Momentum eigenfunctions in an infinite well

[PeterDonis]

Are you telling me that in QM , CM and SR there is no limit on how big momentum can be ?

Yes, that's what they have told you.

Surely v not being able to reach c places a limit on momentum in SR

Instead of waving your hands, do the math. If you have questions about the math, you can start a new thread in the appropriate forum. This thread is closed as the question you asked in your OP has been answered.

 

[Nugatory]

Are you telling me that in QM , CM and SR there is no limit on how big momentum can be ? Surely v not being able to reach c places a limit on momentum in SR

It does not.
The momentum of a particle moving at speed $v$ is $p=mv/\sqrt{1-(v^2/c^2)}$.

That value can be made as large as we please by choosing a value of $v$ sufficiently close to but still less than $c$.