# FTL is not possible via entanglement because you need to compare both the photons

1. Jun 17, 2011

### San K

Faster than light (FTL) is not possible via entanglement because you need to compare both the photons....

or in other words

entanglement is a single system that is "spread out" (across time-space) , the two photon pair behaves as a single system in which

the two photons are within that system with opposite spins fluctuating randomly....

or in other words

there is no information transfer (in entanglement) to begin with/in the first place......(across time-space)...information transfer happens only when you compare both the photons

or in other simpler words

like in the movies....when two people have half of the map each.........both the halves need to be joined...to see the full map....

and you can join the maps only at the speed of light (or slower).....

Last edited: Jun 17, 2011
2. Jun 17, 2011

### SpectraCat

Well ... I might quibble with the details of some the language you used (there is not evidence that the polarization relationship between the entangled photons is "fluctuating randomly", and you don't "compare the photons", you compare results of polarization measurements), but yes, in general you have captured the essence of why entanglement cannot be used for FTL communication.

3. Jun 17, 2011

### San K

thanks, my friend, for validating

4. Jun 17, 2011

### ThomasT

What is singular about a two photon entangled system is the relationship between the underlying quantum disturbances. It is this relationship that's being measured by a global parameter (such as crossed polarizers). And that's what characterizes the nonseparability of the system. The exact qualitative nature of this relationship is an open question. The point of origin of this relationship is an open question. Whether or not the underlying quantum disturbances are 'communicating' ftl is an open question -- and though this is not the most reasonable assumption to start with, it's nonetheless, in some sense, a possibility that hasn't been ruled out.

But even if there is some sort of underlying ftl 'communication' involved, apparently we can't ever hope to use it to send ftl messages with. This seems to have been pretty well demonstrated via formal means, and is related to the fact that, as you indicated, our apprehension of entanglement is entirely dependent on the transfer of information via classical channels. What's really going on in the reality underlying instrumental behavior is not well understood. It might have to do simply with the degree of precision of the relationships involved -- ie., how closely the underlying quantum disturbances are related, and limitations on that. Or it might have to do with something more mysterious than that, that nobody has a clue about yet.

5. Jun 17, 2011

### San K

Thanks ThomasT. Interesting post about the possibility (of FTL) not being ruled out. Do you think even if FTL was possible (via entanglement or other channel) it would violate causality etc in any way?

I don't understand the Andromeda paradox yet.

I guess......Quantum entanglement is like a see-saw with "random fluctuations".

There is no information being transmitted via a see-saw. You cannot transmit information via any see-saw (that "takes care" of only the law of conservation of momentum between two states only).

I presume that one cannot even send any form of purposeful vibration via entanglement because the momentum/energy entanglement is so fundamental/apriori and that's the only thing (and properties similar to that) that can be transmitted faster than light

Last edited: Jun 17, 2011
6. Jun 17, 2011

### rgmcc

It can be interesting to ponder the question of whether it is sometimes useful to "communicate" "random information" over a quantum channel.

It seems there are some applications in e.g. cryptography.

Or, as a very contrived example, it strikes me that you could set up a "quantum lottery" where you would win or lose money based on a "quantum coin toss"... and you could arrange it so that your friend living on Jupiter found out the results of your lottery over an "FTL channel".

In a speculative and far-fetched sense, one might argue that much of the information we would like to communicate is generated by quantum random events "at the source"; so that's a conceptual loophole for workarounds to the "no FTL rule". (But doubtfully useful in many circumstances.)

7. Jun 17, 2011

### San K

yes quantum lottery is possible, but then the payment would happen at the speed of light.

there are many ways to do this quantum lottery simply by classic channel.....and would take same time......when payment time is included....

thus there is no "real/practical" advantage

however in things like cryptography there might be a "real/practical" advantage

agreed...the "start/origin" might be random.... however it no longer stays that way (i.e. random) once "use of free will/purposeful" action by humans comes into place.....we have the ability to "skew/bias" the probabilities, at a more macro level...

8. Jun 17, 2011

### Drakkith

Staff Emeritus
I don't see any paradox honestly. Is it supposed to be one?

9. Jun 17, 2011

### San K

thanks for posting.....neither do I however I have not read much on the andromeda paradox.

I am simply searching for any idea/logic/scenario that would say if FTL happened then causality would be violated.

Last edited: Jun 17, 2011
10. Jun 18, 2011

### ThomasT

Not necessarily.

Maybe ftl in some deeper level of reality isn't possible, and maybe we'll never have any way of knowing or demonstrating that it isn't possible.

Anyway, though it remains unresolved one way or the other, the working assumption in modern physics is that the speed of light is the ultimate in our universe.

I wouldn't worry too much about ftl stuff.

11. Jun 20, 2011

### San K

agreed, however I wonder what is the common/popular thought among physicists to explain that the spins (of the entangled pair), when detected, at moment x, would be different, when detected, at moment y.......(detected = a determinate state if forced)?

Copenhagenites would say.....let's not talk about it...

Last edited: Jun 20, 2011
12. Jun 20, 2011

### SpectraCat

I'm not sure you completely understand what it means to "detect" the spin of an entangled particle. In order to do that, you must project it into some 2-state measurement basis .. we typically use |H> and |V> for linearly polarized light, or |R> and |L> for circularly polarized light. What theory predicts, and measurement has shown, is that there is no preferred basis for measuring the spin of entangled pairs. In other words, if you have a Bell state where the spins of the particles are opposite, e.g.:

$\Psi=\frac{1}{\sqrt{2}}\left[\left|H\right\rangle\left|V\right\rangle + \left|V\right\rangle\left|H\right\rangle\right]$

Then when you make a measurement of one of the spins in any basis, the other spin will be opposite. If you measure particle A with a polarizer angle of 45º and find it in the |V> state, then particle B will be in the |H> state. If you measure particle A with a polarizer angle of 72º and find it in the |H> state, then particle B will be in the |V> state.

As to your question about "what if you measure it a time x vs. time y", the descriptions of entanglement are time-independent, so the only way to answer that question in a sensible way for ANY interpretation of QM (not just CI I think), is to say something like, "the distributions of possible outcomes of measurements are not affected by choice of the time when the measurement is carried out". A question about measurements on time-independent quantum states like, "say you measure particle A at 45º at t=0 and find it to be in state |H>, what would have happened if you waited until t=x and before measuring particle A at 45º?", simply has no meaning in any interpretation of quantum mechanics that I am aware of.

Last edited: Jun 20, 2011
13. Jun 20, 2011

### San K

good info, thanks.

To keep this limited lets just focus on linear polarizers for now:

1. does the polarizer have any effect on (skewing) the probabilities?

2. This is more of a 'technical" question.

What method, instruments etc are used to determine the V/H or L/F state (say after it emerges from the polarizer)?....since we are discussing only linear polarizers, L/F can be ignored...

3. let's consider two kinds of polarizers - Absorptive and Beam Splitter

Absorptive
now when the photon encounters the polarizer, the absorptive will only allow say V ones.

now when the photon encounters the polarizer, its state (V or H) must be decided/determined ....so as to allow only the V ones to pass through?

in other words: the polarizer stops H and lets V pass through. so at the polarizer/check-point the photon would have to come to a determinate state (V or H)?

4. after the V photons emerge from the polarizer, are they now locked into V state?

5. can polarization be between V and H? .....i.e. any angle other than 0, 90, 180, 270 and 360?

This for now, while i re-read on polarizers.

14. Jun 20, 2011

### San K

do we have any experimental proof that spin (of an entangled pair) is time dependent?
i.e. the spin, of the same photon, would be different if measured at various times, at t=0, t= x, t =y etc

15. Jun 20, 2011

### SpectraCat

First, I said time-INDEPENDENT. I will assume that is what you meant to ask about

I am not sure how this works in the fully relativistic theories of QM, but in non-relativistic QM, we take this as a postulate. In other words, we assume that the |H> and |V> basis states are stationary states. Certainly this is consistent with experimental measurements ... if you polarize a photon in a particular basis, and then measure the polarization later in the same basis, you always find it to be in the expected basis state.

For entangled photons, they are described by superpositions of the time-independent basis states ... this is the part where I am not quite sure of the rigorous derivation for massless photons, but I am fairly sure this conclusion is correct ... since the time-independent polarization states are degenerate, then the superposition is also time-independent. This is also consistent with measurements, which always observe a 50-50 probability of observing |H> or |V> for measurements on single photons in superpositions. If the superposition were time-dependent, then the probabilities for observing |H> or |V> would show a time dependence, and that has never been observed. From you can either conclude that the superposition states are time-independent, or that the time-dependence is so fast that it is averaged out on experimentally observable timescales. I assume that the latter possibility is discarded because there is no theoretical basis to expect it.

16. Jun 20, 2011

### San K

opps sorry...i mis-read you...its time-independent....well this gets interesting....specially for entangled photons/states....

the entangled photons also must have a fixed/stationary state then, before/after passing through polarizer...

there is no reason to believe that entangled photons would behave differently than individual photons..when it comes to (stationary) spin etc?

Last edited: Jun 20, 2011
17. Jun 20, 2011

### SpectraCat

Read my last couple of posts again carefully, then perhaps you will better understand why the two statements of yours that I quoted above are incorrect.

18. Jun 20, 2011

### DrChinese

Hi Sanjiv!

4. Yes, this is a pretty fair statement.

5. Yes they can be considered as being polarized at any angle based on the polarizer position. In other words, V and H are arbitrary labels used for convenience. You could also say 0 or 1, + or -, heads or tails, etc.

19. Jun 20, 2011

### San K

good to hear from you again doc

do you mean.......whatever angle the first is (relative to the plane of the polarizer or any arbitrary plane for that matter)...the other would be at ninety degrees to the other photon (...assuming vh kind of polarizer)

ok...so new question...can the entangled pairs be at 60 degrees (relative) to each other instead of 90?

i guess that is not possible....

Last edited: Jun 20, 2011
20. Jun 20, 2011

### San K

i mean to say....is entanglement so different that....

a single photon...and its (stationary) spin....is different from a entangled pair.....?