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<   No. 2859   2010-11-24   >

Comic #2859

1 Caption: ==Mythbusters== Antimatter
2 Jamie: Continuing our testing of myths related to quantum mechanics and modern physics, today we're exploring antimatter.
3 Jamie: According to theory, one gram of antimatter can cause an explosion as large as 43 thousand tons of TNT.
4 Adam: Right. So to test it properly, first we need to establish the baseline!

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The seeds of antimatter theory were planted in the late 19th century by various scientists who proposed forms of "negative matter", with properties the opposite of normal matter. In much the same way as electric charges come in two different forms, one of which acts as a source for electric field and the other of which acts as a sink (see my earlier discussion of divergence of the electric field), these people proposed that if gravity has sources (i.e. massive objects), then it may also have sinks. These sinks would be some form of "negative matter", which would display negative gravity - they would repel other objects.

These ideas came to nothing, except for inspiring a handful of fanciful Victorian notions of antigravity, including one of H. G. Wells' best-known novels: The First Men in the Moon.

The discovery of real antimatter proceeded in a somewhat parallel manner, which illustrates something about how science works. The Victorian-era negative matter version was an idea inspired by analogies drawn from electromagnetic theory. This is a form of discovery that draws on theoretical concepts, and broadly follows a three-step process:

  1. A theory is established which explains some aspect of nature.
  2. Someone speculates that a logical extension of that theory predicts some phenomenon which has never yet been observed.
  3. Experimenters use that prediction to explore previously unconsidered areas, and discover the phenomenon is real. Alternatively, someone working on an experiment closely related to the theory finds a strange result and realises that it fits the prediction.
Of course not all discoveries are made this way. A lot of them are accidental discoveries made in the course of investigating something completely different. But for the cases of antimatter this is the version that applies. Victorian scientists were looking for ways to understand gravity in terms of James Clerk Maxwell's wildly successful formulation of electromagnetism, and this led them to the idea of negative matter. As it turned out, step 3 of the discovery process never happened, because nature obeys its own laws, not the ones we wish it to.

And so the idea of negative matter was quietly forgotten. Until 1928, when Paul Dirac was working out solutions to Erwin Schrödinger's newly formulated (1926) quantum mechanical wave equation, which describes the energy states of particles. Dirac realised that this new theory also allowed solutions in which there existed particles with properties that made them look like negative versions of established subatomic particles. In particular, he found a solution for an "electron" with the same mass, but opposite electric charge. He called this hypothetical particle an anti-electron. Unlike the negative matter hypotheses, Schrödinger's equation predicted the anti-electron to have the exact same mass and gravitational effect as the normal electron.

Step 3 in this case had to wait a mere four years. Carl David Anderson was working on the closely related field of particle decays in cosmic ray interactions, when he found what appeared to be an electron, but with the opposite electric charge! He realised this was Dirac's predicted anti-electron, and went on to win the 1936 Nobel Prize for his discovery. (Don't feel bad for Dirac and Schrödinger and the hard work they put in though - they shared the Prize in 1933.)

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