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Trying to be clear on EPR

  1. Feb 20, 2009 #1
    Trying to be clear on EPR....

    Hi - I've been reading this article on quantum physics and I'm getting a little lost.... I don't know anyone I can ask about this stuff - particularly since most of the math is still a few years away for me and I'm an accountant, not a physicist.... if anyone has a minute, would you maybe take a look at this and let me know if I'm anything like getting it? It's the ERP that's got me stumped....

    That, or recommend a place to start reading and a course of study to get started on (I'm at McGill University as an adult in, you guessed it, accounting)

    Thanks very much from a total stranger who probably shouldn't keep reading physics before bed (it's worse than donuts!)


    From reading: http://library.thinkquest.org/3487/qp.html

    ERP It's an experiment proposed by Einstein to disprove the weirdness they were seeing in the behaviour of quanta - the whole wave particle deal, and the reason for his quote "God does not play dice". Only, the experiment should work and doesn't. Plenty of people make it sound like it's obvious but it isn't to me... let's see: These particles can be measured in two ways - momentum and location, but not at the same time. Because you have to stop them to measure their location, and so you can't get their momentum because you've stopped them. So, Einstein proposed - and 50 years later they had the tech to test it - take two quanta (they move in pairs), separate them and send them off in different directions. Then, stop one and measure it's location, at the same moment you measure the momentum of the other and you have the info for the one you've stopped. So far I understand it. It should work, it makes sense. But then it didn't work. I think because they behave like waves, which are by nature unmeasurable and if you stop one of two things causing a wave it changes the shape of the wave - like two pillars in a river - Ok, so somehow it didn't work because changing one really does affect the other, and instantaneously, which according to relativity is impossible since it requires the two separated particles to communicate/affect each other faster than the speed of light.


    Last edited by a moderator: Apr 24, 2017
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  3. Feb 20, 2009 #2
    Re: Trying to be clear on EPR....

    Hi Tammy

    i am just learning this stuff as well. In a course I just took ERP was described this way.

    Quantum Mechanics predicts a violation of the principle of locality. It says that information can be sent instantaneously without any transitional movement through space. ERP felt that no true theory of Physics could tolerate instantaneous communication. Information should travel at some velocity.

    Their objection appeared in the 1930's but was not resolved for another 30 years. Bell came up with an experiment that could distinguish locality from Quantum non-locality. (The experiment is amazingly simple.) Some years later, Bell's experiment was actually done and it was found that ERP was wrong and QM is right.

    The phenomenon that ERP and Bell addressed was the entangled spin of two particles e.g. two electrons. There are states of these two particles in which niether have a definite spin but once the spin of one of them is measured the spin of the other is known with certainty. This is true even if the particles are on opposite sides of the Universe. This is non-locality and this is what bothered ERP. But it is right and Physicists accept it today.
  4. Feb 20, 2009 #3


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    Re: Trying to be clear on EPR....

    EPR is usually discussed in connection with Bell's Theorem, which stirs up heated discussion here sometimes. In fact, there are a couple of Bell's Theorem threads going on right now!

    For a simple introduction to Bell's Theorem / EPR and its significance, try this page by DrChinese who posts here regularly:

    Bell's Theorem with Easy Math

    [added] Ah, I see I just barely beat him to the punch...
    Last edited: Feb 20, 2009
  5. Feb 20, 2009 #4


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    Re: Trying to be clear on EPR....

    Welcome to PhysicsForums, Tammy!

    First, you are not the only person to have difficulty with this. Remember, we are talking about Einstein-level material.

    Second, it takes reading about this a bit to get the hang of it. Don't go to just one source, I would recommend going to several. There are 3 main papers regarding EPR. The first is EPR itself, 1935, in which the so-called EPR paradox was introduced. The second was Bell, 1965, in which Bell's Theorem is introduced. And finally the experimental realization of the ideas of EPR and Bell, by Aspect, 1981. There have of course been thousands of other papers and variations on the experiments, but these 3 will get you pretty far.

    If you want to see a simple version of these discussed, you might also consider a page I put up for this purpose: Bell's Theorem with Easy Math. If you understand basic probability theory, you can follow this. It also gives a short history. By the way, the reference you provided refers to momentum and position (which is discussed in EPR); however, most experiments relate to particle spin - usually light particles (photons) which are easier to work with in many respects. Spin relates to polarization, which is the same polarization attribute used to make polarized sunglasses work. The rules for polarized light are simple and have been known for 200 years, discovered by a guy named Malus. This is the "cosine square theta" rule, where theta is the angular difference in 2 polarizer settings. Shortened to "cos^2(theta)" often.

    I hope this helps.
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