Laser recoil (was Re: GEVs/Grav/Arty)

> > ** What recoil do you see from a HEL?
> 
> Photons have momentum. They don't have mass, but they do have 
> momentum, calculated as planck's constant / wavelength (I think 
> that's right, ooo my memory is bad today, I think there might be some
> 2*Pi factors in there).
> 
> A laser shoots out a lot of photons. Conservation of momentum has to
> apply. The laser must therefore recoil. 

Okay, memory is slowing coming back. As stated above the momentum (p) 
of a photon is given by the Planck constant (h) divided by the 
wavelength (l). The energy (E) of a photon is given by the Planck 
constant multiplied by the frequency (f). And wavelength multiplied 
by frequency gives the speed of light (c)

p = h/l
E = h * f
c = l * f

So, p = E/c

For a kinetic energy projectile, 
p = mass (m) * velocity (v)
E = 0.5 * m * v^2

So, p = 2 * E/v 
(For relativistic particles, the 2 decreases to 1 as v approaches c).

So for a laser 'pulse' with the same energy as a KE projectile the 
momentum will be a lot lower. Hence, the recoil will be a lot lower.

	Steve

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