4-momentum of a massless particle

[pellman]

The 4-momentum of a massless particle traveling in the z direction is (k, 0, 0, k). What is the significance of the value of k? It does not determine the speed since they always travel at light speed. If one particle has momentum (k, 0, 0, k) and another has (j, 0, 0, j) with j not equal to k, what is the physical difference between them?

 

[Ibix]

The momentum it carries is different. The energy is different. So how hard it would push on a light sail and the transitions it could excite are different. $E=h\nu$, so the frequency and wavelength are different.

 

[Dale]

The 4-momentum of a massless particle traveling in the z direction is (k, 0, 0, k). What is the significance of the value of k? It does not determine the speed since they always travel at light speed. If one particle has momentum (k, 0, 0, k) and another has (j, 0, 0, j) with j not equal to k, what is the physical difference between them?

I second @Ibix reply. The two particles have the same velocity (speed and direction) but energy and momentum are different and consequently wavelength and frequency.

 

[pellman]

I knew it would be something I already knew! Lol! Thank you, guys!