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View Full Version : Can I see behind the EH of a black hole?



tommac
2010-Jun-22, 01:50 PM
On my way to work today I was thinking about a few things ... basically unruh radiation, the big rip, and black holes.

What I was thinking about is during the late stages of a big rip scenerio could a black hole be ripped apart from the universal expansion? It seems that atoms can be ripped apart, why not a black hole.

The next thought was what would we see if there was an active black hole at the edge of our universe at our universal EH? Would we see inside the BH or what would we see?

Then I thought about unruh radiation and then about relative travel with regards to the BH.

So this is my question. If I were to travel at just under the speed of light towards a BH would I be able to see inside the EH and why not ( or why )?

The thought was that I would be "falling" at roughly the same rate as an infaller and in fact I would be blueshifting any light that was escaping. Lets say that I was shadowing the movement of an infaller. the infaller had a flashlight and was falling past a large EH ... As I "fall" ( not really fall but move relative to him ) will I experience the EH in a similar fashion to what I would if I was hovering?

So that is my main question for this thread.

Another thought that i had was in universal expansion ( constantly expanding ) from what I understand the EH of the universe is actually 2x the speed of light correct? So that no matter which direction the light was shining ( towards the observer ) and which way the observer was travelling ( towards the light ) they would never reach each other. Would the same effect happen at a black hole as I travel at the speed of light directly towards it .... would the real EH actually be when space-time is warped at 2x the speed of light rather than just at the speed of light? :shifty:

Cougar
2010-Jun-22, 03:21 PM
You seem to be picturing the universe as a snow globe that you can view from outside. There is no outside.

grant hutchison
2010-Jun-22, 03:32 PM
If you could see "beyond the event horizon", it wouldn't be an event horizon. An event horizon is a global property of spacetime, separating signals that can escape from signals that end at the singularity. Nothing inside the event horizon can get to any point outside the event horizon. If you fall across the event horizon following behind someone who falls just ahead of you, you will receive signals from them continuously as you cross the horizon; but you won't receive any signal they emitted below the event horizon until you have crossed the event horizon yourself. Outside the horizon, you see them by light they emitted outside the horizon; at the horizon, you see them by light they emitted at the horizon; inside the horizon you see them by light they emitted inside the horizon.

Grant Hutchison

tommac
2010-Jun-22, 04:21 PM
You seem to be picturing the universe as a snow globe that you can view from outside. There is no outside.

No I am not ...

tommac
2010-Jun-22, 04:26 PM
If you could see "beyond the event horizon", it wouldn't be an event horizon. An event horizon is a global property of spacetime, separating signals that can escape from signals that end at the singularity. Nothing inside the event horizon can get to any point outside the event horizon.

Yes I realized a few more things about what I was saying that coinside with your post here.
Light from inside of the EH of a BH can never leave the BH, BUT ... assuming that the light was shone directly in the direction of the EH could an external observer travel at near the speed of light towards the EH and "observe" that photon? This is where I think the 2x speed of light EH would be important ... at a point inside the EH there should be another true event horizon where no observer ( could ever observe what is inside ).





If you fall across the event horizon following behind someone who falls just ahead of you, you will receive signals from them continuously as you cross the horizon; but you won't receive any signal they emitted below the event horizon until you have crossed the event horizon yourself. Outside the horizon, you see them by light they emitted outside the horizon; at the horizon, you see them by light they emitted at the horizon; inside the horizon you see them by light they emitted inside the horizon.

Grant Hutchison

This can be true, up until the point where space-time is being curved at a 2x c rate. At which point ... even if the infaller is travelling at the speed of light and the photon is travelling at the speed of light .... well I guess this is my question so dont want to state it like I know what I am talking about ;-)

Argos
2010-Jun-22, 04:48 PM
Light from inside of the EH of a BH can never leave the BH, BUT ... assuming that the light was shone directly in the direction of the EH could an external observer travel at near the speed of light towards the EH and "observe" that photon?

All paths inside the BH lead to the singularity. Itīs the only way you can shine your flashlight to.

grant hutchison
2010-Jun-22, 05:08 PM
Light from inside of the EH of a BH can never leave the BH, BUT ... assuming that the light was shone directly in the direction of the EH could an external observer travel at near the speed of light towards the EH and "observe" that photon?No. There is no such photon to observe.


This is where I think the 2x speed of light EH would be important ... at a point inside the EH there should be another true event horizon where no observer ( could ever observe what is inside ).

This can be true, up until the point where space-time is being curved at a 2x c rate. At which point ... even if the infaller is travelling at the speed of light and the photon is travelling at the speed of light .... well I guess this is my question so dont want to state it like I know what I am talking about ;-)I have no idea where this "2x lightspeed" (cosmological?) event horizon of yours comes from. But the fact remains that no signal gets out of a conventional black hole event horizon, no matter what the state of motion of the external observer.

Grant Hutchison

tommac
2010-Jun-22, 05:55 PM
All paths inside the BH lead to the singularity. Itīs the only way you can shine your flashlight to.

But wouldnt some paths be quicker to the singularity?

tommac
2010-Jun-22, 06:00 PM
I realize that no photon gets out ... what I am saying is can an infaller catch-up to a spaceship ( or photon ) that is trying to escape?


If my friend and I are on a journey into a BH ... and we get seperated where they end up falling into the EH first ... I then hit my boosters to go after him and he hits his boosters to escape ... although it is clear that he will not be able to escape the EH ... what is not clear to me is that could I reach my friend ( I realize that both of us would be inside the EH ).

I guess what I am asking is what is the Event horizon for the freefaller?


No. There is no such photon to observe.

I have no idea where this "2x lightspeed" (cosmological?) event horizon of yours comes from. But the fact remains that no signal gets out of a conventional black hole event horizon, no matter what the state of motion of the external observer.

Grant Hutchison

grant hutchison
2010-Jun-22, 06:38 PM
But wouldnt some paths be quicker to the singularity?Once you're through the event horizon, there's a maximum elapsed time to the event horizon which you can't exceed: you have to manoeuvre to match the path of something that has fallen from rest at the event horizon. Photons don't have that option.


I realize that no photon gets out ... what I am saying is can an infaller catch-up to a spaceship ( or photon ) that is trying to escape?And I've answered that: the infaller can only receive signals which were emitted below the event horizon once he is below the event horizon.


If my friend and I are on a journey into a BH ... and we get seperated where they end up falling into the EH first ... I then hit my boosters to go after him and he hits his boosters to escape ... although it is clear that he will not be able to escape the EH ... what is not clear to me is that could I reach my friend ( I realize that both of us would be inside the EH ).It would depend on your separation, and the acceleration you could generate. But there's nothing to prevent it. If you're falling into a supermassive black hole, there's nothing to prevent you doing knee-bends and toe-touches below the event horizon, and nothing to prevent you seeing your feet. It's just that your head, toes, eyes and photons are all on their way to the singularity at slightly different rates.


I guess what I am asking is what is the Event horizon for the freefaller?The same as for everyone else.

Grant Hutchison

tusenfem
2010-Jun-22, 06:48 PM
This totally belongs in ATM. tommac, you were given an answer that your idea is impossible.
If you want to discuss this further, then only in ATM.

tommac
2010-Jun-22, 07:37 PM
This totally belongs in ATM. tommac, you were given an answer that your idea is impossible.
If you want to discuss this further, then only in ATM.


How is it impossible to fly into a black hole at near the speed of light?

tommac
2010-Jun-22, 07:40 PM
Once you're through the event horizon, there's a maximum elapsed time to the event horizon which you can't exceed: you have to manoeuvre to match the path of something that has fallen from rest at the event horizon. Photons don't have that option.

I get this But if the two infallers are falling one trying to escape and the other one trying to rendezvous, in theory they could rendezvous right?

But ... if they are far enough apart then the space-time between them would be expanding too quickly for a rendezvous .... right?

grant hutchison
2010-Jun-22, 07:50 PM
I get this But if the two infallers are falling one trying to escape and the other one trying to rendezvous, in theory they could rendezvous right?I've answered that already. Yes, they could rendezvous, just as you could reach down and touch your toes while inside the event horizon of a supermassive black hole.


But ... if they are far enough apart then the space-time between them would be expanding too quickly for a rendezvous .... right?If they were too far apart, or incapable of the necessary acceleration, they would be unable to rendezvous before one of them hit the singularity. I doubt if "expanding spacetime" is involved.

Grant Hutchison

tommac
2010-Jun-22, 11:12 PM
I've answered that already. Yes, they could rendezvous, just as you could reach down and touch your toes while inside the event horizon of a supermassive black hole.

If they were too far apart, or incapable of the necessary acceleration, they would be unable to rendezvous before one of them hit the singularity. I doubt if "expanding spacetime" is involved.

Grant Hutchison

thanks ... this answered my questions

PetersCreek
2010-Jun-23, 12:03 AM
Operating on the assumption that all of tommac's questions have been answered and does not wish to defend his ATM statements (i.e.; "another true event horizon"), this thread is closed. As usual, report this post to request that it be reopened.

Let this serve as yet another warning to keep your ATM assertions out of Q&A.