# What is vacuum?

It is well known that where there is nothing there is a vacuum. How is this possible, as we have gas all around world and it is the job of air to fill the empty spaces. So why doesn't air get dispersed and fill those places?

Does a vacuum create pressure? If yes, why is there no pressure between molecules of gases or any other place like outer space? And if no, then it is lengthy! If we start emptying a tumbler it creates pressure (low pressure area), and then why would the tumbler have no pressure at all after ejecting all the air from it?

• a reason why air might not "get dispersed and fill that places" is that there is a wall around this space. Aug 3 '16 at 16:16
• What about in space. Aug 3 '16 at 16:18
• There is gravity which keeps the air in place. Moreover, vacuum isn't really empty there are lot of photons and other particles moving. Some of these particles have mass and some of them have at least momentum which can exert pressure. Aug 3 '16 at 16:21
• Are you saying that here gravity wins over vacuum . But now i am waiting a answer for 'what is vacuum?' Aug 3 '16 at 16:23
• Not just photons. en.wikipedia.org/wiki/Interstellar_medium Aug 3 '16 at 17:45

You are right - "Nature abhors a vacuum". That is - where there is "nothing", "something" will quickly finds its way there. This is why it's hard to keep your desk tidy...

But seriously - if you prevent "something" from reaching the "nothing", then you will continue to have "nothing". Which is why people build vacuum chambers. It is remarkably hard to make a really good vacuum on earth, as any surface that you use to contain the vacuum (of course you are really "keeping the something out", but in vacuum technology one often speaks of keeping the vacuum in) will have a tendency to give off occasional molecules. This can be adsorbed gas (a thin layer of gas molecules that were sticking to the surface), dirt, - anything. To make a good vacuum you need really clean components, and they usually have to be heated to help remove any contamination from the surface.

In outer space, there's a pretty good (but not perfect) vacuum - I have read that there are about 50 hydrogen atoms per cubic meter. So wherever you look, you will from time to time see an atom.

So no- for the purpose of your question, vacuum does not create a pressure to keep things out: it's just a space where either "molecules have been pulled away" or "molecules have a hard time getting to". And there is no "perfect" vacuum.

• No one going to say something about 'virtual particles'?
– Gert
Aug 3 '16 at 16:26
• But vacuum tries to force molecule to desperse??? Aug 3 '16 at 16:33
• @Gert - definitely not. OP is a curious 15 year old... Aug 3 '16 at 16:35
• @dr.honey I have no idea what you just said... Aug 3 '16 at 16:41
• @dr.honey A vacuum is not a thing or a force. It is the absence of gas in a space. Much like the absence of warmth is called cold, or the absence of light is called darkness. It's not like an empty space would "suck" molecules to it with a force. Instead, other molecules tend to push molecules into the vacuum. Aug 3 '16 at 18:15

First the part 2 of your question, as rightly pointed out by others that in outer space 1) the gravity does contain the gases near earth and 2) the escaped gases has lots of volume to fill hence the density become too low.

Now there is nothing like perfect vacuum. Perfect vacuum means $0$ particles in infinite volume.

Atmospheric pressure i.e. ~$1$ bar ($10^3$ mili bar or mbar) has ~$10^{19}$ particles per cc. If you remove the 10% of the atoms from a box nearly 0.9 bar pressure is applied on that box, and if you remove 99% of atoms nearly 1 bar pressure is applied. Hence in terms of pressure $10$ mbar is nearly perfect vacuum.

As pointed out by others a vacuum chamber is required to create vacuum. Vacuum chamber is a vessel which prevents the outside air to fill in and a pump is connected to the chamber which suck the inside air out.

Usually the vacuum chamber is sealed by rubber or metal seals which fills the gap in metal plates and helps in creating vacuum. Ideally one can make a permanently sealed vacuum chamber but that would be of no use for experiments.

We can make a vacuum of the order of $10^{-10}$ mbar on the earth, it still has ~$10^6$ particles per cc. These particles are mainly from the evaporation from metal surface of the chamber. You may call $10^{-10}$ mbar nearly a perfect vacuum.

I will be happy to help further

EDIT: In response to your comment

I think you need to read a little bit about kinetic theory of gases. In the first approximation the gas atoms/molecules are treated as billiard balls. When they collide with each other they bounce elastically. When they collide with the walls of a container then they exert the pressure on the wall. In the first approximation the pressure increases linearly with the density of the molecule. The velocity/energy in the gas molecule comes from thermal energy and it is a measure of the temperature.

Now imagine if there is a container where the pressure inside and outside is same then there is no pressure difference. If you decrease the pressure inside the container then the pressure difference increases.

inside pressure $P_{in} \propto n_{in}$

outside pressure $P_{out} \propto n_{out}$

Pressure difference $\Delta P=P_{out}-P_{in}$

in this case max pressure difference is $\Delta P=P_{out}-0=P_{out}$

Hence with increasing vacuum, pressure difference can not increase linearly but tends towards the limiting pressure difference and that is the atmospheric pressure.

Although we neglect the attraction between gas molecules (but it is there), but that will not make any difference.

Due to attraction between gas molecules the actual pressure of a real gas is little bit smaller than the pressure of a ideal gas, This effect is more pronounced at lower temperatures but since we have removed most of the gas molecules hence the distance between gas molecules is quite large and that makes their mutual attraction negligible.

I have to admit that i could not quite understand part of your question

Does a vacuum create pressure?

I think pressure and vacuum are opposite to each other. Vacuum can generate pressure difference but it is actually absence of pressure.

This is also confusing

If yes, why is there no pressure between molecules of gases or any other place like outer space?

I dont think that there is pressure between molecules of gases. When we talk of kinetic theory of gases there are two types of the quantities i.e. macroscopic (things we can feel or measure) and microscopic (the actual processes that is happening on the level of atoms/molecules). Pressure is a macroscopic quantities. There is no pressure between molecules of gases. There is force between the molecules of gases and this force arises due to the collision of molecules. Hence if pressure (or the molecular density) is too low (as in outer space) the collision become more and more improbable and there is no force exerted on the atoms/molecules (for a very long time.)

• See how we create vacuum.......1)suck air ,, more low pressure area 2)suck more air ,, more low pressure area 3)suck all air ,, large pressure should be created ...but why its not the CASe ..if it is then why air molecule is not by themselves and become denser Aug 4 '16 at 9:18
• Pressure of a gas $p\propto n$. Now if you remove just 99% of molecules the pressure become 1% of initial value. Now if you remove all atoms the pressure difference is increased by 1% hence not much increase. I think this will clear your doubt. Maximum pressure difference between atmosphere and absolute vacuum is ~1 bar. Aug 4 '16 at 9:33
• So the pressure increases but it is negligible ...now my question is why ,, there is no attraction b/w molcule of gases that is an empty space . Aug 4 '16 at 9:40
• @dr.honey please see the edited question for the elaborate answer. Aug 4 '16 at 16:52

It is well known that where there is nothing there is a vacuum.

Well, technically speaking this statement is not true. Moreover it is philosophically doubtful. There is one very nice documentary chaired by Neil Tyson where a few scientists discuss about "nothing". You can see in this documentary that it is not very easy to decide if nothing really exits in universe but vacuum, at least technically, has nothing to do with nothingness.

What is vacuum, then?
Unfortunately, the answer that you get to this question very much depends on who you ask. The concept of vacuum in a theoretical physicist mind is not same as an experimental physicist or an engineer. Considering that your following questions are very much down to earth type, I assume that you are interested in what an experimental physicist think of vacuum. If so, one can define vacuum as a region of space, mostly closed, which has lower pressure then surroundings. An experimental physicist in general defines three different vacuum regimes:
rough vacuum: $1000-10^{^{-4}}\ mbar$
high vacuum: $10^{^{-5}}-10^{^{-9}}\ mbar$
ultrahigh vacuum: $<10^{^{-9}}\ mbar$.

One final comment here: it is technically not possible to have a vessel, container, chamber which is completely, fully, 100% free of any gasses. Why this is the case is somehow explained in the previous answers.

How is this possible, as we have gas all around world and it is the job of air to fill the empty spaces. So why doesn't air get dispersed and fill those places?

I assume here when you say "those spaces" you mean the outer space. Our planet is surrounded by air (our atmosphere) and it is gravity that keeps the air around the earth and not run away to the outher space. For example the gravity of Mars is not strong enough to keep an atmosphere similar to ours around Mars.

Does a vacuum create pressure?

Yes and no. As explained above vacuum is "lack of pressure". If you have vacuum somewhere you have a lower pressure than surroundings. Therefore there is a pressure toward the vacuum from the surroundings but not other way around.

Why is there no pressure between molecules of gases or any other place like outer space?

Pressure is defined as force per area. That means wherever you have some sort of force you can conceptually think that there is some sort of pressure. Since it is almost impossible to have a region of space where there is no particles that means that there is always some sort of pressure everywhere. Maybe sometimes so low that our most fancy gadgets today cannot measure the pressure.