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For use with Chapter 30
The Nucleus

The Tiny Neutrino Has Massive Implications
Posted August 3, 1998

Sometimes reactions don't appear to obey the laws of physics. For example, a nucleus will lose some energy during beta decay. However, energy must be conserved. In 1930, physicist Wolfgang Pauli hypothesized the existence of the neutrino (which is Italian for "little neutral one"). Neutrinos are tiny elementary particles with no electric charge that carry the extra energy out of a nucleus during beta decay.

Neutrinos are incredibly small. Because they have no charge, they can travel right through atoms without being deflected by the electrons or protons. They are so small that one neutrino could travel through a wall of lead 100 light years thick without ever striking a single nucleus. In fact, billions upon billions of neutrinos are slamming through Earth--and you--every second.

So far, scientists have discovered three types or flavors of neutrinos. Until recently, neutrinos were thought to have no mass. However, through a joint effort between Japan and the United States, scientists have discovered that neutrinos do have a very small amount of mass. They are so small that it would take millions of neutrinos to equal the mass of one electron.

However, it is not their size that matters; it is their number. Neutrinos are everywhere. It is estimated that there are roughly a billion neutrinos for every electron and proton in the universe. Scientists now speculate that neutrinos could make up 20% or more of the mass of the entire universe. This could go a long way to explain "dark matter." Dark matter is the matter in the universe that scientists know exists, but cannot see in stars, planets, and gas clouds.

These new discoveries have also led to the possibility that there are even more flavors of neutrinos. How long will this take to find these new flavors? Dr. Sandip Pakvasa of the University of Hawaii says that we might have an answer in as little as five years. Perhaps one day we will discover that the tiny, little neutrinos outnumber the familiar protons, neutrons, and electrons.

Find out how the Japanese and American scientists set up their experiment to determine the mass of the neutrino.


  • Besancon, Robert M., ed. The Encyclopedia of Physics, 3rd Ed. New York: Van Nostrand Reinhold, 1990, pp. 787-8.

  • Browne, Malcolm W. "Mass Found in Elusive Particle; Universe May Never Be the Same." The New York Times, June 5, 1998, pp. A1, A14.

  • "Ghost Particle Sculpts the Universe." Astronomy, Vol. 23, Issue 6, June 1995, pp. 22.

  • Johnson, George. "Elusive Particles Continue to Puzzle Theorists of the Sun." The New York Times on the Web. June 9, 1998.

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