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So we wondered, since sound travels through gases, does that mean you could hear a sound in a nebula? It will have to do with pressure. Pressure is the force per unit area exerted on any object due to the surroundings. Atmospheric pressure on Earth is about 101.3 kPa (kilo-pascals), but it varies up and down a bit depending on the weather.

Absence of pressure is called a vacuum. Outer space is pretty close to a vacuum, being about 0.1 pico-pascals, which is close to zero. Since sound is a wave of pressure, there is a threshold minimum pressure wave humans can hear, and it is about 20 micro-pascals. To hear sound, the surrounding pressure would need to be higher than that, to support a wave of that amplitude.

Wikipedia has a very nice list of significant pressure values, which the list above is partially compiled from.

More information about the Puy de Dome and Vacuum in a vacuum experiments can be found in Designing Experiments and Games of Chance, The Unconventional Science of Blaise Pascal by William R. Shea. It is available free on Google Books (in part) and recommended by the Clear Science staff (in full)!

This brings us to Pascal’s Vacuum-in-a-vacuum experiment. (Or Vide dans le vide, sometimes translated as emptiness in emptiness.) The idea that the space above the mercury in a barometer was nothing, i.e. a vacuum, was controversial.

Pascal used a barometer made from a specially-blown glass tube, which had a curved trap in the middle. Near the trap was a hole, which could be covered to keep air out. By filling the tube and flipping it over into a tub of mercury, just like a regular barometer, one would get the usual result of a raised column of mercury balancing atmospheric air pressure. However, there would also be a collection of mercury in the curved trap.

By uncovering the hole, one allows air to rush into the tube, and the column of mercury falls, as pressure inside and outside equalize. But the mercury from the trap is pushed into the upper part of the tube, in which there is still a vacuum. This results, again, in the familiar mercury column, balancing a vacuum vs atmospheric pressure.

For this reason, the experiment is called vacuum in a vacuum.

In 1647 Pascal wrote Experiences nouvelles touchant le vide (New experiments with the vacuum), in which he explained why the weight of mercury in the tube had to balance the air pressure outside a barometer. For the mercury to be at equilibrium, they must balance in terms of F=ma. (Do you know that one?) Therefore, the height of mercury is a reading of the atmospheric pressure.

In 1648, Pascal got his brother-in-law to carry a barometer to the top of Puy de Dome, a dormant volcano in central France. He found that at the top of the mountain, the mercury level in the barometer was a little more than 3 inches lower than at the bottom of the mountain. This is because at the top of a mountain, there is less air weighing down on you, so the pressure is lower. 

Blaise Pascal performed several groundbreaking experiments using barometers, which were invented in 1643 by Evangelista Torricelli. Any fluid can be used in a barometer, but mercury was a standard in Pascal’s time. If you turn over a tube of mercury in a dish of mercury, you get a column of mercury of some height h, which is exposed to atmospheric pressure at the bottom.

It turns out that the weight of mercury pushing down and the weight of air pushing down at the bottom are the same. So the height of the mercury is really an indication of atmospheric pressure. Mercury will stand 760 mm at 101.3 kPa, and we call that 760 Torr. That’s right: one unit of pressure is named after Torricelli and one after Pascal.

Pascal’s most profound discovery with barometers was that the space at the top of the tube was in fact a vacuum, or nothing. Before that, scientists thought something had to be there, since you could see through it. Their logic was that nothingness was impossible, or would be black.

Let’s talk about something suggested by a Clear Science reader: What is Pascal’s Emptiness in Emptiness experiment? It was an experiment done in the 17th century by Blaise Pascal, who made many contributions to science. First let’s talk about Pascal and what those contributions are.

The SI unit of pressure is the pascal (Pa), taken from his name. Standard atmospheric pressure is 101,300 pascals, or 101.3 kPa. Atmospheric pressure is caused by the weight of air above you, pushing down. If all the air went away, the pressure would feel like you were in outer space, because nothing would push down on you.

(BS alert: It’s not actually “down.” More properly it’s “normal” to any surface. But if you don’t know what normal means, down is an all right way to think of it.)

If you watch the weather report, you’ll notice that the atmospheric pressure changes day to day. This is because the air is constantly in motion, and sometimes there is less on top of you than usual. (And sometimes more.)