# Mass-energy conservation

1. Jan 28, 2014

### john951007

We know the equation E=mc^2 which correlates energy to mass.
If we rearrange the equation:
E/m = c^2 , it turns into the ratio of energy to mass is the square of the speed of light, does it mean for every particle in the universe, the ratio between the energy of the particle and its mass is the square of the speed of light?

2. Jan 28, 2014

### Mordred

correct

3. Jan 28, 2014

### Staff: Mentor

The complete equation is $E^2/c^2-p^2=m^2 c^2$. So the relationship you wrote holds as long as p=0, in other words for a particle at rest.

4. Jan 29, 2014

### john951007

Does photon satisfy the full equation, because photon has momentum but does not have mass?

5. Jan 29, 2014

### BruceW

yep, the photon does satisfy the full equation.