Tuesday, May 01, 2007

We Live in the Past

English astronomers have discovered some very young galaxies, according to a new study (Monthly Notices of the Royal Astronomical Society, abstract available at www.blackwell-synergy.com/doi/abs/, cited by Cowen (2007).

Twenty-one extremely young galaxies were identified. (Audience yells: “How young were they?”) These galaxies were so young, their eyes had just opened even though they had no face yet (see spooky picture above from the Hubble space telescope). Actually, they formed only 1.2 billion years after the Big Bang, which on a human time scale is about 8 years old.

These galaxies have many interesting properties, but what made me stop was the thought that if a giant green monster appeared in the sky and ate these galaxies, we would not know. It takes 12 billion years for light from these galaxies to reach us, traveling at, of course, the speed of light!

This is an old mind-twister that we learn in high school, but I still have trouble with it. As we see these galaxies now, we are looking at light that has reached us after traveling through space for the last 12 billion years, beginning long before the earth itself even existed. We are seeing what they looked like 12 billion years ago. They could have turned pink and formed the word “Hello!” in the sky, then gone back to their present configuration, but such antics would not be visible to us until many more billions of years from now.

The Hubble is a time machine. How do we feel about that? We are looking at these galaxies now, right now in the present moment. Yet we are seeing them as they were 12 billion years ago. What sense does that make?

It’s not like looking at an old picture of the 1880’s. Even though all the people in the picture are long since dead, you don’t get the sense (very much ) that you are peering through the veil of death because the photograph itself has survived (even if it’s a copy of a copy). We understand that many things in the universe survive longer than a human being.

But in this case, we are not looking at an old picture of a galaxy. We are looking at the galaxy as it appears in the natural world today. How can we be looking at something in the natural world today and be seeing it as it was 12 billion years ago, with no possible way to observe it as it is today?

This is also not like reconstructing the ages of things in the fossil record. We can say a certain plant lived 2 billion years ago, based on fossils. There were no humans around to see it, but we are here now and we see the traces it left behind. That’s like seeing someone’s footprints in the snow. The person is gone now, but the footprints reveal their former presence.

We are not looking at these galaxies' footprints or the dust and gas left over after they blew up. That’s them, right there, in the sky now. And yet, we are seeing what they looked like before we got here; before there even was a here here.

And the lifetime of those galaxies, as they grew up from 8 years old to the 80 years old they would be today, on a human scale – that lifetime is in the galaxies’ past, but it is in our future to observe it. In that sense, the galaxies, as they exist today, are 12 billion years in our future.

What’s amazing is that the same principle applies to looking at anything. When I look at my computer screen, I am seeing it in the past, because it takes a certain finite, non-zero amount of time for the light to reach my eyes, just as it does for the light from those distant galaxies. Of course that time is much shorter for the computer screen. In the same way, everything I see, I see in the past. It is not possible, in principle, to ever perceive anything in the present moment. Everything we perceive is history. We just choose to ignore the time lag when convenient. So is there a present moment? There can't be, in principle. We live in the past.

Cowen, R. (2007). Back to (Near) the Beginning: Galactic Springtime. Science News, 171 (April 21) 246.