## Tuesday, April 17, 2012

### The dog at the head of the class

kw: book reviews, nonfiction, physics, relativity, tutorials

The special and general theories of relativity are hard. It took me years to get somewhat comfortable with some parts of special relativity. General relativity, a theory primarily of gravity, has led to some compelling metaphors such as the rubber-sheet model of space as distorted by mass. But if the math of special relativity is daunting (I find it so), the math of general relativity gets positively pathological. As a consequence, I am always ready to read another treatment of these subjects, for any new insights they can offer.

Professor Chad Orzel's new book How to Teach Relativity to Your Dog is the latest. The author's dog Emma is already well educated, being the foil of his 2009 book How to Teach Physics to Your Dog, which climaxed with quantum mechanics. Emma comes across as being rather better educated than the typical new college freshman, though a bit more enthusiastic, particularly when bacon or bunnies get mentioned.

Throughout the book, Emma asks probing questions as the author tries to explain relativity's concepts in terms a dog can understand. Thus, while Einstein authored thought experiments involving trolleys and lamps and pendulums, Dr. Orzel's examples entail observations of Nero the neighbor cat as he streaks across the yard, or Winthrop the beagle on a day he gets to chase bunnies and Emma doesn't.

One great value of the book is the repeated statement that science has to work the same for all observers. The findings of relativity all flow from this simple principle. I recall being totally flummoxed at first, upon learning that shining a light through the forward port of a fast rocket would not make the light go any faster. In my rocket, I could split part of the beam into an interferometer and measure its velocity as 299,792,458 m/s. The beam out the front window, sent into a similar interferometer by Emma and her master as the rocket approached, would be found to have exactly the same speed! This leads to their clock and my clock running at different rates, to foreshortening of distances as each of us measures the other, and so forth.

All this is explained in early chapters, in a simple enough way that you can get the gist of it, whether you understand the math or not. At its most extreme, near the end of the discussion of general relativity, black holes are discussed. Nero the cat is sent into one by Emma the dog (much to her delight). Each is shining a signaling laser at the other. As Nero falls inward, his laser's light as seen by Emma doesn't change velocity, but its color changes to longer and longer wavelengths. Emma observes Nero seeming to slow down and come to rest at the event horizon.

But from Nero's point of view, he falls inward ever faster, even as Emma's light signal shifts wavelength. He passes the event horizon without noticing it. In fact the only change Nero is going to notice is when tidal effects "spaghettify" him as he approaches the singularity at the black hole's center.

There is no point in going over all that the book covers. It is very readable. The author has somehow captured the personality of a super-smart dog with all of a normal dog's appetites (huge) and enthusiasms (overwhelming). His conversations with Emma are in a great tradition employed by Galileo, Plato and others.

I learned a number of things. For example, I learned a better way to determine relativistic kinetic energy, than the clumsy way I was taught some forty years ago. There is also a brief, but clear explanation why the GPS satellites were set to run 38 microseconds slow, having to do with time dilation caused by both their velocity (slowing them a bit) and their altitude (speeding them up even more).

There is one statement I would modify. In a footnote, the author writes, "The electron is known to be smaller than 10-22 m in radius, one-trillionth the size of an atom. In the Standard Model, it is believed to be a true point, with no measurable size."I would not have used "believed to be", but "mathematically treated as". A minor point, but physics doesn't truck in beliefs.

I find it mildly surprising that Dr. Orzel's earlier book brought out quantum mechanics, before this treatment of relativity. I find quantum mechanics the harder subject. Note to self: get 2009 book, pronto!