1. Nov 20, 2005

### EP

If when you accelerate an electron to the speed of light, it will hit a speed limit of C. At C the electron will gain mass from all the energy that is still trying to accelerate it, right? Then my question is that what happens to the mass it gained while it was attempting to pass C? Does it just lose mass as a result of deaccelerating? Or is the electron actually heavier than before?

2. Nov 20, 2005

### SpaceTiger

Staff Emeritus
There are many threads on this issue in the relativity forum. Here's an example:

Several things to quickly note:

1) There are at least two kinds of "mass" in relativity, invariant mass and relativistic mass. The latter increases with speed, the former doesn't.
2) The relativistic mass always increases with speed, even at small speeds. The object doesn't have to hit the speed limit for you to see that effect.
3) Massive objects can never actually reach the speed of light (it requires an infinite amount of energy), but they can get very, very close to it.
4) In most cases, if an object is decelerating, something is taking energy from it. A loss of relativistic mass is effectively a loss of energy. Energy can take many forms, so the answer to this question:

depends on what was causing it to decelerate.

Last edited by a moderator: Apr 21, 2017
3. Nov 21, 2005

### Chronos

Energy conservation is still the rule. The energy required to accelerate a mass appears to accrue to that mass as measured by observers in different inertial reference frames.