1. Dec 12, 2009

ucsaint

My query is this.

In the twin paradox lets assume that one twin Barbara keeps on moving at a relative speed of "c" away from the stationary earth twin Alex.
If I were to be able to be at both Barbara's rocket and the earth at the same instant ( meaning I could travel through space instantaneously ) would I actually see any difference in the ages of the two twins.

Which then gives rise to the question
Lets say we on earth know this planet XEON to be 20light years away from us.
Lets say we have a twin on earth Alex and another twin Barbara moving at speed of light "c" towards the planet and away from earth.
Lets assume the twins meet at an event "p" on the surface of earth. At that event both twins are of the same age and exactly same physical condition. From this event "p" onwards Barbara is moving towards the XEON, while Alex stays behind on earth.
A doctor has give both Alex and Barbara 10 years to live from event "p"

Now Alex knows that he will die in 10years and so will Barbara . I assume he can claim that there is no way Barbara can reach the planet XEON alive since she will take 20 years but she will be dead in 10years. Is it possible for Barbara to reach planet XEON alive or will she die before she reaches it ?

2. Dec 13, 2009

hamster143

In special relativity, "instantaneous travel" is ill-defined, since instantaneousness is reference frame specific.

It is quite possible for Barbara to reach the planet alive, if her spaceship is fast enough. If she has a really fast ship, she can make it there and back to Earth. Of course, by the time she gets back, at least 40 years will have passed by Earth's clock and Alex will be long dead.

3. Dec 13, 2009

pliu123123

Suppose Bob and Alex are both stationary on Earth. We know that though Barbara is moving at high speed away from us, because we know that both Alex and Barbara should describe physical law in the same way, their reference frame relative to Bob should be different:
Standing on view of Bob, Bob counts time in the same way as Alex so if Alex knows he dies after 10years, Bob also know that Alex dies after 10 years.
However, according to relativity principle,Bob ,who cant distinguish whether he moves away from Barbara or Barbara away from him, would change his reference to describe Barbara's life.

Bob can choose like this:
1.) using (ct,x,y,z) to describe a moving Barbara and using (ct,x,y,z) to describe Alex
2.) he can choose (ct',x',y',z') to describe a stationary Barbara and use (ct,x,y,z) to describe Alex
Notice that these description should deal with infinisimal region and should concentrate on one reference frame in each description only

If Bob chooses 2.), he can describe the matter easily.
Think about that if Barbara and Alex and Bob are at rest on Earth, Bob sees that they die at the same "time".
But now the 2nd method requires him to describe Barbara dies after N years,not equal to 10,in the proposed problem. And he says that Alex dies after 10 years ,the same. He cannot say they die at the same "time" anymore, but he can say they die at the same way or same format as his description.

Last edited: Dec 13, 2009
4. Dec 13, 2009

ucsaint

Thanks all for the reply, I think I have got my answers. Its basically the simultaneity of events that was confusing me.