As John Mayer says, “Gravity has a hold on me”. And you. And everything.
The Law of Gravity tells us everything in the universe is attracted to everything else.
Everyone learns about Newton’s apple in elementary school. It is an apocryphal story but illustrates the point that objects of all sizes are attracted to the Earth. Apples fall.
What is missing from the story is the Earth is attracted to the apple. It falls a little bit towards the apple as well. Or it would if the apple and the Earth were the only two objects involved.
Measuring Newton’s gravitational force constant solidified his Law of Gravitation. We have understood the behaviour of gravity for a very long time. However, a theory of gravity was a much more elusive task.
After all, most of us can accept that if we drop an object, it will fall towards the Earth but why? Simply answering “gravity” doesn’t explain why gravity does what it does. It is the equivalent of saying “because.”
One of Albert Einstein’s great contributions to physics was to provide an explanation of gravity in his General Theory of Relativity. Essentially, the equations describe the curvature of space-time as a consequence of the matter and energy in it.
Imagine a rubber sheet stretched over a large area. Drop a marble on the sheet and you get a small dimple in the rubber around the marble. Drop a bowling ball and you get a very large dimple – a deep well – formed in the rubber sheet.
But if you place the bowling ball near to the marble, the deep well will draw the marble in and down to the bowling ball. That’s gravity. The curvature of space-time draws objects together within a four-dimensional structure.
For the marble to get out of the well takes energy. And if the well is deep enough, there will never be enough energy for the marble to escape.
This is what a black hole is. A deep, deep well in space-time from which even photons of light are incapable of escaping. This week, astronomers have finally been able to capture an image of the one at the heart of our galaxy.
Eventually, everything in our galaxy will fall into that black hole.
