Entanglement and Observing Wave Functions

#1
I was thinking recently about a few things:
  • The common acceptance of "you can't prove a negative" in academic papers.
  • The existence of "no signalling" theorems (attempts to prove negatives.)
  • Entanglement doesn't work for signalling because observing the photon for data changes the photon, and currently they have to send the original measurements for comparison.
  • Recent quantum/gravitational wave research involves "partially observing" subjects to avoid changing them.
Pardon my lack of a theoretical physics PhD, but doesn't the idea that we can "cheat" the observation rules and look at waves without collapsing them give more credence to the ability that entanglement could be used for signalling? If the problem is that a full observation changes the state on us, yet we can prove other theorems by observing functions without changing them (as was claimed elsewhere on the board), shouldn't this bring the idea of transferring data in that way back on to the table?
 
#2
I was thinking recently about a few things:
  • The common acceptance of "you can't prove a negative" in academic papers.
  • The existence of "no signalling" theorems (attempts to prove negatives.)
  • Entanglement doesn't work for signalling because observing the photon for data changes the photon, and currently they have to send the original measurements for comparison.
  • Recent quantum/gravitational wave research involves "partially observing" subjects to avoid changing them.
Pardon my lack of a theoretical physics PhD, but doesn't the idea that we can "cheat" the observation rules and look at waves without collapsing them give more credence to the ability that entanglement could be used for signalling? If the problem is that a full observation changes the state on us, yet we can prove other theorems by observing functions without changing them (as was claimed elsewhere on the board), shouldn't this bring the idea of transferring data in that way back on to the table?
This is how Charles Seife explains it in his book Decoding the Universe:

"It is impossible to use an EPR pair to transmit information faster than the speed of light...Even though the states of particles A and B are correlated...[because] there isn't a causal relationship between the two...."A" isn't causing "B"s collapse anymore than "B" is causing "A"s collapse. There is no good explanation for why this is, it just is."

Now I don't agree with Seife on a lot of what he says because he's basically a reductionist/mechanist multiverse proponent with a bias against stuff like psi research. But regardless of whether you buy the explanation or not, this has been born out by experiment. In fact, they have actually breached the speed of light with a signal by as much as 7%, but they have been unsuccessful in sending a causal "message" from A to B or vis-a-versa.

What I think is that entanglement hints at the mechanism for psi, but is not the mechanism itself. Now it could be the mechanism ultimately, its not impossible, because when we think of psi events, they do sometimes take on a "acausal" character and don't always mimic what we understand to be "classical" information exchange. So it could be QE at its source.

However, another possibility is a whole other field lying either outside of, or at the very foundations of, classical space-time. Space-time is often talked about as being imprinted, like a hologram, with information, and as we know the space-time manifold has the capability of traveling (i.e. expand) faster than light. Or it could be something like Bohm's quantum-information potential which doesn't drop off with distance, or the signal is propagating in something similar to a "scalar" field often referred to as the "false vacuum" in mainstream texts (i.e. the region quantum fluctuations tunnel out of).

I can't quite comment on keeping something in a perpetual state of observation as providing a basis for transmitting information faster than light. It might have some relevance, but I can't really wrap my head around why it would matter.

Regards,
John
 
#3
In fact, it is still an open question whether any "collapse" of a wave function actually exists. All we know is that we're moving from a mixed, largely random, set of states toward a definite measured result. The "collapse" is not predicted by quantum theory, and still is not completely explained by quantum theory. It is merely the most fashionable interpretation of experimental events as we understand them. For example, both many-worlds and pilot-wave interpretations don't believe anything is "collapsing".
 
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#4
I was thinking recently about a few things:
  • The common acceptance of "you can't prove a negative" in academic papers.
  • The existence of "no signalling" theorems (attempts to prove negatives.)
  • Entanglement doesn't work for signalling because observing the photon for data changes the photon, and currently they have to send the original measurements for comparison.
  • Recent quantum/gravitational wave research involves "partially observing" subjects to avoid changing them.
Pardon my lack of a theoretical physics PhD, but doesn't the idea that we can "cheat" the observation rules and look at waves without collapsing them give more credence to the ability that entanglement could be used for signalling?
As I understand it, the wavefunction and entanglement are two different things. I photon for example exists as an unknown probability until it is measured; when it is, it assumes a known probability. "Weak Measurement" has been done using photons with a known
Four basic steps
The experiment has four basic steps. The first is to generate a stream of single photons with identical wavefunctions. "It is virtually impossible to measure a wavefunction with just one copy of a quantum system (i.e. one photon), this we are almost sure of," explained Lundeen. The team either used an attenuated laser beam or a process known as spontaneous parametric down-conversion (SPDC) to produce its photon stream.
http://physicsworld.com/cws/article/news/2011/jun/15/catching-sight-of-the-elusive-wavefunction
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Entanglement requires two or more particles linked in some unknown way. Sending a signal via entanglement isn't known to be possible because once an entangled system is disturbed that system is no longer entangled. Sending a signal would be a one shot deal. I think what you wrote above is also true.

If the problem is that a full observation changes the state on us,
Measuring changes the state of the wavefunction to a known state.

yet we can prove other theorems by observing functions without changing them (as was claimed elsewhere on the board), shouldn't this bring the idea of transferring data in that way back on to the table?
I don't know what you are saying. What other theorems?[/quote]
 
#5
Entanglement requires two or more particles linked in some unknown way. Sending a signal via entanglement isn't known to be possible because once an entangled system is disturbed that system is no longer entangled. Sending a signal would be a one shot deal. I think what you wrote above is also true.
It's not so much an issue of maintain coherence between the entangled pair, but the causal character of classical information exchange compared to the acausal character of a collapsed EPR system. For all we can tell with quantum entanglement, we cannot say definitely whether A collapses B or the other way around. Maintaining coherence is a difficulty no doubt, but is no longer much of an issue in simple 2-body experiments in a controlled lab setting. I don't think this necessarily rules out information exchange in a yet unknown/unacknowledged fashion, but as is, it doesn't in any way meet our criteria for a classical "messaging" mechanism.

Not sure what you were going to say about weak measurements (because you didn't finish the sentence), but I'm guessing you were going to point out that certain weak measurements can be made without "disturbing" the system (i.e. collapsing it, or whatever collapse really is). Right?
 
#6
It's not so much an issue of maintain coherence between the entangled pair, but the causal character of classical information exchange compared to the acausal character of a collapsed EPR system. For all we can tell with quantum entanglement, we cannot say definitely whether A collapses B or the other way around. Maintaining coherence is a difficulty no doubt, but is no longer much of an issue in simple 2-body experiments in a controlled lab setting. I don't think this necessarily rules out information exchange in a yet unknown/unacknowledged fashion, but as is, it doesn't in any way meet our criteria for a classical "messaging" mechanism.

Not sure what you were going to say about weak measurements (because you didn't finish the sentence), but I'm guessing you were going to point out that certain weak measurements can be made without "disturbing" the system (i.e. collapsing it, or whatever collapse really is). Right?
The quote finishes the sentence, but yes.
 
#8
Here is my understanding of the idea that information can't be passed using entanglement - based on two entangled electrons with opposite spin.

Suppose I closed my eyes and dropped one shoe of a pair in one box and the other into another, and then mailed one box off to a friend in the next town, or in Andromeda without looking at the contents. When I came to open my box, and found a left shoe (say), I would instantly know what was in my friend's box - a right shoe -, but there would be nothing magical or quantum about that! Also there would be no information transfer.

Now suppose I did the same with an entangled pair of electrons. The crucial difference is that both of us could measure our electron's spin about 3 different possible axes - X, Y, or Z. Suppose I measured in the Z direction, if my friend also measures in the Z direction, the result is analogous to the shoes above, but if he measures the spin about Y or Z he should get a spin which is plus or minus one half with equal probabilities. If this is repeated a lot of times, and all the data is collected, it will be obvious that whenever we both chose the same measurement direction (long after the particles separated) we came up with opposite spin values.

Keep in mind that even if you measure the spin about a different axis, QM requires that the answer is either +1/2 or -1/2.

This means that in some strange way, every time I made a measurement, I transmitted the direction of measurement to my friend.

The reason this doesn't count as information transfer, is that my friend has the choice of measuring the spin about 3 possible axes, and he will get random 50/50 answers on all through axes! He doesn't learn anything from the experiment until all the data is brought together - because he doesn't learn which spin value I actually measured!

That is (I hope) standard QM, but I can't help wondering if there is more to quantum probabilities than is usually assumed. For example, if people can preferentially select quantum states, this might not show up in normal physics experiments - only experiments like Dean Radin's - where he does report such an effect!


David
 
#9
The reason this doesn't count as information transfer, is that my friend has the choice of measuring the spin about 3 possible axes, and he will get random 50/50 answers on all through axes! He doesn't learn anything from the experiment until all the data is brought together - because he doesn't learn which spin value I actually measured!
What stops you both from agreeing to only use one direction of measurement on a single bit for the experiment, though? If post-hoc analysis shows that changes can be determined at all, it seems more like a problem that requires further experimentation and isolating behaviors than simply tossing up and saying its impossible.
 
#10
What stops you both from agreeing to only use one direction of measurement on a single bit for the experiment, though? If post-hoc analysis shows that changes can be determined at all, it seems more like a problem that requires further experimentation and isolating behaviors than simply tossing up and saying its impossible.
Well if you do, you go back to the boring pair of shoes situation. If I open a box and find I have a left shoe, I know my friend has the right shoe, but I haven't transmitted any information because I had no choice as to the outcome when I opened the box. The test is, suppose I sent a whole set of boxes each with one pair of shoes, I couldn't use that as a 'resource' to send a message (devised after the parcels had gone.

Once you introduce other axes, you get into the strange situation that A's choice of axes does seem to get transmitted to B, but there is still no information transferred.

David
 
S

Sciborg_S_Patel

#11
Well if you do, you go back to the boring pair of shoes situation. If I open a box and find I have a left shoe, I know my friend has the right shoe, but I haven't transmitted any information because I had no choice as to the outcome when I opened the box. The test is, suppose I sent a whole set of boxes each with one pair of shoes, I couldn't use that as a 'resource' to send a message (devised after the parcels had gone.

Once you introduce other axes, you get into the strange situation that A's choice of axes does seem to get transmitted to B, but there is still no information transferred.

David
This is how I understood it.

Desginate certain spin measurements as 0, others as 1. If I get 0, I know you got 1, and vice versa. But this offers no way of sending along an encoded bit-string, because neither of us knows anything until we measure.

Though as David said, if you could influence the measurement you got beforehand, via Psi, you would have the means to begin a superluminal transfer.
 
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