# Particle and energy

1. Jan 20, 2016

### zainalinaeem

If we slow down a particle of light or any(photon) speed, would the mass of it will decrease. Reverse to what Einstein said ' as you reaches speed of light or light speed the mass of light it start increasing mass.

2. Jan 20, 2016

### mathman

Photon speed is constant. You can't slow it down.

Things which travel at the speed of light always travel at the speed of light.
Things (with mass) that travel at less than the speed of light never travel at the speed of light.

3. Jan 20, 2016

### Mister T

The notion that mass increases with velocity is not a consequence of Einstein's theory. The erroneous claims that attribute it to Einstein have been disappearing from textbooks for the last 25 years or so, and are now pretty much gone from view. The modern view is that mass does not increase with velocity. Note that we're not talking about a change in the physics here, just in the way we describe it.

You can look at the relativistic factor $\gamma=\frac{1}{\sqrt{1-v^2/c^2}}$ and use it to write expressions like $\gamma mc^2$ for the total energy or $\gamma mv$ for the momentum. What some people used to do is call $\gamma m$ the mass and say that it increases with velocity, but the better way to think about it is to consider $m$ to be the mass, which is constant, and realize that it's $\gamma$ that increases with velocity.

4. Jan 21, 2016

### Staff: Mentor

The attribution to Einstein is not erroneous - it really did come from him (section 10 of the 1905 paper). He eventually came to recognize it as a mistake.

5. Jan 21, 2016

### Mister T

You are correct. I should word it differently to make that clear. The concept had already been present before the 1905 paper, after 1906 Einstein stopped using it, and went on to discard it. It was a very short-lived part of his long career. It was others, not Einstein, who were for the most part responsible for its proliferation.