
‘Time Crystals’ Could Upend Physicists’ Theory of Time - Jaigus
http://www.wired.com/wiredscience/2013/04/time-crystals/all/
======
Xcelerate
Maybe I can help give insight into this topic to those without a physics
background. Our current best theory that describes reality at the small scale
is quantum mechanics. In quantum mechanics, each "system" -- a collection of
particles that you are interested in -- corresponds to one wavefunction
(literally, a mathematical function). Glossing over some details, this is a
function of spatial coordinates (a vector r) and time (t). For example, if
your system is hydrogen, your wavefunction is a function of two coordinates --
the electron's position and the proton's position -- and time. You can perform
some linear algebra on this function to predict what state the system will be
in at any future time. And you can also predict what your measurements
(position, momentum, spin, etc.) of this system will be. The Schrodinger
equation is what governs the evolution of the wavefunction. I'll refer you to
Wikipedia if you want to know the details of that equation.

In chemistry you have what are called stationary states. These are solutions
to the time-independent Schrodinger equation Hψ(r) = Eψ(r) [H is an operator;
E is a scalar]. Now, when you plug ψ(r) into the time-dependent Schrodinger
equation, you get Ψ(r, t) = exp(-iEt/hbar)ψ(r), where i is the imaginary unit,
E is the energy, t is time, and hbar is the reduced Planck's constant. So you
can see there is clearly a time dependence.

However, when you measure some property of a system, you aren't measuring the
wavefunction but rather the results of some linear operator acting on the
wavefunction (each operator corresponds to a probability distribution of what
measurement you will get). So despite the fact that the wavefunction has a
time dependence, your measurement probability distribution functions do not!

Now the other thing you need to know is that so far these stationary states
all correspond to ground states. What is a ground state? It is the lowest
energy level that a system can obtain. You might think that the different
orbitals an atom can have in chemistry are all stationary states, but they're
not. They can spontaneously decay to a lower-energy state. You need quantum
field theory to prove that, and I don't even know how to do that, so I won't.

The deal with these time crystals is that Dr. Wilczek has proposed a lowest-
energy system that corresponds to cyclical time-varying measurement
probability distribution functions. So despite being a stationary state, your
measurements depend on when in the cycle you take them! This has not ever been
done experimentally, so it looks as though the Zhang and Li group are going to
attempt to do so.

~~~
tedsanders
(1) How can the ground state not be an energy eigenstate? By definition, the
lowest-energy state has a well-defined energy (that is, the lowest energy) and
therefore ought to be in an energy eigenstate?

(2) You said: "Our current best theory that describes reality at the small
scale is quantum mechanics." I don't disagree, but I'd like to emphasize that
"at the small scale" might not have been necessary. I think it's a shame when
people think that quantum mechanics only applies to small things.

(3) The way that I understand time crystals is that they are analogous to real
crystals. And just as the translational symmetry of a crystal creates a
corresponding quasi-momentum for the momentum, so does the time-translational
symmetry introduce a quasi-energy similar to energy. Is that correct? Perhaps
it helps answer my first question.

~~~
Xcelerate
1) It still is an energy eigenstate, unless I've confused something somewhere.
The Hamiltonian applied to Ψ(r, t) should give the same value for E regardless
of t.

2) I certainly agree that QM is the basis to everything. However, with the
quantum gravity issue, I didn't want to imply it was a grand unified theory to
someone who wasn't familiar with QM.

3) I haven't taken a solid state physics course, so I'm afraid I don't know
enough to answer that.

------
tedsanders
This article doesn't seem fair to physics. For instance, it says:

"How can something move, and keep moving forever, without expending energy? It
seemed an absurd idea — a major break from the accepted laws of physics."

I don't think that something moving forever is a major break from the laws of
physics. Consider the following:

(1) An asteroid flies through space forever (doesn't violate laws of physics)

(2) A current persists in a superconductor forever (again, no violation)

(3) Heck, currents can ever persist in non-superconductors
(<http://en.wikipedia.org/wiki/Persistent_current>)

(4) The motion of the Earth around the sun (the two-body problem doesn't
violate physics)

(5) Even the motion of an electron around a nucleus is perpetual motion in a
sense

(6) The fact that things have temperature means that their molecules are
always moving!

(7) Etc.

Anyway, there are many examples of perpetual motion in physics. The key point
is that you cannot extract infinite energy from them, just like you can't
extract infinite energy from a time crystal. So why does the article act like
time crystals are a big deal in this respect?

-A grumpy physicist

~~~
ivybridge
I think you're confusing forever with a really long time. (1) Asteroid's
velocity is eventually overcome by external forces. (2, 3) Structure decays.
(4) Bodies converge or diverge. (5) Radioactive decay. (6) You are observing a
high energy system. (7) Etc.

~~~
tedsanders
I don't think I'm confusing forever with a really long time; I'm just assuming
they're effectively the same.

(Aside: nothing exists forever - even time crystals - because eventually the
universe will be a bunch of infinitely far apart black holes or something. So
I'm not sure how that assumption is relevant.)

~~~
kaybe
The distinction still makes sense, I think. There are particles with a decay
time so long they're practically stable (e.g. bound neutrons) and then there
are stable particles.

The examples you gave above are forever; I just started assuming things like
eternity and I can go on to assume no external forces/stuff in the way. (So
not disagreeing with your initial statement, just adding.)

------
ISL
The seminal papers in this field:

Quantum Time Crystals (Wilczek) <http://arxiv.org/abs/1202.2539>

Classical Time Crystals (Shapere, Wilczek) <http://arxiv.org/abs/1202.2537>

Space-time Crystals of Trapped Ions ( Li, Gong, Yin, Quan, Yin, Zhang,
Duan,Zhang ) <http://arxiv.org/abs/1206.4772>

All three papers appeared in a single issue of Physical Review Letters (The
fancy Physics journal).

PRL also issued this Physics Viewpoint, a popular science article:

<http://physics.aps.org/articles/v5/116>

------
whatshisface
_"How can something move, and keep moving forever, without expending energy?
It seemed an absurd idea — a major break from the accepted laws of physics."_

An object in motion will remain in motion, until acted on by an external
force.

The real interesting thing here is that something can move, but have no energy
- potential or otherwise. Unlike things we are used to, if a time crystal
train hit you you wouldn't feel a thing. (Well, not exactly, but it gets the
point across.)

~~~
smosher
He's talking about acceleration, not really movement. But I don't think it's
accelerating either, not really.

~~~
XorNot
A satellite in orbit is always accelerating as well, and will remain in
constant motion if the orbit is far enough out (i.e. well beyond atmospheric
drag).

I'm about half-way through this and it feels more and more like it was written
by someone who would also ask why a permanent magnet can support a load
against gravity without expending any energy - it's the same fundamental
attribution error.

EDIT: In fact finishing it, I'm really confused as to what's new here. Giant
external magnetic field, and stuff just coasting in a circle? Nothing about
this feels implausible in a classical sense with a ring of charged ions.

~~~
coldtea
> _EDIT: In fact finishing it, I'm really confused as to what's new here._

Since 1) the Nobel-winning physicist, 2) the peer reviewed journal AND 3) the
main critic of his theory agree that there is something new there, you
probably ARE confused as to what they really meant.

I think an appeal to authority is appropriate here. Time and again I've seen
simplistic comments in tech forums, putting down new scientific theories based
on some misunderstanding of what new they claim, with the argument that "it's
just like" some other older theory.

And all this criticism usually based on a 10000 miles high explanation of the
idea in some popular science article. As if that is enough to capture all the
nuances of the thing.

I may not be quite proficient in Physics to be able to say what's the case
here (then again I doubt many if any at HN are at the level of the article's
professor), but I've seen the same kind of responses many times in comments
about other fields of study.

~~~
blaabjerg
His criticism is probably aimed at the journalist, not the scientist.

~~~
XorNot
Indeed so. The article does a poor job of explaining - whereas down the other
comments on HN here I now actually grok the significance.

------
dingfeng_quek
Covers the same thing with more technical jargon, but much more accurate and
insightful:

[http://www.scientificamerican.com/article.cfm?id=time-
crysta...](http://www.scientificamerican.com/article.cfm?id=time-crystals-
could-be-legitimate-form-perpetual-motion)

EDIT: A lot of comments appear confused about stuff in the article from Wired.
That's due to the journalism. The Scientific American article addresses many
of issues raised here.

~~~
cpeterso
Thanks for the Scientific American link. It's much clearer than the Wired
article. <:)

------
jlgreco
This article seems like it is abstracting away what would actually make this
experiment more interesting than, say, spinning a dinner plate in a vacuum in
zero-g.

~~~
gwillen
A dinner plate spinning in the vacuum is not in a ground state -- it has
substantial kinetic energy that is being converted into heat and
electromagnetic radiation as it spins, and it will gradually slow down and
stop. (Veeery gradually.)

The theory here is that you can have a system that is already in a ground
state, where it can't decay further, and is still spinning (and will spin
forever without gaining or losing energy.)

I don't have the expertise to properly understand the details, but it seems
like it's along similar lines as something like an electron 'orbiting' a
nucleus in ground state -- unlike the plate, the electron will never stop
'orbiting' because it has no energy to lose.

The electron, though, is delocalized -- it's in every place around the nucleus
at once. So there's no periodic motion involved. By contrast, this experiment
will tag one of the atoms in the ring so we can watch it move periodically.

------
gus_massa
The experiment is interesting and I'll like to read the results. But I don't
like a few details, probably most of them are errors in the press release.

* Perpetual mobile:

It's theoretically possible to build a system that moves forever. But it's
impossible is to connect it to some kind of generator to extract energy for
free, while the system continues moving at the same rate. And the real systems
have some kind of friction that dissipates a part of the energy, so the real
systems usually stop in a while.

The few cases where the movement can last forever is the movement in the
superfluids and the current in the superconductors. They are not very ordered
systems like this 100 Ca ring so they are not cristal-like. But the movement
of the electrons in a superconductor is very similar to the moment of the Ca
ring.

* Tagging a Ca:

The problem with quantum systems is that they act strangely. If the system is
small enough the Ca lost their individuality and become indistinguishable
bosons. (I'm almost sure Ca are bosons, nor fermions.) So to describe their
state you must use Slater permanent (or determinants) and not look at each one
individually. So any perturbation changes the whole system and is not useful
to tag one Ca. (If the system is big enough, you can approximate it
classically, but 100 Ca doesn't appear to be very big.)

* Quantum Gravity:

The Quantum Theory and Special Relativity are joined since 1928 by Dirac. The
same ideas were later used in QED, QCD, and all the Standard Model. So all the
calculations of the collisions in the LHC use a theory that includes Quantum
Theory and Special Relativity. And the only way to use Special Relativity is
to have a common structure for space and time.

Those theories are not related to the continuity or discontinuity or
periodicity of the space or time. Nobody knows how to joint Quantum Theory and
General Relativity, but in my opinion the problem is not related at all to the
existence of space-crystals and the inexistence of time-crystals.

------
ryanthejuggler
Time crystals and ion traps... I must have fallen asleep watching Doctor Who
again.

Joking aside, this could be the E=mc^2 of our generation. We take for granted
that the speed of light is the universal speed limit and that DNA has a
helical shape, but a century ago we knew neither of these things. The
internet, in the scheme of things, is still in its adolescence (at best). The
thing that fascinates and scares me more than anything is that in 50 years
science will have already advanced beyond recognition.

~~~
jerf
"Joking aside, this could be the E=mc^2 of our generation."

No, it's way less interesting than it sounds. It's _interesting_ , but the
inevitable science fiction overtones make it sound way _more_ interesting than
it actually is. It's really "just" another "humdrum" implication of quantum
mechanics. It's also another interesting way of exploring the mathematical
relationship between the time and space dimensions, which itself, while very
interesting, isn't as interesting as putting those words in a science fiction
show would make them sound. It's the hard kind of interesting that involves
years of mathematics study and the resulting profound realizations about the
nature of the universe that raise two questions for every question answered,
not the kind of interesting that produces aliens before the next commercial
break. If you want the profound realizations, they're there for the taking,
but it does take the work.

~~~
redblacktree
And yet, these insights appear to happen instantaneously.

(Note that I'm not disagreeing with you, just pointing out an apparent
paradox)

------
brandon_wirtz
My understanding as explained to me by CERN, You can build a Time Crystal but
it is tough to observe it after. The object don't so much spin through
infinity, as infinity spins through them. Building a Time Crystal changes how
it moves through Space-Time. Since we are moving through Space-Time really
quickly if you change for lack of a better word the inertia of matter by
changing how it moves through Time it won't stand still for very long, (well
not relative to you) this makes observation nearly impossible.

------
drudru11
(Humor)

Something tells me that as soon as they build these, the cast from Time
Bandits is going to bust through the wall and steal them.

------
riemannzeta
Is Noether's theorem preserved since we still have energy conservation? How?
Very cool.

------
exabrial
All I wanna know is, if we dope a time crystal with a transition element like
gallium, can we get a flux capacitor/transistor thing?

