How do we know that vacuum is devoid of matter?

According to Wikipedia:-

Vacuum is a state with no matter particles (hence the name), and no photons.

I don't understand how can we find a part of space where there is no matter present?How is it possible?

• That's wikipedia for you. That is an awful definition. – David Hammen Nov 8 at 2:20
• It isn't the defintion in wikipedia. This is a misquote. – James K Nov 8 at 22:11

What "vacuum" means depends on who you ask:

If you ask a general relativist, they will tell you that a region of space-time is a vacuum if the right-hand side of the Einstein Field Equations is zero. This means no matter or radiation and is thus experimentally impossible. However it has the advantage that there are a number of known solutions to the field equations for vacuums (for example the Schwarzschild solution which describes the vacuum surrounding a spherically symmetrical uncharged non-rotating mass). Real spaces which don't have much matter or energy in them appear to approximate these solutions pretty closely.

If you ask a quantum mechanic, they will tell you that the vacuum is the lowest energy state of all the various fields which pervade space and time. In this state there is still some possibility of observing a particle, which arises from the rules of quantum mechanics.

If you ask a fluid dynamics expert, they will tell you that a vacuum is a region with a low enough gas density that the mean free path of the gas particles (the average distance they travel before they hit one another) is comparable to the size of the region. At this point it stops behaving like a gas with a well-defined pressure and temperature and so on, and started behaving like a bunch of individual particles.

If you ask some engineers, a vacuum is anything at a pressure significantly below the ambient (so heavy industrial vacuum equipment doesn't get the pressure very low, but its enough to suck in air).

If you ask a rocket engineer, they will probably refer to what is outside their rocket once it gets about 100km or so (maybe less) as vacuum.

and so on. There is no single golden defintion.

• An excellent answer! In your list of vacua you might also mention the vacuum that the quoted definition seems to be asking for, which might be called the astronomer's definition (a region of space devoid of matter) and which is very nearly achieved in between galaxies, particularly in voids. – Mark Olson Nov 9 at 17:21

This is just the definition of vacuum.

A vacuum is a space from which everything that we know can be removed has been removed. It still has dark energy and the Higgs field and spacetime geometry in it, because, as far as we can tell, there is no way to get rid of those. If we're wrong, and more can be removed, then what we call the vacuum is actually a false vacuum.

• This doesn't address the major error in the OP's quote -- photons are just fine in a vacuum. – Carl Witthoft Nov 8 at 16:01

You are misquoting and perhaps misreading

A "vacuum is space devoid of matter". This is the definition at the top of the page. It then goes on to note that true vacuums don't exist, but outer space is a very good approximation of a vacuum, with only a few particles of hydrogen per cubic metre in intergalatic space.

It later discusses "Quantum Electro-Dynamic Vacuum" (a state with no particles and no photons). And it immediately notes that "this state is impossible to achieve experimentally." It is impossible to achieve a perfect vacuum, and even if all the particles could be removed, you could not exclude all the photons. It is a theoretical construction in which the average value of the electromagnetic field is zero, but there are local random fluctuations which manifest as virtual particles.

You have misquoted wikipedia and not read the sentence in context.