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## Is the event horizon of a black hole physical?

 Quote by Naty1 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.

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

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 Quote by bcrowell Not true.

 Quote by Naty1 ... 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) in to 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.

 Recognitions: Gold Member 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.

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 Quote by 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. 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".

 Recognitions: Gold Member Coordinates can be tricky. I wouldnt 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 seperate 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.
 Recognitions: Gold Member 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.

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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.

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 Quote by Chalnoth 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.

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 Quote by bcrowell 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.

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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...

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

http://www.physicsforums.com/showthr...=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!!??

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 Quote by Naty1 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 ?

 Recognitions: Gold Member 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 definetely 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.

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