Expert: Courtney Seligman - 10/29/2010

Question
Hi, I am a lawyer with a healthy interest in science.  I read the popular science books and magazines.  In doing so I thought I understood most basic concepts.  It occurred to me today that I don't understand how we determine how far away objects are.  I was reading about a recent Hubble discovery of the most ancient galaxy we have seen yet.  It was dated through red-shift.  I understand the idea is that the older/farther the light is, the more it will shift to the red side of the spectrum.  My question is how does light have a Doppler effect?  As a train comes nearer and passes, I get how the sound waves become more and less compressed.  Einstein's whole deal was based on a train thought experiment where light on a train and off (and between passengers# traveled at the same speed relative to each person.  Shouldn't all the light coming at us stay at the same speed and thus wavelength?  How is old/far light different than light from the sun?  Universal expansion could account for some of that, but is that the whole basis that we use to date how far something is from us?  Am I missing a basic concept that makes light waves expand over distance, but stay the same speed?  In advance, thank  you for considering my #probably silly) question.

Answer
The expansion of the Universe is caused by the expansion of the empty space within it. On the average, a given amount of empty space doubles in size in a few billion years.

As light passes through space it "stretches", increasing its wavelength by the same amount as the space it passes through. If it takes a few billion years to pass through a region, so that the space it passes through doubles in size, then the light will double in "size" or wavelength, as well. If it takes less time, the space it passes through won't grow as much, so the light's wavelength won't increase as much. If it takes more time, the space it passes through will grow more, so the light's wavelength will increase more.

As a result of this line of thought, we can conclude that the further light has to travel to reach us, the longer its wavelength will be. So we can use the amount that its wavelength has changed (its "redshift") to estimate how far it hard to travel. Ignoring complications, if light from one object has lengthened by 1%, and light from a more distant object has lengthened by 10%, then the light from the more distant object must have had to travel ten times farther, and the object must be ten times farther away.