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Friday, 8 November 2013

Who needs to be right?

Christian Doppler was in Prague in 1842 when he wondered if he might be able to explain the colours of binary stars. He came up with an idea that we now call the Doppler effect, to do with the perceived frequency of a wave and the way the relative velocities of the source and the observer affect the observed frequency.

This picture has nothing to do
with this story: I just fell prey to
the pictorial imperative.

Confused by the opaque prose? Not to worry: think about what happens when a train goes through a level crossing where a bell is clanging: as we pass, we stop approaching the bell and start moving away, the tone of the bell seems to drop.

If somebody drives past sounding their horn, the pitch drops in the same way. It happens with sirens as well, but it's harder to spot, because the siren is varying anyhow.

Well, Doppler thought it might have been the reason why stars appear to have different colours. It was a great theory, but completely wrong.

Then in 1845, a clever young Dutch physicist with the curious (to our eyes) name of Buys Ballot said that it was all a load of old cobblers, but he reckoned there was something in the idea itself, even if it didn't cause the colours of binary stars.

He got some musicians loaded on a train on the Utrecht-Amsterdam line, and as they whizzed through Maarssen station at 70 kph, musicians on the platform listened to the calibrated note being played by the other musicians on the train, and the principle was established.

Sadly, even though Doppler was way off course, he had thought of the effect first, so even today, we refer to Doppler shifts, Doppler radar, Doppler this and that. It plays a big role in ultrasound work, and oddly enough, there is a Dopper effect on star colours, though our eyes can't see it.

Stars that are receding from is show a red shift, and the few objects whizzing our way show a blue shift, but we can only detect that by taking a spectrum with a spectroscope, invented by Kirchhoff and Bunsen. We really only recall Bunsen today by the Bunsen burner, which he didn't even invent! Credit in science can be funny like that.

A few years later, in 1848, nationalist uprisings hunted Doppler out of Prague and back to his native Vienna, where a few years later, he played a part in educating young Gregor Mendel, but that's another story.

Poor old Buys Ballot, who was right, and who proved that the effect existed, is hardly even a footnote, a bit like Maarssen station, which is now closed.

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