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As a photon has no mass and must always have velocity c, if I were to shine a laser straight up (so Earth's gravity would be pulling straight back on it), what would the effect be on the photon? It wouldn't slow it down nor divert it, correct? My understanding is that it would reduce the frequency of the photon (as it's kinetic energy must be reduced, just as a classical object would lose kinetic energy). If it's the case that only gravitational redshift would occur given this trajectory (and please correct me if I am wrong there), I have two similar questions:

Would not light leaving a galaxy therefore be affected by a gravitational redshift? Is that included when physicists perform calculations regarding the expansion of galaxies away from us (and how accurate could these calculations be, given general estimates of mass distributions, etc., particularly given dark matter's gravitational effects)? If not, could it be that what we now think is a separation of these galaxies is somewhat, primarily, or even completely just light being affected by gravity?

Also, would not light then be able to escape a black hole provided it entered in precisely perpendicular to the event horizon and the black hole was not moving at all orthogonally to the photon's trajectory? (Or, perhaps more plausibly, if a photon is emitted from inside the black hole with a relative velocity of c towards the event horizon.) And then just come out the other side severely redshifted (to a frequency of almost 0 Hz)? I'm familar with the GR equations for gravitational redshift, but it also does not work inside the Schwarzschild radius (as the denominator becomes a square root of a negative number).

Apologies if this is just confused ramblings of someone who knows just enough to be dangerous.

Matthew Tanous
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For the first question: Sure, light emitted by a galaxy is affected by the gravitational redshift, but the effect is small and independent of the distance of the galaxy from us. (See also the question "Why is “gravitational” red-shift neglected in galaxy and galaxy cluster scales?".)

For the second question: Once inside the black hole, you can't emit the photon towards the horizon, because every valid direction either a photon or a massive particle could travel is towards the center of the black hole. In a sense, trying to avoid the singularity once inside the horizon is like trying to avoid tomorrow when outside.

Mike Rosoft
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The question presupposes that photons would be emitted from the hard core of the black hole - that is, that they would fly into the air and then fall back in.

The black hole is not black on the inside when looking out. On the contrary, on the inside of the black hole, the sky would be bright.

Any laser pointed into the sky would transfer less energy to the sky than it received from it, therefore the net transfer of energy between the sky and the laser (from a laser of ordinary power) would still be into the laser, not out of it.

Incidentally, this does seem to suggest that we could communicate into black holes if there was appropriate equipment on both sides - by measuring the variance in the amount of energy going into a certain point (since a laser pointed out from inside the black hole would attenuate the absorbtion of energy from outside), though the measurement would have to take place over extraordinary amounts of Earth-time relative to time in the black hole.

The effective refractive index of the internal sky of the black hole would also require a laser there of extraordinary fine focus and alignment.

Steve
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