麻豆传媒

I RECENTLY posted an article to Twitter about a question that completely caught me off guard: do we understand why planes stay aloft? It turns out that the answer to that depends on who you ask.

To allay any anxiety you might suddenly be feeling, the first thing to say is that we know which equations to solve and which solutions give us a plane that stays up in the air. The challenge is that we don鈥檛 know how to interpret those solutions to offer an intuitive explanation, something we can all immediately grasp, rooted in physical principles. I grew up hearing that it is all explained by Bernoulli鈥檚 principle, with lift resulting from air flowing faster over the top of the wing and slower underneath. But I learned recently that this can鈥檛 explain why planes can fly inverted.

Whether this is a problem or not depends on what you think counts as a good understanding of physics. The mathematically inclined may be happy with the solutions alone. But as physicists, we are trained to expect that once an equation has been solved, we should be able to reverse-engineer an explanation for why the solution is physically sensible. Sure, physicists often work with complex equations. But we should be able to give everyone a feel for what we have learned, right?

Well, sort of. In my book , I write about the time when British-Iraqi drag performer, writer and film-maker Amrou Al-Kadhi told a Channel 4 News presenter in the UK that . I loved this comment from Al-Kadhi for reasons that perhaps they themselves would find surprising: to me, it suggested that all of the physicists who were shocked to discover that these fundamental objects can be particles and waves at the same time were too stuck in old ideas about gender to have an intuition for it. Perhaps if we all grew up without the gender binary, then wave-particle duality, as this quantum phenomenon is called, would feel as intuitive as gravity does, at least in terms of how it behaves in and around Earth.

That is the thing about intuition. It is a feeling. The fifth definition of 鈥渋ntuition鈥� in the Oxford English Dictionary comes from philosophy and is articulated as follows: 鈥淭he immediate apprehension of an object by the mind without the intervention of any reasoning process; a particular act of such apprehension.鈥� That is to say, intuition is immediately understanding something without really having to think about it. In that sense, I am not sure intuition is possible in physics. We always have to think deeply about the subject at hand to understand what is going on. But it is certainly the case that such thinking becomes faster as we get more used to certain ideas.

Importantly, even if we understand something, that doesn鈥檛 necessarily mean we know how to describe it in words. Alexis Shotwell writes about this in her book of philosophy, . Shotwell describes the phenomenon of 鈥渋mplicit understanding鈥� 鈥� knowledge that we hold but can鈥檛 put into words. Take riding a bicycle. As I wrote this paragraph, I thought about how to describe the mechanism of riding a bike and realised it would probably be a good writing exercise because the words didn鈥檛 come naturally to me. So, while I disagree with some applications of the idea in Shotwell鈥檚 book, I think her concept of implicit understanding can help us to better understand our own perceptions and expectations of what physicists do.

Does it matter if we can鈥檛 put into words why planes fly, as long as we keep them aloft? This is, on some level, a philosophical question. Calling it philosophy doesn鈥檛 mean I am not responsible as a physicist for sorting out an answer, just that I understand I am choosing a position that reflects my own personal view, not something that is rooted in definitive empirical data. As it happens, the way I think about this question has implications for questions that are more relevant to my research than planes are.

A reader of this column wrote to me some months back, for example, asking what it meant that space-time is expanding: where was all of the extra space-time coming from? As I considered how to answer this question, I read other people鈥檚 explanations and realised that it might seem as if I were cheating if I gave similar explanations. None of them is intuitive and some of them boil down to 鈥渢he equations imply this鈥�, which would hardly be satisfying. In the end, the answer is that space-time isn鈥檛 an object but a phenomenon. What does that mean? It may not be immediately obvious, but in my next column I will try to explain, to make sense of it in the most intuitive way I can. Stay tuned.