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Q & A: EPR timing

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Most recent answer: 10/22/2007
Q:
The ERP effect is interesting but there’s one thing I don’t understand. According to the ERP effect:

"It is not until the moment where the measurement is made of a photon that the other aquires its characteristics: if the polarisation of one photon is +1, the other becomes -1 and vice-versa."

It says that the second (not yet observed) photon acquires it characteristics the moment the first one gets observed. But how do you know, experimentally, that this happens in that moment if haven’t observed the second photon yet? There´s no way of knowing when this happen untill you look, is there?

On another note, can’t the state of the 2 photons already be determined the moment they leave the atom? Is it even possible that an uneven number of photons can leave the atom at the same time? My feel on nature is that balance is an absolute necessity. If an uneven number of photons leaves the atom at the same time, then balance is uneven, an opposite has to come out at the same time to keep things in balance, hense they´re each others opposites,.. so you always find a "yin and yan".
- Peter (age 36)
São Paulo, SP, Brazil
A:
Nice question. The issue is whether the photons (or whatever) already somehow know their polarizations when they leave the joint source. The Einstein-Podolsky-Rosen proposal didn't settle that. John Bell's arguments showed that there were experiments that could rule out the photons knowing their polarizations ahead of time. Thee experiments have been done many times now, and it is clear that the photons do NOT know the polarizations ahead of time- unless there's some astounding conspiracy between them and the other remote random quantum proceses used to set the polarization angles of the detectors.
You also ask how the photons can decide at the same moment. That picture does raise enormous problems, because there is no such thing as 'the same moment' for remote events. In relativistic physics the sameness of the moment depends on the reference frame. One possible resolution is found in 'many worlds' interpretations, in which both possible outcomes occur for both photons, so there are no choices made and thus nothing to coordinate. Subsequent communication would be limited to those choices compatible with conservation laws. There is nothing resembling consensus in the physics community on any interpretation of these effects.

Mike W.

(published on 10/22/2007)

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