Can The Doppler Effect Help You Beat The Speed Camera?

By Kyle Hill | March 18, 2014 10:30 am

Doppler ShiftThe shortest answer is no.

Thanks to the curiosities of physics, there is this paradoxical yet plausible notion that you could beat a camera meant to photograph you speeding by going so fast that it won’t pick you up. In theory there is some speed at which the very light reflected off of your car will become undetectable to the human eye. But how fast would that be?

Don’t have time to read? Listen to the whole post below!

When you hear an ambulance headed your way, the blaring sirens increase in pitch* until the vehicle reaches you, and the pitch slides back down as it passes. This is “Doppler Effect.” Sound waves traveling in the same direction can “bunch up,” making them seem at a higher pitch. The same thing can happen with light. Edwin Hubble, the astronomer whose name christened the Hubble Telescope, discovered that galaxies moving away from us had light waves that were stretching apart. Like a fading sound, the light from the galaxies was getting redder—being “red shifted.” What happens as the galaxies gallop away from us means that if you were to go fast enough, there is some point were the light reflecting off your car would be red-shifted below human (or camera) detection.

In a paper from the Journal of Physics Special Topics, authors Worthy, Garner, and Taylor-Ashley do the Doppler number crunching. They assumed that a car would be moving away from the camera when the photo was taken, that the average license plate reflects basically yellow light, and that a license plate is undetectable when the light is red-shifted below 430 terahertz—the human limit.

Using those values and this equation, the authors concluded that the minimum velocity to beat the speed camera with the Doppler Effect is about 0.178c, or 18 percent the speed of light. Unfortunately for your outstanding tickets, even in the fastest supercar ever built, you have no hope of getting to this speed. 18 percent the speed of light is over 33,000 miles per second—if you crashed your car at this speed you would be obliterated by 10 times more energy than was released by the supervolcano Krakatoa.

So no, you can’t red-shift your way out of a ticket. But you could still speed your way out of one.

The Discovery Channel’s Mythbusters actually established that you could outrun the speed camera. So long as you passed the camera at 300 miles per hour or more, you would be far outside of the camera’s range as the photo snapped. If you want to break the law but now the laws of physics, invest in a dragster.


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Image Credit: Roads At Night: She’s Gone by Cayusa on Flickr

Paper Link: Red-Shifted Speed Cameras

Reference: Worthy, D., Garner, R., Gregory, J., & Taylor-Ashley, J. (2013, November 19). Red-shifted Speed Cameras. Journal of Physics Special Topics, 1-2.

*An earlier version of this post incorrectly linked the Doppler Effect to changes in amplitude, not pitch.

CATEGORIZED UNDER: Space & Physics, top posts
  • Quicklime

    “18 percent the speed of light is over 33,000 miles per hour” – I would suggest fact-checking that statement a little more. It would actually be roughly 33480 miles per *SECOND*, not hour. Either way, your car would almost certainly ablate away very rapidly.

    • disqus_atlq8Zmtsd

      To be fair, “over 33,000 miles per hour” is inclusive of 33000 miles per second. =D

    • Kyle Hill

      Noted, fixed! Jeez, today was not a good day for units.

  • disqus_atlq8Zmtsd

    How “loud” something sounds is a matter of wave amplitude while the “bunching” of waves refers to their frequency. The Doppler effect has nothing to do with how loud the ambulance sounds as that is simply an effect of the inverse square law (volume is inversely proportional to the square of the distance from the source). The Doppler effect refers to how the sound is higher pitched when the siren is moving towards you and lower pitched while moving away.

    Basically your article implies that the color of light is a result of intensity by asserting that sound volume is the result of frequency.

    • Kyle Hill

      Noted, fixed! Thanks for catching my blunder.

      • disqus_atlq8Zmtsd

        It happens to the best of us. I submitted a term paper on the Greco-Persian wars once that had all dates AD. EVERY. SINGLE. ONE.


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About Kyle Hill

Kyle Hill is a science writer and communicator who specializes in finding the secret science in your favorite fandom. His work has appeared in Wired, The Boston Globe, Scientific American, Popular Science, Slate, and more. He is a TV correspondent for Al Jazeera America's science and technology show TechKnow and a columnist for Skeptical Inquirer magazine. Find his stream of nerdery on Twitter: @Sci_Phile Email him at sciencebasedlife [at] gmail [dot] com.


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