|MadSci Network: Physics|
Richard Feynman's presentation of quantum mechanics (TFLOP, vol 3) postulates that to calculate the probability of an event that can be reached by various paths, one adds the probability *amplitudes* of paths that reach indistinguishable final states, but one adds the *probabilities* of distinguishable final states. It is the adding of amplitudes that produces interference phenomena, since probability amplitudes can have opposite signs, while probabilities are all nonnegative. This would seem to rule out any possibility of producing an interference pattern by combining the beams of two different lasers, since the final state "photon arrives at point X and Laser 1 has lost some energy" is distinguishable from the final state "photon arrives at point X and Laser 2 has lost some energy." Yet classical waves can interfere without regard to their origins, so viewing photons (or radio waves --- we don't have to limit this to visible wavelengths) as packets of electromagnetic waves would lead to the conclusion that photons should interfere, regardless of where they came from. So ... what happens?
Re: Can light from different lasers interfere?
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