May 29, 2010

Solar Eclipse Experiment Confirms Theory of Relativity – 1919

On this day in 1919, Sir Arthur Eddington led a team of scientists who took the opportunity of a rare total solar eclipse to measure the amount that the Sun's gravity bends starlight.

These measurements agreed with the prediction made by Albert Einstein in his Theory of R
elativity. It was one of the first confirmations of the theory, and Eddington's work quickly became both famous and controversial.

Einstein wasn't the first to suggest that really large objects could bend light; Henry Cavendish and Johann Georg von Soldner both pointed out that Newton's theory of gravity makes this prediction. Einstein came up with his Theory of Relativity in part to explain things not explained by Newtonian principles, and he came up with a different prediction for the amount that light should bend in a gravitational field. Einstein's prediction was twice as large as Soldner's!

Sir Arthur Eddington realized that an upcoming total solar eclipse would allow scientists to test the new Theory of Relativity. During a total solar eclipse, the Moon entirely covers the Sun, and stars that appear in the sky near the Sun can be observed. Eddington's team did observations simultaneously in a city in Brazil and in Sao Tome and Principe on the west coast of Africa, and their measurements found that light was deflected (bent) the amount predicted by Einstein.

Eddington's results, as I said, were widely publicized, making the front page of most newspapers. (How often does the result of a scientific experiment make the front page of newspapers?)
According to Wikipedia, this publicity had the effect of making Einstein and his theory world-famous.

Eddington's Results: Einstein Is Right!

Eddington Right?

I mentioned that Eddington's results were also controversial. Some scientists suggested that there were errors and possibly confirmation bias. Wow! Confirmation bias sounds bad, doesn't it?

Actually, it's really common, and it plagues all of us, not just scientists.
Confirmation bias is the very human tendency to see what we expect or want to see—to only notice the things that confirm a belief or opinion or hope, and not to notice things that conflict with the belief / opinion / ho
pe. Notice that we do this without knowing we are doing it! We cannot completely trust our own brains!

Confirmation bias is one reason that doctors don't often operate on or diagnose loved ones, and detectives don't investigate loved ones. Confirmation bias tells us that, when we keep up with current events, we should read and listen to several different perspectives—keeping in mind the fact that our minds will tend to better remember the stuff we agree with, and will tend to forget the stuff we prefer not be true.

We all indulge, to some extent, in wishful thinking—but if we are aware of this tendency, we can reduce its effect on our memory and reasoning.

try to eliminate confirmation bias by searching for falsifying data (in other words, trying to prove something wrong), doing double-blind tests (in which neither the experimenter nor the subject knows what to expect—for instance, when testing a new medicine, in a double-blind test, neither the experimenters nor the subjects know which bottles hold medicine and which hold flavored and colored water), and by repeating observations.

The people who suggested that Eddington's results might be tainted by this bias were right to question the results. They turned out to be wrong—Eddington was right—but the questioning itself was right-on.

Does that make sense to you?

You see, Eddington was an early supporter of Einstein's theory, and he had alr
eady become the major explainer of the theory in Britain. So he had every reason to want the results of his observations to back up the theory as correct. Naturally, many scientists wanted to repeat the observations to see if the results stood up. And Eddington's results—and Einstein's theory—were backed up by observations made during a solar eclipse in 1922 and many, many times since then.

Play “Guess the Rule”

A hypothesis is a guess about what's going on. It tends to be a thoughtful guess, what we call an “educated guess,” not just some random idea.

If someone said that she had one specific rule in mind, and your goal was to guess her rule, your job would be to come up with one or more hypotheses (guesses) about what the rule is—
and then to test your idea. The person with the rule (let's call her the Ruler) gives you one example that follows her rule:
2, 4, 6
One of the students tests these three numbers:
10, 12, 14
The Ruler says, "Yes, that follows my rule." The next student tests these three numbers:
108, 110, 112
The Ruler says, "Yes, that follows my rule." Another student suggests:
234, 236, 238
The Ruler says, Yes, that follows my rule."

Do you think you know what the rule is?

What three numbers would you test, when it's your turn?
I have the answer below, but I want to mention that this game is set out in more detail on the internet. For example, this excellent group lesson for teens or adults, written from a teacher's point of view, explains this game and its value to students. Another lesson (again meant for older kids and adults) has several “problems,” one of which is this same game. There are links to suggested answers.

Most people, when they see the three numbers given by the Ruler, hypothesize that the rule is counting up with three consecutive even numbers. (Consecutive means the very next one in a particular number sequence.) When they see other students test this theory with more sets of increasing consecutive even numbers, and the teacher agrees that those sets follow the rule, their hypothesis is confirmed over and over, and they become more and more convinced that they are right.

But they are wrong, and they are going about testing the idea in the wrong way.

When you come up with a guess such as “counting up with three consecutive even numbers,” you should test the hypothesis with something that would not fit the hypothesis in just one way. For example, you could try:

  • counting down with three consecutive even numbers
  • counting up with four consecutive even numbers
  • counting up with three non-consecutive even numbers (such as 10, 22, 50)
  • counting up with three consecutive odd numbers

As you try to prove your original hypothesis wrong, you will more quickly and more surely find out whether or not it is right or wrong.

In other words, if you want to confirm an idea, try to prove it wrong.

In this particular game, the rule is: count up. That's it. Doesn't matter if the numbers are even or odd, consecutive or non-consecutive. Doesn't matter how many numbers there are. All that matters is that the numbers increase!

More Games!

You can play the familiar game of Mastermind to practice making confirming-or-refuting tests. If you don't have this game (or if there is nobody around to play), try the online version here.
A similar game using words is called Jotto. Try out that game here.

Going back to Einstein's prediction...
Ummm....Why does light bend around large objects?

It's pretty complicated, but you can look at the diagram here to see how large objects “dent” spacetime, causing spacetime to curve. (Click "Watch Web Preview.) The path of light moving in a “straight” line in a curved universe will itself curve!

If you're interested in more, this website has some upper-level math, but it also has some diagrams you might want to see. Here is another website with some great diagrams about the geometry of the universe. I love this stuff!

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