Redshifting

Jillian's Guide to Gravitational Waves : This Idea Called Redshifting

It happens when light goes through a strong gravitational field and also over large distances of spacetime. Take the spacetime example. Say there's this light ray travelling from many millions of light years away. One would think that light ray would just cross the distance and arrive at the earth exactly as it left, plus or minus a little scattering due to all the dust and gass floating around out there. Not quite. See, as you read this, spacetime is expanding---I hope you've read my section on spacetime---and that affects the light ray. Now, instead of having only 13 million light years to cross, it has a little more. The speed of light is a constant in a vacuum, but now there's this extra distance to go. The length of the light wave, therefore, gets a little longer (very much like newsprint on silly putty that is being stretched). If the length of eave wave increases, the frequency of the waves decreases, for the frequency is inversely related to the wavelength (that is, the frequency does the opposite of the wavelength: if the freq. increases, the wavelength decreases).

Ah, the frequency decreases. What does that mean, exactly, and what does it change for astronomy? Okay, I'm sure you've been shown that nice diagram with all the frequencies of light, fram gamma rays to microwaves and with the tiny section of visible light in there, right? Good. When the frequenc of a light ray decreases, it shifts down that scale. Say our light ray was originally blue. It could conceivaly be redshifted until it was green or even yelow! A light ray could even be redshifted down to a microwave. Ever hear of the cosmic microwave background radiation that was detected by COBE (and has that odd oval picture with blue and red lumps)? Those microwaves are actually light rays from the first instant that light rays could travel through the universe, and they have travelled near 15 billion light years (and years) to get to us. Redshifting can also be caused by strong gravitational fields, such as those around a neutron star or a black hole.

Gravitational waves can also be redshifted by expanding spacetime. It's the same process, really. Here's a quick analogy: the jump rope. Imagine two people loosely holding a jump rope and wiggling it. With a little effort those people can get a standing wave with four loops. Imagine that is really a gravitational wave and the rope is spacetime. If the people pull on the rope and make it taut, the standing wave becomes one of two loops. See? Spacetime expanded and the wavelength increased (thus the frequency decreased). That's not quite scientifically right, but it's as good an explanation as I can give ya.

ALL IMAGES CONTAINED ON THIS WEBSITE ARE THE EXPRESSED PROPERTY OF THE ARTIST, JILLIAN BORNAK, AND SHOULD NOT BE TAKEN OR USED WITHOUT HER PERMISSION.
OFFENDERS WILL BE TOAD.