‘What is light?’ Grandy’s The Speed of Light: Constancy and Cosmos



Light. What is it?














Al-Ghazali, The Niche of Lights: A parallel English-Arabic Text Translated, introduced and annotated by David Buchman, 1998. Islamic Translation Series. Brigham Young University Press. Provo, UT.




  1. Beautiful, Steve. I’ll be looking for this book.

    Yesterday I met with the TAs I supervise for tutorial sections accompanying the third of a sequence of calculus-based introductory physics courses (~ BYU’s Physics 123), and we worked through a tutorial worksheet on simultaneity. It rather elegantly shows that by simply assuming the speed of light to be constant in all reference frames (to which you refer in your footnote), one has to choose between two apparent absurdities: either (1) two events that are simultaneous in one frame are not simultaneous in the other, or (2) something can happen in one frame and not happen in another.

    Einstein’s special theory of relativity selects option (1).

    Of course there is also the choice to dismiss the constancy of the speed of light in all frames, but there is much evidence in support of this notion.

  2. Thank you for telling us about this book — and describing your experience with it so beautifully.

    I had occasion to make the acquaintance of David Grandy a few months ago, and have had a handful of conversations with him, but I did not know about this book–he was too gracious to even mention it.

    Moving it to the top of my Christmas list.

  3. But can God go faster than the speed of light? Can He make light go faster or slower? Would the universe be destroyed?

    One minor correction: it is the speed of light in a vacuum that is a constant cosmic speed limit. Moving through air or water, light slows down. In fact, there is an amazing phenomenon, Cherenkov radiation, where an observer can actually overtake the speed of light. The result is a shock wave (like a sonic boom, except with transverse EM waves instead of pressure waves). In some recent Scientific American, scientists had slowed down light in some medium to several centimeters per second!

  4. Dan – cool reference. “Slowing” of the speed of light in materials is actually due to absorption and reemission. Special relativity is beautiful, though, and the experiments you are talking about are pretty interesting.

    I fondly remember many wonderful debates with David Grandy in my undergrad years when he used to attend a history and philosophy of science club I organized. I usually disagreed with him, but always found him stimulating.

  5. Fascinating, Steve, and beautifully written. Thank you for giving us this glimpse of the book and your experience with it.

  6. Great post, Steve. Light is such a fascinating thing, and they way you were able to describe it, and how it seems the book talks about it is incredible. I am excited to pick it up and read it.

  7. Rob Osborn says:

    Ah, the speed of light and relativity, I love it. I used to stay up at night and draw up various scenerios. My favorite was the one about the “shooter”. Here it is-

    Suppose two men were standing facing each other and they both had very accurate time pieces. The one has a special gun able to shoot bullets at exactly half the velocity of light. The shooter looking through his scope starts his timer the exact second he pulls the trigger and stops it when he sees the guy fall down. The other guy being shot at is looking through his eyepiece starts his timer when he sees the shooter pull the trigger and stops it when he is hit. What do both see? And, how long does it take the event to happen from each persons perspective?

    Even though it takes only a certain amount of time it’s truly all relative to where you are standing. This is where the question of light velocity and time really play on the mind. The shooter records a longer elapsed time than the guy being shot. Th etricky part is- what does the guy being shot at see? His elapsed time is shorter, so does the velocity of the bullet appear faster? By the time the guy being shot starts his timer, the bullet is already half way to him.

  8. Interesting, Rob, though I would be hesitant to call it a relativity problem since both observers are in the same reference frame (i.e., they are not moving relative to each other). The only reason they observe different elapsed times between firing and hitting the target is because the second shooter is not accounting for the delay caused by the finite speed of light.

    Another fun thing to consider is that what the guy being shot at would actually observe is the shooter being suddenly vaporized by the recoil of a gun that shoots bullets at half the speed of light, since conservation of momentum would cause the shooter to be thrown suddenly backward at a speed of about 80,000 miles per hour. Awesome!

  9. The last sentence in the first paragraph should read “…because the second observer is not accounting…”

  10. David Grandy’s work deserves a wide audience among Latter-day Saints. Always self-effacing, he’s also one of the smartest thinkers we have and an excellent writer.

  11. This is just lovely. I adore thinking about Big things like this, an do so so seldom these days. I’m glad I waited until my kids were asleep to check in today. You’re fantastic SteveP.

  12. Thanks. I have to tell someone what I want for a gift in a gift exchange. You’ve solved my problem for me, thank you.

  13. BTW my favorite was the really fast pole vaulter. He has a twenty foot pole. He moves so fast that it is only ten feet long to an observer and he is running towards a ten foot long shed. So the observer sees him fit within the shed.

    Of course from the pole vaulter’s view, the shed has compressed to only five feet, so he just got a twenty foot pole in a five foot shed. ;)


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