Whenever we look at anything, we are seeing it in the past thanks to the simple fact that light does not travel from place to place instantaneously. When we see the sun setting, we are seeing it as it appeared eight minutes ago, not as it exists in our present. Traveling at about 186,000 miles per second, it takes the light that long to make the 93 million mile trip from the sun to our eyes. The moon’s image at night is delayed in arriving on our retinas by about a second and a half, since it takes that long for the light to travel the 240 thousand miles from the Moon to the Earth.
But it’s when we look at the stars that we begin time traveling in earnest. Sirius, the Dog Star, the brightest star visible in the sky besides our Sun, is about eight light years away. Therefore, when we view it on some winter evening we’re seeing it as it appeared about eight years ago. That’s how long the photons hitting our eye have been in transit, traveling 186 thousand miles every second. Other stars in the sky are even further away. When we look at the North Star, Polaris, we’re looking backward in time to a moment about 434 years ago. The light from the North Star that we see tonight left the surface of that distant sun in 1578–before the King James Version of the Bible was translated, before Galileo ever pointed a telescope at the sky, and nearly two hundred years before the United States declared its independence from England.
Betelgeuse, the bright reddish star forming one of Orion’s shoulders is about 640 light years away–so when we see it come December, we’ll be seeing how it looked 640 years ago. If it went supernova when the American Revolution began, we won’t know about it for another 400 years or so.
On May 3, 1986 the astronomer Robert Evans observed a supernova in the galaxy Centaurus A. Centaurus A is about 15 million light-years away. This means he watched a star blow itself to smithereens fifteen million years ago.
The furthest object observed thus far by the Hubble Space Telescope is designated UDFj-39546284. It is 13.2 billion light years away. This means that the Hubble Space Telescope was looking 13.2 billion years into the past. For those who imagine the universe is only 6000 years old, sorry: we can even see with our naked eyes that the universe is older than that. If we went outside tonight and looked at the northeastern sky, just a bit to the right of the constellation Cassiopeia, we’ll find something very ancient. If we’re away from city lights, we’ll see a small oval fuzzy patch. That’s the Andromeda Galaxy, a galaxy of more than a trillion stars. It’s the most distant object that we can see without using a telescope. It is 2.5 million light years away–which means that we’re seeing how it looked 2.5 million years ago. We’re looking into the distant past. Our eyes alone reveal that the universe is more than 6000 years old.
An interesting, science fiction sort of thought came to me tonight; I’m not the first to have thought it, though. But if the warp drive turns out to be possible, then it would be interesting to travel multiple light years and then look at the Earth. If we could travel 2000 light years from Earth in the way the Enterprise did it in Star Trek, then, with a sufficiently large telescope, I wonder if it would be possible to view historical events? Would we be able to build a telescope with the resolution necessary to see the Earth as clearly as a U.S. Government spy satellite can see my back yard? If we could travel 65 million light years away, could we witness the asteroid strike that took out the dinosaurs?
I found it an interesting thought; probably only something for a science fiction story, however. I suspect that it isn’t possible to get that kind of resolution through any telescope that could actually be constructed.