As light travels upward in the earth’s gravitational field, it loses energy, and so its frequency goes down. This is because there is a relation between the energy of light and its frequency (that is, the number of waves of light per second): the greater the energy, the higher the frequency. “Another prediction of general relativity is that time should appear to run slower near a massive body like the earth. Only light, or other waves that have no intrinsic mass, can move at the speed of light.” For this reason, any normal object is forever confined by relativity to move at speeds slower than the speed of light. It can in fact never reach the speed of light, because by then its mass would have become infinite, and by the equivalence of mass and energy, it would have taken an infinite amount of energy to get it there. “As an object approaches the speed of light, its mass rises ever more quickly, so it takes more and more energy to speed it up further. And so the theory itself would determine the outcome of our search for it!” “Yet if there really is a complete unified theory, it would also presumably determine our actions.
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