. Such states are thought to be rare, so it is unclear why one had to exist at all. As it would mark the beginning of time, questions also arise about whether such a state must also coincide with the beginning of the universe, the big bang.
To add insult to injury, there is one other problem: laws of physics at scales much smaller than the whole universe – like the quantum scale of a particle or two – are fully reversible, which means they don’t constrain time’s arrow to fly only in one direction. Pablo Arrighi at Paris-Saclay University in France tells me that this is one of the biggest paradoxes in physics.
“The laws of physics are reversible, but how come what I see in daily life is not?” he asks. Arrighi and his colleagues wanted to see whether they could come up with a very basic “toy universe” – a simplified model of the real universe – where everything is perfectly reversible, but the arrow of time as we know it still exists.
They found that the arrow of time is inevitable if the toy universe does what our own universe does and keeps constantly expanding. This model universe also let the researchers eliminate the past hypothesis: it allowed for the big bang, but didn’t necessitate a special state at which time begins and the arrow of time is launched forwards.
In fact, Arrighi says this work made him reconsider his previously suspicious stance on potential futures like the “big crunch”, where the universe eventually stops expanding and shrinks to a tiny point, and the “big bounce”, where the cosmos gets caught in a never-ending cycle of bang-expand-shrink-crunch-bang.
Strikingly, in this toy universe built only on reversible laws of physics, the big bang doesn’t have to be a singular instance when all physics as we know it breaks. Rather, there is more entropy-driven expansion on the other side – essentially another universe. “Our birth would be caused by their birth. Our matter would come from their past,” says Arrighi of the imagined universe on the other side of the big bang.
Though it may seem radical, the idea of two universes expanding in opposite directions, each with its own sense of time, has been on physicists’ radar before. For instance, in 2014, independent physicist Julian Barbour and his colleagues argued in favour of this scenario. Their work was based on a study of gravity, as opposed to Arrighi’s work, which is much more grounded in a computational mode of argument – and Arrighi’s team’s toy model can be easily simulated on a computer. The idea of abandoning the past hypothesis has also been suggested before by researchers such as Sean Carroll at Johns Hopkins University in Maryland.
To come back to our original question, could the answer be that time comes from nowhere – or at least nowhere special? When I pose this question to David Albert, a philosopher at Columbia University in New York, he cautions me to think more carefully about the word “special”. He isn’t, in fact, convinced that the low-entropy state of the past hypothesis would necessarily be special at all.
“People have some idea that every possible physical state ought to be as probable as every other possible physical state. If you think that way, then these low-entropy states turn out to be highly improbable,” he says. “But my own attitude towards this is that it’s crazy to think that you could get probabilities just from a priori reasoning.” He points out that we really should find the probabilities of any given event by investigating it through observations. As long as we find evidence that the event – in this case, the universe existing in a low-entropy state – had to happen, then it doesn’t matter how improbable that event might be according to abstract arguments, he says.
Albert is all for removing the past hypothesis from our list of absolutely necessary physics edicts – in his view, fewer laws are always better. But he wants that intervention to be grounded in observations above all. The gap between the size of systems where we can manipulate and carefully study quantities like entropy, such as the textbook example of gas particles in a box, and the size of the whole universe is huge. So he advises care and scrutiny about where physicists may be making assumptions as they extrapolate from one to the other.
“But the general project of investigating whether you could get away with not positing the past hypothesis, and just derive it from other laws, I think is interesting. If that can be done, that’s great,” says Albert.
After finishing my call with Albert, I made a reminder to call everyone again in a year and see how time was holding up. Even if I still don’t know how to explain where time comes from, its arrow will certainly push me into a future where I have more conversations about it.
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