The way the word "theory" is used in modern English is increasingly synonymous with some half-baked idea that is unproven at best and almost certainly NOT true. This is why many people when they encounter the word "theory" in a scientific context become confused, skeptical, or downright contrary.
But when scientists use the word "theory" in its technical sense, what do they mean?
To help us through this philosophical maze, let's take a simplified example from astronomy. Let's suppose you look up at night and see a point of light. You suggest to your friend that the point of light is a star. Your friend says, "Nuh uh!" How do we find out who is right? How do we determine objectively whether the point of light you see is in fact a star? How do we eliminate the pointless, useless subjective difference of mere opinion and determine what is fact? This is what science does every day.
An idea, a notion, a concept or a proposition becomes the sole property of Science if that proposition is testable. If the question is specific enough and all the terms can be defined with even moderate precision, and if it is a question that some experimentation or observation could resolve one way or the other, then that notion can be called a Hypothesis.
Your hypothesis is that the visible point of light is a star. This is a testable proposition, and so any further debate is pointless. Anything except observation and experimentation would be a useless waste of time and words.
For example:
You: "Uh huh!"
Friend: "Nuh uh!"
You: "Is so!"
Friend: "Is not!
. . . and so on until shots are fired, or someone gets de-friended.
How would scientists proceed? First, they would establish that the phenomenon in question actually exists in objective reality. Do you both see the point of light? Yes, and many other people see it too. It also shows up in astrophotography and can be detected by computers who are obviously not human and not susceptible to our occasional mass-hallucinations. Therefore its objective existence is reasonably assured for all practical purposes.
Is it the same point of light that you are both referring to? Don't assume this - make sure you're both talking about the same thing by measuring its position quantitatively and seeing that you are in agreement. OK - you are both looking at the same objective phenomenon and still reaching a different conclusion about it. These preliminary steps alone would have eliminated probably half of all so-called scientific debates throughout history. People have argued about things that didn't even exist, or about things that were not even the same thing. Ridiculous, isn't it? We would never do anything that stupid today, would we (ahem).
Now that we have an honest disagreement of a factual, testable nature, it's time to get more data. Your claim is that it is a star. What do we know about stars? Well, their positions are relatively fixed in the sky relative to other fixed luminous objects from night to night and season to season. A couple months' worth of repeated observations should tell us whether the object in question is more star-like or more planet-like (or airplane-like). Suppose it turns out that it is relatively fixed in the sky relative to other stars. Does that prove it is a star?
For fun, let's say your friend is extremely obstinate and skeptical. No, he says, this does not prove it is a star, defined as a sun occupying its own space within a galaxy and producing a broad spectrum of electromagnetic radiation through a process involving the nuclear fusion of hydrogen into helium.
OK, so what else can we do to collect more information? How about using a spectrophotometer? That separates the light coming from the star into specific colors and reveals characteristic thermal and chemical signatures indicating that the object is producing abundant light and heat, along with very specific indicators of the presence of various chemical elements. The results are all consistent with that of a star. Also, very precise measurements of the object's position taken 6 months apart show us that as the earth orbits the sun the apparent position of the object changes so very slightly as to suggest it is located immensely far outside outside the solar system, but not beyond the limits of our galaxy.
So far nothing we have been able to observe or measure suggests that the object is anything but a star. No other possibility has been suggested that is also consistent with all the data. Do we conclude that the object is definitely a star? Based on all we know right now, identifying the object as a star is indistinguishable from whatever the real truth may be.
That is what the word "Theory" means. It is a hypothesis that has been tested using every available means and cannot be disproved with any available data that we have. It is "Indistinguishable from Truth."
But your friend says, "Dummy! You're still wrong! Look through my binoculars once, would ya!"
What you see when you look more closely is that the object you thought was a star is actually a DOUBLE star. "Cool!" you say. "So my hypothesis requires modification."
Notice that it remains true that to the naked eye the object is indistinguishable from a star. Remember three paragraphs ago when we said that calling the object a star was indistinguishable from the truth? Is that statement still accurate? It is! To the naked eye, the object certainly is indistinguishable from a star. Only under high magnification does the fact that there are two stars become apparent, useful, or even a necessary distinction.
Once a Theory attains Theory status it can only be improved, not overturned. That is because it was based on observation and experiment in the first place, and at some previous time was indistinguishable from the truth. The fact that we can observe now with higher resolution and have more technologies available for making observations that were previously impossible doesn't make previous observations wrong. It just makes them superseded by more accurate data or data of previously inaccessible phenomena.
It is a frequently repeated error that somehow Quantum Mechanics and Relativity have overturned Newtonian physics and made it wrong. Newtonian physics was tested and observed countless times in the past and found to be indistinguishable from the truth. And in situations where speeds are not more than a few kilometers per second and objects are larger than a few nanometers, Newtonian physics is still indistinguishable from reality.
In fact, an oft-overlooked conclusion of quantum mechanics is that any object of more than a few thousand individual atoms must by the laws of quantum mechanics behave in a totally Newtonian fashion! And likewise, Relativity requires that objects not moving very fast relative to each other must obey Newtonian physics to an extremely high level of precision, often far beyond any hypothetical chance of detecting any difference. Newtonian physics is not overturned by modern physics, Newtonian physics in everyday situations is what modern physics predicts and requires!
While hypotheses come and go about as often as fad diets, and some things that newspapers incorrectly report as proven facts are later found to be less than accurate or even just plain wrong, a scientific Theory is a much more reliable and constant thing. It takes many decades to establish a scientific Theory, and is never the result of just one experiment. It certainly is nothing like a mere philosophical conjecture.
My definition of a Theory is this: after every conceivable, available objective test and observation that anyone can make, a Theory is that which remains indistinguishable from the truth.
For more on how Science works and what a Theory is, see this:
I Have A Theory!
