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.
(published on 10/22/2007)