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Q & A: photon inertial mass

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Most recent answer: 11/24/2015
Q:
Given that photons have zero rest mass, can the mass of a photon be calculated (with a non-zero result) given its speed of 'c'?
- Tom Gillis (age 72)
Bangor, Maine, USA
A:

You need to know the frequency, f, in your reference frame. The photon energy is given by E=hf, where h is Planck's constant. (This relation is universal, not just for photons.) Since E=mc2, where m is inertial mass, we have m=hf/c2. The momentum p=mc or hf/c. 

You might notice that since E is just a universal constant times inertial m, m and E are really just the same thing measured in different units. Likewise E is just a universal constant times f, so f is also the same thing in another set of units! 

We have some old discussions of these issues, e.g. : 

Mike W.


(published on 11/24/2015)

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