Like all good theories of physics, special relativity makes predictions which can be tested, and explains things in a natural way which might otherwise be hard to explain. Probably the most famous prediction is contained in the equation
– the fundamental equivalence of mass and energy. Mass can be created from energy – easily visualized as a result of the decay of photons – and energy can be created when mass is destroyed, for example, when matter and antimatter particles annihilate each other. The equationpredicts a decrease in mass during radioactive decay – the products of decay have less mass in total than the particles that decay: the difference is called the mass defect, and is released in the form of energy of one form or another.
The theory also predicts that moving bodies appear to be shorter. The faster a body moves, the shorter it appears to be. If the body is a spaceship with people on board, it will not appear to be shorter according to those people. It will appear to be the same length. In fact if the spaceship is moving with uniform velocity, they will not be able to determine if the outside world is stationary or the spaceship. If the velocity of the spaceship relative to the outside world is V, then the velocity of the world relative to the spaceship is -V. Velocity is truly relative, and if the spaceship appears to be shorter relative to an observer, then the observer appears to be shorter relative to the observers on the spaceship.
Time passes more slowly for moving objects. The faster a body moves, the more slowly time appears to pass. If the body is a spaceship with people on board, time will not appear to move slower according to those people. It will appear to be move at the same normal rate that time moves. It will only appear to move more slowly for those observers relative to whom the spaceship is moving. In fact if the spaceship is moving with uniform velocity, they will not be able to determine if the outside world is stationary or the spaceship. If the velocity of the spaceship relative to the outside world is V, then the velocity of the world relative to the spaceship is -V. Velocity is truly relative, and if the time on the spaceship appears to be move more slowly relative to an observer, then the time for the observer appears to be pass more slowly for the observers on the spaceship.