Written in 1980, there was, of course, no observational evidence for gravitational waves, but since they had been predicted by Einstein's equations in the early 1900s, many calculations had been performed as to what they might look like, and how they might be observed. Shipman discussed gravitational waves in a section of the chapter titled "Frontiers and Fringes." In the chapter's introduction he states
There are a few frontier areas of black-hole studies that are properly called fringes, since they represent speculative ventures far beyond the boundaries of experimentally tested or even testable theory [my italics]. These fringe areas are widely publicized. You see reports that black holes are space warps: You can fall into one and come out somewhere else in this universe or in another universe. Although these ideas could be true, they are, at our present level of sophistication, flights of fancy into the never-never land inside the event horizon. It is very easy to believe that black holes are such strange objects that, if you accept their existence, then anything weird, even space-warp stories, that is said about them is true. Do not fall into this trap. Black-hole research, like most of science, contains some results that are true, some that are probably true, and some that are speculation - published because they are interesting if fanciful ideas and just might be true. I have gathered all these ideas and put them in this latter part of this chapter so that you, the reader, will know what is fact and what is not.Shipman is very careful to distinguish fact from fiction, and to explicitly tell the reader which is which. Most of the popular physics books today, although they are by eminent physicists like Brian Greene, are not so careful, and they wind up creating more hype than is justified.
"Testing general relativity" and "Gravitational waves" are two subjects listed under Frontiers. Hulse and Taylor had discovered their binary pulsar PSR 1913+16 in 1975 and had measured the decrease in the orbital period in 1978 that was consistent with the loss of energy via gravitational wave radiation. However, Shipman goes on
Gravity waves represent a speculative research frontier. Einstein's general theory of relativity predicts that they do exist. However, they are very weak and difficult to detect. Direct searches for gravity waves have not been successful. The only real detection of this radiation has been an indirect one, based on careful observations of the orbit of a binary pulsar. But even if you believe [convinced by the evidence - belief is not science - ed.] that gravity waves have not yet been seen, they are still within the mainstream of scientific research.What I especially like about Shipman's exposition is that after all the technical explanations, which I've omitted, he circles back and is very clear about how the non-expert should think about the different stories that are seen in the media.
Finally, at the end of the chapter, he lists all the ideas discussed in preceding chapters, and groups them according to how much weight should be given to them. It is so useful that I often copy it and give it to my students when we discuss the scientific method and how science progresses. Here it is.
- FACT
- White-dwarf stars exist
- Neutron stars are pulsars and exist
- Evolution of stars though the red giant stage
- Theoretical model of a classical black hole (possibly including rotation)
- Black holes have no hair
- PROBABLE FACT
- Cygnus X-1 is a black hole *
- Low-mass stars -> planetary nebulae -> white dwarfs *
- WORKING MODEL
- Medium-mass stars -> supernovae -> neutron stars *
- Massive stars may become black holes *
- Black holes evaporate (very slowly)
- Gravitational radiation exists **
- Einstein's theory of gravitation (general relativity) **
- CONTROVERSY
- Is Epsilon Aurigae a black hole?
- Are globular-cluster x-ray sources giant black holes or neutron stars?
- Are most x-ray sources in the Milky Way galaxy related to dying stars?
- How do pulsars produce radio emission?
- How massive can a neutron star be?
- SPECULATION
- Wormholes, white holes, and space warps
I've placed stars on some of the statements to indicate that today, most scientists view them as more definite than in 1980, and the number of stars indicates how many categories they should "move up." For example, 'massive stars may become black holes' is now a 'probable fact,' and 'gravitational radiation exists' is now a 'fact'. Of course, the word "fact" in science has a meaning that is not 100%, like a mathematical statement. The statement 1 + 1 = 2 is a true mathematical statement, but since science works inductively rather than deductively, there is always the possibility that another explanation will come along that will change our views. But at the present time, these things are 'facts.'
