if photons in motion have mass and energy why don't they knockstuff over

Bjoern Feuerbacher wrote:

wrote:

if photons in motion have mass and energy why don't they knock stuff
over

like little things like blades of grass or ants out of a tree


Do the math. Visible light has a wavelength of 5 * 10^(-9) m. Momentum
of a photon is p = h/lambda. Thus p is approx. 1 * 10^(-25) kg m/s. Even
for gamma rays, whose wavelengths are about 10^(-6) smaller, the
momentum of a photon is only about 1 * 10^(-19) kg m/s. I don't think
that could knock even an ant or a blade of grass over - do you?

Lets take a mol of these photons then.
You can indeed lift glass spheres with a laser.
It needn't be especially powerfull actually.
A few mW are sufficient.

Rene
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