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Question:
If matter consists of mostly empty space, why can't we move our hand through something "solid"?



Replies:
If you look at the cage around a still fan blade, you will see that most of the cage is empty. When the fan blade is spinning, the cage is still mostly empty space, but it would not be advisable to put your hand through it! While it is not full - it is definitely occupied. The electron shell around an atom is indeed mostly empty space, but it is occupied by the electrons by virtue of the fact that the electrons are in constant motion. Not only are the electrons occupying the space, but they repel each other as a result of their electric charges. If you were to try to push the electron field of one atom to intersect with the field of another, the repelling forces become very significant. Without it material in general would have no volume - it would collapse in on itself. Similarly, if you try to push the electron fields of the atoms in your hand into the electron fields of the atoms in a brick wall - then you will appreciate the repulsion involved. It is extremely difficult for the electron fields of two atoms to occupy the same space.

In some substances, the outer layers of the field of each atom may overlap a little with the atoms of adjacent atoms. It is this overlap which allows metals to conduct electricity, and it is what makes some compounds form such nice crystal structures. But it is only the very top layer of atoms which are involved in this kind of overlap. The only way to overcome the repulsion of deeper electrons would involved HUGE forces, such as those inside a nuclear reactor, a sun, or a black hole. Not something you are likely to try and touch.

Nigel Skelton

AUSTRALIA



Although solid matter is mostly open space, the spaces are very small and the particles are very tightly bound together.

Think of a chain link fence such as you see around playgrounds or baseball fields. It is mostly holes. However, each hole is too small to let you get through and the wire is too strong for you to break by walking into it. Solids are the same thing.

On microscopic and atomic scales, there is quite a bit of empty space, but no way to get a human being through in one piece!

Hope this helps.

Bob Avakian
Oklahoma State University Institute of Technology


Maybe a good analogy here is to think of solids like a net. You can see through a net and its cross-section is mostly air (very little rope/string), but you can't walk through the net. Matter is the same way -- the atoms are bonded to each other forming a solid network, and the strength of those bonds is what prevents you from pushing your hand right through.

Hope this helps,

Burr Zimmerman


You have started with an assumption that is incomplete. Specifically, that matter consists "mostly of empty space". Consider carefully what you mean by: "mostly", "empty", and "space".

Those are not easily defined in terms of measurable quantities. Consider first: "mostly" -- do you mean 1%, 10%, 50%, or 80%? And if so how do you measure it? Consider second: "empty" -- at the atomic and sub-atomic scale the presence of matter is defined by probabilities, not by "concrete" boundaries. So it is not clear exactly what "empty" means. In that sense no space is absolutely "empty". Finally consider "space". This term is particularly tricky to define. At the atomic and subatomic scale "space" no longer means "over here" and "over there" with nothing in between. Having said all that, the technical point is that electrons that form the outside of atoms and molecules obey what are called "Fermi statistics" which states that no two particles that obey those rules can have exactly the same set of quantum parameters. They will do what is necessary, like repelling one another, so that the "avoidance rule" is met. Other types of particles, called "Bosons", obey a different set of rules. Those "rules" place no such limits on the particles. These different set of rules give rise to "weird" behavior like superconductivity, superfluidity and other phenomena not observed under "normal" conditions.

It is the "avoidance rule" that gives rise to the "repulsion" that prevents matter passing through matter. This is not the same as the repulsion of like charges, or the repulsion of like poles of a magnet. It a more basic type of "repulsion" that only expresses itself on the atomic and subatomic scale.

Vince Calder



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