This is the most common misconception ( assuming I understood what you mean)
The key point to grasp the theory of relativity is the concept of symmetry

Newton and his maid(his word) gave for granted the existance of a hierachy of observers
A ship is in motion relatively to the earth
The earth is in motion relatively to the sun
The sun...

Newton thought that this chain must have a beginning
An entity at absolute rest (God)

This intuitive but false assumption made classic mechanics a bloody mess, despite of its success, which Newton himself and the others after hime , were aware of

Einstein cleaned it up claiming that all observers are equivalent thus he wrote a set of equations which are valid for any observers

Einstein's logical reasoning entails a non intuitive but true conclusion : There is not an absolute time

Coming to your doubts

The twin on the earth claims that the time on board flows slower
However also the pilot claims that the time on earth flows slower than the time on board
Otherwise you would break the principle odf symmetry

That's wrong. Acceleration can be measured, speed can't. So you can say very explicitly who is accelerating and who isn't. That's the definition of inertial system: coordinate frame and velocity is given, NO acceleration or whatsoever. Thus, the person who stays behind is always in the same inertial system, while you (the traveler) changes them by accelerating.


That has nothing to do with the velocity but with gravity and is again an effect of general relativity. "Time moves slower in space" is not correct (where did you get that from?). It doesn't matter if I'm in space or not, what matters is the curvature of the fourdimensional manifold - and now again we're getting too detailed for this forum. Sorry.


That's the problem we're discussing. It has nothing to do with speed itself, though (see our discussion).

Read Error's answer, it is quite a good one I think ^^.

@Alberto: It's true what you say but I don't get if you're trying to resolve the problem, you're merely restating it - or did I miss something?

well first of all I dont know whether everyboy here is really convinced :

The twin paradox is not a paradox but the plain truth
The pilot returning on the earth is really younger than his twin brother

Are you asking why is it ?

Nobody can really grasp this claim being something beyond our experience

It is not possible to provide an intuitive explanation just a logical explanation is possible , namely :

The symmetry of the observers

At a first glance all of us assume that an observer on the earth is somehow different than an observer on a train

However if you figure out two starships in the deep space there is no way to distinguish them
You can just claim that they are in a relative motion

If the two starships observes the same event, for example a super nova then they can meeasure a differnt duration of the event
However their measures must be symmetrical otherwise you could distinguish the two observers

There is one only way to make the symmetry of the observers compatible with the speed of light, being a costant :

The time on board of the two starships must flow in a different but symmetrical way
The identity is of course a special case off symmetry

In other words

The first pilot claims : the time on board of the other ship flow slower than my time
The second pilot claims : the time on board of the other ship flow slower than my time

It is not something that we can understand it is something that we must accept

Alberto, the twin-paradox actually consists of two paradoxes.

The first is the one you've stated, and formulated as "How can two twins age differently?". This one can quickly be understood -- it follows from the fact that the speed of light is a constant that has the same value to every observer. If you accept that, all the equations follow and this paradox is nothing but a different "line-length" in spacetime.

The second is trickier to solve, and is the one we're talking about. It is as such: "If all observers are equal, how come one ages MORE than the other"?
Even from your last statement:



that paradox is not resolved. Quite the opposite, in fact! The quote implies that BOTH twins still have the same age -- only that they will (somehow) seem younger to the other guy. But that is *NOT* what happens: Twin A is OLDER than Twin B. But how can that be if we've stated that every observer is equal, or everything is symmetrical, as you've stated it?

The reason for that is that in order for the two twins to meet again, Twin B had to turn around, and thus, he "changed inertial systems". Due to that, the problem is very easy to calculate for Twin A (on earth), yet difficult [but not impossible] for Twin B (in spaceship). But hearing "all observers are the same" suggests that the calculation should be the same, though it isn't for the reason given above.



I hope that cleared that up?

Haha,
I belive there is a way to move as fast as light.
But when you have a ship that flys as fast as light, it won`t take long till you crash something
But it could be that the ship would just fly throw the planits (and so on), becose it`s so fast.
Jest like the light that flys throw us all the time, I meen that dosn`t hurt.

You mean neutrinos? They're slower than light...

You still dont grasp the principle of symmetry

B is younger than A according to the A time
A is younger than B according to the B time

In other words
Should the twin on earth (A) reaches his brother (B) on board on the star ship then he would find that his brother is older than him

Anyway I was right to suppose that some people assume that the twin paradx is a... paradox
Nope it is not

The ultimate confirmation of the theory of relativity came from the spatial missions
Before that , a minority of great scientists still believed that the relativism of time was, so to speak, a sort of mathematical trick

I quote by heart

A shuttle remained in the space about 1 year time, travelling at an average speed v = 40.000 km/hour
On return on the earth the clocks on board were t = 3.6 sec late , in compliance with the theory
So the astronauts (if any) were 3.6 sec younger according to the earth time

I would suggest to read the Bertrand Russel's book
It was written before the spatial mssions
Russel said that only the future spatial mission can tell the last word on this question

The last word was : Time is actually relative

This is just avoiding the issues. "B is younger in A time"? The paradox is explicitly stated in a way everybody can understand as to avoid using additional parameters that make comparisions impossible [of course, only because it is possible to state problems that way doesn't alway mean that you can answer them as such -- but in this case, it should be possible].

The question is: Assuming both twins die after they lived the same proper time, who is closer to death when they meet again [in other words: Assume both do exactly the same things afterwards, and die a natural cause (=old age), who dies first]? A physically better way to say this is: What twin had more proper time pass (which equals age)?

Thats a clear question, and it has a clear answer. It is: Twin A stayed in a inertial frame the whole time. Things are easy to calculate in his system, and we find that Twin B will be younger than Twin A (since more proper time passed). Twin B is not staying in an inertial frame the whole time - things are difficult to calculate in his frame. But there really is no need to, since we already got our answer earlier.
The actual, easy, straight answer to all of this is: The twins age differently since their pathes are different in spacetime, and thus have different lengths.


Saying "A time" or "B time" isn't wrong, but it isn't helpful, either. It's good to establish that there is no such thing as absolute time, but we cannot really say anything about A's age in B-time. In other words, your statement:



is correct if Twin B would be moving in one direction at the same velocity forever. But the essence of the twin-paradox is that he's not, he went back, accelerated and thus "changed inertial frames".


EDIT: I read your post again, and now I'm not sure what you're arguing, to be honest. It seems you're stating that there is a paradox* sometimes, yet that there isn't at others.


* - for the sake of the argument, I've called it "paradox", even though we've already established that it isn't one. But even that seems weird, because, as I've stated, the "twin paradox" actually refers to two things. Which are we arguing about? Who can even tell...

The twin on the earth dies first
This is what happen everyday in particle accelerator

I don't get Alberto's arguments either.


That's wrong in general and depends on how A reaches B.

I've another riddle for you (rather easy):
Suppose you fly in a spaceship at a substantial fraction of c towards a human colony on planet Blorb. Blorb has a moon. On the moon, there live the evil Zarg. The constellation is as follows:
(Blorb) ---- (Moon) ---------------- <You|= ----
Your sensors detect an energy outburst on the moon and then you detect an explosion on Blorb.
Have the Zarg caused the explosion? Do you attack them?