Physics by Carlo Rovelli

The Value of Doubt

We like to think that science is completely objective and scientists are absolutely certain of their theories. But ask any scientist how they reach their conclusions and you'll be surprised to see them talk about doubt rather than certainty, about what might or could be rather than what is true.

When Einstein introduced the basic concept of quantum theory, he did not say he was absolutely certain. Instead, he began with these key words:  'It seems to me that the observations associated with [...] transformation of light are more readily understood if one assumes that the energy of a light ray [...] consists of a finite number of 'energy quanta' which are localized at points in space'. Charles Darwin had two little words next to the first sketch of the idea of the tree of life in his notebook:  'I think...' And Michael Faraday, introducing the idea that a physical field is a real entity, the notion on which all modern theoretical physics relies, was not quite so sure: 'And though I should not have raised the argument unless I had thought it both important and likely to be answered ultimately in the affirmative, I still hold the opinion with some hesitation, with as much, indeed, as accompanies any conclusion I endeavour to draw respecting points in the very depths of science'. Just as it is about to discover something unfathomable, genius pauses, hesitates, and wonders.

You could try this yourself: open a prestigious science journal, such as Nature, and take a look at one of their articles. You'll find a range of words that suggest uncertainty — 'presumably', 'it is likely that', 'it seems plausible', and so on. It sounds as if in order to publish in Nature, you have to be completely unsure of what you are saying.  I have met many well-known scientists, of all sorts. There are those who always seem to be very sure of themselves, and know the answer to everything right away.  In my experience, they are not doing the most cutting-edge work.  Those who are, will often look at you, think for a while, then mumble 'perhaps it is this…', then change their mind, and more often than not end up saying 'actually, I am not sure...'

Few people have changed their minds on crucial scientific issues as often as Albert Einstein. By changing his mind a sufficient number of times, he was right more often than most of his colleagues. But to be able to change your mind, you need to be ready to be unsure.

Well, you may object, this all happens during the process of discovery. Once something has been scientifically proven, a scientist must be sure of what has been discovered. Once a theory has been formulated, verified, tested, retested and tested again, we can be certain – isn't that true? Hmm, are you sure about that?  Newton's gravitational theory was the most tested and confirmed theory of humanity. Had everybody been convinced that it was right, there wouldn't have been an Einstein to find a theory that is actually much better.

It is not certainty that characterises scientific thinking; it is uncertainty. The reason is very simple: as we are always doubting everything, we ask more questions and find the best possible solution at that very moment. John Stuart Mill put it this way: 'The beliefs which we have most warrant for, have no safeguard to rest on, but a standing invitation to the whole world to prove them unfounded.' That is, if we are open to doubt, we have a better chance at being right.

If you want to test your teachers, ask them a difficult question: a good teacher is the one who answers 'I am not sure'.

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