the EPR paradox: is it totally retarded, or am i?

Philosophy Forums Science, Technology, and Math
1

the Einstein-Podolsky-Rogen thought experiment was thought up by those guys to illustrate that quantum mechanics is an incomplete system because it appears to be capable of transmitting information instantaneously (“spooky action at a distance”)

Alice <------- electron-emitter---------> Bob

you have something that emits electrons and two observers on either side. whenever the emitter shoots an electron at alice that has a ‘value’ of X+, the emitter simultaneously shoots an electron at bob with a value of X-

so that means that in the time it took alice to learn that the emitter had shot an X+ electron at her, she could know that it also shot an X- electron at bob. but she could have learned that information in less time than it would have taken bob to tell her at the speed of light. she can know what X value bob got without communicating with him.

this violates special relativity because information cant travel faster than the speed of light.

so am i retarded or is this thought experiment? or did i read it wrong, here?

imagine you are watching a lighthouse, which emits light in only one direction as it spins around. as i am on land and see the light shining on me, i know that it is not shining on the water on the opposite side, and i can learn this faster than the speed of light can travel from that water to my brain to inform me directly.

but what about this, einstein, i had to learn about the nature of that lighthouse before the experiment took place. i had to go up there and see, at much slower than the speed of light, that light only comes out one side. i had to learn that part first before i could learn that if its shining on me, its not shining the other way.

alice doesnt know that bob received an electron of opposite X value unless she took the time to learn the nature of that electron emitter and its behavior. her examination would have taken place without violating special relativity, and her subsequent experiments would only be as certain as her initial, non-“spooky” examination.

so why is my interpretation wrong?

2

You’re missing the point. having knowledge of the system is not the issue, the problem is that someone at point A knows the state of affairs at point B quicker than they should do, and thats all. HOW they know it is irrelevant, the fact is that they do.

It isnt breaking Einsteins rules cos no information is passed between the points (the result is random). But physicists are still troubled because the universe seems to be communicating with itself at faster than light speed, or else indeterminate things are secretly determined, or else ‘time’ splits off picking one (or both) scenarios and ensuring the whole universe conforms to it.

The wikipedia page on interpretations of quantum mechanics should help.

3

I may be completely wrong, but I thought it was just knowledge by inferrence from one to the other. ie: you look at one of a ‘matched’ pair, the other being somewhere else, and seeing that one is black, can assume the other white or somefin. All you have to know is the rules of association between the two.

4

Taking tabula’s point further, not only that but it’s not concrete knowledge, at the time of receiving the E+ electron, Alice has no way of verifying that the machine behaved as it was supposed to and actually transmitted the electron to Bob.

Surely this completely invalidates the thought experiment as the knowledge of the state at Bob isn’t actually a priori but a posteori so doesn’t invalidate the theory as the REAL transmission of knowledge actually took a lot more steps than those stated in the thought experiment.

5

if nothing actually goes faster than the speed of light, how does it violate special relativity? how is it even remotely related to it?

the thought experiment might as well be ‘if a guy in africa sees a lion, hes gonna run’ and somebody in africa is running away from lion as i say that and so i somehow knew what was happening without even experiencing the specific instance. what does that have to do with physics?

what is random? if alice gets an X-, bob gets an X+, for sure. i dont follow. im familiar with the zany multiple wave functions collapsing into one at the point of random decisions, but i dont see what this thought experiment has to do with any kind of science at all.

well lets say i throw two balls in opposite directions spinning at the same rate, with a dot drawn on the point on the bottom of the ball. for the next 7 and a half billion years, i can follow one ball, and by seeing where its dot is facing, i can conclude that the other dot is facing the same way, instantly.

except i dont know that the ball hasnt bumped into something. i dont know that the electron traveling to bob didnt spontaneously explode in his face. i dont actually know anything about anything besides exactly what special relativity says i should.

why does this example specifically illustrate something about quantum mechanics? why is this action described as “spooky” if its so intuitively obvious in the regular world?

6

were they simply trying to put into plain words the many more specific examples of action at a distance like the dual slot experiment?

7

Tab and Matt, you are missing the point too. The thought experiment is not concerned with how the knowledge is transmitted, or if it is reliable, or whatever, only that it IS transmitted and that it appears to do so faster than the speed of light.

You appear to think it is the observers who matter. The EPR paradox isnt concerned about this, the observers are there just to observe, and the thought experiment (which has actually been performed btw) accepts that it is only inference that allows the observers to know what goes on elsewhere, and that no information has been communicated, etc. This is all besides the point.

What matters is that the particles themselves seem to have communicated with each other at FTL speeds.

Please note that the way i have to explain it will make an interpretation on quantum mechanics, and this is not uncontroversial.
Before the particles are observed they have both states (50% one, 50% the other). Thats right. They are not one or the other but both at the same time. This is because they are indeterminate.
When they are determined by an observer, the particle (in effect) chooses which state to be in. The act of observation forces the particle to be specific.
Of course, this entails a simultaneously observed particle elsewhere to be the opposite state.

So how is it that the state of the particle can be truly indeterminate before observation, while both particles remain opposite to each other upon observation without them communicating at FTL speeds with each other?

The answers are still up for grabs, including the non-answer.

Ah. You have to be careful. Information cannot be passed at FTL speed, but it is theoretically possible for non information carrying particles to be faster (i think).
The EPR paradox is to do with special relativity’s application to quantum mechanics (i guess). The argument is in this case whether something has gone FTL, and if not, how does the universe make sure that everything works out if there is simultaneous determination of entangled but spatially seperated particles.

The randomness is “who gets which?”, and the problem is that the other particle must know what its partner got, even though it cant (seem to) communicate with it.
Try some wiki if the rest of my post hasnt helped (though obviously i love chatting about it you might get a better explanation).

This is a silly analogy. Nevermind that the property that is being observed is alot more “intrinsic”, any interference is irrelevant to the principle. (like, “ha ha! gravity doesnt work! i proved it in a airplane!”)

Of course something could interact with it and (though i dont know how) could change its spin, but that wouldnt change that the other particle must know what that spin should have been despite never coming into contact with at the time of determination. In effect, the interference becomes a point of determination as well. I’ll say again though, this is irrelevant.

8

the dual slot thing is an entirely different angle. Thats just an experiment that proves the wave-particle duality of all things. A single electron, which is proven to be a particle, acts like a wave by interfering with itself as it travels every possible route simultanously before being determined by the absorbing material.

Just as the EPR particles are indeterminate in that they are not sure what state they are in, so they have all possible states, so the electron is not sure of its velocity or position, so it is all of them at once (until it is determined to take one of them)

9

Hmm. Yes, I remember now. The old cat-box “events determined by observation” and “collapsing probability” etc. I never did buy that one.

I do like the single photon dual-slit experiment though - but I thought it was shadow photons from other universes causing the interference - or should I stop reading Fortean Times…?

10

you are probably not buying particular interpretations, but the facts are the facts. Real experiments show that particles do seem to behave in this manner. They didnt use real cats though, and more’s the pity.

as ever, wiki loves you
en.wikipedia.org/wiki/Double-slit_experiment

its just particles behaving as waves. Exactly how they behave as waves, while simultaneously (and definitely) being particles depends on your interpretation.

en.wikipedia.org/wiki/Interpreta … _mechanics

11

Electron A: “Ooh quick - someone’s looking!!! Get your spin sorted out, you’re a mess.”
Electron B: “Shit run for it…!”
Electron A: “Oh - this one or that one…? I can never decide. Fuck it, I’ll go through all of them at once.”

Hard life being a sub-atomic particle/wave/cross-dresser.

12

Interesting thread…

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(Note: In the below I’m just going to treat knowledge as reasonable belief, simple definition, so makes for easy illustration of my point.)

Oreseo, having re-read I see I posted in haste, but thinking further about it I find myself agreeing with futureman (for once :wink:). I still don’t believe it’s transmitting faster than light, my point is that you will have had to learn that the transmitter was going to do that action first, so the assertion that the knowledge is travelling FTL is a sleight of hand, because without the premises of:

p1 Bob will be standing at point A ready for E
p2 the transmitter will emit E and E+ at the same time in the directions of Bob and Alice respectively

you can’t have the inference:

i1 When Alice receives E, Bob receives E+

The fallacy the thought experiment is commiting is that i1 is travelling FTL, where it’s not, i1 does not exist without p1 and p2, so the time taken to transmit i1 also, by very definition, has to include the time taken to transmit p1 & p2, therefore the transmission of i1 can never be faster then light.

For this thought experiment to work, you essentially have to pretend that the knowledge itself has actually just appeared in Alice like magic, which is faith, not knowledge, and thus has no ramification, as faith is fairy tales.

For her to have knowledge, information, inference, whatever you want to call it, about the reception of the particle at Bob she has to have been in one of the the following:

Scenario A (futureman’s lighthouse scenario):

To reasonably believe that Bob has received E Alice needs:

  1. Knowledge of where Bob is, which can only have been received at light speed.
  2. Knowledge of what the transmitter is going to do, again, only possible at light speed.

So when she receives E+, at light speed, the time the knowledge has taken to get to her isn’t just the time of E+ from transmitter to her, it’s also the time taken to transmit 1 + 2 as well, add it all together and it cannot have possibly been at FTL, the knowledge was transmitted at light speed or lower. If 1 + 2 had not existed, there would be no i1.

Scenario B (you gotta have faith!):

The knowledge that the transmitter was going to transmit E to Bob was sent to Alice at the same time as E+ at light speed.

But in this scenario she doesn’t have a reasonable belief, or any information about E, she is just being told, by a non-verifiable source that maybe there’s an E out there travelling to someone apparantly called Bob.

If you try and get out of this paradox by saying she can trust the information sent at the same time as E+, you’re getting yourself back into Scenario A, where the inference is dependant on light=speed or less premises which result in an inference, andthe inference does not exist without these premises.

The alternative is that for alice to have any information about E she has to take this information on faith, and if we start saying faith can upset special relativity then the whole shebang collapses. Or, she has to verify it, i.e. observe it, and goes and finds and asks Bob whether he did in fact receive an E, which again happens at light speed. Up until that verification the state of E is indeterminate, within the realms of special relativity.

To me this thought experiment essentially seems to be saying, “I have faith that the cat’s dead, ha, Shrodinger, you’re wrong!”

14

Matt, i respect your rigour, but you are off the mark about the experiment’s aims. This is not all that surprising considering the wierdness of the background knowledge required though. :slight_smile:

I can go through your argument point by point (just say so), but i think it would be more helpful if i just started again.

In short: read this:
en.wikipedia.org/wiki/EPR_experiment
and the other wikis ive posted to clear things up.

In long:
Forget bob and alice. Bob and alice are now completely passive unthinking sensors. They are there to absorb the particles and determine what they are. Lets call them A and B. Forget all this ‘knowledge’ business.

The particles can remain E+ and E-. It doesnt really matter though as long as one is necessarily the opposite of the other.

The emitter throws the two E’s out to the sensors. At this moment each E looks like this:

E heading towards A

  • 50%
  • 50%

E heading towards B

  • 50%
  • 50%

This is because the Es are absolutely and fundamentally undetermined (this is what the cat is about). God himself couldnt tell me which is + and which is -. In a very real sense, they are both.

When they hit the sensors (which they will do simultaneously) the act of the E hitting the sensor will determine whether it is + or -. Necessarily, the other E will be determined to be the opposite.

Right up until the E hits its sensor, it could go either way, and yet they MUST go opposite ways. Thus, when the E is hitting the sensor it must be in some sense entangled with the other E making sure it goes the other way, even though they are far apart.

How this is possible depends on your interpretation of quantum mechanics. FTL interaction is only one option.

Is this clearer?

Of course, Tab, afterwards E- asks E+ “are you sure we can do this funky quantum shit?” and E+ says “yeah, I’m positive! Dont be so negative about it!”

15

@Oreso

I believe you are correct, I recall non-information can move FTL. I vaguely remember some exp where the front of a light wave got from a to b FTL but the pulse, capable of carrying info, did not (okay, I remember it quite badly, the point was as above but the precise facts and ‘why’ I’ve forgotten).

Also, imagine a laser rotated 360 degrees a second. Draw a circle round it 300,000 km long and you would witness a dot move at the speed of light, a larger circle and the dot moves faster than light - magic. Useless as no information can move at that speed by this method but it should definitely work.

16

Hm? wouldnt the laser line bend to make sure the dot doesnt exceed c?

Like, from the circle’s POV the circumference of the laser line’s trajectory would decrease even though the radius (the distance between the laser and circle) would remain the same. That’s what bending space time is all about.

Theres a similar experiment my mates and i thought up to challenge our physics teacher:
A spinning disk with its outer edges reaching c.

Just as an object gets squashed (decreases in length) along the axis of its movement when approaching c, the circumference of the spinning disk would grow smaller so that each particle has to move further from the POV of an outside observer.
At light speed the circumference would be 0, and at FTL the volume would invert. And all while the radius is unchanged as there is no movement along it! Measuring pi would be problematic. :smiley:

17

That’s clearer, the key think that I’ve been missing is they’re a linked pair so it’s that the spin is determined, rather than the reception. There still seems to be somethign going on though, can’t one argue that Alice’s observation of Bob’s particle started a lot earlier than her observation of hers, hence dismissing the paradox?

That is that the particles were observered and hence determinite as soon as Bob and Alice agree the experiment?

Although I think I’m arguing circularly now! No, I don’t think you can. I really need to learn QM.

18

observation is a technical term in QM, basically it means that something has been bounced off the object and absorbed by the observer (in common experience, we bounce light and sound off of things).

In QM, observation is an act that will change the object observed (which is why the heisenberg uncertainty principle is the case, as soon as we bounce an object off to determine information about the particle, the impact has no doubt changed that information to some unknown degree).

It is this act of bouncing something off the particles that will determine their states. So, if you can observe the particles throughout the whole experiment then it is invalid.

good stuff,
cheers!

19

Good question… I think not, the properties of the photons at the imagined circumference would be identical to those when they are emitted (ie. c straight ahead, no sideways velocity) the dot would appear to move sideways faster than c but no physical object actually would.

I cannot think of a way that any information could move faster than c in this system, (any ideas?) so there is no breaking of any physical law I know.

PS. If you could view the whole laser at a given time it would be a spiral, but any individual photon would be moving directly away from the center (O) of the circle.

20

lets say you have a big box with evenly spaced, positively charged balls in a grid throughout it. alice and bob and the emitter are in there.

when the electrons come out, and neither of them have observed it, does the grid of balls react to them? if they did then they would be the first observer wouldnt they?

so as soon as a particle has any kind of effect on the outside world, THATS when all of the wave functions collapse and the particle chooses which property it will have. and prior to that moment, the particle contained no properties? or it contained all properties?!

1: how do we know what the property of an object is without interacting with it in any way

2: what does it matter what a particle looks like before anything it ever does has any effect on the universe.

3: where do these property-free particles exist in the universe? where can a particle be created where it is completely impossible to determine what its state is by the mechanics of the machine creating it and that machine’s state at the time?