# Probably an old question

1. Jan 6, 2005

### EMR

I am completely new to the ideas of reletivity, not to mention this forum, and I'm exploring this field independently (I'm only in first year...), so please forgive me if this is already an established theory.

I just started reading J. Richard Gott's book Time Travel in Einstein's Universe (I just got done with Michio Kaku's Hyperspace), and I'm around page 50. Gott uses an excellent example of how time slows down for somebody moving near the speed of light with a beam of light boucing two mirrors and then goes on to show how time seems to be going slower for the moving astronaut.

However, just before this, Gott shows that a charged particle moving by a magnet is accellerated by a magnetic force and that a stationary charge is accellerated by a magnet moving past it by an electric force. It doesn't matter what is moving, just how they are moving relative to each other.

Gott also establishes that there is no such thing as "being still," because "being still" is a relative thing, much as we think we're "being still" on earth, but we're actually traveling 30 km/s. So, for somebody traveling near the speed of light, is it not possible that he would see himself as "being still" and a person on earth as moving near the speed of light? With this in mind, shouldn't time be dialating for both of them and thus the net effect is no dialation at all??

I don't see Gott going any further into the subject of relativity to help explain this to me, because as I peeked forward into the book, the next sections are about why we can't break the light barrier, time as the fourth dimension and the cone diagram of past and present (which I already understand because of Hyperspace ) After that he goes into Flatland, so it seems he's getting away from the topic.

Can anybody shed some light on this for me, please? Thanks in advance

2. Jan 6, 2005

Staff Emeritus
It looks like you uderstood Gott's discussions really well. You are correct that every observer (of positive mass) observes themselves "being still". This is caled the rest frame, and each observer has one. No contraction or dilation happens in the rest frame, Newtonian physics works there, and the only coordinate change of the whole frame is the passage of time. Also the speed of light is the same in every rest frame, c.

Relativistic contraction and dilation only happen (in SR) when you have two observers in relative motion to each other. Then each of them can regard themselves as being in their rest frame, while the other one is moving. This is symmetric. And each of them sees the other's lengths contracted and the other's times dilated. Gott's mirror clock can show how this hapens. As long as both observers remain "inertial", that is not accelerated, this symmetric measurement situation continues.

It is important that the relativistic contraction and dilation ar not illusions. In a sense they are "all the physics there is" beyond the rest frame.

3. Jan 6, 2005

### EMR

If this is the case (that both observers see each others time and length being dialated and it is just not an illusion), then how can Gott say that traveling near the speed of light can be used as a time-travel device to the future? If the astronaut sees time slowing down for the person on earth, then he should get back after a 1,000 year trip (assuming he could live that long :rofl: ) to find that only 25 years have passed on earth, depending on how fast he was going. But then again, if the observer on earth sees time slowing down for the astronaut, then the astronaut would take 1,000 years to take the trip and would only have aged 25 years.

Who's really younger? Do they cancel each other out, and everybody ages 25 or 1,000 years? It's a very confuzzling concept to me.

4. Jan 7, 2005

### mijoon

This is called the twin paradox. There are several threads dealing with it on this board.

5. Jan 7, 2005

### jdavel

EMR,

This isn't a very good place to learn about special relativity, let alone the Twin Paradox that you asked about. While there are plenty of people here who understand SR and are quite good at explaining it, there are others who (intentionally or other wise) clutter every SR thread with incorrect and/or confusing drivel. The monitors are supposed to prevent this, but they don't.

My advice is find a book that's used as the primary text for an introductory course on the Special Theory of Relativity at some good university. Read it from cover to cover, and do all the problems. Then you'll understand the theory, and you'll be able to explain the Twin Paradox yourself.