# Does gravity exist in a vacuum?

My understanding has always been that it does from conventional science courses, but really thinking about it, I was wondering if this is really the case.

To my limited understanding there is a theory that there are gravitons that act as particles to pull two different masses together. If these gravitons really are the physical particles of gravity, then a so called "vacuum" that had gravity wouldn't be a vacuum at all. A real vacuum should lack these particles, and thus, lack gravity?

Anything in the vacuum should then implode due to its own gravitational attraction within itself? If this is the case, could we say in a real vacuum, external gravity does not exist?

• I'm not sure I understand the last point about implosion; if you placed a planet in a universe that was entirely empty you wouldn't expect the planet to implode because it's own internal pressure would be equal to the gravity pressing inwards – Richard Tingle Apr 19 '15 at 15:39
• Even considering classical models only, would you say that by definition light cannot traverse a vacuum, because if it did then the so-called vacuum would contain photons ("corpuscles" as Newton called his theorised particles of light) in transit? If you do define it like that, then that's just one more way in which there's no such thing as a vacuum... – Steve Jessop Apr 20 '15 at 14:05
• When I was in high school, a teacher put a rubber ball and a feather into a clear plastic tube. He tilted the tube back and forth and we saw the ball fall rapidly while the feather drifted down slowly. He then sealed the ends of the tube and used a pump to remove (almost) all of the air inside. He tilted the tube back and forth again and the ball and feather fell side by side at the same rate. Ok, so the tube was not a 100% complete vacuum, and the gravity acting on the ball and feather came from the earth which was not in the tube. But sometimes these simple little demos go a long way. – cobaltduck Apr 20 '15 at 15:47
• @cobaltduck - I think that what he's getting at is whether a vacuum should contain any particles at all, not even theoretical massless gravitons. Your high school teacher probably wasn't able to suck all of the gravitons out of the chamber (and probably didn't surround it with a gravity blocking shield to prevent traversal from gravitons). As you said, the high school lab equipment wasn't even able to achieve a very strong vacuum so many air molecules were left inside the chamber (but not enough to disrupt the experiment). – Johnny Apr 21 '15 at 0:41
• Even without considering gravitons and using a classical model, you must consider exactly what you consider to be "gravity": the field, or the accelerative effect of the field? The field only influences matter, so in an ideal vacuum, there would be no matter for the field to influence, so the field would be undetectable, and no accelerative influence due to gravity would be observed. But presumably if matter were to spontaneously appear within the vacuum, it would be influenced by the field (and would create its own field)--therefore the field can be said to "exist" in some sense. – Kyle Strand Apr 21 '15 at 23:17

Your intuition is good, but you're mixing up some quantum and classical phenomena.

In classical (i.e. non-quantum) physics, a vacuum is a region of space with no matter. You can have electromagnetic fields in a vacuum, so long as the charges creating the fields are in a different region. By the same token you can have gravitational fields in a vacuum, generated by masses somewhere else in space. In this classical description of the universe, there are no such things as photons or gravitons, and everything (for the most part) works out.

In quantum physics, the story is not so easy. As you say, now our force fields are particles, too (photons and gravitons), so maybe a "quantum vacuum" shouldn't include them either? Unfortunately, it turns out that in quantum mechanics (as rob pointed out) it is impossible to have a perfect vacuum, a state with no particles in it at all. One way to see this is through the energy-time uncertainty principle: $\Delta E \ \Delta t > \hbar/2$.

A perfect vacuum, a state with no particles at all, must have exactly zero energy. If the energy is exactly zero, then it is completely certain, and $\Delta E = 0$ which violates the uncertainty principle. So the quantum vacuum is not a state with zero particles, it is a state with probably zero particles. And in different situations you may find useful to alter your definition of "probably," so there are a lot of different things physicists will call a "vacuum" in quantum mechanics.

This idea, that quantum mechanically there are always some particles around in any region of space, has some cool consequences that we've verified in the lab! One is the Casimir Effect. This is a force that shows up when you move two objects in a vacuum so close together the pressure from these "virtual" photons causes them to attract. Another is the particle they discovered at the LHC, the Higgs Boson. The Higgs field has a "vacuum expectation value," a perfect quantum vacuum will have a non-zero Higgs field throughout it. Excitations of this field are the Higgs particles found at the LHC!

• Some great answers here, thank you to everyone. I understand now that vacuum can be relative depending upon the context and need not be an absolute thing of nothingness. Apologies for the lack of upvotes as I lack the rep. – user4779 Apr 19 '15 at 7:15
• A lot of the ideas in this answer are useful, but many of them aren't quite right. Firstly, the time-energy uncertainty principle is often a slippery thing to properly pin down, and can't be applied to get the conclusions here: indeed, the vacuum is an energy eigenstate by definition, so does have an exact energy (though not an exact particle number in an interacting theory). [Aside: this is to say nothing of the usual subtleties of the Hamiltonian in quantum gravity...] Also, the Higgs stuff confuses the field (with a nonzero VEV) with the particle (fluctuations away from this value). – Holographer Apr 21 '15 at 20:47
• @Holographer, I couldn't agree more. I was aiming for a more intuitive answer than rigorous, but should have taken more care. I updated the Higg's discussion, do you have any suggestions for clearing up or replacing energy-time uncertainty argument? – Geoff Ryan Apr 21 '15 at 22:20

The graviton is the hypothetical gauge boson associated with the gravitational field. I say hypothetical because it is far from clear whether gravity can be described by a quantum field theory, so it isn't clear whether gravitons are a useful description.

In any case, you should not take the notion of virtual particles like the graviton too seriously. have a look at Matt Strassler's article on virtual particles. Virtual partices are really just a mathematical device for describing the energy in quantum fields. So even if the graviton is a good description of gravity we shouldn't view the vacuum as being full of gravitons and therefore not really a vacuum.

For example, suppose we put a charged particle in a vacuum. Would you claim the vacuum is not a vacuum because there is an electrical field in it? If so then you would also have to say the vacuum near a massive body isn't a vacuum because there is a gravitational field in it. While I suppose there is some validity to this claim, it seems excessively zealous.

• "would you claim the vacuum is not a vacuum because there is an electrical field in it?" No... I would claim it's not a vacuum because you put a charged particle in it. – Paddling Ghost Apr 21 '15 at 21:56
• @PaddlingGhost: but the field created by a charged body extends into the vacuum surrounding it. – John Rennie Apr 22 '15 at 5:16

You are simply confusing vacuum with "nothingness", which is a philosophical concept. You can check the definition at wiki

Vacuum is space that is devoid of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure.[1] Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space.

There are different theories that try to explain gravitey (curvature of space-time, graviton, etc) but according none of this gravity or gravitons can be considered matter

• Could you adapt this answer to actually answer the question? You haven't talked about gravity at all here. – 299792458 Apr 19 '15 at 6:31

In quantum mechanics, it's impossible to remove all the particles from a vacuum. A volume of space time that contains only photons and gravitons in thermal equilribium (or not) sounds like a perfectly good vacuum to me.

A perfect vacuum never exists as mentioned in multiple other comments. All "messenger particles" are fluctuations of their respective fields (e.g. the graviton a place in the gravitational field that has a non zero energy value). All fields are subject to quantum fluctuations, in essence, they rarely have no energy at one point but the fluctuations average to be zero (that is for most fields, others such as the proposed Higgs field possibly have non-neglibile energy values at their lowest energy state). Since the graviton can also be described as a wave function (much like light; there is theoretically such a thing as gravity waves that warp space-time). This and the point made earlier are some proof why there is no such thing as a perfect vacuum. What may make the situation a bit more complicated is string theory which predicts the graviton to be a close ended string suggesting its ability to interact with more than our three spacial and one time dimension. (All information summarised from Brian Greene's Fabric of the Cosmos

I believe that part of the problem is not having a clear definition of "vacuum."
I can think of at least three types of vacuum. 1) absolute 2) conventional & 3) "practical" vacuum. The practical vacuum is the type you find in a "lab." The conventional vacuum is the one defined as the "absence of matter." The absolute vacuum does not exist, other than "theoretically."
Using the practical and conventional definitions for vacuum, the answer to the question is yes, gravity exists in these types of vacuum. For the absolute definition, the answer is no, because nothing exist (not even fields, photons, fluctuations, gravitons, etc.).

Yes, gravity does exist in a vacuum. A vacuum does not need to be completely devoid of matter, it just needs to have a lower pressure than the area around it.

Consider the syringe above. If I was to put my finger over the end, and then pull on the plunger, an imperfect vacuum would be created. If there was a solid mass in the syringe cavity, it would still obey gravity.