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Q & A: gravity and time

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Most recent answer: 09/04/2016
It is my understanding that because of relativity, an astronaut traveling around a larger-massed object will experience time differently--time will move more slowly for him than for someone down on Earth. My question is: If this astronaut experiencing time at a different rate than we are was in constant communication with us from the moment he began travelling around this object and experiencing this different rate of time, and we talked to him while this was happening, wouldn't we be talking to him still in our current moments--our current moments being what to him is the farther away future at this point (his year corresponds to our decade, so we are in his future as soon as he begins his trip)? We wouldn't be talking to him in our past--what to him is his now? Or would we? If we wouldn't, would this show that there is some kind of overall time... that doesn't change even though he has changed...?
- Gemma Arnold (age 18)

Everybody experiences local time just the same way. The relative rates of your watch, your heart, your daily rhythm, etc. all stay the same. What changes is the perceived rates of things that are at a distance up or down in the gravitational field. (There are also the effects of relative motion, but let's keep it simple here.) So the astronaut who's "down" in some big gravitational field sees our time as going fast, and we see hers as going slow. There's no universal time with which to compare them. 

As for the signals we send her, she does see more of them, e.g. more Earth-years going by, than she would have if she'd stayed home. We see her have fewer birthdays than we would have seen if she'd stayed home. 

Nobody gets into anybody's future. But maybe it would be make for some very interesting sci-fi!

Mike W.

(published on 09/04/2016)

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