Question

On the atomic level, there is some free space between the electrons and nucleus of atoms (and between the protons and neutrons in the nucleus itself). What 'material' fills these spaces? Dark matter?

Asked by: Jeff

Answer

The empty space between the atomic cloud of an atom and its nucleus is just that: empty space, or vacuum. That's the simple answer, but there are a few subtleties:

1) Sub-atomic particles such as electrons, protons and neutrons need to be treated as quantum objects. Thus they have a wavefunction which can be *thought of* as the 'spread' in the particle's location. Electrons are thus 'spread out' quite a bit in their orbits about the nucleus. In fact, the wave-functions for electrons in s-orbitals about a nucleus actually extend all the way down into the nucleus itself. In this sense, then, the space between the electrons and the nucleus isn't really 'empty.'

2) The electrons and the protons/neutrons are constantly interacting, either electromagnetically or through the weak force. In quantum field theory we would say that these particles are constantly exchanging photons (in the case of electromagnetism) or heavy gauge bosons (in the case of the weak force). Thus you might say that the otherwise 'empty' space between the electrons and nucleus is 'filled' with these quanta carrying forces.

Despite these two quantum-mechanical subtleties, it's still correct to say that the space between the electrons and nucleus in atoms is truly empty space. As for dark matter, if only it were sitting inside the atom -- then maybe we would have discovered it by now! Despite the mysterious name 'dark matter,' we actually know quite a lot about what the particles that might make up dark matter can and cannot be. One thing we know is that it isn't likely to be interacting strongly with protons, neutrons and electrons. That means it isn't likely to be found in atoms -- and if any dark matter exists in our planet at all, it would have to be down at the very core of the planet where it has been drawn by the force of gravity. Needless to say, that makes searching for it rather difficult!

Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley

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