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Physicists Create World’s First Time Crystal

technologyreview.com

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Re: Physicists Create World’s First Time Crystal

#51

Post by davesque »

lisper wrote: post #45
davesque wrote: post #38

I figured this article would fly over my head (which it mostly did), but it was actually a very fun read. My main takeaway was the paradox that the interesting arrangement of matter could be in its lowest energy state, yet still moving. And that no energy could be extracted from this movement. I'm probably butchering the concept with my misuse of terminology but whatevs.

You're not that far off. The particles aren't really moving , i.e. they are not going from one spatial location to another. What is happening is that they are changing quantum spin states, and the spin states are changing in such a way that it appears that a circular arrangement of atoms is rotating. This is a reflection of the fact that an atom isn't really a thing (there are no "things", i.e. there are no particles…

Thanks for the great explanation. Serious question: Can I ask how you happen to know so much about this subject?

Re: Physicists Create World’s First Time Crystal

#52

Post by Strilanc »

code51 wrote: post #13

Can someone explain what computations using __zero energy__ from related article [1] actually means ? [1] https://www.technologyreview.com/s/426917/physicists-predict... """ Another is the possibility that it may be possible to exploit time crystals to perform computations using zero energy. As Wilczek puts it, “it is interesting to speculate that a…quantum mechanical system whose states could be interpreted as a col…

They literally mean "not spending any energy doing the computation".

We spend energy on computation for two fundamental reasons: 1) zeroing data and 2) error correction. You avoid zeroing data by doing reversible computation. You avoid error correction by finding really really well behaved / stable systems.

Time crystals are reversible basically by definition, and Wilczek is saying that maybe there might be some time crystals that are also well behaved and do something interesting enough to encode computations.

Re: Physicists Create World’s First Time Crystal

#53

Post by nchallak »

Ever since I saw my parents' initial skepticism to the use of handheld devices (smartphones, tablets etc.) I have always wondered what such a mental block for my generation might be like. It could very well be an invention such as this that is considered speculative today which might become viable overnight. My kid may someday wonder why his dad doesn't understand basic quantum phenomena.

Re: Physicists Create World’s First Time Crystal

#54

Post by semiel »

dysfunctor wrote: post #44
contravariant wrote: post #33

Earlier quoted context omitted.

>They did this, and observed the ions changing in such a way where there was no driving influence and it's implied that the reason that this behavior was observed is because time symmetry was broken, which is just a fancy way of alleging that the universe is non-deterministic, I think. Time translational symmetry and determinism have nothing to do with one another. Even a classical pendulum breaks time translational…

So what are the implications of an object that breaks that symmetry? Would it be possible to observe an object in different times in the same state, or the same time in different states?

> So what are the implications of an object that breaks that symmetry? Would it be possible to observe an object in different times in the same state, or the same time in different states?

The object would be in a repeating set of states. First state 1, then state 2, then state 1, then back to state 2, etc.

This is not in itself surprising. Think of a pendulum, for instance. The difference here is that the motion is the lowest energy state. Over time, a pendulum swing decays. This does not: it will continue forever, if not disturbed.

The obvious next question is: isn't that perpetual motion? According to the normal dictionary definition of those words yes it is, which is why this is a fascinating discovery. However, it doesn't violate the normal arguments against perpetual motion, because there's no way to extract energy from the system. Anything you did to influence the motion would require adding energy.

Re: Physicists Create World’s First Time Crystal

#55

Post by marcosdumay »

jacquesm wrote: post #35
rbanffy wrote: post #20

Earlier quoted context omitted.

It's interesting it is a perpetual motion machine, but it works only as long as you don't use it.

Any atomic nucleus with orbiting electrons fulfills that requirement, you could loosely call those time crystals too. Their half life for most stable atoms is so long that perpetual applies for most practical uses of the word. Which makes me wonder if these time crystals have a half-life too.

It must be able to tunnel into some other structure. The half-life is defined by how much energy it needs to "melt" as the article says.

Re: Physicists Create World’s First Time Crystal

#56

Post by bcherny »

lisper wrote: post #45
davesque wrote: post #38

I figured this article would fly over my head (which it mostly did), but it was actually a very fun read. My main takeaway was the paradox that the interesting arrangement of matter could be in its lowest energy state, yet still moving. And that no energy could be extracted from this movement. I'm probably butchering the concept with my misuse of terminology but whatevs.

You're not that far off. The particles aren't really moving , i.e. they are not going from one spatial location to another. What is happening is that they are changing quantum spin states, and the spin states are changing in such a way that it appears that a circular arrangement of atoms is rotating. This is a reflection of the fact that an atom isn't really a thing (there are no "things", i.e. there are no particles…

This interpretation sounds similar to Ulam/Wolfram's cellular automata :)

Re: Physicists Create World’s First Time Crystal

#57

Post by dangirsh »

Strilanc wrote: post #52
code51 wrote: post #13

Can someone explain what computations using __zero energy__ from related article [1] actually means ? [1] https://www.technologyreview.com/s/426917/physicists-predict... """ Another is the possibility that it may be possible to exploit time crystals to perform computations using zero energy. As Wilczek puts it, “it is interesting to speculate that a…quantum mechanical system whose states could be interpreted as a col…

They literally mean "not spending any energy doing the computation". We spend energy on computation for two fundamental reasons: 1) zeroing data and 2) error correction. You avoid zeroing data by doing reversible computation. You avoid error correction by finding really really well behaved / stable systems. Time crystals are reversible basically by definition, and Wilczek is saying that maybe there might be some time…

IIRC reversible computations only take zero energy if you perform them infinitely slowly.

Re: Physicists Create World’s First Time Crystal

#58

Post by lisper »

Jcol1 wrote: post #49
lisper wrote: post #45

Earlier quoted context omitted.

You're not that far off. The particles aren't really moving , i.e. they are not going from one spatial location to another. What is happening is that they are changing quantum spin states, and the spin states are changing in such a way that it appears that a circular arrangement of atoms is rotating. This is a reflection of the fact that an atom isn't really a thing (there are no "things", i.e. there are no particles…

Isn't the entire universe one quantum system? If so, how can we talk about discrete systems. Also, when you say there are not discrete things, do you mean in the QFT sense of field excitations?

> Isn't the entire universe one quantum system?

Yes.

> If so, how can we talk about discrete systems.

That is an excellent question! And the answer is that when you do the math, the result of taking a subset of the wave function (the mathematical operation is called a "trace") is something that looks like a classical system. If you want the details, see this paper:

http://www.flownet.com/ron/QM.pdf

Or this video

https://www.youtube.com/watch?v=dEaecUuEqfc

which covers the same material.

Re: Physicists Create World’s First Time Crystal

#59

Post by lgas »

dangirsh wrote: post #57
Strilanc wrote: post #52

Earlier quoted context omitted.

They literally mean "not spending any energy doing the computation". We spend energy on computation for two fundamental reasons: 1) zeroing data and 2) error correction. You avoid zeroing data by doing reversible computation. You avoid error correction by finding really really well behaved / stable systems. Time crystals are reversible basically by definition, and Wilczek is saying that maybe there might be some time…

IIRC reversible computations only take zero energy if you perform them infinitely slowly.

Just curious, is there a difference between infinitely slowly and not at all?

Re: Physicists Create World’s First Time Crystal

#60

Post by lisper »

davesque wrote: post #51
lisper wrote: post #45

Earlier quoted context omitted.

You're not that far off. The particles aren't really moving , i.e. they are not going from one spatial location to another. What is happening is that they are changing quantum spin states, and the spin states are changing in such a way that it appears that a circular arrangement of atoms is rotating. This is a reflection of the fact that an atom isn't really a thing (there are no "things", i.e. there are no particles…

Thanks for the great explanation. Serious question: Can I ask how you happen to know so much about this subject?

Years of practice :-)

Seriously though, I don't actually know much about QM, I only know the One Thing that makes the rest of QM easy to understand (kind of like knowing Lisp makes the rest of computer programming easy to understand).

In case you want to know that One Thing, it's written up here: http://www.flownet.com/ron/QM.pdf

There's also a movie version: https://www.youtube.com/watch?v=dEaecUuEqfc

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