Momentum of light

arkain101 · #1 (**permalink**) 01-11-2007

Question on the momentum of photons:

How much momentum does a photon carry for a given intensity?

So lets say we use a specific photon value for a specific frequency, and measure the momentum for (t) 1 second. Can we calculate the value of momentum of a photon for specific frequencies?

I question this in order to understand how much force one would be met with as they travelled faster and faster and experienced a doppler shift in the light that comes towards them.

As I understand, if a source of light traveled directly towards an observer the frequency of the light would increase perportional to the velocity.

as calculated:

So, would mass traveling faster and faster infact be affected by an ever increasing frequency of photons that have an increasing momentum that forces against that masses acceleration?

(assuming I am right in my thinking)
One could test special relativity purely with calculations to decifer whether a paradox forms or not. While checking the relationships of the doppler shift, time dilation, and the velocity considerations. I think..

Farsight · #2 (**permalink**) 01-11-2007

Arkain, this is from Mass Explained:

When we turn our attention from a cannonball to a photon, we have to express the energy and the momentum in a different way. There is no “mass”, so the energy is hf, and the momentum is hf/c. The h here is Planck’s constant of 6.63 x 10-34 Joule-seconds, and is an “action” which is a momentum multiplied by a distance. The f is the frequency per second, and our old friend c is distance over time, which converts a stopping-distance measure into a stopping-time measure. It’s just λ/c or wavelength over frequency, so you can also express the momentum as h/λ...

Yes, the relativistic mass would experience more momentum when running into photons, because they look like they have a higher frequency. It's a bit like driving fast over bumps. I don't think there's any paradoxes to it.