# Treatment of GR and cosmology in Giancoli

1. Jan 22, 2016

### bcrowell

Staff Emeritus
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 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.

2. Jan 22, 2016

### Orodruin

Staff Emeritus
I would be surprised if an author of a high-school textbook actually knew GR, just as I would be surprised if I found a modern treatment of SR which does not include relativistic mass. The point is that knowing relativity at the level required to write this properly is not really a prerequisite for writing a high-school textbook in physics. The focus is generally going to be more on more basic physics like Newton's laws. I would not be surprised to learn that some authors chew off more than they can bite with regards to relativity and cosmology.

3. Jan 22, 2016

### bcrowell

Staff Emeritus
It's marketed as a college textbook.

And big commercial texts like this one usually have a long list of advisors and consultants. However, that elaborate process is often carried out very poorly. For example, it's often found that the people listed as consultants on a K-12 school text didn't actually know they were being listed and had not read the book or provided any feedback.

Considering that Giancoli costs a college student $222, I do think there's a responsibility on the part of the publisher to try to get facts right. 4. Jan 22, 2016 ### alw34 The few percent of kids that will take additional courses after HS will hopefully get the right story. Those that do not won't remember what they learned anyway. Look, the poor author has only had six stabs at getting it right; Whadda ya expect for$222?

I reviewed local HS American History texts twice. It was an abysmal experience about 10 years ago; about 6 years ago the text being used was much more accurate. For example, a separate page supposedly devoted to President Reagan, a staunch conservative, started out with FOUR paragraphs of liberal philosophy!!!

Supposedly Texas, a huge market, got sick of the liberal biases and forced publishers to clean up their act. Anyway, each time I forwarded four or five pages of incorrect and misleading accounts to the course development administrator in my school superintendents office. I found by checking my on line school budget breakdown, they got paid around \$130,000 annually at the time, likely 60% to 80% more than average teacher salary. So go get 'em if you are inclined!!

5. Jan 22, 2016

### Orodruin

Staff Emeritus
I have in general found that pulling out misconceptions from students from earlier courses in modern physics and at high school is a large part of my work when I teach SR. Going back to the textbook that was used when I took my first modern physics course at university, I found that apparently atmospheric muons travel about 700 m in their rest frame in order to reach the Earth's surface ...

6. Jan 22, 2016

### bcrowell

Staff Emeritus
What is wrong with that? I must be missing something.

7. Jan 22, 2016

### George Jones

Staff Emeritus
In a muon's rest frame, it doesn't travel (in space).

Last edited: Jan 22, 2016
8. Jan 22, 2016

### Staff: Mentor

My first exposure to relativity was through a small add-on chapter in Serway. It was similarly bad and I think contributed to how long it took me to learn SR.

9. Jan 22, 2016

### bcrowell

Staff Emeritus
Serway has now been inflicted on students unto the ninth edition. It just goes on and on, like one of those families where grandma introduces the grandkids to her fentanyl dealer.

10. Jan 22, 2016

### Staff: Mentor

My first SR text was Taylor & Wheeler, the first edition. I hadn't realized how lucky I was.

(Of course, my first GR text was MTW, which at the time was way over my head--I couldn't follow much of it beyond the initial SR chapters. So I guess it all balances out. )

11. Jan 22, 2016

### Staff: Mentor

Yeah, mine is third edition. Vintage confusion.

Unfortunately, I suspect that a lot of student's first intro to relativity is through a similar "afterthought".

12. Jan 23, 2016

### Matterwave

This part of the sentence bothers me way too much for me to read the rest of the thread...