Gravity
Gravity is hard to ignore. It holds Earth in orbit around the Sun, so we see the Sun scooting across the sky every day. And when we slip and fall, we feel its effect when our backsides hit the ground.
And yet gravity is the weakest of the basic forces of nature. The other force we're most familiar with is electromagnetism, which produces magnetic fields plus visible light and other forms of energy. Electromagnetism is more than a billion billion billion billion times stronger than gravity.
Even so, gravity is unrelenting. Galaxies that are hundreds of millions of light-years apart move through the universe together because they're tied to each other by gravity.
Isaac Newton devised the first good theory of gravity. He described it as an attractive force between any two bodies. Scientists still use his equations for most everyday calculations.
But Newton's equations don't explain the effects of strong gravitational fields. Those effects are best described by Albert Einstein's theory of General Relativity. Einstein described gravity as a warp in space caused by matter. The more matter you have, the greater the warp -- and the stronger the "pull" between objects. That's why galaxies pull so strongly on each other -- they are as massive as billions of Suns.
Einstein's equations also predict that gravity travels through space in waves. More about that tomorrow.
Script by Damond Benningfield, Copyright 2005
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