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Subject: Confusion over red shift

Date: Wed Aug 11 15:23:33 1999
Posted by John W
Grade level: grad (science) School: No school entered.
City: Lexington State/Province: KY Country: USA
Area of science: Astronomy
ID: 934399413.As Message:
From what I have understood, one of the main reasons for believing that stars, galaxies, etc. are "moving away from us" is the red shift of light from these objects. Objects at the same distance away from us have about the same amount of red shift, and objects farther out have more. But reading recent questions/answers on this site, it appears that light will red shift over distances regardless of the objects' motions (i.e. if there are two objects that are not moving relative to each other, and gravity effects are nominal, then light from one object to the other will red shift, and the greater the distance, the more shift). If this is indeed true, then doesn't this account for the redshift we see in the universe? I am not saying that the "Doppler effect red shift" is not true, but how can we say that these stars/galaxies are moving away from us, when it may just be the "natural red shift" during the trip the light takes? After all, this seems to support that the farther out an object is, the more red shift we see.

I went through the Mad Science archives and didn't see any answers that say that light leaks energy as it travels, but the archives are big and I might have missed them!

This idea, usually called `tired light'', has many predictable consequences, not one of which is seen. There are many things tired light cannot explain, too.

The biggest problem is that we see time dilation in distant supernovae. Exploding stars that are very far away appear to get brighter and dimmer slower than ones that are nearby. This dilation of time is exactly what we expect if the supernova is moving rapidly with respect to us; in other words, the supernova is so far away that the expansion of the universe is carrying it away from us at a healthy fraction of the speed of light. We know time appears to slow down for a clock moving relative to a stationary observer, and the amount of dilation seen in distant supernovae matches very well with the dilation predicted given their velocity.

For more about current thinking in cosmology and a discussion of why tired light doesn't work, check out Ned Wright's excellent Cosmology FAQ (tired light is listed under ``fads and fallacies'').

©2008 Phil Plait. All Rights Reserved.

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