[petschge]

There is a massive working medium. For massless particles such as photons the connection between energy E and momentum p is E = p *c or p = E/c. Since c is large, the momentum p you can carry per energy E is small. For a massive particle with mass m the connection is given by E = p^2 / (2 m) or p = sqrt(2 m E). As long as E is small (so small that the velocity of the particle is small compared to the speed of light, but that is required anyway for the non-relativistic approximation I am using here a) the sqrt doesn't hurt you too much and you can actually carry much more momentum per particle.