# Quantum entanglement question **URL:** \ **Category:** Factual Questions\ **Created:** [February 20, 2009, 5:54am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688 "2009-02-20T05:54:07Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![BlinkingDuck](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@BlinkingDuck](https://boards.straightdope.com/u/BlinkingDuck)\ **Post date:** [February 20, 2009, 9:58pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/41 "2009-02-20T21:58:02Z")
I am not an expert by far…or even a journeyman… but I’ve looked at spooky action at a distance this way: Using the ‘many-worlds’ explanation of quantum mechanics when 2 objects are entangled and one is observed, like I open the shoe box and see the left shoe, then the universe splits into 2 and I went the left shoe ‘path’. The other box is automatically the right shoe in this universe…no splitting is needed…hence ‘entangled’. No idea if that is a good way of visualizing it though… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 20, 2009, 10:04pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/42 "2009-02-20T22:04:29Z")
> [@](#): > > FWIW (and that’s nothing) I’m not quite so convinced that the particles need to be proximate in some sort of sense currently defined within our construct of spacetime. I do recognize that’s one explanation, and no nuttier than non-locality. I think we just haven’t figured out what particles are, and what space is. For sure particles are not shrunken down baseballs. And maybe they aren’t something that flies through space; maybe they are space, moving, and don’t become “particles” until they hit (interact) with something else. So when the waveform describing the system collapses (so to speak), what was space, moving, becomes two disentangled “particles.” Sure, light propagates at a constant speed “in” space but until we figger out what space and particles are, we don’t know that two entangled particles are not actually the same ezzak perturbation of space across as long a distance as we want to imagine. There’s nothing to say space has to behave like an aether; it can have properties all its own. There is nothing wrong with what you’re saying here. On the other hand, there’s also nothing useful with it. If it helps to keep your head from exploding, great (and in fact, I know a few physicists who do like pretty much this explanation, for this reason). But it won’t really tell you anything new about the results of any experiment. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [February 20, 2009, 10:12pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/43 "2009-02-20T22:12:25Z")
How do two particles become entangled? How do you seperate two entangled particles without effecting their entanglement. How do they store and transport those seperated particles during the experiments? Once one is determined to be a left shoe (and hence, the other a right) is that the end of it? Can they be measured again in 10 minutes and then the left is a right and visa versa? Can the particles themselves actually be manipulated (thereby effecting their entangled pair)? Or are we just measuring one and confirming our assumption about the second after measuring the first. Can we actually make the one particle behave as we want by manipulating the particle we have? If not. What’s so special about this? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 20, 2009, 10:42pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/44 "2009-02-20T22:42:39Z")
The easiest way to get two entangled particles is to create them in the same process. For example, if I have a pion that decays into an electron and a positron, the electron and positron will (at least initially) be entangled. They’ll remain entangled until something is done to them which affects the state of their spin (practically speaking, for charged particles, this usually involves putting them in a magnetic field). Once you measure the state of a particle, it is no longer entangled, and will remain in whatever state you measured it to be in until you do something else to affect the state. If you perform the same measurement twice in a row, you’ll get the same result. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [February 20, 2009, 10:48pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/45 "2009-02-20T22:48:56Z")
> [@Bear\_Nenno](#): > > What’s so special about this? When you measure the state of one member of an entangled pair it INSTANTLY influences the state of its partner. The effect is not limited by the speed of light. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [February 21, 2009, 12:46am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/46 "2009-02-21T00:46:30Z")
**Indistinguishable’s** answer is very good. It might help to understand this if you asked yourself the following question: If I want to tell someone that a system is in state 1 how would I do this via correlated pairs? You’ll find you can’t to do it. ---
**Author:** ![The\_Second\_Stone](https://avatars.discourse-cdn.com/v4/letter/t/bbe5ce/32.png) [@The\_Second\_Stone](https://boards.straightdope.com/u/The_Second_Stone)\ **Post date:** [February 21, 2009, 1:15am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/47 "2009-02-21T01:15:19Z")
> [@Chronos](#): > > Quoth **Chief Pedant** :There is relativity, and there is relativity. The Special Theory of Relativity is concerned with things like speeds close to c, and the speed of information transfer. The General Theory of Relativity is concerned with the curvature of spacetime and gravity. Special Relativity is not at all inconsistent with quantum mechanics, and indeed, QED, one of the theories which combines the two, is the best-tested theory in all of science. General Relativity, however, is to the best of our knowledge incompatible with quantum mechanics (which of course implies that the best of our knowledge is currently still inadequate). > > Quoth **Lumpy** :Ah, now we’re finally getting at the point of quantum entanglement. The two experimenters can measure any component of spin they want. No matter what direction I measure spin in, and no matter what electron I use (regardless of whether it’s entangled or not), I’ll always get an answer “positive” or “negative”. Once I make that measurement, the spin is definitely in that state, so if I measure the up component of spin and get “positive”, and then measure the up component of that same particle again, I’ll also get “positive”. Note that this breaks any entanglement the particle might (or might not) have had: A particle in a definite state cannot be entangled. If I take a particle which is definitely in an up-down state (like that one I just measured to be up), and measure it horizontally, I can get either positive or negative, with equal probability. And if I measure it at some diagonal angle to the first measurement, I can get either positive or negative, but not with equal probability: It’s more likely to be in the state which more closely matches the original state. In fact, the correlation between the two measurements is equal to the cosine of the angle between the two measurement directions. > > That’s for a single particle. What about a pair of entangled particles? If both particles in the pair are measured on the same axis (say, for instance, the vertical component of both is measured), then they’ll always be opposite. That’s not too mind-boggling; that could be explained just fine by a hidden-variable theory like those shoes in shoeboxes. If the axes they’re measured on are perpendicular, then there’s no correlation at all: Half the time they’re the same (both positive or both negative), half the time they’re different. This could also be explained by a hidden-variable theory, albeit one slightly more complicated than the shoeboxes. But here’s the kicker: If the angle between the axes is somewhere between parallel and perpendicular, then the correlation between them is equal to the negative of the cosine of the angle between them. So, for instance, if the angle between the measurement axes is 45 degrees, then they’ll have a correlation of -0.707… Bell’s Theorem guarantees that, no matter what local hidden-variable theory you try to come up with, you’ll never be able to devise one consistent with that result. And the answer to my two questions in English is? I ask you because you are the one who seems most likely to have the knowledge to answer it. Or does you being in the know require that I remain uncertain? ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [February 21, 2009, 1:27am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/48 "2009-02-21T01:27:25Z")
> [@The\_Second\_Stone](#): > > And the answer to my two questions in English is? 1. Both theory and experiment so far say “instantly”. Definitely [faster than the speed of light](http://ideafestival.typepad.com/my_weblog/2008/08/entanglement-br.html). 2. The record so far is [144 kilometers](http://www.nature.com/nphys/journal/v3/n7/abs/nphys629.html), I believe. But there’s no theoretical reason to think there’s an upper bound. ---
**Author:** ![The\_Second\_Stone](https://avatars.discourse-cdn.com/v4/letter/t/bbe5ce/32.png) [@The\_Second\_Stone](https://boards.straightdope.com/u/The_Second_Stone)\ **Post date:** [February 21, 2009, 1:33am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/49 "2009-02-21T01:33:17Z")
> [@Pochacco](#): > > 1. Both theory and experiment so far say “instantly”. Definitely [faster than the speed of light](http://ideafestival.typepad.com/my_weblog/2008/08/entanglement-br.html). > > 2. The record so far is [144 kilometers](http://www.nature.com/nphys/journal/v3/n7/abs/nphys629.html), I believe. But there’s no theoretical reason to think there’s an upper bound. Thank you. So if I ever write my sci-fi stories I can posit a communication device that will work instantly anywhere in the universe. And on a more practical level, boomer subs can be in constant communication no matter how deep and have unbreakable code using a one time pad. (One time pad only necessary if you assume someone else has secretly entangled your particles.) ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [February 21, 2009, 1:37am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/50 "2009-02-21T01:37:22Z")
> [@The\_Second\_Stone](#): > > Thank you. So if I ever write my sci-fi stories I can posit a communication device that will work instantly anywhere in the universe. You can’t use quantum entanglement as an information channel (i.e., you can’t send messages over it; you can only read what it produces of its own accord, so to speak). > [@](#): > > And on a more practical level, boomer subs can be in constant communication no matter how deep and have unbreakable code using a one time pad. (One time pad only necessary if you assume someone else has secretly entangled your particles.) Huh? ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [February 21, 2009, 2:45am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/51 "2009-02-21T02:45:58Z")
> [@Pochacco](#): > > When you measure the state of one member of an entangled pair it INSTANTLY influences the state of its partner. The effect is not limited by the speed of light. Instantly in what frame of reference? (Different ones will count different events as being simultaneous.) -FrL- ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [February 21, 2009, 4:03am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/52 "2009-02-21T04:03:39Z")
> [@Frylock](#): > > (Different ones will count different events as being simultaneous.) > > -FrL- So what? No one has said anything about simultaneity. The effect must be instantaneous not necessarily simultaneous. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [February 21, 2009, 4:15am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/53 "2009-02-21T04:15:31Z")
> [@Frylock](#): > > Instantly in what frame of reference? (Different ones will count different events as being simultaneous.) Heh. That just adds to the weirdness! Consider two entangled particles A and B. In some reference frames A may be measured first. In other reference frames B may be measured first. So if a message is being sent, is it travelling from A to B, or B to A? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [February 21, 2009, 4:53am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/54 "2009-02-21T04:53:49Z")
> [@Pochacco](#): > > Heh. That just adds to the weirdness! Consider two entangled particles A and B. In some reference frames A may be measured first. In other reference frames B may be measured first. So if a message is being sent, is it travelling from A to B, or B to A? That’s correct and normally this would mean that causality gets all screwy. But there’s no conceivable measurement that an observer can do that will distinguish the two particles. ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [February 21, 2009, 5:59am UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/55 "2009-02-21T05:59:51Z")
> [@Ring](#): > > So what? No one has said anything about simultaneity. The effect must be instantaneous not necessarily simultaneous. “Well,” the appropriate response will come, “what does instantaneous mean if not ‘The effect is simultaneous with the cause’?”. > [@Pochacco](#): > > Heh. That just adds to the weirdness! Consider two entangled particles A and B. In some reference frames A may be measured first. In other reference frames B may be measured first. So if a message is being sent, is it travelling from A to B, or B to A? As you would likely agree, such a question no more has or needs an answer than a question like “Is X standing still and Y moving, or is Y standing still and X moving?”. All the more reason to adopt the perspective that wavefunction collapse is not a physical phenomenon, as such; at least, it is not the sort of thing which has a genuine spatiotemporal location. There is no substantive question as to whether the wavefunction collapse occurred at A or instead occurred at B. And being not a spatiotemporal event, wavefunction collapse has no legitimate spatiotemporal location in relation to other events; in particular, there is no well-formed question as to whether it is part of a causal chain surpassing the speed of light. Trying to speak that way should be regarded as a category error. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [February 21, 2009, 1:52pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/56 "2009-02-21T13:52:06Z")
Why does it need to be instantaneous? The only way to compare the two measurements is to bring the results together, which can only happen at the speed of light. ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [February 21, 2009, 2:30pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/57 "2009-02-21T14:30:22Z")
> [@The\_Second\_Stone](#): > > And the answer to my two questions in English is? I ask you because you are the one who seems most likely to have the knowledge to answer it. Or does you being in the know require that I remain uncertain? You asked three questions, I think: \*"1. How fast do the changed states travel? Has it been measured or is it just theory? 1. How far apart can this work in theory? Yes, we know in practice that it can be meters, but could it be miles? Light years? Across the Universe? "\* In fairness to Chronos, I think he’s giving you the specifics around the answer to your second question, which was “Has it been measured or is it just theory?” The answer to that question is, “Actual measurements have been made from which we infer an assortment of things, among which is that the assignment of spin (the paramter those variously-angled detectors are measuring) does not occur until a measurement takes place.” This is a critical inference, because there is a huge difference between simply deciding which one of an entangled pair has a previously-assigned spin (this lets you know instantly what the other far-away one must be), and assigning the spin at the moment of measurement (this creates the spooky action at a distance which is both instantaneous and simultaneous or whatever other linguistic term you want to apply to it). So the short answer is, “Yes, there are supporting experimental measurements for the concept.” The longer answer, elaborated by Chronos, is what those measurements involve and why the conclusion is drawn that the spins of a particular particle pair are truly randomly assigned at the time of measurement. Because this assignment is random, the argument goes, it cannot be that we are simply establishing which spin a particle already had when it separated from its entangled partner. The inference drawn from what happens at detection is that the particle has neither up nor down spin until it’s measured, and when it’s it randomly becomes either up or down, immediately constraining its partner to the opposite spin. There’s no weasel room here around time frames, instanteous v simultaneous and so on, because there is no time involved: it’s zero. That’s what makes it spooky in the first place. It’s not some propagation of information across the distance between point A and point B that is somehow “faster” than the speed of light. Speed needs time in the equation to exist. The time involved is not small. It’s zero. ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [February 21, 2009, 2:40pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/58 "2009-02-21T14:40:30Z")
> [@Chief\_Pedant](#): > > …and when it’s it randomly becomes either up or down, immediately constraining its partner to the opposite spin. ETA: Should read, “…and when it’s measured it randomly becomes either up or down…blah blah blah” Missed the edit window for the key point I was babbling on about. ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [February 21, 2009, 2:42pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/59 "2009-02-21T14:42:06Z")
> [@Indistinguishable](#): > > “Well,” the appropriate response will come, “what does instantaneous mean if not ‘The effect is simultaneous with the cause’?”. Exactly. (Though I might not have put the question in terms of causes and effects.) > [@](#): > > All the more reason to adopt the perspective that wavefunction collapse is not a physical phenomenon, as such; at least, it is not the sort of thing which has a genuine spatiotemporal location. There is no substantive question as to whether the wavefunction collapse occurred at A or instead occurred at B. And being not a spatiotemporal event, wavefunction collapse has no legitimate spatiotemporal location in relation to other events; in particular, there is no well-formed question as to whether it is part of a causal chain surpassing the speed of light. Trying to speak that way should be regarded as a category error. I think I see what you mean here. Do you mean to say that a statement like “A waveform collapses” should be treated more like “2+2 = 4” than like “ball A hit ball B”? As in, the usefulness of “2+2 = 4” does not involve the sentence’s being true at a location in space and time? Even if waveform collapse doesn’t happen at a location, it seems like the scientists (at least in their popularizations) make a pretty clear point about the thing that’s happening here happening “instantaneously” or “simultaneously,” and I’ve always wondered what they mean by this. Is it merely loose talk? -FrL- ---
**Author:** ![Chief\_Pedant](https://avatars.discourse-cdn.com/v4/letter/c/a6a055/32.png) [@Chief\_Pedant](https://boards.straightdope.com/u/Chief_Pedant)\ **Post date:** [February 21, 2009, 2:53pm UTC](https://boards.straightdope.com/t/quantum-entanglement-question/486688/60 "2009-02-21T14:53:27Z")
> [@Chronos](#): > > There is nothing wrong with what you’re saying here. On the other hand, there’s also nothing useful with it. If it helps to keep your head from exploding, great (and in fact, I know a few physicists who do like pretty much this explanation, for this reason). But it won’t really tell you anything new about the results of any experiment. Agreed. And as you are well aware, the fun for theoretical physicists is coming up with wacko ideas for which no experimental and/or predictive models can be designed. Any such ruminations are not “useful” exactly, but when general relativity and quantum mechanics are at such odds, it’s obvious that there is something else going on which will more beautifully explain what really happens. And recognizing that does keep my head from exploding over particle entanglement. [Previous page](https://boards.straightdope.com/t/quantum-entanglement-question/486688.md?page=2) [Next page](https://boards.straightdope.com/t/quantum-entanglement-question/486688.md?page=4)