by Chet Raymo
Imagine that the mile represents the distance to the most distant objects we see with our telescope, most of the way back to the big bang itself, 13.7 billion years ago. As I start my walk in the morning, the big bang is a mile away - 13.7 billion light-years.
On this scale, the Milky Way Galaxy is about the size of a dime - one hundred billion stars in a whirling spiral. The next spiral, the Andromeda Galaxy, is another dime (well, maybe a nickel) about as far away as my footprint is long. The Milky Way under my heel, Andromeda under my toe.
I take a step. I span galaxies. As I walk, galaxies flow under my feet. Fling dimes for a mile in every direction, a foot or so apart. Every dime is a galaxy of a hundred billion stars. Every star (perhaps) with planets.
As we look out with our telescopes, we see those other galaxies as they were at some time in the past. No galaxy is closer to the center. There is no center. Nor a boundary. The universe inflates like the surface of a balloon. Space (the surface of the balloon in my analogy) comes into existence as the balloon inflates. The galaxies are dots on the balloon. As the balloon inflates, the dots move apart. But no dot is central, and there is no boundary to the surface. The view from any galaxy is like the view from any other.
Now let the mile of my walk represent time, the 13.7 billion years that the universe as we know it has been in existence. All of recorded human history would snuggle under my little toe. My lifetime is the thickness of a slip of paper.
You've seen the Hubble photographs. You've heard the analogies. But one can hear the story a hundred times and still it's hard to grasp the dimensions of cosmic space and time. I've been writing and teaching this sort of thing for half a lifetime, and I still have a hard time getting my head around it.
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