[quote=@Feisty-Pants]
This is waaaay over my head now. 

Light doesn't have mass, bit it does have energy, right?
[/quote]

You are correct. And it has momentum, and thus it can exert pressure. 

Depending of whether you take it as a particle or a wave it's either E=h*ν or E=p*c

[quote=@thewizardguy]
[@AtomicNut]
In all technicality Photons do have mass. To call the a massless particle is not entirely correct. After all, momentum can only exist when there is mass, however miniscule.

Have you ever read up on the driving principles behind the Lightsail?
[/quote]

No they do not. Radiation Pressure is expressed in terms of momentum.  Second law of newton can also be expressed as derivative of momentum. You can do all the maths for that without using mass even once. So hence it can be a massless particle with momentum. But not mass. 

If it had mass, it would be impossible to travel at c.