PDA

View Full Version : Optical doppler on Earth



Lord Jubjub
2009-Dec-17, 12:49 AM
Thought experiment:

A semi-trailer truck is painted black such that no light is reflected or emitted except through the headlights, running lights and tail lights. These all emit at exactly 510 nanometers (greenish light).

You are sitting by the side of the road with a device that will tell you, to the nearest nanometer, the wavelength of light it detects. As the truck approaches you, the light will appear bluer. You read exactly 510 nm as the truck passes you. The truck then drives away from you with a redder light.

If the truck was driving, relative to you, at 60 mph (100 kmph), would the wavelength differences be detectable?

Hornblower
2009-Dec-17, 12:58 AM
Thought experiment:

A semi-trailer truck is painted black such that no light is reflected or emitted except through the headlights, running lights and tail lights. These all emit at exactly 510 nanometers (greenish light).

You are sitting by the side of the road with a device that will tell you, to the nearest nanometer, the wavelength of light it detects. As the truck approaches you, the light will appear bluer. You read exactly 510 nm as the truck passes you. The truck then drives away from you with a redder light.

If the truck was driving, relative to you, at 60 mph (100 kmph), would the wavelength differences be detectable?

The speed is roughly 1/10,000,000 of the speed of light, so the wavelength change would be roughly 1/20,000 of an nanometer. You would need an instrument that can do much better than the nearest nanometer.

Lord Jubjub
2009-Dec-17, 01:06 AM
So even New Horizons, shortly after launch, only had a redshift in the picometer range?

01101001
2009-Dec-17, 01:09 AM
Manmade stuff is slow!

ngc3314
2009-Dec-17, 03:18 AM
If you don't mind a large massive setup, the fiber-fed temperature-controlled spectrographs used for extrasolar Doppler detection of planets can measure velocity changes of a few meters per second (shifts of a few parts in 10^8 in wavelength).

George
2009-Dec-17, 05:09 AM
If you don't mind a large massive setup, the fiber-fed temperature-controlled spectrographs used for extrasolar Doppler detection of planets can measure velocity changes of a few meters per second (shifts of a few parts in 10^8 in wavelength). That reminds me of SOHO, which is capable of something like 0.01 cm/sec velocities.