# Time Dilation and Aging?

1. Sep 5, 2007

### microtech

Having read a whole lot of discussions about the "twin paradox" in this forum, I have a question that probably only serves to shine a bright light upon my ignorance...

Clocks running slower at near-lightspeed, OK. Sounds reasonable. I'll buy that.

But why does this mean that the space-travelling twin is also aging slower than is his Earth-bound brother? In what magical way does relativistic time dilation "slow down" biological metabolism???

2. Sep 5, 2007

### olgranpappy

the same way it "slows down" clocks, and you bought that, so you should buy this too, it's on sale.

Anyway, you are discussing an interesting fact, but that fact is not the "paradox" part of the "twin paradox"...

3. Sep 5, 2007

### JesseM

There is no objective meaning to "near-lightspeed", your speed can only be measured relative to one inertial frame or another. There is some perfectly valid frame where we are moving at near-lightspeed right now.
Just as the traveling twin has no objective speed, he has no objective "rate of aging" at any particular moment, you can pick a frame where the Earth-twin is aging faster or a frame where the traveling twin is aging faster. There is an objective amount he'll have aged between any two events on his worldline, like the event of leaving Earth and the event of reuniting with the Earth-twin. In order for them to reunite one of them must change speeds to catch up with the other--usually we assume the travelling twin turns around and returns to Earth, but you could also imagine that Earth-twin accelerates away from Earth to a speed large enough to catch up with the original travelling twin--and in this case, whichever twin changes speeds will be the one who'll have aged less between the moment they originally departed and the moment they reunite. This can be thought of as a feature of the geometry of spacetime, in the same way it's a feature of the geometry of ordinary 2D space that if you draw two points on a piece of paper and draw two different paths between them, one of which has a constant slope in your coordinate system (meaning the path is a straight line) and one of which has a changing slope (meaning it's non-straight), the non-straight path always has a greater length than the straight one.

4. Sep 5, 2007

### Janus

Staff Emeritus
It would be much more magical if it only slowed down clocks.

5. Sep 5, 2007

### PatPwnt

You got only a portion of it. It's not just the clock that slows down. It's that all of time slows down when you travel at any speed for that matter (relative to something else). The faster you go, the slower time travels for you compared to a still observer.
If I was watching you in a space ship traveling .90 speed of light from Earth, I would see you moving in slow-mo. If you looked out your window at me, you would see me in slow-mo on Earth. The difference is, you will be the one who accelerates back to Earth so you will be the one who has aged less then I.

6. Sep 11, 2007

### BosonJaw

Slow-mo? I think you should check out the Lorentz tranformaton/contraction theory.

7. Sep 11, 2007

### JesseM

Slow-mo is correct--the Lorentz transformation says that in each observer's inertial rest frame, any clock which is moving relative to that frame will be slowed down in that frame. So if you are moving at 0.99c relative to me, then in my rest frame you are moving in slow motion, while in your rest frame I am moving in slow motion.

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