How does E=mc2 fit together with dark energy and dark matter?
There are three pieces to this question.
The first is Einstein’s famous equation and what it means. It tells you that you can swap mass and energy like you might swap dollars and Euros, and it tells you what the exchange rate is — this is the c2 with c standing for the speed of light. The speed of light is about 186,000 miles a second, so the conversion factor is huge. A tiny fraction of the matter in a nuclear bomb, for example, is converted to an awful lot of energy!
Now for the things referred to as dark matter and dark energy. In this context, ‘‘dark’’ means we can’t see them directly with any telescope that we know how to build. We can only infer their existence indirectly via their gravitational effects.
Dark matter was invented to explain the puzzling fact that if you look at how stars revolve in galaxies (think of a galaxy as a sort of super-huge solar system), the stars farthest out seem to be going around too quickly for them to be held where they are by the gravity of the luminous matter (stars) that we can see with our telescopes.
The idea then is that a huge fraction of the matter in a galaxy is invisible and untouchable and only noticeable by its gravity. Presumably dark matter can, at least in principle, be converted into normal energy at an exchange rate given by Einstein’s equation.
Astronomers have discovered that not only does the universe seem to be expanding, but it’s expanding faster and faster.
To explain this, the idea came about that in addition to dark matter, which helps hold galaxies together, there must be some other stuff that we also can’t see called dark energy that has a sort of weird antigravity effect that drives the ever-accelerating expansion of the universe. Presumably it too would be interconvertible with mass, but the only thing it really has in common with dark matter is the word ‘‘dark.’’
Of course, this is all based on theory, so it’s quite possible that there are other explanations that we haven’t thought of yet!
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or
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