Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Neutrino, very small particles with no electric charge and little or no mass. Neutrinos are elementary particles—that is, they cannot be broken into smaller particles. Neutrinos are so small that they pass right through most material. One important kind of neutrino is created in the nuclear reactions that give the Sun its energy. The Sun produces so many neutrinos that 70 billion neutrinos pass through every square centimeter (0.15 sq in) of the surface of Earth every second. Scientists study neutrinos to learn more about the reactions that give the Sun its energy. Similar reactions occur in radioactive substances, or materials made up of atoms that spontaneously change into other particles (Radioactivity). Neutrinos also help scientists understand these radioactive reactions. Neutrinos play an important part in the theory scientists have developed to explain the elementary particles that make up all matter and energy.

Neutrinos have so little mass that scientists are not sure that neutrinos have any mass at all. Because they have little or no mass, neutrinos move at speeds near the speed of light (300,000 km/sec, or 186,000 mi/sec). Neutrinos are probably true pointlike particles, meaning they have a radius of zero, or no size.

Neutrinos are affected by at least one of the four fundamental forces that exist in nature. These four forces are the strong force, the electromagnetic force, the weak force, and the gravitational force. The strong force is the force that holds together particles in the nucleus of an atom. It does not affect neutrinos. The electromagnetic force causes particles with electric charges to attract or repel each other. Neutrinos have no electric charge, so the electromagnetic force has no effect on them. The weak force allows particles, even elementary particles, to change form. The weak force does affect neutrinos. The gravitational force causes attraction between particles with mass. If neutrinos do indeed have any mass, the gravitational force affects them, but the mass of neutrinos is so tiny that scientists have not been able to measure gravity’s effect on neutrinos.