A view From the Bridge

Revisiting Feynman on physical law

Posted on behalf of Andrea Taroni

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Physics, along with jurisprudence, is principally known for its laws. And physical laws are amazing: they can predict almost anything, from the effects of gravity to why the Sun shines. Explaining them is surprisingly hard, however. Anybody first encountering them in the classroom, typically as mathematical formulae applied to abstract problems, can attest to that. The result is countless hours spent by teachers, educators and popularisers of science devising ways to make physics (and its laws) ‘more interesting’.

Richard Feynman’s The Character of Physical Law – published in 1965 and now newly reissued by MIT with a foreword by Frank Wilczek – stands out as an early example of a successful attempt towards this end. The book is based on a series of lectures the iconic physicist had delivered the previous year at Cornell University. But it’s a layered work, and clearly shows Feynman also drawing from another set of lectures, delivered at the California Institute of Technology from 1961 and 1963. Those would go on to become his most famous work: The Feynman Lectures in Physics (reviewed here).

However, whereas The Feynman Lectures were an attempt to reinvigorate the pedagogical approach to ‘freshman’ physics, The Character of Physical Law is, in Wilczek’s words, far more than an exposition of facts and ideas. It is also a character study of Feynman himself.

By physical law, Feynman is quick to explain that he means “the rhythm and pattern of phenomena of nature which is not apparent to the eye, but only to the eye of analysis”. In other words, the very phenomena we uncover through painstaking empirical observation, and tend to ultimately write down as mathematical equations. But the topic of the lectures is broader still. They focus on the characteristics common to all the laws: “that is another level, if you will, a higher generality over the laws themselves”.

The big picture

What is really striking about The Character of Physical Law is Feynman’s ease in covering broad areas of physics — for instance, the law of gravitation, the relationship between physics and mathematics, the role of symmetry in physical laws. But crucially, he is equally adept at discussing the history of these topics and their relevance to everyday life, and lucidly articulating the reasons why one might be curious about them. It is this combination of skills that allows him to avoid excessive abstraction and philosophising, a common pitfall when looking at the big picture of things.

For instance, Feynman kicks off by discussing the law of gravitation. In plain words, this describes how a particle is attracted to every other particle through a force directly proportional to the product of their masses, and inversely proportional to their distance. Though acknowledging that it is a discovery of the Enlightenment, he argues that by “describing its history and methods, the character of its discovery, its quality”, he recontextualises it for the present.

In the space of a few pages, the reader learns the way mathematician and astronomer Johannes Kepler established how the planets orbit around the sun. And they are provided with a clear description of the Newtonian mechanics that explain what makes them go around — including, of course, a brief explanation that, eventually, even Newton’s laws are found wanting and Einstein’s relativity takes over. At the next level of generality, Feynman also considers other instances in which inverse-square laws appear in nature — for example, to describe the interaction between electrical charges. The reader is invited to think deeper as each layer of description is peeled away, while at the same time keeping in mind the common threads that bind them together. Yet Feynman isn’t afraid to admit when even the boundaries of his knowledge are reached: “instead of having the ability to tell you what the law of physics is, I have to talk about the things that are in common to the various laws; we do not understand the connection between them”.

This approach certainly demonstrates an unusual depth of physics understanding. It also reveals Feynman’s humanity. Feynman was of course famously charming and charismatic — and, arguably, flawed, perhaps propagating the myth of his stage persona a little too enthusiastically. But ultimately he was, in my view, a man driven by a playful, down-to-earth spirit of curiosity, not the dry and abstract reasoning of a detached academic.

Rules of the game

As Wilczek notes in the foreword, a lot has happened in physics since 1965; yet The Character of Physical Law holds up extremely well today. My favourite chapter is the one on symmetry in physics. Feynman starts off by noting that symmetry appears to fascinate the human mind, if only for aesthetic reasons. But he chooses to emphasise the symmetry within the laws of physics themselves. Certain laws can be symmetric with respect to time and space, for example, but not necessarily under changes of scale. The implications of these symmetries are more obvious in some cases than others. But the key point is that by focusing on these underlying rules of the game, one gains an appreciation for the character of the physical laws they apply to.

To underline that, he masterfully explicates the far-reaching implications of charge-parity violation in the weak nuclear force. In his own words, “it is as if 99.99% of nature is indistinguishable right from left, but that there is one little piece which is completely different”. This ultimately explains the preponderance of right-handed molecules, such as proteins, that play a central role in the biochemistry of life. Feynman’s genius as a communicator lies in his ability to explain this connection in a manner that is accessible, fascinating and accurate in equal part.

Ultimately, I wouldn’t go quite as far as Wilczek by describing The Character of the Physical Law as the single best introduction to modern physics. Somehow, I suspect there is a reason why the more incremental approach espoused in The Feynman Lectures in Physics has gained traction with a wider readership over the years. But for the interested reader looking for more, this book offers enlightenment to those exploring its facets.

Andrea Taroni is chief editor of Nature Physics. He tweets at @TaroniAndrea. 

 

For Nature’s full coverage of science in culture, visit www.nature.com/news/booksandarts.

Comments

  1. Pentcho Valev

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    Pentcho Valev said:

    I am afraid Feynman was not honest in The Character of Physical Law (and elsewhere). See this:

    Richard Feynman: “It is evident, is it not, that if you are in a spaceship going at 100,000 miles a second in some direction, while I am standing still, and I shoot a light beam at 186,000 miles a second through a little hole in your ship, then, as it goes through your ship, since you are going at 100,000 miles per second and the light is going at 186,000, the light is only going to look to you as if it is passing at 86,000 miles a second. But it turns out that if you do this experiment it looks to you as if it is going at 186,000 miles a second past you, and to me as if it is going 186,000 miles a second past me! The facts of nature are not so easy to understand, and the fact of the experiment was so obviously counter to common-sense, that there are some people who still do not believe the result! But time after time experiments indicated that the speed is 186,000 miles a second no matter how fast you are moving. The question now is how that could be. Einstein realized, and Poincaré too, that the only possible way in which a person moving and a person standing still could measure the speed to be the same was that their sense of time and their sense of space are not the same, that the clocks inside the space ship are ticking at a different speed from those on the ground, and so forth.”

    By 1905 not a single experiment had shown that the speed of light is constant, let alone experiments demonstrating the constancy “time after time”. Just the opposite. In 1887 (prior to FitzGerald and Lorentz advancing the ad hoc length contraction hypothesis) the Michelson-Morley experiment unequivocally confirmed the variable speed of light predicted by Newton’s emission theory of light and refuted the constant (independent of the speed of the light source) speed of light predicted by the ether theory and later adopted by Einstein as his 1905 second (“light”) postulate:

    To it, we should add that the null result of the Michelson-Morley experiment was unhelpful and possibly counter-productive in Einstein’s investigations of an emission theory of light, for the null result is predicted by an emission theory.

    Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887. […] The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light corpuscles being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v).

    The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE.

    Banesh Hoffmann, Relativity and Its Roots, p.92: There are various remarks to be made about this second principle. For instance, if it is so obvious, how could it turn out to be part of a revolution – especially when the first principle is also a natural one? Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton’s laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether. If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that the introduction of a ‘luminiferous ether’ will prove to be superfluous.

    Also, Poincaré did not believe that the speed of light is independent of the speed of the observer and accordingly did not try to procrusteanize time and space, as Feynman suggests:

    Olivier Darrigol, The Mystery of the Einstein-Poincaré Connection: It is clear from the context that Poincaré meant here to apply the postulate [of constancy of the speed of light] only in an ether-bound frame, in which case he could indeed state that it had been ‘accepted by everybody.’ In 1900 and in later writings he defined the apparent time of a moving observer in such a way that the velocity of light measured by this observer would be the same as if he were at rest (with respect to the ether). This does not mean, however, that he meant the postulate to apply in any inertial frame. From his point of view, the true velocity of light in a moving frame was not a constant but was given by the Galilean law of addition of velocities.

    Pentcho Valev