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Jan 27 2012

RCimT: Spacey particle physicsey sciencey catch-up Friday

Some random science bits and bobs to clear out a bunch of tabs.

Scientists have discovered the speed limit for quantum interactions, and it is much, much slower than the speed of light. It is a little faster than twice the speed of sound in the medium in question, in fact. Yes, apparently also for entangled particles. This effectively hamstrings any woo-peddlers’ attempts to suggest that quantum effects explain things like the imagined effect of the planets’ positions on a person’s fate (I’m looking at you, astrologers).

And despite that speed limit, it looks like quantum computing will allow for effectively perfect encryption, where none of the computer components involved in encryption or decryption will have to actually “see” the data in question; they are told only how to perform the calculations, and the endpoints do all the heavy lifting of interpreting the data. Without knowing the actual initial state of the requests made to a particular server, no actual useable information can be gleaned by a third-party snooping on the stream. This still sounds pie-in-the-sky to me, but it’s potentially groundbreaking.

Meanwhile, in the universe, another Big Bang prediction has been confirmed, with the discovery of so-called pristine gas, gaseous materials from the initial Big Bang event that has not yet mixed with any other materials.

We’ve also figured out exactly how lumpy spacetime can be, thanks to the result of a cosmic race between two photons emitted seven billion years ago. The granularity of spacetime is constrained by the fact that two photons, evidently emitted in the same gamma-ray burst, ended at practically the exact same time. We’re not talking tortoise and the hare stuff either — I strongly doubt one of them flew really fast then started dawdling at the finish line because it was so far ahead it couldn’t possibly lose. I suppose we could try to experiment for that though.

The Japanese physics lab KEK has discovered new subatomic particles made out of quarks, which is apparently both exotic and strange — most particles are only made out of two or three. This is interesting in that it suggests there may be a large number of possible particles we have not yet discovered. And yet, despite the interest I should show in this, I can’t get the idea of four Armin Shimermans out of my head.

Armin Shimerman as the Ferengi Quark from Star Trek: Deep Space 9Armin Shimerman as the Ferengi Quark from Star Trek: Deep Space 9
Armin Shimerman as the Ferengi Quark from Star Trek: Deep Space 9Armin Shimerman as the Ferengi Quark from Star Trek: Deep Space 9

And now neither can you.

Here’s a pretty image to try to help get that out of your head. An algae bloom as seen from space looks like a great huge figure-8.

And if you couldn’t see that because you went blind at the Quarks, good news — it appears that stem cell research might have you covered, where preliminary experiments have helped restore sight to two folks afflicted by degenerative vision diseases.

Hooray for science!

20 comments

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  1. 1
    Randomfactor

    I think you misread that quark story. Looks to me like two new particles, each composed of four (previously-known) quarks? Although I admit I’m not up on my quark soup.

  2. 2
    josh

    Randomfactor is correct. The quarks are, as far as we know, the same ones we already know (6 flavors, 3 colors). Familiar particles (called hadrons as a class) are made up of 2- and 3- quark combinations. (Mesons and baryons respectively.) Recent evidence supports the existence of 4-quark combinations, which would be new as far as known particles goes, and since they weren’t really expected at this mass, might indicate something exciting about the underlying theory (Quantum Chromodynamics), although I don’t know the details enough to comment.

  3. 3
    Jason Thibeault

    Yeah, evidently I bollocksed that one up but good. Josh’s reading is exactly right. The linked article describes particles discovered by KEK that are MADE OUT OF four quarks, which is evidently both novel and portentious. I don’t know what it portends either.

  4. 4
    Zinc Avenger (Sarcasm Tags 3.0 Compliant)

    I think it portends something timey-wimey.

  5. 5
    Robert B.

    About the four quarks: I suppose these new particles (tetraquarks? verybaryons?) are roughly consistent with what we know, though I don’t recall any of my prof’s ever mentioning the possibility. Quantum chromodynamics (the theory that describes how gluons hold quarks together) just says that a particle can’t have a net “color”… okay, let me start over.

    Quarks have a property called “color” that accounts for how they stick together. This “color” has nothing to do with light, it’s just a convenient metaphor. The three possible quark colors are red, green, and blue for matter, and anti-red, anti-green, and anti-blue for antimatter. (If they wanted to be really consistent with their metaphor, they might refer to “anti-red” as cyan, anti-green as magenta, etc. But they don’t. “Anti-red” it is.)

    The rule goes like this: a particle is possible if it has no net color. So you can have, say, a pi+, with a red up quark and an anti-red anti-down quark. Or, in general, you could have lots of two-quark combinations, with one quark of any color, and another quark of its anti-color. The only restriction (or so I thought, more below) is that you can’t have, say, an up and an anti-up quark bond together, because they’ll just annihilate. These two-quark combinations are mesons, “middleweight” particles. (Leptons, the lightweights, actually have nothing to do with quarks, but they didn’t know about quarks when the names were coined.)

    So, a color and an anti-color can cancel out. But there’s another way to be stable, and that’s by having a full set of three colors. A red, green, and blue together will combine to “white,” and have no net color. This makes a stable particle, too. (This three-way combination is the whole reason for the color metaphor in the first place.) So, for example, you might have a red up quark, a green down quark, and a blue down quark, and that makes a neutron. Or you could have any combination of three quarks, as long as they were all matter or all antimatter, and their colors went red, green, blue. The three-quark combinations are baryons, “heavyweight” particles.

    But there’s no “population limit” in the physics that says you can have only two or three quarks stuck together that way. The physics only says “I’ll stick together anything with no net color.” If you put a proton and a neutron next to each other, the red and green quarks don’t “know” that the blue quark from the neutron is not “their” blue quark. A red and a green together will be attracted to any blue. So the proton and neutron basically combine into a six-quark combination – a “deuteron,” a nucleus of a Hydrogen-2 atom. An atomic nucleus, as Dr. Ruiz points out in your link, is basically just a particle composed of quarks in some multiple of three.

    But a four- or five-quark particle is exotic (physics-speak for “weird and awesome.”) I guess they’re formally called tetraquarks and pentaquarks, though I’m tickled by the name “verybaryons” that I just made up. A pentaquark would have a trio and a pair, a color combination like: red, green, blue, red, anti-red. Sort of a baryon and a meson stuck together. A tetraquark would have two pairs, like green, anti-green, blue, anti-blue. It would be sort of like two mesons stuck together. Though, again, the physics doesn’t “know” that two quarks “belong” to one meson, and the other two to another, until they split apart (though this apparently happens quite quickly.) All four (or five) quarks would be held together by the same force, just like the force that holds a proton to a neutron is the exact same force that holds the “proton’s” three quarks to each other.

    The part I find really wild is that the Belle experiment is apparently looking at verybaryons that contain a bottom and an anti-bottom quark together. I didn’t know this was possible – as far as I understand it, if a particle and its antiparticle cross their wavefunctions, they just annihilate. I wouldn’t expect the particle they’re describing to exist, and if it did exist, I’d expect to see it decay to one meson and some gamma rays – matter and anti-matter annihilate, with two quarks left over. Instead it seems to decay to two mesons – four quarks before, and four quarks after, no annihilation. In other words, a particle and its own antiparticle are meeting up, and before they can annihilate, they have time for not one but two other interactions – joining a tetraquark, and then having that tetraquark decay into two mesons. Lolwut.

    (I only took one course in QCD, so I don’t know exactly what’s surprising to the experts about these new tetraquarks. It might be something very different from the annihilation thing that surprised me.)

  6. 6
    F [i'm not here, i'm gone]

    This still sounds pie-in-the-sky to me

    You do realize that quantum cryptography is commercially available, right?

  7. 7
    Jason Thibeault

    Zinc Avenger @4: at the very least wibbly, if not also wobbly.

    Robert B@5: as usual, fantastic explanation, and I sincerely thank you for it. Far more coherent than anything I’ve read trying to learn about this stuff on my own. You may have only had one course, but your explanation at least largely agrees (to a layman) with what I’ve read on Wikipedia and on various other sites like the linked one. Are you, by any chance, on a teacher track?

    Verybaryon is a great name. And if they don’t accept it into mainstream particle physics, I’m at least planning on using it as the name for my Oingo Boingo cover band.

    ;)

    F: “commercially available”? Seriously? Do tell!

  8. 8
    Robert B.

    I’m not formally in teacher training at the moment, but yeah, I’m an educator. A tutor, most successfully.

    I’ve looked at the Quantum Speed Limit article. It’s pretty awesome, I never thought that phonons (the quasiparticles mentioned in the article) would have a maximum speed, though it makes sense. But I’m pretty sure that speed limit does not apply to entangled particles, at least not in the way you seem to mean.

    The speed limit study was performed for a particular phenomenon in a particular kind of system, which the article calls a “lattice.” I’m not sure that’s the word I’d pick for a one-dimensional gas, but what they meant by it is that here we have a system with lots of particles, but each particle is only interacting with a select group, its neighbors. The typical lattice is a crystal, where each particle has particular neighbors because they’re all locked in place; no atom is free to travel around the crystal, because it’s solid. In this case, each particle has particular neighbors because they’re locked onto a line – though they can move around, they can’t get past each other.

    Either way, though, since a particle can interact with its neighbors, and the neighbors can interact with their neighbors, you can make a change in one place and have the effect propagate all the way across the lattice. If the lattice is regular, so will the interactions be, and a regular propagating interaction is just a wave. And on the quantum scale, waves and particles are the same thing, so it also behaves, in every mathematical sense, like a particle (though it’s a “particle” that can’t exist apart from its lattice.) And apparently, each kind of lattice has a maximum possible speed for such particles moving through it – a fairly low one, it seems, on the order of the speed of sound.

    (By the way, I’m pretty sure that this quantum speed limit is slower than the propagation speed of an electric current. I know basically zilch about quantum computing, but if they were planning to build quantum computers around phonon interactions, this is seriously bad news for their processor speeds. I hope that wasn’t the plan, although the main advantage of a quantum computer is not speed per se but rather the different kinds of calculations possible with it.)

    Entangled particles, though, are a whole different thing. Two particles are “entangled” when the come from a common source, in a way that puts a constraint on their quantum numbers. The classic case is two entangled fermions traveling in different directions from their common source. They each have something called spin, which might be +1/2, or -1/2, but either way the two spins must be opposite, and total to zero. So if you know the spin of one, the spin of the other has been logically determined. There’s a quirk of statistics, though, that makes this logical determination measurable. If you start measuring the spins of one beam of fermions, someone watching the other beam would see different results from the moment you started your measurement. This change “travels” from one observer to the other, not only faster than this new quantum speed limit, but faster than light. (There’s a trick to it, at least according to Many Worlds – the change in the entangled particles turns out not to be a true interaction, or any kind of signal. So it doesn’t break General Relativity, in the same way that the shadow of a moving object might travel across a distant surface faster than light, without breaking GR.)

    You can see how this is entirely different from what the Wired article is describing. The phonons whose speed limit we have just now measured aren’t involved in entangled particles at all; in fact they can’t even exist in an entanglement experiment (which has no lattice.) We’re talking about an entirely different kind of thing.

    Fortunately, any quantum mechanism for astrology was debunked way back in 1924 by Louis DeBroglie. Our friend Louis basically explained that big things just aren’t quantum in any meaningful way, which makes sense because otherwise quantum mechanics would be part of our everyday lives. And planets are very, very big. The range at which Mars (for example) can have quantum interactions is on the order of its DeBroglie wavelength, which is about 10^-62 meters. That’s not quite as much as the distance between you and Mars. :-D In fact, if you were to crash into Mars, and smear yourself in a thin organic film across its surface, you are still a trillion trillion trillion trillion times too far away from Mars to have a quantum interaction with it.

  9. 9
    Dan M.

    Robert B@

    Your comments are fantastic! I’m very happy to hear that you teach some. Do you have a blog where you collect these great descriptions?

  10. 10
    Robert B.

    @ Dan M.:

    No, but maybe I should.

  11. 11
    CompulsoryAccount7746, Sky Captain

    @Robert B#8

    If you start measuring the spins of one beam of fermions, someone watching the other beam would see different results from the moment you started your measurement.

    My education on this is informal, but I was under the impression that entanglement was synchronized noise, that you could only recognize happened to be synchronized after comparing (lots of) measurement results with someone who did the same experiment with the other particle(s).

    If someone else is watching first, they’re measuring it, and your measurement results would be sync’d to them. If you’re first, theirs would be sync’d to you.

    It’s not that entanglement travels fast and causes a noticeable change in front of your eyes, it’s that entanglement does its magic while nobody’s looking (but it’s not decided the moment of initial entangling with hidden variables because of Bell’s Theorem).

    Or am I mistaken?

  12. 12
    CompulsoryAccount7746, Sky Captain

    @Myself#11:
    Correction: “while somebody’s not looking”

  13. 13
    jon

    @ Dan M.:

    No, but maybe I should.

    please do!

  14. 14
    jon

    erm….. I meant

    Robert B: please start to blog, I know I’d be a regular lurker (at least)

  15. 15
    Robert B.

    @ Compulsory Account:

    Hm. “Noise” isn’t quite the right word, but entanglement is synchronized probability distributions, yes. Looking at a single particle could never be conclusive. But the statistics of beam #2 change from the moment beam #1 is observed – that is, starting with the beam #2 partner of the first-observed beam #1 particle. This can happen even if the distance between those two observations is space-like – that is, even if there’s no time for a signal to get from one particle to the other within the speed-of-light limit. This, of course, is a very strong hint that whatever is happening, beam #1 cannot actually be causing any change in beam #2. Whenever you have to describe some physics phenomenon as “magic” you can be pretty sure there is something wrong with your thinking. (Not that it’s your fault, the expert you heard/read it from probably used a magical description, explicitly or otherwise. I’ve certainly heard plenty of those, from people who ought to know better.)

    Oh, and watch out when using the phrase “hidden variable” with respect to QM. Hidden Variable theory (or theories, I think there’s more than one) are opposed to both the Copenhagen Interpretation and Many Worlds theory. Hidden Variable supposes that the apparent random nature of quantum events is an illusion, caused by limits in our ability to measure, and that all particles have a real exact position and definite spin and all the rest. I was briefly fond of Hidden Variable in grad school, but more recently I judge it to be unlikely – it complicates the theory while making no additional predictions, and it’s even worse at explaining entanglement than Copenhagen.

  16. 16
    CompulsoryAccount7746, Sky Captain

    Whenever you have to describe some physics phenomenon as “magic” you can be pretty sure there is something wrong with your thinking.

    I meant that as a tongue-in-cheek euphamism for an effect without a classical analogue.
     

    watch out when using the phrase “hidden variable” with respect to QM.

    Uh, I said “not decided the moment of initial entangling with hidden variables because of Bell’s Theorem”
     

    “Noise” isn’t quite the right word, but entanglement is synchronized probability distributions, yes.

    But the statistics of beam #2 change from the moment beam #1 is observed

    In what way would #2 change? If they’re synchronized, and #1 is still appearing to local experimenters as-if-unentangled, that would make #2 locally appear as-if-unentangled in the same way #1 was.

    Each individual measurement is randomly assigned a result within the probable range (‘noise’, as opposed to a forced-value humans would abuse to send messages). And when entanglement is involved, those results happen to be correlated, which someone couldn’t know until someone compared their data.

  17. 17
    CompulsoryAccount7746, Sky Captain

    If you mean from the moment #1 is measured, a #2 particle will lose its superposition to take on a definite value, the #2 experimenters can only ever see it post-collapse anyway (entangled or not).

    For elaborate rigs built to have different statistical outcomes when a particle’s attribute ‘was ever determined’ vs ‘remained in a superposition throughout’… measuring its entangled partner would count. But it’s still a hindsight “shall have been” finding, not an “Ah ha I saw it change mid-experiment. Joe must’ve prodded his electron just now.”

  18. 18
    Robert B.

    @ CompulsoryAccount:

    I think I must have misunderstood the sentence where you used the phrase “hidden variables.”

    And about the entanglement: there’s a twist I’ve been omitting, partly because I didn’t quite recall it myself. If you perform the exact same measurement on both beams – measuring polarity or spin along the same axis, for example – your measurements will match every time, and as you say, the distribution can’t change based on whether the other beam is being watched.

    However, the real force of these experiments comes from making measurements on each beam that are not quite lined up exactly – for example, measuring polarity or spin along axes separated by 22.5 degrees. Keep in mind that these experiments can only return results of “up” or “down” (or, in the case of photon polarity, “zero.” You can’t measure two spins to “sort of” match, the detector either says they do or they don’t. If you run the same electron through two detectors in succession, the chance of both detectors reading the same result varies as the cosine of the angle between them.

    And if you run two entangled electrons through different detectors tilted at different angles, the chance of a match (or rather, a counter-match, opposite results) has that same dependence on the cosine of the angle. It can’t be that the two electrons have nothing to do with each other, or the odds of a counter-match would be 50/50 for any angle. It can’t be that the two electrons have “real” exact spins at specific angles that they carried with them from the start, because then the odds of a match would be linear with the angle. And it can’t be that a signal is passing between the detectors that controls this relation, because it still works when the detection events are separated by a space-like interval, and space-like causality makes the baby Einstein cry.

    But this can never be used to send a signal, because you can only see what happened when you compare the results from both detectors, at which point you’ve obviously had to send a separate signal between the two detectors anyway. As you say, it’s only observable in hindsight, a very important point. You can’t use this effect to build an ansible.

  19. 19
    Jenora Feuer

    @Robert B:
    Very nice descriptions. A couple of points:
    Particles consisting of a quark and a matched anti-quark exist: see the Neutral Pion as an example. They aren’t stable, of course; the neutral pion has a lifespan measured in fractions of a femtosecond, but it does exist briefly and its existence has effects. Especially when you get one traveling at near-lightspeed as a collision product from cosmic radiation.

    The chance of both detectors reading the same result is actually the square of the cosine of the angle between them. So an angle of 30 degrees difference means that you get 3/4 matches, and an angle of 60 degrees difference means you get only 1/4 matches. Which is where Bell’s Inequality comes in: as turning one detector 30 degrees off loses you 1/4 of the matches, turning the other detector 30 degrees off loses you 1/4 of the matches, but somehow turning both 30 degrees in opposite directions means you lose 3/4 of the matches, rather than the expected 1/2 or less…

    Of course, as you say, this can only be measured in hindsight, and the results are all statistical. You can never re-run the same experiment with the same photons to see what the results would be if you change the polarization further, so you can never really know what would have happened, only the rather limited actual results.

  20. 20
    Robert B.

    @ Jenora:

    Right, the square. It’s always the square of the wavefunction amplitude. (I’m absolutely terrible with math details like that, even though I’m quite good at math and I’ve used them a million times. When I took my oral qualifying exam in grad school, I forgot the 1/2 in KE = 1/2 mv^2, and the professors looked at me like I must have wandered into the room by mistake.)

    And about the pi0 – okay, I should have known better there, too. But a quick wikipedia refresher reminds me that pi0 decays by annihilation, as I’d expect: pi0 -> gamma + gamma, or pi0 -> gamma + e- + e+. And as you say, the annihilation happens really fast, so that the pi-zero is many orders of magnitude shorter-lived than the pi+ and pi-. But the new tetraquark decays to two mesons; there’s no way it should have time for that before the bottom and anti-bottom quarks annihilate. Any ideas? Are tetraquarks that short-lived? Is the annihilation forbidden? If so, by what?

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