I’m reading Death By Black Hole by Neil deGrasse Tyson, and I’m on the chapter on density. He claims that in intergalactic space, a cube 200000 kilometers on a side will have about the same number of atom as the air in your fridge. Then he goes on to say that this is probably the closest to an absolute vacuum as you get in space.
Now, it seems to me that if you could look at a small enough spot in that space, that would an area with no atoms. So does a spot have to be a big enough area with no atoms to be considered a vacuum? What’s the deal?
Your fridge is filled with air, which is a mixture of gases. Gases consist of bunches of atoms and molecules zipping around and banging into stuff and bouncing off again. Isolate a small enough section of your fridge, and you will find a space with no atoms.
But remember that gas atoms are constantly moving. So, soon enough, some atoms will fly through your spot, and some other spot will be empty. So when it comes to gases, it only makes sense to talk about the average number of atoms that you’ll find in any one spot, which for your fridge (and outer space) is a number greater than zero.
Pressure, like temperature, is a statistical averaging of atomic behavior. So atomic density of space is rough 1x10[sup]6[/sup] atoms for every m[sup]3[/sup], atomic density of earth at sea level is 6x10[sup]23[/sup] atoms per m[sup]3[/sup]. Now P=nRt/V so to compare P[sub]space[/sub] to P[sub]earth[/sub] you wind up with
P[sub]space[/sub] = p[sub]earth[/sub]*[n[sub]space[/sub]T[sub]space[/sub]/n[sub]earth[/sub]T[sub]earth[/sub]] since the Vs will be equivalent. n is the number of particles in question so we know that
Lets say that T[sub]space[/sub] could be 10K with T[sub]earth[/sub] being about 300 so that gives up P[sub]space[/sub] = 5x10[sup]-20[/sup]*P[sub]earth[/sub]
A high vacuum state would be around 10[sup]-14[/sup]*P[sub]earth[/sub]
If we assume air has 1.245 x 10[sup]24[/sup] atoms in a cubic foot as stated here (note that it takes into account diatomic molecules in air), and the average refigerator is 22 cubic feet, then we get a total of 2.739 x 10[sup]25[/sup] atoms.
Distributed over a cube of 200,000 km, which has volume 8.0 x 10[sup]15[/sup] km[sup]3[/sup], that is about 3.0 x 10[sup]9[/sup] atoms per cubic km, or 3.0 x 10[sup]0[/sup] atoms per cubic meter.
That is, 3 atoms in a space a little bigger than that same refrigerator. My statistics are a little rusty but you could figure out what contiguous volume in that space you could select at random and have an expectation of 95% that there would be zero atoms.
And as **DSeid **suggests, you have virtual particles constantly appearing and disappearing. However, I don’t think those partcles are atoms, and since they come in and out of existence spontaneously in a vacuum they may not disqualify the vacuum from being conceptually “absolute.” That is, the OP probably has to answer his own question of what is an “absolute” vacuum. If you mean a vacuum devoid of *all *matter, then you can’t get there except for possibly short durations. If you exclude virtual particles then you just need to get rid of those three pesky atoms per cubic meter.
I agree with that. As a consequence, pressure can only be defined or measured over a large enough volume that you can can count the number of atoms/molecules. Or measured over long enough time period that you can count the number of atoms/molecules that pass through a given volume.
Comedienne Rosanne Barr once said that she wouldn’t clean carpets until somebody made a riding vacuum. Electrolux made one for her, but by then, she could afford to hire it done. That was impressive, but not absolute.
