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Testing the mass-energy relationship

15 Jun 2013, 19:42 UTC
Testing the mass-energy relationship
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How do we know the most famous equation in physics — E = mc2 — is actually correct?
The answer, of course, involves experiments. It doesn’t matter how brilliant Einstein (or anyone) is, no theory is good if it doesn’t hold up under experimental tests. The relation between mass and rest energy is no different. Indirect tests exist everywhere: without the physics underlying E = mc2, nuclear and particle physics wouldn’t work at all. However, it’s challenging to measure the mass-energy relation directly, and the key is finding the mass of two systems that differ only in energy content.
A Penning trap, used to measure the masses of ions and other electrically charged particles. [Credit: Imperial College]The experiment: confine sulfur atoms in a trap. Bombarding those atoms with neutrons makes a new type (isotope) of sulfur, but in an “excited state”: one in which the protons and neutrons in the nucleus are bubbling around a bit more rapidly than usual. The excited state isn’t stable: it “decays” by emitting a gamma ray photon without changing its nuclear composition. In other words, before and after the emission, a given ion has the same number of protons and neutrons; the only difference ...

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