[HN Gopher] Physicists link two time crystals in seemingly impos... ___________________________________________________________________ Physicists link two time crystals in seemingly impossible experiment Author : galaxyLogic Score : 77 points Date : 2022-06-16 14:58 UTC (1 days ago) (HTM) web link (www.space.com) (TXT) w3m dump (www.space.com) | theteapot wrote: | As is usually the case with any advanced physics / astrophysics | article I read, I can't get into the article because I'm stuck | mulling over a premise. Article states: | | > The laws of physics are symmetric through space ... But in a | crystal, this gorgeous symmetry gets broken. The molecules of a | crystal arrange themselves in a preferred direction, creating a | repeating spatial structure. In the jargon of physicists, a | crystal is a perfect example of "spontaneous symmetry breaking" | -- the fundamental laws of physics remain symmetric, but the | arrangement of the molecules is not. | | I don't understand how crystals break spatial symmetry. Are we | talking about some _absolute_ spatial directional bias? If it 's | just relative the crystal lattice itself I can't see how that | breaks symmetry. | c1ccccc1 wrote: | There's two things that can have symmetry here: The laws of | physics themselves, and the system under investigation. The | symmetries of the laws of physics don't get broken, but those | of the system do. Compare a crystal to a gas: In a gas, the | atoms are all bouncing around pretty randomly, so at any given | point in space, there's roughly the same chance of finding an | atom. Shift the gas to the left by distance x, and the local | probability distribution of atom positions looks pretty much | the same. In a crystal on the other hand, the atoms are still | moving around and vibrating (so there's still some uncertainty | in the positions of the atoms), but they tend to stay pretty | close to their proper position in the crystal lattice. So the | atoms are more likely to be in positions that line up with the | rest of the crystal lattice than anywhere else. This breaks the | symmetry. Shift by distance x and the peaks of that probability | distribution no longer line up. The exception to this is if x | is a multiple of the spacing of atoms in the crystal. Then | you're shifting the peaks by exactly the right amount that they | line up again when you're done. So a crystal doesn't completely | break the symmetry of space, but it reduces it from a | continuous symmetry (you can translate by any amount in any | direction) to a much weaker discrete symmetry (only certain | translations of space will preserve the symmetry). | | A time crystal is similar to an ordinary crystal except that | instead of reducing symmetry of translations in space from a | continuous symmetry to a discrete symmetry, it reduces symmetry | of translations in time from a continuous symmetry to a | discrete symmetry. | | EDIT: It's a little ironic that if you ask most people, they | would say that a crystal is more symmetric that a gas, since a | gas will look completely random and asymmetric if you take a | snapshot of the positions of all the atoms at a single time. | But since physicists care about the _probability distribution_ | of atom positions, they say that the gas is more symmetric than | the crystal. | philipov wrote: | What you've described is a repeating loop. What makes a time | crystal more than that? | staindk wrote: | Your question reminded me of this video about "Homochirality: | Why Nature Never Makes Mirror Molecules"[1] - even though it's | not directly related I think it may be interesting to you. | | [1] https://www.youtube.com/watch?v=SKhcan8pk2w | sigmoid10 wrote: | Symmetry becomes much more easy to grasp if you think of it | only in terms of transformations - in this case coordinate | transformations. If you for example rotate your system by a few | degrees, does it look the same if you were to overlay it with | the initial state (imagine the crystal as an infinite lattice). | If yes, you have found a symmetry. For a crystal structure, you | usually only have some discrete symmetries, i.e. you can maybe | rotate by multiples of 90 degrees or shift axes by multiples of | a certain length, but apart from these things the "inherent" | rotational and translational symmetry of empty space is gone. | What they're calling "spontaneous symmetry breaking" here is | technically correct, but in this context it's a pretty trivial | observation (I mean, yeah, it is a lattice after all) without | any deep insight, as opposed to the Higgs mechanism for | example. | zarzavat wrote: | It's counterintuitive because most people are more familiar | with the mathematical concept of symmetry whereby a lattice | has more symmetry than a random set of points (which almost | certainly has no symmetry at all). However from the physics | point of view, the random set of points is more symmetrical | than the lattice because there's no way of telling which way | a random set of points is oriented. | dannyz wrote: | If you imagine every molecule in the crystal can be oriented | randomly, then there is a very large number of possible global | configurations that are equally likely and we say the crystal | is "symmetric" with respect to these outcomes. If the | orientations become ordered in some fashion as the article is | saying we say the symmetry is broken. | kadoban wrote: | > If it's just relative the crystal lattice itself I can't see | how that breaks symmetry. | | Yeah, just relative to the crystal lattice. | pdonis wrote: | _> I don 't understand how crystals break spatial symmetry._ | | Imagine doing some experiment in a vacuum. It will work the | same no matter which direction you orient the experiment or | where in space you put it. | | Now imagine doing the same experiment inside a crystal. Now it | _won 't_ work the same no matter which direction you orient the | experiment (because some directions will cause something in the | experiment to hit one of the atoms of the crystal, and other | directions won't) or where in space you put it (because there | are crystal atoms in some places but not in others). | | That's how the crystal breaks spatial symmetry. | hangonhn wrote: | In case anyone else is as confused about time crystals as I was, | Physics Girl recently released a video on YouTube that does a | decent job of explaining it: | https://www.youtube.com/watch?v=ieDIpgso4no | drc500free wrote: | This article perfectly bookends my abandoned physics degree. | | 1. This reminds me a bit of the delayed-choice quantum eraser, | which is one of the weirdest scientific outcomes and is the sort | of thing that inspired me to pursue a physics degree. It implies | a certain kind of time travel is possible, in the sense that in | the present moment we can cause some past moments to collapse. | | 2. As my TA, Frank Wilczek successfully scared me off that | physics degree by simply being so smart and having complicated | things come so easily to him. Being confronted with the kind of | horsepower needed to be successful in academic physics was eye- | opening. | moffkalast wrote: | > time crystals, which are strange quantum systems that are stuck | in an endless loop to which the normal laws of thermodynamics do | not apply | | And here I was thinking they were talking about crystal | oscillators. | mensetmanusman wrote: | Replace 'time crystals' with MS Windows | [deleted] | bee_rider wrote: | > It wouldn't mean free energy -- the motion associated with a | time crystal doesn't have kinetic energy in the usual sense, but | it could be used for quantum computing. | | I'm not smart enough to have anything really insightful to say | about the article. But I don't know if it is more amusing or | vaguely annoying that a technobabble phrase like "we'll have to | pick up more time crystals for the ship's navigation computer to | keep functioning" could be realistic in the future. Or, if it | isn't realistic, the real show-stopper could just be the lack | practical long distance space travel. | colpabar wrote: | Ha - I came here to post pretty much the same thing. It always | makes me wonder which came first. Did sci-fi/literature-at- | large start using crystals this way after we started using them | for time purposes, or did humans just always think crystals | were cool and somehow supernatural and it just turns out we can | use crystals to keep time? Probably the latter, given that | humans do love shiny rocks, and they've existed much longer | than we have. | ghostly_s wrote: | We've been using crystals to keep time for over 100 years. | ncmncm wrote: | And the better watches had them for more centuries. | bee_rider wrote: | Apparently (based on the article) these time crystals were | thought up in 2012. I'm sure you could find the phrase "time | crystal" in sci-fi previously. In fact I bet the person who | thought the idea up was extremely pleased that they could get | such a cool sci-fi sounding name for their thought | experiment. | | On the other hand, crystal oscillators (riffing off your | "using crystals [...] for time purposes") go way back, and | pre-date Star Trek style technobabble I guess. | | But the idea of "magical crystals" goes back even further, | and the thing that makes them interestingly shiny is tied to | their structure. So I guess we knew there was something kind | of funky going on there but didn't have the science to | describe it really well. | | And what's sci-fi anyway? If someone in like 1800 wrote a | story about teaching rocks to think, we'd probably call it | fantasy. It just so happens that we managed to pull that idea | from magic to reality. | | (reference to): | | https://twitter.com/daisyowl/status/841802094361235456 | | Actually, the more I think about it, the more I think your | "or" should just be treated as an inclusive or, and answered | with "yes." | SoftTalker wrote: | Yeah "time crystal" will always make me think of Doctor | Who. Not sure if they were ever actually mentioned in any | story lines, but it sounds like something their writers | would come up with. ___________________________________________________________________ (page generated 2022-06-17 23:00 UTC)