Is the event horizon of a black hole physical?

[phinds]

phinds:


and yet the cosmological horizon is essential for particle production during the inflationary epic of the universe.

So once again, it HAS physical effects...which does not necessarily make it 'physical'...however, I'm still unsure what 'physical' means...

.

You lost me on that one. I thought the cosmological horizon was simple the radius from wherever you happen to be standing out to the place where light emitted from particles can no longer reach you. What does that have to do with particle production in the early universe ?

 

[Mordred]

I guess in both cases if its merely a description dependant upon set conditions.
Much like a border. Then even though the region is physical. The descriptive is not.

In the cosmological horizon that horizon is veiw point dependant. So saying it and of itself as physical is kind of stretching the term physical. The exact location definitely is but one cosmological horizon is not the same for someone in another galaxy for example. I suppose the same could be said if the event horizon.

Lol just goes to show the trickiness of terminology.

 

[Chalnoth]

You lost me on that one. I thought the cosmological horizon was simple the radius from wherever you happen to be standing out to the place where light emitted from particles can no longer reach you. What does that have to do with particle production in the early universe ?

In an accelerating universe, you get Hawking radiation at this horizon for the exact same reason that you get Hawking radiation from the horizon of a black hole (and again, it's proportional to the area of the horizon).

 

[Mordred]

I've never heard of that, could you provide a reference. I' d definitely interested in reading it

 

[phinds]

In an accelerating universe, you get Hawking radiation at this horizon for the exact same reason that you get Hawking radiation from the horizon of a black hole (and again, it's proportional to the area of the horizon).

But no matter WHERE a virtual particle pair is created, it is on such a horizon from some point, therefore according that logic, no virtual particles should EVER recombine, which is clearly nonsense. What am I missing?

 

[Chalnoth]

I've never heard of that, could you provide a reference. I' d definitely interested in reading it

Here's one:
http://arxiv.org/abs/1007.4044

 

[Mordred]

Many thanks mate. Took a quick glance at it. Its going to take me a bit to study. At first glance its definitely worth studying.

 

[Naty1]

What does that have to do with particle production in the early universe ?

Ah, now I see why you express some skepticism about horizons...they are not so simple!

All horizons I know of induce real particle production...where quantum fluctuations/perturbations [ 'virtual particles'] become 'real', that is 'physical',detectable.
These result in the Unruh effect, Hawking radiation, and particle production during inflation. The number of particles in a region is not well defined. Changing geometry induces perturbations which result in particles! Just like early inflation. While each of these is based on different geometry, Schwarzschild, Rindler, de Sitter,etc, they share some really interesting features.

I've even seen a research paper that says the Hubble sphere can induce particles. An idea along these lines is if the proper separation distance between virtual particles expands faster than the proper path distance between them, the particle pair will not annihilate.

String theory suggests that it may be the configuration of higher dimensional spaces that influences string [particle] properties...and their creation...so when spacetime jiggles around or morphs from one region to another [expansion inflation] it seems plausible that our perception/detection of particles might also change...because they change as a result of geometry changing.

Rovelli says it this way:

..uniquely-defined particle states do not exist in general, in QFT on a curved spacetime. ... in general, particle states are difficult to define in a background-independent quantum theory of gravity...

Discussion here...

https://www.physicsforums.com/showthread.php?t=590798&page=2

Particle creation in an accelerating Universe?

bapowell

...these are vacuum fluctuations -- virtual particles. How do you suppose they become real? Now, particle production via changing gravitational fields and expansion is a real phenomenon, and might be relevant to the origin of matter.

Quantum fluctuations in the inflationary vacuum become quanta [particles]
at super horizon scales.Research paper here:
Stimulated creation of quanta during inflation and the observable universe
http://arxiv.org/abs/1106.4240Ivan Agullo, Leonard Parker
(Submitted on 21 Jun 2011)

Inflation provides a natural mechanism to account for the origin of cosmic structures. The generation of primordial inhomogeneities during inflation can be understood via the spontaneous creation of quanta from the vacuum. We show that when the corresponding stimulated creation of quanta is considered, the characteristics of the state of the universe at the onset of inflation are not diluted by the inflationary

expansion and can be imprinted in the spectrum of primordial inhomogeneities.

edit: I see Chalnoth answered...although I suspect he meant Hawking radiation is inversely proportional to the horizon area...anyway, the TEMPERATURE is inversely proportional to the area explaining why a small black hole, for example, is hotter than a large one.

 

[Naty1]

phinds,

if you are more mathematically inclined,
see post #17 in the "Particle creation in an accelerating Universe?"

discussion...BrianPowell does a quick summary..

just don't ask ME questions about the math [LOL].

 

[phinds]

... An idea along these lines is if the proper separation distance between virtual particles expands faster than the proper path distance between them, the particle pair will not annihilate. ...

Yes, THAT I can see but I cannot see how that has anything to do with our cosmological horizon, and in fact I cannot see how the above could occur at all except in the final stages of a "big rip" scenario.

AGAIN, I say, and would like to see a response to, that ALL virtual particles are at SOMEBODY'S cosmological horizon, so if they fail to annihilate at a cosomological horizon, they ALL fail to annihalate (I am not conflating this with your sentence above, which is a different situation).

EDIT: by the way, I'm beginning to sound a bit argumentative here, but that's just my inherently nasty personality. I'm acutally just trying to learn what's going on.

 

[Chalnoth]

Yes, THAT I can see but I cannot see how that has anything to do with our cosmological horizon, and in fact I cannot see how the above could occur at all except in the final stages of a "big rip" scenario.

Because the cosmological horizon for an accelerated expanding universe is an event horizon, it produces Hawking radiation.

 

[Naty1]

phinds:

...that ALL virtual particles are at SOMEBODY'S cosmological horizon,

I know what you are getting at...and I have my own list of questions, for example, if cosmological horizons are changing size today, are particles popping out?

Anyway, the first paradigm you'll likely have to forgo is that everybody observes the same particles...they don't according the Unruh effect, for example. Just like time and space are relative: different observers make different observations. A free falling observer passes the horizon of a black hole in normal local time; yet an observer trying to hover just outside the horizon is blasted by ionizing radiation while they experience the passage of time differently. so not much is what it might appear.

I can offer some possible partial suggested answers, intuitive hints, rather than based on the mathematics. Perhaps they will antagonize an expert into lashing out and we might get a precise answer!

For example, since temperature is inversely proportion to the horizon area, stuff like the Unruh effect is too small to measure. Not much radiation results. And cosmological sized horizons in today's environment would likely be even 'colder'. [Smaller black hole Hawking radiation might have enough to heat to be detected.] You might find the discussion below of interest...you'll find lots more such clues...and much uncertainty.

What is a particle??
https://www.physicsforums.com/showthread.php?t=386051

Perhaps sounds a bit silly, but when one starts thinking about it, the answer is not simple.

edit: another analogy: if you are familiar with the idea that an electron changes size when confined, say in an orbital versus, say in an orbital in a lattice, you get the idea of 'confinement'...like binding the ends of a vibrating string limits natural modes ...bringing discrete quanta into existence where non appeared before...I think horizons may act similarly...[maybe that will REALLY antagonize someone who knows what is happening [LOL]]...

 

[phinds]

Anyway, the first paradigm you'll likely have to forgo is that everybody observes the same particles...they don't according the Unruh effect, for example. Just like time and space are relative: different observers make different observations.

Yes, but I don't get what that has to do with what has now become my fundamental question based on responses so far.

I appreciate the effort being put into helping me out with this but I still am left feeling that I have not gotten ANY answer to my fundamental statement/question:

EVERY particle pair is at SOMEBODY'S cosmological horizon ... etc (as previously stated)

 

[Chalnoth]

Yes, but I don't get what that has to do with what has now become my fundamental question based on responses so far.

I appreciate the effort being put into helping me out with this but I still am left feeling that I have not gotten ANY answer to my fundamental statement/question:

EVERY particle pair is at SOMEBODY'S cosmological horizon ... etc (as previously stated)

This is the exact same issue as that with the Unruh effect.

 

[phinds]

OK, that to me comes across as one of those things that translate very badly from math to English and I don't get the math so it just sounds like nonsense to me.

Sounds like you, and/or the Unruh effect, are saying that every vitrual paritcle-pair both does and does not annihilate.

 

[Chalnoth]

OK, that to me comes across as one of those things that translate very badly from math to English and I don't get the math so it just sounds like nonsense to me.

Sounds like you, and/or the Unruh effect, are saying that every vitrual paritcle-pair both does and does not annihilate.

Rather it's saying that the existence of a particle is, in some cases, a matter of perspective. Also, my understanding is that the virtual particle/anti-particle pair description of Hawking radiation is more a heuristic device to get the idea across rather than an accurate description of what's going on.

 

[Mordred]

After reading the articles everyone posted. I can see what is being stated. I' m not sure I fully agree. However that's more likely due to my limitted understanding of The Unrah effect.

 

[Naty1]

Chalnoth:

Also, my understanding is that the virtual particle/anti-particle pair description of Hawking radiation is more a heuristic device to get the idea across rather than an accurate description of what's going on.

yes: I saw somewhere where Hawking himself wrote that...as intuitive explanation...and he said it was not as a direct representation of his math.phinds:

...you are saying every vitrual paritcle-pair both does and does not annihilate.

I don't think so...I believe what is being explained is that everyone makes slightly different observations...like the analogies to time,distance,simultaneity in my prior post.

I'm frankly amazed we can even detect 'physical' particles what with all the [general relativity] 'activity' going on behind the scenes, [LOL] and that's even before one gets to changes in quantum state...check this out, particles can be evanescent:

You can briefly check out creation and annihilation operators in quantum mechanics,

In the context of the quantum harmonic oscillator, we reinterpret the ladder operators as creation and annihilation operators, adding or subtracting fixed quanta of energy to the oscillator system.

or if like me, you like illustrations better than math, try the ones here:

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

Also note the existence of zero point energy IS the vacuum energy of particle production.

... Because of the zero-point energy, the position and momentum of the oscillator in the ground state are not fixed (as they would be in a classical oscillator), but have a small range of variance, in accordance with the Heisenberg uncertainty principle. The zero-point energy also has important implications in quantum field theory and quantum gravity.

The way I picture this is probably found in the thread links I already posted:

There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields

so the energy for particles, 'quanta', is all around us globally in the 'vacuum'...what does it take to localize it...turn it to detectable quanta??

which means I am repeating myself, you seem to be repeating yourself, so I'll cease now...

 

[lukesfn]

AGAIN, I say, and would like to see a response to, that ALL virtual particles are at SOMEBODY'S cosmological horizon, so if they fail to annihilate at a cosomological horizon, they ALL fail to annihalate (I am not conflating this with your sentence above, which is a different situation).

I don't know anything, but perhaps, in the case of most pair productions, one particle will pass through the horizon just before the other then recombine afterwards, but this obviously would not be seen from out side the horizon. Perhaps a very small number of pairs become real, and one escapes the horizon, but in this case the particles would not recombine in any reference frame.

Maybe it is the same at a black hole horizon. Most times when a pair is created near the horizon, both fall in, it is just that occasionally one escapes.

Perhaps you are in error to assume that all virtual particles must not be able to annihilate to cause the observed radiation, may be it is only a small fraction.

 

[phinds]

Perhaps you are in error to assume that all virtual particles must not be able to annihilate to cause the observed radiation, may be it is only a small fraction.

So what you are saying is that a small fraction of all virtual particle pairs fail to annihilate only because there happens to be a frame of reference from which they are at the cosmological horizon! Why would that be? It still makes absolutely no sense to me.

 

[Mordred]

I agree with you Phinds. I can find no reason why the cosmological horizon would have any of the following, Hawking radiation, blackbody radiation or Unruh effect. The cosmological horizon is an observational limit of what we can see and measure. The only possibility of how it would have any of the previous is that we would never be able to measure beyond that point. In that case I can see one particle going beyond that limit, the other staying where we can measure. In that case the models could describe it as one of the aforementioned. However that is the only possibility I can see. In the case of a BH event horizon, that event horizon exists for a different reason in that outside the event horizon particles and light can still escape. I've looked for papers that described any of the mentioned radiations and have yet to find one that justified the cosmological horizon portion of the articles.

 

[phinds]

I agree with you Phinds. I can find no reason why the cosmological horizon would have any of the following, Hawking radiation, blackbody radiation or Unruh effect. The cosmological horizon is an observational limit of what we can see and measure. The only possibility of how it would have any of the previous is that we would never be able to measure beyond that point. In that case I can see one particle going beyond that limit, the other staying where we can measure. In that case the models could describe it as one of the aforementioned. However that is the only possibility I can see. In the case of a BH event horizon, that event horizon exists for a different reason in that outside the event horizon particles and light can still escape. I've looked for papers that described any of the mentioned radiations and have yet to find one that justified the cosmological horizon portion of the articles.

Thanks. I'm glad to see at least someone agrees w/ me. (and of course I agree about the black hole EH, which IS a different thing)

 

[Naty1]

Lukesfn posts:

Perhaps a very small number of pairs become real, and one escapes the horizon, but in this case the particles would not recombine in any reference frame.

phinds posts:

Sounds like you, and/or the Unruh effect, are saying that every vitrual paritcle-pair both does and does not annihilate.

This is pretty much one of the aspects I already posted...with multiple perspectives...here is another: Everyone has a different causal history, a different past lightcone...

By coincidence I was reading this:

[via an illustrious set of theorists!]

Holography in a quantum spacetime
Fotini Markopoulou and Lee Smolin
http://arxiv.org/PS_cache/hep-th/pdf/9910/9910146v1.pdf

Synopsis... the set theory and subsequent math is WAY over my head, but
the the interpretations intriguing:

[For reasons I don't understand all this involves FINITE causal sets...information is limited right from the foundational start.]

...One important lesson, discovered first by Fischler and Susskind[3] and developed by Bousso[4], is that in a cosmological spacetime the holographic principle must be formulated in a way that employs the lightcones of the spacetime. .. we need a framework for a cosmological quantum theory which incorporates causal structure. .. As each observer receives information from a distinct past, the algebra of observables they can measure, and hence the (finite-dimensional) Hilbert spaces on which what they observe can be represented, vary over the history. Consequently, the algebra of observables of the theory is represented on a collection of Hilbert spaces. These replace the single wavefunction and single Hilbert space of other approaches to quantum cosmology… Quantum causal histories were originally motivated by the need to provide a general framework to understand what observables are in background independent approaches to quantum gravity, such as those proposed in [16, 18, 19, 20]. In these theories a causal quantum spacetime is constructed from local changes in a spin network or a network of abstract surfaces...

Did you notice the last sentence SPIN network! That is the basis for Marcus' interest in TOMITA TIME in another thread...this AGAIN leads me to suspect that because no universal time in GR or cosmology has yet been achieved, there is no universal past history.

...A key feature of the weak holographic principle# is that a complete description of the universe requires more than one screen [horizon]. This is simply because in a generic cosmological history there is no single screen whose past is the entire universe. Thus, a cosmological holographic theory must be a many-screens theory, each screen recording information about its causal past...we expect that no cosmological form of the holographic principle, even one that holds in the semiclassical limit, can escape the fact that many screens are necessary to give a complete description of a cosmological spacetime.,,,

#: a 'weak holographic principle' is the subject of this paper and is background independent, like GR and cosmology, which means it affords a dynamic rather than fixed space-time background.

 

[Mordred]

This article may shed some light on the Unruh effect, page 22 describes a similar scenario that they referred to as the Rindler wedge

http://www-gat.univ-lille1.fr/~debievre/Talks/unruhrolla09A.pdf

EXPLANATION: the quanta are Rindler quanta. Fulling proved (1973) that the
notion of vacuum and hence of particle or better, quantum, is observer-dependent
and notably that on the Rindler wedge the vacuum with respect to the Rindler time is
not identical to the Minkowski vacuum, associated with inertial observers

this PF thread seems to cover the Unruh topic best.

https://www.physicsforums.com/showthread.php?t=574548

 

[lukesfn]

So what you are saying is that a small fraction of all virtual particle pairs fail to annihilate only because there happens to be a frame of reference from which they are at the cosmological horizon! Why would that be? It still makes absolutely no sense to me.

The point I was trying to make was that the horizon may not the the cause of the fail to annihilate. Maybe it is just the fact that space is stretching in the case of a cosmic horizon, or perhaps it is a purely spontaneous process of some virtual pairs becoming real by chance collisions. This creates a radiation at the horizon, but it would not be caused by the horizon, it would just be an apparent effect. (Not that I know anything though, I am curious about how this is meant to work my self)

 

[Mordred]

Thats one part I had considered. Spacetime expansion as far as I understand occurs everywhere that is not graviationally bound. If the cause is spacetime expansion itself then
then one would think that failure to annihilate would occur in any cosmological vacuum state. Seems to me I read something to that effect when I was looking over the universe from nothing model supported by Lawrence Krauss.
Following the mathematics of these articles is not a skill I possesses lol

 

[Chronos]

The 'location' of a black hole's event horizon is a frame dependent measurement [i.e., relative]. What a remote observer perceives is not the same as that of an infalling observer, and reconciling the two perceptions is no trivial matter. On this basis it could be argued the event horizon is purely subjective, lacking any objective sense of 'physicality'. For discussion, see http://www-e.uni-magdeburg.de/mertens/teaching/seminar/themen/touching_ghosts.pdf

 

[PAllen]

The 'location' of a black hole's event horizon is a frame dependent measurement [i.e., relative]. What a remote observer perceives is not the same as that of an infalling observer, and reconciling the two perceptions is no trivial matter. On this basis it could be argued the event horizon is purely subjective, lacking any objective sense of 'physicality'. For discussion, see http://www-e.uni-magdeburg.de/mertens/teaching/seminar/themen/touching_ghosts.pdf

Hmm. The technical definition of true event horizon makes it a feature of global geometry, completely independent of coordinates: the boundary separating null geodesics that reach null infinity from those that don't. Apparent horizons (local trapping surfaces) are coordinate dependent.

Certainly, classically, there is nothing to see at an event horizon. If you have two nearby, comoving, radial infallers, they see nothing distinguishing at the event horizon; including no discontinuity in appearance of the 'outside', and both can still send signals to each other (until one hits the singularity).

 

[Mordred]

Good article definitely changed my perspective on how the event horizon alters perspectives. Also makes me relook over Unruh effect

 

[Naty1]

Chronos posts:

The 'location' of a black hole's event horizon is a frame dependent measurement [i.e., relative].

PAllen:

The technical definition of true event horizon makes it a feature of global geometry, completely independent of coordinates: the boundary separating null geodesics that reach null infinity from those that don't. Apparent horizons (local trapping surfaces) are coordinate dependent.

Maybe you two can comment on this explanation:

My understanding from reading over time in these forums what you experts are trying to explain is that both posts are correct. That 'time stops' [acceleration 'g' grows unbounded] at an absolute horizon from the perspective of a static distant observer is an idealization: an observer at infinity. So we CAN see stuff fall into a black hole, say in another galaxy, from a finite distance in a finite time.

That a static local observer would experience an unbounded gravitational force g at the [absolute] horizon requires she be hovering there for all time while a free falling observer experiences no such 'singularity'.

What I am unsure about is that I think the space-times in our models are static... but when actual matter or energy or detection device with mass passes the event horizon the space-time actually becomes dynamic. It is not clear to me how that model static space-time idealization may affect the descriptions I gave. In other words, strictly speaking the models only apply to a "test object" [observer] falling into the hole...no effect on the space-time.

In any case, for me so far, most physics is about local phenomena in finite time.

edit: Maybe what I should have asked is how hovering or falling towards a black hole horizon is any different than hovering or falling toward some other mass, like a star, dense planet,etc...