I "Black holes can only get bigger" - huh?

[PeterDonis]

It concludes

Note that this conclusion does not mean Hawking radiation definitely does not happen, or should no longer be considered as a possible prediction. It means we don't really know either way. As the same paper says in its abstract:

"[A] definitive theoretical treatment will require an understanding of quantum gravity in at least some regimes. Until then, no compelling theoretical case for or against radiation by black holes is likely to be made."

 

[Sunil]

That's not what the paper you referenced shows. That paper simply assumes that it is possible to have a trajectory for an ingoing shell in which the shell asymptotically approaches some areal radius $R$ which is a little bit larger than $2M$, and computes what outgoing radiation visible at infinity would be present in such a case assuming a scalar quantum field and s-wave emission only. It does not show any "modification" of GR, whether in the trans-Planckian domain or not, that implies such a trajectory; in fact, it does not appear to claim that GR needs any modification at all.

That's why I added "for the mathematics". I should have written "for the mathematics of how fast the Hawking radiation stops if the collapse stops".

At the bottom of p. 3, the paper says that this possibility is unlikely:

"These results strongly suggest that the conventional wisdom as regards the formation of black hole is correct and that the semiclassical radiation does not prevent the formation of the event horizon."

I have not claimed that in semiclassical GR semiclassical radiation prevents the formation of the event horizon. A paper which makes such a claim is

Gerlach, U.H. (1976). The mechanism of blackbody radiation from an incipient black hole, Phys Rev D 14(6) 1479-1508.

I'm neutral about this question. My point is simply that QG is unknown, that we can have QG effects if the surface time dilation reaches factors like $10^{100} t_{age-of-universe}/t_{Planck}$, and if we assume that unknown QG effects stop the collapse once such a factor is reached, then we can used the Paranjape paper to see that there will be no Hawking radiation. (If we think that this needs more justification than simply "stable stars don't radiate".) For this sufficiently simple argument I have no reference in the literature.

You are mis-stating the analogy. Unruh radiation is an easily derived property of a quantum field in flat spacetime in the presence of an accelerated observer; it associates the radiation with the presence of a Rindler horizon for such an observer. The analogy with Hawking radiation relies on the fact that, for an accelerated observer close enough to the event horizon of a black hole, the hole's event horizon is the same as the observer's Rindler horizon. But that only holds for a black hole (and for an observer accelerating to "hover" close enough to the hole's horizon); it does not hold for an ordinary star, since the star is not a black hole and has no event horizon.

Whatever, it does not work. What can cause radiation has to be inside the backward light cone. There is no event horizon in that backward light cone.

 

[PeterDonis]

If we think that this needs more justification than simply "stable stars don't radiate".

Which, as I have already pointed out, is an invalid argument since the radiation, in the conventional view, is associated with an event horizon, and stable stars have no event horizon.

Whatever, it does not work. What can cause radiation has to be inside the backward light cone. There is no event horizon in that backward light cone.

If this argument is correct, it is an argument that there is no Unruh radiation. Do you have a refutation of Unruh's derivation of Unruh radiation? If you don't, you should consider the possibility that it is your argument here that is wrong, not the derivation of Unruh radiation.

 

[Sunil]

Which, as I have already pointed out, is an invalid argument since the radiation, in the conventional view, is associated with an event horizon, and stable stars have no event horizon.

Not in the context of that argument (I assume that some unknown QG effects stop the collapse when surface time dilation becomes astronomical.)

If this argument is correct, it is an argument that there is no Unruh radiation. Do you have a refutation of Unruh's derivation of Unruh radiation? If you don't, you should consider the possibility that it is your argument here that is wrong, not the derivation of Unruh radiation.

No, Unruh radiation is in Minkowski space, everything is time symmetric. My argument uses the time asymmetry - in the past lightcone, the observer sees the collapsing star before horizon formation. I don't consider eternal BHs and don't think it applies to them.

Note that this conclusion does not mean Hawking radiation definitely does not happen, or should no longer be considered as a possible prediction. It means we don't really know either way. As the same paper says in its abstract:

"[A] definitive theoretical treatment will require an understanding of quantum gravity in at least some regimes. Until then, no compelling theoretical case for or against radiation by black holes is likely to be made."

Correct. AFAIK nobody claims that it is an impossible prediction.

 

[PeterDonis]

Not in the context of that argument (I assume that some unknown QG effects stop the collapse when surface time dilation becomes astronomical.)

Such an object would not be a "stable star" since it is impossible for such an object to be made of stress-energy that obeys energy conditions (which all ordinary matter and radiation does), because of the Buchdahl Theorem: an object made of ordinary matter can only be stable if its radius is greater than 9/8 the Schwarzschild radius for its mass.

It is known that quantum fields can have an effective stress-energy tensor that violates energy conditions; however, the usual effect of such violations is instability: objects either collapse or explode. So the most natural assumption of the "unknown effects" you assume to be present is not that a collapse would just stop at some radius slightly larger than the Schwarzschild radius, and form a stable object of that radius made of exotic matter (i.e., "quantum stuff" that violates energy conditions). The most natural assumption is that the collapse would turn into an explosion, as in the various quantum "bounce"models.

 

[PeterDonis]

Unruh radiation is in Minkowski space

As the background spacetime, yes. But the background spacetime is not all that is present. See below.

everything is time symmetric.

Wrong. The process of detecting Unruh radiation is not time symmetric: a particle detector carried by the accelerated observer can be taken from its ground state to an excited state by interacting with the quantum field; the observer interprets this as "detection of a particle". The quantum field undergoes a corresponding transition. This is a time asymmetric process.

Another way of seeing the time asymmetry is as follows: the Unruh radiation is coming out of the Rindler horizon, but there is no radiation going in to the Rindler horizon (because the quantum field state is the vacuum state with respect to inertial observers, and that state contains no radiation coming in from past infinity). This is a time asymmetric condition.

 

[Sunil]

As the background spacetime, yes. But the background spacetime is not all that is present.

Ok, my "everytyhing" was sloppy, sorry.

Such an object would not be a "stable star" since it is impossible for such an object to be made of stress-energy that obeys energy conditions (which all ordinary matter and radiation does), because of the Buchdahl Theorem: an object made of ordinary matter can only be stable if its radius is greater than 9/8 the Schwarzschild radius for its mass.

It would not be a stable star in classical GR. So what? As I said, I presuppose that unknown QG effects stop the collapse.

The most natural assumption is that the collapse would turn into an explosion, as in the various quantum "bounce"models.

It is not my intention to speculate about the QG details. But my intuition tells me that there will be not that much difference between an atom and a BH - they both will have a stable ground state.

 

[PeterDonis]

my intuition tells me

"Intuition" is not what we're supposed to be basing discussion on. We understand what your opinion is, but your opinion is not the same as established fact, nor is it a valid reference.

 

[Sunil]

"Intuition" is not what we're supposed to be basing discussion on. We understand what your opinion is, but your opinion is not the same as established fact, nor is it a valid reference.

No problem - I have simply answered a claim about a "most natural assumption ... that the collapse would turn into an explosion" by giving another assumption. Is is obvious that speculations about unknown quantum gravity effects cannot be established facts and cannot be supported by valid references. The best one can support it is by some vague analogies, which I have done, with the reference to the ground state as the most important quantum effect for atoms.

 

[Fra]

Is is obvious that speculations about unknown quantum gravity effects cannot be established facts and cannot be supported by valid references. The best one can support it is by some vague analogies, which I have done, with the reference to the ground state as the most important quantum effect for atoms.

I am guessing that what Peter means is that in these the rules suggest that only "published speculations" should be discussed? not becase they are necessarily right but because it at least prevents random personal and pedestrian speculations from degrading forum quality.

But I agree this sometimes makes the discussions "difficult" or constrained, from a creative perspective or idea exchange perspective, I have no personal issues with speculation as long as its reasonble and not complete random layman speculation. I think the moderators are doing a good job still here to keep the forum sane.

Maybe the thread would have been better in BTSM or QM foundations? How can one talk about GR+QM without touching upon QM foundations? This was what I meant with post 17 as well.

/Fredrik

 

[Sunil]

I am guessing that what Peter means is that in these the rules suggest that only "published speculations" should be discussed? not becase they are necessarily right but because it at least prevents random personal and pedestrian speculations from degrading forum quality.

I take care about my posts not degrading a particular thread, especially if mods are involved. Peter has not given a reference to his speculation, and in such situations I conclude this is fine in this context.

 

[PeterDonis]

I am guessing that what Peter means is that in these the rules suggest that only "published speculations" should be discussed?

And even that only in the Beyond the Standard Model forum. Which this isn't.

Granted, any quantum discussion of black holes is kind of borderline here, since we don't have an established theory of quantum gravity. But for purposes of this thread, we are discussing the closest thing to a "standard" model of black hole evaporation, the model originally proposed by Hawking. It's good to be aware that there are issues that have been raised with this model and that there are other proposed models, but if we really want to get into a detailed discussion of those issues we probably need to start a separate thread in the BTSM forum.

not becase they are necessarily right but because it at least prevents random personal and pedestrian speculations from degrading forum quality.

Maintaining the signal to noise ratio of PF is one reason, yes. There are at least two others: first, PF is not a platform for conducting original research, and personal speculations/theories that aren't in the published literature are original research; and second, the published literature at least provides a common basis for discussion.