Laws, theories and hypotheses
Laws, principles, theories, hypotheses and conjectures
In science all of these terms have different shades of meaning but none of them are cast in stone (no matter what some text books might suggest) and none of them are the same as a guess.
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I have sometimes used the word theory where perhaps hypothesis or conjecture might have been better. This is just to try and use an everyday word rather than a technical one.
Here is a simplified account of some of the ways these words might be used.
A theory is a big complex structure that's accepted as being true for the moment
Let's look at the Big Bang theory for the beginning of the universe as an example.
It's fairly easy to think that the Big Bang theory simply says that a long time ago there was this big explosion that started the universe. End of story.
If this was all there was to it, it would only deserve to be called the Big Bang conjecture or even the Big Bang complete guess.
But the Big Bang theory, like all proper theories, is extensive and complex.
Here are some of the features that are common to all scientific theories
- It is broadly accepted by the tens of thousands of scientists in the particular scientific community as being the best explanation going.
- There is a lot of evidence, over a long period of time, conducted by large numbers of scientists from many different fields that support the theory.
- It has a broadly defined scope of the sort of things it can and can't explain. For example it might try and explain the way galaxies are distributed but not why birds often build nests.
- It contains implicit assumptions about the sort of things the universe (or some part of it) is made from. For example the universe is made from space that changes shape depending on what's in it and how it's moving (Einstein's view), rather than space being absolute and unchanging (Newton's view).
- There is a language that describes what you see that only really makes sense if you accept the theory. For example a galaxy looking redder than it should is accepted as meaning that the galaxy is moving away from us - hence 'expansion of the universe'.
- There are mathematical techniques that allow you to answer questions with numbers like For how long? How big? How many? How far? For example the (fiendishly difficult) mathematics of General Relativity and Quantum theory, rather than any of the stuff you do at school.
- There are accepted experimental techniques and large numbers of specialist scientists who continually refine and expand them. For example the use of huge telescopes and particle accelerators, rather than bunsen burners and test tubes.
- The theory makes predictions that can be tested. These predictions often use mathematics and come up with a number. For example the Big Bang theory predicts the existence of more matter than we can account for. This leads scientists to look for 'dark matter'.
- The theory is often modified round its edges and there are always puzzles that need solving. For example the Big Bang theory can't account for why there is more matter in the universe than anti-matter.
- It's possible that the theory may ultimately be replaced with another one that covers a bigger scope or has a closer match to observations.
Even if not all of these things make sense immediately it should be clear that a theory is more than just a simple statement. A theory is the widest-ranging and most complex structure in science.
A law is a pithy part of a theory
The major difference between a scientific law and a scientific theory is that theories are huge, complex structures with raggedy edges that would take a book to describe. A law can be written in a single sentence.
There are several types of scientific law.
A scientific law may be a short statement or simple equation that sums up a universal truth that over time has never been seen to be violated.
- The law of conservation of energy: Energy can neither be created nor destroyed.
- The law of conservation of matter: Matter can neither be created nor destroyed.
Some laws are simply true by definition (these are sometimes called principles)
- Newton's 2nd Law: Force = mass x acceleration, serves to define what a force is
- the principle of conservation of momentum is true under all circumstances simply by defining what masses you're keeping track of
- the principle of moments is just a neat piece of mathematics that helps you calculate useful things like when a lever will lift a weight
Sometimes things that are called laws just apply to very small parts of the universe in very specific circumstances.
- Ohm's law for electric circuits (voltage is proportional to current if the temperature doesn't change)
- Boyle's law for gases (pressure is inversely proportional to volume at constant temperature)
- Hooke's law for springs (force is proportional to extension if you don't stretch a spring too much)
Laws are typically part of a specific theory, though this may not always be obvious. For example, the conservation of matter is part of the Newtonian world view that things can't just appear from nowhere. In quantum physics they can, so this law has to be modified.
A hypothesis is normally a statement about one part of a theory that you can go out and test by experiment or observation.
For example in the 1820s astronomers noticed that the planet Uranus didn't move in the way it should have done if they used Newton's theories to do the calculations. Several astronomers independently came up with the hypothesis that there was another planet outside Uranus. The hypothesis was mathematical and predicted where in the sky to look for this planet.
Later astronomers found the planet, which they called Neptune.
When you make a prediction about an experiment at school you should really say that 'My hypothesis is that...' rather than 'My theory is that...'
A conjecture is at least one degree weaker than a hypothesis.
A hypothesis is normally based on fairly well accepted grounds (e.g. Uranus isn't following the orbit predicted by Newton's theories) and can be fairly easily tested (let's see if this new planet is where we say it is).
A conjecture takes the form:
We have good grounds to accept that such-and-such is the case though it may not be. Let's accept that it is and build other ideas on top of it.
We've heard that the new boy in the school is a good swimmer even though we've never seen him. Let's assume he is and ask him to train with the swimming team.