Dimensions of momentum?

Question

I am learning realitivity in college and in our class our lecturer explained four-momentum. When I was reading a book in QFT.
it writes the momentum as $p^{mu} = (E,p^i)$. Why is one of the components energy? Energy and momentum have different dimensions
or is it different in quantum field theory?

J. Murray · Answer

It is common in QFT (and relativity in general) to use natural units in which $c=1$. In SI units, the 4-momentum takes the form $mathbf p = (E/c, vec p)$ where $vec p = gamma m vec v$ is the 3-momentum.

Answered by J. Murray on February 3, 2021

Dr jh · Answer

Actually, you are correct in that energy and momentum have different dimensions. What is actually happening is that
in the book you are reading, the author is using units (called "natural units") in which the speed of light $c=1$.
The momentum
four vector can be written explicitly as $hat p = [frac{E}{c},p_i]$.
Click here for more about natural units.

Dimensions of momentum?

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