If light is a form of energy and can be converted into matter then does light slow down since no mass full object can reach the speed of light
3$\begingroup$ A single photon cannot turn into matter for essentially that reason (conversion of momentum: there is a frame in which a system of particles has zero total momentum, but for a single photon that is not possible). Two photons, unless they have parallel momenta do have a frame in which they have zero total momentum, and they can indeed be converted into matter. $\endgroup$– doetoeMar 7, 2016 at 17:09
$\begingroup$ Anyway you would be desappointed of how few mass it would generate (for mass concentrates a huge amout of energy : $c^2$ per unit mass). $\endgroup$– Fabrice NEYRETMar 7, 2016 at 21:20
$\begingroup$ What do you mean by a system of paricles which have 0 momentum $\endgroup$– Jayadev AshokMar 18, 2016 at 17:41
Light or photons always travel with speed of light.
Conversion of the light's energy into matter (e.g. electrons and positrons) takes place at high photon energies (gamma rays, >1.022 MeV) and near atomic nuclei. In this process however, there is no slowdown but rather an energy conversion from photon energy into rest energy of the particle and its kinetic energy.
In the end the residual light is still travelling with speed of light, but with less energy (i.e. a smaller frequency).
$\begingroup$ Momentum must be conserved. How do you resolve that? $\endgroup$– Bill NMar 7, 2016 at 17:43
$\begingroup$ Nuclear recoil can account for that. $\endgroup$ Mar 7, 2016 at 17:50
$\begingroup$ My point is that you don't mention it in your answer. Don't leave it to be assumed because that is what limits a single massive object from absorbing all the photon energy. $\endgroup$– Bill NMar 7, 2016 at 19:18