The scientific method has been one of the best tools for understanding the Universe we live in since the 17th century. Darwin’s Theory of Evolution, Einstein’s Theory of Relativity, The Germ Theory of Disease, The Big Bang Theory are some of the examples of science’s greatest breakthroughs and achievements which helped us understand Nature much better than any of our ancestors ever did. However some people don’t realize the weight of these achievements because something about all this does not sit well with them. “Isn’t it all just a theory?” they ask. This is a common mistake and you could be forgiven if you also thought the same. But the truth is that the use of the word ‘theory’ in Science is very unlike its use in colloquial language. To understand this difference, we need to understand how the scientific method works.
THE SCIENTIFIC METHOD
Anything in science begins with an observation of the natural world. Newton began studying gravity when he observed falling objects. It may not exactly be an apple from a tree, but you get the idea. Charles Darwin started studying evolution after observing the variation in the beak structure of birds called finches on the Galapagos Islands. The Big Bang theory has its roots in the discovery that the Universe was expanding made by observations of the redshift of various galaxies by Edwin Hubble (more on that later). The point is that everything that we study in science is based on some observation.
However, an observation is not enough. Science has to explain these observations which means we need ‘hypotheses’ or ‘models’ which are proposed explanations for our observation. Once we have a good hypothesis in hand, we need to test whether our hypothesis holds weight. To do this the hypothesis needs to be falsifiable and by that I mean that there must be some experiment that can help us either confirm or reject our hypothesis.
To give an example of this process, I will cite the Big Bang Theory as it is currently one of the best models on the origins of the Universe as we know it. As I mentioned before, the Big Bang Theory was initially proposed as an explanation for Hubble’s discovery of an expanding Universe. Hubble discovered that the Universe was expanding by measuring the redshift of various galaxies. A redshift is just like the Doppler Effect, but on light. You may have noticed an ambulance siren change pitch as it races past you. Similarly galaxies which emit light, seem to appear redder as they move away from us. This is what we call redshift. The redshift of galaxies indicated that they were all moving away from us which indicated an expanding Universe. This was our initial observation.
Now how do we explain this observation. A sensible model would be one that claims that if the Universe is expanding, then there must have been a time when it was smaller. Therefore if we rewind the clocks far enough, we get to a point when the Universe was point sized. This point sized “singularity” as we call it, then rapidly expanded to give us the Universe we see today. That is now a proposed hypothesis or model. There were many other observations that lead to many more features being added to this model through theoretical calculations, like the exact chronology of events that happened after the birth of our Universe. However none of these could be taken seriously until we tested this hypothesis through experiment.
One of the features of the Big Bang model was that according to it, the Universe was extremely hot till about 380,000 years after the Big Bang. So hot that protons and electrons could not combine to form atoms because of their high energies. Due to this the free electrons would continuously scatter photons, i.e. the particles of light. This meant that photons were unable to travel far along a straight line which rendered the Universe opaque. You would see a bright orange glow wherever you looked instead of the inky black you see in the night sky today.
But how would we know if this hypothesis is true? There has to be some experiment we could perform that could help us confirm or reject this model. Such an experiment was surely conducted and it actually confirmed our hypothesis. The experiment in question was the detection of what we now call the Cosmic Microwave Background Radiation (CMBR). Basically once the Universe was cool enough, the protons and electrons would combine to form atoms and the light that was being scattered all around the place would now be able to travel freely in straight lines essentially forever. In theory we should be able to observe this light. Surely the expanding Universe has stretched this light to frequencies that we cannot see with our naked eye, but we must still be able to see it in the microwave spectrum. And guess what? Scientists have been able to accurately map the CMBR coming from all directions to generate this beautiful picture.
It is only after we test our model or hypothesis through repeated experiments can we promote it to the ranks of a theory. Another example would be of Einstein’s Theory of Relativity. Before it was a theory, it was just a model developed by Einstein to describe gravity as the bending of space and time itself, rather than a force. Apples fall to the Earth but not because there is an invisible force acting on the apple but because space itself curves toward Earth while the apple just follows this curved space. But a model always has to be verified with repeated tests. One such test was performed by British astrophysicist Arthur Eddington who observed a solar eclipse from the island of Principe off the west coast of Africa on 29th May 1919. If Einstein’s model was correct then light from the stars that appear near the covered Sun must be deflected as shown.
This is because space and time itself curve due to the Sun’s gravity which in turn deflect starlight which was exactly what was observed. Moreover the amount the starlight was deflected was in perfect accordance to what Einstein’s equations predicted. Several experiments like this have been conducted which tested Einstein’s proposed model like detection of gravitational waves, observing the orbit of Mercury and so on. After passing each of these tests, we could finally promote Einstein’s model to a theory.
So to summarize, a scientific theory is not just a wild guess someone came up with at 3 in the morning and it should not be treated that way. It is a model or a hypothesis that has undergone rigorous experimental tests and has been verified to be the best contender to explain certain phenomenon. The chart below explains this perfectly.
LAWS VS THEORIES
Now a common question arises. What about scientific laws? Many hold the misconception that theories eventually become laws. But that is not true. The status of a scientific theory is the highest status any model can achieve. Nothing goes past this stage. As you can see in the above flowchart, a theory can only be tested again and again until we find one experiment where the prediction of theory does not match observation. If that happens we get new ideas to incorporate into our model, test them and check if our modified model can accurately predict the outcome of an experiment before we perform it.
However, a scientific law is something else entirely. A law is something that explains what will happen in a certain situation. A theory explains why it happens. Taking the example of gravity, Newton’s Law of Gravity explains what will happen if two bodies with masses m1 and m2 are kept with a separation ‘d’ between them. That is that they will seemingly attract each other with a force inversely proportional to the square of their distance.
However a theory is something that explains why it happens. For example Einstein’s Theory of General Relativity tells us that gravity is just an apparent force which is felt because anything with mass bends space and time itself around it. Objects following curved paths in this dent “fall” toward the massive body.
Therefore a law and a theory differ only in an explanatory power. A theory explains whereas a law simply states and quantifies phenomenon. A theory does NOT, with sufficient evidence become a law. They are not successive stages.
I would like to point out however that in some cases, the word theory is not used with its scientific definition, even in science. For example String Theory is not exactly a ‘scientific theory’. It is still in its model or hypothesis stage. This is because although it seems to be mathematically consistent, it has not been tested well enough through experiment. The experiments required to test the predictions of this model require tremendous amount of energy which unfortunately our current technology cannot generate.
So can theories never be wrong? Yes, they can be. In fact no model can be 100% confirmed to be true. There is always a possibility that a future experiment will disprove a theory or maybe a certain aspect of it. Then we will have to develop new models and test them again to fix the gap. However, the more a theory is confirmed with experiment, the less the chance of that happening.
Does that mean we never take current theories about the Universe like the Big Bang theory, General Relativity, The Theory of Evolution seriously? Can we invent our own beliefs to replace these theories just because they are theories? This is again not true. Any model that wishes to replace existing ones will need to have undergone the same rigorous testing as the current one and should be able to explain phenomenon where the previous model failed. Just because the Big Bang Theory is a ‘theory’ does not mean we can discard it and come up with our own idea no matter how outlandish. Although loosely used in some cases, a theory in science is not the same as a theory (a wild guess) in common language.