Hypothesis, Theories and Laws: what’s the difference?

Posted on October 21, 2017

The basis of Science relies on the Scientific Method, the best method we have to understand nature. Everything starts with an observation (e.g. plants do not grow well -or at all – in heavy metal-contaminated soils)  that leads us to make a question (how well can a plant grow in heavy metal-contaminated soils?). A scientific hypothesis is formulated according to this question, and it is the first step in any experimental research. A hypothesis has to be probed or rejected, by designing a set of experiments which results need to provide a clear proof to accept/reject the hypothesis. If the results of the experiments don’t leave room for distrust, a conclusion is drawn and communicated to the scientific community, commonly by peer reviewed articles in renowned scientific journals.

So, now that we have an understanding of what a hypothesis is and the purpose of formulating them, we can move on to the point of this article: what is a Scientific Theory and a Scientific Law ? When enough proof about a topic is accumulated over time, a Theory is born. A Theory explain an aspect of nature, based on all the accumulated knowledge about it. In Science, a Theory is the highest rank of knowledge. However, that doesn’t mean that they are rigid and unchangeable. If new evidence contradicts an explanation offered by a Theory, the Theory needs to be reviewed, changed or replaced by a better Theory that offers a better and more complete explanation. Let’s put an example: the Theory of Evolution through natural selection, by Charles Darwin. This is maybe the most controversial Theory for the general public, although there is not controversy at all in the scientific community (as I said, if there was any piece of evidence that debunked the Theory of Evolution, it would have been taken down long time ago, but on the contrary, every year there is newer and more detailed evidences of it).

Evolutionary tree showing the divergence of modern species from their common ancestor in the centre.[294] The three domains are coloured, with bacteria blue, archaea green and eukaryotes red. Source: Wikipedia

However, we use the word theory (note that it is not written with capital T) every day in a non scientific context. We often use phrases such as “In theory, tomorrow I have classes during the whole day” or “In theory, my new camera should take better photos than my old one”. In this context, we use “in theory” as a way to explain a future event based on our current knowledge or our intuitive understanding of how things work (based on my last week timetable, I should have the same classes schedule tomorrow, or based on the specs of my new camera, it should perform much better than my old one). However, none of those meanings for the word “theory” are related to the vast accumulation of independent proofs to explain a natural event. That usage of the word theory falls far from the vast accumulation of knowledge obtained through tested, reviewed and discussed independent experimental data, which defines a Theory (with capital T).

Charles Darwin (left) and Sir Isaac Newton (right)

On the other hand, we have Scientific Laws. Although commonly a Scientific Law is regarded as a superior level of knowledge than a Scientific Theory (even from some scientists, sadly), they are complementary of each other. A Scientific Law provides a mathematical explanation for a Scientific Theory. The best example would be the Theory of Gravitation, which explains that any object with mass applies a force over other objects, which depends on the mass of the objects involved and the distance that separates them. The Law of Gravitation offers a very neat equation that relates all these parts:

F=G\frac{m_{1}m_{2}}{r^2}

where F is the Force between the two objects, G is the gravitational constant (6.674×10−11 N · (m/kg)2), m1 and m2 are the masses of the two objects , and r is the distance between the center of the two masses.

Unfortunately, there are a lot of Scientific Theories that cannot be explained in such an elegant and direct mathematical way. The Theory of Evolution through Natural Selection, for example, is so complex and involves so many different biological and abiological factors, that would be impossible to explain in such a way. However, that does not mean that it is not backed up by mathematics, or that it is not probed altogether. As  many other Theories in Biology, it is just too complex to be explained by a Law.

I hope that this brings some light over the terms Hypothesis, Theory and Law that are so often misused (some times even with mean intentions) and misunderstood.

Some more detailed explanations about this topic can be found in the Wikipedia:


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