Most recent answer: 01/17/2015
- John (age 56)
Burnsville NC USA
Those are excellent questions. I'll deal with them on at a time, in random order.
1. Why don't stars burn up all at once? This one is straightforward physics. Although stars are powered by nuclear fusion, you can see the same principle at work in fission. What happens if a more-than-critical mass of plutonium comes together, but not forced together by an explosion? A very rapid chain reaction starts, but then it shuts down because the resulting heat makes the metal expand enough for the neutrons to escape more easily. (Two of my fathers friends died in accidents of this sort. Nothing was blown up but they got huge radiation doses.) The reason it's hard to get controlled fusion to work here is that it's hard to keep the fusion fuel together. The energy released by the fusion tends to blow the fuel apart, stopping the reaction. This negative feedback leads to a steady-state rate in stars, where gravity keeps the fuel from completely blowing apart.
2. How can we see neutrinos or photons from just after the Big Bang? If neutrinos, which almost always just pass through things, were whizzing around everywhere, they still would be. Where would they have gone? As space has expanded, however, the density (number per volume) of those neutrinos has gone down far below what it sued to be. After most of the hydrogen formed uncharged atoms, things also became nearly transparent for most photons. (This happened far later than when things became transparent for neutrons.) So the photons that were around then are mostly still whizzing around, by the same argument. Their density is down, and their wavelengths have been stretched out, but they're still here as the cosmic microwave background. I'm not quite sure why you see a problem. Perhaps you're unconsciously picturing a finite universe with edges?
3. Space is definitely not the dark matter itself. The evidence for dark matter is primarily the gravitational effects of its clumping up around galaxies. Thus it doesn't seem to be any more intrinsic to space than are more familiar types of matter. As for dark energy, which does seem to be everywhere, it is conceivable that there is just plain a fixed cosmological constant intrinsic to any spacetime. I don't think many people believe that, in part because during the apparent early strong inflationary period the "constant" was enormously larger. That suggests more that the constant is some dynamic feature of the physical state of the vacuum. That leads to your question about "ether".
4. Is space like an ether? Once general Relativity replaced Special Relativity, people noticed that "empty" space was playing a more active role. Now, with a cosmological constant (dark energy of some sort?), a pervasive Higgs field, etc., empty space seems even more dynamic. Sort of ether-like! The words don't matter much- the key thing is that the symmetry of this background is different from the symmetry of an ordinary fluid, the sort of thing that an ether picture typically calls to mind. Still, it's sort of ironic that the philosophical belief that empty space should play no role led via the relativity route to a picture in which it's more active than in the older pictures.
5. Why use pictures of gravity on 2-D spaces? The problem here is that our minds are deeply tuned to picturing only 3 spatial dimensions, and to implicitly assuming that the geometry is Euclid's. We can easily, however, picture curved 2-D subspaces of that 3-D space, e.g. the surface of a sphere. Those provide nice visual examples of non-Euclidean geometry. Then we can take the math intuition developed for those 2-D spaces to help think about 3-D non-Euclidean spaces or 3+1-D spacetime. One aspect of the pictures often used for gravity, however, is truly "dumbing down". The pictures often show a "downward" bulge in some sheet, as if up and down existed as something other than internal features of the sheet. That encourages people to think of something rolling "down" into the bulge. It would be better to picture the sheet sideways, so that it's non-Euclidean geometry wouldn't get all mixed up with some external gravity from an irrelevant Newtonian picture. Just trying to follow the geodesics (closest thing to straight lines) on the curved space already shows some of the interesting non-Euclidean effects of gravity, including lines that intersect at two points.
posted without vetting until Lee gets back
(published on 01/17/2015)