Neutrinos are very small particles which also have very weak interactions with matter. To give a dramatic example of this, right now, as you read this, there are about a trillion neutrinos streaming through your body every second (this number varies from source to source, but suffice to say there are a ton [no pun intended] of these little buggers going through you) which are being produced by our sun. There are also neutrinos going through your body from cosmic sources but these number far fewer than those of solar origin. To give another example of how weakly these particles interact with matter, a neutrino could pass through 9 light-years (remember that a light-year is 6 trillion miles) of lead before interacting with another particle. Suffice to say, such a particle would have very little mass indeed, if any at all. For years, many believed neutrinos were something like photons, massless. But now this new experimental data from the neutrino observatory in Japan has shown us that they do indeed have mass, or so they think. This changes a lot of things, the standard model of particle physics, cosmology and so forth. Neutrinos, I believe, if I remember correctly, are electricly neutral. When a star goes supernova (a certain kind of supernova) its core emmits a huge amount of neutrinos which are so numerous that despite the particles weak interaction, tears the outter layers of the star apart in a great explosion. Anyway, hope this shead some light on this for you. For more information on this get a good basic particle physics book.

-Paul

: I was just reading a new report that says that neutrinos have a mass and that this may tip the scales just enough to make the universe contracting rather than expanding. My question is what is a neutrino, and what was believed about them formerly