eli5 Why is a perfect vacuum so hard to create?

Question

My university has a sputtering machine which is this crazy expensive piece of equipment that has to have a really strong vacuum pump and wacky copper seals and if it loses power for even a minute it has to spend 16 hours pumping it’s vacuum back down.

I know people talk about how a perfect vacuum is like near impossible, but why? We can pressurize things really easily, like air soft co2 canisters or compressed air, which is way above 1 atmosphere in pressure, so why is going below 1 atmosphere so hard? I feel dumb asking this as a senior mechanical engineering student but like I have no clue lol.

In: 3143

Answer 1

Let’s do a 2d anolgy.

You have a hockey rink with 10,000 hockey pucks. You bump one, you’ll likely make all of them move, eventually. If everything is slick enough, they’ll never stop moving, sliding and bumping each other. So consider them all in constant motion from bumping into each other.

Now you want to empty the hockey rink of hockey pucks. You make a hole in the ice, but it takes a lot of energy to get keep that hole open so that hockey pucks fall out and don’t come back in.

Because the pucks are in the constant motion from colliding with each other other, at first, hocky pucks fall down the hole at a high rate, but as the pucks drop out, there are fewer and fewer pucks on the ice. They don’t bump into each other as often. They move slower and slower because there are fewer and fewer collisions.

Eventually you’re sitting there watching the last ten or so pucks meandering around the ice, sometimes bumping the wall or each other, but they’re going slowly and its just stupid luck when they even get close to the the hole (which still requires a lot of energy to keep open and to keep pucks from coming back in).

That’s why. There’s no real force moving the last molecules out of the container as it gets more and more empty. If they get close enough to the exit, they’ll leave, but unlike the start, when there are a lot of collisions moving things around, its just a blind luck that they’ll get close enough to the pump to get the boot.