Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

‘Time Crystals’ Could Upend Physicists’ Theory of Time

wired.com

Post reply on HN
Print view View original

31–40 of 52 posts

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#31

Post by gus_massa »

Xcelerate wrote: post #25
gus_massa wrote: post #24

Earlier quoted context omitted.

The problem is that they get a varying measurement after tagging an atom, so the system is not anymore in the ground state, because the tagged atom is exited and it will sooner or later decay to the ground state.

It seems as though they avoid that problem by resetting the experiment after the measurement. From Li's paper: > Then we can use a global probe laser which is only scattered by the mark ion (state dependent fluorescence) to measure its angular displacement after a time separation Δt. [...] After the measurement, we can cool the ions back to the ground state and repeat the experiment again.

No. The problem is that they are not measuring the ground state; they are measuring an exited state. It doesn't mater what they do with the system later.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#32

Post by dingfeng_quek »

Covers the same thing with more technical jargon, but much more accurate and insightful:

http://www.scientificamerican.com/article.cfm?id=time-crysta...

EDIT: A lot of comments appear confused about stuff in the article from Wired. That's due to the journalism. The Scientific American article addresses many of issues raised here.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#33

Post by ivybridge »

tedsanders wrote: post #21

This article doesn't seem fair to physics. For instance, it says: "How can something move, and keep moving forever, without expending energy? It seemed an absurd idea — a major break from the accepted laws of physics." I don't think that something moving forever is a major break from the laws of physics. Consider the following: (1) An asteroid flies through space forever (doesn't violate laws of physics) (2) A curren…

I think you're confusing forever with a really long time. (1) Asteroid's velocity is eventually overcome by external forces. (2, 3) Structure decays. (4) Bodies converge or diverge. (5) Radioactive decay. (6) You are observing a high energy system. (7) Etc.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#34

Post by Xcelerate »

gus_massa wrote: post #31
Xcelerate wrote: post #25

Earlier quoted context omitted.

It seems as though they avoid that problem by resetting the experiment after the measurement. From Li's paper: > Then we can use a global probe laser which is only scattered by the mark ion (state dependent fluorescence) to measure its angular displacement after a time separation Δt. [...] After the measurement, we can cool the ions back to the ground state and repeat the experiment again.

No. The problem is that they are not measuring the ground state; they are measuring an exited state. It doesn't mater what they do with the system later.

Okay, hold on. I think I see what you're saying. Let me think about this.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#35

Post by ISL »

The seminal papers in this field:

Quantum Time Crystals (Wilczek) http://arxiv.org/abs/1202.2539

Classical Time Crystals (Shapere, Wilczek) http://arxiv.org/abs/1202.2537

Space-time Crystals of Trapped Ions ( Li, Gong, Yin, Quan, Yin, Zhang, Duan,Zhang ) http://arxiv.org/abs/1206.4772

All three papers appeared in a single issue of Physical Review Letters (The fancy Physics journal).

PRL also issued this Physics Viewpoint, a popular science article:

http://physics.aps.org/articles/v5/116

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#36

Post by ISL »

gus_massa wrote: post #31
Xcelerate wrote: post #25

Earlier quoted context omitted.

It seems as though they avoid that problem by resetting the experiment after the measurement. From Li's paper: > Then we can use a global probe laser which is only scattered by the mark ion (state dependent fluorescence) to measure its angular displacement after a time separation Δt. [...] After the measurement, we can cool the ions back to the ground state and repeat the experiment again.

No. The problem is that they are not measuring the ground state; they are measuring an exited state. It doesn't mater what they do with the system later.

It's my impression that the crystal arrangement really refers to the distribution of ions in the trap. Exciting one atom does add energy to the system, but in a near-eigenstate that's quite orthogonal to the crystal's structure.

Whether the distinguishability of one ion over the others changes the quantum statistics is an important and subtle problem.

You're right to be skeptical. When the time comes to interpret the results of this experiment, you'll find yourself in good company. Doubly so if the experiment resolves T violation.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#37

Post by cpeterso »

dingfeng_quek wrote: post #32

Covers the same thing with more technical jargon, but much more accurate and insightful: http://www.scientificamerican.com/article.cfm?id=time-crysta... EDIT: A lot of comments appear confused about stuff in the article from Wired. That's due to the journalism. The Scientific American article addresses many of issues raised here.

Thanks for the Scientific American link. It's much clearer than the Wired article. <:)

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#38

Post by tedsanders »

ivybridge wrote: post #33
tedsanders wrote: post #21

This article doesn't seem fair to physics. For instance, it says: "How can something move, and keep moving forever, without expending energy? It seemed an absurd idea — a major break from the accepted laws of physics." I don't think that something moving forever is a major break from the laws of physics. Consider the following: (1) An asteroid flies through space forever (doesn't violate laws of physics) (2) A curren…

I think you're confusing forever with a really long time. (1) Asteroid's velocity is eventually overcome by external forces. (2, 3) Structure decays. (4) Bodies converge or diverge. (5) Radioactive decay. (6) You are observing a high energy system. (7) Etc.

I don't think I'm confusing forever with a really long time; I'm just assuming they're effectively the same.

(Aside: nothing exists forever - even time crystals - because eventually the universe will be a bunch of infinitely far apart black holes or something. So I'm not sure how that assumption is relevant.)

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#39

Post by royalsporks »

dingfeng_quek wrote: post #32

Covers the same thing with more technical jargon, but much more accurate and insightful: http://www.scientificamerican.com/article.cfm?id=time-crysta... EDIT: A lot of comments appear confused about stuff in the article from Wired. That's due to the journalism. The Scientific American article addresses many of issues raised here.

Thanks for sharing the article!! The link is worth reading.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#40

Post by gwillen »

gus_massa wrote: post #31
Xcelerate wrote: post #25

Earlier quoted context omitted.

It seems as though they avoid that problem by resetting the experiment after the measurement. From Li's paper: > Then we can use a global probe laser which is only scattered by the mark ion (state dependent fluorescence) to measure its angular displacement after a time separation Δt. [...] After the measurement, we can cool the ions back to the ground state and repeat the experiment again.

No. The problem is that they are not measuring the ground state; they are measuring an exited state. It doesn't mater what they do with the system later.

It seems like you could easily fix this problem by substituting a different atom instead of exciting one of them. It would be interesting to see if the results are the same.
Post reply on HN