# Mass of a moving proton

how is a mass of a proton moving at a high speed calculated experimentally?

phinds
Gold Member
2021 Award
how is a mass of a proton moving at a high speed calculated experimentally?
"calculated" is not the same as "experimentally". Which are you asking about?

EDIT: damn. I did what I frequently do. You have said "proton" and I saw it as "photon"

Last edited:
mfb
Mentor
There is nothing to calculate, the mass of a proton does not depend on its motion

phinds
Gold Member
2021 Award
There is nothing to calculate, the mass of a proton does not depend on its motion
I'm assuming he's referring to "relativistic mass". And yes I know that's a seriously deprecated term which is why I put it in quotes, but I don't know what else to call it. Momentum, perhaps?

OOPS: I see now he's talking about protons, not photons. Forget I said anything

Last edited:
mathman
In the LHC the proton "mass" must include the relativistic correction to make it work.

steph17
mfb
Mentor
The concept of relativistic mass was used decades ago, then scientists got rid of it because it just leads to weird consequences without helping anywhere.
"Mass" now always means "rest mass" and does not depend on the motion of the object.

@mathman: No. You just need energy, momentum, the fixed proton mass of ~938 MeV and relativistic mechanics.

steph17
Staff Emeritus
2021 Award
Rather than turning this into a discussion on what we think the OP means, wouldn't it be better to wait for the follow-up? "Calculated experimentally" makes no sense, so let's see what is actually meant.

Last edited:
steph17
i do mean the relativistic mass, there is a formula for the correction of mass for proton or some other particle moving at velocity comparable to that of the light's, how is it verified experimentally? do they measure the force exerted by the proton in some way or change in its momentum and thereby calculate the mass?

The concept of relativistic mass was used decades ago, then scientists got rid of it because it just leads to weird consequences without helping anywhere.
"Mass" now always means "rest mass" and does not depend on the motion of the object.
i didnt know that

mfb
Mentor
They measure the energy - the deflection in the dipole magnets depends on the energy and you have to control it very precisely otherwise the protons crash into the inner or outer wall - the protons follow a ring with a diameter of about 8 km, and a deviation of 1 cm would mean they get lost.

steph17