I'm not an expert at physics, but this is the most profoundly interesting thing I've heard of in physics : The EPR Paradox (http://en.wikipedia.org/wiki/EPR_paradox)

From the article linked :
"The EPR paradox draws on a phenomenon predicted by quantum mechanics, known as quantum entanglement, to show that measurements performed on spatially separated parts of a quantum system can apparently have an instantaneous influence on one another. This effect is now known as "nonlocal behavior" (or colloquially as "quantum weirdness" or "spooky action at a distance"). In order to illustrate this, let us consider a simplified version of the EPR thought experiment put forth by David Bohm."...


So basically, it sounds like entangled quanta can instantaneously "communicate" with each other; when one's acted on -> the other one reacts instantly, no matter how far apart they are. Right ?

Could one extract from this "non locality" that the universe exhibits a reality which implys EVERYTHING is actually in one spot, and distance is an illusion ?

The only other way I could think of explaining it is if the universe is being run by something like a (very advanced) computer program. The 4D universe is the program running, and the code behind it could have objects any distance effect one another instantaneously. :shrug:

So basically, it sounds like entangled quanta can instantaneously "communicate" with each other; when one's acted on -> the other one reacts instantly, no matter how far apart they are. Right ?

Wrong. Sorry.

The idea is that the entangled particles live on the same waefunction. The wavefunction itself carries no ``information''. So the idea of two ends of the wavefunction communicating is not right---there can be no exchange of information because there IS no information.

Specifically, let's say that you and I set up an experiment to measure the spins of an entangled particle. I live in New York and you live in California. Let's imagine pions or something being made in nebraska, so that when the pion decays, you get one photon of one spin, and I get another photon of another spin, so that the two spins add together to make a spin zero. So when I measure a photon, I see it has one spin, and i KNOW that you measured another photon with the opposite spin.

When cased this way, it's clear that no information can be communicated between us. I measure spin up, but in order to see what YOU measured, I have to call you on the phone, or something.

Further, we have to know that it was a pion decaying in the first place! That means that someone has to call me and you on the telephone to TELL us that we should be ready to detect a pion.

The only other way I could think of explaining it is if the universe is being run by something like a (very advanced) computer program. The 4D universe is the program running, and the code behind it could have objects any distance effect one another instantaneously. :shrug:

This is actually not as far fetched as it sounds, but I feel that these kinds of questions aren't scientific. And either way, it doesn't really follow from your previous statements.

Ohhh, I knew this was some deep stuff. :bugeye:

Leaving out the "communication bit", isn't it correct to say if one of the entangled particles is acted on, the other one reacts instantaneously ? (observed or not observed) ?

Hmmmm maybe not. It's not a reaction, per se. If it was a reaction, then it would be non-local :)

Doesn't one observer know that the entangled spins have been observed, or the "other end" has a known spin state, so has a 50/50 chance of guessing which it is?

I remember a Alice/Bob scenario where the remote party can determine that this has happened, but only knows with a probability of 0.5 what's been observed.

Except the whole thing has to be pre-arranged before Alice and Bob part company, to transmit the signal, and the distance between them still affects what they can arrange to do, given the known states of these entangled photons.

The pre-arranged bit is like having a long telegraph wire, that gets connected briefly, so the message is: "there's a connection", but no further information is available on the quantum entanglement line. You have to use ordinary speed-of-light photons for that.

The two pions are on the same wavefunction, but when you measure the spin of one of the pions, isn't the collapse of the wavefunction occurring instantly also for the other pion?

The two pions are on the same wavefunction, but when you measure the spin of one of the pions, isn't the collapse of the wavefunction occurring instantly also for the other pion?

Sure. The collapse of the wavefunction is instantaneous. But both photons live on the same wavefunction. It's the wavefunction that tells the photons how to behave, not vice versa.

How can a wavefunction collapse instantaneously regardless of distance? If I measure a photon in Alpha Centauri, its entagled partner's wavefunction INSTANTLY collapses even if its here on earth. This is the most befuddling thing in physics.

If I measure a photon in Alpha Centauri, its entagled partner's wavefunction INSTANTLY collapses even if its here on earth.

On the face of it, it is impossible.
I think such claim is result of mistaken interpretation.

How can a wavefunction collapse instantaneously regardless of distance? If I measure a photon in Alpha Centauri, its entagled partner's wavefunction INSTANTLY collapses even if its here on earth. This is the most befuddling thing in physics.Not if you consider that the entangled photons are two halves: of the same wavefunction, with identical but opposite (in sign) quantum "numbers": spin, direction, frequency.

Distance, if you're a photon, doesn't come into it, a photon is an excitation with an indefinite "lifetime". Photons don't decay or "dissipate" - but they do stretch, along with space itself (cosmic expansion); obviously the distance between two entangled photons is zero distance, as far as photons are concerned. Or the wavefunction is the connection, and it's represented as two exactly equal but "opposite" photons, travelling in exactly opposite directions (of course, the direction of a photon can be changed as it propagates).

The change of direction implies that entanglement is immune to the direction of either of the pair of photons - although when the original photon "splits" into two equal halves, they must propagate at 180 deg to each other.
Subsequently the wavefunction "ignores" changes in the direction of propagation (and so the distance) of either of the entangled pair.

I hear people constantly say that entangled particles dont communicate instantly over great distances but they never give any reason for this. In fact their thinking seems circular to me. 'we know that particles dont communicate ftl because we know that ftl is impossible'. but what if it is possible. As the op pointed out:

if the universe is being run by something like a (very advanced) computer program...and the code behind it could have objects any distance effect one another instantaneously

there seems, therefore, to be no reason to just assume that ftl is impossible. instant communication to any point in the universe would mean that there is in fact an absolute time but relativity doest say that absolute time doesnt exist just that we cant determine what it is without flt.

There's difficulty in pinning down what, exactly, is communicated.
For the simple notion of entangled photons, there's no more apparent communication involved than if the photons were spin up / spin down right from the beginning of their journey. Of course one will be up and the other will be down, and of course you won't know which is which until you measure it.


Further investigations (the Aspect experiment) make it clear that it's not quite that simple, but the point remains... what (if anything) is being communicated?

dont know about photons but if you change the spin of one entangled particle doesnt that change the spin of the other?

if entanglement is just the fact that when produced 2 particles have opposite spin then the whole concept seems rather trivial. even physicists arent that dumb. there must be something more to it than that.

You don't "change" the spins, you "observe" them.

If you observe say the plane (or circular) polarization of one of an entangled pair of photons, that means you know the polarization of the other half (of the wavefunction). At the "other end", an observer can determine that a measurement has been made, is all, not what the measurement was.

the question was what if you change the spin?

Again, you only "measure" spin.

A photon of light has an unknown polarisation until it's measured. You can't "measure" an unpolarised photon - you have to polarise it with a filter of some kind.

The filter "measures" polarisations of incoming photons, and allows those with the same plane polarised "direction" through. IOW you can't tell anything about photon polarisation until you polarise some photons.

I specifically said that I wasnt referring to photons.

Most interactions (changing the spin, for example) would break the entanglement.
I wish I understood the Aspect experiment (http://www.electrodynamics-of-special-relativity.com/Aspect-s-Experiment) - I think it's the definitive one that showed that something very spooky is going on, but I don't have a good handle on what that spookiness is.

I have a question about the question: how do you "change" the spin of a particle (that isn't a photon), like an electron or a proton, say?
How do experimenters go about changing particle spins exactly, or what do they need to do?

so this entanglement...how exactly can it be used in teleportation?

Another (perhaps somewhat desperate) point to point out is that photons, while massless, as far as QM is concerned are particles, with all the properties (except mass) that particles have: momentum, spin, direction of motion (propagation), etc.

Photons are definitely particles of something, but they look like waves as well. This is our "mistake": we think that a wave can't be a point-like particle, it has to behave like one or the other, but unfortunately for our sensibilities, quantised particles behave like both anyway.

Quantum teleportation is the translocation of a quantum state. Since the "target" is left with an identical set of quantum numbers, it's the same particle.

Why not just a physics of the lightsaber draqon and be done with it hehe.

Why not just a physics of the lightsaber draqon and be done with it hehe.

well thats the thing in Star Wars universe the lightsaber just springs out from nowhere, they don't explain it, it just works u see :p ...if lightsaber was in Star Trek universe they would have had some sort of parascientific proof for it

well thats the thing in Star Wars universe the lightsaber just springs out from nowhere, they don't explain it, it just works u see :p ...if lightsaber was in Star Trek universe they would have had some sort of parascientific proof for it

Forcing a response, are we?

You don't "change" the spins, you "observe" them.

If you observe say the plane (or circular) polarization of one of an entangled pair of photons, that means you know the polarization of the other half (of the wavefunction). At the "other end", an observer can determine that a measurement has been made, is all, not what the measurement was.If the observer can determine that a measurement has been made then that is information. Could be used bitwise so that 1 equals observation and 0 no observation, thus transferring all kind of digital information without concern of earth horizon or distance at all. (?)

I guess that depends what sort of information a single bit is? Remember, unless the "other end" knows beforehand what particular quantum variable will be "determined", they won't know anything at all.
Putting aside the practicalities of transporting an entangled photon, or any other kind of quantum, any significant distance (i.e. storing it in some container), encoding is a big issue - you've got exactly 1 bit.

obviously you would have to use a continuous stream of them to transit a message.

Even then you can't transmit a predetermined message - you can only share a random string of bits.

That's the promise of quantum encryption - you can generate a random string of bits that is guaranteed to be known by only the receiver and the sender. No eavesdropper can listen in without breaking the entanglement.

The sender can then use that random string to encrypt the real message, which is then sent through ordinary channels.

If the observer can determine that a measurement has been made then that is information. Could be used bitwise so that 1 equals observation and 0 no observation, thus transferring all kind of digital information without concern of earth horizon or distance at all. (?)

The observer of one of the entangled quanta can't tell if the other one has been measured. They can only tell what such a measurement would be, if it were made.

Right. The way to see entanglement and two spatially separated observers, is that they need to agree what to measure (spin or polarisation, say), in which case, there's a 50% chance that they see the same (or opposite) states, or halves of the same circle.

They both establish an arbitrary direction for polarisation for example, and so can't both establish the same "up" or "down". Because there's no absolute or "pure state" that can be known by either party, there's only a 50/50 chance of guessing what the other observer sees.

I suppose it's like two different observers seeing the same photon with a random frequency, i.e. the same random stream of bits (or its inverse, both observers would have the same expectation). They only have a probability of 0.5 of agreeing on what they saw, when they meet up or communicate.

If the observer can determine that a measurement has been made then that is information. Could be used bitwise so that 1 equals observation and 0 no observation, thus transferring all kind of digital information without concern of earth horizon or distance at all. (?)

But the observer has to first KNOW that a measurement has been made. But this requires some communication between the two observers.

http://www.nature.com/nature/journal/v403/n6769/full/403515a0.html


Further investigations (the Aspect experiment) make it clear that it's not quite that simple, Maybe from the outsider's point of view Zeilinger's stuff sharpens the focus on exactly what is strange here.

Sorry for misleading with: "the other observer can tell a measurement has been made" thing. This is only because they have pre-arranged to measure their half of the entangled state, so please disregard any implication that information is transmitted because of one observer "fixing" something.
If you know the spin state of one half, you know the spin state of the other, but you have to communicate this observation to a remote observer, who sees their own measurement (with a 50% chance of it correlating).
At least I think that's how it goes.

IOW the only thing both parties know is that they are both looking at the same entangled state/ random photon from the void. But from individual reference frames.