On the 29 May 1919, the world witnessed a total eclipse of the sun. The path of totality passed from northern Brazil, across the Atlantic Ocean and finally to Africa. The best place to view that eclipse was the island of Principe, off the West Coast of Africa. On that day, a small team of scientists sat huddled on the island around a bunch of telescopes, waiting to take photographs of the eclipse. The leader of the team was the astronomer Arthur Eddington, and the photograph he eventually took revolutionised our understanding of the universe.
When Albert Einstein published his general theory of relativity in 1915, he realised that, if his theory was a valid description of reality, we would expect to make particular and specific observations. His theory stated, for example, that gravity curves space. If this were true, Einstein thought, we would expect the sun's gravity to curve light from distant stars. In other words, if the sun moved in front of a specific star, that star would still be visible to us because its light would curve around the sun. Not only did Einstein's theory make this prediction, but he also used the theory to calculate the exact amount by which the sun's gravity would bend light. The answer: light would be deflected by 1.7 seconds of arc, less than one thousandth of a degree.
The 1919 eclipse (read this article for a full account) was the perfect opportunity to test this prediction, and although there is some doubt over Arthur Eddington's original calculations, the photograph he took proved that light from distant stars was deflected by the sun's gravity, by almost the exact amount which Einstein predicted. This result has been consistently confirmed many times over by many astronomers since 1919, and it stands as one of the many examples in science where a theory was tested and confirmed by empirical evidence.
In 1963, Karl Popper, probably the most well known contributor to the philosophy of science, published an article titled Science as Falsification. Drawing on the confirmation of Einstein's theory by the 1919 eclipse, Popper outlined, within just a few pages, his own view on what separates scientific ideas from non-scientific ideas. I suggest you read the entire paper here, but his main points are as follows:
- It is easy to find confirmations for any kind of theory. So to determine the validity of a theory, we have find some way to test it.
- Scientists test ideas by making risky predictions (i.e., examples of what evidence we would expect, and not expect, to observe in the world around us if a specific theory is correct). In the example above, Einstein's prediction was that the sun's gravity would bend light, but he made it a risky prediction by calculating by how much it would bend: by 1.7 seconds of arc.
- By testing a theory in this way, we make it possible to refute (i.e., to falsify) the theory. For example, if we observed that light was not deflected by the sun, then Einstein's theory would be invalidated. According to Popper, a theory which is not refutable, at least in principle, by any conceivable event is non-scientific (solipsism is an example of a 'theory' that is irrefutable and thus non-scientific).
In other words, a theory is scientific if it makes risky predictions that can be tested against physical observations, and is thus liable to falsification.
This is just a basic outline of what makes a theory scientific. For a much more overall and comprehensive description of science as a method, read here.
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