Why can a photon only interfere with itself across multiple universes but not with other photons in the same universe?

[c***@public.gmane.org]

This is something that has bugged me for a while:


Why can a photon only interfere with itself across multiple universes (interference), but can't interact with other photos in the same universe? i.e, why don't two photons interact when fired directly at each other?, and yet the same photon can interfere with itself across multiple universes?



Thanks,
Layman

[Gary Oberbrunner]

Post by c***@public.gmane.org
Why can a photon only interfere with itself across multiple universes
(interference), but can't interact with other photos in the same universe?
i.e, why don't two photons interact when fired directly at each other?,
and yet the same photon can interfere with itself across multiple universes?

Great question!

Those two photons are completely uncorrelated; their state vectors are orthogonal so they can't interact. They go right through each other. To see this simply, notice that their phases will be random and their frequencies may be ever so slightly different even if they come from the same source.

[c***@public.gmane.org]

Post by David Deutsch

Why can a photon only interfere with itself across multiple universes (interference), but can't interact with other photos in the same universe? i.e, why don't two photons interact when fired directly at each other?, and yet the same photon can interfere with itself across multiple universes?

Quantum theory describes nature according to certain laws of motion.

These laws do not refer to universes or interference. Those are approximate terms. The laws predict that photons in a vacuum do not interact with each other, but that a single photon in a suitable interferometer interacts strongly with itself. Two or more photons in a suitable interferometer interact with each other as well as with themselves.

Thanks. If you could build a light source that emitted light at the same phase and frequency, would the photos interfere with each other? Or is there more to it?

[smitra-RSh1/+X/PmFmR6Xm/]

Post by David Deutsch

Post by David Deutsch

Post by c***@public.gmane.org
Why can a photon only interfere with itself across multiple
universes (interference), but can't interact with other photos in
the same universe? i.e, why don't two photons interact when fired
directly at each other?, and yet the same photon can interfere with
itself across multiple universes?

Quantum theory describes nature according to certain laws of motion.

These laws do not refer to universes or interference. Those are
approximate terms. The laws predict that photons in a vacuum do not
interact with each other, but that a single photon in a suitable
interferometer interacts strongly with itself. Two or more photons in
a suitable interferometer interact with each other as well as with
themselves.
Thanks. If you could build a light source that emitted light at the
same phase and frequency, would the photos interfere with each other?
Or is there more to it?

What happens when you get interference from two coherent light sources
is that each photon can be emitted from eiter source and these two
possibilities interfere with each other, precisely analogous to the two
slit experiment.

Saibal

[David Deutsch]

Post by c***@public.gmane.org

Post by David Deutsch

Why can a photon only interfere with itself across multiple universes (interference), but can't interact with other photos in the same universe? i.e, why don't two photons interact when fired directly at each other?, and yet the same photon can interfere with itself across multiple universes?

Quantum theory describes nature according to certain laws of motion.
These laws do not refer to universes or interference. Those are approximate terms. The laws predict that photons in a vacuum do not interact with each other, but that a single photon in a suitable interferometer interacts strongly with itself. Two or more photons in a suitable interferometer interact with each other as well as with themselves.

Thanks. If you could build a light source that emitted light at the same phase and frequency, would the photos interfere with each other? Or is there more to it?

There's necessarily a bit more to it (after all, two photons isn't one photon) but essentially yes:

http://en.wikipedia.org/wiki/Hong–Ou–Mandel_effect

-- David Deutsch



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[David Deutsch]

Post by c***@public.gmane.org
Why can a photon only interfere with itself across multiple universes (interference), but can't interact with other photos in the same universe? i.e, why don't two photons interact when fired directly at each other?, and yet the same photon can interfere with itself across multiple universes?

Quantum theory describes nature according to certain laws of motion.

These laws do not refer to universes or interference. Those are approximate terms. The laws predict that photons in a vacuum do not interact with each other, but that a single photon in a suitable interferometer interacts strongly with itself. Two or more photons in a suitable interferometer interact with each other as well as with themselves.

-- David Deutsch