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Apologies to all if this has been asked before, I searched but was unable to find one similar.

This is a question that has been bugging me for a while that i haven't really been able to find a suitable answer for.

I am aware that the space between an atoms nucleus and its electron cloud is teeming with virtual particles that allow the exchange of energy that give electrons an assigned energy level or 'shell' but what bugs me is about the space in between atoms.

What is in between atoms? is it classifiable as a vacuum where nothing at all exists?

I would find it hard to believe that atoms are pushed right up against each other at all times due to repulsive charges on the nucleus acting upon any other.

I accept that the gap is unbelievably small but on the scale of atoms and electrons, how small are we talking? Is there even a gap at all? Do we know what is in between or is it unknown? is it a similar process to the virtual particles between nucleus and electrons?

It is to my limited understanding that when particles "collide" there is no physical interaction, rather an exchange of energy through virtual photons. Is that what exists in all of these gaps? a constant exchange of virtual energy that acts as a consistent repulsion between all atoms?

Malcolm Brown
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  • Related: http://physics.stackexchange.com/q/34049/2451 and http://physics.stackexchange.com/q/7615/2451 – Qmechanic Jan 11 '13 at 15:04
  • while my question is most certainly related to those (indeed my reading of both led me to finally ask) i don't think either covered my particular issue in enough depth. The second one lends itself to the inner machinations of transfer between nucleus and cloud, whereas mine is more focusing on exchange between atom and atom in a non vacuum environment, vacuum was merely (incorrectly perhaps) tagged as i theorized the area between particles on earth might well be a vacuum –  Jan 11 '13 at 15:10
  • I should clarify what theorists mean by vacuum. Vacuum is full of fields! We say that it is not a vacuum if there are real particles (quantised excitations of a field carrying a definite energy). When there aren't any real particles present we call it vacuum, even though the fields are still there and the fields are still fluctuating (virtual particles). Vacuum is the state of lowest energy. – Michael Jan 11 '13 at 15:35

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The space between atoms depends very much on the medium you are talking about. In solids the typical distance between atoms is about the same as the size of the atoms themselves. In everyday gases at room temperature and pressure the distance between molecules is many times their size, and in deep space you can get densities as low as one proton per cubic centimetre!

You can get a rough idea of the average separation $\ell$ between atoms by using

$$ \ell \approx \left(\frac{m}{\rho}\right)^{1/3} $$

where $m$ is the mass of an atom and $\rho$ is the (mass) density of the material. This can be compared to the size of an atom, which for all elements is about the same at $\approx 10^{-10} - 10^{-9}\ \mathrm{m}$.

Space is full of fields like the electric and magnetic fields. You can think of certain types of "vibrations" of these fields as virtual particles, but the common view of modern physics is that the field picture is more fundamental. There are fields for all of the elementary particles, and the fields are constantly fluctuating due to quantum mechanics. You can think of temporary ripples in the fields as virtual particles, which are responsible for transmitting disturbances through space. Real particles are quantised excitations (or vibrations) in a field which propagate long distances.

Matt Strassler has gone to great lengths to explain this point of view in his popular articles.

Frederic Brünner brings up an important point about virtual particles. Physicists use an approximation called perturbation theory to do most of their calculations (because the calculations are really hard to do without making approximations). Virtual particles are a convenient way to organise these calculations, but you should not think of them as physical objects like real particles. In a sense virtual particles are misleading. What they really represent are rapid fluctuations of the fields (what I called ripples before). At large distances these fluctuations don't matter except when they average out to a smooth classical field. For the interactions between atoms, and even most of the interactions between electrons and nuclei, the classical field is all you need.

Michael
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  • @RhysW There is one field of each type. For example, there is one photon field (more commonly called the electro-magnetic field), one electron field, etc. that fill all of space; not a seperate field belonging to each particle. You can actually go a long way to understanding the physics without thinking about virtual particles. At very large distances (compared to $\approx10^{-13}m$) the net effect of all virtual photons is to create a classical electric field around charges (http://en.wikipedia.org/wiki/Coulomb%27s_law). Like charges repel and opposite charges attract. cont.-- – Michael Jan 11 '13 at 15:21
  • cont. The fields produced by different particles simply add together, though since electrons and protons have opposite charges their fields have opposite signs and tend to cancel out. As a result the electric fields between atoms are very weak compared to the fields inside atoms, and the net force between atoms is a very weak residual of the original electrical force. Roughly speaking, atoms which are far apart attract each other (weakly) but atoms which are very close repel very strongly. This what enables many atoms to come together to form organised structures. – Michael Jan 11 '13 at 15:24
  • cont. To answer your question (finally): You can visualise irregular materials that way, as long as you remember that any "membrane" is imaginary and arbitrarily defined. The field of an atom doesn't stop; it goes on forever, gradually weakening to the point of being immesurable. Atoms in certain other materials - crystals - form a regular repeating arrangement. The atoms in a crystal only wiggle around their regularly spaced equilibrium positions, without moving past each other. The Feynman Lectures on Physics are great reading. The man loved to talk about how everything is atoms jiggling. :) – Michael Jan 11 '13 at 15:30
  • I have to disagree with the explanation regarding the virtual particles. They are mathematical artifacts of a perturbative expansion within the framework of QFT, and as such shouldn't be used as a physical explanation for phenomena within an atom. Calling them "ripples" doesn't change that fact. – Frederic Brünner Jan 11 '13 at 15:34
  • @FredericBrünner I understand your point. I was following Matt Strassler's popular articles on the subject (and probably butchering it). There is a basis for moving the discussion from virtual particles to ripples. The ripples (by which I really mean quantum fluctuations) are there whether or not you are doing perturbation theory, but in the context of perturbation theory you can naturally identify them as virtual particles. The point is that the "ripples" language is more accurate. It doesn't depend on perturbation theory. And they are necessary - c.f. the Lamb shift. ;) – Michael Jan 11 '13 at 15:43
  • @FredericBrünner I would agree with you - I wish the notion of virtual particles never got popularized. I was merely trying to communicate to a layman the concept that virtual particles are really trying to explain (quantum fluctuations of a field). – Michael Jan 11 '13 at 15:45
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    @RhysW Frederic brings up an important point about virtual particles. Physicists use an approximation called perturbation theory to do most of their calculations (because the calculations are really hard to do without making approximations). Virtual particles are a convenient way to organise these calculations, but you should not think of them as physical objects like real particles. What they really represent are rapid fluctuations of the fields (what I called ripples before). At large distances these fluctuations don't matter except when they average out to a smooth classical field. – Michael Jan 11 '13 at 15:50
  • Ah ok that description really helps, i never assumed virtual particles were actual entities, being as they are called 'virtual' and all, but i did not know the alternative, thanks to you both for the stellar description –  Jan 11 '13 at 15:52
  • @MichaelBrown Well, the virtual particles are called "particles" because they carry the quantum numbers of the corresponding on shell particles . In any case even the terminology "particle" at the quantum level is misleading people to expect classical particle behavior, which leads to all those arguments about waves and particles. – anna v Jan 11 '13 at 16:02
  • @MichaelBrown I still do not see the necessity of the "ripple" picture, except for maybe some heuristic explanation (which is not much better than the virtual particle exchange concept). QED allows one to calculate a modification to the Coulomb potential at small length scales, resulting in the Lamb shift. I do not see how it is necessary to explain this in terms of ripples, fluctuations or exchange. These concepts imply time-dependence of some sort, which just isn't there. – Frederic Brünner Jan 11 '13 at 16:34
  • @FredericBrünner You raise another valid technical point. When you do the relevant Feynman integrals the time dependence cancels out (when you have "nailed down", i.e. static charges). I'm perfectly happy leaving a layman with a heuristic understanding. When the OP is ready for it I'm also happy to show him how to derive the Coulomb potential from QED. Feel free to write up your fluctuation&virtual-particle-free layman's explanation of quantum field theory. I'm not sure how it could be more than "we have this math that I can't show you, but it works," but I look forward to reading it. :-) – Michael Jan 12 '13 at 00:27
  • I prefer to refrain from giving such heuristic explanations if the consequence is a complete misunderstanding of the situation. The layman usually does not know when to take an explanation with a grain of salt. ;) – Frederic Brünner Jan 14 '13 at 13:43
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I have thought about this question for quite some time. The theory that I highly believe in is about string theory. The theory states that the space in between subatomic particles are almost massless strings from type 2a string theory. These string have so little mass (roughly .83 x 10 to the -5 GeV) that they are undetectable. Of course this theory relies solely on string theory. Thank you for your time.

dmckee --- ex-moderator kitten
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Jack Moody
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  • The FAQ is very down on "Pitches for your own personal theories or work". Also, please do not sign your posts--your usercard is appended automatically. – dmckee --- ex-moderator kitten Mar 08 '13 at 17:56
  • @RhysW I have always thought that the strings are so tightly interwoven that there is minimal if any space in between these so called strings. But it is interesting to think about it. Back to the theory, the space in between these atoms or quarks are these tightly interwoven strings. Once again they have such little mass that they are undetectable. – Jack Moody Mar 08 '13 at 18:40
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    This is a complete misunderstanding of string theory. – Michael Mar 09 '13 at 00:14
  • there are many understandings of string theory due to the many types. this is just one of the types. – Jack Moody Mar 09 '13 at 16:39