# Stress energy components as a 3 or 4 density?

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1. Jan 26, 2016

### Ryan Rankin

I usually come across expressions for the Stress energy tensor showing them as three densities (normally over a space-like slice.
In wikipedia's article on the spin tensor, they clearly (it does not appear to be an error) both write about and express the components of the stress energy tensor as four-densities. Is the three density just an approximation then?
For example:

I usually come across this as an integral over three-space.

2. Jan 26, 2016

### Orodruin

Staff Emeritus
Wikipedia is wrong. In fact the statement itself makes no sense whatsoever. You cannot get momentum at time t if you have integrated t out!

Also, please note that it is customary (and required!) to link to your sources. Others should not have to go through the process of finding something you talk about to verify what you have said.

3. Jan 26, 2016

### Ryan Rankin

https://en.m.wikipedia.org/wiki/Spin_tensor
Apologies.
Thank you for your response, originally I had thought time would necessarily be explicit still as a limit of integration