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## Homework Statement

An inertial observer O bounces a radar signal off an arbitrary event P. If the signal is emitted and received by O at times τ1 and τ2 respectively, as indicated by O’s clock, prove that the squared interval ∆s 2 between O’s origin event (i.e., its spatial origin at time τ = 0) and P is c 2 τ1τ2.

## Homework Equations

∆s^2=ct^2-|r|^2

## The Attempt at a Solution

u'=γ(u1-βu4)?? I am not even sure if I need to do a Lorentz transformation. Please give me some direction!