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Measuring instant changes in particles

  1. May 1, 2007 #1
    There are many experiments that supposedly prove that two entangled particles instantly effect each other when one is touched. I know I don't have to explain this since you all seem to understand it so well, but basically the two particles fly apart and we measure one's spin and the other is instantly effected beyond the speed of light.

    Well, my question is: What on earth are we using to make such a determination? Do we have some camera that is capable of measuring frames for each light second? To us, light instantly flies across the room, but we know this is not true. But to measure such a thing is no simple exercise. So during the test where we entangled two particles, watched the fly apart and then attempt to measure one, Who is standing across the room or across the ocean on another island that can say that the measure of the first instantly effected the second?

    Thanks for you explanations,

  2. jcsd
  3. May 10, 2007 #2
    Your answer in short is : Parametric down conversion, ie PDC. There is a nice paper that explains the experimental setup in a fairly easy language. Just google for it or do a search on this forum for PDC !

  4. May 10, 2007 #3
    1. One can ensure that the speed of light was exceeded by using synchronized clocks. No big deal about that.

    2. There is no proof that something traveled instantly between the two detectors. It is one possible explanation, but not the only one. It has been proven that no information can be sent faster than light by using entanglement so the whole thing could be an illusion.
  5. May 14, 2007 #4
    Thanks, I just think about how fast light truely travels, consider two people at two locations a few miles apart at most trying to detect the difference between two events and doubt very seriously they'd detect such a minute thing.

    I'm also still studying up on the PDC experiment but from what I've learned so far it seems the particles can only be measured once. Its not like they physically can be stuck in a box and someone can pop the lid open over and over on them and say, green, red, red, green... and check it on the other end to see what they are doing. At least I'm not seeing how this would be done at this point in my studies.

    If this is the case then I would question whether anything at all is changing, or if the particles are always oposite of each other after the point of entanglement and we're just placing beliefs in something that may not exist at all.

    Again, I'm only talking out loud as I"m not done reading on the experiment yet.

    Thanks again,

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