According to the theory of special relativity, all massless particles, such as photons (particles of light), travel at the same velocity in a vacuum, which is the speed of light denoted by 'c'. This means that regardless of their energy or frequency, all photons move at the same speed.
On the other hand, for particles with mass, special relativity tells us that their speed is always less than the speed of light. The exact speed at which a massive particle moves depends on its energy, momentum, and mass. As a massive particle accelerates to higher energies, its velocity approaches but never reaches the speed of light.
In summary, massless particles always travel at the speed of light ('c') in a vacuum, while particles with mass always travel at speeds less than the speed of light.