- #1

- 6,723

- 423

## Main Question or Discussion Point

My daughter's high school physics class is using Giancoli (6th ed.) as their text. I'd previously looked at the treatment of SR, and it seemed OK, although old-fashioned. But this morning I started flipping through ch. 33, "Astrophysics and Cosmology," and I was pretty shocked. It reads like a catalog of common mistakes and misconceptions. The impression I get is that the author simply doesn't know GR.

He states the equivalence principle, and uses it to show that a beam of light crossing an accelerating elevator appears curved. Then: "If a light beam can follow a curved path, as discussed above, then perhaps we can say that

In a section on black holes, he forgets about curvature and reverts to calling gravity a force. Light emitted from within the event horizon is "pulled back in by gravity."

The discussion of cosmology seems to have been written backwards compared to the logical order, and it contains mistakes. First he makes assertions about the "curvature of the universe." Then he introduces the special-relativistic equation for the Doppler shift of light, and speaks as though it can be applied to cosmologically distant objects, which is wrong. (He admits in a footnote that it's wrong, which raises the question of why he did it that way.) He uses this as evidence for Hubble expansion. For some reason there is a section on the steady-state model, which hasn't been viable for half a century, and some of the technical details are wrong. (He says the steady-state model violates conservation of mass-energy, whereas in fact the stress-energy tensor has zero divergence in the steady-state model.) Then he discusses the CMB.

He states the equivalence principle, and uses it to show that a beam of light crossing an accelerating elevator appears curved. Then: "If a light beam can follow a curved path, as discussed above, then perhaps we can say that

*space itself is curved*and that it is the gravitational mass that causes the curvature." [his italics] After that he briefly mentions that the curvature is really a curvature of spacetime, but after that he abandons spacetime curvature and talks only about curved space. He talks about the "curvature of the universe" when he means the spatial curvature. The idea that's completely absent is that gravity *is* the curvature of spacetime.In a section on black holes, he forgets about curvature and reverts to calling gravity a force. Light emitted from within the event horizon is "pulled back in by gravity."

The discussion of cosmology seems to have been written backwards compared to the logical order, and it contains mistakes. First he makes assertions about the "curvature of the universe." Then he introduces the special-relativistic equation for the Doppler shift of light, and speaks as though it can be applied to cosmologically distant objects, which is wrong. (He admits in a footnote that it's wrong, which raises the question of why he did it that way.) He uses this as evidence for Hubble expansion. For some reason there is a section on the steady-state model, which hasn't been viable for half a century, and some of the technical details are wrong. (He says the steady-state model violates conservation of mass-energy, whereas in fact the stress-energy tensor has zero divergence in the steady-state model.) Then he discusses the CMB.