Can you truly cross an event horizon?, Event horizon depends on observer

I was just thinking about what a black hole might look like from a closer perspective and realized the mathematical event horizon is abstract and unobservable. It's based upon the point at which light would have enough of an escape velocity to forever leave the grip of a black hole, but real observations are necessarily made at shorter distances.

As a matter of fact an observer would never subjectively see themself as having crossed any event horizon. The effective event horizon would always be further inside a black hole. So, for example if you orbitted near the event horizon of a black hole, you could see inside the event horizon and even passing the event horizon would of course not be noticeable (though it would theoretically be an inevitable one way trip) and observations of the "edge"/event horizon of the black hole would continually lie further inside at least up to some point.

Hello All

What do you think a black hole is?


There is no way you could see into a black hole. If light cannot escape than you have no light to communicate.


Too many people think of black holes as we see them in the movies.

Have a look at this link for more info
Not that I agree with the info. Its always a theory.

http://cosmology.berkeley.edu/Education/BHfaq.html

The pop-sci concept of the event horizon of the black hole is a bit oversimplified. The "true" model is a bit more complicated, as you have managed to figure out. The event horizon is not just a universal "point of no return". It's actually very subjective, and different for every observer. For each observer, the event horizon is the surface beyond which nothing crossing it will ever be able to return to the point of observation. A photon may be able to get back across this surface itself with a little energy left, but this remaining energy would be red-shifted down to nothing before it could reach you.

There is also another event horizon to consider: the edge of the observable universe. I.e. the distance at which the additive Hubble expansion of the intervening space exceeds the speed of light. This one is a little different in that it is a two-way horizon. Not only will nothing beyond it ever be able to reach you, but you will also never be able to reach anything beyond that horizon.

These two are "duals", in a manner of speaking. They must come together. One can't exist without the other. As you move in closer to a black hole not only will the event horizon of the hole move farther in with respect to you, but the event horizon of the edge of the observable universe will also close in around you.

Take this example scenario... You start out x meters from the event horizon of the black hole. Then you move x meters closer to the black hole, arriving at the point where you previously observed the event horizon to be. Now you observe two major changes. 1) The event horizon of the black hole still appears to lie some distance ahead of you. 2) The edge of the observable universe (observable to you, that is) has moved in a great deal.

For observers between the old location and the new location of your event horizon, they will not perceive this change. They can still come to you, but you cannot go to them. So the two-way event horizon actually becomes two opposite one-way horizons separated by some distance. This is because Hubble expansion is, as far as we know, directionless, whereas gravity is not. Depending on where you stand gravity may push against you or pull with you, but Hubble expansion always pushes against from every direction.

Things that you could previously see will be "eaten up" by the advancing shadow of the "edge of the universe". They effectively pass out of existence as far as you are concerned.

Remarkably the view from inside the observable universe looking out is theoretically not too different from the view near a black hole looking out, with the exception of distance scales. This has led some to propose that the observable universe is in fact a black hole itself.

This post has been edited by WaterBreath on Jun 2 2006, 03:36 PM

The answer to the post question is a resounding NO!

Light slows down in gravity.It doesn't matter its direction of travel. Where time slows down light does. Because light falls we have to wait for it.

The actual limit of gravity would prevent an event horizon. It looks like the event horizon really is a singularity where gravity is at its maximum. Since an event horizon defines the place where time stops light could no longer move. It couldn't even enter a so called black hole. The real black holes are naked singularities.

is possible yes cross the enent horizon.beyond of that regions must occur breakdown of simmetries,generating sundries types of exotic particles(creation due strong gravitation that separate regions with virtual particles and antiparticles,that violate reversibility space-time)that are negative-negative matter-that compensates the osccilations of spinors,leading to multiples manifolds of space-time.these holes of energies recuperate information "lost" in black holes there are regions completely connecteds that retransmit the energy in form of packets of waves.There is global conservation of energy-time( THE SPACE IS GENERATED BY INFINTIES HOLES THAT DEFORM THE REGIONS,AND CREATING THE CONNEXION LEFT_HANDED AND RIGHT_HANDED,GENERATING THE INFINITIES FIGURES GEOMETRIC_TOPOLOGIC IN THE CONTINUITIES OF SPACE_TIME( AS JACOBI PERIODIC FUNCTIONS WITH TWO ORBITAL FOR TIME
DOUBLING SPACE_METRIC exp(-iwt)'(+iwt)'with spherical harmonics with propre time&,-&,encurving in each point of space-time associted to violation of PT linked to OCTONIOS that GENERATE 2-TORSION ASYMMETRICS THAT GENERATE ELLIPTIC GEOMETRIC -that has derivatives of times,with incresed of velocities-that are conformal and isometric with mobius transformations-non linear equaions-it is defrees of curvatures of curves
THOSE FRACTIONAL TRANSFORMATIONS COUPLED 2-bidirectional of times ict and-ict' create the infinities osccilations of spinors( measure with the TIME that double the 2-torsions in infinities imagens-the regions of black hole are holographycs-beTWEEN THAT REGIONSNON CAUSAL APPEAR THE DARKMATTER THAT ARE THE NEGATIVE ENERGY THAT SUFFER REPULSION(accelerating the waves for out of black holes creating the HOLES INTO THE BLACK HOLES-RADIATION OF ENERGIES-THAT ARE MICRO WHITE HOLES,DECREASING THE INTERACTIONS INTERNES aPPERING IN THAT BREAKDOWN OF HELICITIES OF THE SPINORS IN GRAVITATION,ELECTROMAGNETIC(GRAVIPHOTONS) being given by non linear equations and STRONG AND WEAK INTERACTIONS-that deform topologic do appear the quantum-through of the non-linearity quantic.
the EVENTSHORIZON CAN APPEAR BEACUSE OCCUR DEFORMATIONS OF SPACE_TIME DIFFERENTIANG THE OBSERVERS VARIABLES WITH EXISTENCE MOST NEXT OR NOT OF THE BH.THE BH CAN BE HORIZONS OF QUASARS?

A.CARLOS MOTTA

is possible yes cross the enent horizon.beyond of that regions must occur breakdown of simmetries,generating sundries types of exotic particles(creation due strong gravitation that separate regions with virtual particles and antiparticles,that violate reversibility space-time)that are negative-negative matter-that compensates the osccilations of spinors,leading to multiples manifolds of space-time.these holes of energies recuperate information "lost" in black holes there are regions completely connecteds that retransmit the energy in form of packets of waves.There is global conservation of energy-time( THE SPACE IS GENERATED BY INFINTIES HOLES THAT DEFORM THE REGIONS,AND CREATING THE CONNEXION LEFT_HANDED AND RIGHT_HANDED,GENERATING THE INFINITIES FIGURES GEOMETRIC_TOPOLOGIC IN THE CONTINUITIES OF SPACE_TIME( AS JACOBI PERIODIC FUNCTIONS WITH TWO ORBITAL FOR TIME
DOUBLING SPACE_METRIC exp(-iwt)'(+iwt)'with spherical harmonics with propre time&,-&,encurving in each point of space-time associted to violation of PT linked to OCTONIOS that GENERATE 2-TORSION ASYMMETRICS THAT GENERATE ELLIPTIC GEOMETRIC -that has derivatives of times,with incresed of velocities-that are conformal and isometric with mobius transformations-non linear equaions-it is defrees of curvatures of curves
THOSE FRACTIONAL TRANSFORMATIONS COUPLED 2-bidirectional of times ict and-ict' create the infinities osccilations of spinors( measure with the TIME that double the 2-torsions in infinities imagens-the regions of black hole are holographycs-beTWEEN THAT REGIONSNON CAUSAL APPEAR THE DARKMATTER THAT ARE THE NEGATIVE ENERGY THAT SUFFER REPULSION(accelerating the waves for out of black holes creating the HOLES INTO THE BLACK HOLES-RADIATION OF ENERGIES-THAT ARE MICRO WHITE HOLES,DECREASING THE INTERACTIONS INTERNES aPPERING IN THAT BREAKDOWN OF HELICITIES OF THE SPINORS IN GRAVITATION,ELECTROMAGNETIC(GRAVIPHOTONS) being given by non linear equations and STRONG AND WEAK INTERACTIONS-that deform topologic do appear the quantum-through of the non-linearity quantic.
the EVENTSHORIZON CAN APPEAR BEACUSE OCCUR DEFORMATIONS OF SPACE_TIME DIFFERENTIANG THE OBSERVERS VARIABLES WITH EXISTENCE MOST NEXT OR NOT OF THE BH.THE BH CAN BE HORIZONS OF QUASARS?

A.CARLOS MOTTA

NO MUST THERE IS NAKED SINGULARITIES,BECAUSE REGIONS ARE NOT CONNECTS,SMOOTH,AND PARA-COMPACT WITH CLOSED SPACE-TIME LINES AND OPENED SPACE-TIME LINES WHERE LIGHT RAYS IF DEFORM AND FOLLOW THE CLOSED AND OPENED CURVATURES OF SPACE_TIME CONNECTING AND DESCONNECTING CAUSAL AND NON-CAUSAL REGIONS THROUGH OF PASSAGE OF ENERGIES(VIOLATING CONSERVATION OF ENERGY),BUT CONSERVED TO OUT OF REGION OF BH WITH SUPERLUMINAL SIGNALS-THE THE SPEED FASTER THAT SPEEDLIGHT DOES CONSERVE MATTER_ENERGY THROUGH OF VIOLATION AND CONSERVATION OF THE ASYMMETRIES OF THE SPINOR MATRICES AND BOTH LORENTZ'sTRANSFORMATIONS
,

Thank you all for the replies.

A question in my mind, along the lines of what WaterBreath posted, is whether or not an event horizon is symmetric in both directions.

An event horizon is normally defined as an abstract construction at some infinite distance from the black hole (and outside any gravitational effects) ... of course this also would be a point that would take light an infinite amount of time to travel so for all intents and purposes the light might as well have been sucked into the back hole as you aren't going to see any of it from an infinite distance away.

A more useful use for an event horizon would be in determining whether or not 2 points have any "line of sight" between them after taking into consider gravitational effects.

Now a question that seems to stick around is whether or not this effect can be unidirecitonal - it's generally accepted with strong evidence also that light can't escape (easily) from a black hole but there's a nagging question for me at least of whether or not the reverse is true - is the event horizon bidirectional where an event horizon applies in both directions, similar to the expansion of the universe where the distance between objects can exceed the speed of light?

This could be possible if the density of space (from an external perspective) was increasing over time inside a black hole. If you look at an event horizon, it's static from an external view but a moving front when you get closer ... it expands outward at lightspeed (though we assume light always appears to travel at light speed locally, so you would have no way to determine locally where the event horizon was by measuring the speed of light). So an event horizon is something that only exists in a relative fashion between separate locations in space.

There was someone else who posted a comment about gravitational forces not simply being compressive but also expansive and able to tear objects apart. If you look at tidal forces near a black hole, we assume it would actually tend to stretch objects toward the center. Though it would place a compressive force to distances perpendicular to the center, it would be expansive for distances parallel toward the center. So space could be seen as being stretched in a sense toward the center but compressed in directions perpendicular to this. I don't know exactly what that means except to point out that not all forces are compressive near a black hole ... to actually hit a singularity and be compressed by it - some matter would actually have to pass through it and be pulled backward ... just like falling through the air doesn't kill you - it's the impact with the ground that hurts

Anyway, I've also posted elsewhere that if we viewed the singularity as a just an infinitesmal point in space with everything falling and being compressed into it, then there would be a lot of activity in a small area. If time is determined by # interactions between objects per unit of time, then an entire universe worth of interactions and time might occur virtually instantaneously from an outside perspective. It's likely these interactions would be performed by properties of matter that don't normally play a large role in their interaction (for example the strong force normally only interacts with a few sub atomic particles at a time, whereas in a black hole that force for a single particle might interact with many solar masses all in an instant of time for an external observer).

On the other hand, if space is altered entering a black hole and effectively the density of it increased (alternately this could be viewed locally as an expansion of space) then event horizons would exist in both directions and you could never truly see a singularity, nor could you see outside a black hole but instead would continually exist inside a slice of the universe bounded on either side by event horizons. (just the point at which the expansion of space makes it impossible to see past it).

An interesting question would be what Hawking Radiation would look like for an observer inside a black hole? It's a bidirectional effect - energy is split continually in space and not all of it recombines ... the event horizon isn't fixed but depends upon the observers location. This energy doesn't only leave a black hole, half of it falls into a balck hole and would seem to be an effect that creates radiation in both directions. Would this radiation look like background radiation or a Big Bang event?

can you see a black hole?

No, the event horizon is defined as the surface furthest from the singularity where no light path moves outwards. Photons cannot escape at all, no matter their energy, because the space-time curvature doesn't give them a path to the outside universe.

The horizon at the edge of the visible universe is a particle horizon, not an event horizon. There's a subtle difference.

To someone falling into the black hole they will cross the event horizon in a finite time and then hit the singularity in a finite time. To observers it'll take the person falling into the black hole an infinite time to hit the event horizon.

No, you couldn't. Even if you were 1 millimetre above the event horizon, you'd not be able to see into it.

Nick's just saying his usual "OMG black holes are wrong!" bit, despite being corrected before and I'm not exactly what antonio pocobi is on about, but I suspect he's playing 'randomly generate a sentence using science words' like most of his posts.

You'd notice it from the warping of the view of the sky, but for a sufficently large black hole you'd not notice the gravitational acceleration very much.

Hello All

Can you cross an EVENT horizon?

Yes you can 100%

But! not in one piece.

Your atomic structure will be broken down to atoms.

Than your atoms would be brobken down to neutrons

Than quarks

Than preons

Than,,,,,and so on, or should I say so long.

It depends on how massive the black hole is. If it was 5 times the mass of the Sun, it would shred you apart before you got close to the event horizon. If it was a million trillion times more massive than the Sun you'd pass through the event horizon without feeling any kind of tug on your body trying to pull you apart. While the total pull of a supermassive black hole is stronger, the pull at the event horizon decreases with mass, it's proportional to 1/M.

Rest assured, you'd be shreded to bits eventually, it just doesn't have to be at the event horizon.

Would it be right to say there is no discontinuity in spacetime at the event horizon .. it's just a bit of an oddity seen from certain points of view?
-C2.

That's true, there's no discontinuity in space-time curvature. You can verify this by computing things like the Ricci scalar R or R_{abcd}R^{abcd} and evaluating them on the horizon. They are non-singular on the horizon, but are at the singularity.

Many thanks AlphNumeric.

If there is no discontinuity at an event horizon then it follows there are no zeros or infinities at an event horizon.

Could you clarify please .. is this "no light path moves outwards or can escape from the singularity? "

Any small enough region of space can be considered locally flat .. this must apply to a region that straddles the event horizon.. can we guess how large a region of space has to be before it contains enough curvature to bend all light through (say) 180 degrees (in space) ? The answer may already be implicit in the definition of the event horizon.

-C2.

No necessary infinities at the event horizon. No physical infinities at the event horizon. The original Schwarzschild solution attempts to impose a coordinate system naively (it was 1916) on the whole of space time and this resulted in some infinities at the event horizon, just like some map projections make Canada look over-sized and Antarctica infinite.

Yup. You can think of the space-time as "Flat-but-tilted". Just like a blown up curved surface is locally flat, so is space-time locally flat. But flat doesn't mean that it's tilted the same way as the greater outside universe.
At exactly the event horizon, the null geodesics (edges of the light cones) are such the nothing with a time-like geodesic moves away from the singularity. Once you cross the event horizon, the light cones are tilted over so that the very concept of "time" is conflated with diminishing distance from the singularity. Talk about predestination! :-)