# Entanglement, can we predict the outcome?

• I
My understanding is that when two particles are entangled, then when you get a measurement of one particle, then you already know the outcome of a similar measurement for the other particle. But as far as the measurement of the other particle, I thought it would be probabilistic. But entanglement suggests that the outcome of the second particle can be predicted. What's going on?

## Answers and Replies

But as far as the measurement of the other particle, I thought it would be probabilistic. But entanglement suggests that the outcome of the second particle can be predicted. What's going on?

Once you know one of the outcomes, the other outcome can be predicted. But without this knowledge, it is probabilistic. This has an entirely classical interpretation. If a pair of gloves was hidden to you. You don't know whether the first one that you will find will be the left or the right one. But once you have found one, you know what the other one is. (I'm not suggesting that entanglement is classical, but this feature of entanglement is.)

bhobba
Mentor
My understanding is that when two particles are entangled, then when you get a measurement of one particle, then you already know the outcome of a similar measurement for the other particle. But as far as the measurement of the other particle, I thought it would be probabilistic. But entanglement suggests that the outcome of the second particle can be predicted. What's going on?

They are 100% correlated. Its like putting a red slip of paper in an envelope and a green slip in another. If you open one randomly you automatically know the other.

Thanks
Bill

morrobay
Gold Member
They are 100% correlated. Its like putting a red slip of paper in an envelope and a green slip in another. If you open one randomly you automatically know the other.

Thanks
Bill

If A measures a horizontal polarized photon then she knows immediately that B must find a vertically polarized photon ,
or in the reverse order.
While the probability of the first measurement being 50/50 vertical or horizontal , Bell showed that those values are not fixed or predetermined.
Then is it correct to say that while photons A and B are correlated their polarization is oscillating vertically and horizontally in tandem.
In tandem meaning when A photon is vertical , B photon is horizontal. When A is horizontal , B is vertical ...

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bhobba
Mentor
Bell showed that those values are not fixed or predetermined.

That's not what Bell showed. He showed the correlation had statistical properties different from the assumption it had properties at all times with the out you can have it if you violate locality.

Then is it correct to say that while photons A and B are correlated their polarization is oscillating vertically and horizontally in tandem.

That's wrong. Can I humbly suggest you actually learn what it says:
http://www.drchinese.com/Bells_Theorem.htm

Thanks
Bill

morrobay
Gold Member
My example above (oscillating) is an analogy of the entangled superposition of two photons |ψ} = (| HV} - |VH})/√2
with ill defined polarization. The very well worn out colored slips of paper/right,left gloves example is what Bell showed was false:
The EPR notion that because the polarization of photon A measurement outcome is vertical and B measurement outcome is horizontal polarization , that therefore photon B horizontal polarization must have been pre existing and independent of A measurement.
Again the counterfactual definiteness in gloves and colored paper analogy has been shown by Bell to be false regarding entangled particle correlations.
And by the way I understand the basics of the inequality: A' A' B, B' each equal to ± 1. ( AB) + (AB') + (A'B) - (A'B') ≤ 2. Approaching 2√2
I dont pretend to be above a B level participant here, but the OP has a BSEE and should be able to appreciate a higher level of replies.
The kind @vanhees71 gives on the subject.

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bhobba
Mentor
My example above (oscillating) is an analogy of the entangled superposition of two photons

It may just be me but even as an analogy I don't get it. But I am not a visual person - I think in terms of equations.

IMHO entanglement is a peculiar quantum phenomena without any classical analogy.

In fact I think its what sets QM apart from standard probability theory:
https://arxiv.org/abs/0911.0695

Thanks
Bill

vanhees71
bhobba
Mentor
The very well worn out colored slips of paper/right,left gloves example is what Bell showed was false:

I don't want this to degenerate into semantics which, IMHO, is one of the silliest things you can argue about, but my interpretation of the above is its not what he showed at all. He showed if you wanted it like the papers (ie with properties regardless of observation) then you need FTL. Of course slips of paper do not communicate FTL so in that sense its not like the paper slips. If that's what you meant then I agree - but as written its not clear to me that's what was intended.

Thanks
Bill

My example above (oscillating) is an analogy of the entangled superposition of two photons |ψ} = (| HV} - |VH})/√2 with ill defined polarization. The very well worn out colored slips of paper/right,left gloves example is what Bell showed was false
The OP's question has nothing whatsoever to do with Bell's theorem. It can be understood classically as I and bhobba said. To invoke Bell's theorem, one has to make measurements on the two particles at different angles.

And I'm pretty sure your local hidden variables would also be ruled out by Bell's theorem if you measure the two particles at the same time.

bhobba
Mentor
It can be understood classically as I and bhobba said.

Exactly. Its simply a correlation - we see things like that classically all the time - nothing wierd etc etc.

The weirdness is entangled particles in QM have statistical properties different than classically - but correlations are nothing weird.

Thanks
Bill

Truecrimson
morrobay
Gold Member
I don't want this to degenerate into semantics which, IMHO, is one of the silliest things you can argue about, but my interpretation of the above is its not what he showed at all. He showed if you wanted it like the papers (ie with properties regardless of observation) then you need FTL. Of course slips of paper do not communicate FTL so in that sense its not like the paper slips. If that's what you meant then I agree - but as written its not clear to me that's what was intended.

Thanks
Bill
I agree , and as you recently stated: ( post #11 , "Steering' thread ) if we insist that they have values before observation then you need FTL communication ( with violations) but only if you insist.
So what I intended to say is that maintaining locality - counterfactual definiteness/ EPR realism was shown invalid

bhobba
vanhees71
Gold Member
2021 Award
Exactly. Its simply a correlation - we see things like that classically all the time - nothing wierd etc etc.

The weirdness is entangled particles in QM have statistical properties different than classically - but correlations are nothing weird.

Thanks
Bill
Well, as discussed at length previously in this forum, it's not weird, it's how nature behaves according to observations. What's weird is the classical picture, because it's an approximation that is limited by the quantum behavior of matter, and that's why quantum theory has been discovered in the first place. It started with the solution of the ultraviolet catastrophe of black-body radiation as predicted by classical theory, which is (fortunately for us and all living beings!) wrong (Planck 1900). A bit later it solved the contradictions of old quantum theory concerning atomic structure, which was a semi-classical theory with some quantum ad-hoc assumptions (Bohr/Sommerfeld ~1913-1916), predicting that hydrogen atoms are disklike in shape although known to be spherical (Heisenberg/Born/Jordan/Pauli 1925, Schrödinger 1926).

bhobba
bhobba
Mentor
Well, as discussed at length previously in this forum, it's not weird, it's how nature behaves according to observations.

That's correct. Weird is undoubtedly a bad choice of words. A different kind of correlation would be a better way of expressing it. It's no more or less weird than other correlations - just different.

Thanks
Bill