# B what exactly is an event-horizion?

#### brianhurren

Summary
approaching a black-hole event-horizon at the speed of light.
Summary: approaching a black-hole event-horizon at the speed of light.

as one falls towards the singularity of a black-hole, one exelerats towards the speed of light. At the speed of light one crosses the event-horrizion. once past it information cant be sent out into the universe because it cant escape out past the event-horision. So my question is, is the event-horision an actual physical thing like a physical boundry or is it just location like the speed of sound is. ie is the event horrizion a thing or a place?

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#### Drakkith

Staff Emeritus
2018 Award
At the speed of light one crosses the event-horrizion.
Not true. All massive objects (meaning objects with mass, not 'large') are limited to sub-c velocities. They can never reach the speed of light in any reference frame.

So my question is, is the event-horision an actual physical thing like a physical boundry or is it just location like the speed of sound is. ie is the event horrizion a thing or a place?
It's just the region where the curvature of space reaches a certain amount. In other words, it's a place.

#### PeterDonis

Mentor
as one falls towards the singularity of a black-hole, one exelerats towards the speed of light
Speed is relative. It is true that, relative to stationary observers (i.e., observers hovering at a fixed altitude above the horizon), the speed of an infalling observer approaches the speed of light as the observer approaches the horizon. But there are no hovering observers at or inside the horizon; they can't exist. There are never any observers relative to whom an infalling observer is moving at the speed of light.

ie is the event horrizion a thing or a place?
It's an outgoing null surface. That doesn't really meet the ordinary person's intuitive sense of either "thing" or "place". Ordinary language is not always capable of describing physics.

#### Orodruin

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It's just the region where the curvature of space reaches a certain amount. In other words, it's a place.
The actual curvature at the event horizon depends on the size of the black hole. A supermassive black hole has little curvature at the event horizon.

The event horizon is a null surface. It moves at the speed of light for any observer it passes. It is therefore doubtfull it should be called a place.

#### Ibix

Summary: approaching a black-hole event-horizon at the speed of light.
As noted by others, this is not an accurate description of crossing the horizon. It's perfectly possible (in principle) to control your fall with a rocket, so two objects crossing the event horizon side-by-side can have any velocity relative to each other. And distant observers cannot assign a velocity to them in a non-arbitrary way.
So my question is, is the event-horision an actual physical thing like a physical boundry
As noted by others, this is a difficult question to answer in ordinary language because it's so far outside normal experience that there are only technical terms for it.

We can say that the horizon isn't a physical thing like the surface of a star or planet - there's nothing there to crash into. If you are small enough relative to the black hole that you don't get torn apart by tidal forces then you can pass through without noticing anything unusual (your imminent death is inevitable once you pass through, but it doesn't kill you itself). It's just a boundary.

However, it's a boundary in spacetime, not in space. And it turns out that there are three distinct types of spacetime boundary, timelike, spacelike and null. Timelike boundaries correspond to what we think of as boundaries in space - there's a "this side" and a "that side", and you can freely cross backwards and forwards, or choose not to cross. Spacelike boundaries correspond to what we think of as moments in time - there's a "before" and an "after", and once you are after you can't be before again, and you can't opt out of crossing. (If the naming convention seems backwards, consider that a moment in time is all of space at a given time.)

Unfortunately event horizons are members of the third group, null surfaces. They are an inbetween case that has no ordinary language description. They have some of the characteristics of boundary in space (you can choose not to cross) and some of a moment in time (you can't cross moe than once and can only cross in one direction). But they aren't really either.

Just to note, you could build a spherical surface (a physical realisation of a boundary in space) completely enclosing a black hole event horizon. And you could make it arbitrarily close to the horizon, although you'd need rockets to help support it at some point. This is why it's easy to think of the horizon (naively, from a distance) as a boundary in space. But curved spacetime is a peculiar place and the horizon itself is a different animal.

Finally, note that all of this is prediction using our current best theory of gravity, general relativity. Any or all of it may be subject to change without notice when we figure out what quantum gravity looks like.

#### Orodruin

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there's a "before" and an "after", and once you are after you can't be before again, and you can't opt out of crossing.
This is not true. It is true in Minkowski space, but not in a more general setting. As long as you remain outside the event horizon you can certainly avoid the constant r spacelike surfaces inside.

"what exactly is an event-horizion?"

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