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## Homework Statement

In a particular frame of reference a particle with 4-momentum ##P_p## is observed by an observer moving with 4-momentum ##P_o##. Derive an expression for the speed of the particle relative to the observer in terms of the invariant ##P_p.P_o##

I am completely stuck on this question. Any help would be appreciated.

## Homework Equations

## The Attempt at a Solution

I used the equations

##P_p = m_p\gamma(u_p, ic)##

and

##P_o = m_o\gamma(u_o, ic)##

I tried to obtain ##P_p.P_o## in the rest frame of the observer and obtained

##P_p.P_o = m_pm_o\gamma(-c^2)## where ##\gamma## is in terms of ##u_p##

This is because ##\gamma## in terms of ##u_o## is equal to 1 in the rest frame of the observer

I'm pretty sure that my answer is incorrect. However even if it is correct I am unsure how I would be able to determine the relative velocity from it.