0

I'm just looking for a good (better) analogy.

How do massless photons put pressure on a surface especially when it's a mirror? Using the analogy of the wind (atmosphere) on a sail breaks down when I think that an air sail catches the wind and that the wind has mass and is an object in motion but how can a mirror catch a proton and how would it matter if it does if it's massless? Where does the force come from? The one article that mentions red shift was a math equation so it did not help me get my brain around it.

Does it have more to do with the momentum energy of the proton and can that be explained in simple terms?

As a child I imagined a solar wind sail but a light sail is a different concept.

I hope it's not too dumb of a question and that this type of question is appropriate here.

John H
  • 87

2 Answers2

1

This is newton second law: $$ \mathbf F = \frac{\Delta\mathbf p}{\Delta t} $$

As you can see, its variation of momentum that brings force. So, its the transfer of momentum. Granted, photons are massless, but they do have momentum.

There are two ways of a photon transfer momentum to a solar sail: The photon is absorbed (also heats up the sail), or, the photon is reflected.

Physicist137
  • 3,354
0

Most people believe that you need mass to transmit a force, or even to be able make one, but the more fundamental concept happens not the force but the momentum. The momentum is the capacity to interact with another entity and transfer it some change in speed. In the case of the photon, it is massless but however has momentum and energy. The energy and momentum of a photon depend only on its frequency ($\nu$) or inversely, its wavelength ($\lambda$):

$E=\hbar\omega=h\nu=\frac{hc}{\lambda}$

$\boldsymbol{p}=\hbar\boldsymbol{k}$