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Q & A: Matter, Energy, and Relativistic Speeds

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Q:
By the theroy of ralativity if accelarate an object at the speed of light it will turn to energy. What whould happen if you slowed down a partical of energy by the speed of light? Would it turn into matter? Or would it stay energy?
- Daniel Nordness (age 16)
Urbana, Il U.S.A.
A:
Hello, Daniel
You have the right idea, but you are mistaken about what relativity theory actually says. It is a common misconception about relativity theory that by accelerating matter up to the speed of light, that the matter will turn into energy. What relativity theory says is that matter (something with mass) cannot move as fast or faster than light, no matter how much you accelerate it. The only thing that can move as fast as light (according to relativity theory) is energy, since energy has no mass.
Okay, I know someone reading this is saying, "What about massless particles?" Yes, massless particles can move as fast as light as far as we know.
What may confuse people about matter and energy is Einstein's famous equation: E = mc^2. This equation which relates mass to energy, does not require that an object be accelerated towards the speed of light. What you can say is that if you convert some matter into energy (by matter-antimatter collisions for example), then the amount of energy you get is equal to the mass times the speed of light squared. Conversely, if you convert energy into matter, then it would have mass equal to the energy divided by the speed of light squared.
So you have the right idea, just in the wrong order. The energy released when matter is converted into energy can travel at the speed of light. And by converting energy into matter, energy is slowed down from the speed of light.

I hope this helps out.


-math dan

(published on 10/22/2007)

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