' Solidity is an illusion '...

By Kelly Cline
June 6, 2012


Your body is mostly empty space. A rock, a tree, a table, all these things are mostly empty space. These objects act as if they were solid all the way through. But a series of extraordinary experiments have shown that this is just an illusion.

The experiments began with the discovery of radiation in the late 1800s, when scientists found that X-rays could go right through human flesh or other solid objects.

In 1909, an astonishing experiment was performed in the laboratory of the English physicist Ernest Rutherford. A beam of alpha radiation particles was fired at a sheet of gold foil. Most of the alpha particles went straight through the sheet of solid gold. However, to everyone’s great surprise, a few of the alpha particles bounced off and came right back. Why did most of the alpha particles go through? Why did a few of them bounce back?

Based on this strange experiment, Rutherford proposed a new idea of what an atom is. Rutherford said that most of an atom is empty space, where negatively charged electrons orbit. The only solid part of an atom is the nucleus, where the positively charge protons are found. The few alpha particles that bounced back were ones that just happened to hit the nucleus itself. Rutherford’s calculations showed that the nucleus is amazingly small. If an atom was the size of a basketball, its nucleus would be too small to be seen with the eye.

So, why do objects feel solid? If my hand is mostly empty space, why can’t I push it right through a table?

Things feel solid because of the electric force. The atoms of my hand have equal numbers of negatively charged electrons and positively charged protons. A table has the same. The electrons in my hand are repelled by the electrons in the table, but they are attracted by the protons in the table. As a result, if I hold my hand an inch above the table, I feel no force.

However, if I try to push my hand through the table, the situation changes. Now the atoms of my hand are smashed right up next to the atoms of the table. Atoms keep their electrons on the outside, so the electrons of my hand are pushed up right next to the atoms of the table. As a result, these electrons push away from each other very strongly. The electrons of my hand are attracted to the protons in the table, but those protons are a bit further away, because protons are found in the center of an atom.

The electron-electron repulsion force is stronger than the electron-proton attraction force because the electrons are much closer together. As a result, I can’t put my hand through a table, no matter how hard I try. The electric force makes the table feel like a solid object, even though it’s mostly empty space.

Scientific discoveries allow us to see the world around us in strange and surprising ways. Look carefully at a rock, a tree, a table or your hand. Those objects are really just clouds of tiny particles held together by powerful electrical forces. Solid objects are mostly empty space.



-Kelly Cline, Ph.D., is associate professor of astronomy and mathematics at Carroll College