Matej,

I visited the Czech Republic with my wife last year, and we loved it. I'm jealous of you living there - access to all those wonderful beers!

Anyway, to add my thoughts to Sparkman's ...

Relativity says matter and energy are equivalent (they can be converted into each other in line with Einstein's equation E=mc^2). So if photons have energy (which of course they do) then they can have gravitational effects just like mass can.

Also, Newton's formula F=m.a is just a special case of the more general form F=dp/dt - i.e. force equals the rate of change of momentum with respect to time (p=momentum, dp=change in momentum, dt=change in time).

For classical material objects, p=m.v (mass x velocity), so dp/dt = d(m.v)/dt = m.dv/dt = m.a (as acceleration, a=dv/dt, is just the rate of change of velocity with respect to time).

However, light still has momentum, even though it doesn't have mass. The momentum of a photon is calculated as p=h/w, where h is Planck's constant and w is the wavelength of the photon's light beam.

So you still need to exert a (non-zero) force to change the momentum of a photon - and, conversely, a photon can still exert a force on another object.

Try doing a web search on "Compton scattering" to find out more about the experiments that were done in the first quarter of the 1900s to prove that photons do actually have momentum and can be used to apply a force to (change the motion of) electrons.

A final word: When trying to understand light, relativity and quantum physics, you need to think in completely different ways, and give up relying on classical concepts like mass (and the conservation of mass). It's best to just focus on energy. Mass is just one form of energy, which can be transformed into other forms of energy (just as heat energy can be transformed into kinetic energy - e.g. a steam engine being used to drive a turbine). And energy can be transformed into mass, of course - photons, for example, can be converted into mass when they transform into virtual electron-positron pairs.

Hope this helps.

Solvay.