1. I'm not sure if this goes here or in the relativity subject but since a photon is a light quanta I picked this topic.

My problem is I'm told photons have no mass and momentum is mass x velocity. If that is so light has no momentum and therefore no energy according to E2 = (mc2)2 x pc2.

Can anyone tell me what's "the quantum way" to get light's momentum?

P.S. -- This is what you learn in high school although it's wrong for it tells you light is "energyless".

2. Originally Posted by tcrosa27
I'm not sure if this goes here or in the relativity subject but since a photon is a light quanta I picked this topic.

My problem is I'm told photons have no mass and momentum is mass x velocity. If that is so light has no momentum and therefore no energy according to E2 = (mc2)2 x pc2.

Can anyone tell me what's "the quantum way" to get light's momentum?

P.S. -- This is what you learn in high school although it's wrong for it tells you light is "energyless".
Photon - Wikipedia, the free encyclopedia

Look under physical properties for definition of energy and momentum.

3. Originally Posted by tcrosa27
My problem is I'm told photons have no mass and momentum is mass x velocity.
That is true only for massive objects, not for photons. The photon momentum is defined as

4. Originally Posted by Markus Hanke
That is true only for massive objects, not for photons. The photon momentum is defined as

h is plank's constant, lambda is wavelength but k is what? never mind it's the wave vector I just found it. and I just also found that h with a dash above is the reduce plank constant too.

5. Originally Posted by tcrosa27
h is plank's constant, lambda is wavelength but k is what? never mind it's the wave vector I just found it. and I just also found that h with a dash above is the reduce plank constant too.
Yes, that's right.

6. That is true only for massive objects, not for photons. The photon momentum is defined as
Is this equation of momentum applicable to larger objects as well?

7. Originally Posted by MaxPayne
Is this equation of momentum applicable to larger objects as well?
No, this equation is specifically for photons.

8. No, this equation is specifically for photons.
And for all other mass-less particles?
Yes, I should vibrate 10^51 times per second for that to be true.The idea is dizzying!

9. Originally Posted by MaxPayne
And for all other mass-less particles?
Yes, I should vibrate 10^51 times per second for that to be true.The idea is dizzying!
The only other known and confirmed massless particle is the gluon - I admittedly don't know how to calculate its momentum, but I have a feeling that it would be a whole lot more complicated than for a photon.

10. E^2=mc^2 +p^2c^4 applies to everything massive or not. For low velocity where v << c the momentum p is approximately mv.

11. Originally Posted by Jilan
E^2=mc^2 +p^2c^4 applies to everything massive or not. For low velocity where v << c the momentum p is approximately mv.
M^2c^4 +p^2c^4=e^2. That's better!

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