Professor Maxwell's Duplicitous Demon

by Brian Clegg

  Even so, Maxwell had done a remarkable amount to improve the standing of physics at Cambridge – and raised his own profile with that of the Cavendish Laboratory. Maxwell ensured that his position of Cavendish Professor of Experimental Physics would be passed on (there was some uncertainty as to whether it should be a one-off position initially). It is a chair that has continued to this day, with just nine individuals so far holding the post. These have included several of the ‘big beasts’ of physics, including J.J. Thomson, Ernest Rutherford and William Bragg††† (the younger of the Bragg father-and-son combo who won the Nobel Prize together). At the time of writing it is held by Richard Friend, a specialist in carbon semiconductors.

  Notes

  1 – The details of the Grace of the Senate of the University of Cambridge of 9 February 1871 are reproduced in Lewis Campbell and William Garnett, The Life of James Clerk Maxwell (London: Macmillan, 1882), p. 350.

  2 – Blore’s letter to Maxwell telling him about the Cavendish Professorship is reproduced in Peter Harman (ed.), The Scientific Letters and Papers of James Clerk Maxwell, Vol. 2 (Cambridge: Cambridge University Press, 1995), p. 611.

  3 – Maxwell’s reply to Blore about the Cavendish Professorship is reproduced in Peter Harman (ed.), The Scientific Letters and Papers of James Clerk Maxwell, Vol. 2 (Cambridge: Cambridge University Press, 1995), p. 611.

  4 – Lord Rayleigh’s letter to Maxwell hoping that Maxwell will take the new professorship was written from Cambridge on 14 February 1871 and is quoted in Lewis Campbell and William Garnett, The Life of James Clerk Maxwell (London: Macmillan, 1882), p. 349.

  5 – The initial quote from Maxwell’s inaugural Cambridge lecture is taken from William Davidson Niven (ed.), The Scientific Papers of James Clerk Maxwell, Vol. 2 (Cambridge: Cambridge University Press, 1890), p. 250.

  6 – The later quote from Maxwell’s inaugural Cambridge lecture is taken from Lewis Campbell and William Garnett, The Life of James Clerk Maxwell (London: Macmillan, 1882), p. 356.

  7 – George Stokes’ comment about the difficulty of getting a house in Cambridge is reproduced in Peter Harman (ed.), The Scientific Letters and Papers of James Clerk Maxwell, Vol. 2 (Cambridge: Cambridge University Press, 1995), p. 615.

  8 – Maxwell’s comments on John Hunter in a letter to Peter Tait are recorded in Peter Harman (ed.), The Scientific Letters and Papers of James Clerk Maxwell, Vol. 2 (Cambridge: Cambridge University Press, 1995), p. 836.

  9 – George Bettany’s complaint of the difficulty imposed by the Mathematics Tripos on Natural Science at Cambridge is from George Bettany, ‘Practical Science at Cambridge’, Nature, 11 (1874): 132–3.

  10 – Isaac Todhunter’s 1873 dismissal of the need for students to see experiments is from Isaac Todhunter, The Conflict of Studies (Cambridge: Cambridge University Press, 2014), p. 17.

  11 – The attempt by Corpus Christi College to sue the university because the Cavendish Laboratory blocked its light is described in R. Wills and J.W. Clerk, The Architectural History of the University of Cambridge, Vol. 3 (Cambridge: Cambridge University Press, 1886), p. 183.

  12 – Maxwell’s comment about moving around like a cuckoo without a fixed location to lecture is from William Davidson Niven (ed.), The Scientific Papers of James Clerk Maxwell, Vol. 2 (Cambridge: Cambridge University Press, 1890), p. 760.

  13 – Maxwell’s letter to Katherine written in London and dated 20 March 1871 about the targets of the laboratory is reproduced in Lewis Campbell and William Garnett, The Life of James Clerk Maxwell (London: Macmillan, 1882), p. 381.

  14 – Maxwell’s list of requirements for the Cavendish Laboratory is from a letter to William Thomson written at the Athenaeum Club on 21 March 1871 and reproduced in Peter Harman (ed.), The Scientific Letters and Papers of James Clerk Maxwell, Vol. 2 (Cambridge: Cambridge University Press, 1995), pp. 624–8.

  15 – Trotter’s letter to Maxwell on the benefits of giving researchers their own space is quoted in Isobel Falconer’s chapter on ‘Cambridge and the Building of the Cavendish Laboratory’ in Raymond Flood, Mark McCartney and Andrew Whitaker (eds.), James Clerk Maxwell: Perspectives on his Life and Work (Oxford: Oxford University Press, 2014), p. 84.

  16 – The Nature editorial comparing Cambridge unfavourably with continental universities was in ‘A Voice from Cambridge’, Nature, Vol. 8 (1873): 21.

  17 – Maxwell’s undated poem ‘Lectures to Women on Physical Science I’ is quoted in Lewis Campbell and William Garnett, The Life of James Clerk Maxwell (London: Macmillan, 1882), p. 631.

  18 – Garnett’s observation on Maxwell’s reluctant acceptance of women on a Long Vacation course is quoted in Isobel Falconer’s chapter on ‘Cambridge and the Building of the Cavendish Laboratory’ in Raymond Flood, Mark McCartney and Andrew Whitaker (eds.), James Clerk Maxwell: Perspectives on his Life and Work (Oxford: Oxford University Press, 2014), p. 86.

  19 (footnote) – The Telegraph’s reaction to a woman coming top in the Mathematics Tripos is quoted in Caroline Series, ‘And what became of the women?’, Mathematical Spectrum, Vol. 30 (1997/8), pp. 49–52.

  * The idea that physics would become largely about adding another decimal place was very clearly in the mind of the physicist Phillip von Jolly, when in 1874 he encouraged the student Max Planck (who went on to win the first Nobel Prize in Physics) to study music rather than physics. This was because von Jolly believed that physics was almost complete, and with a couple of small problems cleared up, all that remained was better measurement. In the early twentieth century Planck and Einstein would shatter this idea with the culmination of those small problems: relativity and quantum theory.

  † This was Maxwell in the entertainingly flippant mode he often used in letters to friends. Hunter had researched the absorption of vapours.

  ‡ Maxwell clearly had a cruel streak when it came to demons.

  § And, yes, it still was just men.

  ¶ No relation to William Thomson.

  || Cambridge would eventually bite the bullet on this and move the main physics site to the New Cavendish, located on wide open spaces to the west of Cambridge at the start of the 1970s. Some physics work continued in Maxwell’s old Cavendish building for a number of years, but at the time of writing, part of the Cavendish is scheduled for demolition to provide better access to other buildings. There is a hope that the core of Maxwell’s structure can be retained and opened as a visitor centre.

  ** Ditto.

  †† Maxwell’s sense of humour to the fore. He was not seriously suggesting employing the university’s crew for the boat race as the motive power for experiments.

  ‡‡ It is interesting that just twelve years after the publication of Darwin’s Origin of Species (a book some would even today regard as distinctly demonic), Trotter was comfortable using the concept of natural selection as a generally understood shorthand.

  §§ A very barbed comment, bearing in mind the antipathy that was still felt for France in the UK at the time.

  ¶¶ The Daily Telegraph noted: ‘And now the last trench has been carried by Amazonian assault, and the whole citadel of learning lies open and defenceless before the victorious students of Newnham and Girton. There is no longer any field of learning in which the lady student does not excel.’

  |||| The second was definitely in 1874, but the first was undated.

  *** The name given to the summer vacation at Cambridge. Teaching during the Long Vacation, particularly between second and third years of the Natural Sciences Tripos, still takes place.

  ††† He of the ‘God runs electromagnetics on Monday …’ quote – see page 201.

  Demonic Interlude VII

  In which the demon’s memory is challenged

  JCM absolutely transformed physics at Cambridge – and we can see this as the last step of the huge changes he made to physics and your everyday world as a whole. Before his time, physics was in many ways an amateur discipline and (as far as experimental physics went) one where mathematics had a limited
role. Post-Maxwell, to be a professional physicist was a significant position – people don’t say ‘It’s not rocket science’ for nothing – and maths was crucial to its development.

  Of course, it’s impossible to say for certain how things would be in your world without JCM. Most importantly, of course, you wouldn’t have me. But at the very least, work on electricity and magnetism would have been put back significantly, and JCM’s work would not only form the backbone of the technological developments of the twentieth century but would also be necessary to introduce both relativity and quantum theory, the two pillars of modern physics.

  Without the Maxwell touch, Einstein would have been left foundering and quantum physics would never have been developed, dependent as it was on wholly mathematical models. Despite all this, our man, James Clerk Maxwell, for some reason barely crosses the awareness of the general public. It’s remarkable that 78 per cent of readers of this book had never heard of Maxwell before coming across it.*

  Forgetting is never easy

  So, JCM’s position in history is solidly established. But what about me?

  You may remember that we left my status somewhat battered by Leo Szilard’s suggestion that there would have to be a use of energy, increasing entropy, for me to make a measurement and store the information in my brain. However, as information theory became better developed, it was discovered that it is perfectly possible to store data and make computations without the expenditure of energy. With a bound, it seemed, I was free – I could do the job my creator had designed me for without using energy and pushing up entropy levels.

  Unfortunately, there was a sting in the tail of this route to freedom. The physicist behind the breakthrough, Rolf Landauer, made a second, distinctly counter-intuitive discovery. While it need not take energy to store information or to make calculations, erasing information does result in exactly the amount of energy Szilard had calculated being put into the system, countering the apparent problem of the reduced entropy. Bear in mind, if you can put energy into the system it is perfectly possible to reduce entropy – to produce the low-entropy arrangement of letters in this book, for example, as opposed to a scrambled-up set of letters, the author had to exert energy to make it happen.† Similarly, refrigerators manage to reduce entropy, shifting heat from a warmer to a colder place because energy is pumped into them from their power source.

  So, it seemed that Szilard wasn’t entirely wrong, but he assigned the energy requirement to the wrong part of the process, to acts of measurement and storing data away, rather than forgetting. The actual justification for the erasure of information requiring energy is complex, but it depends on the concept of reversibility. If a process can be run forwards or backwards without distinction it is known as a reversible process and does not increase entropy. But if it is not possible to reverse it without energy consequences, it will increase entropy.

  If we consider the parallel of what is supposed to be going on in my demonic brain as a simple sum such as 2 + 5 = 7, given the sum, the process is reversible. If I know what operation is involved, given any two numbers from it, I can recreate the rest. But if I erase the two values that make up the left-hand side of the sum and am left with only the 7, I can’t get back to the 2 and the 5. It’s not the action of computing the sum that makes it irreversible and hence increases entropy, it’s the process of forgetting what the sum actually was.

  What has all this information theory stuff got to do with me and the molecules? Followers of Landauer argued that however big my memory was, I would eventually run out of storage as a result of having to deal with so many billions of gas molecules, and so would have to erase what was stored in order to continue with the process – as a result, I would end up countering the benefit I had produced.

  In 2017 researchers published a paper entitled ‘Observing a quantum Maxwell demon at work’, which they claimed gave them insight into my mind. Their ‘demon’ was a decidedly inferior object in the form of a superconducting cavity which held microwaves. It interacted with a small superconducting circuit which could give off or absorb a photon of light. The demon controlled the system, ensuring light could only be drained from the system, not absorbed, transferring the energy in one direction.

  The team allegedly probed the demon’s memory using something called quantum tomography that allowed them to use multiple runs of the system to build up a picture of what was happening in the memory – and they apparently showed that, in order to work, the demon had to keep information about the state of the system, seeming to prove the assertion that I could only bend the second law if I didn’t forget things.

  For many, the whole business of the erasure of memory was the end of my story. However, it’s entirely possible that they really hadn’t thought things through particularly well, as we shall discover. But we should first return to JCM at Cambridge, as he continues in his new role as Cavendish Professor.

  Notes

  1 – The paper apparently probing a quantum demon’s mind is Nathanaël Cottet, Sébastien Jezouin, Landry Bretheau, Philippe Campagne-Ibarcq, Quentin Ficheux, Janet Anders, Alexia Auffèves, Rémi Azouit, Pierre Rouchon and Benjamin Huard, ‘Observing a quantum Maxwell demon at work’, Proceedings of the National Academy of Sciences, 114(29) (2017), pp. 7561–64.

  * I can say this because I am a demon and so can make up statistics. In reality, I have no idea how many readers had never heard of him, but Maxwell has always scored badly for name recognition compared with the likes of Newton, Faraday and Einstein, or even Schrödinger and Heisenberg (though most younger people probably think the latter is just a character in Breaking Bad).

  † Apart from these demonic interludes, which he allowed me to put together, so he could sit back and do nothing.

  Chapter 9

  The last work

  Maxwell had a belief in the openness of science, and as a result he probably did not provide enough direction and guidance to build effective staffing for the Cavendish during his time, a failing that would later have to be corrected. His approach was a result both of limitations on his time and a philosophy that was strongly geared to giving researchers their heads. He was quoted by Manchester-based physicist Arthur Schuster in Schuster’s contribution to a 1910 history of the Cavendish Laboratory as saying: ‘I never try to dissuade a man from trying an experiment. If he does not find what he wants, he may find out something else.’

  Books and the power of light

  Instead, a lot of Maxwell’s time during his Cambridge years was dedicated to his writing. It was while at Cambridge that he published both his Theory of Heat and Treatise on Electricity and Magnetism, each of which attracted a far wider readership than we would now expect for a technical science book. The Theory was reviewed so widely that even The Ironmonger felt the need to comment, saying that ‘the language throughout is simple and the conclusions striking’. To be fair, Maxwell’s subtitle (which now seems somewhat condescending) was ‘Adapted for the use of artisans and students in public and science schools’. The Treatise was arguably Maxwell’s written masterpiece and was still in use as a textbook well into the twentieth century.

  For the first year or so, the move to Cambridge and getting work started on the Cavendish Laboratory got in the way of Maxwell finishing his 1,000-page masterpiece on electromagnetism – but the Treatise was finally brought out in 1873, at the same time as parts of the new building were starting to be usable.

  As always, Maxwell was not happy to leave his work as it was, but insisted on pulling at the threads of new ideas, looking for gaps to fill and inaccuracies to iron out. As he did so, he realised a strange implication of his predictions on the nature of electromagnetic waves. Not only did these waves correspond so clearly to light, they appeared to be capable of something that no one suspected light could do – electromagnetic waves should be able to apply pressure to matter. If his theory was correct, an insubstantial beam of light should be able to push a solid object.

  This seems a crazy concept. Alm
ost all of our experience of light suggests that its effects on physical objects would not be able to move them – yet Maxwell calculated that there should be a small force generated by the interaction between electromagnetic waves and the matter they illuminated. There was one physical phenomenon that did suggest such an effect could be occurring. As we have seen (page 173), Maxwell had already given some consideration to the way that a comet’s tail always points away from the Sun. The idea of pressure from light was a possible explanation for this. Maxwell gave this hypothesis a theoretical backing by showing that the energy of absorbed light should result in momentum being added to the body absorbing it.

  The reason we don’t see this happening as a rule, Maxwell suggested, was because the effect was so small. He calculated that the Sun, the most powerful light source in our vicinity, would only produce the equivalent pressure of 7 grams (0.015 pounds) across a whole hectare (2.47 acres) of area. Maxwell would never see an experimental demonstration of the effect that he had predicted, but 25 years after his theory was published, the Russian physicist Pyotr Lebedev demonstrated this ‘radiation pressure’ for the first time.

  The concept of radiation pressure is not just a partial explanation for comets’ tails (in practice the effect is primarily due to the ‘solar wind’, a flow of particles from the Sun). It has also been suggested as a possible way of powering spaceships by light, using immense sails to catch the light from the Sun or from a battery of lasers. But most important of all, it is an essential phenomenon to be able to understand how stars operate. Without it, the Sun would not function. The matter in stars is subject to immense inward pressure from gravity – it is the radiation pressure of the vast numbers of photons of light produced inside the star that is the first level of defence against gravity’s attempts to make the whole thing collapse.