## Monday, November 15, 2010

### The Basic Principle of General Relativity - Principle of Equivalence

Einstein published General Relativity in 1915. Similar to special relativity, where he only considered frames of reference moving with constant relative velocity, general relativity is based on simple principles that lead to fantastic consequences. In this case, Einstein reasoned that the Principle of Equivalence was the fundamental building block concept for gravity. It states that the effects of acceleration on a system are indistinguishable from the effects of gravity on that system. This video shows an example of this principle in action, and shows how you can quickly figure out gravity should bend the path of light, even though light has no mass (so Newton would say this cannot happen)! This principle also explains why inertial mass (the mass in F = ma) is equivalent to gravitational mass (the mass in F = GMm/r^2), which is why all objects fall at the same rate in a gravitational field. Note that general relativity leads to the Big Bang, black holes, stellar evolution, gravitational red shifts, precession of planetary orbits, effects on time (which are needed for GPS to be so accurate), frame dragging as planets and objects move through space-time, and so on. All are confirmed through observation and experiments. Check it out!

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