# How to define a vacuum

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During a recent, gaseous, conversation with a partner, I defined a vacuum to be "the elimination of space between particles." I drew the following picture.

o o o

o o o

o o ==> o o

o o o

o o o

I claimed that the group of particles to the right were experiencing a vacuum. Not a total vacuum, it seems apparent now. Is it an oxymoron to describe such a thing as a partial vacuum, or would this simply be a case of negative pressure? In the group of particles to the right, it seems more likely to describe the change as an increase in pressure/reduction in voume. In such case, the space between particles hasn't quite so much been eliminated as it has been filled by particles found now more closely together. Perhaps I should have considered the surrounding volume.

What states do the following diagrams indicate?

------------------

| |

| | ---------

| o o o | | o o o |

| o o | | o o |

| o o o | | o o o |

| | ---------

| |

-----------------

Anyway, my partner responded by drawing the following.

o o o o o

o o ==> o

o o o o o

Claiming that the group of particles to the right were experiencing a vacuum, to which I responded was merely "a reduction of matter." He assumed the surrounding volume in each case remains the same.

Would any of you be so considerate to inform me as to how wrongly stated these claims of mine were?

EDIT: Those diagrams have been screwed up

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One could make the argument that a vacuum is the increasing of space between particles of a gas, not the elimination. A vacuum is a region of reduced pressure with respect to atmosphere as a baseline, i.e. the removal of particles. (there are a few definitions; lets stick to that classical one lest this get bogged down in semantics and/or QM). One can measure the quality of the vacuum by measuring the pressure or pressure differential. "Negative pressure" implies that you are not using P=0 as a baseline for a differential measurement.

("Partial vacuum" would be acceptable in the context of vacuum being the complete elimination of particles, though this is unattainable due to the QM issue I alluded to earlier, so all vacuums are partial vacuums. So lets just call them all vacuums)

PV = nRT to some approximation, so you can reduce pressure by reducing the number of particles (n) or the temperature (T), or by increasing the volume (V).

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The thing about particles in a vacuum is that there aren't any.

OK QM makes that impossible in theory and real life makes it impossible in practice, but the idea of a vacuum is that it's a space where there are no particles.

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The thing about particles in a vacuum is that there aren't any.

That's one definition of vacuum, and I already stated I wasn't using that one.

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"A vacuum is a region of reduced pressure with respect to atmosphere as a baseline, i.e. the removal of particles."

"PV = nRT to some approximation, so you can reduce pressure by reducing the number of particles (n) or the temperature (T), or by increasing the volume (V)."

~Clearly. How vulgar of me.

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How come it's impossible to make a perfect vacuum? what's that QM issue you alluded to?

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How come it's impossible to make a perfect vacuum? what's that QM issue you alluded to?

Are you familiar with Hawking Radiation?

If you want more specific answers on QM and the impossibility of a perfect vacuum, consider the impossibility of certainty implied by QM.

Was that Heisenberg over there? I like his hat.

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Like iNow had stated, even if you had removed every particle in a defined space, new ones would pop into existence via Hawking Radiation.

There can never be nothing in a defined space, and thats the principle behind zero-point energy. There is actually quite a bit of something where there is nothing... and it may very well be that something is merely the result of a lack of that larger amount of something... if that makes sense at all.

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Hawking radiation is the result under specific conditions and in one description, of the underlying phenomenon: vacuum fluctuations (which are related to quantum foam). Virtual particle/antiparticle pairs are continually popping into existence and then annihilating. Hawking radiation is specific to behavior in the vicinity of black holes, where (in this description) one of the virtual particles becomes real with the addition of energy.

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actually i've heard that before that particles can sometimes appear out of nowhere. but how can this be possible. I understand that this has probably been observed that particles seem to pop out of nowhere but wouldn't observing that particles seem pop out of nowhere and particles actually popping out of nowhere be a different thing? the uncertainty principle though i think i understand for particles but these links talk about uncertainty principle of space time. what do they mean by that exactly?

can't i know exactly where i am in space-time at any given moment? unless space-time is moving around me. I guess I could see how space could be doing this and if the universe is expanding, i think it must be, but isn't time pegged to my rate of change of position? and is only relative, therefore, in order for it to mean anything i must be able to locate two objects in space accurately or else i can't really say that they are relative. perhaps the motion of space is only significant for quantum size materials? if so, then uncertainty of space must mean uncertainty of time. but maybe i'm just way off here. i'll stop talking now.

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"That's one definition of vacuum, and I already stated I wasn't using that one."

Thanks Swansont, but I was actually thinking about the original poster who started talking about particles in a vacuum.

Anyway, its not so much one definition of vacuum, it's essentially the definition of vacuum, for example here's Wiki's version

"A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than standard atmospheric pressure. The root of the word vacuum is the Latin adjective vacuus which means "empty," but space can never be perfectly empty"

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"That's one definition of vacuum, and I already stated I wasn't using that one."

Thanks Swansont, but I was actually thinking about the original poster who started talking about particles in a vacuum.

Anyway, its not so much one definition of vacuum, it's essentially the definition of vacuum, for example here's Wiki's version

"A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than standard atmospheric pressure. The root of the word vacuum is the Latin adjective vacuus which means "empty," but space can never be perfectly empty"

I disagree, because people who do experimental physics talk about vacuums all the time, and none of them are discussing regions completely devoid of matter. They talk of regions relatively devoid of matter, and often quantify them in terms like low/rough vacuum, high vacuum and ultra-high vacuum. And so does the wiki entry: regions where the "gaseous pressure is much less than standard atmospheric pressure" are not devoid of matter.

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I disagree, because people who do experimental physics talk about vacuums all the time, and none of them are discussing regions completely devoid of matter. They talk of regions relatively devoid of matter, and often quantify them in terms like low/rough vacuum, high vacuum and ultra-high vacuum. And so does the wiki entry: regions where the "gaseous pressure is much less than standard atmospheric pressure" are not devoid of matter.

This is also my experience of both physics and engineering.

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someone stated earliar that if the vacuum got really empty, particals would pop up out of no way, would'nt that be againts the law of conservation,,, or which ever law it is that states that matter can not be created or distoryed and speaking about that law, how is that law justified/explained, if matter cannot be created than where did all the matter come from, ( its been here since the biggering,,,,,,,, the biggering of what?)

staying on the topic,,, lets just say that we managed to create a chamber that was capible of creating a 100% vacuum, is they any theory that suggests that atleast from a theorical stand point 100% vacuum can be achived(and space would not blowup in our faces).

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someone stated earliar that if the vacuum got really empty, particals would pop up out of no way, would'nt that be againts the law of conservation,,, or which ever law it is that states that matter can not be created or distoryed and speaking about that law, how is that law justified/explained, if matter cannot be created than where did all the matter come from, ( its been here since the biggering,,,,,,,, the biggering of what?)

staying on the topic,,, lets just say that we managed to create a chamber that was capible of creating a 100% vacuum, is they any theory that suggests that atleast from a theorical stand point 100% vacuum can be achived(and space would not blowup in our faces).

That's part of the weirdness of quantum mechanics. The Heisenberg Uncertainty Principle allows it, and the energy of empty space isn't zero. The "zero-point" energy isn't actually zero energy, it's just that we can't tap into that reservoir. You can tell it's there, though, via cavity QED experiments and the Casimir force. (and you can use the search function to find some of the discussions we've had on these topics)

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Look at the original post. It says " Idefined a vacuum to be 'the elimination of space between particles.' "

The point I was making is that a vacuum is more or less the complete oposite of that. It is what you get when you eliminate (to a greater or lesser extent) the particles from the space.

Let's make this clear; every post I made in this thread has stated that the vacuua that people talk about are not, and cannot be, absolute; they are always partial. So what's the point of telling me that "people who do experimental physics talk about vacuums all the time, and none of them are discussing regions completely devoid of matter"? It certainly doesn't detract from the fact that the deffinition given in the OP is odd, to say the least.

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Look at the original post. It says " Idefined a vacuum to be 'the elimination of space between particles.' "

The point I was making is that a vacuum is more or less the complete oposite of that. It is what you get when you eliminate (to a greater or lesser extent) the particles from the space.

Let's make this clear; every post I made in this thread has stated that the vacuua that people talk about are not, and cannot be, absolute; they are always partial. So what's the point of telling me that "people who do experimental physics talk about vacuums all the time, and none of them are discussing regions completely devoid of matter"? It certainly doesn't detract from the fact that the deffinition given in the OP is odd, to say the least.

It is odd, and I pointed that out. All I was doing was pointing out that there's more than one definition of vacuum. You seemed to be implying that the only definition was a region devoid of particles. What you are calling a partial vacuum is what other people call a vacuum. If I've misread things, then I apologize.

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• 1 month later...

Perhaps The solution lies in defining nothing.

Nothing also means ‘without dimensions’ and this is the point (excuse the pun). How can a point exist in nothing. In order to have dimensions space must be something.

In a partial vacuum the vacuum force acts on the centre but, infinity does not have a centre. As a result infinity divides the partial vacuum into a field of small vacuums of equal force.

Next ask which is the elementary particle; is it the vacuum zero point or is it the vacuum zero point and its associated vacuum field? Particles are defined as point like objects that is to say: a point that acts as if it had dimensions. It seems as if some people are trying to eat their cake and have it to.

Normal practice is to refer to the background as the minimum energy of infinity but, what is energy? Would it not be better to refer to the background as the lowest density level of matter; then the energy is related to matter (particle mass) in the normal manner.

You might like to call this a graviton field but then, what is gravity?

Do virtual particles exist at this stage or are they a product of later developments? But then, what is a virtual particle; is it not just another name for something we do not understand?

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• 1 month later...

Virtual particles are not really particles, we just use a particle model to make our calcuations easier.

Besides, if space was infinitely indivisible, and the point-like particles reside in them. Wouldn't the smallest bit of space be completely filled up by the smallest particle, with the purest density (i.e. all space is taken up, since it is the smallest particle in the smallest space), wouldn't the gravity of that point be enough to form a mini-black hole?

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The technical definition of a vacuum is simply the lowest energy state of space-time. If the Higgs mechanism is correct, then the vacuum is not really empty at all - it will be filled with the Higgs field since this is more energetically favourable than nothing at all.

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