As is so often the case, the simple questions can be the deepest, and this is no exception!
Suppose you take a box and try to suck the air out of it. This is a first natural try at making a perfect vacuum, and, as you note, it soon gets hard to do well since the air around outside will keep trying to find a way inside, in keeping with "nature abhors a vacuum."
You might go out into space and find a place where there wasn't much matter to avoid the "air trying to get in" problem. Of course, even space has gas clouds and whatnot in it. However, even if you chose a little volume somewhere with no atoms around you'd still be in trouble since there is something called the "cosmic microwave background radiation." This is leftover heat from the big bang, and it would give you about 6,000 photons (bits of heat radiation) per cubic inch of empty space.
In fact, any conceivable box you make would have walls of some temperature, and they would emit photons into the inside. The only way to avoid this is to cool the walls to absolute zero. Unfortunately, a law of thermodynamics tells you that there's no way to get all the way down to absolute zero. You might have guessed this already since it seems like something that might be hard to do.
You might be clever and make the box so small that there's no way for a photon to fit inside. Sadly, you'd be out of luck again since photons come in all sizes, and in fact the act of assembling the box would invariably produce photons anyway and mess things up. If you tried to make a very tiny box, this would eventually require so much energy that more particles would be produced and spoil your attempts.
In short, as far as current scientific knowledge tells us, you can't make a perfect vacuum.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.
