The Exquisite Precision of Time Crystals | Hacker News Reader

The Exquisite Precision of Time Crystals

scientificamerican.com

83 pointslaurex20 comments

I like that his idea arose when he was going to teach a course on symmetry in physics. Nice reminder that teaching is absolutely useful to a person whose job it is to think.

Timing crystals are also exquisitely precise. For example, top-of-the-line ovenized crystals achieve uncertainties of around 1 part per billion. This seems somewhat surreal for an effect that is based on mechanical resonance.

Double ovenized crystal oscillators have lower phase noise (short term variations) than rubidium standard oscillators. Atomic oscillators are ridiculously cool and it's quite surprising to me that they perform worse in phase noise than a mechanical system that is thousands of times larger. The best explanation I've heard is that the signal that comes out of the physics package is a weakly modulated light source that has a (relatively) low SNR. This isn't a big deal because that signal is used to discipline a series of other oscillators. If each of those disciplined oscillators were very low phase noise you'd probably have a very nice rubidium standard, though you'd be paying many thousand dollars.

https://www.youtube.com/watch?v=ymV9LwhD0W0

In my last days at a test lab I examined the effect of an external reference clock on jitter noise floor measurements. It was a new BERT (1 year old) and an equivalent time oscilloscope that had not been serviced in 4 years. The external clock was a NIST TMAS rubidium standard. The measured jitter was lowest when using the instrument internal oscillators, implying the instruments have pretty excellent oscillators (no surprise) and that the effect of aging/lack of calibration had not been a serious issue. There is also the possibility that the external clock training circuits were not as nice as the internal clock training circuits.

You might be interested in this documentary from 1943, which shows the manufacture of quartz timing crystals from start to finish. The amount of skilled labour required to produce a single finished crystal was utterly staggering and highlights just how essential radio communication was to the war effort.

https://archive.org/details/6101_Crystals_Go_to_War_01_20_16...

Viewing the film caused me to look around for the history of the crystal oscillator (happens). Which led my to this 1948 Bell document ... ' The Evolution of the Quartz Crystal Clock' ... which starts at 1500 B.C. (They were very thorough back then.)

https://web.archive.org/web/20110717061023/http://www.ieee-u...

Time crystals mentioned in TFA are an entirelly different thing from crystal oscillators.

Can you help me understand that? The concept of "temporal symmetry breaking" sounds exactly like oscillators.

Is it that it's a quantum-scale effect, creating a truly identical system at each cycle, as opposed to the approximate classical-scale system of a conventional oscillator?

If I understand the article correctly, it is a less strong constraint on the signal than being periodic, like an oscillator would be. The signal is only periodic for specific value of time, in between those value of time the signal is not periodic.

To give a music analogy, the constraint is like always play the same chord on the first beat of each measure. In between you are free to play whatever chords you like.

How does a physical system can acquire such property? I have no idea that's the mystery a physicist will have to explain.

Can it be just a sum of periodic signals? For example if one signal has a period of 7 and another of 11, the period of their sum is 7x11.

No, that's something different, there is a definite state that is visited at a regular interval.

I get your idea of using various periods : If instead of a sum of periodic signals, you do something completely nonphysical like a product cos(x)cos(x/3)cos(x/5)cos(x/7)any_f(x) you can observe indeed that the zero crossings come back periodically. (Edit: something kind of an amplitude modulation may not be so nonphysical after all)

I think the difference is that an oscillator requires a continual driving force to stay periodic. Stop applying energy and it winds down. A time crystal, once it gets going maintains its periodicity.

Mapping it to spatial terms, I think: You could take a bag of beans and lay them out in a nice repeating grid pattern if you want, giving them spatial symmetry. But you have to keep making the grid yourself. The "bean crystal" does not naturally spread on its own. When a real crystal cools, the entire substance aligns into a regular repeating pattern on its own without any external force needed.

hence the "also" in the comment, one imagines.

Sorry, I misunderstood then. But still, personally I think it's about as relevant as if the post was about dark matter, and someone commented "well, dark chocolate is also quite hard to detect if the fridge light goes off".

Also technically correct.

And potentially yummy, if discovered.

'Time Crystals' sounds very retro Sci fi.

I'm sure Dr Who has fought a rubber masked alien or 2 over Time Crystals.

There was an entire season arc where the Fourth Doctor collected the pieces of the Key to Time, which were "crystals" that assembled into a little plastic cube...

Isn't that the plot to Through the Dragons Eye[1]? Although that came 15 years later, it could just be ripping off a combination of The Chronicles of Narnia, and The Dark Crystal though.

[1] https://m.youtube.com/watch?v=v9MXx9xej10 (I remember the special effects being a lot better)

Bonus points for guessing how old I am from those references.

Yeah, "collect all the pieces of the macguffin[0]" is a really common plot device, especially in anime and obviously RPGs. It's a really simple way to show "progress" without needing to bother with character development or power scaling. Bonus points if each piece of the thing has a unique power that turns out to be the cause of the conflict of the week.

[0]https://tvtropes.org/pmwiki/pmwiki.php/Main/DismantledMacGuf...

https://blog.mathspace.co/stones-and-apples/

This is a useful description of space time.

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