Is this true? Mass always has momentum (in some frame) but momentum isn’t always allied to mass.

I think so. Light, for example, always has momentum, but no mass. (The amount of momentum is frame dependent, but can never be zero.)

13 minutes ago, Strange said:

I think so. Light, for example, always has momentum, but no mass. (The amount of momentum is frame dependent, but can never be zero.)

Cheers. That's what I was thinking of. Must an object always have momentum, even a co-moving object, because of the fact that there is no such thing as truly stationary.

Edited December 7, 20178 yr by StringJunky

Since you have posted this question in classical Physics, where bodies are allowed to be 'at rest' I think it should be pointed out that a body at rest has no momentum, but may have mass. Its mass may be modelled as' light' that is insignificant in application.

Edited December 7, 20178 yr by studiot

4 minutes ago, studiot said:

Since you have posted this question in classical Physics, where bodies are allowed to be 'at rest' I think it should be pointed out that a body at rest has no momentum, but may have mass. Its mass may be modelled as' light' that is insignificant in application.

Right. What would the Relativity case be?

16 minutes ago, StringJunky said:

Right. What would the Relativity case be?

Relativity falls into the classical case. It's QM where objects have to have momentum, and for that you can look at the Heisenberg uncertainty principle. You can never say the momentum is zero, unless you have no knowledge of where it is.

14 minutes ago, swansont said:

Relativity falls into the classical case. It's QM where objects have to have momentum, and for that you can look at the Heisenberg uncertainty principle. You can never say the momentum is zero, unless you have no knowledge of where it is.

Right. Cheers. I forgot Relativity is a classical theory. 

Edited December 7, 20178 yr by StringJunky

They both have gravity as Momentum and Rest Mass both create energy-stress despite which of these it is light has only momentum while it may not have Rest Mass still causes curvature as energy-stress under GR.

Edited December 21, 20178 yr by Vmedvil