Firstly, I’m not a physicist or cosmologist. I enjoy reading about physics and cosmology because I’m interested in the implications for technology and because I want to be able to spot when stupid philosophers and fiction writers make huge, unfounded claims based on small scientific oddities. One thing that my very casual study of cosmology has taught me is that common sense is not reliable.
I remember sitting around a campfire with my brother and grandfather talking about the expansion of the universe. Of course only now do I understand why it is that theories of the universe are done with experimental physics and not guys sitting around campfires. We were talking about the redshift that Edwin Hubble discovered in the 1920s establishing the expansion of the universe. (If you don’t know about the red shift, google it. It’s fascinating.) The conversation that followed went something like this:
Grandfather: So when we look out into the universe we see that all the galaxies are flying away from us?
Grandfather: Does that mean that we’re in the middle?
Grandfather: Why not?
Brother: …I don’t know.
I pondered this question for weeks! Where is the middle? If we’re not in it then wouldn’t the galaxies on our side of the middle all be flying away from that middle in the same direction? It took Lawrence Krauss’ video lecture called “The Universe From Nothing” to explain to me exactly why we’re in the middle. When it was explained I almost fell off my chair with the surprise of how obvious it is.The trick is to get outside the universe. Imagine the universe as a two dimensional object. If all the galaxies were in a grid and spaced apart equally. (A) Then imagine that the grid expands so that the space between each galaxy is larger but still equal. (B) From universe has indeed expanded around a center point. But from the perspective of any one galaxy, all the other galaxies are farther away. You can see this by superimposing the expanded universe onto the original and aligning any galaxy with itself. (C,D)
The common sense answer, that we’re in the middle, is the same common sense that told astronomers that the earth was the center of the solar-system. It’s the bias of being the observer. If you’re looking at something big, wherever you’re standing looks like the middle. The mindset is even manifest in our language: when someone is lost at sea they are “in the middle of the ocean.”
You are in fact not in the middle of anything at all. The universe is a humbling place. Most importantly, common sense is not a pathway to truth. It serves for everyday things but, for any question worth talking about, you have to leave your common sense at the door.
Some people say that common sense isn’t too common; I say it doesn’t make much sense.