1. Jul 18, 2009

### supreabajaj

Can anybody please tell me about the TWIN PARADOX and that why does a person's internal clock run slower in space as compared to the person who's on earth??

2. Jul 18, 2009

### A.T.

http://en.lmgtfy.com/?q=TWIN+PARADOX" [Broken] and if you have specific questions come back and ask.

Last edited by a moderator: May 4, 2017
3. Jul 18, 2009

### Hermit

I agree with A.T., but I only wanted to say that this sentence is false : in special relativity, you must set the system of reference.

4. Jul 18, 2009

### Fredrik

Staff Emeritus

You only have to specify the coordinate system if you're talking about coordinates. He asked about an actual measurement by an actual clock. In the mathematical model, this corresponds to the proper time of the curve in spacetime that represents the clock's motion, and proper time is a coordinate independent property of a curve.

5. Jul 18, 2009

### Hurkyl

Staff Emeritus

Brief answer: the twin paradox is an famous common error people make when they start pondering the idea of an observers frame of reference, but haven't yet learned to pay attention to the difference between inertial and noninertial frames.

(There are some variations that arise from other kinds of errors, but the above is the most typical version)

6. Jul 18, 2009

### supreabajaj

But what I want to know is that why does a person's internal biological clock run slower when he is in space?? And that why does a clock attached to the non-inertial frame of reference ( ie. the spacecraft) lose time?? And even if it does, what has it to do with the aging process?? Shouldn't time run at equal speeds at all the places?? Please help me guys...

7. Jul 18, 2009

### atyy

But what is time? In physics, we define quantities by saying what physical operations we do to measure them. To define a time we use a clock made of light. It turns out that light (or the electromagnetic field) has some strange symmetries, which make "time" as defined by light clocks run fast or slow.

8. Jul 18, 2009

### Hurkyl

Staff Emeritus

You have to understand what "runs slower" means.

The biggest, most important thing to understand is that there is no absolute notion of time -- you can't ask the universe "what time is it?" and have it tell you "12:06 AM, EDT".

It doesn't make any sense to say things like "his clock is running slow". You need to understand why this is nonsense before you can understand anything on this subject.

The only thing we can do is to compare different clocks. (And unless they're right next to each other, we need to devise some protocol for making the comparison)

What would be correct to say is that the Earthbound observer chose some specific method to compare his clock to the spacebound clock, and found when using this method, the space clock was running at a slower rate.

As for why they had aged differently when they met back up, that's simply because they took different routes through space-time, and the space-bound twin's route was shorter in duration than the Earth-bound twin's route.

(This is very closely related to the fact of Euclidean geometry that a straight line is the shortest distance between two points -- any other path between two points is longer)

9. Jul 18, 2009