Looking back in time…

As archaeologists dig through deeper layers of pristine soil, in the
process they uncover artifacts from increasingly older civilizations.
Similarly, as astronomers look to greater distances from Earth, in the
process we see further back in time. Let’s see if we can get our
heads wrapped around this concept. It’s certainly not true that when
we look to the opposite side of a football field we see into the past.
If so, then the people farther away from us would look younger. This
would be the case though if you put your friend on a planet orbiting a
nearby star. The reason for this ‘anti-aging’ effect has to do with a
basic property of light. We can only see other people, whether at a
sporting event or on a distant planet, if light bounces off of that
person and makes it back to our eyes. This normally happens very
quickly because the speed of light is so fast, 670,000,000 mi/h. At
the same time, the speed of light is not infinitely fast. When you
look at a nearby star in the sky, it is so much farther than the other
side of a football stadium that in fact it takes the light many years
to travel to Earth. For example, imagine that light from a nearby
star takes 10 years to reach us. If something dramatic were to happen
to that star in that 10 year travel time, like a stellar flare or even
complete annihilation of the star (a supernova), then you would not
know about it on average until a handful of years after the fact! The
situation gets more extreme as we look beyond the stars to objects
even further away from us such as galaxies. Light from the nearest
galaxy to us from the northern hemisphere takes over two million years
to reach us. If we decide to transmit a radio message today to
Andromeda broadcasting the simple message that “We are here,” such a
hypothetical message would take over two million years to reach
Andromeda. Even if such a message was received and translated by any
potential intelligent creatures living on a planet in this galaxy,
they would know that we had sent this message over 2 millions years
ago. How will we be doing in two million years?