*SIBAPRASAD DAS asked the Naked Scientists:*

Einstein in his STR concluded mass changes with the speed and tends to infinity as speed of the object approaches the speed of light . He inferred on the basis of electro dynamics and Lorentz trans formation . But did he ever think that the driving agent's speed matters the most ? Scientists always tried to speed up particle with the electromagnetic field whose own speed limit is the speed of light . So aslong as they would try with the particle accelerator of electromagnetic field would come to the same conclusion and never be able to reach the speed of . Think otherwise .

Mass never change .

*What do you think?*

Einstein's conclusion of the light speed limit was derived from the invariant nature of the speed of light. Basically it comes down to that once you establish that there is a finite speed that is measured as being the same for all inertial frames, that speed is also the speed limit for the universe. How you attempt to reach that speed has no bearing on your inability to reach it.

But let's approach this from the practical example of a particle accelerator. It is true that we use electromagnetic fields to get the particles up to speed. But we also note that the greater the speed at which we want to accelerate the particle to, the greater the amount of energy we have to pump into those fields and this amount of energy matches what Einsteins predicted. Now you might try to argue is that this increase in energy is due to the decreasing inefficiency of the fields acting on the particle as it gets closer to the speed of light, but hen you would have to explain where the excess energy did go.

In addition, if you are going to argue that it is the lack of the EM fields ability to push the particle up to light speed for the reason you suggest, then the particle after being accelerated up to light speed should have no more kinetic energy than what Newtonian Physics would predict for a particle moving at that speed. However,we don't just stop with accelerating the particles up to near light speeds, we smash them in to things and examine the results. The results of those experiments show that the kinetic energy of those particles well exceeds that predicted by Newton and agrees with That predicted by Einstein. That extra energy we pumped electromagnetic fields went into the particle, but smaller and smaller amounts of that energy resulted in an increased speed.

You are correct about one thing, in the modern view of Relativity, the Mass of the particle does not change. This is not however due to any modification of the theory itself, but due to the change in convention. At one time it was common to talk about "rest mass" and "Relativistic mass". Relativistic mass was what increased as an object accelerated. This became a bit cumbersome, and since it was known that the Relativistic Mass was just the mass equivalent of the Kinetic energy, it was decided that the term "mass" would only apply to the Rest or invariant mass, and relativistic mass would be just considered "energy". In this way the mass of the particle is considered as constant, and it is its energy that increases by the rules of Relativity as the particle accelerates.

Outside of the energy requirements, there is another line of reasoning that sets the speed of light as a limit. It uses the addition of velocities.

We normally assume that velocities add linearly. Thus for instance, if we are traveling at a speed of w with respect to the road and throw a ball forward at a speed of v relative to ourselves, the ball will be moving at w+v with respect to the road. Einstein showed that this was not the case, and due to the invariant nature of the speed c, the correct equation would be (u+v)/1+uv/c^2).

Now as long as u and v are small compared to c, the answer comes out to be very very close to u+v. So close, that in normal life speeds, we could never measure the difference.

But is does become noticeable as v and w become larger. The thing to note is that as long as you start with values of u and v that are smaller than c, the answer will be smaller than c. So for example, if you start out from A at 0.5c and then at some point drop a marker that continues t move at 0.5 relative to A, and then accelerate until you are, by your measurement, moving at 0.5c relative to the marker, you would find that you measure your speed relative to A as being only 0.8c. If you drop another marker, and accelerate to 0.5c relative to it, you will now be moving at ~0.92857c. You can keep dropping markers and accelerating to 0.5c with respect to them forever and you will never reach a speed of c relative to A.