Difference between hypothesis, theory and law?

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Many people say that in science, a theory is an experiment that was proven and can be replicated. Like the theory of evolution, which is proven beyond doubt (I can feel the creationnists coming, turns out evolution is still true, but that's not the point). However we also say string theory despite the fact that it wasn't proven.

Also, why do Newtonians Laws of Physiscs are called Laws and not Theories? I mean nobody is doubting the existence of gravity.

Somebody told me that a law was a theory that could be resumed to a mathematic equation, while someone else told me it was something that applied EVERYWHERE, to ANYBODY in the universe, which was why evolution wasn't a law, because while it is still proven on Earth, it's not 100% sure if there is also life somewhere else than Earth and if natural selection applies to it too.

Anyway, it's all confusing, so can somebody help?

An idea is a well thought-out opinion.

An hypothesis is a testable idea.

A theory is a former hypothesis that has been proven to be valid by repeated demonstrations.

A principle (a "Law") is a generally accepted theory, usually expressed in mathematical terms.

So...string theory was proven valid? Since when?

"String Theory" is a misnomer. So far, it qualifies only as an hypothesis (maybe ... is it testable?), although it has been "proven" through mathematical simulations. Same for Dark Matter and Dark Energy - Hypotheses (maybe ... are they testable?) that seem to explain certain phenomena without any practical demonstration or test verify its accuracy.

A theory has wider scope than a hypothesis.

A law has the widest scope of all. What makes a law a law (in the scientific sense) is not only empirical corroboration but great generality. For example the various conservation laws. They hold in numerous applications and contexts and are near the logical base or core of various physical theories. From the conservation laws many other results can be derived.

ruveyn

I'm glad you chimed in, Ruveyn. Your knowledge of science seems to run much deeper and wider than mine, or you seem to remember the physics lectures better than I.

Either way, thanx!

Well, technicaly, Dark Matter and Dark Energy has been proven, because there is something (or is it...someone ) f**ing up with the movement of galaxies. It's just that we haven't observed it directly.

See -The Trouble With Physics- by Lee Smolin

ruveyn

A hypothesis is an idea that can be tested, as Fnord points out. I would like to add to the definitions of theory and law though based on what I've gathered from forty years of studying the social controversy over the teaching of evolution.

Laws are not "better" than theories, and a theory doesn't graduate to become a law (unlike what some people believe).

To call something a "law" of science is misleading in a way, because they are not laws in the ordinary sense but simply observed regularities of nature. A theory in some ways is better than a law because instead of just being descriptive, a good theory also makes testable predictions. A theory must account for all the observed facts and be falsified by none, so in some ways theories are also stronger than isolated facts.

Also, a theory can never be "proven" true. In order to be a scientific theory though, it must be falsifiable, that is, there must be some predictions it makes that can conceivably be proven false. In that case, the theory is either modified to account for all the existing evidence plus the new evidence without contradiction, or it is discarded. Scientists do not knowingly hang on to ideas that are not supported by evidence.

Evolution is a fact of nature, just as gravity is a fact of nature. And, like gravity, there are various theories of evolution that explain various aspects of this fact of nature. Some theories account for some aspects of reality better than other theories, or apply within limits or conditions outside of which they do not apply. Gravity didn't go away when Einstein proposed a deeper explanation of it than Newton did. Evolution won't go away either, as much as the creationists want it to.

As far as I'm aware, the difference between something being labelled 'a law' or 'a theory' is only down to when it was discovered.

As science matured we realised that nothing is truly 'universally applicable' everything has limits and breaks down at some point when pushed to an extreme so 'laws' are only laws within a certain scope. So we stopped calling things 'laws' and settled on the less arrogant 'theory'.

Granted if you want to start breaking down things like light or time you need to recreate the conditions at the very start of the big bang, but they are still breakable...

Despite what Lee Smolin and Peter Woit might lead you to believe, string theory actually does make testable predictions. In fact, a whole class of the many solutions of string theory have already been ruled out empirically by experiments done recently at the LHC.

Also, in my opinion, neither of those two have really provided compelling reasons for string theory to be abandoned, in either of their respective books. That's just my opinion though, and I know other colleagues of mine who would disagree with me.

Lee Smolin, by the way, still supports loop quantum gravity, his own alternative to string theory, even though it has already mostly been proven wrong by observations made by NASA's Fermi satellite. That's despite the fact that he promoted it over string string for years, on the basis that it had more "testable predictions" yet when it came to testing those predictions, they turned out to be wrong. But that goes without saying.

Unfortunately there is no one and single string theory. There are many string theories which makes empirical testing rather difficult.

It is much better when theories grow out of facts. But such simplicity is gone forever.

The game was up when Dirac predicted anti-particles on purely mathematical grounds.

ruveyn

Metaphorically speaking, it seems that all of the low-hanging fruit that humans can reach from the ground have already been picked from the tree. Literally, it used to be that with a little glassware, some wire, a few magnets, and a source of electricity, one man working in a potter's shed could demonstrate almost any principle of electricity. Now it takes something the size of CERN to even hint at the cosmic mysteries awaiting our discovery.