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Q & A: Relativistic vs rest mass in light

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Most recent answer: 04/22/2014
Q:
If I understand correctly E=mc2 means even light has mass. Very very little mass but mass none the less. But I thought nothing with mass could go the speed of light. So if both of these points are correct then how can light go the speed of light?
- Chris (age 33)
Texas
A:

Hi Chris,

As with most questions we get about light and mass, the confusion arises because there are two definitions of the word "mass," and they are often confused.

Nothing with rest mass can go at the speed of light. Light has no rest mass, but it does have what we call relativistic mass, which arises from both rest mass and kinetic energy.

If you think you found a mass-based paradox by asking a simple question, you probably mixed up your definitions of mass.

Cheers,

David


(published on 04/22/2014)

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