Gas pressure requires a container

Example of a meme showing flerfs' misunderstanding of gas pressure.

Flat Earthers like to claim that gas (specifically the gas in Earth’s atmosphere) can only exert pressure if it is contained by some tangible, airtight barrier, often citing the second law of thermodynamics as the reason for this. The argument is used to imply the existence of a firmament that contains the atmosphere.

Common adages include:

• Gas pressure requires a container
• You can’t have gas pressure without a container
• The necessary antecedent to gas pressure is a container
• Gas pressure is defined as gas molecules pressing against the walls of the container

Of course, Earth is sufficiently massive for gravity to contain the atmosphere. Many flerfs think that gas isn't affected by gravity, but this is incorrect. Gas has mass and is affected by gravity just as much as any other form of matter. In a gravitational field, gas settles into a pressure gradient such that the upward force due to pressure differences perfectly balances the downward force due to gravity; a situation known as hydrostatic balance. This pressure gradient is observed in the atmosphere and is easily measurable, even with a mobile phone app. Contrary to popular belief, the gradient is not necessarily caused by the fact that gravity is weaker further from Earth's surface.

If the firmament were exclusively responsible for containing the atmosphere, there would not be a pressure gradient. Instead, air pressure would equalise everywhere. Even if the firmament were real, it would not be what is containing the atmosphere.

Vacuum

Flerfs often make the mistake of thinking that vacuums pull things towards them. This is because we intuitively think of "sucking" as a pulling force, when in actuality, it is the pressure of a substance that pushes it towards free space. Vacuums are simply regions that are devoid of matter. They don't create forces and don't have any kind of "sucking power", contrary to the belief held by flerfs that the vacuum of space should immediately suck all the air out of the atmosphere. Most if not all of outer space is only an approximate vacuum; even interstellar space contains around one atom per cubic centimetre. This equates to about 1×10^-17 torr.

Housekeeping question

The afformentioned pressure of interstellar space led to the famous housekeeping quesion, "is 10^-17 a negative number?". Flerfs have struggled to answer this. Rather humorously, some flerfs think that negative gas pressure is possible.

Vacuum chambers and pressurised containers

Flerfs like to point out that if you open a vacuum chamber, air immediately rushes in to fill the space and equalise in pressure, regardless of the orientation of the opening. Likewise, if you release the valve on a bottle of gas pressurised above atmospheric pressure, the air quickly rushes out of the container into the lower pressure of the atmosphere. They use these demonstrations in an attempt to prove that gravity has no effect on restricting the dispersal of gas.

In actuality, gravity is acting on gas in both of these situations, but the force due to pressure differences far exceeds the force due to gravity. Flerfs are generally unable to understand that multiple forces can act on something at the same time. All gas in the atmosphere eventually settles into the specific pressure gradient that is perfectly balanced by gravity.

Second law of thermodynamics

Flerfs assert that the atmosphere is a violation of the second law of thermodynamics, claiming entropy implies air should disperse into outer space. Ironically, flerfs cite physical laws when they think it suits them, while rejecting others like the law of gravity.

The second law of thermodynamics actually deals with heat transfer. A very basic way the second law is described is by two bodies with different temperatures in contact: heat from the warmer body will transfer to the cooler body until the two are the same temperature. The second law has been demonstrated to students by illustrating a volume filled with gas next to a volume with a vacuum. When the wall dividing the two is removed, the gas will disperse to fill the available volume. This is only an analogy for the second law and does not define it.

If the sun were to disappear, the atmosphere would slowly radiate heat out into space, but it would still be held onto by Earth's gravity.

Flerf demonstrations

Flerfs have attempted to prove that gas pressure requires a container through physical demonstrations:

• Nathan Oakley tried to show that the atmosphere should get sucked into space by bursting a balloon.

Simulations

Screenshot of Mick West's simulation on Metabunk, showing how the gravity of a planet creates an atmosphere with a pressure gradient.

Very basic atmospheric simulations show that it is possible for gravity to contain air. Mick West and okreylos have coded their own, and both show that gravity will result in a pressure gradient.

Mick West's interactive simulation is available on Metabunk.org.

See also

• Ideal Gas Law
• Brian Sweet
• Misconception about the Second Law of Thermodynamics by FlatEarth.ws