Nature’s nifty tricks

Herman BondiA few weeks ago, and thanks to this post over at A Neighborghood of Infinity, i discovered a little jewel: Herman Bondi‘s book Relativity and Common Sense, an insightful and original introduction to special relativity for the layman. The above mentioned post gives an overview of Bondi’s approach, based on what is known as k-calculus, arguably simple enough to be taught at highschools. Bondi makes the case for an understanding of SR as an evolution of Newtonian ideas, rather than as the revolution we all thought it was. Whether successful or not, this attempt leads Bondi to start his book with a delicious review of classical physics, illustrating the concepts and principles with very well chosen common life phenomena and their corresponding explanations. You know, trying to come to grips with quantum mechanics interpretations or finding a unified theory of everything is all very well, but it’s the marvel of being able to explain those other nature’s nifty tricks what draw me to physics in the first place. These little and easy to understand principles convey in a very real and fun sense the magic of ours world’s understandability. Bondi touches a lot of these magic things in the first fifty pages of his book, and i thought some of you may amuse yourselves finding (or just remembering) the explanations to these down-to-earth phenomena:

  • Conservation of momentum. Your baby is sleeping in her pram. Would you put the brakes on or off to ensure she will still be there when you come back and find that she awakened in the meantime?
  • Spinning cats. Can you think of a way of exploiting angular momentum conservation to explain how a cat manages to land always on all fours? (For extra score, how’s this problem related to Yang-Mills theory?)
  • Coriolis force. Can you explain it in simple terms (involving, say, a disk and a little ball)? Does it have anything to do with kitchen sinks? And what about weather? How does it explain cyclones and anticyclones?
  • Wave phenomena: the sonic boom. Can you explain how the explosion associated with a plane travelling at ultrasonic velocities is produced?
  • More on waves: Doppler effect. You surely know about the proverbial band on a train wagon, but what about putting the band on the station and yourself in the train? Will be the frequency shift identical? Why or why not?

These look like simple, even basic, questions, but their solution does not lack subtle points. I’m with Bondi in that one should better understand everyday physics before jumping to the unification of forces or even, more modestly, special relativity. At least in my case, and with the benefit of hindsight, i regret having jumped too early into mathematical physics and abstract stuff, only later learning about funny things like those above: i’m sure i’d be a better physicist had i spent more time in the wonderful world of understandable experiments… but, oh well, you already know the line: you’re never too old… ;-)

Turning back to Bondi’s book, never mind some of the unfavourable comments at Amazon. Even if you don’t buy his claims about SR being common sense (i don’t), the book is just excellent.


3 Responses to “Nature’s nifty tricks”

  1. sigfpe Says:

    Neat book innit?

    I do have a tiny bit of sympathy with the negative reviews at amazon. Even with knowledge of what Bondi was setting out to argue it was sometimes tricky to follow his long verbal descriptions. In fact, I basically reread paragraphs until something clicked and I recognised what point he was trying to make, at which point I could work backwards and figure out what he was actually saying. But someone new to relativity would have to reconstruct his scenarios reading forward, and that’s probably a lot trickier.

    Nonetheless, it;s still a great book.

  2. jao Says:

    Yeah, definitely neat. I agree in that Bondi’s reasoning is not always crystal-clear, precisely because of its originality; but this only adds to the fun once you get it! :) On the other hand, i think that trickiness only shows that the author’s claims about SR being ‘common sense’ are a bit of an overstatement. In fact, my only quibble with Bondi’s exposition is the easiness with which he dimisses as obviously absurd some pre-relativistic concepts like the ether, or absolute time, and as obviously obvious relativistic ones… sounds a bit whiggish to me. But these are minor quibbles.

    (BTW, great blog that of yours: keep up the good work!)

  3. Luis Sanchez Says:

    One of the first books (the first one was the excellent one by James Hartle) I used when learning relativity was Ray D’Inverno’s Introducing Einstein’s Relativity that used K-calculus, to be honest I didn’t really liked the approach at that time but when after learning SR (with Hartle and the classic Spacetime Physics by Taylor and Wheeler) I ended up liking Bondi’s methods. By the way, does someone knows if Bondi was the one who devised the sticky bead argument?

    80% of the faculty here seems to think that silly argument that Coriolis force causes the direction in which water flushes, your post actually reminded me of many discussions I had with them.

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