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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.

Try Why Do Astronauts Float by Julian Hamm

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

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.

For example:

Some laws are simply true by definition (these are sometimes called principles)

Sometimes things that are called laws just apply to very small parts of the universe in very specific circumstances.

For example:

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.

For example

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.

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