Why Does Time Go Forwards, Not Backwards? - PART 2

If there is an arrow of time, where did it come from in the first place?

"The answer is embedded in the beginning of the Universe," says Carroll. "The answer is because the Big Bang had low entropy. And still, 14 billion years later we are swimming in the aftermath of that tsunami that started near the Big Bang. That's why time has a direction for us."

The extraordinarily low entropy of the Universe at the Big Bang is both an answer and an enormous question. "The thing we understand the least about the nature of time, is why the Big Bang had low entropy, why the early Universe was like that," says Carroll. "And I think honestly, as a working cosmologist, I think that my fellow cosmologists have dropped the ball on this one. They don't really take that problem seriously enough."

Carroll published a paper in 2004 with his colleague Jennifer Chen, in which they aimed to explain why the Universe had such low entropy close to the Big Bang, rather than just assuming or accepting this was the case. "There's plenty of loopholes in the theory, plenty of aspects of it that are not completely baked – but I also think it is by far the best theory on the market," says Carroll. "It doesn't cheat."

Other cosmologists agree that it is indeed time to turn serious thought to this problem of the Universe's low entropy origins. "The likelihood of our current Universe having initial conditions of this kind, and not any other kind, is around one in 10 to the 10 to 124 (1:10^10^124)," says Cortês. (Another way of saying it is that the event had a probability of 0.00…01 – with 10^(10^124) zeroes omitted – a number so large it's awkward to express in conventional maths, Cortês notes.) "I mean I could safely say, this is the largest number in modern physics, outside of philosophy or mathematics."

Simply taking such unlikely low-entropy origins as given is a grand case of "shoving the problem under the rug", Cortês says. "If physicists keep doing this, after a while it's going to be a very big pile under the rug. It's left to us cosmologists to explain why time only moves forward."

Even if we don't yet know why, the Universe's low entropy past is a plausible source of time's arrow. Like most things that have a beginning, the arrow will also have an end. The first person to spot this was, once again, the Austrian physicist Ludwig Boltzmann.

"Boltzmann thought, 'ah, entropy is growing in the Universe and maybe it's going to maximum at some point'," says Rovelli. At that point, heat would be evenly distributed throughout the Universe, no longer flowing from one place to another.

There would be no energy available in a useful form for doing work – in other words, almost nothing interesting would be happening throughout the entire Universe. As astrophysicist Katie Mack describes it, "As that process continues, everything is decaying so much that all that’s left is the waste heat of everything that ever existed in the Universe." This fate is known as the thermal death of the Universe, or heat death.

"Stars will stop burning, nothing will happen anymore. There will be nothing but small thermal fluctuations," says Rovelli. "Suppose this happens – and we don't know for certain if it's going to happen, but suppose it does – should we say that there is no time direction there? Of course there's no time direction, because every phenomenon that happened one way could also go one way or the other. Nothing will distinguish the two directions of time."

This is perhaps the strangest thing about the arrow of time: "It only lasts for a little while," says Carroll.

It's very hard to picture what might happen if the arrow of time eventually vanishes. "When we think we produce heat in our neurons," says Rovelli. "Thinking is a process in which the neuron needs entropy to work. Our sense of time passing is just what entropy does to our brain."

The arrow of time that arises from entropy brings us a long way closer to understanding why time only goes forward. But there may be more arrows of time than this one – in fact there is arguably an entire volley of arrows of time pointing from the past to the future. To understand these, we have to step from physics into philosophy.

Human Time

The ways that we intuitively understand and experience time shouldn't be taken lightly, says Jenann Ismael, professor of philosophy at Columbia University, New York. If you think about your own experience of time, you may soon be able to recognise several of the psychological arrows that form a core part of human experience. One of these arrows is what Ismael terms "flow".

"If you look out at the world, you don't experience a purely static representation of the instantaneous state of the world," she says, like in a movie made up of a number of static frames every second. "We see directly that the world is changing."

This experience of the flow of time is built into our perception. "Vision isn't like a movie camera at all," says Ismael. "Actually what happens is your brain is collecting information over some temporal period. It's integrating that information so that at any given moment, what you're seeing is a computation that the brain has done. So that you not only see that things are moving, you see how fast they're moving, the direction in which they're moving. So the whole time, your brain is integrating information over temporal intervals and giving you the result. So you see time, in a way."

There's a second feature of time that Ismael distinguishes from flow, which she terms "passage".

The idea of passage is closely bound up with time-oriented experiences such as memory and anticipation. Take the example of a wedding, or any much-anticipated life event. Our experience of these moments has many layers – from the fractious planning stages, to the intensity of the day itself, to recollections that stay with us for years. There is a directionality to these different experiences: the way we anticipate an event in the future is fundamentally different from how we remember it when it's passed.

"All of that is part of what I think of as the experience of passage, this idea that we experience every event as anticipated from the past, experienced in the present, remembered in retrospect," says Ismael. "It's kind of Proustian in its density."

These aspects of the directionality of psychological time – as well as many others, like the sense of openness we have about the future but not the past – could all trace their roots back to the arrow of time born of the Industrial Revolution.

"I think it does all come back to entropy," says Ismael. "I see no reason now to think that the kinds of arrows that are involved in human psychology are anything but ultimately rooted in the entropic arrow. But it's an empirical question. This project to understand human experience in relation to the entropic arrow, I've no reason to think it's going to fail."

That project is what Carroll hopes to do, taking several features of our experience of time and relating them back to entropy. His first target is causality, another element of the arrow of time, as causes happen before their effects.

To say the least, this project is a major undertaking for all physicists and philosophers involved. And still, lurking in the shadows behind all such efforts, there remains that nagging question about why entropy was so low in the earliest Universe.

"I think we understand why we have this sense of flowing," says Rovelli. "We understand why the past seems fixed to us that the future seems open. We understand why there are irreversible phenomena, and we can reduce all that to the second law of thermodynamics, to the rise of entropy.

"It's very much related to the fact that if we trace it back, back, back, to fact that the Universe started very small, in a very peculiar situation. Then somehow, it's falling down from that peculiar situation.

"But of course there's one question open, I mean, why? Why did it start in that particular way?"

- BBC