Redshift

This book has some good points and was clearly a labour of love for the author. Unfortunately, I am not able to recommend it to readers of Physics Education. The author sets out to discuss redshift, with particular reference to its importance in astronomy and cosmology. The main text is fairly descriptive, with `mathematical notes' giving a more quantitative treatment. There are separate chapters dealing with the nature of light, the Doppler effect, gravitational redshift, cosmological redshift and unconventional interpretations of redshift. Quite rightly, the author is very firm about the fact that cosmological redshift is not a Doppler shift. So what are the problems? First, and least important, is the production and presentation of the book. The text is set in a large sans-serif typeface which I found uninviting to read, and the diagrams are highly stylized, often with broad black lines and sometimes with sharp corners where there should be smooth curves, e.g. in graphs of redshift as a function of velocity. The book would benefit from the attentions of a professional editor - there are numerous minor, but irritating, misspellings, inconsistent symbols and proof-reading errors (for example, `Niel' Bohr and `Kirchoff' for Niels Bohr and Kirchhoff; a θ mysteriously becomes a J in mid-derivation; some figure labels are incomplete or misplaced; a vital `exp' is missing from a formula for a Gaussian line profile; megaparsec is abbreviated throughout as mpc (milliparsec) rather than Mpc). Far more serious, though, are the problems with the science. Despite the author's laudable aim of presenting the `laws of light' and explaining the various distinct ways in which redshifts might arise, the treatment given here has many shortcomings, exemplified by one particularly serious error that occurs in the first chapter. Following some discussion of the origins of special relativity, the author provides an example using regular pulses of light being sent from a moving to a stationary car in which he shows that the time interval between successive pulses being received at the stationary car is longer than the interval between successive pulses being sent from the receding car. However, this has nothing to do with relativity. It is merely a consequence of light travelling at a finite speed and the fact that the observers have measured the time intervals between two different pairs of events. The author seems to have completely missed the crucial relativistic effect, namely that two observers in relative motion will disagree about the time interval between the same two events (e.g. the sending of two light pulses) and that the disagreement depends only on the relative speed and not on whether the two are approaching or receding. The author extends his initial derivation to arrive at what he quite correctly describes as the non-relativistic Doppler effect, but then goes on to reiterate that it illustrates special relativity. Similiar confusions are to be found elsewhere in the book. By no means everything is incorrect, but clarity and physical insight are sadly lacking. At the beginning, I did refer to `good points', and in fairness to the author I should mention some of them. In the first chapter, there are some good descriptions of early attempts to measure the speed of light (I liked Galileo's, which involved two people some miles apart using buckets to cover and uncover candles). There is a good account of the discovery and interpretation of `nebulae' which led to Hubble's model of the universe and the birth of modern cosmology. The transverse Doppler shift is included. There are some nice diagrams that illustrate the distinction between cosmological and Doppler shifts. The final chapter (on 'unconventional interpretations') makes interesting reading, though it does mix the respectable with the unorthodox. The overall plan of the book is well thought out, and (despite needing an editor) it is written in a readable style. However, these redeeming features do not alter the fact that anyone using this book to study `the laws of light' is likely to end up as confused as the author seems to be.