Physicists Create World’s First Time Crystal
technologyreview.com
technologyreview.com
Also, when you say there are not discrete things, do you mean in the QFT sense of field excitations?
Yes.
> If so, how can we talk about discrete systems.
That is an excellent question! And the answer is that when you do the math, the result of taking a subset of the wave function (the mathematical operation is called a "trace") is something that looks like a classical system. If you want the details, see this paper:
http://www.flownet.com/ron/QM.pdf
Or this video
https://www.youtube.com/watch?v=dEaecUuEqfc
which covers the same material.
Your statement just connected to unrelated subjects in my head that might mean something. As I studied hardware, I found that underneath these nice, mathematical blocks that we build digital with are analog components that appear to operate on messy, kind-of-chaotic waves of electricity they hand-tune into the digital cells. Then, you say the clean building blocks of classical systems are composed of messy waves in quantum. Worded like that, it makes me wonder if analog vs digital could teach us something about quantum vs classical. Or some universal principle at work. Reason I wonder is some of the math keeps showing up in different disciplines.
What you think? Is my brain overreaching on this?
But the ability of classical math to model all this is truly extraordinary.
https://www.dartmouth.edu/~matc/MathDrama/reading/Wigner.htm...
Seriously though, I don't actually know much about QM, I only know the One Thing that makes the rest of QM easy to understand (kind of like knowing Lisp makes the rest of computer programming easy to understand).
In case you want to know that One Thing, it's written up here: http://www.flownet.com/ron/QM.pdf
There's also a movie version: https://www.youtube.com/watch?v=dEaecUuEqfc
When I first wrote the paper 15 years ago I submitted it to Physics Today. It was rejected, not because it was wrong, but because the reviewers thought that it was nothing new, that everyone already knew this stuff. Since then the physics community has bifurcated rather neatly into two camps: the ones who agree with the PT reviewers, and the ones who think I'm a "category 5 loon" (that was Lubos Motl's term). Personally, I think the existence of people like Motl and your anonymous "dismissive" QFT experts -- and the fact that I can still stump card-carrying physicists with the EPRG paradox -- falsifies the hypothesis that "everyone already knows this stuff."
I will also say, because I'm feeling rather annoying by all this, that in 15 years no one has ever pointed out any actual mistakes in the paper (except for a few typos).
Edit: downvote me all you like, but at least be accurate. I'm referring to the title of the tech talk video linked several times in this thread: "The Quantum Conspiracy: What Popularizers of QM Don't Want You to Know"
So I just wanted to ask in respect of "no one has ever pointed out any actual mistakes" - have you seen the stackexchange discussion at http://physics.stackexchange.com/questions/208609/does-the-f... , and do you have any comment on the assertion there that your statement of EPRG misses the point that "even if Alice does let her system produce interference, the other system will not produce interference either"?
The thing that people who critique my work generally miss is that it is not about physics, it's about pedagogy. It's about how the physics is explained. Everyone agrees on the physics. It's the explanation that is at issue. The whole first half of the talk is the problem statement.
(God damn this is annoying: "I'm afraid I won't have time to critique whatever it is he says in the second half of the video." Well, the second half is where I explain why the EPRG paradox doesn't actually work. By the time I gave that talk I already knew that people got this wrong, so I went to great lengths to highlight the fact that EPRG was a straw man. And then people jump into the middle of the video, see the story out of context, and say "This is wrong." Well, duh, of course it's wrong. That's the whole fucking point!)
I did suspect that "Garret then claims that this can be used for superluminal communication" missed that you were using this as a kind of reductio, but I then took it to mean that your reductio was not working how you expected it to. Probably I should re-watch the video - it's been a couple of years.
> your reductio was not working how you expected it to
Actually, it pretty much works exactly how I expected, and has for 25 years (I started down this road in 1990 when the EPRG paradox first occurred to me. It took me ten years to find someone who knew the answer.)
The problem is when people look at EPRG out of context and think that my thesis is that I've invented FTL and therefore I am the World's Greatest Physicist. (And now some moron is probably going to cite this very comment and say that I've claimed to be the Worlds Greatest Physicist because they saw that I wrote "... I am the World's Greatest Physicist" and they don't have time to read the part that they elided.)
But I think you give too short a shrift to Many-Worlds because it's not intuitive to you. Plenty of things about QM are not intuitive; I don't think that's sufficient reason to reject it.
[UPDATE] Yes, my recollection is correct. Go watch it again starting at the 58 minute mark or a little earlier.
just to confirm my interpretation, its saying that:
- fundamentally the universe is governed by quantum mechanics
- classical physics doesn't truly exist
- the classical physics we see and intuit can be explained by entropy (information theory) of the quantum state
- measurement and entanglement are the same in the sense that both imparts "information" into the system. this in effect acts as a "filter", an example of which is the double-slit experiment where the waves are "filtered" to particles when being measured
- our conscious mind is a classical construct so its naturally difficult to intuit quantum laws
is that mostly correct? also, how popular is this theory in the physics community?
Mostly.
> also, how popular is this theory in the physics community?
Not very. The fashionable way to think about this stuff is something called "decoherence" which really amounts to the same thing but phrased in different terms.
so if i were to do further research into this, I would use the keyword "decoherence"?
also not being a physicist, could I naively think this would be a good candidate for the "theory of everything"?
Decoherence, relational quantum mechanics, quantum information theory, the Von Neuman measurement model -- all of these turn out to be the same thing.
> also not being a physicist, could I naively think this would be a good candidate for the "theory of everything"?
No, the "theory of everything" refers to the unification of quantum mechanics and relativity. Completely different topic.
Aren't Planck constants all about that ?
Spacetime is a sort of cartesian representation of space and time, with probably 4 axes (x, y, z, time).
Many relativity concepts obey this, for example stuff moving in space at the speed of light, move literally zero on the time axis. And you can calculate the relationship between time and speed by rotating vectors.
So back to quanta: we don't know if time is discrete. We don't even know if time actually exists, or its properties, because the math right now has results that are quite weird (like implying moving backwards in time should be normal and common as moving forward...), also there are arguments over the shape of "spacetime" with most people assuming it is a 4d cube, but maybe it ins't.
Thus you don't have a "full dimensional vector {x,y,z}" but you have a bunch of particles each keeping a list of variables with distances to other particles around them.
Maybe I'm not explaining it right, but no one has proposed anything like this as far as I know.
In the standard cosmology, we use quantum fields that permeate the whole of spacetime and carry values at each point in spacetime. Some of the fields have values at each point that correspond to the presence of absence of a particle at that point (e.g. the electron field), while other fields provide a mechanism for one field to influence the state of another. In General Relativity we have the metric tensor, which is a field that carries information relevant to the causal structure of spacetime, and is determined by the values of the other fields (roughly, matter determines curvature, which in this case is a component of the distance in space and time from one configuration of matter to the next).
So, rather than saying spacetime doesn't exist, we say instead that spacetime is only relevant when there is a configuration of matter within it that can be used to established one or more systems of coordinates (e.g. the matter states converge, diverge or oscillate).
However, in Special Relativity there is no spacetime curvature (by definition) and so we do not need to use a metric tensor (we could naturally use the Minkowski tensor to describe the Minkowski spacetime, which is the spacetime of Special Relativity), but commonly attach a 4-vector to particles or bound collections of them. As with any spacetime in General Relativity, there is nothing special about a Minkowski spacetime that has nothing in it, or that has any other always-unchanging configuration of mass-energy.
The Standard Model of Particle Physics is a group theory with the Poincaré group as a subgroup; the Poincaré group is the isometry group of Minkowski spacetime, or in other words, in Special Relativity, when you move something following the rules of the Poincaré group, you do not change the fundamental states of the thing you are moving. The Poincaré group in 3+1 spacetime has 3 (bidirectional) linear spacelike translation components, 3 components of (bidirectional) rotation, 3 components of Lorentz boost, and one timelike translation. So if you do a particle-smashing experiment today and the same experiment tomorrow (or in a lab across campus), you will get the same result.
One can represent the action of the Poincaré group in several different ways. There is no harm in either approach in your second paragraph, as long as one is careful about how one manipulates either representation (especially, for example, swapping one representation for the other).
No, on a Minkowski diagram with the x axis as a one space-like dimension coordinate (e.g. radius in metres) and the y axis as the time-like coordinate (e.g. -ct, where t is in seconds and c is the speed of light; - because of the sign convention[0]), something travelling at the speed of light traces out a 45 degree angle.[1]
This is a fundamental property of Minkowski spacetime, the spacetime of Special Relativity (and the spacetime in local sections of the fibre bundle in General Relativity).
Indeed, in a hyperbolic solution in General Relativity, everything moves at one second per second on the timelike axis measuring each object's proper time. This is one of many properties of time we do know very well; what's unclear is whether the property is emergent in the behaviour of systems of small objects, or whether it is fundamental to physics and applies to every object everywhere.
General Relativity requires that the manifold be infinitely differentiable, and thus continuous, rather than discrete. Anything that seeks to explain gravitation and the observables from tests of relativity will have tremendous trouble introducing discretization on the background while retaining compatibility with those tests. For all practical purposes, that's true for everything outside the event horizon of a black hole and more recent than the first 1e-38 seconds after the big bang.
[0] https://en.wikipedia.org/wiki/Sign_convention [1] https://en.wikipedia.org/wiki/File:Minkowski_diagram_-_photo...
First let us observe that even in the classical universe there are discrete quantities. Atoms, for example, are discrete. You can't have half a uranium atom because that's not a uranium atom any more, it's some other kind of atom. But aside from these discrete quantities most of the things that describe the state of the classical world seem to be continuous: you can position an atom anywhere, you can move it at any speed (including zero), etc. and hence the atom can have any value for its total energy.
This turns out to be only an approximation to the truth. It turns out that classical systems in fact can only have energies that are discrete multiples of some very small number. This number is not Planck's constant, but it's derived from Planck's constant in a way that depends on the physical setup. This is why, for example, to see quantum effects you have to control the setup very carefully, which generally means making things very cold so they don't jiggle around too much. This is one of the complications that I can't get into here. But the details don't really matter. What matters is that it turns out that some of the quantities we thought were continuous are in fact discrete, it's just that the units are so small that it's not apparent until things are very small and/or very cold. And in particular, energy is discrete.
But the underlying math of QM is not discrete, it's continuous. So how do we get from the continuous math of the quantum wave function to the discrete behavior we observe in reality? We don't really know. If we really understood that we could explain why Planck's constant has the value that it does, and we can't do that (yet). One possibility is that space and time are discrete just like energy it, but it's just not apparent because the units into which nature slices up space and time are too small for us to access experimentally. If space and time do turn out to be discrete, then the continuous math of QM will turn out to be merely a very (very!) good approximation.
But the distinction that really matters between the quantum and the classical is that states in the quantum universe are described by complex numbers and states in the classical universe are described by real numbers. That is the thing that makes quantum and classical fundamentally different, and it's the thing that makes the quantum world "weird" to us, because that's how you get things like entanglement and destructive interference. The quantization part is, ironically, a relatively unimportant detail, at least when it comes to talking about things like time crystals. (It's incredibly important for other things, like semiconductors.)
So a time crystal appears to be made up of real matter. The article mentions ions that are arranged in a certain particular way, cooled in order to reduce their energy, and this still makes sense so far.
So, the idea is to create a closed loop of sorts from a temporal perspective. I'm imagining an object as a set of states that the object can be observed in, and if there was a loop, it'd be possible to observe a certain sequence of states over and over.
The arrangement of the ions is important, and one of the properties is the spin, which is possible to change with a laser beam, because obviously.
Anyways, I guess the frequency of the oscillation of the ions changing spin, since they interact with each other in a domino fashion, can be controlled with the frequency of the laser.
They did this, and observed the ions changing in such a way where there was no driving influence and it's implied that the reason that this behavior was observed is because time symmetry was broken, which is just a fancy way of alleging that the universe is non-deterministic, I think.
Maybe this will eventually be useful?
I was under the impression that the whole idea of time symmetry revolved around being able to "zip/fold/traverse/map" both forwards and backwards in time. It's my understanding that such a concept would imply determinism, because time is really just a set of states, in the smallest possible interval of time.
I also don't understand why the time crystal loses energy over time if it is trapped in a temporal loop. I mean obviously there's not such a thing as perpetual energy, but I would think that the energy of a certain object at a particular frame in the set of states that is time, then when it returns to that state from either direction, it should be the same.
Edit: This is meant to be inquisitive, not necessarily interrogative. Thanks!
Time translational symmetry and determinism have nothing to do with one another. Even a classical pendulum breaks time translational symmetry yet it's fully deterministic. In this cases they do break the symmetry in a more fundamental way, but it still doesn't say anything about determinism.
The object would be in a repeating set of states. First state 1, then state 2, then state 1, then back to state 2, etc.
This is not in itself surprising. Think of a pendulum, for instance. The difference here is that the motion is the lowest energy state. Over time, a pendulum swing decays. This does not: it will continue forever, if not disturbed.
The obvious next question is: isn't that perpetual motion? According to the normal dictionary definition of those words yes it is, which is why this is a fascinating discovery. However, it doesn't violate the normal arguments against perpetual motion, because there's no way to extract energy from the system. Anything you did to influence the motion would require adding energy.
That article is from 2012 and quotes:
"If that’s the case, we ought to be reading the details about the first space-time crystal sometime in the days and weeks to come. "
Was there something non obvious they had to overcome that took the extra few years to produce these space-time crystals?
(I use the term non obvious as if this was not all way beyond my ken!)
In any event, they sure seem to have made a lot of progress in just a few years. I look forward to reading more about this, as well as listening to Neil deGrasse Tyson describe it in his infectiously enthusiastic way.
[1] Back to the future (1985) http://www.imdb.com/title/tt0088763/
[1] https://www.technologyreview.com/s/426917/physicists-predict...
""" Another is the possibility that it may be possible to exploit time crystals to perform computations using zero energy. As Wilczek puts it, “it is interesting to speculate that a…quantum mechanical system whose states could be interpreted as a collection of qubits, could be engineered to traverse a programmed landscape of structured states in Hilbert space over time.” """
https://en.wikipedia.org/wiki/Reversible_computing?wprov=sfl...
We spend energy on computation for two fundamental reasons: 1) zeroing data and 2) error correction. You avoid zeroing data by doing reversible computation. You avoid error correction by finding really really well behaved / stable systems.
Time crystals are reversible basically by definition, and Wilczek is saying that maybe there might be some time crystals that are also well behaved and do something interesting enough to encode computations.
As a concrete example, most of thermodynamics becomes insanely complicated for very rapid changes in pressure and temperature. And in many cases, you cannot actually compute the state of the system (without making a bunch of approximations). But, if you make some assumptions and approximations, thermodynamics becomes much more simple even though you've added some error factors due to your approximations (but it's definitely "good enough").
In general, the first thing you learn as part of a physics degree is that approximations and assumptions are just part of how you do science. Without them, we'd all be stuck dealing with insanely complicated equations that probably are not solvable. And at the end of the day, the unsolvable part of whatever equation you're dealing with wouldn't actually have a significant effect on the result (so you can generally ignore it).
His work on anyons coupled with his work on time crystals makes him highly qualified to speak on what would be useful computational structures.
Time locked crypto that doesn't rely on proof of work jumps to mind, but I'm by no means an expert on either cryptography or quantum mechanics.
"One reason why space-time crystals are interesting is that their periodicity in time makes them natural clocks. So there should be plenty of people with more than a passing interesting in making one."
So I took from that, yes, this could be an alternative to atomic clocks.
- matter / energy is neither added nor removed to retain shape/movement
- the energy/matter within the "crystal" change state/position in a repeating and/or predictable matter
- pattern retains even when the energy is at extreme lows
This would be analogous to a water cycle. The water changes state in a predictable manner, water heats up (due to the sun) and evaporates. As the molecules get closer to space, the cool and attract each other to form clouds. The clouds hit a maximum and rain occurs flowing back into bodies of water again.
The difference is that the water cycle needs the presence of heat/cooling from external sources while the time crystal can exist at "zero degree" temperatures.
...and can someone tell me where the actual time comes into play?
Sure, one can say it is "something not unlike" the manipulation of time, and yet... somehow... it seems like we're just deciding to call it something it is not, since it certainly seems similar enough.
If I carve a piece of wood, such that it resembles a gun, it can play the part of a gun, but only until a person with an actual gun challenges me to a duel.
Can someone explain what this means? It makes sense that physical laws are symmetrical in space. But in time? I thought that would not be the case because of entropy/irreversible processes etc. What am I missing? The whole article rests on this premise.
When you look at each particle in the thermodynamic system, you can easily see there is indeed simmetry in time.
https://en.m.wikipedia.org/wiki/Noether%27s_theorem
Groundbreaking stuff. But it is not something you would want to go over in a weekend.
However, it should be noted that the arrow of time only applies to the macroscopic universe. All of the laws of physics (in this article's case it's quantum mechanics) are time symmetric (if you play the experiment in reverse you couldn't tell it was being done in reverse -- which is what you can see in Feynman diagrams). So at the heart of it, the universe is very symmetric in time.
To quote Primer: "What's the one variable in all of the equations of heat and matter? What's the one variable that you can turn negative and still get rational answers? Its not energy, it's not mass..." [It's time].
https://www.youtube.com/watch?v=QR9reQrumS0&list=PL1FA30AE29...
Spolier alert: they don't really know whats going on, and they disagree pretty strongly. Decoherence seems related to the thermodynamic arrow of time: you can't un-measure something, but famously we don't know how to interpret that, many worlds blah blah.
Time police
A "normal" crystal has a repeating structure in space; a time crystal is a construct with a repeating temporal structure.
Edit: have just looked it up. Looks like it's basically 3-D structure with repeated motion over time.
Their half life for most stable atoms is so long that perpetual applies for most practical uses of the word.
Which makes me wonder if these time crystals have a half-life too.
it does seem to be breaking some rules.. but I guess that's the achievement here. Intuitively there should be some concept of friction/heat loss but maybe on this scale things are different. Maybe it could be exploited as a battery? A way of capturing energy into a stable kinetic "state". (Like a flywheel..)
But normally that oscillation represents stored energy.
They key to a time crystal is that it's oscillating while it's in a minimum energy state. It would take energy to make it stop moving.