1. ## Some questions about relativity

Well, there are some questions about relativity/the speed of light that I have.

We all know about the example of brother A and brother B. Brother A stays on earth and brother B goes off at the speed of light. When the brothers meet, they realize less time has passed for brother B.

But who's to say it's brother B that sped off? One could be perfectly justified by saying that brother B wasn't moving and it was in fact A that was moving away...or they were both moving. If any motion is relative, who knows who's really moving?

Or we could make it a bit different. Lets say you have some galaxy that somehow stays together while rotating at a significant portion of c. If brother B turns his ship around and goes against the motion of the galaxy, in my spaceship it looks like B is the one staying still (when B started, he was moving with the galaxy) and A is on the planet earth going at breakneck speed. So then I should see brother A as younger, and brother B should see A as older, but then you could say that brother B should see A as younger because A was moving away at almost c. Either way, outside the galaxy, I'm one hell of an old fart when they see me.

So how do you decide who is moving? And wouldn't you have the two brothers each thinking that they were the younger one, and that they were both the older one, at the same time? Hopefully I've managed to make some snese with my question, so can anyone help me out with how this works?

The Twins

Perhaps the most famous of the paradoxes of special relativity, which was still being hotly debated in national journals in the fifties, is the twin paradox. The scenario is as follows. One of two identical twins is an astronaut. He sets off in a relativistic spaceship to alpha-centauri, four light-years away, at a speed close to that of light. When he gets there, he immediately turns around and comes back. As seen by his brother on earth, the astronauts clocks ran very slowly, so although the trip took over eight years by earth time, the astronaut has only aged by, say, one year. So as he steps down out of the spaceship, he is seven years younger than his twin brother. But wait a minute-how does this look from the astronaut's point of view? He sees the earth to be moving close to the speed of light, first away from him then towards him. So he must see the clock of his brother on earth to be running slowly. So doesn't he expect his brother on earth to be the younger one after this trip?

The key to this paradox is that this situation is not as symmetrical as it looks. The two brothers have quite different experiences. The one on the spaceship is not in an inertial frame during the initial acceleration and the turnaround and braking periods. (To get an idea of the speeds involved, to get close to the speed of light at the acceleration of a falling stone would take about a year.) Suppose the two kept in touch with each other by flashing a light once a month, using their own calendars, so the other could keep tabs on his brother's age by counting flashes. After the initial acceleration, the two are parting at a constant rate, and each will see the other's flashes at some steady rate which will be less frequent than monthly, because the other guy's clock appears to be running slowly, and also the distance between them is increasing, so each succeeding flash has further to travel. When the astronaut turns around, however, halfway through his trip, he will be traveling towards the light flashes, each succeeding one has less far to travel, so he will see the flashes coming in at a faster rate, in other words, he will see his brother on earth to be aging rapidly. From the earthbound brother's point of view, after he sees the astronaut to turn around, he sees the astronaut to age at the rapid rate. But since the astronaut is traveling close to the speed of light, he arrives back on earth very shortly after his brother on earth sees him turn around! Thus for the brother on earth, watching the signals coming in from the spaceship, he sees them coming in at the slower rate for almost the entire trip, whereas the astronaut looking at the signals from earth sees them to come in at the slow rate for the first half of the trip, until he turns around, and after that, they come in at the fast rate-so the brother on earth ages more.

3. Another interesting perspective:

http://www.ebtx.com/ntx/ntx28c.htm

4. The thing that distinguishes them is the acceleration, I believe. On the other hand, is it possible to distinguish acceleration from gravity when inside a sealed box? Hmmm.

5. I suppose I wasn't comepletely clear with my first post, so I'll restate it: Basically, who is the one moving? How do you decide which one is younger? The fact that one is attached to a lump of metal that provides the acceleration isn't realy enough to say which is the one going almost c.

6. Originally posted here by thesecretfire
I suppose I wasn't comepletely clear with my first post, so I'll restate it: Basically, who is the one moving? How do you decide which one is younger? The fact that one is attached to a lump of metal that provides the acceleration isn't realy enough to say which is the one going almost c.
Hmm. I hate trying to answer these questions, I end up confusing myself over time. The thing is, brother B experienced acceleration when he sped up and left and when he slowed/changed direction to come back. The stationary brother didn't. I guess you could say that motion is relative, but inertia is not.

And then we get into how acceleration can be indistinguishable from gravity, under certain conditions... bleh.

7. Your point about acceleration is good, I hadn't considered that. But then again, the whole thing with time (and space and weight) dilation is just based on velocity, so acceleration shouldn't matter, should it?

8. Acceleration? What?

Really, relativity can't be applied in terms of a quantity of matter like us. It's more available at particles level. The greater the size, the less appliability (is it the word?) of relativity. Anyway, you can continue...

9. When you accelerate, you change your speed. But it isn't speed itself that matters, it is the acceleration. Acceleration uses energy, and that energy pools to mass.... blah blah blah changing metersticks blah blah blah. "Speed" is just a byproduct, in a sense...

10. Remember all, speed=distance/time
Acceleration is just the rate of increase in speed.

What I will say is that, everything is relative. That is the one thing that must be accepted to first begin to understand Physics. Also remember that the speed of light is a constant. Its speed is always that of c. This constant is not changed by velocity of the object which it leaves. Also spacetime is determined by the position that one is relative to the other object. This is the other end of the constant theory. You can never see time "go faster" than it is currently moving for you. If you are moving away from a point "let's say earth" in a space ship, earth's time will appear to be moving slower than your own, this would be the same as that of the earth observers watching you. So to answer your question, the 2 brother theory remains correct regardless the relative speed of the object the observer is on "earth". And no, you will not age slower on earth because you are moving relative some point outside the galaxy. The brother that left will age slower because he left the point you are on, and you will age faster becuse your speed remained slower than his, as you stayed on the earth.

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