Matter and light
The Universe is composed of two things, matter and light. Even these are only different manifestations of the same thing being able to be converted from one to the other. The study of the composition and history of the Universe is called cosmology Matter Matter comes in four states: solids, liquids, gases and plasma. Light E = mc2 Nuclear fusion reactions which occur in nuclear reactors and nuclear fission reactions which occur in the Sun and stars, generate energy from matter in accordance with Einstein's equation. Converting light energy to matter is another matter (no pun intended) altogether, however, and may have only occurred in the original "Big Bang" when our Universes was formed. Baryonic matter The baryonic matter with which we are familiar is composed of atoms which are themselves built up of subatomic particles called protons, neutrons and electrons. Our knowledge of "dark matter" is still evolving and ranges from newly discovered "black holes" at the centre of our galaxy to neutrinos which travel near the speed of light and pass through the Earth and us as though we do not exist at all. The study of matter in all its forms and the evolution of the Universe is called Cosmology, and is well worth further study. The atomic and kinetic theories of
matter Today, scientists believe that the many and varied properties of matter can be explained by the presence of atoms and combinations of atoms that form molecules and ionic crystals. The second important theory of matter is "The kinetic theory". This theory describes how atoms interact to form the four physical states of matter: solids, liquids, gases and plasma. The kinetic theory is used to explain the properties of the four physical states and the processes which occur during a change in state, like melting and boiling, in terms of the movement of particles and the forces of attraction between particles.
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