# Observer S' on a rocket vs an observer S on Earth

1. Jun 29, 2013

### 71GA

This is a basic question regarding Lorentz transformations. Lets say we have two observers - S on Earth and S' which we put on a rocket headed for Alpha Centauri (A.C) =).

If i choose 2 events like this:
1. rocket leaves Earth
2. rocket arrives on A.C

These two events clearly do not happen on a same place from perspective of an observer S. But what about an observer S'? Would he say that they happen on a same place?

I want to know this so i can use $\Delta x' = 0$ in Lorentz transformations.

2. Jun 29, 2013

### tiny-tim

hi 71GA!
yes

but remember that that only applies if S' is an inertial observer,

ie if his velocity is constant throughout

so this would have to be a spaceship that goes past earth and ac, without landing!

3. Jun 29, 2013

### ghwellsjr

Here are a couple spacetime diagrams depicting your scenario. Earth is shown in blue. A.C. is shown in red. The rocket is shown in black. The inertial observer that tiny-tim mentioned is shown in grey. The dots show one-year intervals of Proper Time for each observer/object. I picked a speed of 0.8c for the rocket to travel from Earth to A.C. The first diagram is for the rest frame of observer S on Earth:

You can clearly see the two events that you specified where the black rocket leaves Earth and where it arrives on A.C.

Now if we transform the coordinates of all the events (the dots) to the rest frame of the grey observer:

You can clearly see that your two events are at the same location. You can do the math to verify this.

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4. Jun 29, 2013

### 71GA

Nice MDs :). Thank you.