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

On 6 Jan 2014, at 22:48, <[email protected]> <chris@christophe=
r-richards.net> wrote:

> David Deutsch wrote:
>=20
>>> This is something that has bugged me for a while:
>>>=20
>>> 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?
>>=20
>> Quantum theory describes nature according to certain laws of motion.
>>=20
>> These laws do not refer to universes or interference. Those are approxim=
ate terms. The laws predict that photons in a vacuum do not interact with e=
ach other, but that a single photon in a suitable interferometer interacts =
strongly with itself. Two or more photons in a suitable interferometer inte=
ract with each other as well as with themselves.
>=20
> 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 ther=
e more to it?

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

http://en.wikipedia.org/wiki/Hong=96Ou=96Mandel_effect

-- David Deutsch



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