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Why does time go forwards, not backwards? - BBC

Why does time go forwards, not backwards? - BBC

Why does time go forwards, not backwards? - BBC
Oct 04, 2022 3 mins, 48 secs

"The interesting feature of Newton's laws, which wasn't appreciated till much later, is that they don't distinguish between the past and the future," says the theoretical physicist and philosopher Sean Carroll, who discusses the nature of time in his latest book The Biggest Ideas in the Universe.

Virtually all of the cornerstone theories of physics since then have worked just as well going forward in time as they do backwards, says physicist Carlo Rovelli of the Centre for Theoretical Physics in Marseille, France, and the author of books including The Order of Time?

"Starting from Newton, and then Maxwell's theory of electromagnetism, then Einstein's work, and then quantum mechanics, quantum field theory, general relativity, and even quantum gravity – there is no distinction between past and future," Rovelli says.

How does a clear direction of time emerge from these descriptions of the Universe, which all lack their own arrow of time.

As Rovelli stresses in The Order of Time, this is the only basic law of physics that can tell apart the past from the future.

Here's the problem: when you zoom in to the level of, say, one water molecule colliding and bouncing off another, the arrow of time disappears.

This means that the arrow of time from past to future only emerges when you take a step back from the microscopic world to the macroscopic – something first appreciated by the Austrian physicist-philosopher Ludwig Boltzmann.

"So the direction of time comes from the fact that we look at big things, we don't look at the details," says Rovelli.

"It's not that the world is fundamentally oriented in space and time," Rovelli says.

While entropy does seem to be inextricably bound up with the arrow of time, it feels a bit surprising – perhaps even disconcerting – that the one law of physics that has a strong directionality of time built into it loses this directionality when you look at very small things.

The question of why time only ever goes forward leads us remarkably quickly back to the origin of the Universe itself (Credit: Edouard Taufenbach courtesy of Gallery C).

"What is entropy?" Rovelli says.

"The answer is embedded in the beginning of the Universe," says Carroll.

"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.

"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.

Even if we don't yet know why, the Universe's low entropy past is a plausible source of time's arrow.

"Boltzmann thought, 'ah, entropy is growing in the Universe and maybe it's going to maximum at some point'," says Rovelli.

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.

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

"When we think we produce heat in our neurons," says Rovelli.

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.

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?

The answer why time goes forward is rooted in the way the Universe began (Credit: Edouard Taufenbach courtesy of Gallery C).

"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

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

That project is what Carroll hopes to do, taking several features of our experience of time and relating them back to entropy

"I think we understand why we have this sense of flowing," says Rovelli

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