There is something I do not understand and its bothering me. If I have a twin and we are both in our 20's. Now let's assume I take a trip to space close to the speed of light, when I will come back I will be younger according to my twin (from the perspective of my twin in earth), but will I myself feel like an old man? I mean, will my biology be still as (old man as we know it) even that my trip was really short (according to my clock).

I hope I was clear enough in what I actually not understand here.

  • $\begingroup$ I've removed a number of comments that were attempting to answer the question and/or responses to them. Please keep in mind that comments should be used for suggesting improvements and requesting clarification on the question, not for answering. $\endgroup$
    – David Z
    May 29, 2020 at 3:56

1 Answer 1


There are a few things to remember here.

  • First, that your bodily activities act as your biological clock.

  • Second that time runs slower for the moving twin, and the twin who switches direction comes back younger.

Now If you have experienced to have aged by 5 years, depending on your speed your twin must have aged by more than 5 years. Your internal clocks say that all your cells have aged by 5 years, while your twin's cells have aged by more than 5 years.


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