right on rajeshhamal. When a particle/object travels close to the speed of light, and is thus considered "relativistic", the energy (since momentum is a measure of energy) of the particle is expressed as momentum = (gamma)*mass*velocity. Here gamma is a relativistic factor that is greater than unity. So, one could consider the factor (gamma)*mass a new mass, one that is larger than the mass of the particle when it is at rest. This is why you hear that mass increases when you approach the speed of light. It can be argued that it is only an appearance of greater mass, or that it depends on how you look at the problem. In short, it is all relative. ;) It should be noted, however, that in order for an object to actually reach the speed of light, it must have no mass, since E=mass*speed of light^2. This is true of massless particles such as the photon, the "particle" that transports light. (Notation:* means multiply by and ^2 means squared). various versions of the argument that who is greater exists, but as Newton said, and someone previously pointed out "We are tall only because we stand in the shoulders of giants". I doubt that Einstein would have been Einstein had Newton not formulated his laws a couple of hundred years ago and we must also remember the contributions of all the scientists in between from Gauss, Maxwell, DeBroglie etc to the modern day scientists like Feynmann et al.