# Why does moving near light speed keep you young? [duplicate]

As was shown in the movie Interstellar, when you go close to a black hole or move near the speed of light in space, then your clock runs slower than a clock on Earth.

In the movie, Matthew McConaughey's character was younger than his daughter because he was moving in space.

Question: Even a person moving in space near the speed of light, but his body clock moves to same rate compare to earth. How could travelling near the speed of light keep you young? The only effect you can see is demonstrated with time and a clock, but how could it effect a person's body?

• You should read about time dilation. – ACuriousMind Jan 16 '15 at 18:10
• @ACuriousMind I got that point, but I am comparing Time with Human body. You can see time different but how you can compare human body with other human body, Even person moving speed of light but his/her body will get age out, You body cell don't know how fast your are moving.. – Satish Jan 16 '15 at 18:14
• Satish: when you are in a moving reference frame, your time actually IS moving slower relative to everything else. All physics down to the basest level runs with slower time. – Jerry Schirmer Jan 16 '15 at 18:30
• I would love to see an experiment proving this for living matter, even accelerating a small bacterium that normally would divide, say once a minute while showing slower division rates near $c$ speed would be interesting. Short of that I stay skeptical... biology is not physics, yet. – hyportnex Jan 16 '15 at 19:39
• @user31748 Time itself slows down. It would seem ridiculous for the laws of physics to know what sort of things we call "clocks" and only slow them down. If there was any bacterium that didn't slow down then we could measure an absolute velocity of something by comparing that bacterium to a clock. – or1426 Jan 16 '15 at 21:45

That's because the special relativity theory predicts that when you are standing in a moving frame that travels at very high speed $v \rightarrow c$, i.e. you're travelling to that speed, then the time experimented by every object in your frame (including you) will be different than the time experimented by the objects relative to whom you're moving at speed $v$, therefore when Matt travels at very high speed or stands near to the horizon of a black hole (which is equivalent), the time that he (and his clock) experiment goes much slower than the time experimented by his daughter on earth, that's why he gets older slower than her.

It's difficult to understand if you're not familiar with it, but this is a theory that has been proved experimentally many times, as a matter of fact, the GPS system in your phone/car wouldn't work if this theory wasn't true.

I hope that it was useful!

• Curious to know how the GPS uses theory of relativity? – dreamerkumar Jan 16 '15 at 19:11
• @manuel91 : no movement at near light-speed will keep you young. While living in the fast moving rocket (with respect to the Earth) you will feel the time passing on you exactly as you would feel on Earth. Of course, while on the rocket passes a year, on the Earth passes much more (so would say the keeper of the Earth's clock), but your body doesn't care of the keeper on the Earth says. You won't live as many years as he says, but the no. of years that pass on the rocket. – Sofia Jan 16 '15 at 19:46
• That's what I said..., sorry if it was confusing. – manuel91 Jan 16 '15 at 19:52

From what I understand of special relativity, moving at near light speed doesn't keep you young. All observers are at rest relative to their own reference frame (there is no absolute frame of reference). So, from your point-of-view, it is everyone else that is moving near the speed of light and it is they that are staying young, while you get old! :-D

(disclaimer: I'm only talking about special relativity here, neglecting gravitational/acceleration effects)