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Counterargument to "the Planck scale is blown up to be visible": The expanding universe itself has (or very probably has) stretched by a factor starting to push

by Steuard 12y ago

Counterargument to "the Planck scale is blown up to be visible": The expanding universe itself has (or very probably has) stretched by a factor starting to push into "practical infinity" territory since the Big Bang. (As I recall, common models of cosmic inflation require a minimum of 60 "e-foldings" of expansion in the first moments of the universe's existence: a factor of 10^26 or so, not counting the considerable expansion since then.) By that argument, we ought to be dangerously close to "voiding the warranty" on spacetime already, everywhere in the universe. As far as I know, the notion that curved space can "enlarge the Planck-scale granularity of spacetime" is not a universal feature of quantum gravity theories. It's not even clear to me what that would mean. (I've only seen one or two string theory talks that suggested such a thing, for instance, and most of the experts in the audience seemed fairly convinced that the idea had no compelling basis in our context. I don't recall whether that's a feature of LQG.)

PaulHoule 12y ago
Well, any funny business that happened at the Planck scale during the big bang is hidden from us by an ionized cloud of gas. Maybe there is some trace left in the CMB, but there could be other physics in the way. People have often said that "physics breaks down" at a gravitational singularity, but it is right to say that the theory of GR breaks down. Singularities generally are a sign that your theory is incomplete and aren't something that occurs in nature. My argument is that the coordinate transformation and analytic continuation of the schwarzchild singularity is invalid, not that anything specific happens because of high curvature. The semiclassical view is that the event horizon is a quantum object, and if that is the case why believe the classical theory of the interior. There is also the classical story that an outside observer sees it take forever for something to hit the event horizon but that the semiclassical theory says the black hole decays so presumably an outside observer sees the in falling object get burned up by hawking radiation or otherwise destroyed by something that happens at the eh. Somehow the physics establishment denies there is a contradiction too and it is certainly true that the wall of fire has it's own problems, but that is because our current theories don't entirely work.
- Steuard 12y ago
  When I'm talking about the Planck scale being "blown up" to observable size during the Big Bang, I'm not talking about any need to look past the CMB, I'm talking about right here, where we're sitting. If there's some issue in which significant "stretching" of spacetime (as via a coordinate transformation) can make Planck scale effects important even in regions of finite curvature, then we ought to be sitting in such a region right now, here on Earth. Based just on what you've said here, you seem to be conflating the notion of a true curvature singularity (where "physics breaks down") with a mere coordinate singularity (like the event horizon of a Schwarzchild black hole in asymptotically flat spherical coordinates, or the north/south poles of the earth in latitude/longitude coordinates: there's an "infinitely stretched coordinate transformation" required to resolve the latter, too). Specifically, it sounds like you're worried about the coordinate transformation that's necessary to eliminate the infinity that shows up at the event horizon in "normal" spherical coordinates. But why should I think that those coordinates (which are optimized to describe the flat space infinitely far from the black hole) reflect any special physical "truth"? Just for instance, if you set up a local coordinate system based on the proper time and length as measured by a freely-falling infalling observer, there's no trace of any infinity in the coordinates or metric at the event horizon. And there's no coordinate-independent quantity in classical GR that diverges at the event horizon. This is why most physicists aren't worried about the event horizon being a meaningful singularity in classical GR. Now, bringing in a semiclassical analysis (or presumably a full quantum analysis, if you've got one; also, call Stockholm) does lead to interesting effects related to the event horizon. But that has nothing to do with coordinate singularities in classical GR and everything to do with the causal structure of spacetime. (Personally, I do harbor some significant doubts about whether we should trust the classical description of the black hole interior once we have a full quantum theory, so I've been quite interested to see the recent firewall questions come up. But the arguments casting doubt on those conclusions are pretty strong, too.) But regardless of my hunches about what will be true in a "final" theory, semiclassical theory is really quite clearly in agreement with classical GR that a local observer won't see anything special as they cross the event horizon.

by Steuard 12y ago

PaulHoule 12y ago

Well, any funny business that happened at the Planck scale during the big bang is hidden from us by an ionized cloud of gas. Maybe there is some trace left in the CMB, but there could be other physics in the way. People have often said that "physics breaks down" at a gravitational singularity, but it is right to say that the theory of GR breaks down. Singularities generally are a sign that your theory is incomplete and aren't something that occurs in nature. My argument is that the coordinate transformation and analytic continuation of the schwarzchild singularity is invalid, not that anything specific happens because of high curvature. The semiclassical view is that the event horizon is a quantum object, and if that is the case why believe the classical theory of the interior. There is also the classical story that an outside observer sees it take forever for something to hit the event horizon but that the semiclassical theory says the black hole decays so presumably an outside observer sees the in falling object get burned up by hawking radiation or otherwise destroyed by something that happens at the eh. Somehow the physics establishment denies there is a contradiction too and it is certainly true that the wall of fire has it's own problems, but that is because our current theories don't entirely work.

Steuard 12y ago
When I'm talking about the Planck scale being "blown up" to observable size during the Big Bang, I'm not talking about any need to look past the CMB, I'm talking about right here, where we're sitting. If there's some issue in which significant "stretching" of spacetime (as via a coordinate transformation) can make Planck scale effects important even in regions of finite curvature, then we ought to be sitting in such a region right now, here on Earth. Based just on what you've said here, you seem to be conflating the notion of a true curvature singularity (where "physics breaks down") with a mere coordinate singularity (like the event horizon of a Schwarzchild black hole in asymptotically flat spherical coordinates, or the north/south poles of the earth in latitude/longitude coordinates: there's an "infinitely stretched coordinate transformation" required to resolve the latter, too). Specifically, it sounds like you're worried about the coordinate transformation that's necessary to eliminate the infinity that shows up at the event horizon in "normal" spherical coordinates. But why should I think that those coordinates (which are optimized to describe the flat space infinitely far from the black hole) reflect any special physical "truth"? Just for instance, if you set up a local coordinate system based on the proper time and length as measured by a freely-falling infalling observer, there's no trace of any infinity in the coordinates or metric at the event horizon. And there's no coordinate-independent quantity in classical GR that diverges at the event horizon. This is why most physicists aren't worried about the event horizon being a meaningful singularity in classical GR. Now, bringing in a semiclassical analysis (or presumably a full quantum analysis, if you've got one; also, call Stockholm) does lead to interesting effects related to the event horizon. But that has nothing to do with coordinate singularities in classical GR and everything to do with the causal structure of spacetime. (Personally, I do harbor some significant doubts about whether we should trust the classical description of the black hole interior once we have a full quantum theory, so I've been quite interested to see the recent firewall questions come up. But the arguments casting doubt on those conclusions are pretty strong, too.) But regardless of my hunches about what will be true in a "final" theory, semiclassical theory is really quite clearly in agreement with classical GR that a local observer won't see anything special as they cross the event horizon.