allan@kpno.UUCP (05/18/84)
> There is a large list of commonly-accepted (these days) ideas in > physics and especially cosmology of which I am not convinced (after > graduate training in Physics and much reading of the technical > literature). Maybe the discussion could be steered toward an > investigation of the evidence for: > > neutron stars > black holes > renormalization methodology > quarks and other unobservable building blocks > magnetic monopoles > gravitational radiation > gravitons Here are my thoughts on these subjects. Any disagreements are welcomed and will be filed in the appropriate place. 1) neutron stars The evidence that neutron stars exist is very good. The best evidence comes from observations of pulsars, since it is virtually impossible to come up with a mechanism that gives pulses with the constant pulse rate that pulsars have without invoking rotation of the source. (Yes, I know that pulsars speed up and slow down, but this is a very small effect). The only type of star that can rotate as fast as is needed without being broken apart is a neutron star. This is especially true of the 'fast pulsar' discovered last year as even with a neutron star the rotation speed at the equator is about one third of the speed of light. An object larger than a neutron star (e.g. white dwarf) would have a rotation speed greater than the speed of light in this case. If you want to go to a smaller source that leads us to ... 2) black holes Before I go on I should say that black holes do NOT provide good models of pulsars. The evidence for the existence of black holes is not as firm as that for neutron stars. There are two main instances where black holes are invoked in astrophysics. Firstly there is the case of an apparent binary star system where we see only one star. Since this is a binary system we can measure the masses of the two stars, and there are cases where the mass of the unseen star is larger than the upper limit that we believe neutron stars can have. There are two things to say about this. If you do not like black holes then you can point out that calculating the upper mass that a neutron star can have is notoriously difficult and the value has been revised many times in the past and may we be in the future. However, if you believe that at least one of the stars is above the neutron star mass limit then what you have left is a black hole. Even if you do not like the normal description of black holes, the object that is left is so like what we normally think of as a black hole that in practice it makes no difference. (A difference which makes no difference IS no difference). Secondly there is the case of active galaxies, quasars and the like. This is the field in which I work so I know exactly how little we do know about this. The reason that black holes are used as models for active galaxies is that we need to explain the large amounts of energy that quasars emit from very small volumes. Black holes provide the simplest and most efficient way of doing this. There are many more complicated ways of generating the energy, but all of these models are going to end up making a black hole, which will then produce more energy than the fancy model did, so you may as well just bite the bullet as use a black hole model in the first place. However, I will be the first to admit that this is only circumstantial evidence. 3) renormalization methodology Anyone who has worked with this will know that the whole thing is one big kludge. However, it does allow us to make calculations that agree with experiments to one part in a billion (not bad, eh?), and as it the only thing that we know how to do at present then we will continue to use it. Ideally it will someday be replaced by a better understanding of how to calculate things properly. 4) quarks and other unobservable building blocks The evidence that quarks exist is pretty good. When high energy electrons are scattered off protons, the results indicate that the protons looks just like three point particles (i.e. quarks). Also the agreement between experimental data and predictions from the quark model is sufficiently good to make one believe that the quarks really do exist. Of course you can get all metaphysical and ask if a quark can be said to really 'exist' if it can not be seen in isolation, however, current models indicate that at temperatures above about 200MeV, quarks are no longer confined. 5) magnetic monopoles The only good evidence for the existence of a magnetic monopole was the Cabrerra (hope I spelled that right) result. It seems that it has not been possible to repeat the measurement and so there is currently less belief that the measurement is valid. Monopoles are required on general theoretical grounds, but I do not believe that the theories are in such a good state that I would bet too much money one them. 6) gravitational radiation The best evidence that gravitational radiation exists comes from observations of the binary pulsar. This is a pulsar in a binary system so that we can measure the parameters of the orbit very accurately. As the orbit is a close one there should be significant gravitational radiation which will make the orbit decay. Sure enough, that is exactly what is observed. The agreement between the observations and the predictions (of general relativity) are so good that this provides strong evidence for the existence of gravitational radiation, and slightly less strong evidence for the validity of general relativity. Aside : [[ The data on the binary pulsar is so good that it is difficult to plot meaningful error bars. Apparently Joe Taylor got fed up with people asking "where's the errors?" so he put some small but respectable error bars on his plots. When the doubter are told that these error bars represent 1000 sigma, they stop complaining. ]] 7) gravitons The experimental evidence for the existence of gravitons is zero and is likely to remain that way. However, if you believe that all the forces are mediated by the exchange of some particles, which is the current theory, then the particle that does that for gravity is, by definition, the graviton. Peter (theories to go) Allan Kitt Peak National Observatory Tucson, Az
nather@utastro.UUCP (Ed Nather) (05/19/84)
[] >From: allan@kpno.UUCP > >The evidence that neutron stars exist is very good. The best evidence comes >from observations of pulsars, since it is virtually impossible to come up >with a mechanism that gives pulses with the constant pulse rate that pulsars >have without invoking rotation of the source. This is a commonly-held belief; it is also completely wrong. The pulsating white dwarf star G117-B15A, one of many very stable pulsators located in the ZZ Ceti instability strip, has a primary pulsational period now known to be constant with 1 part in ten to the 14th power. Most pulsars are far poorer clocks than this. The pulsation mechanism (hydrogen ionization zone driving) is well understood, and does not involve rotation. (The slight rotation of the object causes modal splitting of the pulsations, by making the star slightly oblate, and is measurable.) What *does* seem to be required for such phenomenal stability involves a lot of mass, at very high density -- but rotation is not the only mechanism that can evoke it. -- Ed Nather ihnp4!{ut-sally,kpno}!utastro!nather Astronomy Dept., U. of Texas, Austin
allan@noao.UUCP (05/22/84)
I stand corrected about rotation being the only way of producing a stable frequency of pulsation. However, the difference between pulsars and the ZZ Ceti stars is that pulsars give strong radio pulses, not optical pulses. While I agree that optical pulses can be produced by non rotational methods, radio pulses are much more difficult. Peter (theories to go) Allan Kitt Peak National Observatory Tucson, Az