Subject: Can light from different lasers interfere?

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?