[chrisser]

I googled a few articles about this, and I still don’t get it - if photons have no mass, how can they provide acceleration in a vacuum?

[RightWhale]

Photons carry momentum.

[wastedyears]

That’s the same question I had when NASA said they wanted to test a Scramjet for space propulsion. They’ll need to carry a lot of air on board.

[r9etb]

if photons have no mass, how can they provide acceleration in a vacuum?

They have no rest mass. But they have a small mass by virtue of the fact that they have energy:

E_photon = mc², so m = E_photon/c²

And they have an exhaust velocity = c.

Thrust = mass flow rate * exhaust velocity. So if you have some number N photons per second, then

T = NE_photon/c² * c = NE_photon/c.

I don't rightly know how you'd count photons, but it occurs to me that NE_photon is the total photon energy flux, so you don't really need to count photons, you just need to know the photon energy output by the thrust "beam".