Register to reply

Relative energy of a black hole.

by cragar
Tags: black, energy, hole, relative
Share this thread:
DaleSpam
#19
Feb23-12, 07:26 AM
Mentor
P: 16,950
Quote Quote by Q-reeus View Post
And this is the last plug here re entry #10. Why no takers? It only fuels suspicions there are no decent and believable answers. So come on, someone give it a shot please.
I don't see anything new in #10 that hasn't been asked and answered several times on this forum already. Try doing a brief search.
Q-reeus
#20
Feb23-12, 08:31 AM
P: 1,115
Quote Quote by DaleSpam View Post
I don't see anything new in #10 that hasn't been asked and answered several times on this forum already. Try doing a brief search.
Not quite the response I was hoping for, but then you and I did cross swords on those matters some time back. There was imo certainly no satisfactory resolution then, which is why I have raised it anew in the faint hope of a better outcome now. Strange to me that yourself and many others here can devote page after page of detailed response to such mundane issues as proving Gauss's law in EM, or magnetism in straight wires, or choo-choo trains with mirrors and flashing lights etc. etc. Yet when it comes to this apparently taboo topic (if not taboo, why the silence from the experts in GR?), I get a terse directive to 'just search'.

Don't want to appear sulky about that, but honestly, if you actually believe the issues in #10 have been well answered, how about your own detailed summary afresh? I have never been able to figure out effective searching using PF. As someone willing to endlessly discuss oft repeated SR/GR topics, this shouldn't be a hard ask surely. I could and if pressed am willing to reference back to that earlier thread, but prefer we leave that baggage behind. Oh, and I should thank you for at least stepping up and actually breaking the silence. Now if it seems like thread hijacking just say so and I will launch the issue(s) under my own banner.
DaleSpam
#21
Feb23-12, 08:55 AM
Mentor
P: 16,950
Quote Quote by Q-reeus View Post
I have never been able to figure out effective searching using PF.
I typically use a google search with the delimiter:

site:www.physicsforums.com

Regarding the rest, the fact that you are aware of the previous responses on this topic and consider them all unsatisfactory seems to indicate that further discussion will probably lead to the same result. As you point out, when available we are often willing to devote many fruitless pages to topics that we notice which we consider interesting at the time.

It appears that nobody with any current interest and availability has noticed your #10. For me, the lack of interest is simple exhaustion on the topic, which is a temporary condition but present now.
Sam Gralla
#22
Feb23-12, 09:15 AM
P: 95
Quote Quote by pervect View Post
There is nothing in this paper that says that a single particle moving at a high velocity will become a black hole.

It does say that a pair of particles colliding can become a black hole (which is true) - but it does not say that a single particle moving at high speed will become a black hole - because this statement, as previously mentioned, is false.
If the "observer" is a real observer and has any finite amount of mass, then a particle moving very fast by it is in fact a particle collision, and a black hole may very well be formed. But I repeat myself.
Q-reeus
#23
Feb23-12, 09:43 AM
P: 1,115
Quote Quote by DaleSpam View Post
I typically use a google search with the delimiter:
site:www.physicsforums.com
Thanks for the tip.
Regarding the rest, the fact that you are aware of the previous responses on this topic and consider them all unsatisfactory seems to indicate that further discussion will probably lead to the same result.
Only active and willing participation this time round can decide that. I'm certainly open to serious feedback. And for the record, here's the link to that thread: http://www.physicsforums.com/showthread.php?t=508950 (begin at #3). Apart from a single oblique cautionary response, there was no participation from any real GR pro there. Disappointing to say the least. As I said in #18 here, that just tends to fuel suspicions.
It appears that nobody with any current interest and availability has noticed your #10. For me, the lack of interest is simple exhaustion on the topic, which is a temporary condition but present now.
A call for more patience I take it, but I suspect #10 has not slipped under more than a few of the resident experts radar. Just can't quite figure a reason for the extreme reticence. And what, you have no ready answers personally willing to offer here? (saw that cute 'no peaking' trick you inserted in another thread - think about applying it to yourself here and try answering maybe, without referencing the link I gave above). For sure I could try starting a new thread, but if nothing here, then most likely nothing there either, which would be so sad.
PeterDonis
#24
Feb23-12, 05:44 PM
Physics
Sci Advisor
PF Gold
P: 6,038
Hi Q-reeus, I just came across this thread and read your post #10. It's been some time, but I know I've given the simple classical answer to this in at least one thread on PF. (Unfortunately it doesn't appear that that thread was one of the ones you've participated in). The simple classical answer is that the "field of the black hole" doesn't actually come from inside the hole; it comes from the past, from the object that originally collapsed to form the hole. That is actually what the Usenet Physics FAQ page you linked to means when it says that the hole's field is a "fossil field"; it's a stationary remnant left by the collapsing object.

IMO, the FAQ page's mention of virtual particles is a little misleading since it invites the interpretation that the BH field actually does come from inside the hole, but somehow virtual gravitons can escape because they can move faster than light. Even on a quantum view of gravity I don't think that's correct; I think the explanation I gave above still applies. (Which is not to say that there is not a sense in which virtual particles can travel faster than light; there is. I just don't think that sort of thing is needed to resolve the question of "how gravity gets out of the black hole"--it doesn't have to get out because that's not where it's actually coming from to begin with.)
Q-reeus
#25
Feb23-12, 10:39 PM
P: 1,115
Quote Quote by PeterDonis View Post
Hi Q-reeus, I just came across this thread and read your post #10. It's been some time, but I know I've given the simple classical answer to this in at least one thread on PF. (Unfortunately it doesn't appear that that thread was one of the ones you've participated in). The simple classical answer is that the "field of the black hole" doesn't actually come from inside the hole; it comes from the past, from the object that originally collapsed to form the hole. That is actually what the Usenet Physics FAQ page you linked to means when it says that the hole's field is a "fossil field"; it's a stationary remnant left by the collapsing object.
Hi Peter. Well you qualify as GR expert so good at least there is some useful feedback. I understand that by 'fossil field' it is meant that the static exterior spacetime curvature persists though causally cut off from the original source. But this begs the question - if the source matter has effectively vanished wrt the exterior region, how can this fossil field persist there? As stated in #10, the inescapable implication surely is that the field acts as it's own source. And that's what makes sense to me, but then it has to be explained how there is no contribution whatsoever from field curvature in the stress-energy tensor, which in GR is the sole source of such curvature!

Now it has been stated that one must 'read between the lines'; meaning in effect that the non-linear character of gravity in GR is evidence 'gravity gravitates' there in some de facto sense. I'm not convinced of that reasoning. The mere fact that matter induces curvature demands non-linearity - rulers and clocks necessarily vary with distance from source, but this of itself does not imply any further contribution to curvature from that curvature. I don't particularly like citing authorities, but to back that point up, here's a quote from a Sascha Vongehr who at least styles himself as knowledgeable in GR:
"All you need to be clear about is that pressure is part of the pressure energy tensor, i.e. part of the physical source of what happens in general relativity, one side of the Einstein equation. Gravity is on the other side, and it is not a source. It does not somehow walk around to the other side and become pressure, it is plain curvature in space-time. It is non-linear and so on, so it looks to us like a force that self-interacts (= being its own source)." (last paragraph at: http://www.science20.com/comments/50..._gravitational)
Don't know whether that qualifies him as 'crackpot' but clearly he has studied the subject and came to that conclusion. Is there actually an overwhelming consensus position on 'gravity does/does not gravitate'?
IMO, the FAQ page's mention of virtual particles is a little misleading since it invites the interpretation that the BH field actually does come from inside the hole, but somehow virtual gravitons can escape because they can move faster than light. Even on a quantum view of gravity I don't think that's correct; I think the explanation I gave above still applies. (Which is not to say that there is not a sense in which virtual particles can travel faster than light; there is. I just don't think that sort of thing is needed to resolve the question of "how gravity gets out of the black hole"--it doesn't have to get out because that's not where it's actually coming from to begin with.)
I have stated elsewhere that if the virtual graviton picture is wrong as suggested, then it surely spells instant death to what is styled as the leading candidate for a TOE - string/superstring/M theory. Because in that theory gravity is described in terms of a spin-2 virtual graviton (and real gravitons for GW's). Unless I suppose vg's are their own source in that theory - I don't really know. That would place it in the 'gravity gravitates' camp. If so though, it gets back to why this is not correspondingly reflected in GR where 'officially' only non-gravitational energy contributes to curvature.

Regardless of the source of BH gravity itself, the issue of charged BH presents a headache for GR imo. Again as stated in #10, one either posit that a static electric (or magnetic) field can act as it's own source (and where would that leave Maxwell's linear eqn's and the definitions of EM fields wrt to potentials, which in turn are defined wrt source charge/current), or one posits something truly magical imo. Namely that infalling charged matter, already at or interior to the EH, can continue to act as a continual source of vp (virtual photon) exchange with the exterior regions - despite all temporal processes ceasing wrt the exterior. If I'm missing something obvious here, please enlighen.
Bet you wished already to have not bitten this one!
PeterDonis
#26
Feb24-12, 12:01 AM
Physics
Sci Advisor
PF Gold
P: 6,038
Quote Quote by Q-reeus View Post
I understand that by 'fossil field' it is meant that the static exterior spacetime curvature persists though causally cut off from the original source.
It appears that by "the original source" you mean the actual black hole region of the spacetime. But that is *not* the "original source" of the observed field in the exterior region. The "original source" is the collapsing object, while it was still collapsing, i.e., before the hole's event horizon was formed; in other words, it's matter with a nonzero stress-energy tensor in a region of spacetime that is *not* causally cut off from the event where the field is being observed.

Quote Quote by Q-reeus View Post
if the source matter has effectively vanished wrt the exterior region, how can this fossil field persist there?
Because the "imprint" of the collapsing matter is propagated throughout the vacuum exterior region of the black hole spacetime.

Quote Quote by Q-reeus View Post
As stated in #10, the inescapable implication surely is that the field acts as it's own source.
No, it doesn't have to. A "gravitational field", in the sense of a nonzero Riemann curvature tensor, can propagate through source-free space (space with a stress-energy tensor of zero), just as an electromagnetic field, in the sense of a nonzero Faraday tensor, can propagate through charge-free space (space with a charge-current density 4-vector of zero). Some people might say that this propagation means "the field acts as its own source", but that usage of the term "source" is imprecise; the precise meaning of "source" is the SET in GR, or the charge-current 4-vector in EM.

Quote Quote by Q-reeus View Post
And that's what makes sense to me, but then it has to be explained how there is no contribution whatsoever from field curvature in the stress-energy tensor, which in GR is the sole source of such curvature!
No, it isn't. See above. The correct statement here is that the SET is the source of *Ricci* curvature; the Einstein tensor, which is what appears on the LHS of the Einstein Field Equation, is the Ricci tensor with a trace term added. But the "gravitational field" of a black hole, what you observe as the "field" in the exterior region, is entirely Weyl curvature; the Ricci tensor is zero (because it's a vacuum solution of the EFE), but the Weyl tensor is nonzero. The Weyl tensor doesn't appear at all in the EFE, so it can be nonzero in regions where the SET is zero.

Quote Quote by Q-reeus View Post
Now it has been stated that one must 'read between the lines'; meaning in effect that the non-linear character of gravity in GR is evidence 'gravity gravitates' there in some de facto sense. I'm not convinced of that reasoning.
People can mean lots of different things by "gravity gravitates". The main thing this illustrates to me is that you have to be very careful reasoning from statements in ordinary English about GR (or indeed about any scientific theory). The true theory is the math, the equations and the precise predictions that they give rise to. Statements like "gravity gravitates" are attempts to capture some aspect of the math in ordinary English, but I would never want to hang my hat on an implication of such a statement without checking it against the actual math, to be sure my reasoning wasn't being led astray by a mismatch between the actual math and the approximation to it in English.

Quote Quote by Q-reeus View Post
"All you need to be clear about is that pressure is part of the pressure energy tensor, i.e. part of the physical source of what happens in general relativity, one side of the Einstein equation. Gravity is on the other side, and it is not a source. It does not somehow walk around to the other side and become pressure, it is plain curvature in space-time.
This looks OK to me; I was saying the same sort of thing above when I talked about precisely defining the term "source".

Quote Quote by Q-reeus View Post
It is non-linear and so on, so it looks to us like a force that self-interacts (= being its own source)." (last paragraph at: http://www.science20.com/comments/50..._gravitational)
People can also mean different things by "self-interacts". The precise meaning of that term really requires quantum theory; in quantum gravity, gravitons can interact with other gravitons directly, i.e., there are graviton-graviton vertices in the allowed set of Feynman diagrams. By contrast, quantum electrodynamics is not self-interacting: there are no direct photon-photon vertices.

Quote Quote by Q-reeus View Post
I have stated elsewhere that if the virtual graviton picture is wrong as suggested, then it surely spells instant death to what is styled as the leading candidate for a TOE - string/superstring/M theory. Because in that theory gravity is described in terms of a spin-2 virtual graviton (and real gravitons for GW's). Unless I suppose vg's are their own source in that theory - I don't really know. That would place it in the 'gravity gravitates' camp. If so though, it gets back to why this is not correspondingly reflected in GR where 'officially' only non-gravitational energy contributes to curvature.
Just to be clear, I was not saying that the quantum picture of "virtual gravitons" was wrong per se; I was only saying that it was not necessary for understanding where the observed field around a black hole comes from. The general question of the validity of the "virtual graviton picture" is, IMO, a topic for a separate thread.

Quote Quote by Q-reeus View Post
Regardless of the source of BH gravity itself, the issue of charged BH presents a headache for GR imo.
This question has the same answer as the answer for gravity: the "source" of the observed EM field around a charged BH is the charge-current density in the collapsing matter; i.e., the observed field at any event in the exterior region is entirely determined by field propagation from charge-current density in the past light cone of that event.
Q-reeus
#27
Feb24-12, 03:49 AM
P: 1,115
Quote Quote by PeterDonis View Post
Only if by "the original source" you mean the actual black hole region of the spacetime. But that is *not* the "original source" of the observed field in the exterior region. The "original source" is the collapsing object, while it was still collapsing, i.e., before the hole's event horizon was formed; in other words, it's matter with a nonzero stress-energy tensor in a region of spacetime that is *not* causally cut off from the event where the field is being observed.
Umm - first part above is past sense 'while it was still collapsing', but ends with a present sense 'where the field is being observed.' Still not seeing how the present - exterior field, is continuously sustained by the past - collapsing matter that is now at or behind an EH.
Q-reeus: "As stated in #10, the inescapable implication surely is that the field acts as it's own source."
No, it doesn't have to. A "gravitational field", in the sense of a nonzero Riemann curvature tensor, can propagate through source-free space (space with a stress-energy tensor of zero), just as an electromagnetic field, in the sense of a nonzero Faraday tensor, can propagate through charge-free space (space with a charge-current density 4-vector of zero).
I'm presuming that is referring primarily to static fields (radiation fields or time variation of fields in general are a non-issue here). If so then seems to me the ability to 'propagate' through source-free space means nothing more than the ability to exist at all in that spatial region, otherwise there is no meaning to the term field. What I'm having trouble with is the notion, unique to GR afaik, that a static field can exist apart from it's original source (and you have I think made it clear that source in GR cannot be curved spacetime itself).
...The correct statement here is that the SET is the source of *Ricci* curvature; the Einstein tensor, which is what appears on the LHS of the Einstein Field Equation, is the Ricci tensor with a trace term added. But the "gravitational field" of a black hole, what you observe as the "field" in the exterior region, is entirely Weyl curvature; the Ricci tensor is zero (because it's a vacuum solution of the EFE), but the Weyl tensor is nonzero. The Weyl tensor doesn't appear at all in the EFE, so it can be nonzero in regions where the SET is zero.
Now you have me really confused. From the wiki article on Weyl tensor at http://en.wikipedia.org/wiki/Weyl_tensor
"The Weyl tensor differs from the Riemann curvature tensor in that it does not convey information on how the volume of the body changes, but rather only how the shape of the body is distorted by the tidal force. The Ricci curvature, or trace component of the Riemann tensor contains precisely the information about how volumes change in the presence of tidal forces, so the Weyl tensor is the traceless component of the Riemann tensor."

That part is fine, and intuitively it's obvious that in a non-zero matter density region curvature will be different than in vacuum regions. Which has similarity to the case in EM where divergence is non-zero only where charge density is non-zero. But then:
"In general relativity, the Weyl curvature is the only part of the curvature that exists in free space a solution of the vacuum Einstein equation and it governs the propagation of gravitational radiation through regions of space devoid of matter. More generally, the Weyl curvature is the only component of curvature for Ricci-flat manifolds and always governs the characteristics of the field equations of an Einstein manifold."

What I get from that is that the exterior static field is a solution of the vacuum Einstein eq'n. So then, you are saying in effect that the latter is quite disconnected from the SET? But what then is the mass of a gravitating body other than an integration over the relevant SET for the matter region? And surely you cannot deny that for any static gravitating body, it's mass M determines the field.
Going back to the analogy with EM, while it's true divergence is zero exterior to a charged region, it's equally true the divergence-free field in that exterior region derives from the charge and nothing else! Yet it seems something totally different applies in GR. I need a clear statement: if the exterior, Weyl curvature field has no source in the SET (i.e., mass = integrated stress-energy-momnetum), what is it's source?
"All you need to be clear about is that pressure is part of the pressure energy tensor, i.e. part of the physical source of what happens in general relativity, one side of the Einstein equation. Gravity is on the other side, and it is not a source. It does not somehow walk around to the other side and become pressure, it is plain curvature in space-time."
This looks OK to me; I was saying the same sort of thing above when I talked about precisely defining the term "source".
Right but as per above I am still very unclear on what does constitute source. And I can't find it now but pretty sure Clifford Will is on record as stating that gravity is a source of further gravity. Not really arguing from authority, but it does seem there are diverging opinions in the GR community.
People can also mean different things by "self-interacts". The precise meaning of that term really requires quantum theory; in quantum gravity, gravitons can interact with other gravitons directly, i.e., there are graviton-graviton vertices in the allowed set of Feynman diagrams. By contrast, quantum electrodynamics is not self-interacting: there are no direct photon-photon vertices.
Interesting but mutual interaction suggests to me that 'gravitons gravitate', but know way too little here to press on that one.
Just to be clear, I was not saying that the quantum picture of "virtual gravitons" was wrong per se; I was only saying that it was not necessary for understanding where the observed field around a black hole comes from. The general question of the validity of the "virtual graviton picture" is, IMO, a topic for a separate thread.
Fair enough on that last point, but it has to be conceeded surely that without a 'gravitons gravitate' picture, field as vg's runs into real trouble in a BH scenario. As does EM field as vp exchange. And I'm not particularly arguing against vg's or vp's - I suspect the problem is with 'BH'.
Q-reeus: "Regardless of the source of BH gravity itself, the issue of charged BH presents a headache for GR imo."
This question has the same answer as the answer for gravity: the "source" of the observed EM field around a charged BH is the charge-current density in the collapsing matter; i.e., the observed field at any event in the exterior region is entirely determined by field propagation from charge-current density in the past light cone of that event.
That is a position statement, but as per my opening paragraph here I'm not seeing how it addresses the specific points raised in #10. If the EM field is not made somehow to be it's own source (but Maxwell's eqn's?!), how is it continuosly sustained? It just hangs there by fiat? Where does that leave the vp exchange picture popular in QFT? The only alternative I see is to posit a connection to the source charge/current. And that seems impossible given the temporal problem mentioned there.

So far this is how I see it. Your position is that both gravitational and electric fields exterior to a BH can be indefinitely sustained, based on past connection to mass/charge. But is that more than just bare postulate, albeit a standard one in GR? Without a position such as 'gravity gravitates', or an attempt to explain connection between exterior and interior 'original' source matter/charge, seems to me such fields are held up by mathematical fiat lacking any real underlying principle. My layman's prejudice perhaps.
PeterDonis
#28
Feb24-12, 08:39 AM
Physics
Sci Advisor
PF Gold
P: 6,038
Quote Quote by Q-reeus View Post
Umm - first part above is past sense 'while it was still collapsing', but ends with a present sense 'where the field is being observed.' Still not seeing how the present - exterior field, is continuously sustained by the past - collapsing matter that is now at or behind an EH.
You have to be careful using words like "present" in a BH spacetime; "present" according to whose time? But anyway, if you are thinking that the field somehow has to be "sustained" by the collapsed matter behind the EH, you are thinking of it wrong. Consider the case of an ordinary gravitating body that is *not* a BH, like the Sun. The field that the Earth experiences "now" due to the Sun is *not* "sustained" by the Sun "now"; it is determined by the way the Sun was eight minutes ago. In other words, it is determined by the way the Sun was in the Earth's past light cone.

The BH case is the same thing, except that, because the gravitating body has collapsed behind an EH, the past light cone of any event in the exterior region only includes the collapsing body *before* it fell behind the EH. That collapsing matter in the past, before it fell beneath the EH, is what corresponds to the Sun eight minutes ago for us on Earth. The collapsed matter inside the EH is irrelevant to the field observed in the exterior, just as the Sun's state "now" is irrelevant to the field we experience on Earth "now". Of course, in the case of the Sun, we will soon know what the Sun's state is "now", by observing the field eight minutes from now, whereas an observer in the exterior region will *never* know the state of the collapsing matter inside the EH, because he will never get an "update" from that region; but again, that is simply because the matter in the BH case has collapsed behind an EH, whereas the Sun has not.

Quote Quote by Q-reeus View Post
I'm presuming that is referring primarily to static fields (radiation fields or time variation of fields in general are a non-issue here). If so then seems to me the ability to 'propagate' through source-free space means nothing more than the ability to exist at all in that spatial region, otherwise there is no meaning to the term field. What I'm having trouble with is the notion, unique to GR afaik, that a static field can exist apart from it's original source (and you have I think made it clear that source in GR cannot be curved spacetime itself).
Again, you are interpreting things incorrectly if you think the static field can exist apart from its source. The source is the collapsing matter; you just have to be careful about specifying what parts of the collapsing matter's worldline act as "source" for the field in the exterior region of the BH. There is no claim, in GR or anywhere else, that the field can exist with no "sources" anywhere. (Technically, an "eternal" BH spacetime does have vacuum everywhere, but it also has a white hole singularity, which is in the past light cone of any observer in the exterior region of the BH; that singularity effectively becomes the "source" of the exterior field. However, the "eternal" BH spacetime is unphysical, and I think it's much better for this discussion to consider the physically realistic case of a BH formed by collapsing matter.)

Quote Quote by Q-reeus View Post
What I get from that is that the exterior static field is a solution of the vacuum Einstein eq'n. So then, you are saying in effect that the latter is quite disconnected from the SET?
No, I'm saying that the exterior static field will only exist in the exterior vacuum region if, somewhere in the past light cone of that region, there is a region of collapsing matter with a nonzero SET. The Weyl tensor at a given event in the exterior vacuum region is then determined by "propagation" (which may not be the best word here, but I don't know of any better one) of the field from the nonzero SET region in the past light cone of that event.

Quote Quote by Q-reeus View Post
But what then is the mass of a gravitating body other than an integration over the relevant SET for the matter region?
That's one *component* of the externally observed mass, yes. But it's not the only one; if you just naively do the integral you have described, you will get the wrong answer. For example, if you just integrate the Sun's SET over its volume you will not get the Sun's actual observed mass; you will get a number that, roughly speaking, corresponds to the Sun's actual observed mass plus its gravitational binding energy, the energy it would take to "disassemble" the Sun and move all of its parts to spatial infinity, so they were no longer gravitationally bound to each other. See, for example, this Wiki page:

http://en.wikipedia.org/wiki/Mass_in_general_relativity

(See the section on the Newtonian limit for nearly flat spacetimes.)

For a BH spacetime, you can't assume that the spacetime is "nearly flat", so the Newtonian limit doesn't apply. But the more general methods discussed in the Wiki article (ADM mass, Komar mass, etc.) still work, because they only require the spacetime to be stationary and/or asymptotically flat, and a BH spacetime meets both of those requirements. But you'll notice that none of those methods require a nonzero SET! Basically, in a stationary and/or asymptotically flat spacetime, you can come up with a workable definition of "energy stored in the gravitational field", which cannot be done in a generic spacetime without those special properties. It then turns out that a BH's mass is *entirely* composed of "energy stored in the gravitational field".

Quote Quote by Q-reeus View Post
Right but as per above I am still very unclear on what does constitute source. And I can't find it now but pretty sure Clifford Will is on record as stating that gravity is a source of further gravity. Not really arguing from authority, but it does seem there are diverging opinions in the GR community.
Generally, I would expect statements like the one you refer to from Will to be talking about the fact that gravitational waves carry energy. (The graviton-graviton interactions I talked about before are the quantum version of this.) Since they carry energy, they can also gravitate. But since gravitational waves can carry energy through a region of zero SET, figuring out *how* they gravitate is not straightforward. You end up having to work backwards to the original source of the waves, which must be a body or a system of bodies with nonzero SET that is vibrating or oscillating in some way. (For example, a binary pulsar system.)

Quote Quote by Q-reeus View Post
Interesting but mutual interaction suggests to me that 'gravitons gravitate'
They do. See just above.

Quote Quote by Q-reeus View Post
Fair enough on that last point, but it has to be conceeded surely that without a 'gravitons gravitate' picture, field as vg's runs into real trouble in a BH scenario. As does EM field as vp exchange. And I'm not particularly arguing against vg's or vp's - I suspect the problem is with 'BH'.
No, the problem is your apparent assumption that, at the quantum level, viewing a static field as due to virtual particle exchange is the only option. It isn't. But again, I think that's a topic for a separate thread.

Quote Quote by Q-reeus View Post
If the EM field is not made somehow to be it's own source (but Maxwell's eqn's?!), how is it continuosly sustained?
Same answer as for gravity above, as I said. The EM field at a given event is ultimately due to a source--a region of nonzero charge-current density--somewhere in the past light cone of that event. The field "propagates" (again, not the best word IMO but I don't have a better one) from that source to the event where the field is observed.

Quote Quote by Q-reeus View Post
Your position is that both gravitational and electric fields exterior to a BH can be indefinitely sustained, based on past connection to mass/charge.
Your use of the word "sustained" is an indication of the conceptual problem you are having. You are thinking of the field as a two-way interaction between "source" and "observer". It isn't. The field is just "propagated" in one direction--from the source in the observer's past light cone, to the observer (more precisely, to a specific event at which the observer measures the field). That can be explained entirely in terms of the model I have given. Nothing has to propagate back from the observer to the source.
Q-reeus
#29
Feb24-12, 11:28 AM
P: 1,115
Quote Quote by PeterDonis View Post
...But anyway, if you are thinking that the field somehow has to be "sustained" by the collapsed matter behind the EH, you are thinking of it wrong.
Thought I made my position clear enough in #25: "As stated in #10, the inescapable implication surely is that the field acts as it's own source. And that's what makes sense to me,..." All along have pointed out what I see as the impossibility of connection between exterior and interior regions.
Consider the case of an ordinary gravitating body that is *not* a BH, like the Sun. The field that the Earth experiences "now" due to the Sun is *not* "sustained" by the Sun "now"; it is determined by the way the Sun was eight minutes ago. In other words, it is determined by the way the Sun was in the Earth's past light cone.
Sure, but once established, there is in that situation a continuous exchange process occurring based on a QFT (charge) or string (gravity also) model. One that can't sensibly apply if source is behind a BH EH.
Q-reeus: "...What I'm having trouble with is the notion, unique to GR afaik, that a static field can exist apart from it's original source (and you have I think made it clear that source in GR cannot be curved spacetime itself)."
Again, you are interpreting things incorrectly if you think the static field can exist apart from its source.
No, as per earlier comment, I've argued the opposite (particularly in the case of charge), but believe that for consistency gravity should be part of it's own source.
Q-reeus: "But what then is the mass of a gravitating body other than an integration over the relevant SET for the matter region?"
That's one *component* of the externally observed mass, yes. But it's not the only one; if you just naively do the integral you have described, you will get the wrong answer.
Agreed - but then I was arguing there assuming a position based on certain of your statements made earlier - and I will quote you on them later here!!
For example, if you just integrate the Sun's SET over its volume you will not get the Sun's actual observed mass; you will get a number that, roughly speaking, corresponds to the Sun's actual observed mass plus its gravitational binding energy, the energy it would take to "disassemble" the Sun and move all of its parts to spatial infinity, so they were no longer gravitationally bound to each other.
Warming up, not quite there yet. But wait!:
...Basically, in a stationary and/or asymptotically flat spacetime, you can come up with a workable definition of "energy stored in the gravitational field", which cannot be done in a generic spacetime without those special properties. It then turns out that a BH's mass is *entirely* composed of "energy stored in the gravitational field".
Oh wow! This looks awfully like at least a substantial convergence of viewpoint, but it leaves me baffled. Said above I would quote you on earlier statement(s). Here they are, both from #26:
Q-reeus: "As stated in #10, the inescapable implication surely is that the field acts as it's own source." (referred specifically to a BH scenario)
No, it doesn't have to. A "gravitational field", in the sense of a nonzero Riemann curvature tensor, can propagate through source-free space (space with a stress-energy tensor of zero), just as an electromagnetic field, in the sense of a nonzero Faraday tensor, can propagate through charge-free space (space with a charge-current density 4-vector of zero). Some people might say that this propagation means "the field acts as its own source", but that usage of the term "source" is imprecise; the precise meaning of "source" is the SET in GR, or the charge-current 4-vector in EM.
Q-reeus: "All you need to be clear about is that pressure is part of the pressure energy tensor, i.e. part of the physical source of what happens in general relativity, one side of the Einstein equation. Gravity is on the other side, and it is not a source. It does not somehow walk around to the other side and become pressure, it is plain curvature in space-time." (I was quoting someone else there)
This looks OK to me; I was saying the same sort of thing above when I talked about precisely defining the term "source".
Pardon my confusion Peter, but am having difficulty reconciling those quotes with what you now say: "It then turns out that a BH's mass is *entirely* composed of "energy stored in the gravitational field"." So surely to goodness gracious this is saying 'gravity gravitates' after all?
Q-reeus: "Interesting but mutual interaction suggests to me that 'gravitons gravitate'"
They do. See just above.
Fine - but - oh well - fine.
Q-reeus: ...but it has to be conceeded surely that without a 'gravitons gravitate' picture, field as vg's runs into real trouble in a BH scenario. As does EM field as vp exchange. And I'm not particularly arguing against vg's or vp's - I suspect the problem is with 'BH'.
No, the problem is your apparent assumption that, at the quantum level, viewing a static field as due to virtual particle exchange is the only option. It isn't. But again, I think that's a topic for a separate thread.
But if one accepts that QFT is fundamentally where it's at viz a viz EM, it has to be the only option! Realize many dismiss vp's as a perturbative mathematical trick, but is there any QFT expert who can explain EM in purely classical terms? Exchange processes are afaik absolutely integral to interactions in general in any quantum theory. If that can be made to work for a BH fossil E field, this would be real news to me. We seem to agree that the 'explanation' given in the link quoted in #10 just isn't. But what is?
Q-reeus: "If the EM field is not made somehow to be it's own source (but Maxwell's eqn's?!), how is it continuously sustained?"
Same answer as for gravity above, as I said. The EM field at a given event is ultimately due to a source--a region of nonzero charge-current density--somewhere in the past light cone of that event. The field "propagates" (again, not the best word IMO but I don't have a better one) from that source to the event where the field is observed.
As per previous remark, there is imo a particularly severe problem explaining a 'fossil E field' using a vp exchange (or equivalent in QFT). Unlike with 'gravity gravitates', for sure charge is not generated by an E field, classically or in QFT. If you insist the past light cone as source does the job, say goodbye to QFT!
Your use of the word "sustained" is an indication of the conceptual problem you are having. You are thinking of the field as a two-way interaction between "source" and "observer". It isn't. The field is just "propagated" in one direction--from the source in the observer's past light cone, to the observer (more precisely, to a specific event at which the observer measures the field). That can be explained entirely in terms of the model I have given. Nothing has to propagate back from the observer to the source.
If only QFT agreed with that, but I think not. Certainly not in the case of 'static' E/B fields. Still I cautiously think we have come to a common understanding on the matter of whether a gravitational field can be a further source of gravity. Yes?
PeterDonis
#30
Feb24-12, 01:35 PM
Physics
Sci Advisor
PF Gold
P: 6,038
Quote Quote by Q-reeus View Post
"As stated in #10, the inescapable implication surely is that the field acts as it's own source.
No, as I have said several times already, this apparent "implication" is wrong, and I've explained why: the "source" is in the past light cone of the event where the "field" is being measured. See below.

Quote Quote by Q-reeus View Post
Sure, but once established, there is in that situation a continuous exchange process occurring based on a QFT (charge) or string (gravity also) model.
And I have said that this apparent "continuous exchange" is also wrong. There is no continuous exchange; more precisely, no "continuous exchange" model is required to explain the observed field. Models of virtual particle exchange may be useful, but that doesn't mean they are required; you can't take what they appear to be telling you about "reality" as absolute truth. See further comments below.

Quote Quote by Q-reeus View Post
No, as per earlier comment, I've argued the opposite (particularly in the case of charge), but believe that for consistency gravity should be part of it's own source.
Not if "source" is defined in the precise way I defined it.

Quote Quote by Q-reeus View Post
Pardon my confusion Peter, but am having difficulty reconciling those quotes with what you now say: "It then turns out that a BH's mass is *entirely* composed of "energy stored in the gravitational field"." So surely to goodness gracious this is saying 'gravity gravitates' after all?
No, at least not the way you mean it. I did warn you about trying to reason about these things, and draw conclusions from statements about them, using English instead of math. However, I can point out one thing in English that may help to resolve your confusion: even if energy is "stored in the gravitational field", it still needs to have got there from somewhere. The "mass" of a BH ultimately comes from the stress-energy of the object that collapsed to form it; but once the BH is formed, its mass can be viewed as being "stored in the gravitational field" that is left behind once the collapsing object has hit the singularity at r = 0 and vanished.

But even the above English version has problems (one is that I have implicitly assumed a notion of time in several places in the above, without saying what notion of time it is). Ultimately, if you want to be sure you are understanding things correctly, you *have* to look at the math. The best summation of the math that I can give in English is this:

(1) The observed "field" at any given event is entirely determined, ultimately, by what "sources" are in the past light cone of that event;

(2) A "source", as used in #1 above, is a region with a non-zero stress-energy tensor.

If you find yourself asking questions, in English, that don't seem to be addressed by the above, then that's probably a sign that English is leading you astray. "Does gravity gravitate?" is, IMO, one such question; see further comments below.

Quote Quote by Q-reeus View Post
But if one accepts that QFT is fundamentally where it's at viz a viz EM, it has to be the only option!
You are assuming that "QFT is the only option" is equivalent to "virtual particles are the only option". That's not the case. As I keep saying, this should be a separate thread, but briefly, "virtual particles" are an aspect of one particular way of treating certain problems in QFT, based on perturbation theory, using approximations appropriate for those types of problems; they are not "fundamental" parts of QFT that must be accounted for in an explanation of quantum "reality".

Quote Quote by Q-reeus View Post
We seem to agree that the 'explanation' given in the link quoted in #10 just isn't. But what is?
As I said before, classically, the EM field of a charged BH is explained the same way as the gravity field: at any given event, the EM field observed at that event is ultimately due to the presence of EM sources in the past light cone of that event. An EM "source" here is a region with a nonzero charge-current density 4-vector.

The plain answer to how this picture is expanded to account for quantum phenomena is that nobody knows for sure. We don't have a theory of quantum gravity. We do know how to do QFT in a curved spacetime, where the spacetime itself is treated classically (but possibly with quantum "back reaction" terms added to the stress-energy tensor); in that picture, causality still holds and the explanation I gave above of fields being determined by sources in the past light cone, is still essentially correct (both for gravity itself and for the EM field of a charged BH). The only thing that QFT in curved spacetime really adds to the classical picture at this level is Hawking radiation: black holes can slowly evaporate. But that doesn't change anything I said above.

My personal opinion is that, even when we do have a good theory of quantum gravity, its classical limit will still be GR, so it will still look like the picture I described above. The only way a complete quantum gravity theory could change that picture, IMO, would be if we somehow discovered that BH's can't exist at all: that there is some large quantum correction to the classical behavior that prevents event horizons from ever forming in the first place. But that would take away the problem we are discussing here altogether.

Quote Quote by Q-reeus View Post
If only QFT agreed with that, but I think not. Certainly not in the case of 'static' E/B fields.
No; see above. QFT, at least as we have developed it so far, does not contradict the picture of fields being determined by sources in the past light cone. See my comments above about virtual particles; virtual particle exchange is not the only way to think about static fields in a quantum context, and in some cases (such as this one), the virtual particle view can be misleading.

Quote Quote by Q-reeus View Post
Still I cautiously think we have come to a common understanding on the matter of whether a gravitational field can be a further source of gravity. Yes?
It depends on what you mean. See my caution above about trying to use English to describe this stuff instead of math. I don't think the question "does gravity gravitate?" is a good one to ask, because it's not a good translation of the math into English in the first place. For the precise definition of the term "source", which I gave above, the plain answer to the question above as you asked it is "no": a "gravitational field" by itself can exist in a region with a zero stress-energy tensor, so it can't be a "source".
Q-reeus
#31
Feb25-12, 04:53 AM
P: 1,115
Peter; on the matter of BH E field source. You at least have a consistent position, in the sense of not changing position from one entry to another. I fundamentally disagree with your viewpoint so there can be no headway and best we drop that part here. I have something in mind for a new thread attacking it all differently, but later. Now on the issue of whether gravity is a source of further gravity, I cannot see a consistent position, even though in any given response you come across as presenting one, when I check against other statements I'm getting a mixed picture. On the one hand, there is this from #28:
1:
Basically, in a stationary and/or asymptotically flat spacetime, you can come up with a workable definition of "energy stored in the gravitational field", which cannot be done in a generic spacetime without those special properties. It then turns out that a BH's mass is *entirely* composed of "energy stored in the gravitational field".
2:
Q-reeus: "Right but as per above I am still very unclear on what does constitute source. And I can't find it now but pretty sure Clifford Will is on record as stating that gravity is a source of further gravity. Not really arguing from authority, but it does seem there are diverging opinions in the GR community."
Generally, I would expect statements like the one you refer to from Will to be talking about the fact that gravitational waves carry energy. (The graviton-graviton interactions I talked about before are the quantum version of this.) Since they carry energy, they can also gravitate.
3:
Q-reeus: "Interesting but mutual interaction suggests to me that 'gravitons gravitate'"
They do. See just above.
Sure seems crystal clear that 1,2,3 here all say in essence the same thing. That a gravitational field, whether static ('virtual gravitons'), or radiative ('real gravitons'), carries energy, and *therefore* gravitates (acts as a source of further gravity). Plain english perfectly adequate at this level. And 1: is specific - "a BH's mass is *entirely* composed of "energy stored in the gravitational field"." A plain english statement that the field must here entirely be it's own source. And yet you will probably say no!
But then this from #30:
Q-reeus: "As stated in #10, the inescapable implication surely is that the field acts as it's own source."
No, as I have said several times already, this apparent "implication" is wrong, and I've explained why: the "source" is in the past light cone of the event where the "field" is being measured.
My own plain english attempt to sensibly synthesize the above would be to say that 'the "source" is in the past light cone' has to be *synonymous* with "a BH's mass is *entirely* composed of "energy stored in the gravitational field"." And I note; that field is all exterior to the EH and accessible in the here and 'now'. And as far as your repeated comments that expressing this in english is leading me astray, i would respond that plain english staements regarding conceptual basis take precedence everytime over just learnig a mathematical framework that may have a suspect conceptual basis.

If I didn't know better, could swear you're out to drive me insane. Saw both movie versions of Nineteen Eighty Four a while ago. Recommended viewing. In the end, O'brien the interrogator breaks poor old Winston Smith, who, with tears of joy, truly believes that when four fingers are held up to his face, there are really five. I have no tears of joy, just a frown. A little melodramatic, but what I'm saying is, please explain what seems to me are irreconcilable statements.
DaleSpam
#32
Feb25-12, 06:26 AM
Mentor
P: 16,950
Quote Quote by Q-reeus View Post
Quote Quote by DaleSpam View Post
Regarding the rest, the fact that you are aware of the previous responses on this topic and consider them all unsatisfactory seems to indicate that further discussion will probably lead to the same result.
Only active and willing participation this time round can decide that. I'm certainly open to serious feedback.
The preliminary data doesn't contradict my hypothesis.
Q-reeus
#33
Feb25-12, 06:37 AM
P: 1,115
Quote Quote by DaleSpam View Post
The preliminary data doesn't contradict my hypothesis.
Possibly, but it's not over yet. If you have some personal insights feel free to share. And just for your quip, I'm inspired to remind you of a long outstanding committment: http://physicsforums.com/showpost.ph...5&postcount=42
Prepared to make good on that one - like 'soon'?
DaleSpam
#34
Feb25-12, 06:56 AM
Mentor
P: 16,950
Quote Quote by Q-reeus View Post
Possibly, but it's not over yet. If you have some personal insights feel free to share.
Correct, it is not over yet, which is why I characterized the data as "preliminary". Nevertheless, the preliminary data suggests that sharing my personal insights will not make any difference. I will continue to monitor the data and, should my hypothesis be falsified, I would be willing to make the effort in the future.

Quote Quote by Q-reeus View Post
I'm inspired to remind you of a long outstanding committment: http://physicsforums.com/showpost.ph...5&postcount=42
Prepared to make good on that one - like 'soon'?
Yeah, you are justified in that, I definitely overpromised. I worked on it a few days, got stuck, and dropped it. The difficulty was that the matter distribution involves some discontinuities, which makes things messy both analytically and numerically.
Q-reeus
#35
Feb25-12, 07:05 AM
P: 1,115
Quote Quote by DaleSpam View Post
Correct, it is not over yet, which is why I characterized the data as "preliminary". Nevertheless, the preliminary data suggests that sharing my personal insights will not make any difference. I will continue to monitor the data and, should my hypothesis be falsified, I would be willing to make the effort in the future.
Splendidly diplomatic. Consider an overseas embassy posting.
Yeah, you are justified in that, I definitely overpromised. I worked on it a few days, got stuck, and dropped it. The difficulty was that the matter distribution involves some discontinuities, which makes things messy both analytically and numerically.
OK then won't press any further, at least this squares the ledger in a way.
DaleSpam
#36
Feb25-12, 07:20 AM
Mentor
P: 16,950
Quote Quote by Q-reeus View Post
Splendidly diplomatic. Consider an overseas embassy posting.
Thanks!

Quote Quote by Q-reeus View Post
OK then won't press any further, at least this squares the ledger in a way.
I put a similar comment in the original thread also, so it would be easier to find.


Register to reply

Related Discussions
Relativistic mass increase appearing to form a black hole vs. a real black hole Special & General Relativity 9
Time dilation and black-hole-black-hole mergers, and ringdown gravitational waves Special & General Relativity 13
Is black hole existence relative to the reference system? Special & General Relativity 8
Relative to a black hole... Special & General Relativity 12
V relative to black hole Astronomy & Astrophysics 2