harrylin said:
So here's my thought process: Schwartzschild's solution is the one that I heard about in the literature, and it happens to be the one that I happened to stumble on in the first papers that I read about this topic, dating from 2007 and 1939.
A clarification about terminology: there are several concepts/objects that can be referred to as "Schwarzschild's solution", or "the Schwarzschild metric":
(1) The full, maximally extended vacuum, spherically symmetric solution to the Einstein Field Equation, considered as a geometric object independent of coordinates.
(2) Some portion of #1, such as the exterior region (region I on the Kruskal chart), or that plus the future black hole region (region II on the Kruskal chart), again considered as a geometric object or objects independent of coordinates.
(3) The Schwarzschild coordinate chart--strictly speaking, the *exterior* Schwarzschild coordinate chart--that covers region I only, considered as a particular way of describing the geometric object which is region I of #1 or #2 above.
(4) And for good measure, one can also use a portion of the Schwarzschild exterior chart to cover the vacuum spacetime outside of a massive body like the Earth or the Sun.
My reason for making these distinctions will be evident in a moment.
harrylin said:
It is obviously a valid reference system according to GR, and it is "privileged" in the same sense as inertial frames and centre of mass systems are "privileged": it allows for the most simple mathematics, so simple that one doesn't need to be an expert to understand it. Thus it is a natural choice in a public discussion about predictions of GR. And the way I understand the first paper that I read about this, it's probably all I will ever need to understand this topic.
Here you are talking about #3 above; but you have to bear in mind a key point about GR: all of the actual physics in the theory is independent of which coordinate chart you decide to express it in; i.e., all of the physics must be capable of being expressed in terms of invariants, things that don't change when you change coordinate charts. You can use a particular chart as a starting point, so to speak, to get you to the invariants; but if you're not talking about invariants, at the end of the day, you're not talking about the actual physics.
For example, when you say that Schwarzschild coordinate time goes to infinity at the horizon, you are not talking about an invariant; there is no invariant physical quantity that goes to infinity at the horizon. So this is not a statement about any actual physics; it's only a statement about a particular coordinate chart, with no real physical content. In order to make this a statement about the actual physics, you would have to be able to translate it into a statement about invariants, and you can't; all invariant quantities are finite at the horizon.
Also, the converse of the statement I made above is *not* true: there is no requirement that any physical invariant must be expressible in *every* coordinate chart. It is perfectly possible to have physical quantities (such as, for example, the proper time on an infalling observer's worldline at an event inside the horizon) that can't be expressed in some coordinate charts, because those charts don't cover that portion of spacetime.
In view of the above, I would have to disagree that understanding things in terms of the Schwarzschild exterior coordinate chart is "all you will ever need to understand about this topic". By limiting yourself to understanding things in terms of that chart, you are limiting yourself to understanding things outside the horizon only. You can't understand what happens at or inside the horizon if the only tool you have is the Schwarzschild exterior chart, because the relevant physical quantities simply can't be expressed in that chart.
Finally, a few words about the way, if any, in which the Schwarzschild exterior chart is "privileged". This chart is indeed a "valid reference system" in that region, and it has the attractive property, as I said before, of having surfaces of simultaneity that match up exactly with the surfaces of simultaneity of observers who are static--i.e., who "hover" at a constant radius above the horizon. This allows you to simplify your view of the physics, so that the connection with your intuitive understanding of Newtonian gravity is evident (for example, concepts like "potential energy" can be defined). So the math in terms of this chart does "look simple"; but the price you pay for that is, as I just said, only being able to express physical quantities in the region outside the horizon.
It's tempting to think that, since things look so nice and intuitive outside the horizon when expressed in the Schwarzschild exterior chart, it must be sufficient to express *all* of the physics everywhere in the spacetime, and therefore the region outside the horizon, since it's the region where the Schwarzschild exterior chart works, must *be* the entire spacetime. But it isn't; when you actually work through the solution of the Einstein Field Equation, in either case #1 above (mathematically simple, because the entire spacetime is vacuum, but physically unreasonable) or case #2 above (more complex because there is collapsing matter present in a portion of the spacetime, but physically more reasonable, though still highly idealized), you find that any solution that only includes the region outside the horizon is incomplete; the EFE itself tells you that that region cannot be the entire spacetime. That's why you can't get a complete understanding by just using the Schwarzschild exterior chart.
harrylin said:
This is an essential clarification for me; it is exactly the kind of disagreement that I tried to illustrate with Earth maps. It doesn't solve the issue, but at least we agree that maps are used that disagree about events - which is a thing that was for me unheard of.
The only sense in which the maps "disagree about events" is that one map (SC coordinates) can't assign coordinates to some events (those on or inside the horizon), while another map (e.g., Painleve coordinates) can. But this is only a "disagreement" in the same sense as a Mercator projection "disagrees" with, say, a polar projection of the Earth's surface; the former can't assign coordinates to the North Pole, while the latter can. Yet both charts can assign coordinates to, say, Big Ben in London. So someone in London could choose either chart to map his surroundings, but one choice would allow him to map the North Pole on the same chart, while the other wouldn't.
These all seem like innocuous statements about coordinate charts on geometric manifolds to me. Apparently they seem like huge issues to you; I'm not sure why. Why is it such a big deal that some charts can cover points or regions that others can't?