# Relativity doubts

#### meteor

Let's see if i can grasp this. Then, what happens is:
1)The clock, indeed,objetively, nor subjectively, rotates at the same velocity at rest and moving
2)If i'm moving in the same frame of reference of the clock, i will not notice nothing strange in it. It will seem to me that is rotating at the same velocity that if we are stopped
3)if i'm stopped and the clock go away from me, it will seem to me that the clock is rotating more slow than normal (but indeed is rotating at normal velocity)
Is this? Thanks for your patience

#### chroot

Staff Emeritus
Gold Member
That is correct. The clock appears to move slowly to an observer at a high relative velocity to it. To an observer riding along right beside it, nothing changes.

- Warren

#### meteor

uhmm..., interesting. So, the twin paradox i always thought that the traveler twin agess less only for the fact that he has travelled, but now i see that this is not the case. It seems that the key of all is the fact that the traveller hass to accelerate to take off ,turn around in the middle of the voyage and deccelerate at the end. It seems quite reasonable. But some people says that the twin paradox still holds true even if there's not accelleration during the trip, and i can't comprehend this.
Respect to the FitzGerald-Lorentz contraction of,say, a box, if you are moving in comoving coordinates to the box, you will not notice contraction, but if you apply the Lorentz transformations, it results that if you are in other frame of reference, you will measure that the box has shrunk. How can be this possible? It's a trick of the brain?

#### chroot

Staff Emeritus
Gold Member
Originally posted by meteor
But some people says that the twin paradox still holds true even if there's not accelleration during the trip
By definition, there MUST BE acceleration somewhere -- the travelling twin has to turn around to come home!

- Warren

#### Hurkyl

Staff Emeritus
Gold Member
But some people says that the twin paradox still holds true even if there's not accelleration during the trip, and i can't comprehend this.
In special relativity, the twins cannot meet again if there is no acceleration. For there to be no acceleration and meet again, one has to appeal to General Relativity and arrange an object for the spacebound twin to provide the right gravitational pull to deflect the twin back to his starting point... but when gravitation is invovled the laws of special relativity do not hold on the large scale, so one cannot derive the paradox.

Respect to the FitzGerald-Lorentz contraction of,say, a box, if you are moving in comoving coordinates to the box, you will not notice contraction, but if you apply the Lorentz transformations, it results that if you are in other frame of reference, you will measure that the box has shrunk. How can be this possible? It's a trick of the brain?
It's a matter of perspective. It's the higher dimensional analogue of the fact that if you look down the edge of a playing card it looks, well, paper thin, but if you look at its face, it is fairly wide. According to Special Relativity, changing your velocity is essentially equivalent to changing the direction you are looking in space-time (the geometry is different; it's Minowski, not Euclidean, but the concept behind the effect is identical). If you bring an object with you as you accelerate, it's rotating right along with you, so you see it the same, but the other observer will see it get thinner as it changes its rotates in space-time.

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