Time Capsule

While looking through old physics books for alternate takes on my quals problems, I found a copy of Sir James Jeans’ Electricity and Magnetism (5th edition, 1925). It’s a fascinating time capsule of early views on relativity and what we know call the “old quantum theory,” that is, the attempt to understand atomic and molecular phenomena by adding some constraints to fundamentally classical physics. Jeans builds up Maxwellian electromagnetism starting from the assumption of the aether. Then, in chapter 20, which was added in the fourth edition (1919), he goes into special relativity, beginning with the Michelson–Morley experiment. Only after discussing many examples in detail does he, near the end of the chapter, say

If, then, we continue to believe in the existence of an ether we are compelled to believe not only that all electromagnetic phenomena are in a conspiracy to conceal from us the speed of our motion through the ether, but also that gravitational phenomena, which so far as is known have nothing to do with the ether, are parties to the same conspiracy. The simpler view seems to be that there is no ether. If we accept this view, there is no conspiracy of concealment for the simple reason that there is no longer anything to conceal.

This passage in particular caught my eye:

If we still wish to retain the hypothesis of an ether through which light and electromagnetic phenomena are propagated, we must adjust the properties of this ether to agree with experiment. Now we have seen (§688) that, no matter how an observer and a source of light move, the wave-surface formed by the light emitted at any instant will be a sphere having the observer as its centre. If the observed constant velocity of light is simply the constant velocity of propagation through an ethereal medium, it would seem to follow that each observer must carry a complete ether about with him. This at least robs the ether of the greater part of its reality. We cannot quite go so far as to assert that the ether is reduced to a subjective imagination, as a simple analogy will shew. A number of travellers may all see what they would describe in ordinary language as being the same rainbow. The angle of the rainbow would be the same for each traveller, and no amount of travelling towards the rainbow would cause it to subtend a greater angle. If the travellers compared observations they would have to conclude that each traveller carried his own rainbow about with him. This would not, however, prove the rainbow to be merely a subjective illusion; when the rainbow disappeared for one traveller it would disappear for all. Considerations such as we have mentioned do not prove in strictness that light cannot be propagated through an ether; what they prove is that if an ether exists, it must be something very different from the absolutely objective ether imagined by Maxwell and Faraday.

Rainbows and quantum states, droplets of mist as quantum systems….

The last chapter in Jeans’ book is “The Electrical Structure of Matter.” It’s a textbook presentation of the old quantum theory, going as far as Sommerfeld’s elliptical orbits. For Jeans, the move from classical to quantum physics involves discretizing the possible values of first integrals of dynamical equations, and then allowing stochastic jumps from one discrete level to another.

It is usual to think of these jumps as occurring absolutely spontaneously, although this conception is probably only a cover for our ignorance of some underlying mechanism.

The preface to this edition is dated March 1925. In May of that year, Heisenberg would set about his observable-quantity-driven attempt to understand atomic physics, arriving the next month on the idea of noncommuting observables. His paper on matrix mechanics would be received at Zeitschrift für Physik on 29 July; the next month, Max Born would postulate the canonical commutation relation, which by September Weyl would recast in finite-dimensional form. So, this book is about as new as the old quantum theory could get!

Written as an e-mail back in May, and called back to my attention while updating an old post of mine.