The definition of the word "theory" in the context of science is sometimes hotly argued by non-scientists

The origin of this confusion has it's roots in the history of the development of science. When we speak of early, classical physics, we talk about laws, Newton's laws of motion for instance, the ideas have the weight of veracity. After all, the word "law" has a serious and strictly defined meaning in our culture. Back when Newton declared his laws, he believed them to be absolute descriptions of how the universe worked. At the time, they were irrefutable. We now know that his laws are in fact approximations, rules that work when describing motion on the macroscopic scale but which break at the quantum scale.

Since that time, science has gotten warier about describing anything as being absolute. Science, and physics in particular, is a tool to root out the true nature of reality. It can describe only what it observes which may or may not be true in every case. In order to say if something is absolutely true, every single possible case of a particular phenomena must be observed. In a universe as vast as ours, that's completely impractical. Science can say if something is probably true all the time if observations of a phenomena are the same in many cases. This tiny bit of waffling bothers many people who are not familiar with the inner workings of science. Shouldn't something be always true if it is true at all? Science just can't commit all the way to absolute - otherwise it wouldn't be science, it would be faith.

So science has tossed the use of "law" in favor of "theory". This "theory" does not mean "hypothesis" which is a speculation. In this case, think of music theory - definitely not a hypothesis, but a working set of rules that define a body of knowledge.

The line between theory and hypothesis can become blurry when it comes to very active and new areas of science. For instance, M-theory, an extension of string theory, is a body of knowledge that attempts to define how everything in the universe works, explaining quantum phenomena along with cosmological and everything in between. Unfortunately, M-theory is largely unproven. It makes a lot of sense (as far as descriptions of the quantum world make sense), but hasn't really been tested yet. M-theory can be more precisely be described as a hypothetical theory.

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