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## Homework Statement

Hello everyone, thanks for reading.

This might be a little more math than physics (don't run away though!), but it's an excercise on my General relativity text :)

I need to prove that there exists an analog formula, like a*b = abcos(theta) for 3-vectors, only for 4-vectors, in which:

**a***

**b**= abcosh(theta), where

**a**and

**b**are 4-vectors, a and b are defined: a = (-

**a*a**)^-0.5, b = (-

**b*b**)^-0.5, and theta is a parameter that describes lorenz boost between the frame where an observer whose world line points along

**a**is at rest and the frame where an observer whose world line points along

**b**is at rest.

I have no idea how to work with this theta :-\

Thanks!

p.s.

**a**and

**b**are time-like 4 vectors.

## Homework Equations

**a*b**= -a0*b0 + a1*b1 + a2*b2 = a3*b3

## The Attempt at a Solution

I just tried to look for examples in the book and work with the definition... But I got nowhere :-\