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I have watched the movie Interstellar and like most of the people who watch it, I was intrigued by the effects of time travel on the astronauts. After reading some articles and watching some videos, I understood that such time travel is not like the "old science fiction type of time travel" where for example you can go back to 1939 or go to 1950 while still in Earth, rather it's mostly about the duration of days or years in each planet meaning that if in Earth the days are 24 hours long, in planet "XYZ" a day might last a year. So it's mostly relative time.

With that said, my question is: when the astronauts of interstellar traveled to other planets, shouldn't they have been aging according to earth's time, rather than the local time, since they're "Earthicans"? By aging I mean biological age, appearance, death of the body cells etc.

If perhaps Interstellar is right, does that mean that if there were two twins and one of them decided to live in Earth and the other Jupiter, one of them would be getting older than the other faster?

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    $\begingroup$ You've essentially described the twin "paradox": en.wikipedia.org/wiki/Twin_paradox. And what difference do you expect to exist between two countries in this regard? $\endgroup$
    – AV23
    Commented May 9, 2015 at 17:49
  • $\begingroup$ It has also been discussed many times on this site: physics.stackexchange.com/search?q=twin+paradox $\endgroup$
    – AV23
    Commented May 9, 2015 at 17:52
  • $\begingroup$ @AV23 hmm sorry I didn't know there was a "Twin Paradox" out there. Forgive my poor understanding of the topic, I've changed it from countries to planets. $\endgroup$ Commented May 9, 2015 at 18:02

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If two twins (or clocks) decide to go live in differently strong gravity wells, they will age at different rates. It isn't based on where you were born. In particular it is not just that different planets spin at different rates or orbit their stars at different rates.

Clocks just tick slower in a deeper gravity well. Hearts beat slower. Synapses fire slower. Food is digested slower. Lungs extract air slower. Molecules move slower. DNA breaks down slower. Absolutely every single thing happens more slowly when in a deeper gravity well. That's why we say it is time itself that runs more slowly in a deep gravity well. And this happens because gravity curves time itself (as well as space).

The only reason we don't notice this is everyday life is that most of our gravity wells near here are actually pretty shallow. So we don't really notice that we age a bit slower than people out in deep space because we only age so very very very much slower than them. But in a truly deep gravity well, this could be closer to a factor of two. And if you want to fire your engines to escape an even deeper well, you might have even bigger factors to deal with. Assuming you have a larger enough object to slingshot by that you can be deep in a well and still have the tidal forces be small. Hence why they wanted a very very large black hole in the movie.

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    $\begingroup$ I don't disagree with anything you say, but I believe it's important to emphasize that the "slower" you mention several times in the second paragraph is "slower compared to clocks, etc which are in shallower gravity wells." Non-physicists and beginners tend to think there is an absolutely correct clock which we access for comparison. Also, a caution about using "deep space." No human has been beyond the moon. That's still very much in the Sun's gravity well. $\endgroup$
    – Bill N
    Commented May 10, 2015 at 0:10
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I can easily buy into gravity curving space time but to assume the twin paradox would impact biological progression is border line fiction and at best a theory. Time would certainly pass but the aging process of a human both chemical and mechanical remain in sync with it's intended modes of operation. If the deep space twin returned to earth 20 years later (earth time) having only experienced 1 hour in a large gravity well, he would have aged 20 years as a biological human at an unprecedented rate of speed. The twin would appear to be aging extremely rapidly to beings who have evolved in this hypothetical larger gravity well. Essentially, the likely hood of surviving passage through a gravity well big enough to make a significant difference is slim and then expecting a survivable biological adaption makes this paradox a fun topic to teach as an intriguing hook to a boring lesson in physics.

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    $\begingroup$ Please do not post "answers" to questions about topics on which you are not knowledgeable. $\endgroup$
    – WillO
    Commented Oct 28, 2017 at 0:54

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