Physicists find that ageing is reversible -- in glass (sorry) (Patreon)

[This is a transcript with links to references.]

One of the reasons I am doing these science news episodes is that sometimes there’s a news item which touches on a really deep question, but it’s so technical that no one dares talking about it. This new research is such a case. It’s an experiment that looked at the question whether aging is reversible, in glass . Yes, I’d be more interesting if it was people rather than glass, but we have to start somewhere, right, so why not with glass. And to their surprise, the researchers found that aging in glass is indeed reversible, with some caveats. Let’s have a look.

The deep question which this research touches on is why time in our universe seems to pass only in one way. Why do volcanoes erupt but not un-erupt, why do eggs break but not unbreak, why do we get older but never younger.

Yes, it’s entropy increase, or so we are told. But the thing is, if aging is just entropy increase, then you should be able to reverse it by decreasing entropy, right?

This is what Maxwell’s famous thought experiment with the demon illustrates. Maxwell said, suppose I have two boxes of gas, one with high temperature, that’s red, and one with low temperature, that’s blue. As long as the boxes are separated the gases are very ordered, and entropy is low. But now I mix the gasses. Entropy will increase until they’ll reach some equilibrium temperature. The second law of thermodynamic now says that once you’ve reached maximum entropy it can’t decrease. So the gasses won’t spontaneously unmix, and that’s certainly what we observe in nature.

Maxwell’s demon now puts a wall in the middle of the gas box with an opening. Each time there’s a fast gas molecule approaching the opening, he lets it through. If there’s a slow one, he keeps it closed. The effect is that over time, the temperature on the right side increases again. So, entropy goes down.

Maxwell came up with this to say that something is fishy with the second law of thermodynamics. But physicists understand today though that what’s actually happening is that the demon needs to have information about which gas molecules are fast and which are not. And if he has information, he needs to have order, so low entropy. The demon can therefore use this information to decrease the entropy. But the total entropy of the demon and the gas still increases. So no problem with the second law of thermodynamics.

The thing is now that for all we know this is generally and always true. If you have information, you can decrease entropy. And with that you can in principle, turn around any process in time, including aging, of materials or anything really. In principle. In practice, well, we can’t. Otherwise Bryan Johnson would spend his money on physics and not on dietary supplements.

But in the past decade or so physicists have managed to actually do this time-reversal in some cases. This is maybe one of the most-exciting research you’ve never heard of. It makes headlines every now and then as saying that physicists have reversed time or something like that.  There was for example a really neat experiment by a group of researchers from MIT in 2017, where they set up a cloud of atoms so that heat actually flowed from the cold to the hot part. I know this sounds kinda lame, heat flows what, but look, this the time reverse of what would naturally happen.

 But in their experiment they exploited quantum effects, or entanglement in particular. This can be done because entanglement can store information when can then be used to seemingly decrease entropy.

The issue is that so long as we’re talking about quantum stuff the time-changes are fairly simple, both mathematically and in reality. They’re easy to understand and keep track of. Make the thing larger, move it out of the quantum realm, and you have irreversibility creeping back in. Physicists believe that this is just seemingly irreversible. But believing and seeing are two different things.

And this now brings me to the new paper. The authors of the paper looked very closely at a type of glass. Why glass? Because glass is known to age at a period that we can observe. It settles rather slowly into it’s final state. Or, as the researchers put it in their paper, they are “subject to physical ageing because [their] state relaxes continuously towards a state of metastable equilibrium.” I think that a continuous relaxation towards a state of metastable equilibrium is generally a good description for aging.

The thing is now that whether this aging process is reversible or not depends on the type of fluctuations in the material. The researchers measured this by scattering light on the material sample and then very precisely looking at the tiny changes in what came back. To their own surprise they found that the fluctuations they saw are of the reversible type. That’s a surprise because the theoretical prediction was that it should be irreversible. , As they write in the paper:

“We are not aware of a theoretical framework that predicts material-time statistical reversibility of fluctuations during ageing.”

Now let me be clear, it’s not like they reversed these fluctuation, they just found out that they’re reversible. Though they also caution in the paper that just because one type of process seems to be reversible in those materials doesn’t mean all of them are. So, more work is needed.

Still I find this very interesting because it sheds some light on our understanding of how stuff ages. And who knows maybe they’ll figure out how to actually make glass younger again in a bit. And then it's just a small step from glass to us.