If an entangled pair of photons are measured they both display the same polarity. Why is this considered entanglement and not simply a coincidence that the photon was polarized by the same crystal? My question is in regards to FTL communication, although I know that many consider FTL impossible, I think like an engineer, not a physicist. Maybe thats why I'm struggling to understand entanglement as something spooky or connected. I'm on a mission to either understand why it's not possible or prove it is possible to achieve FTL comms.
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1Imagine that Bob and Alice have a photon A for Bob and B for Alice. A and B are entangled. If Bob measure its photon just few instant before Alice and he gets a given measure, he can deduce what Alice will measure at the same time (or few moment after) because they are entangled. But he can't choose what he will measure, and thus he can't choose what Alice will measure. So they can't communicate with this – StarBucK Jun 10 '17 at 23:33
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You could follow the references given there https://en.wikipedia.org/wiki/No-communication_theorem – Jun 11 '17 at 01:00
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1Use of entanglement is not really different in quantum mechanics and classical physics. In everyday life if you know Jim and Jack are twins, and somebody tells you that one of the twins went to New York, if Jim enters your office you immediately know that jack is in New York – anna v Jun 11 '17 at 03:12
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Understanding entanglement and understanding why FTL communication is impossible are two entirely separate things.
If you want to understand why entanglement is not "simply a coincidence", read this question and my accepted answer thereto.
If you want to understand why FTL communication is impossible, you need to study the special theory of relativity. The bottom line is that if I can send you information FTL, then at least one observer will say that you received the information before I sent it.
Finally, once you fully understand both quantum entanglement and special relativity, you might come to believe that, despite all that, quantum entanglement appears (paradoxically) to create an avenue for FTL communication. (You would not be the first to fall into this trap.) If that occurs, the remedy is to write out very clearly, specifically, and step-by-step the protocol by which you think you could use entanglement to communicate FTL. Probably the mistake will jump out at you in the process; if not, your writeup will at least allow others to pinpoint the exact locus of your confusion.
WillO
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Yes, the two photons are mathematically entangled. Experimental results also match the mathematical predictions. However, no one has been able to prove (or disprove) that the two particles are physically entangled. There are other possible ways that the experimental results can possibly be explained. Those ways are termed loopholes. That means that entanglement is considered real and anything else that can describe the same experimental results needs to be ruled out.
In any case, entanglement seems to be spooky and you are among numerous people who get baffled by it.
Reality in physics sometimes beats common sense. But, slowly, with time, common sense catches up with scientific/mathematical explanation of the reality. And thus the reality does not remain against common sense any more. In other words, our common sense evolves. Good example of this evolution is theory of relativity (special, and general). They both appear to evade common sense, but then people start understanding them with their sense with time.
Quantum entanglement on the other hand, has not synced with common sense in spite of being baffling for 100 years. Therefore I personally think that entanglement has not been scrutinized as much as it has been hyped.
With utmost respect to mathematics, I would say that quantum entanglement will likely turn out to be a notorious case of mathematical camouflaging.
I found this recent article that may shed some light on classical possibility of entanglement - https://www.nature.com/articles/ncomms14829
kpv
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Thanks all for the help, I suppose my theory on how it could be used for FTL comms, is that: if you can force an attribute such as polarity to one of the entangled pair without a measurement or by simply using a polarizing film to wait until you get one photon of the pair polarized with a top spin then take a measurement of another attribute on that particle(alice). You would then know two attributes of particle (bob) while only taking one measurement from alice. – Kyle Dixon Jun 12 '17 at 00:48
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@KyleDixon: Per current understanding of entanglement, You can not "force without measuring and without breaking the entanglement". So, even if entanglement collapsed at FTL, it would not have been possible to transmit information via collapse. So, FTL comm is out of question, you may keep trying various combinations and will not be able to succeed. The article I have pointed to, is meant to indicate that the current physical understanding of entanglement is likely flawed even though the quantitative formulation is correct - hence "mathematical camouflaging". – kpv Jun 12 '17 at 01:41
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@kpv appreciate the clarification. I'm new to the world of quantum physics and am learning quite a bit. "Will not be able to succeed" - my favorite words. You are most likely correct and I know there are a lot of people out there working on exactly this. My ultimate goal is to lower satellite ping times, and chose this as a first option to look at. I've been experimenting at my office with UV lasers and if I make any world-changing discoveries I'll let you know ;) – Kyle Dixon Jun 12 '17 at 08:24
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You cannot send information faster than light with entangled particles (photons or otherwise) because the person doing the measurement on one of the particles cannot use their measurement to send information to the person making the measurement on the other particle.
What you can do is use the emitter to send information to one of the people doing the measurement. But that information will travel no faster than the speed of light.
The reason you cannot use the correlation between the entangled particles to transfer information between the measurement devices is that you cannot tell in advance what is the outcome of the measurement, that being one of the defining features of quantum mechanics.
As to what's the difference between entanglement and classical correlations, I'm sure this question has been answered elsewhere. It's what Bell's theorem is all about and I think it's unnecessary that I repeat this once again.
WIMP
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