Real Science

Monday, 12 March 2007

Alphabet soup

This story exposed a few gaps in my knowledge. Until it crossed my desk, I’d never heard of the YORP effect, nor indeed of the separate scientists whose fortunately varied surnames form the acronym – Yarkovsky, O'Keefe, Radzievskii and Paddack.

Just imagine if they’d been called Blane, Underwood, Radzievskii and Paddack. Or Yarkovsky, Edwards, Lang and Paddack. Or … but you get the idea. Wouldn’t have worked at all.

Makes you wonder if just a couple of them discovered the effect, then signed the other guys up to get a good acronym out of it.

This is not as daft as it sounds. Physicists are not always grown-up and sensible. In fact as a good rule of thumb if you’re talking to a scientist who seems very serious, he’s either not a physicist or his dog just died.

One of the most famous science papers of all time is known as the alphabet article, because it was authored by Ralph Alpher, Hans Bethe and George Gamow – α, β and γ being the first three letters of the Greek alphabet.

The paper was actually written by Alpher, who was studying for a PhD at the time. Gamow his supervisor thought it would be a good joke to add the name of his buddy Bethe, who hadn’t been involved.

The young graduate student did not appreciate the humour. The presence of two eminent physicists’ names on his work meant his contribution to the important research it reported – on the origin of the Universe – might not get the recognition it deserved. He was right.

Mathematicians generally aren’t as human as physicists. They spend most of their time away from real science, in a Platonic world of beautiful objects the rest of us can’t even see. But sometimes they too get supposedly humorous ideas for papers. (If this one seems a bit rarefied, follow the links to Bacon number, which is back in something like the real world again.)

I digress. In trying to figure out the science behind the press release on YORP and the asteroid 2000 PH5, I couldn't see at first how sunlight could produce the asymmetric force needed to give it a turning moment. Then I realised it’s a small asteroid, on which gravity is far too weak to produce a smoothly spherical object – and the science started to make sense:

The Cornell astronomers bounced radar off the little chunk of rock to build a 3-D picture of it. Together with its temperature and the thermal properties of the material it’s made from – as well as distance from the sun and solar radiation output – this enabled them to calculate a value for the asymmetric force and hence the torque. Its moment of inertia – again from the 3-D model – then let them predict its rotational acceleration.

Meanwhile the optical guys were using tiny changes in brightness to actually measure its rotational velocity and, over a long period of time, rotational acceleration.

Comparing the calculated with the measured values of this allowed the scientists to test the hypothesis that it was the YORP effect that was speeding up the spinning. The agreement was good, so they announced the first observation of YORP in action on a body in the solar system.


I still want to know more about Yarkovsky, O'Keefe, Radzievskii and Paddack though.


  • O'Keefe was my father. He was also responsible for discovering that the earth is (slightly) pear shaped, for which reason, we received many lugs of pears for Christmas for several years. He figured out how to map China when going from flat to round was unusual (during WWII) and he figured out that a satellite would help us to map the earth. He said tektites were from the Moon, and when nobody would listen, we put it on his funeral program so he'd have one more chance. He liked Wyeth and always talked about his appreciation of how light looks red when it comes through smoke or fog, but looks blue if it bounces off. I found his earliest comments about this in a letter to my mother before they married; he had read Goethe's Farbenlehre.
    He believed very strongly that, as a civil servant, he was in the employ of the citizens of the US, who had a claim on his attention. Therefore those who wrote to him always received polite answers, even the little old ladies who wanted him to repent of his belief that the earth is round, or that it circuits the sun.
    He always wanted to be an astronomer; now he has an asteroid and an astral effect named after him. Good.

    By Mary, At 27 March 2007 06:44  

  • This is lovely. Thank you for sharing it with me. It conveys so much more about the man than the Wikipaedia entry, which is all I really knew before.

    It is so good in fact that I've pulled it out of the comments section and given it a post of its own. I've also put it up at another science blog I contribute to -

    By Douglas, At 27 March 2007 18:22  

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