
Time crystals might exist after all - dnetesn
http://phys.org/news/2016-09-crystals.html
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cohomologo
Physicist here with some context:

The systems considered here have periodic drives (in the article, "Floquet").
This means that time-translational symmetry is already partially broken. The
system is only the same after waiting times that are multiples of the period T
of the drive.

The time-translational symmetry breaking occurs because the state of the
system is not periodic with period T as would normally happen but periodic
with period 2T.

In terms of frequencies, if the drive frequency is f = 1/T, then this system
responds at a frequency f/2, whereas normal systems can only respond at
frequencies f, 2f, 3f, ... that correspond to harmonics.

Additionally, this time-translational symmetry breaking makes a stable phase
of matter -- that is, you don't have to fine tune any parameters of the system
to see the effect, and experimental noise won't destroy it. It also doesn't
matter which initial state you prepare your experiment in. While not as exotic
as a time-translational symmetry breaking without a drive to partially break
the symmetry first, it is still pretty surprising that this type of phase
exists at all. It is likely that spontaneous breaking of full time-
translational symmetry can never be stable in the same sense.

~~~
Pyxl101
> In terms of frequencies, if the drive frequency is f = 1/T, then this system
> responds at a frequency f/2, whereas normal systems can only respond at
> frequencies f, 2f, 3f, ... that correspond to harmonics.

Interesting. I heard it described once that a particle with spin of 1/2 is
like a particle that you have to rotate twice (through two 360 degree
rotations) before it's in the same configuration. Wikipedia actually has a
visualization that seems to depict this reasonably:
[https://en.wikipedia.org/wiki/Spin-%C2%BD](https://en.wikipedia.org/wiki/Spin-%C2%BD)

Anyway, your description makes it sound like this system by responding at f/2
might have an analogous property with time. Is this at all a reasonable or
correct analogy?

~~~
cortiver
This is a great question. (I am the first author on the "Floquet time
crystals" paper referred to btw). The analogy is not perfect because what
actually happens to a spin-1/2 particle under a 360 degree rotation is it
comes back to the _same_ configuration, but its wavefunction picks up a
quantum phase factor of (-1), which is not observable. On the other hand, a
Floquet time crystal does actually does go to an _observably_ different state
under a time shift. The best analogy is really to, for example, a magnet,
which does go to an observably different state if you rotate it (because the
north and south poles rotate).

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pcl
The article doesn't really describe time crystals themselves in terms I could
understand. Wikipedia to the rescue: [https://en.m.wikipedia.org/wiki/Space-
time_crystal](https://en.m.wikipedia.org/wiki/Space-time_crystal)

~~~
userbinator
To those who aren't knowledgeable about the subject, it sounds as bizarre and
incomprehensible as
[https://en.wikipedia.org/wiki/Time_Cube](https://en.wikipedia.org/wiki/Time_Cube)

~~~
jerf
While this isn't quite what a "time crystal" is, imagine a two-dimensional
universe that contains just one arrow in it. Now spin the arrow. Now plot the
full state of the system, using the third dimension as time. You get a sort of
spiral thing going along the dimension you used for time. This periodic
structure is sort of like what they mean by "time crystal"; it's a periodic
structure in the time dimension.

Now, there's more to it than that. I'm not proposing this as a full
explanation of what it means, just trying to get the B-class sci-fi ideas out
of the way to sketch out the complicated bit of what they mean. As a crystal
is a periodic atomic structure in space, a time crystal is a periodic atomic
structure in time. There's more to both, of course, but that should help get
the woo-woo out of the way.

~~~
lisper
How is a "time crystal" different from a plain old [EDIT: quantum] harmonic
oscillator?

~~~
wyager
The other responses have not addressed the possibility that you are referring
to a quantum harmonic oscillator. If you are, that is a very good question. I
believe the resolution here is that while the phase of a quantum harmonic
oscillator in the ground state changes with time, the results of bracketing
any hermitian operator (i.e. measuring a physical quantity) does not change
throughout time. Therefore in the ground state of the quantum harmonic
oscillator, the system is time translation invariant. So a quantum harmonic
oscillator does not form a time crystal.

~~~
lisper
Well, yeah, I thought it was a given that we're in quantum-land here. In
classical physics the lowest energy state of a system is necessarily static.

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adjwilli
I got redirected to the http
//www.thesofttic.com/ZiWblMur/win-f/gi/?SiteID=4905&conversion=11ee233b-a3f6-4b44-8d5f-9c578b58eb29
from the link. Anyone who know why or what that is? Is phys.org really
infected with malware?

~~~
0x0
Might want to reformat the URL so it's not clickable, looks like a malware
drive-by site...

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n72
Oh no. This is going to get picked up by one of those nutbag spiritual
clickbait sites and tomorrow my girlfriend is going to tell me about an
amazing discovery about crystals which can be used to power a time travel
device.

~~~
hexane360
For only $200 I can build a perpetual motion machine based on TIME CRYSTALS!
Physics said so!

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blackoil
Interesting, that Microsoft sponsors such a research that I can't see being
used by the business for visible future. I am not a shareholder so can't make
a view of this is good or not.

~~~
ianai
It's probably a tax write off. If it is then I'd almost prefer it to them
paying taxes to be divided out to DOD

~~~
sksnxjis
Where T<1 is the tax rate, spending X on a write-off allows you to avoid
paying X*T in tax. But you still end up with X(1-T) less money in the end.

~~~
denim_chicken
What is your point???

~~~
zo1
The point is that they are still paying money. Calling it a "tax write-off" to
the layman sounds like they're just doing it to reduce how much they pay in
taxes. I.e. It sounds like tax-evasion/fraud.

~~~
Pyxl101
His point specifically is that tax write-offs don't reduce how much you have
to pay overall. They may even increase it. They just redirect some of it to
other purposes such as research instead of paying taxes.

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mrtree
I read it and understood nothing.

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mrfusion
Why can't regular crystals survive the heat death of the universe?

~~~
tlb
Heat death === no remaining thermodynamic free energy. The universe is all one
temperature, though theories vary widely on what this temperature might be.

At high temperatures crystals vaporize, absorbing heat. At low temperatures,
gases sublimate onto the crystal, releasing heat. Either way, if you have
crystals, the universe hasn't completed its heat death yet because you could
use the crystal to do work.

~~~
p1mrx
By that definition, wouldn't the heat death never actually happen? Space is
expanding, so there should be some particles that zip off and never collide
with anything.

Also, couldn't a sublimating gas disrupt a time crystal?

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joelthelion
Wouldn't a simple pendulum (or an orbiting planet) qualify as a time crystal?

If not, why not?

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dschuetz
I always thought that making physics more complicated made physics potentially
more wrong.

But then I read "Microsoft Station Q" in the article. Go figure. If there is
"Microsoft" on it there is a flaw in everything.

~~~
ZenoArrow
> "I always thought that making physics more complicated made physics
> potentially more wrong."

There's no correlation between complexity and correctness. An explanation can
be too simple as well as too complex. Nature is what it is, our understanding
of it evolves over time.

