- #71
magneticnorth
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Uh oh, my bad , expiated was the wrong word . I "sinned " ! would you believe I meant "explained " . Trust me fellas if I meant to be religious I would have used promulgated
I figured as much ... I was just raggin' youmagneticnorth said:Uh oh, my bad , expiated was the wrong word . I "sinned " ! would you believe I meant "explained " . Trust me fellas if I meant to be religious I would have used promulgated
phinds said:The term "singularity" means "the place where our model breaks down and we don't know what was going on".
It has never, in the context of the big bang, meant a "point in space"
This "point in space" interpretation of "singularity" in reference to the big bang singularity is pop science woo woo.
Ken G said:Then the issue is in what it means for a scientific theory to "break down." To me, that has nothing to do with mathematics. Newton's theory is perfectly sound mathematics-- yet it "breaks down" when speeds approach c. So to me, breaking down does not mean there are mathematical problems, it means there are physical problems.
martinbn said:I agree, but there is a differenece when it comes to general relativity. In the case of Newton's theory there are observations and experiments that show the theory "breaks down" when speeds approach c. With singularities in GR there is just the maths and it is perfectly fine. So why the statement that it breaks down!
It's true that we can't point to an observation and say "GR is wrong there," but when a theory predicts there was an origin in a singularity, yet provides no physics of origination, nor any way to give physical credence to a concept of an infinite kinetic energy density, then we can say the theory is incomplete. We can even wonder how much of the problem traces to the cosmological principle, which is not really part of GR, it is part of making GR solvable. So what "breaks down" is really GR with the cosmological principle. Which raises an interesting question: what can be said about the singularity without adopting the cosmological principle?martinbn said:I agree, but there is a differenece when it comes to general relativity. In the case of Newton's theory there are observations and experiments that show the theory "breaks down" when speeds approach c. With singularities in GR there is just the maths and it is perfectly fine. So why the statement that it breaks down!
Ken G said:It's true that we can't point to an observation and say "GR is wrong there," but when a theory predicts there was an origin in a singularity, yet provides no physics of origination, nor any way to give physical credence to a concept of an infinite kinetic energy density, then we can say the theory is incomplete.
We can even wonder how much of the problem traces to the cosmological principle, which is not really part of GR, it is part of making GR solvable. So what "breaks down" is really GR with the cosmological principle. Which raises an interesting question: what can be said about the singularity without adopting the cosmological principle?
That's in the mathematics, where the singularity can be regarded as not in the spacetime. That was the point of that nice article you cited. But note that science is not just mathematical theories, it is taking mathematical theories to try to tell a story about what is happening, and how it could be predicted. So if the mathematics can avoid the singularity, the physics cannot-- there are frames that could be populated by hypothetical observers, as we always do in physics to say what is happening, and those frames will get unbounded observables like kinetic energy density and temperature. What's more, they only comprise a finite duration in proper time, with no accounting of t=0 itself. The mathematics can simply not extend to t=0, but in a physical description, that's an incompleteness. It is that incompleteness that causes people to make wholly unsubstantiated statements like "time itself began at t=0," or "time itself began with the Big Bang", and worse, to claim that this claim is part of the Big Bang model, when it certainly is not.martinbn said:That's the whole point, there was no origin in a singularity. The thing, that's not even defined, and we want to call singularity is not part of space-time. In space-time everything is finte and perfectly well behaved.
OK, I was wondering about that. But let me stop you and ask, what do you mean by "physically reasonable conditions", and how do we know those are not the problem, rather than GR itself? Maybe the physically reasonable conditions are wrong, and since we have no observations to say those conditions hold at arbitrarily early times, we cannot then claim that a singularity is a prediction of GR, it is merely among the things we would like to test. I'm sure I don't need to remind you of all the "physically reasonable conditions" in the history of physics that turned out to not test out at all!The cosmological principle is not the problem, that's what the singularity theorems say. Under physically reasonable conditions Lorenzian manifolds are geodesically incomplete.
atyy said:I don't think classical GR breaks down, only quantum GR.
martinbn said:Under physically reasonable conditions Lorenzian manifolds are geodesically incomplete.
Which to me sounds a lot like "the theory breaks down ... "PeterDonis said:Yes, and the question is whether a geodesically incomplete manifold is physically reasonable. One of the main reasons for pursuing a quantum theory of gravity is that a lot of physicists think the answer to that is "no", and therefore, since the classical theory unavoidably implies geodesic incompleteness, the classical theory cannot be physically reasonable as it stands--more precisely, its domain of validity cannot extend arbitrarily close to a singularity.