(Submitted April 16, 2007)
Galaxies at the fartherst reaches of the universe are travelling at near the
speed of light correct? Does this mean that time within those galaxies is at
a slower rate?
Yes, from our point of view. From their point of view, time is moving
slower in our galaxy. The situation is completely symmetric. We can see
this by imagining that there is a clock in each galaxy sending out
regular tick signals. We specify that the clocks are constructed exactly
the same. We can count the ticks, compare it with our clock, and see how
time is changing in the other galaxy.
Well, there are such clocks: atoms of various elements that have definite
frequencies at which they emit light. A frequency is just a number of
ticks per second, and all atoms of a given kind, say sodium, have the
same characteristic frequencies. Looking at a distant galaxy, we can see
that their sodium atoms run slow, the light is redder than it should be.
They see the same thing, our sodium atoms look redder to them than theirs.
In addition to the time dilation effect, distant galaxies show the Doppler
effect of reddening because they are travelling away from us. The time
dilation effect exists for any direction of relative motion.
The fact that each observer sees the other's clock running slow appears
to be a paradox because of our limited experience with high speed. But
that is just the way things are. For more information on the apparent
Jay Cummings and Jeff Livas
for Ask an Astrophysicist