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?