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Einstein's General Relativity says gravity warps spacetime. Consider a hypothetical scenario:

  1. A person travels into space from Earth.
  2. He landed on a different planet in some far off galaxy where time runs slower than Earth. 1 hour on that planet is about 7 years on Earth. The person does not know anything about time dilation or relativity, so he's not aware that gravity slowed down the time.

I was wondering how that person's mind will perceive time. Does the brain think that lot of time has passed but in reality only 1 hour passed? Or is it like, the brain also slows down and adapts to time dilation?

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This is a common confusion thinking that there is such thing as a "real" amount of time. Time literally runs slower. The brain will think that an hour has passed, and an hour will have passed at that location. Saying "in reality 7 years passed" is incorrect. "7 years passed on Earth" would be correct.

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All physical processes will slow down in that gravity well; all clocks will run slow. Since the brain contains physical processes (chemical reactions, electrical impulses, etc.) these will run slow too and for this reason, a person in that gravity well who looks at their wristwatch will not be able to detect the fact that it is running slow, because their brain is too.

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    $\begingroup$ Isn't it more correct to say all processes will slow down relative to an observer not in the gravity well or in a different location within that well, but not in any absolute sense of slowing down? None of these three observers will notice any difference in the passage of their time. $\endgroup$ Jul 23, 2020 at 20:28
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Our brain is a bad device to measure time. Depending on several conditions we feel that time passes slowly or fast. And it happens in a gravity well or not.

That is why clocks were invented, and small differences as that caused by gravitational wells need very precise clocks to be detected.

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