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‘Time Crystals’ Could Upend Physicists’ Theory of Time

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Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#21

Post by 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

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#22

Post by coldtea »

XorNot wrote: post #7
smosher wrote: post #4

Earlier quoted context omitted.

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

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 finishi…

>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.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#23

Post by Xcelerate »

tedsanders wrote: post #20
Xcelerate wrote: post #14

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)…

(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…

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.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#24

Post by gus_massa »

Xcelerate wrote: post #14

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)…

The problem is that they get a varying measurement after tagging an atom, so the system is not anymore in the ground state, because the tagged atom is exited and it will sooner or later decay to the ground state.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#25

Post by Xcelerate »

gus_massa wrote: post #24
Xcelerate wrote: post #14

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)…

The problem is that they get a varying measurement after tagging an atom, so the system is not anymore in the ground state, because the tagged atom is exited and it will sooner or later decay to the ground state.

It seems as though they avoid that problem by resetting the experiment after the measurement. From Li's paper:

> Then we can use a global probe laser which is only scattered by the mark ion (state dependent fluorescence) to measure its angular displacement after a time separation Δt. [...] After the measurement, we can cool the ions back to the ground state and repeat the experiment again.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#26

Post by redblacktree »

jerf wrote: post #15
ryanthejuggler wrote: post #8

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 mo…

"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 i…

And yet, these insights appear to happen instantaneously.

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

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#27

Post by eru »

baddox wrote: post #12
tzs wrote: post #10

Earlier quoted context omitted.

> 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). If general relativity is correct, that system will generate gravity waves which will take away energy and the orbit will decay. This is a small effect, however, so we generally don't have to worry about it. E.g., Earth's orbit is decaying by about the width of a…

Wouldn't tidal forces influence Earth's orbit much more than gravitational radiation? Although I suspect both are far too small to measure on a human time scale.

If memory serves right, tidal forces are quite measurable. A centimetre a year or so extra in the distance between moon and earth.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#28

Post by blaabjerg »

coldtea wrote: post #22
XorNot wrote: post #7

Earlier quoted context omitted.

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 finishi…

> 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 theor…

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

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#29

Post by caf »

XorNot wrote: post #7
smosher wrote: post #4

Earlier quoted context omitted.

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

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 finishi…

The difference here is that normally when you have something coasting in a circle like that, it's possible to remove energy from the system and have it stop doing that. For example, tidal forces are constantly removing energy from the Earth-Moon system.

Here, the theory is that the system is already in its lowest-energy state, despite the fact that part of it is still in motion.

Re: ‘Time Crystals’ Could Upend Physicists’ Theory of Time

#30

Post by smosher »

XorNot wrote: post #7
smosher wrote: post #4

Earlier quoted context omitted.

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

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 finishi…

The point is, iiuc, the only rest state is rotating and at a particular rate at that. That's quite a bit different than coasting, which could have various different speeds (or ~none) depending on the amount of inertia in the system—but I think even that is removed here.
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