The Physicist Judge

Posted on Jan 31, 2017 | 0 comments


What do cannabis sativa, viagra, carcinogens and epidemiology, the BRCA1 polypeptide, and a Norwegian roll-on/roll-off container ship have in common? This was the question posed by Justice Michelle Gordon in a recent paper she delivered at the University of Western Australia. The answer is (of course) (now former) Chief Justice Robert French, whose successor, Susan Kiefel was sworn in yesterday.

French (like me) has qualifications in both science and law and he told the following blogworthy anecdote about his early legal career in a paper he gave in 2011:

“Nearly 40 years ago, as a newly minted lawyer who happened to have a science degree with a physics major, I was keen to take on cases which involved scientific questions. One such case concerned a young man riding a motor bike who was clocked at twice the speed limit by a radar gun. He insisted that he had only been travelling at the speed limit. The radar gun works by transmitting a radar beam at a certain frequency. When that beam hits a moving object it is reflected back and its frequency shifts upwards. This is called the Doppler Effect. The radar gun combines the reflected signal with the outgoing signal to produce a resultant frequency called the ‘beat frequency’ which is a function of velocity. According to that beat frequency, the gun produces a readout of speed.

I remembered from my basic physics that the speed of a wheel at the top is twice the speed at the axle. So if the motor bike were travelling at a speed ‘V’, the spokes at the top of the wheel would be travelling at the speed 2V relative to an external observer. Could this be the explanation for the disputed reading? Had some of the reflected beam come off spokes travelling at twice the speed limit even though the bike itself was travelling within the law? Could my client have been telling the truth? I engaged the services of a PhD student. We brought to court a bicycle wheel, a radio frequency generator and a couple of oscilloscopes. The magistrate was transfixed by the evidence. However, he didn’t know very much about physics. In the end he said he would rely upon the policeman’s personal estimate of the speed and convicted my client. He was probably right to do so.”

Creative commons acknowledgment for the photograph

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