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Since the article doesn't do a good job of explaining what a time crystal is, here's my attempt at summarizing the Wikipedia article [0]. Normally, a system wh
by yccs27 6y ago
Since the article doesn't do a good job of explaining what a time crystal is, here's my attempt at summarizing the Wikipedia article [0].
Normally, a system which is driven by an external frequency oscillates at the same, or a multiple of the driving frequency. This is because the external conditions are symmetric under a time translation (i.e. a delay) by the period duration. This symmetry is normally preserved by the system.
A time crystal, however, will oscillate with a fraction (or rational multiple) of the driving frequency. This breaks the original time translation symmetry.
The analogy with normal 'spatial' crystals is through this symmetry breaking: Empty space is symmetric under all translations, but a crystal spontaneously breaks the symmetry group down to the smaller set of 'translations by a multiple of the lattice constant'.
The continuous symmetry of time translations cannot be spontaneously broken, due to thermodynamic arguments. Instead, we explicitly break down the time translation symmetry to 'delays by a multiple of the period duration'. A time crystal then spontaneously breaks the symmetry down even further, by multiplying the period duration / dividing the frequency.
[0]: https://en.wikipedia.org/wiki/Time_crystal https://en.wikipedia.org/wiki/Time_crystal
- centimeter 6y agoHow does this compare to the behavior of nonlinear optical crystals like lithium triborate? They are also pumped at a lower frequency (e.g. 1064nm photons) but lase at twice that (532nm).
- yccs27 6y agoIn some way, it's the inverse: a nonlinear optical system can multiply frequencies, while a time crystal divides a frequency. That's not as easy as multiplying, due to the symmetry concerns I mentioned. It's basically a frequency divider, but as a system of cold atoms instead of an electronic circuit, and in a different frequency regime.
- mwilcox 6y agoI would say that gravitational waves break the idea that empty space is symmetrical
- RIMR 6y agoCould you elaborate on that?
- chovybizzass 6y agono. i can't sorry
- deleted 6y ago[deleted]
- yccs27 6y agoI mean, you're technically correct (the best kind of correct). But in the lab, we're pretty safe with neglecting the effects of general relativity.
- Darkphibre 6y agoI don't know... atomic clocks are approaching precision that can measure the effects of time dilation due to gravity on the order of inches. They have to take into account daily tectonic tides. > The precision of current optical clocks is astounding. You may have heard that time goes more slowly when gravity is strong due to general relativity. Optical clocks are so sensitive they can measure the different flows of time 2cm apart in height. If I lay a book on the table, the bottom of the book is slightly closer to the center of the Earth than the top, so experiences slightly stronger gravity. This difference is measurable with an optical clock. Optical clocks are so sensitive we can no longer average the time of multiple clocks together—the ground you or a clock are sitting on typically rises and falls by ~5cm a day due to land tides. The seismic motion of the ground currently limits our ability to measure time. https://arstechnica.com/science/2021/01/a-curious-observers-guide-to-quantum-mechanics-pt-2-the-particle-melting-pot/ https://arstechnica.com/science/2021/01/a-curious-observers-...
- wholinator2 6y agoCertainly for atomic clocks (nearly the very definition of precision), but the vast majority of lab work isn't anywhere near that level of precision. So yes, you may need to factor in GR if your goal is to measure the smallest possible thing, but for general work with macroscopic items, it's such a small difference it is completely unnecessary.
- k__ 6y agoWhat are potential applications? Do the allow some kind of special optical devices?
- dekhn 6y agoWhen I hear "time crystal", I think "the wielder of this object can travel through time, or at least do something far out, like see past events", not "a crystal that when vibrated breaks symmetry".
- 7800 6y agoAnd whenever I hear time crystal, I think of bad sci-fantasy 80s movies. “Mondor, you must grab the time crystal before the second moon equinox, or the Flarborg will be released from their electronic pen and ravage us like they’ve never ravaged before!” “Oh. Well, we wouldn’t want that would we?”
- p1mrx 6y agoOh, but you can see past events while wielding (or not wielding) a time crystal. Vision is far out: https://www.youtube.com/watch?v=FjHJ7FmV0M4 https://www.youtube.com/watch?v=FjHJ7FmV0M4
- amelius 6y agoI think of quartz crystals, used to measure time.
- nom 6y agoMaybe they can. Who knows what strange realms of physics this research leads us to.
- dnautics 6y agoI believe the most recent iteration of star trek is particularly guilty of stealing "time crystals" from the science news headlines and applying it to this sci-fi trope.
- deleted 6y ago[deleted]
- ww520 6y agoEvery time the discussion of time crystal comes up, a simple analogy can explain what it is. Regular crystal has repeatable structure in space, i.e. its lattice is the same shape over and over again across in space. Time crystal is the kind of material that has repeatable structure across time, at the lowest energy state. It just means it changes its shape periodically over time. An example of that is quartz crystal vibration where its structure bends and unbends over time. Quartz bending and unbending (vibration) is due to the piezoelectric effect when an external electric force is applied. The external force requirement disqualifies it as a time crystal. The "at the lowest energy state" requirement means time crystal can "vibrate" by itself without external force applied. "Vibrate" is used loosely here. It could be the nuclear-spin flipped periodically, or some other structural changes that need no outside influence.
- zbyte64 6y agoWould it be accurate to say that the "Vibration" or nuclear-spin flipping doesn't emit energy or is neutral in the thermodynamic sense?
- ww520 6y agoI guess if it’s emitting energy then it’s not at the lowest energy state. The emitted energy draws the energy state down further.
- klyrs 6y agoIf you could coax work out of it in a ground state, that would be a perpetual motion machine, so no, it had better not emit energy. But you could detect the spin flips to make a clock
- kergonath 6y agoSpin waves are similar to vibrations (phonons) in a lot of respects, you still need energy to change a spin. That said, some aspects of thermodynamics are counter-intuitive (well, actually most of it is). Vibrations are present even at 0 K because of the uncertainty principle, and that’s true for spin waves as well as actual vibrations. It does not mean that any energy is created. The difference with time crystals is that usually these vibrations don’t involve any symmetry breaking. Time crystals are consistent with standard thermodynamics.
- _8091149529 6y agoFor anyone struggling to make sense of this: I have a physics PhD, I have spent most of my career in fundamental research, I have read the Wikipedia article and other references multiple times, and attended several talks, and I still don't understand what qualifies as a time crystal. All I know that it seems to have something to do with subharmonic response.
- systemvoltage 6y agoThe more deeper you get in a particular field, the more you realize how no one else has a clue. It's time we put credentials aside, I found this amusing: https://en.wikipedia.org/wiki/Michael_Crichton#GellMannAmnesiaEffect https://en.wikipedia.org/wiki/Michael_Crichton#GellMannAmnes...
- dingoegret 6y agoThat was a good little read and mirrors my sentiments about media consumption exactly. People who have a specialized knowledge about a subject will often find themselves reading false information about serious topics from media about their subject of knowledge. Lowering the credibility of that media in their eyes and preparing themselves to be more skeptical of information output from that media. Then flip to another subject they are totally uninformed on and believe word for word everything that is said to them no matter how utterly ridiculous.
- boibombeiro 6y agoThe irony is that this applies to hacker news.
- WildParser 6y agoI don't really know what those people in physics do, but as a mathematician my definition of a time-crystal would be a 4-dimensional discrete structure. You have some 4-dimensional symmetries. Just rotating in 3D is a bit boring. Interesting symmetries would be where you get the time-axis involved. 4D is most likely a bit boring - having more dimensions and more time-dimensions helps greatly to get more symmetries to work with...