From Newsgroup: sci.physics

JTEM <jtem01@gmail.com> wrote or quoted:
|Because photons don't experience "Distance" either. No time, no
|distance, so just as two entangled photons appear to "Magically"
|send information across vast distances, they also appear to send info |backwards in time. Because for them they are always in the same place
|and it's always the same time.

You mixed some right points with some others that are correct
but not really relevant here. - For any path an object
moves on, you can find a moving coordinate system where the
object is always at rest, but that doesn't matter for this
interference experiment.

I came up with something that shows "sending information back
in time" in a way that's much simpler:

A photon goes through a hole and gets scattered. Even though it
could hit anywhere, let's just think about two spots, +x and -x.

| | #|+x
| | .-'#|
| | .-' |
| Photon ---> : |
| | '-. |
| | '-.#|
| | #|-x

Right before the photon hits the screen, the chance of hitting +x is
the same as -x. So if you buy into locality, right before the photon
is detected, there's got to be something in the space before +x that
contains something that may let the photon show up there, and the
same for the space before -x. I marked those spots with "#" above.

If the photon actually lands at +x, it can't also land at -x
because there's only one photon, not two. So if the photon gets
detected at +x, the signal about that has to get to the area before
-x right away, so it's not detected there too. This would mean a
faster-than-light signal, basically like sending signals to the past.

But that's not a problem for the rule that info can't travel
faster than light, 'cause we can't modulate this signal from
outside to use it to send info. You could call this a "signal" or
"information," but it's a special kind of information that travels
inside a single quantum system, so I say "signal" to keep it clear.

If you want to read more about this, there's the "Afterword"
in "Introduction to Quantum Mechanics" by David J. Griffiths,
where he asks, "Why are physicists so alarmed at the idea
of superliminal influences?" at the end of section "A.2".

He does not use my simple thought experiment from above; it was
me who made this up, but otherwise he says something similar
about the consequences: It's a superluminal influence, but
can't be used to send outside information superluminally.


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