# How can light have no mass?

1. May 14, 2013

### danielpherlio

Ive been looking at the E = mc2 explained on youtube and apparently light has no mass how can it have no mass but you you can feel heat on your skin from light

2. May 14, 2013

### ModusPwnd

Why should feeling heat imply mass? That logic doesnt follow.

The equation you posted is for a massive particle at rest. Look up the full version of the equation for a clue on how to apply it to light.

3. May 14, 2013

### Staff: Mentor

E = mc2 applies only to things that have mass when they are at rest - and light is always moving with speed c, never at rest, so the equation doesn't apply. Instead, we have to use the more general equation:
$$E^2 = (m_{0}c^2)^2 + (pc)^2$$

Here p is the momentum and $m_0$ is the "rest mass" of the object; when people say that light has no mass they mean that its rest mass is zero. Light does however carry momentum (p is non-zero) so there's still some energy to warm your skin.

4. May 14, 2013

### danielpherlio

Thanks nugatory