Re: relative motion and aging


"Curious" <anthonyroseuk-curious@xxxxxxxxxxx> wrote in message news:1116515987.722078.295920@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Is it true that relativity predicts that, for example in the twin
> paradox, one twin is younger than the other when re-united?

Yes.
Because one twin remains in an inertial frame, while the
other must either
- jump on an outgoing rocket (frame),
then jump again on an incoming rocket,
and then jump of the rocket onto the Earth frame again,
or
- accelerate, decelerate, turn around, accelerate, decelerate, stop.

So the traveller does not remain in an inertial frame all
the time. He will end up younger when the twins are
reunited.

> Is it true that in any general case, if one twin moves faster than the
> other, it predicts that they will be aged differently?

You must specify faster with respect to what.
If you have triplets, the one who moves the fastest with
respect to the inertial frame, will, when reunited, be younger
than the one who moves less fast with respect to the inertial
frame of the stay at home.
When during the entire trip the velocity is known as a function
of the Earth time, the total travel time (proper time) can be
calculated with a simple integral.

> Is it also true that relativity applies in this way to any two bodies
> moving wrt each other? No matter which body you pick as the 'at rest'
> body?

See above.

> If so, does relativity predict that both bodies will be younger than
> each other, depending on which you view as moving wrt to the other?

The bodies will only be said (in a very sloppy way) to be "younger
than each other" when they are *not together*.
This is the same as when two people look at each other through a
small gap between their fingers. Each person says that the other
person is smaller than himself.
Age can only be meaningfully compared when they both are
either together, or not moving with respect to each other.

Dirk Vdm


.

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