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<   No. 2874   2010-12-09   >

Comic #2874

1 Jamie: An important property of antimatter is that an antiparticle and its counterpart particle meeting will annihilate one another.
2 Jamie: This produces two gamma ray photons of equivalent energy, according to Einstein's mass-energy relation, E=mc2.
3 Jamie: It's possible in unusual circumstances for other numbers of photons to be produced, but two is by far the most common.
4 Jamie: Richard Feynman invented a neat way of showing particle interactions, now called Feynman diagrams.
5 Jamie: Here's one showing the annihilation of an electron and its antimatter counterpart, a positron.
6 Jamie: Here's another one showing another common phenomenon: the scattering of light off an electron.
7 Jamie: Feynman noticed that if you turn that diagram through 90°, interchanging the roles of time and space, it's the same reaction...
8 Jamie: ... except one photon is reversed and the positron is replaced by an electron travelling backwards in time.
9 Jamie: But a photon is its own antiparticle, and the positron is an anti-electron. So antimatter is equivalent to normal matter going backwards through time!
10 Adam: At least, that's the myth! How are we going to test this one, Jamie?
11 Jamie: We build a powerful gamma ray generator and stick something in it.
11 Adam: Me!
12 Jamie: Are you sure?
12 Adam: I'll either travel through time or become the Incredible Hulk! I can't lose!

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Wow, the Wikipedia article on Feynman diagrams is insanely technical. Most people don't need to know that Feynman diagrams are "graphical representations of a perturbative contribution to the transition amplitude or correlation function of a quantum mechanical or statistical field theory".

In simpler terms, Feynman diagrams are basically just diagrams showing how a particle interaction works, in a nice graphical, easy-to-understand way. Isn't that a nicer description?

Pretty much everything I want to say about Feynman diagrams is actually in the comic. You don't need scary-looking equations to explain scientific concepts. (Though sometimes the equations do make neat summaries.)

I remember vividly learning about these in my university quantum physics courses, and being utterly blown away by the fact that one simple interaction (electron-positron annihilation) and another simple interaction that looks at first glance to be completely unrelated (photon scattering off an electron) look exactly the same if you swap around time and space and think of antiparticles as normal particles moving backwards in time. It's not just the case for this interaction either - there are plenty of other interactions involving all sorts of other particles that work just as well when rotated this way.

This is not necessarily to say that antiparticles are normal particles moving backwards through time. It's not really clear what that would actually mean in practical terms. But if you think of them that way, quantum physics still works, and becomes considerably simpler in the process. So it's a powerful way of thinking about it than can help you solve real world problems.

In many ways, this is actually one of the less weird aspects of quantum mechanics.

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