by oraclelive
 P: 25 I may not be right exactly but however i stand corrected where necessary. Twin paradox experiment explains time dilation in space time. My understanding is like this, assuming twins at 10yrs each. If one makes a space travel and the other is at stationed on earth (both being at initial reference frame). After say 20yrs, the space twin returns to earth and found out that he's younger (ie, less than 30yrs) than his twin on earth in time. Which implies that gravity slows time in space. Now what about ageing?
 PF Patron Sci Advisor Emeritus P: 2,351 It isn't gravity that causes the (time)age difference, but the speed at which the space twin travels. For the space twin to age 10 yrs while his Earth twin aged 20, he would have had to make the trip at 0.866c. I'm not quite sure about what your last question is asking, but I'll take a guess. When they reunite, the Earth twin will have aged 20 yrs and be 30 years old and the Space twin will have aged 10 yrs and be 20 yrs old.
 P: 25 Thanks for the clarity about the cause of the time difference. But the question ain't clear yet. Look at it this way, the space twin is younger when they reunite. In ageing, does the space twin look younger (facially) than the earth twin and if not, why?
Mentor
P: 40,226

 Quote by oraclelive In ageing, does the space twin look younger (facially) than the earth twin and if not, why?
Sure. That twin really is younger in the normal sense of the word.
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Emeritus
P: 2,351
 Quote by oraclelive Thanks for the clarity about the cause of the time difference. But the question ain't clear yet. Look at it this way, the space twin is younger when they reunite. In ageing, does the space twin look younger (facially) than the earth twin and if not, why?
Yes, that is what "the space twin is younger when they reunite" means, he will have only physically aged 10 yrs while his brother has physically aged 20 yrs.
 P: 25 If so, does it now mean that the speed at which the space twin travels affect the growth hormones of the body system?
 P: 18 Time in the space twin's frame of reference slows down wrt the earthbound twin's frame of reference. His clock slows down His heartbeat slows All known physical and biological functions within his reference frame slow down ...all with respect to his earhbound twin. In his spaceship, everything is normal for him. Everything is time dilated in the same way. He ages more slowly than the earthbound twin as a result of this.
 P: 25 Thanks Huttate for the much clearification. If this true in practice, it implies that space travel will actually help man slow down in age.
Mentor
P: 40,226
 Quote by oraclelive If this true in practice, it implies that space travel will actually help man slow down in age.
Yes, but only compared to those left behind. From the perspective of the space travelers themselves, they age just like usual.
 PF Patron Sci Advisor Thanks Emeritus P: 38,387 The point is that it is not just "aging" that is slowed down. For the traveler, as compared to the stay at home, time itself has slowed down. When the traveler returns he will have experienced ten years of travel- he will have lived for 10 years while the stay at home twin will have lived for 20 years.
 P: 18 Oraclelive : You must have this clear in your mind: The space traveller's ageing process does not slow down! Let me be as clear as I can. The space traveller experiences everything just as he would on earth. His seconds are seconds as he experienced them on earth. He beard grows as it did on earth. He ages as he did on earth. He is only ageing more slowly with respect to the earthbound twin. Relativity is all about frames of reference. It is essential that you understand this point otherwise you will end up confused and misinformed. To be more clear [I hope] both twins see everything as happening quite normally in their own frames of reference. It is only when we compare how time and space behaves in the two separate frames of reference with respect to each other that we notice that one twin has aged more slowly than the other.
 PF Patron P: 4,153 Huttate: You must have this clear in your mind: Nature does what it does. It doesn't know anything about frames of reference. Let me be as clear as I can. The Theory of Special Relativity is all about frames of reference and are purely a human construct that helps us understand and coordinate things that are happening to different objects and observers who are traveling at different relative speeds. To be more clear [I hope] both twins see everything as happening quite normally to themselves but abnornally to the other twin. It is only when we compare how time and space behaves in a single arbitrarily selected frame of reference that we can analyze how one twin has aged more slowly than the other. Where did you get the idea that it was necessary or advantageous to assign each twin to their own frame at the exclusion of the other one?
 P: 18 ghwellsjr: pedantic and adds nothing. I was emphasising frames of reference as this IS the starting point for understanding event separation across spacetime. The poster appeared convinced that spaceflight would lengthen his life and I wanted to disabuse him of the notion. The final paragraph is clear as it stands and to change to the 3rd person and not stress the reference frames would miss the point. I don't care what your anthopomorphic nature thinks.
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P: 4,153
Huttate: you should go back and read what you said in your previous two posts. In the first one you said:
 Quote by Huttate Time in the space twin's frame of reference slows down wrt the earthbound twin's frame of reference. His clock slows down His heartbeat slows All known physical and biological functions within his reference frame slow down ...all with respect to his earhbound twin. In his spaceship, everything is normal for him. Everything is time dilated in the same way. He ages more slowly than the earthbound twin as a result of this.
And then you say the opposite in your second post:
 Quote by Huttate Oraclelive : You must have this clear in your mind: The space traveller's ageing process does not slow down! Let me be as clear as I can. The space traveller experiences everything just as he would on earth. His seconds are seconds as he experienced them on earth. He beard grows as it did on earth. He ages as he did on earth. He is only ageing more slowly with respect to the earthbound twin. Relativity is all about frames of reference. It is essential that you understand this point otherwise you will end up confused and misinformed. To be more clear [I hope] both twins see everything as happening quite normally in their own frames of reference. It is only when we compare how time and space behaves in the two separate frames of reference with respect to each other that we notice that one twin has aged more slowly than the other.
Here's what you should have said:

Time for the space twin slows down in the initial reference frame that oraclelive mentioned in his first post. But he cannot tell that anything has slowed down because everything in his spaceship is time dilated in the same way and so everything is normal for him.

Expressions like "the space twin's frame of reference" or "the earthbound twin's frame of reference" are commonly understood to mean a frame of reference in which an observer is at rest. The fundamental tenet of Einstein's Special Relativity is that observers and objects at rest in any inertial reference frame will experience no time dilation but other observers and objects not at rest in that same reference frame will experience time dilation. This is why you need to present one frame of reference when talking about the reduced ageing of the traveling twin.
 P: 25 Yea, thanks folks for your contributions. Time indeed slows everything in spaceflight.
 P: 1,064 Since there is talk in this thread regarding how someones beared grows, hormones release timely ect, while travelling close to C, has anything even remotely identifiable as matter to the naked eye acheived this? I get that particals can travel at significant fractions of C, but molocules? I thought the "time traveling" example was an annalogy for what happens to particles.
 Mentor P: 21,648 GPS satellites travel at a high enough fraction of C for these effects to have a critical impact on their functionality.
P: 1,064
 Quote by russ_watters GPS satellites travel at a high enough fraction of C for these effects to have a critical impact on their functionality.
Oh, I didn't think of those. I supose astronauts too.

Still, there is fast and then there is .85C.

So going fast through space have no effect on the thing going fast? No forces pop-up?

That's pretty cool, its like the one free thing.

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