Theory and Evidence

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What Is It?

The terms theory and evidence go hand in hand. A theory is a formal explanation of the relationship among a set of observations. The observations provide evidence for the theory. Pieces of evidence support a theory in the way that legs support a table or columns support the roof of a building.

Scientists develop theories to explain natural phenomena. The Big Bang Theory is a well-known theory about the origin of the universe. It states that the universe originated billions of years ago in an explosion from a single point of nearly infinite energy density. The observation that the known universe appears to be expanding away from a central point is used as one piece of evidence supporting this theory.

Theory making is a process. A hypothesis is a sort of immature theory that has little or no supporting evidence. Once sufficient evidence has been collected to confirm the hypothesis, it becomes a theory. Once a theory becomes sufficiently grounded in evidence, it can be become a fact. In the time of Galileo, the notion that the Earth and other planets revolve around the sun was a controversial theory. Today, because of supporting evidence, it is accepted as a fact.

One good test of a theory is whether or not it can be used to make reliable predictions. Using theories about orbital mechanics and the force of gravity, astronomers were able to predict the existence of the planet Pluto before it was actually observed by Clyde Tombaugh in 1930. The accuracy of this prediction helped confirm the prevailing theory about the behavior of objects in the solar system.

A valid theory cannot be proposed without evidence. A good example of how intricately theory and evidence are intertwined is to consider proposing a theory, such as the Big Bang, without evidence. Without the evidence that supports claims about the age and origins of matter, the theory is no more valid than any other. Without evidence, a theory has no credibility and is really no more than an opinion.

Facts and Observations Interpretations and Questions
There isn't any water on the floor or around the base of the tank. It can't be a leak. If it was, we'd see water on the floor. Maybe the fish are drinking the water?
We got another tank of the same size without any fish. We measured the water level every day. The water level went down the same amount in that tank too—about three centimeters each day. It can't be the fish drinking the water, because the water level went down in the tank that didn't have any fish. Also, even if the fish drink the water, the level shouldn't go down because the water is still in the tank, in the fish.
We put a cover on the tank and the water level didn't go down. It must be that the water is getting out into the air in the tank when the cover isn't on.
We saw some water drops on the underneath of the cover. The water must have gone through the air to get to the bottom of the cover.
We can't see any water in the air. The water in the air must be invisible.
The water level in the tank goes down because the water goes up into the air where it is invisible. Our evidence is that the water must be going into the air because the tank is not leaking and the fish are not drinking it. We know the fish are not drinking it because we measured the water in a tank without any fish and the water level in that tank went down too. Also, we checked for leaks, so it was not leaking out. When we put a cover on the tank, we measured that the water level did not go down. We saw some drops of water on the underneath of the cover, so we think it went through the air but the cover stopped it. That's our theory. Our teacher says this is "evaporation."

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