5.6: Einstein's Theory of Gravity

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Why is jumping on a trampoline so much fun? Its springy surface helps you jump higher than you ever could on the hard ground. But even on a trampoline, you can’t keep jumping higher and higher. Gravity always pulls you back down. You’ve probably known about gravity since you were in elementary school. It’s the force that pulls all objects on Earth down toward the ground. Is that all there is to gravity? Absolutely not!

It Started with Newton

In the late 1600s, Isaac Newton introduced his law of gravity, which identifies gravity as a force of attraction between all objects with mass in the universe. The law also states that the strength of gravity between two objects depends on their mass and distance apart. Newton’s law of gravity was accepted for more than two centuries. It can predict the motion of most objects and was even used by NASA to land astronauts on the moon. It’s still used for most practical purposes. However, Newton’s law doesn’t explain why gravity occurs. It only describes how gravity seems to affect objects. There are also some cases in which Newton’s law doesn’t even describe what happens.

Q: Newton expressed his ideas about gravity as a law. A law in science is a description of what always occurs in nature. For example, according to Newton’s law, objects on Earth always fall down, not up. What is needed to explain gravity?

A: A theory is needed to explain gravity. In science, a theory is a broad explanation that is supported by a great deal of evidence.

Einstein Explained It All

In the early 1900s, Albert Einstein came up with a theory of gravity that actually explains gravity rather than simply describing its effects. Einstein showed mathematically that gravity is not really a force that of attraction between all objects with mass, as Newton thought. Instead, Einstein showed that gravity is a result of the warping, or curving, of space and time, which made up the same space-time “fabric.” These ideas about space-time and gravity became known as Einstein’s theory of general relativity.

Visualizing Einstein’s Ideas

Einstein derived his theory using mathematics. However, you can get a good grasp of it with the help of a simple visual analogy. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Now imagine placing a lighter ball at the edge of the trampoline. What will happen? It will roll down toward the bowling ball. This apparent attraction to the bowling ball occurs because the trampoline curves downward, not because the two balls are actually attracted to one another by an invisible force called gravity.

Einstein theorized that the sun and other very massive bodies affect space and time around them in a way that is similar to the effect of the bowling ball on the trampoline. The more massive a body is, the more it causes space-time to curve. This idea is represented by the Figure below. According to Einstein, objects move toward one another because of the curves in space-time, not because they are pulling on each other with a force of attraction. Einstein’s theory is supported by evidence and widely accepted today, although Newton’s law is still used for many calculations.

This diagram shows how Earth’s mass bends the “fabric” of space and time around it, causing smaller objects such as satellites to move toward Earth.

Could we ever get close enough to black hole to observe its effect on space and time? Well,we really don’t know what black holes look like up close but we can use our understanding of physics to make accurate predictions and simulate what might happen. Try placing the space probe or clock near the event horizon of the black hole in the simulation below to learn more:

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Summary

Newton’s law of gravity can predict the motion of most but not all objects. It also doesn’t explain gravity. It only describes its effects on the motion of objects.
Einstein showed mathematically that gravity is not really a force of attraction between all objects with mass, as Newton thought. Instead, gravity is a result of the warping of space-time.
Einstein’s ideas have been supported by evidence and are widely accepted today.