# Is the event horizon of a black hole physical?

1. Jan 11, 2013

### 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.

2. Jan 11, 2013

### Naty1

Re: is the event horizon physical ?

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?

Last edited: Jan 11, 2013
3. Jan 11, 2013

### phinds

Re: is the event horizon physical ?

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.

4. Jan 11, 2013

### Mordred

Re: is the event horizon physical ?

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?

Last edited: Jan 11, 2013
5. Jan 11, 2013

### Nabeshin

Re: is the event horizon physical ?

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.

6. Jan 11, 2013

### Chronos

Re: is the event horizon physical ?

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

7. Jan 11, 2013

### phinds

Re: is the event horizon physical ?

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.

8. Jan 11, 2013

### phinds

Re: is the event horizon physical ?

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

9. Jan 11, 2013

### phinds

Re: is the event horizon physical ?

Good point. Thanks.

10. Jan 11, 2013

### phinds

Re: is the event horizon physical ?

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.

11. Jan 11, 2013

### Mordred

Re: is the event horizon physical ?

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

Last edited: Jan 11, 2013
12. Jan 12, 2013

### Chalnoth

13. Jan 18, 2013

### Naty1

Re: is the event horizon physical ?

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] [Broken]

and at a talk Susskind gave at Princeton and Rutgers Universities: "where you could not get away with half baked claims"...
edit: this happened in 1993.

Last edited by a moderator: May 6, 2017
14. Jan 18, 2013

### Drakkith

Staff Emeritus
Re: is the event horizon physical ?

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.

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

I think this goes beyond the usual meaning of physical.

15. Jan 18, 2013

### Naty1

Re: is the event horizon physical ?

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."

16. Jan 18, 2013

### bcrowell

Staff Emeritus
Re: is the event horizon physical ?

Not true.

17. Jan 18, 2013

### Drakkith

Staff Emeritus
Re: is the event horizon physical ?

What?

18. Jan 18, 2013

### phinds

Re: is the event horizon physical ?

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.

19. Jan 18, 2013

### phinds

Re: is the event horizon physical ?

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

20. Jan 18, 2013

### Drakkith

Staff Emeritus
Re: is the event horizon physical ?