- #1

bananabandana

- 113

- 5

## Homework Statement

$$ E = -\vec{v_{obs}} \cdot \vec{p} $$

Where ## \vec{p} ## is the four momentum, and ## \vec{v_{obs}}## the velocity of the observer.

## Homework Equations

## The Attempt at a Solution

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This was a stated result in a GR course. I look through my SR notes and find that I have actually never seen a proof that ## p^{0}##

*is exactly,definitely*the energy.

I understand that the norm of the four momentum will be invariant etc...

But the phrase in the notes I have "For a free, non-interacting particle there is no physically meaningful property of "energy"" is making me uncomfortable - have I fundamentally misunderstood something?

Thanks!