Redshifts and Redshift Rendezvous

December 29th, 2008

I think I may have touched on this topic in the past, but reading Redshift Rendezvous the other day reminded me of a common misconception with regard to astronomy.

Again, I recommend John Stith’s novel of mystery and action involving a hyperspace ship on which the speed of light is quite slow and relativistic effects are readily observed.   It’s a quick read with a hook and a fast-paced plot and some interesting science along the way.   I don’t think John got this next bit wrong given some assumptions that are probably valid, but I recall thinking that some subtle points might have been missed, and are certainly misunderstood in many quarters.

Anyway, objects moving away from us at close to the speed of light have a redshifted spectrum, while those moving toward us have a blueshifted spectrum.

A redshifted object may appear red, but it could also appear blue, or white, or any other color.   Likewise a blueshifted object doesn’t necessarily look blue.   It could, but doesn’t have to.

When a spectrum is shifted, ALL the colors move in a particular direction.   For a highly redshifted object to look red, it has to lose it’s blue light.   The blue light in a redshifted object is originally emitted (or reflected) in the ultraviolet.   An ultraviolet-bright object that is significantly redshifted is going to look blue, and in fact most intermediate redshift quasars were originally found because of their extremely blue colors.   It’s only in very high-redshift quasars, in which the far and extreme ultraviolet part of the spectrum is redshifted into the optical blue, that we see quasars that appear red due to the redshift effects.

I need some figures here, but I don’t have the time or inclination to make them right now.   Here is a link to a page about redshifts, quasars, and cosmology that explains this topic in more detail.

Oh, and back to Redshift Rendezvous…most lights and clothing we wear are designed for the optical spectrum.   It’s hard to get lights to put out a lot of ultraviolet, and clothes to reflect it.   Also redshifting light decreases its overall intensity.   I expect that most redshifted objects on board the ship would look pretty red.   I’m a little less certain that the blueshifted objects would look blue or violet-tinged.   As discussed last summer, a lot of our lights are cool and put out a lot of infrared light, but also as discussed last summer, the key to how things looks seems to be the light level.   Highly blueshifted objects are going to be “Doppler boosted” and brighter than normal.     Intense lights with particular colors tend to look white.   So, I think redshifted objects onboard are likely to look red, but blueshifted objects are just likely to appear brighter than normal.

Anyway, really interesting novel and I recommend it.

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