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> This is the same fallacy as the fallacy that nothing can actually fall into a black hole because it looks like it takes forever from the outside. Oppenheimer

> This is the same fallacy as the fallacy that nothing can actually fall into a black hole because it looks like it takes forever from the outside. Oppenheimer and Snyder published a mathematical model way back in 1939 that shows how a black hole can form in a finite time from the gravitational collapse of a massive object, as seen by an observer falling inward on the surface of the object. The collapse appears to take forever as seen by a distant observer, but this is an optical illusion caused by the effect of spacetime curvature on the paths of light rays. This has been studied for decades and is thoroughly understood. But isn't equating the two observers incorrect? That is, when speaking of 'formation time', we are speaking of the formation time by observers at infinity, free from the effects of local curvature. What I understand is that all observers must agree on reality, thus on whether the BH forms at all: if the infalling observer makes it through an EH and into a singularity, then those exist (by definition) as unique reality. But evaporation seems to complicate this classical GR view understood for a while: since now outside observers see finite-time quasi-evaporation, infalling observers must observe this phenomenom while infalling, and actually before reaching the EH. There's no way I can see that: 1) EHs and singularities (i.e. Black Holes as a steady-state phenomenon) exist; 2) All observers agree on reality of their formation and evaporation. I defiantly conclude they don't exist (or I'm missing something major) :) That is, black holes in nature would be something akin to a extremely time-dilated "Object trap" without singularties (although perhaps it must gain an apparent horizon?).

pdonis 8y ago
> isn't equating the two observers incorrect? There is no "equating the two observers". > when speaking of 'formation time', we are speaking of the formation time by observers at infinity No, we aren't. The hole isn't formed at infinity. The time that is being talked about for observers at infinity (or more correctly, very far away) is the time it takes light rays, coming outward from the object that is collapsing into a black hole, to reach those observers. The curvature of spacetime near the horizon is such that it takes longer and longer for those light rays to get out to the observer far away, the closer to the horizon they are emitted. At the horizon, outgoing light rays do not move outward at all, due to the curvature of spacetime there, so they never reach the observer far away--hence that observer never sees the horizon form. But, as I said, that is an optical illusion caused by the curvature of spacetime. > What I understand is that all observers must agree on reality, thus on whether the BH forms at all All observers agree on observable events and measurement results that they can all observe, yes. But if an observer cannot observe a particular event (such as the horizon forming) because spacetime curvature prevents light rays from that event from ever reaching him, then he can say nothing at all about whether that event happens or not. > evaporation seems to complicate this classical GR view understood for a while It complicates it in the sense I said before: that observers far away, instead of never seeing infalling objects cross the horizon, now see those objects cross the horizon (more precisely: see light rays emitted outward by those objects when they crossed the horizon) at the same time they see the hole evaporate (more precisely: see light rays from the hole's final evaporation). It does not mean what you claim, that infalling observers will see the hole evaporate before they fall in. The spacetime geometry of an evaporating black hole (i.e., including quantum effects) is different from the spacetime geometry of a classical black hole that never evaporates. The statement that a distant observer never sees an object cross the horizon is only true of the latter, not the former. > I defiantly conclude they don't exist (or I'm missing something major) :) The second is the correct choice. You need to stop being defiant and start learning what the models of black holes in physics actually say. > black holes in nature would be something akin to a extremely time-dilated "Object trap" without singularties (although perhaps it must gain an apparent horizon?) To us far away, an apparent horizon looks the same as an event horizon, and works the same--all the things I said above would still be true (at least for the time we have been observing--see below for how long it would take for differences to become observable) if the objects we observe and call black holes actually have only apparent horizons (because of hypothetical quantum gravity effects that, as I've already said, are an open topic of research), not event horizons. And all of your criticisms would still be wrong. (In at least some of the hypothesized quantum gravity models, there is no singularity and no event horizon, only an apparent horizon; but to us far away, the object looks and works the same into the very far future; it takes something like 10^70 years for the difference to be observable.)
- darkmighty 8y ago
  > No, we aren't. The hole isn't formed at infinity. The time that is being talked about for observers at infinity (or more correctly, very far away) is the time it takes light rays, coming outward from the object that is collapsing into a black hole, to reach those observers. The curvature of spacetime near the horizon is such that it takes longer and longer for those light rays to get out to the observer far away, the closer to the horizon they are emitted. At the horizon, outgoing light rays do not move outward at all, due to the curvature of spacetime there, so they never reach the observer far away--hence that observer never sees the horizon form. But, as I said, that is an optical illusion caused by the curvature of spacetime. I think you have a misconception about GR here. In GR, spacetime isn't just an "optical device" that delays rays cast in an absolute reference. In relativity, the behavior of light rays is actually closely related to interpretation of time and space. You can't separate the two. In fact, the definition of (relative) local time is given exactly by the rate a "light clock" ticks when communicating periodically to a "light clock" located elsewhere. Approaching the event horizon, the rate of clock ticking goes to 0 relative to an outside observer (to the the limited extent that coordinate frames can be defined in GR). So classically you can't say that to the outside observer the BH forms in finite time, I don't think. What matters in the end is what the observer sees, which, according to you, would be (for Bob, far away): Alice falls into BH -> Alice approaches EH -> Alice's BH evaporates -> Alice finishes falling into the BH That's not possible! The light rays from evaporation, which happens, in Alice's own frame, after she falls into the BH, cannot precede the light rays from her fall. So she cannot truly go past the EH.
  - pdonis 8y ago
    > I think you have a misconception about GR here. No, you do. Everything I am saying is taken straight from GR textbooks--mainly Misner, Thorne, & Wheeler (1973) and Wald (1984)--and peer-reviewed papers, such as Hawking's original paper on black hole evaporation. You need to go read them. > In GR, spacetime isn't just an "optical device" that delays rays cast in an absolute reference. I never said it was. The paths of light rays in GR are determined by the geometry of spacetime. "Optical illusion" is one way of trying to describe the effect of that in the case under discussion. But the effect is there regardless of what you call it. > In relativity, the behavior of light rays is actually closely related to interpretation of time and space. You can't separate the two. This is just another way of saying that the paths of light rays in GR are determined by the geometry of spacetime. Which is true, but to figure out what that means in a specific scenario, you need to properly understand the spacetime geometry in that scenario. You evidently do not. > In fact, the definition of (relative) local time is given exactly by the rate a "light clock" ticks when communicating periodically to a "light clock" located elsewhere. There is a limited set of scenarios in which this works, but that limited set does not include the case under discussion (Alice falls into a black hole while Bob remains far away). In general in GR, there is no well-defined way of comparing "times" for spatially separated observers. The only well-defined time is local--proper time along a particular observer's worldline. > Alice falls into BH -> Alice approaches EH -> Alice's BH evaporates -> Alice finishes falling into the BH I never said that was the sequence of events. It isn't. The correct sequence of events is (note: this is what happens to Alice, Bob does not see all of these events--see my other post in response) Alice falls into BH -> Alice approaches EH -> Alice finishes falling into BH -> Alice's BH evaporates If you don't understand how that happens, you need to go study GR textbooks. The model that gives the sequence of events I just described has been well understood for several decades now--Hawking published the first paper using it in the 1970s.

pdonis 8y ago

> isn't equating the two observers incorrect? There is no "equating the two observers". > when speaking of 'formation time', we are speaking of the formation time by observers at infinity No, we aren't. The hole isn't formed at infinity. The time that is being talked about for observers at infinity (or more correctly, very far away) is the time it takes light rays, coming outward from the object that is collapsing into a black hole, to reach those observers. The curvature of spacetime near the horizon is such that it takes longer and longer for those light rays to get out to the observer far away, the closer to the horizon they are emitted. At the horizon, outgoing light rays do not move outward at all, due to the curvature of spacetime there, so they never reach the observer far away--hence that observer never sees the horizon form. But, as I said, that is an optical illusion caused by the curvature of spacetime. > What I understand is that all observers must agree on reality, thus on whether the BH forms at all All observers agree on observable events and measurement results that they can all observe, yes. But if an observer cannot observe a particular event (such as the horizon forming) because spacetime curvature prevents light rays from that event from ever reaching him, then he can say nothing at all about whether that event happens or not. > evaporation seems to complicate this classical GR view understood for a while It complicates it in the sense I said before: that observers far away, instead of never seeing infalling objects cross the horizon, now see those objects cross the horizon (more precisely: see light rays emitted outward by those objects when they crossed the horizon) at the same time they see the hole evaporate (more precisely: see light rays from the hole's final evaporation). It does not mean what you claim, that infalling observers will see the hole evaporate before they fall in. The spacetime geometry of an evaporating black hole (i.e., including quantum effects) is different from the spacetime geometry of a classical black hole that never evaporates. The statement that a distant observer never sees an object cross the horizon is only true of the latter, not the former. > I defiantly conclude they don't exist (or I'm missing something major) :) The second is the correct choice. You need to stop being defiant and start learning what the models of black holes in physics actually say. > black holes in nature would be something akin to a extremely time-dilated "Object trap" without singularties (although perhaps it must gain an apparent horizon?) To us far away, an apparent horizon looks the same as an event horizon, and works the same--all the things I said above would still be true (at least for the time we have been observing--see below for how long it would take for differences to become observable) if the objects we observe and call black holes actually have only apparent horizons (because of hypothetical quantum gravity effects that, as I've already said, are an open topic of research), not event horizons. And all of your criticisms would still be wrong. (In at least some of the hypothesized quantum gravity models, there is no singularity and no event horizon, only an apparent horizon; but to us far away, the object looks and works the same into the very far future; it takes something like 10^70 years for the difference to be observable.)

darkmighty 8y ago
> No, we aren't. The hole isn't formed at infinity. The time that is being talked about for observers at infinity (or more correctly, very far away) is the time it takes light rays, coming outward from the object that is collapsing into a black hole, to reach those observers. The curvature of spacetime near the horizon is such that it takes longer and longer for those light rays to get out to the observer far away, the closer to the horizon they are emitted. At the horizon, outgoing light rays do not move outward at all, due to the curvature of spacetime there, so they never reach the observer far away--hence that observer never sees the horizon form. But, as I said, that is an optical illusion caused by the curvature of spacetime. I think you have a misconception about GR here. In GR, spacetime isn't just an "optical device" that delays rays cast in an absolute reference. In relativity, the behavior of light rays is actually closely related to interpretation of time and space. You can't separate the two. In fact, the definition of (relative) local time is given exactly by the rate a "light clock" ticks when communicating periodically to a "light clock" located elsewhere. Approaching the event horizon, the rate of clock ticking goes to 0 relative to an outside observer (to the the limited extent that coordinate frames can be defined in GR). So classically you can't say that to the outside observer the BH forms in finite time, I don't think. What matters in the end is what the observer sees, which, according to you, would be (for Bob, far away): Alice falls into BH -> Alice approaches EH -> Alice's BH evaporates -> Alice finishes falling into the BH That's not possible! The light rays from evaporation, which happens, in Alice's own frame, after she falls into the BH, cannot precede the light rays from her fall. So she cannot truly go past the EH.
- pdonis 8y ago
  > I think you have a misconception about GR here. No, you do. Everything I am saying is taken straight from GR textbooks--mainly Misner, Thorne, & Wheeler (1973) and Wald (1984)--and peer-reviewed papers, such as Hawking's original paper on black hole evaporation. You need to go read them. > In GR, spacetime isn't just an "optical device" that delays rays cast in an absolute reference. I never said it was. The paths of light rays in GR are determined by the geometry of spacetime. "Optical illusion" is one way of trying to describe the effect of that in the case under discussion. But the effect is there regardless of what you call it. > In relativity, the behavior of light rays is actually closely related to interpretation of time and space. You can't separate the two. This is just another way of saying that the paths of light rays in GR are determined by the geometry of spacetime. Which is true, but to figure out what that means in a specific scenario, you need to properly understand the spacetime geometry in that scenario. You evidently do not. > In fact, the definition of (relative) local time is given exactly by the rate a "light clock" ticks when communicating periodically to a "light clock" located elsewhere. There is a limited set of scenarios in which this works, but that limited set does not include the case under discussion (Alice falls into a black hole while Bob remains far away). In general in GR, there is no well-defined way of comparing "times" for spatially separated observers. The only well-defined time is local--proper time along a particular observer's worldline. > Alice falls into BH -> Alice approaches EH -> Alice's BH evaporates -> Alice finishes falling into the BH I never said that was the sequence of events. It isn't. The correct sequence of events is (note: this is what happens to Alice, Bob does not see all of these events--see my other post in response) Alice falls into BH -> Alice approaches EH -> Alice finishes falling into BH -> Alice's BH evaporates If you don't understand how that happens, you need to go study GR textbooks. The model that gives the sequence of events I just described has been well understood for several decades now--Hawking published the first paper using it in the 1970s.