Thursday, 8 August 2013

Why don't we expand with space?

It's often said that space is expanding, and that raises the question of why we don't expand with space.   The thing is that space is not expanding.  Instead, gravitational fields are kind of telling everything to move apart.  It's easier to think of thing when dealing with gravitational attraction, so... what exactly is gravitational attraction?

According to Einstein, gravitational attraction is what happens when space is curved in a way that means that something that isn't doing anything - not changing direction or accelerating off somewhere - will move towards a gravitational source.   Gravitation attraction is when the default action of something is to move.  If you are in free fall in space and you get near a planet or a star you will find yourself moving towards that object without feeling anything at all.  That's gravitational attraction.  But - when you hit the surface of a planet (or land safely) then you do feel a force.  That force is not gravity, but the surface of the planet pushing back.  It feels like an acceleration, because that is what it is - your default action is to speed up towards the centre of the planet but the planet is constantly slowing you down.

So, if the universe is filled with an expansion force (as it seems to be), and this force is repulsive gravity (which it seems to be), then the default action of everything will be to separate.  However, the repulsive gravity force is EXTREMELY small, so small that your body has no problem at all resisting it.  But, it has an effect:  just like you are a tiny bit smaller on the surface of the Earth because of the resistance to gravity than you would be if you were in free fall, you are a very tiny bit larger than you would be if there was no expansion effect of the universe.  You don't continue to expand for the same reason that you aren't crushed by gravity on the surface of the Earth - your body can easily maintain its shape against such forces.

So, you don't expand with the universe, you, and the Earth, and the Solar System, and the galaxy, are all just a tiny bit larger than you would be if the expansion force wasn't there.

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