OK I have noticed that this forum has basically turned into being all about Relativity so I got interested and started reading about it on the internet and I got Einstein's little book.

I understand the spaceship clock moving more slowly, that is easy. I understand that length contraction is necessary so the light clock is still able to tick when it is pointed in the direction of the spaceship's travel, that's easy too.

But as the guy looks out the spaceship at the clock that is "at rest", he is supposed to see it as moving more quickly? How do you know that the clock that is supposed to be "at rest" is really "at rest"?

I am just trying to understand Relativity at this point, not in any way trying to disprove it or come up with my own stupid theories.

Yay, even more.

Concerning your "at rest" issue, you might want to include more details. If you are just considering 2 objects with a constant relative velocity, either can be assumed at rest and the other in motion - but you can only analyze each object "at rest" separately.

I mean "at rest" in the same way Einstein used the embankment in his book as compared to the moving train. Maybe some guy sitting in his backyard drinking beer?

That is kind of my question, what is "at rest"? Why wouldn't the guy in the spaceship see the exact same length contraction as the guy on Earth sees when he looks at the spaceship? If he does, what does he see the Earth clock doing?

That's a very good question for funkstar et al. I will just tell you that of course the guy in the spaceship sees things for Earth length contracted just as much as Earth sees things for the spaceship length contracted.

That is kind of my question, what is "at rest"? Why wouldn't the guy in the spaceship see the exact same length contraction as the guy on Earth sees when he looks at the spaceship? If he does, what does he see the Earth clock doing?
It's time dilating. And yes, he would see the same length contraction. This is a beautiful symmetry of relativity.

"At rest" is commonly described as being in an inertial frame. Look it up if you get confused, it's a bit subtle. Basically, it's an equivalence principle: You know Newton's first law, right? It's like that. If you can't detect your motion, or feel any external force (like gravity), you're in an inertial frame.

"At rest" is commonly described as being in an inertial frame.
Just don't say that when James R is around. He'll claim you are an idiot for saying an object goes from one inertial frame to another inertial frame :D

So the guy on Earth sees his clock moving more quickly than the guy in the spaceship and the guy in the spaceship sees his clock running more quickly than the guy on Earth. Is that correct?

Yep

So the spaceship goes on a journey, does the clock in the ship and the one on Earth supposed to read the same when it comes home?

No.

The spaceship that went on the journey changed spacetime frames (by accelerating/decelerating) and therefore destroys it's symmetry wrt the earth frame.

No.
What if he goes on a journey through the sun and meets the Earth on the other side half a year later :m:

The intense temperature of the suns core will melt the spacetime around the ship. thus rendering the experiment moot. :m:

The spaceship that went on the journey changed spacetime frames (by accelerating/decelerating) and therefore destroys it's symmetry wrt the earth frame.
I fixed that for you with my journey to the sun and back example.

The intense temperature of the suns core will melt the spacetime around the ship. thus rendering the experiment moot. :m: I don't think so, I have some damn fine thermal protection outfitted on my craft.

Molten spacetime is powerful stuff.

That is pretty amazing. It was simple up until that point. Comparing what each observer sees and bringing the results together in the end.

Disregarding the acceleration-deceleration, if you were to freeze the time of 2 different velocity frames, the observers would see each other's clock as moving more slowly up until that point, but actually the clocks are keeping the SAME TIME.

Disregarding the acceleration-deceleration, if you were to freeze the time of 2 different velocity frames, the observers would see each other's clock as moving more slowly up until that point, but actually the clocks are keeping the SAME TIME.

Here we go...

Each observer sees the other guy as dilated (slower) while they are in uniform motion. There is no "actually". The only thing any observer can ever say about another, is what he observes. This is a HUGE sticking point with other members here. There is no absolute reality against which you can say what is "really" real for all frames. Can't do it.

I guess that's why they call it relativity. Go figure...

So if you freeze time and someone goes and looks at the 2 frozen clocks, he sees the same time, correct?

Ok. Hang on there.

First of all, "freezing" time in a theory that is all about space AND time leads to all sorts of problems. What you are asking is really, if you stop the earth and ship relative to each other (bring everything to a relative screeching halt) what will you see?

Important: By bringing everything to a relative screeching halt, you are matching the two frames. This is the only way you can make a "real" comparison between the clocks and see what they "really" say.

So, it depends on who accelerates or decelerates.

The one that accelerates to match frames will show less accumulated time on their clock than the one who dosen't (asymmetric spacetime trajectory). If they both accelerate/decelerate equally to match frames, the clicks will read the same.

"If they both accelerate equally to match frames, the clocks will read the same."

Yes!

Yes!!! BooYah!

Just don't say that when James R is around. He'll claim you are an idiot for saying an object goes from one inertial frame to another inertial frame :D
Well, it's just a standard. Of course, as James R rightly points out, everything is in all frames. When I say something "is in" some particular frame, I implicitly mean "at rest" in that frame.

Yes, I know you know this, but haynewp probably doesn't.

I'd like to analyze a train of humans "observing" a monkey while the monkey is "observing" the train of humans. Who is really in the zoo is a matter of relativity...

But anyway, our train travels at .9c and each cabin is .9 light seconds apart. If each human wears a clock on their forehead and each human is in his own cabin and all of the clocks are in sync according to the rest frame of the train, then when the monkey takes a picture of each human in the train, what is the time captured for each picture?

ooh ohh ahh ahh?

If you want, I can change the animal to an african grey parrot that has learned to speak english.

Better.

OK - monkey is the name of our african grey parrot, reread the problem.

"If they both accelerate equally to match frames, the clocks will read the same."

Yes!
Don't read to much into it. It isn't generally true. It's quite possible for previously synchronised clocks to match frames in the future, where this won't be true. The proper time (meaning the time elapsed on the clock) for any two trajectories in spacetime will not in general be the same, even if they do end up matching frames like described.

For instance, in the twin paradox, accelerating to the speed of the rocket returning to Earth (as it zips by, perhaps) won't undo the relativistic effects to any high degree. The "travelling" twin won't suddenly age all the years he's "saved". His clock will tick faster than yours while you accelerate, true (because in any frame, your trajectory while accelerating is curved while his is straight, and the Lorentz metric (roughly) means that Euclidially longer trajectories equal less proper time), but compared to the length of his total trajectory, this will probably be all but negligable.

Yes. I was beginning from a state in which both clocks are synchronized and in uniform motion and each accelerates equally to match frames. A rare thing indeed.

And furthermore, this "equal acceleration" would have to be from the viewpoint of the endframe, I think...

There is a confusion many people get when first encountering str, in that they think that even though the clocks were out of synch when in different frames, they will end up in synch and reading the same when brought into the same frame. As the twin paradox shows, this simply isn't so.

Once you start thinking about the length (in the Lorentz metric) of spacetime trajectories as the same as proper time, a lot of the common sense difficulties simply go away (and the analogue in GR, but on curved spacetime, makes much more sense). It's a pity there isn't a simple sketch function associated with posting here, as pictures (with the exception of Geistkiesel's, of course) tend to clarify things a lot.

His clock will tick faster than yours while you accelerate, true (because in any frame, your trajectory while accelerating is curved while his is straight, and the Lorentz metric (roughly) means that Euclidially longer trajectories equal less proper time)
If I may say so, I think that this a very nice explanation of gravitational time dilation.

That is all.