PastCast: The Quantum Theory

Kerri Smith

This is the Nature PastCast, each month raiding Nature’s archive and looking at key moments in science. This show rewinds to 1920 and tracks the nascent field of quantum physics.

Music: Le Tombeau de Couperin

Voice of Nature: John Howe

Nature. Thursday, December 16^th, 1920. Contents. Instruments for the navigation of aircraft, The Study of Weeds, Tragic Death Feint of a Snake, The Quantum Theory.

Music: Le Tombeau de Couperin

Philip Ball

The early 1920s is an interesting time because it’s a time when the basic idea of the quantum hypothesis of energy being divided into these little packets, into these quanta, that had begun to be accepted, but it hadn’t yet become apparent where that was going to lead.

Music: Le Tombeau de Couperin

David Kaiser

It was not a single moment of eureka and now we have found it. It was actually a decade plus of really grappling with this very unexpected notion that there could be a basic fundamental ‘packagedness’ or discreteness to the way the world works.

Music: Le Tombeau de Couperin

Voice of Nature: John Howe

Page 508. The Quantum Theory. Professor Max Planck was awarded the Nobel Prize for physics this year, and his address on the occasion of receiving it makes extraordinarily interesting reading. He describes in some detail the discovery of the quantum.

David Kaiser

My name is David Kaiser. I’m an historian of science and a physicist and I teach at the Massachusetts Institute of Technology. It’s a very brief article and it really is just a kind of summary of the lecture that Max Planck has recently delivered on the occasion of Planck winning the Nobel Prize. So, Planck won the prize for work that he had published largely in the year 1900. It was a very famous paper he’d read out to a scientific society I think in December in 1900 and it was work that introduced piecemeal, introduced in stages ideas that would coalesce into what became quantum mechanics.

Philip Ball

I’m Philip Ball and I’m a science writer. Planck proposed this idea of energy quantisation to try to solve a problem called the black body radiation problem which was basically a problem about how a warm body radiates energy, radiates electromagnetic radiation, and Planck had found that the only way he could find to develop a mathematical equation that seemed to fit the experimental data for how that happened was one that supposed that all the atoms that were vibrating and releasing this radiation did so in a way that was quantised – they only could take certain energies and not others.

Music: Le Tombeau de Couperin

Philip Ball

Planck was a fundamentally conservative character, both in terms of his social outlook and his scientific outlook, and he was quite concerned to try to match up what he proposed for black body radiation with the classical physics that everyone knew until that time, and according to classical physics, things were continuous and energy was continuous, and he was always resistant or at best conservative about introducing this idea, this quantum hypothesis, into our sort of physical picture of what reality is like. He literally said that this idea of quantisation of energy should be used as conservatively as possible, and I think that sums Plank up in a nutshell. Everything for him had to be done as conservatively as possible.

Music: Le Tombeau de Couperin

David Kaiser

Planck himself has often been called a kind of reluctant revolutionary. So, with hindsight, Planck himself came to see what he’d done was really distinctive but that was an awakening that took either months or arguably a decade or more to get to something like the recognition that we would now take for granted about what was relevant about his work.

Music: Le Tombeau de Couperin

Voice of Nature: John Howe

Page 508. Professor Max Planck was awarded the Nobel prize for physics this year. He describes in some detail the discovery of the quantum, and to anyone engaged in research the description will be very encouraging, for it shows through what darkness the mind of a great discoverer must grope, and what false tracks he will follow, before he sees the light of the truth.

David Kaiser

The author’s done a wonderful job sort of reporting on this German language lecture that Planck had just delivered on the occasion of the Nobel Prize, and the author of the English language report says that Planck’s lecture shows…

Voice of Nature: John Howe

…through what darkness the mind of a great discoverer must grope…

David Kaiser

That’s very dramatic, right?

Voice of Nature: John Howe

…and what false tracks he will follow, before he sees the light of the truth.

David Kaiser

May we all see the light of the truth someday, right? It’s all about again, the sort of not bumbling but the piecemeal, the false starts, the backward motions, the confusions that turn out to be dead ends, and that that is part of what science is and I think to capture this kind of groping in the darkness, the language that the author has used, I thought was true to the time.

Music: Le Tombeau de Couperin

Philip Ball

It was Einstein five years later in 1905 who really came forward and said we have to take this idea seriously, that energy really does come in these packets, and not just the energy of atoms vibrating about in a material but Einstein said perhaps light itself, perhaps the electromagnetic radiation itself that these black bodies are releasing is also quantised, also comes in little packets, and this was really a revolutionary idea because until that time, everyone had believed that electromagnetic radiation, that light and other forms of radiation, radio waves and so forth, that they are waves, so they’re completely continuous things, but Einstein suggested that actually they may have a kind of particle character to them.

Music: Spirit of Play

Philip Ball

To Planck, being conservative, was very reluctant to accept the ideas that Einstein was putting forward, sharpened by the fact that it was Planck’s own ideas that Einstein was toying with, and Einstein got quite impatient. But gradually, the two of them became reconciled and became very close friends, so it was really Einstein rather than Planck who pressed forward with a physical interpretation of what Planck had said mathematically, and that interpretation was what became quantum theory.

Music

Voice of Nature: John Howe

The success of the quantum theory has been, and continues to be, so enormous that it often appears as if writers had forgotten that the whole present system of physics is based on a perfectly definite set of mutually contradictory axioms.

Philip Ball

I think what the writer in this piece is referring to as being mutually contradictory is the principles of classical physics and quantum physics. It seems clear that they’re in some sense not compatible, that in the everyday world we’re using classical physics but we know that underpinning those principles must be quantum rules for how things behave on very small scales, and it certainly wasn’t clear at that stage and it’s still only becoming clear today how our classical world emerges from the underlying quantum world. But in some ways, things have become even more contradictory and more difficult since that comment was made because it’s now become much more clear that quantum physics seems to be incompatible with Einstein’s theory of special and general relativity because that doesn’t start from a basically quantum picture. It starts form a kind of classical picture of what physics is and we have no idea of how to marry those two theories together. That’s one of the fundamental challenges that physicists are now faced with.

Voice of Nature: John Howe

Unfortunately, Planck does not seem nearer the solution than the rest of us. We may expect to wait a long time for the Maxwell of the subject to appear and reconcile the seemingly irreconcilable.

Philip Ball

In a way, I think you could say that the contradictions have multiplied and there are some people who feel that it’s for this reason that quantum physics still hasn’t got to the core of the problem, that perhaps there’s still something missing from at least the way we have formulated quantum physics that prevents us from having some sort of physical intuitive picture and that prevents us from being able to get around these contradictions that seem to arise.

Music: Le Tombeau de Couperin

Voice of Nature: John Howe

The remainder of the address is occupied with the later history of the quantum, and it is scarcely too much to say that this is simply the history of modern physics.

David Kaiser

It’s fair enough. I think that if we have append to that relativity then we’ve got nearly all of it. I think that’s right. Certainly, from the view from 1920, I think that was a very fair assessment.

Philip Ball

It’s a very striking claim to have made at this point because it’s absolutely true, and it shows that whoever is writing this piece is very prescient because to recognise by the 1920s that quantum physics was going to be so central to all of physics, really, during the 20^th century, that’s quite striking.

David Kaiser

And I think it captures, it’s the future of modern physics and it’s an unfinished future, again, as he ends, saying we still need our grand synthesiser to put these puzzle pieces together. But by 1920, by 1915 even, I think it was clear to most experts that this was not something that was going to go away, this was not an accidental thing, it’s not reducible to the great axioms that had governed physics for 100-300 years. It’s something new and it’s here to stay and it’s still a work in progress.

Music: Le Tombeau de Couperin

Kerri Smith

You’ve been listening to the Nature PastCast produced by me Kerri Smith, with contributions from Philip Ball and David Kaiser. The music was by Maurice Ravel and Spirit of Play. Next time, the powerful invisible force of the X-ray.