Why do we call it ‘modern’ physics and why should anybody care about the reasons why?

A rambling mess about my PhD

About 4 years ago, I applied for a PhD to study the transition from classical to modern physics in Britain. I knew relatively little about physics, having dropped it at school because it was boring and I was never able to understand how a fridge works. And I wasn’t really familiar with the categories of ‘classical’ and ‘modern’ physics. But I thought the project sounded interesting, and I really didn’t like my job. Miraculously, I was given money to quit and move to Manchester (which is something I thoroughly recommend doing). And I quickly set about trying to find out what it is I was supposed to be studying. As is so often the case with history, I was given a ‘received interpretation’ and told to look into it.

Here’s how the basic story goes (bear with me):
As the end of the nineteenth century approached, physicists were pretty pleased with themselves. They had the aether, this weird stuff that was everywhere and explained everything (slight oversimplification). They had a nice theoretical framework for important technologies like steam power. They had decided, and mostly convinced everybody else, that their discipline underpinned all others. And they could namedrop Newton whenever they liked. But then, with the twentieth century just round the corner, weird things started happening. Wilhelm Röntgen discovered X-rays. Henri Becquerel discovered radioactivity. J. J. Thomson ‘discovered’ the electron (although he called it a ‘corpuscle’ and didn’t conceptually relate it to the electron that his theory-inclined peers had been discussing). It would seem that matter was stranger than we’d first thought. And of course it got stranger. In the early twentieth century, quantum theory emerged, postulating that energy jumped around in little packets. This was a BIG problem for physicists of the Victorian tradition, who liked everything to be continuous and connected. Yes, matter had been discontinuous for some time, and was getting more so, as the atom was split up into little pieces revealing a void of empty space within. But physicists had managed to reconcile continuity with atomism by arguing that all these broken up bits of matter were swimming in the aether. Everything was still connected. But when discontinuity infected energy as well, it got a little bit harder to argue for continuity. And then there was also relativity theory to deal with, messing up time and space irreparably.

At some point after all these developments, people started referring to some types of physics as ‘classical’ and others as ‘modern’. And it stuck. Pretty soon we were looking back at the year 1900, conveniently situated between two centuries, and seeing a revolution in thought. X-rays, radioactivity and subatomic particles were all ‘classical’ – they didn’t defy the laws of Newtonian mechanics. Relativity and quantum theory were too weird – they became ‘modern physics’. Nowadays these categories seem quite obvious. Except that some people don’t really consider relativity theory to be modern physics, but rather an extension of ‘classical’ mechanics. So there’s clearly something of a problem here.

My job (if you can call it that) was to go back to the first few decades of the twentieth century and look at how physicists were actually using these terms, and for what purpose. Why was I doing this? What was the point? Well, that’s what historians do – they look at stuff that happened and say “Hey, this actually occurred slightly differently from what you thought. So? Um, well, you know, nuances.” You have to have some kind of purpose in your life. I do actually occasionally read very good arguments about why history is important, but then I forget them, like I forget everything, because I’m a bad academic in that respect. So I’m pretty sure there’s a fairly good reason for all of this, I just can’t quite remember. Maybe I’ll figure it out. Something to do with helping us think about why we think the things we think about science.
(While I was writing this, Rebekah Higgitt and James Wilsdon were writing something much more important about how history can be used by policy-makers, which answers some of the ‘why’ questions)

Moving beyond whether this is at all important in the larger sense, my research was certainly important for historians of early twentieth century physics (of which there are at least four). Because when you separate an entire discipline into ‘classical’ and ‘modern’, there is the temptation to only look at the ‘modern’ stuff. And then it all gets a bit Whiggish. But classical and modern physics weren’t two separate entities for quite a long time. Our retrospective artificial separation results in an incomplete picture and incorrect characterizations of the people involved. Certain ‘classical’ scientists and institutions get dismissed. We start to see them as obsessed with the ether and continuity, stubbornly clinging to theories that are clearly wrong. We fall into the trap of assuming that people believe ‘wrong’ theories because of stupid philosophical commitments, and that people believe ‘right’ theories purely because they’re correct. Our own biases cause us to only ask certain questions.

Not every historian makes this mistake – in the 70s Paul Forman argued that the development of quantum mechanics in German-speaking countries was directly influenced by the wider intellectual culture of Weimar Germany, which rejected notions of causality and determinism. It was such a good idea, that people now refer to this as the ‘Forman thesis’, which must be pretty cool for Forman. (I met Forman once, in a Chinese restaurant in Washington, but embarrassingly I hadn’t heard of him or his thesis before then. And, more embarrassingly, I fled the restaurant after about two minutes because the humid Washington air was making me miserable. I made excuses about my weak English constitution, went back to my poorly air-conditioned B&B and watched Jersey Shore for 2 hours. It was a high point in my academic career.)

People believe things for all sorts of reasons. And physicists in the early-twentieth century often believed multiple things that, from our current point of view, seem incompatible. It was not unheard of for a physicist to simultaneously praise quantum theory and the aether. So you can’t lump everybody into two categories, particularly if they weren’t even using the categories at the time.

So why did these categories come about? We can find some clues if we look more broadly at other stuff that was going on at the same time. Modernist literature and modern art both emerged towards the end of the nineteenth century. The Church of England had its own modernism, with Anglican Modernists challenging the orthodox view of Christ’s divine status. What these all had in common was a challenging of past authorities and a sense of disconnect with the past. Physics was facing this very problem.

God knows why all these various disciplines all started using the same terms – somebody was definitely copying somebody else. But it’s sort of not surprising that physicists latched on, as these categories are particularly useful for science. Unlike many other disciplines, science is supposed to be progressive, and it tends to make a big deal out of this fact. An artist can quite happily decide to start drawing faces that don’t look like faces, and then get into a philosophical discussion about what art even is, but they sort of get away with it. (Apologies for my description of art – I’m a philistine. Also a lot of modern artists and writers actually did initially struggle with the problem of rejecting old ideas, so what I’ve written isn’t entirely true) But when physicists start devising theories of motion that reject everything that had been believed for the past 200 years, then we have problems. Science had built up a reputation as this great route to knowledge, better than anything else, and physics puts itself at the very centre of this. We’re supposed to trust it. But how can we continue trusting it, when the physicists themselves admit that they’d been wrong all along about the very fundamentals of what they were doing?

We can see the classical / modern divide as a way around this. By designating Newtonian physics as ‘classical’, a word with grand connotations, physicists were able to push him to the side a bit without rejecting him completely. He wasn’t wrong, just different. So they were doing ‘modern’ physics, whilst ignoring ‘classical’ physics, but arguing that they were both physics and they were both good and please don’t lose faith in us, we’re doing our best.

The end.

I was going to put some actual historical evidence in this blog post, but I typed for too long, so never mind. Maybe another time.

P.S. postmodern physics