Column: Aristotle didn’t know it all about nature and vacuums

Aristotle famously claimed that nature abhors a vacuum. True, air does rush into the space from which air has been evacuated, but not because nature has feelings of any kind about the vacuum.

A vacuum is the total absence of matter. In practice, a true vacuum does not exist even in deep space.

On Earth, only the most sophisticated scientific equipment can create a vacuum as good as in outer space, but studying even partial vacuums gives us an understanding that is simple, reducible mathematically to the perfect vacuum and also consistent with the rest of the huge body of physics.

Understanding the vacuum requires the concepts of air pressure and the kinetic movement of air molecules.

Living as we do at the bottom of a sea of air, we are normally unaware of the effects of its pressure. This is because the pressure is equalized inside and outside our bodies. It is only when that equilibrium is disturbed that we notice: Ears pop in airplanes, and we feel short of breath when high in the mountains.

A drinking straw is an excellent example of the partial vacuum. We suck on one end and draw liquid up through the straw. But how can any such sucking action exert a force on the liquid in the glass to cause it to rise through the straw? It cannot!

In physics we learn that matter changes its motion only when unbalanced forces act on it (Newton’s second law). No amount of action on the air in the straw can cause the liquid to move. The unbalanced forces come from the lungs at one end of the straw and air pressure at the other.

We lower the air pressure inside the straw as atmospheric pressure on the liquid outside the straw pushes it through the straw in order to equalize the pressure inside.

Sure enough, it seems as if something inside the vacuum cleaner is magically drawing air into it. The vacuum cleaner is like a big straw, only here the action is provided by an electric motor to pump air out of the canister so that outside air pressure can push air through the hose carrying dust, dirt, pollen, mites and cat hair.

An airliner flying 6 or 7 miles high maintains artificial atmospheric pressure in the cabin, albeit lower than outside at sea level (roughly equivalent to 8,000 feet elevation). Outside there is very little air pressure, too little for us to survive.

If the hull is punctured, we say incorrectly that items are ‘sucked’ out of the plane. Invoking Newton’s second law we ask, Where is the force that sucks the air out?

Easy answer: There is no sucking force. However, the air pressure inside the cabin blows the air out in the form of a mild explosion.

Aristotle was a brilliant man, but he lived in the ignorance of his times, without the tools of science and technology that we now have to investigate beyond the direct input of the senses.

These explanations required the understanding of air pressure as introduced by Galileo in the seventeenth century, the gas laws as formulated by Boyle and Charles and the kinetic theory of gases as visualized by Bernoulli in the eighteenth century and formalized by Maxwell in the nineteenth century, and finalized in the twentieth century.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.