Communicating with Black Hole Time Warp: A to B and Back Again

[RajatX]

Suppose A is on a planet orbiting a black hole and B is far off such that due to time warp, every hour A experiences is equal to a year for B. Could they communicate using radio devices? Would an hour-long message from A be year-long for B? How much extra time it would take for a radio message to reach from A to B and B to A?

 

[jbriggs444]

Suppose A is on a planet orbiting a black hole and B is far off such that due to time warp, every hour A experiences is equal to a year for B. Could they communicate using radio devices? Would an hour-long message from A be year-long for B? How much extra time it would take for a radio message to reach from A to B and B to A?

The scenario is problematic. See https://www.physicsforums.com/threads/using-black-holes-to-time-travel-into-the-future.938858/

 

[Ibix]

@jbriggs444 - I think with a near-extreme rotating black hole you can manage to orbit close enough to get a high time dilation factor (I'll check later). What you can't do is what stevendaryl wanted to do in that thread, which is a free-fall flyby with a significant time dilation. And you can certainly hover as close as you like to a black hole, assuming you've got sufficient rocket power and resistance to acceleration.

@RajatX - someone close to the black hole (possibly hovering rather than orbiting) could certainly use a radio to communicate with a distant friend (although remember that the radio frequency will be redshifted by a factor of about 9,000, so very careful thought would need to go into equipment design). Their one-hour message would be spread out over a year. This is actually just a more extreme version of the general relativity correction built into GPS satellite clocks.

I don't understand the last question.