# If you drive fast enough at a red light, it will appear green **URL:** \ **Category:** Factual Questions\ **Created:** [January 20, 2003, 5:31pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822 "2003-01-20T17:31:49Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Logical\_Phallacy](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Logical\_Phallacy](https://boards.straightdope.com/u/Logical_Phallacy)\ **Post date:** [January 20, 2003, 5:31pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/1 "2003-01-20T17:31:49Z")
Alright so I’ve heard of this annoying thing called the speed of light and all, but still the above seemed like a cute quote. Now given that one isn’t going to be going faster than the speed of light, does that necessarilly mean that one can’t be going fast enough to distort light? Is there any vehicle we’ve invented capable of going fast enough to distort our perception of light sufficiently to make it seem like a different color? ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [January 20, 2003, 5:35pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/2 "2003-01-20T17:35:11Z")
You’d have to be going at relativistic speeds, which, IIRC, are upwards of 90%+ the speed of light. So pick a car that does at least 170,000 miles per second, and you’ll notice some blue shift, I suppose. ---
**Author:** ![Logical\_Phallacy](https://avatars.discourse-cdn.com/v4/letter/l/848f3c/32.png) [@Logical\_Phallacy](https://boards.straightdope.com/u/Logical_Phallacy)\ **Post date:** [January 20, 2003, 6:23pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/3 "2003-01-20T18:23:04Z")
neato! After a good deal of googling I still can’t figure out just what relativistic speeds are? Why is it that this anomoly would occur? ---
**Author:** ![Sock\_Munkey](https://avatars.discourse-cdn.com/v4/letter/s/aeb1de/32.png) [@Sock\_Munkey](https://boards.straightdope.com/u/Sock_Munkey)\ **Post date:** [January 20, 2003, 6:41pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/4 "2003-01-20T18:41:04Z")
Its’ called doppler shift. ---
**Author:** ![cj\_finn](https://avatars.discourse-cdn.com/v4/letter/c/4bbf92/32.png) [@cj\_finn](https://boards.straightdope.com/u/cj_finn)\ **Post date:** [January 20, 2003, 6:42pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/5 "2003-01-20T18:42:54Z")
“Relativistic” refers to speeds that approach the speed of light, i.e. your speed relative to the speed of light. Why does blue shift happen? For some of the same reasons that the Doppler effect happens, i.e. light will look differently depending on your speed in the same way a car sounds differently as it approaches, passes, and moves away from you. This analogy is simplistic and will probably be panned, but gives you an easy way to think about. Ultimately, the whole color shift thing occurs because the speed of light is not a constant. It changes depending on how fast you are going. I recall that time is the actual constant but then I also recall reading something indicated that time isn’t constant either. This is all clear blue right? cjfinn ---
**Author:** ![Don\_Roberto](https://avatars.discourse-cdn.com/v4/letter/d/e47774/32.png) [@Don\_Roberto](https://boards.straightdope.com/u/Don_Roberto)\ **Post date:** [January 20, 2003, 6:43pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/6 "2003-01-20T18:43:18Z")
Actually, you’ll need to search for the **relativistic doppler shift**. The plain old doppler shift generally refers to sound waves in air. ---
**Author:** ![lno](https://avatars.discourse-cdn.com/v4/letter/l/a8b319/32.png) [@lno](https://boards.straightdope.com/u/lno)\ **Post date:** [January 20, 2003, 6:44pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/7 "2003-01-20T18:44:46Z")
It’s like a Doppler Shift. From [howstuffworks.com](http://howstuffworks.com)’s [article on relativity](http://www.howstuffworks.com/relativity3.htm): > [@](#): > > The Doppler Effect basically says that there is an observed frequency shift in electromagnetic waves due to motion. The direction of the shift is dependent on whether the relative motion is traveling towards you or away from you (or vice versa). Also, the amplitude of the shift is dependent on the speed of the source (or the speed of the receiver). A good place to start in understanding the Doppler effect would be to first look at sound waves. There is a Doppler Shift associated with sound waves that you should recognize easily. When a sound source approaches you, the frequency of the sound increases and likewise, when the sound source moves away from you, the frequency of the sound decreases. Think about an approaching train blowing its whistle. As the train approaches, you hear the whistle tone as a high note. When the train passes you, you can hear the whistle tone change to a lower note. Another example occurs when cars race around a racetrack. You can hear a definite shift in the sound of the car as it passes where you are standing. One last example is the change in tone you hear when a police car passes you with its siren on. I’m sure that at some point in our lives, all of us have imitated the sound of a passing car or passing police car; we imitated the Doppler Shift. This Doppler shift also affects light (electromagnetic radiation) in the same manner with one critical exception; the shift will not allow you to determine if the light source is approaching you or if you are approaching the source and vice versa for moving away. To sum, the frequency of the light wave (or sound wave) will change if you (or the source) is moving towards or away from the other. If you drove very, very fast at a red light, the color would shift towards blue as **Qadgop** said. Similarly, if the red light were to move very, very fast towards you, the color would shift in the same manner. ---
**Author:** ![Don\_Roberto](https://avatars.discourse-cdn.com/v4/letter/d/e47774/32.png) [@Don\_Roberto](https://boards.straightdope.com/u/Don_Roberto)\ **Post date:** [January 20, 2003, 6:47pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/8 "2003-01-20T18:47:17Z")
> [@](#): > > Ultimately, the whole color shift thing occurs because the speed of light is not a constant. It changes depending on how fast you are going. I recall that time is the actual constant This is incorrect. The speed of light is constant, no matter how fast one is moving–it is in fact one of the few true constants in the universe. The fact that the speed of light is constant is what causes **time** to be not constant. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [January 20, 2003, 6:49pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/9 "2003-01-20T18:49:11Z")
try this site: [http://www.arachnoid.com/sky/redshift.html](http://www.arachnoid.com/sky/redshift.html) ---
**Author:** ![Tony\_Montana](https://avatars.discourse-cdn.com/v4/letter/t/278dde/32.png) [@Tony\_Montana](https://boards.straightdope.com/u/Tony_Montana)\ **Post date:** [January 20, 2003, 6:55pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/10 "2003-01-20T18:55:21Z")
But, I thought the light shift would be red if you’re headed _towards_ a light source and blue if away,no? /hijack Also, is it true that you can see these effects star-gazing?(stars twinkling from white/blue or white/red) /lojack ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [January 20, 2003, 6:57pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/11 "2003-01-20T18:57:04Z")
No, it’s red for away and blue for toward. Although these things are very useful in astronomy, you will not see the effect visually in stars in our galaxy; this sort of thing is measured with a spectrometer. One of my old physics texts had a problem that used this idea. A motorist stopped for speeding appeals because the blueshift made the light look green to him. The judge drops the light-running charge and immediately charges him with speeding, one dollar per MPH over the speed limit he was going at the tmie. The problem was to figure out what his fine was. IIRC it was on the order of $100 Million. ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [January 20, 2003, 7:02pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/12 "2003-01-20T19:02:06Z")
[This page](http://www.fnal.gov/pub/inquiring/more/light/light_page20.html) describes the very situation in the OP, and you only need to be going about .17c to observe red as green. The formula for the frequency shift is : f[sub]obs[/sub] / f[sub]0[/sub] = [1 / (1 + v/c[sub]0[/sub]) ] \* sqrt ( 1 - v / _c_[sup]2[/sup] ) where f is the frequency observed, f[sub]0[/sub] is the frequency emitted, and v is the speed of the observer or transmitter (the formula can be restated with both moving). c[sub]0[/sub] is the speed of the wave in the medium ( since this applies to any wave, such as sound), but _c_ is the universal constant speed of light in vacuum. So for low-speed waves like sound or water, the relativistic correction term is small (v/_c_ is small), but for light, c[sub]0[/sub] =\> _c_ and the formula simplifies to the one shown in the linked page. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [January 20, 2003, 7:09pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/13 "2003-01-20T19:09:02Z")
Hmmmm, **panamajack** ’s link says that the guy was driving at 31875 MPH, but 0.17c is actually 114,004,827 MPH, I think. But it’s still pretty cool. ---
**Author:** ![panamajack](https://avatars.discourse-cdn.com/v4/letter/p/47e85d/32.png) [@panamajack](https://boards.straightdope.com/u/panamajack)\ **Post date:** [January 20, 2003, 7:25pm UTC](https://boards.straightdope.com/t/if-you-drive-fast-enough-at-a-red-light-it-will-appear-green/149822/14 "2003-01-20T19:25:42Z")
The 114 million MPH is right; it makes it 31,875 miles per _second_, an error on their part. I made a mistake in my formula, too : the sqrt term should have (v/_c_)[sup]2[/sup], not v/(_c_[sup]2[/sup]). I remember a comic strip (for a college newspaper) that showed a guy taking a test. The question was what the Doppler Effect was; he wrote, “The Doppler Effect is what makes the lights on cars look white when they’re coming toward you, and red when they’re moving away.”