Whizz for anti-atoms!

The field of anti-matter spectroscopy began this week

At the Guardian. See also Chapter 7.3 of Smashing Physics.

Talking to people who have seen or read “Angels and Demons” by Dan Brown, which features CERN quite heavily, they expect to be disabused. Yeh, yeh, of course we don’t have private jets1, or parachute training tower whatsits. I don’t even own a white coat. It’s fiction, enjoy it!

Antimatter? Oh, no that’s real, obviously.

In fact anti-electrons (i.e. positrons), anti-protons and other anti-particles are commonplace in even moderate-energy physics facilities. Even in hospitals. But they are usually moving quite fast. Slowing them down, making positrons and anti-protons combine to make anti-atoms of hydrogen; all the while stopping them from annihilating with all the normal matter hanging around; and then keeping them long enough to study their properties, is the trick. The ALPHA experiment just did this, and published the results in nature this week2.

What they have done is not only trap atoms of antihydrogen, but fire photons (microwaves in this case) at them. These photons make the positrons inside the antihydrogen change energy levels. Or quantum state. Yes, something like the ones in Brian’s diamond. When this happens, the magnetic properties of the anti-atom change, which means it isn’t trapped anymore, it escapes, and then annihilates.

The frequency to which the microwave has to be tuned in order to cause this gives a measure of the difference between the different energy levels inside the anti-hydrogen. The difference between them is the same as it is for hydrogen.

This is the first time we’ve measured inside anti-atoms. The result – “it looks the same” – is not a surprise. It would have been a huge surprise if it had been different. In fact, the symmetry between matter and anti-matter as far as electromagnetism goes is built into our theory very deeply. The existence of antimatter was in fact predicted by combining quantum theory and special relativity in the Dirac equation3, and within this theory the electric charge of the electron has to be exactly opposite that of the positron, for instance.

The current measurements aren’t very precise, but they are the first. More precise ones will come. At one level we know the result already; anti-hydrogen has the same energy levels as hydrogen, or theory says so anyway. But this is just extrapolation from theory (albeit very well-tested theory).

There’s nothing like an actual measurement. And the field of anti-matter spectroscopy began this week.

There are some lovely short films on the antimatter experiments (and other stuff) here.

By the way, when I first read Angels and Demons, we didn’t have those retina scanner thingies. But we do now. Hmm.

If you are wondering about the title, you really must read the Molesworth books by Willans and Searle. Really.

1 tell me about it, I’m in the Geneva departure lounge yet again.

2 there is a link to the paper from this press release. It’s in Nature. I can’t read it because I’m not at UCL now (see previous footnote). You can’t read it because you (probably) don’t have a subscription. I thought CERN enforced open access, so I am a bit puzzled by this. Support open access.

3Let me take the opportunity to recommend (again) the biography of Paul Dirac by Graham Farmelo for further excellent reading.

About Jon Butterworth

UCL Physics prof, works on LHC, writes (books, Cosmic Shambles and elsewhere). Citizen of England, UK, Europe & Nowhere, apparently.
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