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In General Relativity events in your past light cone can be events in your future light cone. In Loop Quantum Gravity, the idea of a past and future lightcone b

In General Relativity events in your past light cone can be events in your future light cone. In Loop Quantum Gravity, the idea of a past and future lightcone become discrete and entirely local to a specific element of spacetime. Consequently, assigning elements of a past light cone to "reality" becomes ambiguous: _which_ past light cone? At the level of LQG all you can describe is relations between points of spacetime and the idea of past/future becomes ambiguous. Another modern approach to quantum gravity (energetic causal set theory) does re-impose a sort of global ordering (in the sense reality is a series of states updated with a single rule depending on the momenta of previous realizations of the state elements) but in that theory the past really is gone. This is similar to the (more classical) approach of Shape Dynamics (a general relativity-like theory of gravity) where all that relates a "current" snapshot of time to a "previous" snapshot is that it minimizes a particular mathematical quantity. I am definitely not an expert in LQG or GR or Shape Dynamics, but the story isn't quite so obvious and simple as you make it out to be and that may be why you are unsatisfied with how physicists talk about it.

pdonis 4y ago
> In General Relativity events in your past light cone can be events in your future light cone. Only in spacetimes with closed timelike curves, such as the Godel Universe. Those spacetimes are considered by physicists to not be physically realistic. All physically realistic spacetimes, including all of the ones we use to actually model things in the actual universe, are globally hyperbolic, and in a globally hyperbolic spacetime the same event cannot be in both the past and the future light cone of some other event.
- nathan_compton 4y ago
  They appear in the Kerr metric, which isn't particularly exotic except for the fact that realistic spinning black holes clearly don't have all the symmetries of a Kerr black hole. I would not be surprised to learn that the question of CTCs in realistic spinning black hole solutions is open, but I am not an expert in General Relativity. We certainly use the Kerr black hole solution to model things in the actual universe. At the very least, the fact that CTCs appear in solutions to GR but seem physically implausible suggest that the basic physics of time are not fully understood (as do the other more outre theoretical avenues I pointed out). I think its pointless to be glib about the reality of the past, given that fact.
  - pdonis 4y ago
    > They appear in the Kerr metric Only inside the inner horizon, which virtually all physicists working in the field consider to be unphysical because of the "infinite blueshift" effect at the inner horizon and the fact that it is a Cauchy horizon: you actually can't predict what will be inside it based on any data from outside it. (Note that the outer horizon, like the horizon of a Schwarzschild black hole, does not have these properties.) The prevailing opinion seems to be that the interior of a rotating black hole will end up being qualitatively similar to the interior of a non-rotating black hole, with the inner horizon and region inside it of the idealized Kerr solution being replaced by something that looks more like a Schwarzschild interior. (And of course if we figure out quantum gravity that might change things even further.) > We certainly use the Kerr black hole solution to model things in the actual universe. We use its exterior to model things in the actual universe, yes. But none of those models depend on the interior having the properties that the idealized Kerr solution has. They certainly don't depend on their being CTCs somewhere inside. > the fact that CTCs appear in solutions to GR but seem physically implausible suggest that the basic physics of time are not fully understood No, it suggests that, as with any physical theory, you have to look at actual data to see which of the infinite number of mathematically possible solutions to the theory's equations describe things that actually exist. All of the solutions that describe things that actually exist are globally hyperbolic and do not contain CTCs or any other weird "time" properties. That suggests that, as far as things that actually exist go, we have pretty good models of "the basic physics of time".

pdonis 4y ago

> In General Relativity events in your past light cone can be events in your future light cone. Only in spacetimes with closed timelike curves, such as the Godel Universe. Those spacetimes are considered by physicists to not be physically realistic. All physically realistic spacetimes, including all of the ones we use to actually model things in the actual universe, are globally hyperbolic, and in a globally hyperbolic spacetime the same event cannot be in both the past and the future light cone of some other event.

nathan_compton 4y ago
They appear in the Kerr metric, which isn't particularly exotic except for the fact that realistic spinning black holes clearly don't have all the symmetries of a Kerr black hole. I would not be surprised to learn that the question of CTCs in realistic spinning black hole solutions is open, but I am not an expert in General Relativity. We certainly use the Kerr black hole solution to model things in the actual universe. At the very least, the fact that CTCs appear in solutions to GR but seem physically implausible suggest that the basic physics of time are not fully understood (as do the other more outre theoretical avenues I pointed out). I think its pointless to be glib about the reality of the past, given that fact.
- pdonis 4y ago
  > They appear in the Kerr metric Only inside the inner horizon, which virtually all physicists working in the field consider to be unphysical because of the "infinite blueshift" effect at the inner horizon and the fact that it is a Cauchy horizon: you actually can't predict what will be inside it based on any data from outside it. (Note that the outer horizon, like the horizon of a Schwarzschild black hole, does not have these properties.) The prevailing opinion seems to be that the interior of a rotating black hole will end up being qualitatively similar to the interior of a non-rotating black hole, with the inner horizon and region inside it of the idealized Kerr solution being replaced by something that looks more like a Schwarzschild interior. (And of course if we figure out quantum gravity that might change things even further.) > We certainly use the Kerr black hole solution to model things in the actual universe. We use its exterior to model things in the actual universe, yes. But none of those models depend on the interior having the properties that the idealized Kerr solution has. They certainly don't depend on their being CTCs somewhere inside. > the fact that CTCs appear in solutions to GR but seem physically implausible suggest that the basic physics of time are not fully understood No, it suggests that, as with any physical theory, you have to look at actual data to see which of the infinite number of mathematically possible solutions to the theory's equations describe things that actually exist. All of the solutions that describe things that actually exist are globally hyperbolic and do not contain CTCs or any other weird "time" properties. That suggests that, as far as things that actually exist go, we have pretty good models of "the basic physics of time".