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mass of a photon

MrDudeMan

MrDudeMan

Lifer
ive read MANY websites on this particular subject, but i was wondering if someone could paraphrase their knowledge of the subject in an attempt to help me understand it somewhat.

basically what ive read is that they have no rest mass, but that isnt the same as their moving mass...

i find it hard to believe, especially after taking physics, that a particle can have momentum but no mass. maybe i am approaching this the wrong way and i am trying to relate two separate definitions and concepts that dont belong together.
 
I'd imagine it has something to do with the Uncertainty Principle.
Either that or the fact that objects effectively gain mass as they accelerate.
 
This has nothing do do with the uncertainty principle, it is a consequence of special relativity .
First of all: A photon has no mass (and obviously no rest mass, it is never at rest since it is always traveling at c).
I think your problem is that you have studied classical physics where momentum is always connected to mass; but this is simply not correct in modern physics.
There is no "reason" for this but the fact that a photon carries momentum becomes quite obvious if you look at the derivation of the dynamical equations that are a result of SR.


 
From what I've gathered photons behave so much like waves that they are basically not a particle, and thus can only be treated using quantum mechanics and not classical physics. Classical physics talk about "regular" mass; you have to go into other physics to work with photons. I think. 😛
 
Even massless particles are affected by gravity.
Black holes changes the curvature of space-time and since photons travel in a straight line in space-time they can be trapped.
Actually the predictions of SR was first tested by observing how much our sun bends light, same mechanism.


 
high-frequency EM radiation can't be described as either a particle or a wave - it's more of a "wavicle"

and light has no mass (if it did, since it's travelling at c, it would have infinite mass because of relativity), but it does have momentum, which is described by
p = E/c

so the higher the frequency, the more momentum... but still no mass
 
The photons have mass when travelling at lightspeed. However, if you have them at rest, their mass became 0 (mass equal mass at speed 0 demultiplied by the square root of the (1 - v^2/c^2), where v is the speed at which that particle travels. So, the mass of a moving photon being known, the rest mass is zero
 
relativistic mass = rest mass * sqrt(1/(1-v^2/c^2))

For a photon, v=c, so v^2/c^2 = 1, and we get relativistic mass = rest mass * sqrt(1/0)

Division by zero makes this undefined, but the mass can be inferred from the momentum, or from E=hf=mc^2, so m=hf/c^2.
 
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