Is the event horizon of a black hole physical?

[phinds]

In another thread, which I don't want to derail, the issue came up as to whether or not the event horizon of a black hole is physical.

Some contend that it is physical but I contend that it is merely a set of coordinates (most easily represented by the spherical coordinate R).

I DO recognize that any photon emitted exactly at the EH, and trying to propagate away from the singularity, is just going to stay right there at the EH, but somehow I just don't "get" that that makes the EH physical.

One argument that I found particularly unacceptable was that just because you can say whether or not a coordinate-specified point is on or off of the EH, THAT makes the EH physical. I see that as nonsense. You could equally well say that any point exactly 50,000 miles from the center of the Earth is automatically a physical point BECAUSE it is exactly 50,000 miles from the center of the Earth. Using this definition there is no such thing as a set of coordinates that do not represent a physical place, and I just can't see that.

Comments appreciated.

 

[Naty1]

too late [LOL] did you see my last post...

irrelevent, I think, if you consider the horizon itself physical...they have physical effects...

I think you'll have to define physical to get a good answer.

a horizon is a global construct...has no local significance...but it DOES have physical effects...

One argument that I found particularly unacceptable was that just because you can say whether or not a coordinate-specified point is on or off of the EH, THAT makes the EH physical.

I agree...

Is the orbit of the Earth physical?? How about a light cone? A cosmological event horizon?

 

[phinds]

too late [LOL] did you see my last post...

irrelevent, I think, if you consider the horizon itself physical...they have physical effects...

I think you'll have to define physical to get a good answer.

a horizon is a global construct...has no local significance...but it DOES have physical effects...



I agree...

Is the orbit of the Earth physical?? How about a light cone? A cosmological event horizon?

Yeah, the physical effects I agree with, and yeah it DOES depend on how you define "physical". I think of physical as something I can touch, but that may be a bit sloppy and naive.

 

[Mordred]

Look at these three expressions.

1) Physical universe
2) Virtual universe
3) Imaginary universe

the first expression would descibe something you can see and touch, the second expression you can see but not touch the third you can neither see or touch. For purpose of EH replace touch with interact with if you were close enough

Is it incorrect to describe a point in spacetime as a physical point in space-time?

 

[Nabeshin]

I think of physical as something I can touch, but that may be a bit sloppy and naive.

This is sensible. The EH is of course not a physical surface in this sense: I notice nothing out of the ordinary when I cross it (classically). Rather, it's a mathematical boundary.

I think the case is even stronger if you think that the EH knows about the entire causal future of the spacetime -- that's simply how it's defined. Any physical surface or object should be completely specifiable in terms of the past, not future, of the spacetime. So the EH in that sense is also not a physical surface.

 

[Chronos]

Event horizons are not 'structures' in any sense of the word, they fluctuate continuously due to mass accretion and quantum effects.

 

[phinds]

...
Is it incorrect to describe a point in spacetime as a physical point in space-time?

Yes, I think it is incorrect. It is a coordinate, not something physical. If there is something material AT the coordinate, then that thing is of course physical, but its location is not physical.

 

[phinds]

This is sensible. The EH is of course not a physical surface in this sense: I notice nothing out of the ordinary when I cross it (classically). Rather, it's a mathematical boundary.

I think the case is even stronger if you think that the EH knows about the entire causal future of the spacetime -- that's simply how it's defined. Any physical surface or object should be completely specifiable in terms of the past, not future, of the spacetime. So the EH in that sense is also not a physical surface.

Bolded point is very interesting and I had not thought of it that way. Thanks.

 

[phinds]

Event horizons are not 'structures' in any sense of the word, they fluctuate continuously due to mass accretion and quantum effects.

Good point. Thanks.

 

[phinds]

So, basically, what I think I'm getting is that you have to have a pretty contorted defintion of "physical" to in any way think of the event horizon as physical.

 

[Mordred]

As you pointed out earlier it depends on how you describe physical.

Physical as defined in my webster copy

a) Of or relating to the body as distinquished from the mind or spirit.

b) Involving or characterized ny vigorous bodily activity.

c) Of or relating to matter or energy.

The event horizon is definitely about energy and matter. If were close enough we can interact with it.

The event horizon can also be descibes by its influences on the physical.

If you think about it even virtual particles are also physical however short lived as they involve energy

 

[Chalnoth]

Well, I suppose if there is a firewall there, then it is indeed physical:
http://quantumfrontiers.com/2012/12/03/is-alice-burning-the-black-hole-firewall-controversy/

 

[Naty1]

phinds:

Here is a description of a horizon from THE BLACK HOLE WAR by Leonard Susskind...
he is describing here work begun by T'Hooft and extended by himself

[described here: http://en.wikipedia.org/wiki/Holographic_principle]

...The picture of a black hole horizon that was emerging was a tangle of string flattended out onto the horizon by gravity...quantum fluctuations would cause...parts of the string to stick out..and these bits would be the mysterious horizon atoms,,

and at a talk Susskind gave at Princeton and Rutgers Universities: "where you could not get away with half baked claims"...

...Witten's response was to accept...the proposition that a black hole horizon is composed of bitsofstring. He even worked out how strings evaporate in a manner similar to black hole evaporation.

edit: this happened in 1993.

 

[Drakkith]

The event horizon is definitely about energy and matter. If were close enough we can interact with it.

I disagree. The EH is simply the point where spacetime curvature reaches a certain amount. That in itself would not be a physical effect to me. And you cannot interact with the EH as far as I know. What could you do to it? You can't touch it, see it, etc. An infalling observer would never know when they passed it.

The event horizon can also be descibes by its influences on the physical.

I'd say that it's not the event horizon that's doing anything, its the mass behind it.

If you think about it even virtual particles are also physical however short lived as they involve energy

I think this goes beyond the usual meaning of physical.

 

[Naty1]

I'd say that it's not the event horizon that's doing anything, its the mass behind it.

There is no 'mass behind it'...
the horizon is a causal boundary.
it's the ultimate 'roach motel' "You can get in but you can't get out."

 

[bcrowell]

The EH is simply the point where spacetime curvature reaches a certain amount.

Not true.

 

[Drakkith]

There is no 'mass behind it'...
the horizon is a causal boundary.
it's the ultimate 'roach motel' "You can get in but you can't get out."

What?

 

[phinds]

There is no 'mass behind it'...
the horizon is a causal boundary.
it's the ultimate 'roach motel' "You can get in but you can't get out."

I agree w/ all that, but feel that none of that makes it physical in any sense.

I am familiar w/ the holographic principle and have banged my head against it a couple of times coming away with the thought that while I don't believe in such a thing at the event horizon of a black hole, I certainly can't refute it, BUT ... when you apply it to the cosomological horizon, which CERTAINLY is not physical in any way, it just falls to pieces for me and I just see it as nonsense.

 

[phinds]

What?

I think the "no mass behind it" mean DIRECTLY behind it, not that there isn't mass at the singularity.

 

[Drakkith]

Not true.

Elaborate please.

 

[Grimstone]

...
irrelevent, I think, if you consider the horizon itself physical...they have physical effects...

Just because there is a physical effect on something does not make the Eh physical.

Just as if you was to float in space and fall (drawn) into the sun. at some point the heat will ignite you, burn you, roast, crispy critter. That does NOT make the point of which you became a marshmallow a physical part of the sun.
just the point the affect causing the effect.

 

[Mordred]

No its not a physical point of the sun. It is a real point in space. Not an imaginary, spiritual or virtual point. Hence it is a physical point.

One of the problems with the word physical is that people tend to think it means materialistic.
Thats incorrect. When you describe a real energy state by definition your describing a physical state.
Actually in several dictionary they take the trrm one step further. Several dictionaries will state that anything physics study is the study of the physical
One example I mentioned before on virtual particles. Virtal particles from what I understand is used to describe extremely short lived quantum disturbances. Even if that particle is not materialistic its still a description of an energy state. If that description is strictly mathematical then its not physical. However if its a description of real disturbances then it is.

 

[phinds]

No its not a physical point of the sun. It is a real point in space. Not an imaginary, spiritual or virtual point. Hence it is a physical point.
l

I'd call that a coordinate, not a physical point.

You make a good point about materialistic v.s. physical and looking at it that way, I'm definitely saying it's not a material(istic) point, but I think splitting materialistic from physical is splitting hairs. Still, as someone said earlier, it DOES depend on how you define "physical".

 

[Mordred]

Coordinates can be tricky. I wouldn't call a coordinate on a map or math model as physical. Those I would call imaginary or virtual. But a coordinate itself (not the numbers used to describe that point) that is located in spacetime. That point does have physical properties whether its only an energy state is physical in nature.
I think it would lead to confusion trying to separate real coordinates from the realm of physical descriptions.
Personally I prefer the descriptions the way they are.
virtual coordinate, mathematical or imaginary coordinate or physical coordinate. Saves time in descriptions.

 

[Mordred]

Actually calling a real location a coordinate is contradictory. The term coordinate is a term used to describe a physical. Virtual or imaginary location. To call a physical point a coordinate moves it from a real location to merely a representation of that real location in a sense. Not to imply there is anything wrong to referring it as a coordinate.

 

[Chalnoth]

Elaborate please.

The space-time curvature at the event horizon depends upon the mass of the black hole. Smaller-mass black holes have more curvature than larger-mass black holes at the horizon.

 

[bcrowell]

The space-time curvature at the event horizon depends upon the mass of the black hole. Smaller-mass black holes have more curvature than larger-mass black holes at the horizon.

Also, curvature is a tensor, not a scalar, and the curvature is not necessarily constant throughout the event horizon for a non-Schwarzschild black hole. The more relevant quantity would be the potential, not the curvature.

 

[Chalnoth]

Also, curvature is a tensor, not a scalar, and the curvature is not necessarily constant throughout the event horizon for a non-Schwarzschild black hole. The more relevant quantity would be the potential, not the curvature.

The Ricci curvature scalar is, however, a scalar. But regardless, if we want to be pedantic, the actually relevant term here is the surface gravity \kappa, which is higher for small-mass black holes at the horizon.

 

[Naty1]

phinds:

BUT ... when you apply it to the cosomological horizon, which CERTAINLY is not physical in any way, it just falls to pieces for me and I just see it as nonsense.

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...

.

 

[Naty1]

From a prior thread...an actual description of an event horizon...more fodder!

https://www.physicsforums.com/showthread.php?t=631987&page=3...

..The event horizon of a black hole is actually lightlike. This follows from it being a null surface, and you can even think of the event horizon as being "trapped light". “the EH is a null surface--more precisely, it has two spacelike and one null dimension.”
PAllen & PeterDonis…… the event horizon is a 3-surface whose tangent space at each point can be given a basis that has two spacelike basis vectors and one null basis vector…the EH is not a "thing". It's just a boundary between two regions of the spacetime.,,,,

My notes suggest PAllen and PeterDonis actually agreed on this explanation!??

 

[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.