[HN Gopher] Physicists link two time crystals in seemingly impos...
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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.
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