# Find velocity given distance in one reference frame and time in another (relativity)

1. Feb 9, 2012

### bunnyrabid

1. The problem statement, all variables and given/known data

The pilot of a rocket wants to reach a star in 1 year. The rocket is currently on Earth. The star is 4 light years away, measured from an observer on Earth. What velocity is needed for the pilot to reach the star in a year in the pilot's/rocket's time?

2. Relevant equations

See below. Relativistic Length = Proper Length / Lorentz factor is used below, to make the substitution from Line 2 to Line 3 of L to L'/(Lorentz factor). The gamma symbol indicates the Lorentz factor.

3. The attempt at a solution

I have been given a solution, I just don't understand it. Is someone able to explain the jump from lines 6 to 7 in this solution? http://i.imgur.com/F70Oy.jpg . T' and L' indicate the proper time (time is from pilot's perspective, hence 1 year, distance from Earth's perspective, hence 4 light years). Tp is used the in first line, it is the same as T', hence where the T' comes from in Line 4.

I have been struggling with this for some time. Any help is appreciated, thank you!

Edit: I believe there is a substitution of a formula from this section of Wikipedia but I am unsure which one. Also, Wikipedia uses different symbols (in each formula, variables without a dash indicate measurements made in the stationary reference frame, unlike mine). https://en.wikipedia.org/wiki/Special_relativity#Time_dilation_and_length_contraction

Last edited: Feb 9, 2012
2. Feb 9, 2012

### Mindscrape

Re: Find velocity given distance in one reference frame and time in another (relativi

Why not just do it yourself? I'll get you started

t'=d'/v