• Question: How big is a neutrino? And where are they found?

    Asked by Jesschan02 to Susan on 17 Jun 2015. This question was also asked by wolf.
    • Photo: Susan Cartwright

      Susan Cartwright answered on 17 Jun 2015:

      Extremely small, and everywhere!
      The size of a neutrino is very difficult to state precisely, because like all quantum objects it is both a particle and a wave, and its wavelike nature means its apparent size depends on its energy. What we can define is its mass. We know the mass is not zero, but we do not have an exact value. However, we know that it is less than 0.000000000000000000000000000000000004 kg (or 4 millionths of the mass of an electron, or 2 billionths of the mass of a hydrogen atom).

      About 65 billion neutrinos pass through every square cm of the Earth every second, from the Sun. There are also about 400 neutrinos, on average, in every cubic centimetre everywhere in the universe, from the big bang. So neutrinos really are everywhere.

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