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Entropy Theories in State of Disorder

Image of Stephen Hawking

Stephen Hawking in freefall flight on board a modified Boeing 727 jet that completes a series of steep ascents and dives to create short periods of weightlessness due to freefall. During this flight Hawking experienced eight such periods. Now one of his theories about entropy is in freefall too. Photo: NASA

By Stephen Luntz

Australian researchers have found that there is more disorder in the universe than previously realised – and that one of Stephen Hawking’s assumptions is probably wrong.

Radio broadcaster Terry Lane used to claim that he lay in bed at night worrying that entropy is increasing. It seems he now has more to worry about than he thought following the discovery that there’s more entropy in the universe than previously realised.

Entropy is the amount of disorder in the universe. As time goes on the universe becomes less ordered, eventually leading to the potential for “heat death”, when the energy in the universe is so evenly distributed that work – and therefore life – becomes impossible. The humour in Lane’s concern lies in the fact that such a fate is hundreds of billions of years away.

However, PhD student Chas Egan and his supervisor Dr Charley Lineweaver from the Australian National University’s Research School of Astronomy and Astrophysics have raised the possibility that there may be far more entropy in the universe than we have realised.

Egan stresses that this doesn’t necessarily mean we’re closer to the end of the universe than previously thought. “It’s very hard to estimate the time to the end of the universe because entropy is increasing so slowly,” he explains.

Egan and Lineweaver have concluded that the entropy in the universe has increased in a series of dramatic steps, with slow increases in between. “What our calculation means in terms of time depends on what you think will happen in the future, and we’re still not sure about that,” Egan says. “There are no foreseeable steps that will cause large jumps in entropy.”

But if the only form of increase in entropy is a very slow one, the end of the universe is not even remotely nigh.

Entropy Reservoirs

Stellar black holes, photons, neutrinos, dark matter and stars all show quantifiable amounts of entropy, which is measured as the amount of energy per degree of temperature. To calculate the amount of entropy in the universe “we divided the universe into different components and calculated each, and added them together,” Egan says. In one sense, however, the process was unnecessary.

Supermassive black holes dominate the equation. By Egan and Lineweaver’s assessment they have almost ten million times as much entropy as the next largest component of the universe, a consequence of their high energy at low temperatures.

For most of the components of the universe, Egan and Lineweaver produced figures similar to what others have come up with before. However, their estimate for supermassive black holes is 30–1000 times greater than previous estimates. Egan explains that this higher estimate relies on work at ANU by Dr Alistair Graham, who found that there are far more supermassive black holes in the universe than had previously been recognised – and they were larger too. The entropy of a black hole rises sharply with its size, so Graham’s recalculation of the size of supermassive black holes greatly affected the entropy calculations.

The supermassive black holes formed relatively suddenly less than a billion years after the universe’s formation. causing a steep rise in the universe’s entropy.

What Does It Mean?

The implications of Egan and Lineweaver’s work are not clear. Certainly, if confirmed, we will learn something about the amount of energy in the universe. Egan says that “the total amount of energy is the same” as previous calculations but “our work decreases the amount of free useable energy. It started off the same, but a whole swag got grabbed by black holes.”

The implications of this are less apparent, although there is no reason to think that entropy will accumulate faster than was previously anticipated. But if we’ve lost more of the useable energy that was originally in the universe, yet the rate of loss is now no faster than we had thought, what does that say about how the universe will end?

“The question of the end of the universe has flipped back and forwards for a few hundred years,” Egan says. For much of that time the dominant theory has been of “heat death”. In this scenario, all of the universe’s energy becomes unusable entropy.

Lord Kelvin proposed this fate for the universe in 1850. The logic was simple: entropy always increases in a closed system. Since the universe is the ultimate closed system, entropy should increase until it can go no higher because no other form of energy exists.

This idea has not gone unchallenged. An alternative view – “cold death” – holds that the universe will expand so much that energy will be too diffuse to sustain life. For any intelligent beings alive at the time, the two scenarios would look not dissimilar, but from a cosmological point of view the differences are interesting. “If you did a poll amongst cosmologists, most would say it will be a heat death,” Egan says. “But lots more work needs to be done before we can be sure. One thing we’re working on is establishing whether the universe’s acceleration caused by dark energy will continue forever.

“The maximum entropy is crudely known,” Egan continues. “People who anticipate a heat death say the maximum entropy is 10123 J/°K” This is 18 orders of magnitude more than Egan and Lineweaver’s estimate for the current amount of entropy.

Being a factor of 1018 below the end rather than 1020 or 1021 makes no difference on timescales we can imagine, but may alter the final outcome.

Can Computers Run Backwards?

One question that fascinates cosmologists is whether it is really necessary for entropy to increase forever. Is it possible there could be universes in which entropy decreases? Some theorists have gone further, suggesting that even within our own universe there could be pockets where entropy is falling, often popularised as “time running backwards”.

Renowned physicist Stephen Hawking has written several papers suggesting that if one was to stumble into such a pocket of decreasing entropy we wouldn’t be able to tell as our memories would also be reversed, with our past becoming our future.

Dr Owen Maroney of the University of Sydney’s School of Physics has questioned this conclusion. Maroney has been determining how much heat computers must put out in order to do their computations, a figure much lower than what is produced by an actual desktop computer. He contemplated the heat emitted by computers in an entropy-decreasing universe. “People had assumed that it wouldn’t change things” he said, “but I realised that’s all it was – an assumption,” Maroney says.

“Computers get hot,” Maroney says, and therefore they increase the amount of entropy. “So Hawking had said that this means computers can’t work in the reversed universe. They’d have to start running backwards. The brain is a kind of computer, so you’d go into reverse too.”

When we break an egg, entropy increases. We can’t put Humpty Dumpty together again without applying a large amount of energy, so we don’t see eggs spontaneously reforming. But the reverse would be the case in an entropy-decreasing universe – eggs would come together from their components. Then again, if our brains had to operate backwards in such circumstances we’d still see eggs breaking apart even if they were really coming together.

However, Maroney has concluded that things might work differently. “In a reversed universe, computers will absorb heat instead. They’ll get cold. That makes entropy go down, so they can work after all. And that means so can you.”

This doesn’t mean that Maroney necessarily believes that it’s possible for consciousness to operate in a world where entropy decreases. “I’ve disproved one attempt to show consciousness can’t work in an entropy-decreasing world. That doesn’t mean it can happen. I don’t know of any strong arguments to show consciousness can’t work in an entropy-decreasing world, but I can’t imagine how we would interact with it and what it would look like.”

Maroney bases his conclusions on his previous work on the heat that computers give out. He says that what Hawking seems to have missed is that there are two correlations between computers and the rest of the universe as calculations are done – the macroscopic and the microscopic – and that these operate quite differently.

Microscopic interactions can be triggered by factors as subtle as photons passing through a gas or minute gravitational effects. They can only be reversed if every factor is reversed. So if we wanted to run the weather exactly backwards we would need to reverse everything from the sun’s light to the miniscule influences of the gravitational fields of other planets.

Maroney’s point is that while these microscopic effects exist, the interaction between a computer and the universe it is calculating about is macroscopic. “Decreases in one does not imply decreases in the other,” he says. He concludes that macroscopic effects outweigh microscopic effects, and enable a computer to operate in a reversed universe, albeit in a heat-absorbing manner we struggle to imagine.

So far Maroney has only based the calculations on classical computers. He says that quantum computers operate differently, and it is possible that quantum computers could give off so much heat that they could not operate in an entropy-decreasing universe.

How Can We Tell?

All this raises the question of whether we can really know how things would operate in a universe we can’t observe, and may not even be possible in the first place. “It comes down to the question of why entropy is increasing everywhere in our universe,” Maroney says. “We don’t really understand why this is.

“It would seem more likely that the universe would have been formed with a lot of black holes, which would have evaporated as it expanded, decreasing entropy. It looks as though the universe has this imposed boundary condition of a low entropy start,” Maroney says.

Since we don’t know what an entropy-decreasing universe would be like, Maroney says that all cosmologists can do is reverse the situation, postulating a low entropy end to the universe and running the calculations when this is enforced.

We know how some things would work in such a situation. If entropy was decreasing we would not see ice cubes spontaneously melt, but Maroney says it remains unclear how many other things would reverse along with the laws of thermodynamics, as illustrated by his question of whether light would still radiate when we flick a light switch.

Likewise, in our universe we cannot reverse causation in time. “I can decide I’m hungry and that I’ll eat a sandwich in 15 minutes,” Maroney says, “but I can’t decide to have eaten the sandwich
15 minutes ago to prevent the hunger.”

Decades ago Carl Sagan popularised the idea that if the universe ever stopped expanding and began to contract we would see a reversal in entropy with events running backwards. This idea had been disproved even before the discovery of the universe’s dark energy-induced expansion made the idea moot.

Nevertheless, Maroney says the idea that entropy may one day reverse is not dead, even if we accept an ever-expanding universe. “It’s possible the universe could achieve heat death and be at maximum entropy and then some ripples could occur that seem at first to be random fluctuations that gradually come together to form ordered structures, from which point entropy starts decreasing.”

An even more intriguing possibility is one where some parts of the universe flip over and start to lose entropy while others continue to increase. “You could have a situation where entropy is increasing here but decreasing in the Andromeda Galaxy,” Maroney says.

“It’s even been shown it would be possible to temporarily communicate between civilisations in the two different states, but it would have to be weak communication only to avoid disrupting each other. Too much communication would be catastrophic for both sides.”

Perhaps we’ve just found something else for Lane to worry about.