# How fast would I need to go...

**URL:** <https://boards.straightdope.com/t/how-fast-would-i-need-to-go/86386>\
**Category:** Factual Questions\
**Created:** [October 9, 2001, 7:42pm UTC](https://boards.straightdope.com/t/how-fast-would-i-need-to-go/86386 "2001-10-09T19:42:39Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\
**Post date:** [October 9, 2001, 7:42pm UTC](https://boards.straightdope.com/t/how-fast-would-i-need-to-go/86386/1 "2001-10-09T19:42:39Z")

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For doppler shift to make a red traffic light look green. I would imagine its much faster than a car could ever go but I was just curious about the facts of a joke I heard.

“but officer I was going so fast it looked green to me”

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**Author:** ![geepee](https://avatars.discourse-cdn.com/v4/letter/g/ebca7d/32.png) [@geepee](https://boards.straightdope.com/u/geepee)\
**Post date:** [October 9, 2001, 7:50pm UTC](https://boards.straightdope.com/t/how-fast-would-i-need-to-go/86386/2 "2001-10-09T19:50:17Z")

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very very very fast , things moving away (such as galaxies)  
give off red shift , but then things moving towards give blueshift , it would have to be about 80% of what the speed the galaxies are moving apart which in your average car you can’t do , i recall somebody trying this in court once and it was thrown out because the prosecuting lawyer had done his homework and the defense did not

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [October 9, 2001, 8:33pm UTC](https://boards.straightdope.com/t/how-fast-would-i-need-to-go/86386/3 "2001-10-09T20:33:45Z")

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A red LED stoplight emits light with a wavelength ( [sym]l[/sym] ) of 700 nanometers. A green LED emits light at 525 nm.  
For nonrelativistic speeds [sym]l[/sym][sub]observed[/sub] = [sym]l[/sym][sub]emitted[/sub] (1+v/c)  
so 525 = 700(1 +v/300,000)  
or v = 75,000 kilometers per second, towards the stoplight

That’s roughly 162 million miles an hour, which is pretty fast for a car.

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**Author:** ![Kamandi](https://avatars.discourse-cdn.com/v4/letter/k/5daacb/32.png) [@Kamandi](https://boards.straightdope.com/u/Kamandi)\
**Post date:** [October 9, 2001, 9:38pm UTC](https://boards.straightdope.com/t/how-fast-would-i-need-to-go/86386/4 "2001-10-09T21:38:12Z")

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My Rambler could do that, but only in second gear.

Beep beep!

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**Author:** ![screech-owl](https://avatars.discourse-cdn.com/v4/letter/s/82dd89/32.png) [@screech-owl](https://boards.straightdope.com/u/screech-owl)\
**Post date:** [October 9, 2001, 10:16pm UTC](https://boards.straightdope.com/t/how-fast-would-i-need-to-go/86386/5 "2001-10-09T22:16:16Z")

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Bumper sticker I’ve seen:

Next to a picture of a traffic light with all green lights -  
“At Warp Factor 9, they ALL look green to me”

[sub]Good one, Kamandi[/sub]