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Space Science

Beginner's Guide to Quantum Entanglement 186

Posted by Zonk from the in-case-you-need-to-make-your-own-universe dept.
No Fortune writes "Einstein called it 'Spooky action at a distance.' This article describes, in scientific layman's terms, how spooky action is created." From the article: "Normally the photons exit the crystal such that one is aligned in a horizontally (H) polarized light cone, the other aligned vertically (V). By adjusting the experiment, the horizontal and vertical light cones can be made to overlap. Even though the polarization of the individual photons is unknown, the nature of quantum mechanics demands they differ."
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Beginner's Guide to Quantum Entanglement More

Beginner's Guide to Quantum Entanglement

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  • Jesus Zonk... (Score:5, Funny)

    by grasshoppa ( 657393 ) on Saturday November 19, 2005 @11:27PM (#14073664) Homepage
    ...what the hell is the matter with you anyway?

    Throwing that kind of physics at us on a Saturday evening when you *know* most of us are half drunk?

    Bastard.

    • Zonk is Jesus?! (Score:5, Funny)

      by Anonymous Coward on Saturday November 19, 2005 @11:28PM (#14073667)
      Lord help us... but anyway, you've got be half-drunk to even start getting quantum physics. Everyone knows that... well, they do when you ask, they didn't before you asked.
      • Well, fortune should have it that I have stumbled upon this article half (well, in fact, at least three-quarters) drunk, and as I read the article, I was thinking "gee, I'm half (well, in fact, at least three-quarters) drunk, and I _can_ understand this shit", so either, yes, Anonymous Coward, your postulate is correct, or this begginers guide is quite well written (perhaps better titled "Drunk's Guide to Quantum Entanglement").

        What I do not understand, however (possibly due to my drunken state) is why it s

        • Re:Zonk is Jesus?! (Score:3, Interesting)

          by jonadab ( 583620 )
          > if I send a baseball in one direction spinning topwise, while at the same isntant sending a
          > baseball in another direction spinning bottomwise, their spins will be opposite and continue
          > to do so, without any interaction between the baseballs.

          Yes, but baseballs are not subatomic particles. Among other things, looking at which direction they're spinning hardly changes their spin at all. The traditional line of thinking is that the laws of physics are different at the macroscopic level versus the

        • I don't see how the example in the article shows any 'spooky interaction' whatsoever. There is no interaction, it's the uncertainty principle that is being circumvented by a clever experiment.

          Here's my reasoning (not that anyone needs encouragement, but feel free to poke holes in this if you see something I'm missing):

          The experiment is setup so that they are creating two different waves from a single one - taking a UV light wave and splitting it such that you get (for example) a red wave an a blue wave. N

    • Perhaps Zonk is half drunk as well.

    • Re:Jesus Zonk... (Score:4, Funny)

      by ZachPruckowski ( 918562 ) <zachary.pruckowski@gmail.com> on Saturday November 19, 2005 @11:34PM (#14073681)
      More to the point, it's 11:33 PM on a Saturday at UVA, and I'm reading about quantum stuff. I'm far worse than Zonk right now. :(
    • The ones that understand this stuff would never go out on a Saturday night... or Friday... or Monday... well, you get my point.
      • Actually, I think it's the large amounts of alcohol that let's me understand it. Unfortunately, passionately trying to explain this stuff to the table full of drunks you're talking to on a Friday night causes nothing more than a few blank stares and a yawn in the pretty young woman who sat down next to you.
    • Half drunk? Bah.
    • Half drunk my Arse... I'm fully drunk! Paid Darn good money for that hooch too.
    • This is the best article I've read in a long time. Being drunk just means I was able to not think too hard about the philosophical repercussions and understand the point he was trying to make.

      That being said, I've bookmarked it to read it again while I'm sober.
    • My level of drunkeness was a wave function until I collapsed.

  • Damppuss (Score:2, Funny)

    by Anonymous Coward
    Did you mean: fleabiteus ?

  • In what? (Score:5, Funny)

    by Transcendent ( 204992 ) on Saturday November 19, 2005 @11:31PM (#14073672)
    in scientific layman's terms

    Ah, oxymoron terms... the best kind.

  • Finally! (Score:3, Funny)

    by Anonymous Coward on Saturday November 19, 2005 @11:31PM (#14073674)
    I've been waiting for Teach Yourself Spooky Action in 24 hours.

  • Great Article! (Score:5, Funny)

    by CynicalGuy ( 866115 ) on Saturday November 19, 2005 @11:34PM (#14073679)
    +1 dugg

  • Einstein called it 'Spooky action at a distance.' (Score:3, Funny)

    by st0rmshad0w ( 412661 ) on Saturday November 19, 2005 @11:35PM (#14073685)
    I call it saturday night.....

  • "Quantum Entanglement"? (Score:5, Funny)

    by artemis67 ( 93453 ) on Saturday November 19, 2005 @11:40PM (#14073696)
    Is that what the geek kids are calling it these days?

    "Excuse me, but you stimulate the neurons in my hypothalamus. Would you like to come over to my place and study quantum entanglement?"

  • sigh, digg (Score:1, Offtopic)

    by alatesystems ( 51331 )
    I saw this on Digg [digg.com] about 2 days ago. Even the title is a direct ripoff. I see so many ripoffs from Digg on /. every single day (except a few days later).

    I read Slashdot about once every 2 days now, instead of my normal all throughout the day that I used to. My surfing time is now devoted to digg and Engadget(and other Weblogs Inc Blogs). You get better quality news with less dupes, better quality writeups, often humor, and just a better web experience in general.

    But I do like the CSS on /. now. It make
    • What other title would you recommend they use? Its a... *gasp* Beginners guide... and its on... Quantum Entanglement. Shocking that the titles are so similar!

    • Re:sigh, digg (Score:1, Insightful)

      by Anonymous Coward
      Since Digg doesn't have a nested comment structure it really isnt't possible to have any meaningful discussion to speak of. It's the discussion that people enjoy here, the articles are secondary (as evidenced by the lack of RTFA.) So please stay there and let us discuss the topic like adults, rather than staying here and bitching like a little crybaby. The articles are no less true because someone else posted them elsewhere a day or two ago. But then again, you're probably going to go bitch on Digg when

    • Re:sigh, digg (Score:4, Insightful)

      by njh ( 24312 ) on Sunday November 20, 2005 @12:07AM (#14073777) Homepage
      I just had a look through digg and it lacks the one thing that makes me come back to slashdot - insightful comments. The comments were 50% juvenile drunken-louts-at-school level and the other 50% were people who didn't understand the question.

    • Re:sigh, digg (Score:3, Informative)

      by MichaelSmith ( 789609 )
      I saw this on Digg about 2 days ago

      And the top article on digg links to newsforge, a stablemate of slashdot. I didn't know of any other site which puts in links to newsforge. It's a pretty obscure site outside the slashdot community.

      I only had a quick look, and correct me if I am wrong, but digg seems to want people to register to view discussions. From the POV of making money from a site this is a better way to go. So would OSDN care if people moved over to digg? Dunno, have to think about this.

    • A lot of sites try to be slashdot and come and go. Anyone remember when kuroshin people were in here years back trying to convince everyone that kuroshin was going to be the end of slashdot? Slashdot persists for a reason.

    • Re:sigh, digg (Score:1, Funny)

      by Anonymous Coward
      You get better quality news with less dupes, better quality writeups, often humor, and just a better web experience in general.

      What???

      Since when has editing required no dupes, correct information, reliable and accountable sources or actual information (rather than advertising another product/site)? Keep up the effort /.!

      Besides with all the weed I smoke I need to read /. to remember what I read somewhere else yesterday/last week/last month. I also need to read it at least twice so that it replaces the brai

    • Re:sigh, digg (Score:3, Interesting)

      by Stalyn ( 662 )
      Dupes and the terrible editing give slashdot its charm. Come on. Also great trolling here on slashdot.... superb. Slashdot has an entire culture around it now. The crap on the front page doesn't even matter anymore. Cmdrtaco knows it. Thats why he doesn't give a shit about dupes and everything else. You could post a story that is nothing but random letters and still generate 100+ comments.

  • It has some errors ... (Score:4, Insightful)

    by Y2 ( 733949 ) on Saturday November 19, 2005 @11:48PM (#14073714)
    Aside from the inconsistent use of plural and singular quanta/quantum, I'd be very surprised if no one was baffed by this: "In all cases, a photon's axis must be 90 degrees to its motion." Axis (of spin) and direction of polarization are linked, but not in a way simple enough to fit in this zero-math article.

    "Figure 5.2 is an enhanced photograph of a photon ..." - That is more than just misleading.

    • Now, the big issue that I haven't figured out is to what extent the noncommunication theorems are valid (the ones that argue that spooky action at a distance does not lead to spooky communication at a distance).

      Personally I doubt the noncommunication theorems, but this may be due to my understanding. They seem entirely untestable for the reason that if you take things from a Copenhagen viewpoint, the fact is that the obvserver can't test this without breaking the experiment.
    • Got to love this one thoug: Electrons do not eat apples to make a quantum leap (their mouth is too small).
    • Yup, confusing. Does that mean it's spinning round an axis which is aligned with its direction of motion?

      Also, why is having the cones overlap effective? I suspect there's logic to that that a layman _could_ understand, but it's left unstated. Is it related to the interference patterns of ripples from two stones in water?

  • Weird thought (Score:5, Interesting)

    by achurch ( 201270 ) on Saturday November 19, 2005 @11:49PM (#14073715) Homepage
    In the midst of serious sleep deprivation, the following weird analogy for quantum entanglement came to mind. Maybe some of the physics folks here can tell me why it's wrong:

    Suppose you take a coin and spin it on a frictionless surface in a vacuum, so that it's perfectly balanced and doesn't wobble. In theory, it will keep on spinning at the same rate forever.

    Now suppose you take a second coin, identical in all respects to the first, and start both coins spinning at the same time--but with one of them 90 degrees out of phase compared to the other, so when one is "horizontal" when viewed from above, the other is "vertical".

    Finally, suppose you have a way to move the coins without affecting their rotation. Move one of the coins as far away as you like from the other.

    Reach out a finger and stop one of the coins. Suppose that at the instant you stopped it, the coin was horizontal. You now know that, at that particular instant, the second coin was vertical--not because the coins somehow "communicated" with each other, but simply because they both followed the same laws of physics up until you interfered.

    Granted, I'm oversimplifying tremendously, but is this a semi-reasonable explanation of why quantum entanglement has nothing to do with instantaneous communication, or do I just need to get to sleep?

    • Re:Weird thought (Score:5, Informative)

      by Y2 ( 733949 ) on Sunday November 20, 2005 @12:06AM (#14073769)
      Suppose that at the instant you stopped it, the coin was horizontal. You now know that, at that particular instant, the second coin was vertical

      Sorry, no. If the coins aren't at the same place, then this term "at that particular instant" is not well defined.

      The tantalizing notions of instant communication involve choosing which of two or more possible measurements to make on one of the photons (after they are separated) and the effects of that choice on the possible outcomes of a fixed or independently-chosen experiment on the other photon. Google "EPR Paradox" for a primer.

      • Neat, so there really is some weirdness going on. Thanks for the pointer! (I had to laugh at the Wikipedia observation that a theory that violated causality would be "deeply unsatisfactory".)

        I guess I'll try and get to sleep now . . .

      • faster than light communication? (Score:4, Informative)

        by jlowery ( 47102 ) on Sunday November 20, 2005 @01:30AM (#14074014)
        Two points, A and B, are within fixed distand of pulsar P.

        A and B have agreed that certain measurements of quantum entangled particles will be made a various time intervals as determined by P.

        Point C lies between A and B, closer to A than B. C sends quantum entangled particles of definite polarization to A and B. At the agreed-to intervals, A does polarization measuments of particles coming from C; B measures for same polarization at the same interval (accounting for the extra time for the entangle photon to reach B).

        Question: does A's collapsing of the state of the entangled photon mean that B will see less entangle photons with that polarization? In other words, will it mean that successful polarization pass-thru's at A's filter has the consequence that B will notice a distinct drop of photons with that polarization passing through its filter? If that were the case, then communication is occuring between A and B in the time it takes for light to travel the shorter distance from C to B.

        Change quantum particles to entangle coin tosses. A has the head filter activated, meaning all heads that pass thru A result in no heads at B. With the filter on a A, does B see less heads than he would at other, 'normal', intervals?

        • You have some problem called the noncommunication theorem but I suspect that this suffers from the same logical flaw as Shroedinger's cat. In essence I assume that the cat is able to observe whether he or she is alive or dead, so you have the question of what role the cat's observation has in the colapse of the quantum state.

          Similarly here, you have the problem that the experiment observer has no way of knowing whether the spooky communication at a distance has occurred until the observer measures the com

        • Re:faster than light communication? (Score:4, Informative)

          by ArsenneLupin ( 766289 ) on Sunday November 20, 2005 @04:34AM (#14074499)
          Question: does A's collapsing of the state of the entangled photon mean that B will see less entangle photons with that polarization?

          No. Every measure that B does along the same "axis" must correspond to what A measured. I.e. if the the entanglement is such that polarization must be opposite, B will always see the opposite of the polarization observed. by A, no matter whether B performed the measurement "before", "after" or "at the same time", (whatever that means on two spatially separed points...).

          However, if A and B perform the measurement on different "axis" (i.e. in a direction offset by 45 degrees from each other), they are in effect measuring two different things, and no correlation exists. The interesting thing is that any measurement on an axis shifted by 45 degrees destroys any information that was there along the original axis, and vice-versa. I.e. if a photon passes successively through a 0 degrees instrument, then a 45 degrees, and then again a 0 degrees instrument, the two 0 degrees measurements are not necessarily the same!

          The point of the "spooky action at a distance" experiment is that A and B randomly chose the direction across which they measure polarization (0 degrees or 45 degrees), and later compare notes:

          • For those times where they happened to use the same axis, polarization will always be opposite.
          • For those where they happend to use an incompatible axis, the observations will be uncorrelated with each other.
          Because at the start of emission, it is not yet known along which axis the measurement will be performed, the measured result cannot be an intrinsic information stored in the photon at creation (the rotating coin example), so we do indeed have "action" at a distance. If the information was intrinsic, the experiment would allow us to know the polarization along two incompatible axis (0 degrees and 45 degrees) at the same time, which is not possible according to quantum mechanics.

          However, because the observers have no way of influencing the outcome of their measurement, the phenomenon cannot be used for communication. However, it can be used to generate a shared one-time pad to be used along with classical communication (quantum cryptography).

          The experiment would work the same way with any pair of incompatible quantum observables, for which an entangled pair of particles could be produced.

    • Re:Weird thought (Score:4, Informative)

      by mburns ( 246458 ) on Sunday November 20, 2005 @12:25AM (#14073829) Homepage Journal
      It is just that your example is purely classical physics, it is causal, so that the QM entanglement is not shown at all, and QM entanglement for your example is not predicted by any one.

      Caused states must not communicate at a distance, this is classical behavior. But, uncaused quantum transitions have the appearance of at-a-distance communication simply because quantum states do not have a classical position. Only the classical manifestations of a quantum state have their separate positions.

      Quantum states themselves resemble categorical propositions in their lack of having a location. For example, where is the proposition, "Roses are red.", located? It becomes much more atractive than one first imagines to state that quantum states are actually a sort of categorical propostion.

      • Thanks (Score:3, Insightful)

        by achurch ( 201270 )
        Okay, I've got it now. Or at least, I've got enough of it to realize that I need to go take some more physics classes to have a decent chance of getting it . . .

    • How experiments say no (Score:3, Informative)

      by Anonymous Coward
      Brian Greene has a beautiful explanation of how your idea was proven incorrect in The Fabric of the Cosmos. Now comes my horrible attempt to further simplify it so it fits in a Slashdot post.

      Your idea would be a fine alternative explanation, if there was only one property being measured. Essentially, there are an infinite number of ways to "stop the coin", different angles if you will. I.e. there are many different properties which can be measured, each of which have the same two possible values. If you
      • If there were an infinite amount of properties though, the chances of you hitting the same property would be infinitely small, thus zero. Would it not, then, make sense that the same value is obtained exactly 50% of the time?
      • It is a little more complex than that.

        As per the entangled electron and light experiments, effecting one side of it induces an effect on the other side. For example, if I take the light polarized groups and twist one half of them, say, 10 degrees, then the other side will spontaneously shift as well so as to ensure that you still have a 90% angle. Same with flipping electron spins.

        The bigger issue is one of spooky communication at a distance. The noncommunication theorem states that faster-than-light com
    • If your analogy worked, we could communicate 'instantaneously' using 'rotating coins': just stop it at a particular position and the other side would be able to read a message.

      Whereas what's occuring in entanglement is much weirder: it is an 'instantaneous distant' action and yet it cannot be used to send data faster than light.
    • Let's say you "split" photon A into photons X and Y.

      The, you ram photon X into something, like a brick, turning it into heat.

      What happens to Y? If they're actually entangled and communicating with each other in some form, something should happen to photon Y. If not, it's just a matter of finding out Y by looking at X.

      So, what happens to Y in this scenario? Anyone care to enlighten us?
    • It's weirder, more like opening a Schrodinger box at one end and taking out a live cat, thus instantly killing another cat a light-year away.

    • ach, no (Score:5, Insightful)

      by Quadraginta ( 902985 ) on Sunday November 20, 2005 @05:16AM (#14074585)
      Goodness, no, this is not an accurate analogy. Here's a formally equivalent analogy that should raise your suspicions:

      Take two bullets and fire them in opposite directions from identical guns. If you measure the distance of bullet #1 at time t, you will find, amazingly, that bullet #2 has traveled exactly the same distance, but in the opposite direction. Hopefully that doesn't strike you as an amazing result.

      What both of our thought experiments say is that if you establish a correlation in a composite system (the two bullets, or your two coins), and you expand the system without doing anything to mess the correlation up, then, amazingly (not!) the correlation will be preserved no matter how large the system gets.

      What QE involves is something different: it says you can create a correlation after you have expanded the system, and in less time than it would take any kind of signal or force to cross the distance involved (in fact, instantaneously as far as anyone knows). The correlation can't be used for communication because you can only verify the results of the correlation by communicating the results of measurements on the two parts of the system, which, of course, you can only do at the speed of light.

      The whole business arises from the fact that we don't yet understand what happens when the "wavefunction collapses." We know that measuring a quantum system instantly transfers it from the quantum state it was in into a new one (the one consistent with our measurement). So far as we know, this happens instantly over the entire volume that the wavefunction occupies. The problem with this is that it seems dangerously close to violating relativity, because it seems something is being transmitted instantaneously over finite, possibly large, distances.

      Relativity is not yet in trouble because we have no good theory of quantum measurement, no knowledge of how a wavefunction collapses, so we can't apply the restrictions of relativity to the internal workings of the collapse. Relativity may never be in trouble, because the collapse may be an epiphenomenon, an event that seems to involve transmission of information but which really doesn't.

      Here's an example of an epiphenomenon: point the world's biggest laser at the Moon and look through a telescope at the dot. Aim the laser at one side of the Moon, and then swing it over to the other side quickly. If it takes you 0.25 seconds to move the laser's aim, how long will it take the dot to "travel" across the face of the Moon? 0.25 seconds, clearly, for a "speed" of 22,000 km/s. If you can change the aim of the laser in less than about 0.15 seconds your dot will "travel" across the surface of the Moon faster than the speed of light.

      But that's because nothing is really moving. The "motion" of the dot is just a fiction in your mind you create to help describe what you're seeing, because what you are seeing looks superficially similar to what you see when a real object moves. But there's no more real motion here than there is horizontal motion when a group in a stadium does "the wave". In the same way, the "transmission" of information in a QE experiment may turn out to be an epiphenomenon of a higher order, something that "looks" like transmission but really isn't.

  • Do not read the article before going to bed (Score:3, Funny)

    by Muhammar ( 659468 ) on Saturday November 19, 2005 @11:53PM (#14073723)
    If you do, they will find your wave function collapsed. In seven days.

  • Polarization problems (Score:4, Interesting)

    by Derling Whirvish ( 636322 ) on Saturday November 19, 2005 @11:53PM (#14073727) Journal
    Normally the photons exit the crystal such that one is aligned in a horizontally (H) polarized light cone, the other aligned vertically (V). By adjusting the experiment, the horizontal and vertical light cones can be made to overlap.

    That's a too simple description of polarization. It doesn't work that way. Take a polarizing filter and shine a light through it. Add another polarizing filter but rotate it 90 degrees from the other. The light is cut off from passing all the way through both. So far, so good. Now here's the tricky part. Take a third polarizing filter and place it in between the two previous ones. Rotate it around. WOW! At some intervals you can now see through all three! With two if you rotate the second you get total blockage when the filter is at 90 and 270 degrees from the first. You get more blockage points around the 360 degrees with the in-between third one (Extra ponts: how many?)! Strange. Add another. You get even more blockage points. (How many now?) Very strange indeed. Does the experiment account for this, the real behavior of polarizing filters and not the simplistic one in the article?

    • Re:Polarization problems (Score:3, Informative)

      by shawb ( 16347 )
      The article tried to explain it without math, and IIRC that requires some basic trigonomic functions, IIRC Nsin(theta.) Ask most people about trig functions and you will just get a blank stare, or a vague memory of hearing about it in high school. Besides, the entangled photon pair in question are indeed perpendicular to each other, and so the action of polarization at other than 90degrees is a moot point.
      • Besides, the entangled photon pair in question are indeed perpendicular to each other, and so the action of polarization at other than 90degrees is a moot point.

        And the polarization of the two polarizing filters that are perpendicular to each other can be defeated by adding a third in-between the others. There are circumstances (adding additional filters) where the polarization depolarizes. The model of polarized light as being filtered through a vertical gate is not wholly correct. It's much more involv

  • So if that now-slashdotted server had a mirror on Jupiter, the mirror would be crawling to a hault also?
    • Yes.

      Also, the Slashdot server can be entangled with any other server simply by linking it (no crystals required). When the /. server is entangled, we know that at any given instant, if the /. server is running, the entangled server will not and vice versa.

      Tell me it ain't true - I dare ya!!
  • This is like being allowed to make 20 waves in the rope each second, or 30 waves, or 40 waves, but never 25, 37, or 42. We all know that photons having a frequency of 42 is the only possible way to explain life, the universe and everything. So why won't he let us make 42 waves per second?

  • Introduction to Quantum Computer (Score:4, Informative)

    by kahrytan ( 913147 ) on Sunday November 20, 2005 @12:45AM (#14073878)
    While we are on the subject of Quantum Mechanics. Check out Caltech's website [caltech.edu] on Quantum Computers.

    I would also like to put you towards HP's Research [hp.com] on it.

    The future is quantum mechanics [utoronto.ca], no matter the subject.

  • I never understood the communication aspect... (Score:4, Interesting)

    by Vellmont ( 569020 ) on Sunday November 20, 2005 @01:14AM (#14073965) Homepage
    Why does one photon have to "communicate" to the other? Take two photons, one is polarized 90 degrees from the other. You don't know anything else. At some point you observe one, and now know the polarization of the other. Why is their communication taking place?

    To make an analogy,say I flip a coin and don't look at it. Then I cut the coin in half between the two sides (without looking at which side is which). I take one side across town to my friend, and keep one. I have no idea which side I have until I look at it, but once I do I also know which side my friend has across town. Where's the mystery here, because I've never been able to understand why there's any spooky action at a distance?
    • Without going into a long winded explanation, we know that 'spooky action at a distance' (more technically called non-locality) is a real phenomenon based on a theorem called 'Bell's inequality'.

      Here's what that theorem says, in fairly simple terms:

      If the system is merely the measuring of characteristics that pre-exist, but are unknown (like your pennies), there is a certain statistical distribution that will occur over a series of measurments of those characteristics.

      Quantum mechanics predicts a different
    • until you measure one. Firstly, for quantum entanglement to be there, both photons must have come from a single event, like an electron-positron collision. They come out of that event with no particular polarization, but rather a quantum superposition of polarizations. This is evidenced by the fact that they have a 50-50 chance of passing through any polarization filter, regardless of its orientation.

      However, once one of them *has* passed through a polarization filter, the other one must have a polarizat
  • Can anyone shed any light (unpolarized) on the 10^33 states of polorization that light can have? Where did that number come from????

  • Not Just a Good Idea (Score:2, Interesting)

    by Doc Ruby ( 173196 )
    That tutorial isn't very good. The science is accurate, but it's presented in a tone that makes it seem like things "obey" physical laws, rather than physical laws accurately describing what things do. That makes it hard for a student to fill in the gaps with our imagination, because laws seem arbitrary rather than reflecting observed consistent behavior.

  • Someone link me to an explanation? (Score:3, Interesting)

    by mrjb ( 547783 ) on Sunday November 20, 2005 @05:00AM (#14074560)
    I'm obviously don't have my PhD so bear with me.

    The part that I *do* get is:

    You cannot measure a system without altering it. That is, if you stick a multimeter in a computer you may crash it. The instrument of measurement is too course to see the state of a system without altering it. Shed light on electrons and they'll 'fly away'. In quantum physics, we're dealing with such elementary particles that absolutely every means of actually measuring the system will interfere with it.

    It is statistically correct to say 'the particle is 50% here and 50% there', if chances are 50-50 for it being in one place or another.

    The part that I don't get (so kindly link me to an explanation) is, just because there is no way of measuring where a given particle is, that doesn't mean it's in two places at the same time. It just means we don't know.

    Two rockets fly in opposite directions at the spead of light for a year. One of them is known to carry a closed envelope saying "white", the other one carries an envelope saying "black". The envelopes are in a time-locked safe. We don't know which rocket carries which envelope. Statistically we might as well say both rockets carry an envelope saying 'grey'. After a year of travel, the captain of the first rocket opens his envelope and reads a single word. Instantly he knows what the contents of the envelope of the other rocket are. Yikes! Spooky action at a distance?

    Someone hit me with a clue-bat, *please*?

    • Re:Someone link me to an explanation? (Score:5, Informative)

      by PaSTE ( 88128 ) <paste AT mps DOT ohio-state DOT edu> on Sunday November 20, 2005 @07:01AM (#14074786) Homepage
      Actually, what you've hit upon is something called the "hidden variables" theory of quantum mechanics. For a while after the spookiness of entanglement was figured out, vis. the Einstein-Podolsky-Rosen paradox [wikipedia.org], many physicists thought just like you do--that there was some hidden variable within the system that we could not measure, but which determined the state of the system exactly. They figured for instance that, if a certain decay process produced two photons, one left-polarized and one right-polarized, then there was some feature of the decay which determined which othe the photons is left-polarized and which right-polarized, so that our measurement of one did not "change the state" of the other photon, if merely revealed its pre-determined polarization.

      The hidden variables theory of quantum mechanics was disproven by a physicists named John Bell. In his method, he began by assuming that these "hidden variables" existed, then, using geometric arguments and the postulates of quantum mechanics, derived a set of inequalities which showed no physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics. [wikipedia.org]

      It's not intuitive at all, but Bell's argument is sound. Entanglement and action-at-a-distance is real, and not due to the system's state being pre-determined by hidden variables.

    • Re:Someone link me to an explanation? (Score:4, Informative)

      by stevelinton ( 4044 ) <sal@dcs.st-and.ac.uk> on Sunday November 20, 2005 @07:08AM (#14074801) Homepage
      The part that I don't get (so kindly link me to an explanation) is, just because there is no way of measuring where a given particle is, that doesn't mean it's in two places at the same time. It just means we don't know.

      What you're hankering after is a "hidden variable model". There is a variable that we can't observe, but it has a definite value. Unfortunately, no simple hidden variable model can explain observations. There are lots of ways of demonstrating this, but all have some complexity.

      One of my favourties is due to John Conway and some other people, and it goes like this.

      Physics tells us that if we pick any set of three directions at right anfles (eg up, backwards and left) and measure the squared spin of a simple particle (like an electron) in each of them, we get two 0s and a 1 in suitable units. The order of the three measurements doesn't matter.

      Now, Conway et al found a set of points on a sphere (ie a set of directions) out of which you can choose lots of triples that are all at right angles. What you can't do is label these points 0 and 1 in such a way that every such triple has two 0s and a 1. So there can't be a hidden variable for the squared spin in each direction, because which one you get depends on which other ones you measure, even though these measurements don't interfere with each other. Using entangled particles and a bit of jiggery pokery you could even do the three measurements at the same time and far apart so there would be no time for information interchange.

      A similar, although more subtle effect occurs in EPR. You give each "rocket captain" a choice of directions to measure the polarisation in, and you find a degree of correlation that you could not expect purely from a hidden variable model.

    • Forgive me if this is wrong, my physics ended at A level. As I understand it (continuing your analogy) if you rub out black on your card and write white instead, the other card will now read black. With photons if you rotate the polarity of one the other also rotates, that's the spooky bit.
  • I too am not a physicist, but still enjoy reading about topics like this. I want to make a comment however, by way of analogy, on the nature of Heisenberg Uncertainty and DeBroglie Wavelength.

    When I was in highschool, our biology teacher asked us to look throgh a microscope at a drop of water under a slide. In the view I could see a fuzzy/blurry-looking speck of dust, which was apparently jiggling. The teacher explained that the jiggling of the speck was due to it being battered by water molecules in a phen
  • The article was good for me which I do not have a formal education in physics. One thing the article says it will explain, but it does not, is how does the link between two particles work. The theory says that if one of the particle's spin changes, the other's spin will change accordingly, no matter what the distance. But how does that work? what is the link between the two particles?
  • For a few years I've wondered if you could use entanglement to communicate back in time.

    Anyone familiar with "realistic" time travel predictions knows if you have a worm hole and spin move one end around really fast then each end will be in a different 'time frame'. Entering one end will bring you out in a different time.

    By the same thinking I wonder could you take a pair of entangled particles and move one around really fast......then spinning one should cause the other to respond, but in a different time.

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