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AriAstronomer
2010-Oct-28, 10:35 PM
"One experiment that is in agreement with the effect of entanglement "traveling faster than light" was performed in 2008. This experiment found that the "speed" of quantum entanglement has a minimum lower bound of 10,000 times the speed of light."

How does this not conflict with our laws that nothing can travel faster than the speed of light? How is it that quantum entanglement can act at large distances at instantaneous speeds and not violate anything?

Shaula
2010-Oct-28, 10:39 PM
Because the law is that information cannot be transferred faster than light - entanglement works in such a way as to obey that law.

AriAstronomer
2010-Oct-28, 10:42 PM
Can you (or anyone) be more specific, and not just say 'entanglement works in such a way that it obeys that law'?

In addition, how exactly do particles become entangled in the first place?

ShinAce
2010-Oct-28, 11:07 PM
Let's make up our own example.

First, entanglement is the result of many particles sharing a creation. So if a gamma ray suddenly splits into positron and electron, each will have opposite spin. If the position(P) has spin up, the electron(E) will have spin down.

Now, let's assume we have a device that only lets particles go through if the spin is 'up'.

We now produce a bunch of electrons and positrons and do some tests.

We notice that all particles get through when we measure the spin after our spin selector device is passed. Half the time, we measure spin up, and the other half, spin down. How did a spin down particle get through? It must have been spin up AND down to get through. Measuring it got rid of one spin.

We now measure spin of one particle before our cool device. When we measure spin down, the partner particle gets through and we find it is spin up. Everytime. When we measure a spin up, the other particle does not get through and we can measure it to see that it is in fact, spin down.

You can time it so that the particles are miles apart from each, but within an each of our device. We can only guess that measuring one spin causes the other particle spin to be well defined, and opposite, instantly. The time it takes the particle to move an inch versus the time it takes the 'spin' signal to cover a few miles is extraordinary.

That is my layman's explanation of the phenomena. Obviously, it could be improved.

Ken G
2010-Oct-29, 04:31 AM
ShinAce's example wasn't really quite right, but it did capture the basic spirit of the situation. It is actually very tricky to detect any unusual behavior by measuring spin of entangled particles, you have to have spin measurements that are inclined at an angle to each other on the two entangled particles (not just spin up or down). But without getting into that kind of technicality, in my view there is a relatively simple way to think about entanglement that resolves all this "faster than light influence" hooey. The bottom line, however, is that Shaula is right-- no information travels faster than c. Thus there is no need to frame the situation in terms of any "influences" moving at high speed either.

First of all, let us appreciate several things:
1) an entangled system is just that-- a system. So anything you do to any part of a system, you do to the system.
2) bizarre behavior in entangled systems never show up when you only do measurements on one part of a system, only when you do measurements on two parts of a system and check the correlations between the measurements.
3) if you only do measurements on one part of a system, you never even know if it is entangled or not-- nothing it does says "I'm entangled with something else", and that's why there is no need to frame the behavior in terms of an "influence" on a piece of the system.
4) you cannot check correlations between measurements at a speed faster than c, because that really requires comparing information, and information never travels faster than c.
5) the behavior you get when you check for correlations cannot be explained by imagining the system "stores" all its information independently in its pieces, the system must contain some form of "holistic" information that is information about the system as a whole, but all that information appears in correlations only, not individual measurements, so the bottom line is, if the questions you ask treat the system as a unified whole (i.e., correlations), then the answers you get will treat anything you do to any part of the system as if it was something you did to the whole system-- without needing for there to be any "influences" moving around within that system.

Or if that's not clear enough, let's take another tack. There is a trivial classical form of "entanglement", which presents no bizarreness, when you reach into a box of shoes and randomly pull one out. You know that if you pull out a left shoe, the one still in the box is a right shoe. Why don't you think the one you pulled out somehow "influenced" the other one? Because you imagine the one you pulled out was always a left shoe, and the other always a right, so your learning that information in no way changed the reality-- so there's no need for any influence there. But in quantum entanglement, that thinking doesn't work-- Bell's theorem tells us that the results of certain (rather complicated) experiments you can do cannot be interpreted as simply saying one part of a system (say the electron from ShinAce's example) was always a certain way, and you are just now finding out about it (like a poker player calling their opponent's hand). Instead, it must have some higher-order information contained in the system as a whole, which produces correlated outcomes when you compare two measurements that cannot be thought of as two independent measurements. However, that does not require that there be any moving influences happening there-- only that a quantum system as a whole stores an unusual variety of holistic information that can be teased out by correlation measurements.

So that's the bottom line, separate measurements on pieces of an entangled system are not independent measurements, they are measurements on a combined system and that system will respond by revealing some correlated connections between its parts that we never see classically (classically the system will always just act like a collection of independent parts, and any correlations that are there are like the two shoes, interpretable as always having been there even before they were measured). But this surprising holistic character of quantum systems never requires that we interpret there as being any "influences" passing back and forth when we do our measurements-- we are simply doing measurements on a whole system, and it is responding as a whole system when we answer questions about correlations (but not when we only answer questions about the individual parts). Normally, true influences between parts, such as the type that could propagate information, would not satisfy that parenthetical remark.

In fact, I'm always surprised at how much hay people make out of entanglement, which is relatively subtle and rarely has measurable consequences, when they are routinely ignoring a much more obvious and significant type of holistic behavior of quantum systems-- the indistinguishability of its identical particles. This indistinguishability (which for Fermions gives us the Pauli Exclusion Principle) has daily consequences in all the solid material around us-- there are "exchange energies" in molecules and atoms that only exist because a quantum system of identical particles (or the electrons therein) must not specify which particle is in which place, the reality itself does not appear to be privy to that information. That's a classic example of "holistic" behavior, the system is not storing that information locally in its parts-- and it has enormous implications for the behavior of materials.

WayneFrancis
2010-Oct-29, 04:40 AM
"One experiment that is in agreement with the effect of entanglement "traveling faster than light" was performed in 2008. This experiment found that the "speed" of quantum entanglement has a minimum lower bound of 10,000 times the speed of light."

How does this not conflict with our laws that nothing can travel faster than the speed of light? How is it that quantum entanglement can act at large distances at instantaneous speeds and not violate anything?

As I understand it there is no information passed and no way, even in theory, how it could be used to transmit information. But I'll love to read other peoples explanations.

WayneFrancis
2010-Oct-29, 04:44 AM
ShinAce's example wasn't really quite right, but it did capture the basic spirit of the situation. It is actually very tricky ...

Ken G words it very well as always :)

speedfreek
2010-Oct-30, 10:28 AM
To supplement the excellent explanation by Ken G, perhaps I might offer an example to illustrate how, although quantum entanglement works faster than light, no information can be obtained faster than light.

We start with the classic "dual slit" experiment. Light that is directed towards those slits forms an interference pattern on a screen behind those slits, as if light is passing through both slits and acting as wave that interferes with itself. But surely, if light is comprised of individual photons, a photon cannot pass through both slits - it has to pass through either one or the other slit. But, even if we send photons (or electrons!) at the slits one at a time, we find the pattern on the screen behind gradually builds up into an interference pattern. This could be interpreted as if each photon "knows" it is part of wave.

However, if we try to actually detect which slit a photon passed through, we lose that interference pattern on the screen behind. As soon as we "know" which slit the photon passed through - which path the photon took, we lose the interference pattern and the light is seen to act as a particle rather than a wave. We cannot "know" which path the photon took without interfering with the photon in some way.

To try to get around this restriction, we can put an apparatus called a "down converter" behind the slits that splits each photon into two entangled photons, each with half the energy of the original. One of each pair of entangled photons (known as the "signal" photon) is sent directly to a detector screen, but we now have an entangled partner for each of those signal photons (known as the "idler" photon) that we can play with!

We send the signal photons from each slit directly to a detector screen, but we separate the paths of the "idler" photons, so we have two streams of "idler" photons, one stream for each slit. So, we know which path those idler photons entangled signal partners took. The "which path" information exists in the system. As a result of this, when the signal photons reach the detector screen, they show no interference.

If we keep the two streams of idler photons separate, and send them to different detectors, we end up with no interference pattern at those detector screens either. The system contains the "which path" information, so we know whether a given idler photon passed through one slit or the other - hence no interference pattern.

But - if we mix all the idler photons together again before we send them through a splitter to a pair of detectors, we lose the "which path" information. If we do this, we find that the idler photons form interference patterns on their detector screens! We have "erased" the "which path" information from the system - the information as to which path the photons took has been lost to the universe!

It doesn't matter if we erase that "which path" information from those idler photons after their entangled signal photons have been detected, we still get an interference pattern when those idlers are detected.

And if we use a "coincidence counter", a device which correlates the timings of all the photons so we can find out which hits on the signal photon detector correspond with their entangled partners hits on the idler detector, we find that, even if we erased the "which path" information from the system after the signal photons were detected, if we correlate the hits on one of the idler detectors with their entangled partners on the signal screen, those signal photons did form an interference pattern, a pattern that was hidden amongst all the hits from the signal photons entangled with the idlers for the other path!

This can be interpreted as if the signal photons "knew" we were going to erase the "which path" information, before we erased it! The "spooky action at a distance" of quantum entanglement seems to work faster than light, and seems to work backwards through time!

But of course, we cannot find out which signal photon corresponds to a given idler photon until we have detected the all the idlers and used the coincidence counter to compare the results at each detector, so the information as to the nature of the signal photons can only be found at the end of the experiment, and passed between observers at a maximum of the speed of light.

Hlafordlaes
2010-Oct-30, 11:03 AM
Another chance to strut my cluelessness on BAUT. Sigh. Anyway, questions arise from my muddled thinking:


This can be interpreted as if the signal photons "knew" we were going to erase the "which path" information, before we erased it! The "spooky action at a distance" of quantum entanglement seems to work faster than light, and seems to work backwards through time!

Is it not held that photons "live" in a sort of eternal "now" in that at light speed, time stops? Can this have anything to do with producing these intuitively bizarre results?


KenG: So that's the bottom line, separate measurements on pieces of an entangled system are not independent measurements, they are measurements on a combined system and that system will respond by revealing some correlated connections between its parts that we never see classically

I've always tried to conceptualize this as the system=wave and pieces=particles. Am I waaay wrong, and do you have any more examples to help me pretend I have some grasp of what you are saying?

kevin1981
2010-Oct-30, 12:36 PM
How about "time symmetry reversal" ? I dont fully understand it but saw it on youtube !

Ken G
2010-Oct-30, 01:46 PM
Is it not held that photons "live" in a sort of eternal "now" in that at light speed, time stops? Can this have anything to do with producing these intuitively bizarre results?
It's not required, you could do something analogous with any quantum system, even electrons moving at nonrelativistic speeds. Any system describable with a "joint wave function", a wave function that involves a superposition of various correlations, would give similar behavior-- that's the source of it.



I've always tried to conceptualize this as the system=wave and pieces=particles. That's not a bad way to think about it.

Substantia Innominata
2010-Oct-30, 10:52 PM
Because the law is that information cannot be transferred faster than light - entanglement works in such a way as to obey that law.

Nah, if anything, then the law works in such a way as to obey entanglement. ;) (And at the same time conserve GR.) I don't think the wording in the original proposition was about nor included the term 'information'. That came later. I'm nearly certain that in all of the earlier formulations one would've used the term 'interaction' instead. (At least Einstein did not use the term 'information'.) So there cannot be an interaction, between anything (be it particles, or whatever) acting faster than the speed of light. Unfortunately, that is what appears to happen in a system involving quantum entanglement. Therefore what about a quick sleight of hand?! Now we speak of 'information' instead that, clearly, must be transmitted also, and otherwise there's no 'true' interaction. It fends off the mess we were in, otherwise.

In the fundamentally local world of Einstein, there would've been no entanglement. Could not possibly. Consequently he suspected a spook -- in other words, the incompleteness of the theory of quantum physics. The later fielded requirement of (measureable!) information, that must be present so as to constitute an interaction, for me and seen in this light, always smacked, even if slightly, like a makeshift. Admittedly, this could be due to misunderstanding. :think:

Ken G
2010-Oct-30, 11:21 PM
The later fielded requirement of (measureable!) information, that must be present so as to constitute an interaction, for me and seen in this light, always smacked, even if slightly, like a makeshift. Admittedly, this could be due to misunderstanding. It's true that the role of information wasn't immediately appreciated, but it isn't a makeshift-- it's quite fundamental to seeing what is really going on. It actually gibes quite well with views you yourself have expressed, in regard to the difference between "sound" and "noise," in respect to the difference between what we could properly say is happening in nature, versus what is happening in the way we interact with and process nature.

The first thing to recognize is that physics isn't what nature is doing, it is what physicists are doing. Then we have a concept of "information", which is what the physicist manipulates to understand and predict nature. So if we don't see the information as being "out there", but rather "in here" [pointing to my head], then we regard the wave function (and the other tools of quantum mechanics) as being a mental construct we build to incorporate whatever information we are going on, with an eye to answer certain types of questions (statistically predicting observations). The wave function need not be unique to the system, for two physicists with different information and different questions (say, one interested only in one particle, the other interested in correlations over the whole system) will successfully use different wave functions for the same physical event.

When we liberate ourselves from the need to regard the wave function as being something "nature possesses", and we relax the idea that information is "contained in the system itself", we can properly see what is happening when we correlate measurements within an entangled system. The actual measurements that get done on any part of the system, when we become privy to their outcomes, changes the information we have about that whole system-- and we update our wave functions appropriately. The updated wave functions can then be used to understand the correlations we see across the system. None of that happens faster than c, because it all happens in the head of the physicist, and the distances to cross in there are very short.

fcunnane
2010-Oct-31, 05:54 PM
I have a related question, yell if you would prefer a separate post.

Is it possible to entangle information onto a carrier system? Meaning if we have light traveling at a wavelength, can we entangle other particles with the light and de-tangle them (logically) with measurements elsewhere.

Snippet: (http://en.wikipedia.org/wiki/Quantum_entanglement) Quantum entanglement, also called the quantum non-local connection, is a property of certain states of a quantum system containing two or more distinct objects, in which the information describing the objects is inextricably linked such that performing a measurement on one immediately alters properties of the other, even when separated at arbitrary distances.

I would measure the system on the receiving end which would alter the (carrier signal) light and then I would be able to extrapolate the information based on a known algorithm applied to the measurements.

Edit: keep in mind, when measuring the entanglement, we would completely remove the possibility of measuring the (carrier signal) light, but who cares if you already know what it is...

PetersCreek
2010-Oct-31, 07:46 PM
fcunnane,

When quoting a source, please cite the source.

fcunnane
2010-Oct-31, 07:54 PM
Taken care of.

AriAstronomer
2010-Nov-02, 08:01 PM
Thanks alot guys. As usual, Ken G nails it home, but speedfreak and fcunnane gave some really good insight too. I'm so happy to have found this forum, it really gives the opportunity to learn so much (especially the neat stuff!) that one wouldnt encounter in a classroom. Places like wikipedia are nice, but there's nothing like a person explaining it to you, responding to your questions.

caveman1917
2010-Nov-03, 06:40 PM
How about "time symmetry reversal" ? I dont fully understand it but saw it on youtube !

It is (as far as i'm aware) not related to entanglement. T-symmetry is the statement that the laws of physics work the same when you reverse time. Note that this statement is false, the weak force breaks time symmetry. The full symmetry that the SM obeys is CPT-symmetry. Reverse charge, parity and time, and the laws work the same. Simply put, if we were to exist in a universe in which all three are reversed, we would never notice - it would be the same universe.

dennyman2
2010-Nov-03, 09:08 PM
I think its really cool that entanglement has been proven. Its such a strange phenomena but I cant wait until we relaly understand it well.