gravity

Did You Know? Isaac Newton discovered gravity, which he saw as the mutual attraction that two masses have for each other. Newton developed an equation that showed that any two objects in the universe, no matter how far apart or how small, exert an instantaneous gravitational effect on each other. These effects diminish, however, as the distance between the objects gets larger and as the masses of the objects get smaller, so that for many distant objects or objects with barely any mass, the effects of gravity are very small. Newton seemed to have the last word on gravity, until Albert Einstein came along. He noted that gravity's effects could not be instantaneous, since they would have to travel at infinite velocities, violating his theory of relativity, which states that nothing can travel faster than light. He also showed that gravity and acceleration are related. Imagine, he said, that you are an astronaut standing in a stationary rocket on Earth: because of Earth's gravity your feet are pressed against the rocket's floor with a force equal to your weight. Now imagine that you are in the same rocket, in outer space, in an area that has no gravitational pull. Even though you are weightless, if the rocket is accelerating and its floor is pushing against your feet, you feel as if you are being acted upon by a gravitational field. Unless you look out the window you have no idea whether the rocket is at rest on Earth or accelerating through space.