Does an event Horizon have the 2 meanings of horizon?

[michel123456]

In layman terms a horizon has a double characteristic:

1. it is a kind of limit to the observation, a surrounding circular line that defines the limit between the sky & the earth: what is observable & what is not

2. It is a boundary that is tied to the observer: you can walk toward a point of the horizon on Earth but you will never be able to reach it because the horizon is your particular horizon and it is moving with you. IOW the horizon is relative to the observer.

Does an Event Horizon of a Black Hole follow the 2nd characteristic? And if not, why?

I mean, if you try to approach the EH, will it go further the same amount as you have progressed? (because of the deformation of Spacetime) Or can you eventually reach an EH (although you cannot reach the horizon on Earth).

Note: the question arises from the fact that the theory surrounding BH comes from the Theory of Relativity, which is relative.

Or to state the question otherwise: what part of the Black Hole observation is relative to the observer and what part of it is absolute?

Edited April 17, 2019 by michel123456

[Strange]

33 minutes ago, michel123456 said:

Note: the question arises from the fact that the theory surrounding BH comes from the Theory of Relativity, which is relative.

Not everything in relativity is relative. For example, the speed of light is the same for all observers (invariant).

Similarly, the event horizon is invariant. All observers (including someone in free fall) will say that the EH is in the same place. Therefore as you fall towards a black hole at some point you will pass through the event horizon (and will no longer be causally connected to the outside universe).

35 minutes ago, michel123456 said:

Or to state the question otherwise: what part of the Black Hole observation is relative to the observer and what part of it is absolute?

Other invariant attributes are the mass, charge and angular momentum.

[michel123456]

20 hours ago, Strange said:

Not everything in relativity is relative. For example, the speed of light is the same for all observers (invariant).

No not this again:

The Speed of light is invariant AND relative. Take the analogy of the horizon on Earth: the distance to the horizon is the same for everybody (for a specific height) aka it is constant. And it is relative to the observer. The same goes for SOL but at another level, it is not distance that is constant & relative, it is velocity.

20 hours ago, Strange said:

Similarly, the event horizon is invariant. All observers (including someone in free fall) will say that the EH is in the same place

That is my question: why are you so sure of this? If the Theory you are using to calculate the EH is Relativity, then how can you be sure that the result of your calculation is absolute?

[Strange]

5 hours ago, michel123456 said:

The Speed of light is invariant AND relative.

No. It is not relative. It is the same for all observers.

You need to understand what terms like "invariant" and "relative" mean in the theory of relativity, not just as English words.

Invariant means all observers will measure the same thing. The speed of light is invariant. Mass (rest mass) is invariant. Electric charge is invariant. Distances between events (locations in spacetime) are invariant.

Relative (or observer dependent)  means that different observers will measure different values depending on their state of motion, etc. Distance in space is relative. Time is relative. Energy is relative. 

5 hours ago, michel123456 said:

That is my question: why are you so sure of this? If the Theory you are using to calculate the EH is Relativity, then how can you be sure that the result of your calculation is absolute?

Because the event horizon is a set of spacetime coordinates.

(You keep using the word "absolute" but I don't know what that means in this context.)

[michel123456]

1 hour ago, Strange said:

No. It is not relative. It is the same for all observers.

You need to understand what terms like "invariant" and "relative" mean in the theory of relativity, not just as English words.

Invariant means all observers will measure the same thing. The speed of light is invariant. Mass (rest mass) is invariant. Electric charge is invariant. Distances between events (locations in spacetime) are invariant.

Relative (or observer dependent)  means that different observers will measure different values depending on their state of motion, etc. Distance is relative. Time is relative. Energy is relative. 

I keep not getting it.

Take the example of horizon that divides sky with earth. Isnt'it observer dependent? Isnt'it at a different spacetime coordinate for any observer? Can you look at the same horizon than anyone else? Doesn't its spacetime coordinates depend on your state of motion?

[Strange]

48 minutes ago, michel123456 said:

I keep not getting it.

Take the example of horizon that divides sky with earth. Isnt'it observer dependent? Isnt'it at a different spacetime coordinate for any observer? Can you look at the same horizon than anyone else? Doesn't its spacetime coordinates depend on your state of motion?

That clearly depends on your position in 3D space and (to a tiny extent) your speed. 

The first has nothing to do with the theory of relativity, while the latter does. 

[beecee]

On 4/17/2019 at 8:29 PM, michel123456 said:

In layman terms a horizon has a double characteristic:

1. it is a kind of limit to the observation, a surrounding circular line that defines the limit between the sky & the earth: what is observable & what is not

2. It is a boundary that is tied to the observer: you can walk toward a point of the horizon on Earth but you will never be able to reach it because the horizon is your particular horizon and it is moving with you. IOW the horizon is relative to the observer.

Does an Event Horizon of a Black Hole follow the 2nd characteristic? And if not, why?

No, it follows the first. The EH is simply the parameters of spacetime/gravity around a mass where the escape velocity equals that of the speed of light. If it is as is most likely rotating, then there are two horizons which we won't go into as it isn't really a factor in what you are asking. 

Quote

Or to state the question otherwise: what part of the Black Hole observation is relative to the observer and what part of it is absolute?

The only "absolute" is the speed of light in any frame of reference. Add to that its finite speed, and observers in different frames of references, will see different things [each is correct] and conclude differently. eg: You watch  me approaching a BH from afar. You will see me gradually further and further redshifted along the spectrum, but never crossing the EH, just fading from view. From my position, I will carry on approaching the EH and cross it as per normal [ignoring tidal gravitational effects] and proceed on to my doom. 

[michel123456]

10 hours ago, beecee said:

The only "absolute" is the speed of light in any frame of reference. Add to that its finite speed, and observers in different frames of references, will see different things [each is correct] and conclude differently. eg: You watch  me approaching a BH from afar. You will see me gradually further and further redshifted along the spectrum, but never crossing the EH, just fading from view. From my position, I will carry on approaching the EH and cross it as per normal [ignoring tidal gravitational effects] and proceed on to my doom. 

All this description makes a BH look like a deep well. It does not look like a solid material, like stone or steel (which are examples more massive than water). How comes?

 

[Strange]

46 minutes ago, michel123456 said:

All this description makes a BH look like a deep well. It does not look like a solid material, like stone or steel (which are examples more massive than water). How comes?

A will is quite a good analogy, in some ways. You look into the well and all you see is darkness (the inside of the well is effectively outside the universe). You can't tell what happens to stuff that falls into the well. The odd thing is, as you throw more stuff in, the well gets bigger.