If atoms have so much empty space in them, and humans are made up of atoms, why can't things pass right through us?
Because that empty space is only "empty" of matter. Their are other things
that call fill space. Among them are fields, electric and magnetic fields are
common examples. In the case of atoms the space is filled with electric
fields. All atoms have tiny positively charged nuclei surrounded by
(relatively) large regions of space that are "filled" with a negative electric
field from the electrons in the "orbits" about the nucleus. When something
else tries to pass through our "empty space" the negative electric fields
interact and prevent it from passing through.
To show that fields can prevent things from passing through empty space, get
two strong magnetic fields and try to bring like poles together (e.g., north
pole to north pole). Depending on your strength and the strength of the
magnets you may be able to get them to touch but you will certainly have to
work hard to do it. You will also notice that the closer you get them together
the harder you have to work to get them closer.
Amanda: it depends what the 'things' are. Some things, like x-rays, can
pass through us...other things, like a can of pop, can't. It all depends on
their size and what type of 'thing' it is.
A more accurate answer is that the particles that make up our bodies
and other materials can interact with each other through empty space. When
you stub your toe on a rock, your toe does not actually touch the rock.
Instead, it gets very close, close enough to transfer energy to it and
experience force from it. At close enough distances, the rock and your foot
repel each other. The force of the repulsion is enough to keep the toe and
rock from passing through each other.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
Click here to return to the Physics Archives
Update: June 2012