The Sciences

The Laws of Physics, Officer, Outrank the Laws of California

DiscoblogBy Veronique GreenwoodApr 18, 2012 1:38 PM


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I think this picture says it all, officer. Clear as day!

To all those police officers out there on traffic duty: Be real careful about ticketing physicists. You might be proven wrong in elaborate mathematical detail. Dmitri Krioukov, a physicist at UC San Diego, was pulled over for running a stop sign. However, he had not in fact run it, and his sense of injustice was apparently so inflamed that he undertook a rigorous mathematical explanation of what had happened, eventually posting a paper on the ArXiv showing that the police officer had fallen prey to a perceptual illusion

(although the paper was posted on April 1, if it's a joke, Krioukov is sticking to his guns; he's spoken to PhysicsCentral

about the work). At the stop sign, he had seen Krioukov's car, a Toyota Yaris, disappear on the far side of a station wagon in the lane closest to the officer and subsequently accelerate away, but he mistakenly concluded that Krioukov had not stopped during that moment, because---this is the clincher---he had been visually measuring not the linear but the angular speed of the car! To put it in Krioukov's own words:

"Police officer O made a mistake, confusing the real spacetime trajectory of car C1—which moved at approximately constant linear deceleration, came to a complete stop at the stop sign, and then started moving again with the same acceleration, the blue solid line in Fig. 5—for a trajectory of a hypothetical object moving at approximately constant linear speed without stopping at the stop sign." "However, this mistake is fully justified, and it was made possible by a combination of the following three factors: 1. O was not measuring the linear speed of C1 by any special devices; instead, he was estimating the visual angular speed of C1; 2. the linear deceleration and acceleration of C1 were relatively high; and 3. the O’s view of C1 was briefly obstructed by another car C2 around time t = 0.

Basically, he both stopped and started his car so quickly that, to someone observing from the far side of a large vehicle, he essentially looked like he never stopped moving. To err is human, though, and Krioukov munificently allows that through no fault of his own, "the O’s perception of reality did not properly reflect reality." Krioukov did not have to pay the fine of $400. He does not say, however, whether that's because the judge found his argument sound, or because the judge took one look at the paper, blanched, and let him off the hook. He also mentioned to PhysicsCentral that he wanted readers to test the strength of his argument. If you see something amiss in the paper, post in the comments.

Image courtesy of Krioukov

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