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**[SOLVED] Relativity and the twin paradox**

**1. Homework Statement**

A friend of yours who is the same age as you travels to the star Alph Centauri, which is 4 light years away, and returns immediately. He claims that the entire trip took just 5 years. How fast did he travel?

**2. Homework Equations**

Time dialation: [tex]\Delta[/tex]t = [tex]\gamma[/tex][tex]\Delta[/tex]t'

Length contraction: L = Lp/[tex]\gamma[/tex]

where Lp is the proper length and gamma is:

[tex]\gamma[/tex] = 1/[tex]\sqrt{1-(v^2/c^2)}[/tex]

**3. The Attempt at a Solution**

I know that the twin paradox utilizes both time dialation and length contraction, but I'm stuck as to how to account for both.

Thanks in advance! =)

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