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In the debate between religion and science a common argument is that the theory of evolution, or the Big Bang theory, or some other scientific theory is "just a theory". "It hasn't been proved!", they argue. "That means my theory is just as valid."
This argument shows a very common misconception about science that needs to be cleared up. Science never proves anything. Ever. Let me repeat that with different emphasis: Science never proves anything.
For instance, gravity is "just a theory". Like evolution it cannot be proved correct. I pointed this out in a debate recently, and my opponent attempted to mock the example. He, like most people who make these arguments, did not understand how science works or its limitations.
Science is based on inductive reasoning, which is a method of drawing generalized conclusions based on finite observations. Inductive reasoning can be used to disprove a theory, but it cannot be used to prove one. for example, take the following observation:
- The grass outside my window is green.
Using inductive reasoning I can conclude:
- All grass is green.
This conclusion is valid in inductive reasoning so long as all observations support it. As soon as an observation contradicts the conclusion, the conclusion is proved false. However, the only way to prove theory is true would be to observe all grass. This limitation is due to the fact that a conclusion is being drawn from a subset of possible observations.
Since science, by its very nature, attempts to draw conclusions from observations of the natural world inductive reasoning is necessary. In science, though, it is literally impossible to make every possible observation to prove a rule. Therefore, it is also impossible to prove any theory in science. Every conclusion science has ever made is an unproved theory, including gravity.
Still not convinced? Consider the theory itself (in a Newtonian sense for simplicity): all mass is attracted to all other mass in a manner which fits a specific equation (F = G*m1*m2/d^2).
How would we test this? We can try dropping objects with various masses, measuring their acceleration, and then use the above equation (along with Earth's mass for the second object) to verify. If the math works out then the theory is supported. Up until Einstein (maybe even up until now, but I'm not sure) this always worked. However, the only way to prove that the equation is right would be to test every object in the Universe against every other object in the Universe. This is (essentially) an infinite number of observations, which can't be done.
Some might argue that this only applies to the equation itself, but not the fact that gravity occurs. They would say "I can prove gravity by dropping something". However, they are wrong. The only thing you can prove by dropping something is that gravity worked for that test. The only thing they're changing in their test versus the test I described above is loosening the requirements for success: instead of requiring that the equation fit the observation, they are checking that the acceleration is positive (F > 0). The reasoning being used to prove the theory hasn't changed. It's still inductive, and you would still have to make an essentially infinite number of observations in order to prove that gravity works in that way every time.
Some people find this unsettling. If nothing can be proved then how can we know anything? What good is science? The problem is that people are wanting more from science than we actually need. We don't need to know that gravity always works. We just need to be confident that it works under given circumstances. After enough successful tests a theory may be considered a fact in practicality, even when it is not technically proved correct.
Science is not about proving things. It never has been, and it never will be. Instead, science is about observing the Universe around us and using those observations to try to understand how the Universe works. This understanding is always subject to change no matter how confident we are.
