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## Main Question or Discussion Point

**Is SR "real"?**

What I mean to say is that as SR predicts time dilation, do the biological processes also slow down?

Suppose A and B are two twin brothers.one of them(A) goes for a space tour in a spaceship with a velocity comparable with c, while the other(B)remains on earth. Clearly, for B the time will run slowly in A's spaceship, while for A it will be normal in his frame. Further suppose that A returns to earth after t years(according to earth frame). Again suppose that A does not trim his hair while on spaceship and his hair has grown a bit during tour.The same applies to B. Now what I want to ask that if we compare the lengths of both brothers' hair ,whose length will be greater and why?

This question I was discussing with my friend and he says that biological processes are not affected in SR both lengths will be same. I argued that time dilation takes place in everything including biological processes and therefore length of A's hair will be smaller. However I too was confused therefore sought to help here.Hoping some comments.

Thanks