QUESTION #198

# What experiment or phenomena proved that light was bent by gravity?

Einstein predicted that light should be bent by gravity. Sir Arthur Eddington lead an expedition to photograph the 1919 Total Eclipse of the Sun. The photographs revealed stars whose light had passed near to the Sun. Their positions showed that the light had been bent exactly as Einstein had predicted. The experiment was repeated in 1922 with another eclipse with the same confirmation.
Answered by: John Pindar, B.S., Physics teacher (Secondary), Newton-le-Willows, UK

This is a really cool question. But, did you know that it wasn't exactly gravity, in the Newtonian sense, that bent the light? I wish I were alive when this happened because I am sure that it must have been a life changing event for the people that were there. Can you imagine what it would be like if your science teacher held a large steel ball bearing in the palm of his hand and right before your very eyes he turned his hand over and the ball bearing did not fall to the ground. What if you were sure that this was not a magic trick; that this man really did find a way to control gravity. Would your blood run cold? Would you get goose bumps? Would you see your entire belief structure crumble before you? Would you hear the noise of that destruction? If you can imagine all of this than you can imagine what it would have been like to witness the experiment you are asking about!

Before Einstein came along everyone took for granted that Newton had it right. Well, Newton had everything right! Newton had built for us a clock-work universe. God wound up the clock and then He rested. If you had enough information you could calculate just about anything in this Newtonian universe. After Newton the work of Science was to gather as much information as possible so that whatever you wanted to calculate you could.

One of the things Newton said is that the force of gravity is a function of mass. Objects exert a force of gravity because they have mass and the more mass they have the stronger the force of gravity they exert. He did not know why objects with mass have gravity; nor did he know why the force of gravity is always attractive. But, his ideas on gravity were time honored and proven mathematical certainties. No one would dare question them. Well, no one but Einstein!

Einstein said that gravity is a function of mass alright but not as a force exerted by the mass! To the scientists of his day these words amounted to scientific heresy. If gravity is not a force, they asked, then what is it? Einstein's answer is no less than life changing! Gravity, he said, is the result of the mass of an object bending space! Bending Space! Can you imagine? I cannot!

What Einstein was saying is that objects do not fall because of an attractive force reaching out of an object and pulling it to itself, as Newton said. Rather, Einstein said, objects fall because the space they are in is, well, bent. If you place a ball on a bent, or slanted table it will fall. This is what happens when you place an object in space near a massive object: It follows the curve of space and it falls. Try it. Place an object in the space in front of you and let it go. You will observe that it follows the curve of the space that is bent due to the presence of earth.

Another way to imagine this is to think of four people holding a bed sheet at the corners and you place a heavy ball bearing in the middle. You will note that there is a kind of hole with the ball bearing at the bottom. Anything else you put on this sheet will fall down the sides of the hole to the ball bearing. In just this way objects fall to earth when you let them go.

So, to the experiment. If Einstein was right then the sun is bending space and we should be able to observe this by looking at the stars behind the sun. In other words, if the sun were between us and certain stars whose exact positions in the sky we know to six decimal places then we should see the light of those stars bending through space as that light passes the position of the sun on its way to earth. As a result the stars should appear to us to be out of place by a certain calculable number of degrees. Yes, not only did Einstein say the the sun would bend the light of the stars, he also knew how much bending would be done.

Well, you can't see stars that are behind the sun because the sun is too bright! Even Einstein knew that! So, they all had to wait for an eclipse. And when it came there were lots of people praying for a clear sky! And as the sky got darker and darker more and more stars came into view until it got just dark enough the see one particular star that everyone had agreed to measure the exact position of that day. And sure enough! The star was exactly where Einstein said it would appear to be and that place was not where God, nor Newton, put it! It appeared off position by the exact amount that Einstein's theory said it would be. Of course, Einstein, ever the man of humility, pointed out that an infinite number of experiments could prove him right; but it would take only one to prove him wrong!

As a result of this experiment we now know that the universe is anything but a clock wound up by God. And, whatever gravity is, it is not what we think!

I hope this long winded answer helps you to understand the profound effect Einstein had on all of us!
Answered by: Tom Young, B.S., Science Teacher, Whitehouse High School