text: if photon has zero rest mass but posses momentum"p=m.v".this means it has mass at"c"? then why a powefull lazer beam can not move a small mass.  secondly how anything that travel at "c" can have a mass ?  please note light propulsion works on combustion of air not momenum photons are massless, but then do possess momentum. a photon's momentum comes from the energy it carries. e = mc^2 + pc where e is the photon's energy, m is its rest mass (= 0), c is the speed of light, and p is the momentum. p = e / c  it is possible to use the momentum of light to move objects.  since momentum must be conserved, a photon's momentum is transferred to the object in a collision, thereby applying a force and causing an acceleration.  the energies (and this momentums) of single photons are very small, and thus their momentum changing effects on other objects are also very small and difficult to measure under normal circumstances.  nothing with mass can ever travel at, or exceed, the speed of light.  to accelerate a mass to the speed of light it would take an infinite amount of energy, and in the entire universe, there is not an infinite amount of energy.