What is the event horizon of a freefaller into a black hole? Does a free faller have a different event horizon than an external observer ...
Lets assume that the free faller has no notion of free falling and has no way to counteract the freefall
Order of Kilopi
What is the event horizon of a freefaller into a black hole? Does a free faller have a different event horizon than an external observer ...
Lets assume that the free faller has no notion of free falling and has no way to counteract the freefall
Order of Kilopi
It's the same event horizon as for everyone else. The event horizon is a global property of spacetime. It separates the region in which photons can escape to infinity from the region in which all photons hit the singularity.
You're not going to revisit this discussion, are you?
Grant Hutchison
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From a previous thread, it almost sounded like you expected there to be an event at the horizon; something the person passing through it would perceive. But the event horizon isn't a "thing" in that sense.
Simplistically, perhaps, it is the boundary where information about events on the inside can never reach the outside; hence the name.
Order of Kilopi
No ... this is a quick "hows your father" type question ... but I would like a bit more detail.Originally Posted by grant hutchison
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My question has nothing to do with that other thread ... or at least ... I dont see the connection. I understand that what the EH is.
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Here is my confusion:
if I am freefalling ... lets say in an elevator. I dont know that I am in a gravitational field and free falling.
Can I see light from all directions around me? When I look forwards ( into the BH that I dont know that I am falling into ) I can see light hitting me ... and when I look back ( away from the BH that I dont know that I am falling into ) I see light hitting me.
In front of me though there would be some point ( and this is an assumption ... so let me reask as a question ) .... would there be some point in front of me that I as the free faller would see as my EH ... from which point light would never reach me ( me being a free faller free to fall past the real EH ) ... Would there be a point that from which would be at near infinite redshift and then a place from which I as a free faller would never be able to communicate with ?
Order of Kilopi
Now that is the kind of detail that should have been in post #1
Get up, a get-get, get down.
Order of Kilopi
This is EXACTLY like that other post!
There is NO point in which your light from your toes will not get to your head UNLESS you find a way to accelerate your head away from the EH of the black hole after your toes have passed the EH but your head has not. Then you have bigger problem because you've ripped yourself in 2 essentially having your toes going at c in one direction and your head travelling at almost c in the other.
To get the effect ... oh wait tommac has me on ignore.
Order of Kilopi
Lets look at this
How does tommac not "see the connection"
to
The difference is an external/distant observer will not see the freefaller pass the event horizon because of, take your pick, the time dilation or the extreme curvature of space.
Order of Kilopi
If I have understood the question correctly, then I think it is quite interesting. If I rephrase (and tommac, please tell me if I have missed the point):
As we approach a black hole (I assume it makes no difference if you are freefalling or (somehow) approaching more slowly) the event horizon will just be "black" (ignoring any effects of acceleration of infalling matter). As we pass the event horizon (which we know won't appear as anything special), will we suddenly be able to see what is going on inside the black hole (like passing through a curtain)? (I am guessing this is not the case.)
Or is the curvature of spacetime such that within the black hole we still do not see any photons coming from some distance "ahead" of us (whatever "ahead" means in this sort of curved spacetime)? So do we continue to see (or, more accurately, not see) a dark event horizon ahead of us, even after passing the external observer's event horizon?
Or something else?
Established Member
How a photon cannot escape to infinity ? Its speed is "c" , whatever the wavelength. It is the maximum speed allowed in our universe . And the photon has no mass so it cannot slow.
I understand the frequency of the photon can be be red-shifted by gravity but iI cannot see how it can be stopped .
Order of Kilopi
It is not that the photon is stopped or slowed, rather that spacetime is so curved there is no path from inside the event horizon to outside.
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Any light we see inside the event horizon was emitted by objects falling towards the singularity. When any object falls into the singularity, it leaves behind a "supply" of photons inside the event horizon: those emitted near the event horizon wind down their radial coordinate only slowly initially; those emitted just above the singularity hit the singularity almost immediately. So when we fall below the event horizon, we are presented with photons, coming from ahead, emitted by objects that preceded us into the black hole. If we fall in immediately after the preceding object, we'll be able to watch almost all of its trip to the singularity during our own trip to the singularity. If we fall in some time afterwards, we'll catch up to signals emitted by the preceding object during only the initial part of its fall: it will take us our whole fall to observer photons from (say) just the first fraction of a second of the preceding object's fall.
So there's an event horizon of sorts, marking off a region of spacetime from which we can never receive signals before our own appointment with the singularity: but it slopes through time, cutting off long-ago events close to the inside of the event horizon and more recent events close to the singularity. But at any given moment of our own fall, we will (at least in potentiality) have signals from all around, from objects falling ahead of us, alongside us, and behind us.
This all connects seamlessly to the external view:
As we fall towards the event horizon, we encounter photons from objects that fell into the black hole ahead of us. The longer ago the object fell in, the more delayed and sparser are these photons, bringing us old views of the object's last moments above the event horizon, which we can watch over some extended period during our fall. As we pass through the horizon, we (potentially) see images of everything that went through the horizon ahead of us: but the sheaf of photons in the vicinity of the event horizon disperses with time, so the images of objects that passed through long ago will eventually wear down to their last photon and disappear. Inside the horizon, we encounter images from photons emitted inside the horizon. The more recently the object preceded us through the horizon, the more complete the history we can observer during our own remaining time before the singularity.
Grant Hutchison
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As Strange says, it isn't stopped. There's just no direction a photon can travel in which takes it outwards; inside the event horizon all light-like paths lead to the singularity.
Grant Hutchison
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Thanks, Grant. I'm not sure if that is what tommac was after, but I enjoyed it!
Established Member
What is curved ? you mean , our photon don't go in a straight line anymore but turn left or right and then cannot escape the black hole ? That's really strange ! (Oops ! sorry !)
Did we see the beginning of this when observing white dwarfs or neutron stars ?
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Stranger than that: the photon is turned in a time-like direction. What's space for the photon looks like time to those of us outside the black hole, and vice versa.
Grant Hutchison
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We see the most direct evidence of this with gravitational lensing: http://en.wikipedia.org/wiki/Gravitational_lensing
Established Member
You will not see any light from the event horizon, because it emits no light and reflects no light. A high speed approach does not change that. You may see infrared light from the accretion disk blue shifted into the visible range, because of your high approach speed which is accelerating due to free fall. Part of the accretion disk will be red shifted as it's particles are moving away from you at almost light speed, so that part may appear brighter since most of the emissions of the accretion disk are ultraviolet and higher frequency.
For low mass blackholes, if any, the Hawkings radiation may be visible, and it may appear to be originating just outside the event horizon. Can we call that the Hawkings horizon? Is the hawking horizon an oblate sphere which appears to be rotating at a rapid rate for black holes that rotate= there may be no non-rotating blackholes in our galaxy. Neil
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Hawking radiation doesn't exist for freefallers. A free-falling observer sees nothing but the usual seethe of virtual pairs as he crosses the event horizon.
Grant Hutchison
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Well it can stop when the space-time curvature wraps back around ... then nothing can go forward right? The photon continues in a straight line ... it is space that is curved.
In any case this is slightly off the point of the OQ ...
I would like to keep everything as referenced from someone freefalling. That person doesnt know that he is free falling and I am just curious what that person observes and if the EH holds up from the point of view of the free faller.
Order of Kilopi
OK ... this is really juicy stuff here. So for the freefaller there is an event horizon .... Is there a way to calculate what that EH would be? It would seem that if it would also be defined by the assumption that the freefaller does not accelerate relative to the gravitational flow that light from inside of the EH can reach him ( but not escape the EH from the view point of som observer outside of the EH ), he can escape by accelerating as long as he is outside of the EH ... but then he would run away from the light ????
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Did you miss this
Well, I suppose you could say he is running away from light inside the black hole - but then we all areIt would seem that if it would also be defined by the assumption that the freefaller does not accelerate relative to the gravitational flow that light from inside of the EH can reach him ( but not escape the EH from the view point of som observer outside of the EH ), he can escape by accelerating as long as he is outside of the EH ... but then he would run away from the light ????
Order of Kilopi
By time you mean that the future is the only way you can travel ... and that future is the singularity?
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Did you miss this:
I have a bit of a problem or at least I still have a question about the word "of sorts" my bold above.So there's an event horizon of sorts, marking off a region of spacetime from which we can never receive signals before our own appointment with the singularity: but it slopes through time, cutting off long-ago events close to the inside of the event horizon and more recent events close to the singularity. But at any given moment of our own fall, we will (at least in potentiality) have signals from all around, from objects falling ahead of us, alongside us, and behind us.
If I am in free fall that EH would seem very real to me wouldnt it? Now from an external observer ... it is obvious that I cant escape the system but for me since I am freefalling ... wouldnt I be part of the system and thus I may not be aware of it. I can accelerate in every direction ... I can move out to infinity in all directions ( except that I will eventually hit the singularity but I am not aware of that ....
Is this wrong?
Order of Kilopi
There's the event horizon of the black hole, which is the same for everyone. That's easy to calculate, and is a global property of spacetime.
There's an event horizon for someone inside the black hole's event horizon, which is determined by the fact that there's a singularity in his future, and in the future of everything else inside the black hole. An infaller's view of his surroundings is therefore limited because photons from distant objects terminate in the singularity before the infaller sees them. The farther ahead of him an object is, the less of its sub-event-horizon history the infaller can (in principle) see. That event horizon is observer-dependent, and is also calculable, but it would be a complicated bit of maths. For a simple "eternal" black hole, it's drawn by the worldline of a photon emitted at the event horizon in the infinite past, and terminating at the singularity along with the worldline of the infaller. No event pastwards of that worldline can communicate with the infaller, because all photons from such events terminate in the singularity before the infaller gets there.
I have no idea what this means.
Grant Hutchison
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Ah, I see. That is not "the" event horizon, which as Grant pointed out is the same for everyone, that is a sort of "pseudo event horizon" (brought up in response to my question) caused by the fact that something ahead of of you is being accelerated faster than you are and so you will eventually stop receiving photons from it. So, if I understod it correctly, you will always have limited visibility ahead of you. In other words, everyone inside the (real) event horizon will only be able to see things a limited distance ahead of them (so they all have their personal "event horizon" but that is not the real event horizon of the black hole).
Errr, did that help!
I'm not sure it would be "real" to you; you would just see things accelerating away from you and disappearing - the closer you get to the singularity the less you will see.If I am in free fall that EH would seem very real to me wouldnt it? Now from an external observer ... it is obvious that I cant escape the system but for me since I am freefalling ... wouldnt I be part of the system and thus I may not be aware of it. I can accelerate in every direction ... I can move out to infinity in all directions ( except that I will eventually hit the singularity but I am not aware of that ....
Order of Kilopi
It wouldn't, no. Because it's a feature of your future. It arises because your future terminates. If you look around as you fall, you can see arbitrarily distant objects, although they will grow fainter and more redshifted with distance. Unless you know about the singularity in your future, you've no reason to assume that you won't be able to watch distant objects evolve for an arbitrarily long time.
See above. You can't move to infinity because there's a singularity ahead. If you make your observer ignorant of that fact, then the personal event horizon I've described can't be intuited by that observer either. If you allow your observer to make observations and understand GR, then both event horizons can be observed and understood by the infaller.
Grant Hutchison
Order of Kilopi
Although this discussion is in terms of an abstract observer, I guess we don't really know if a real observer would be able to survive let alone observe; do we even know whether chemistry would still work with this swapping of time-like and space-like dimensions?
Similarly, if you could be dangled through the event horizon on a wire (bear with me!) then wouldn't the wire break at the event horizon because the forces holding the molecules together could be transmitted back to the cable outside?
Order of Kilopi
It's not quite like that, I think.
Maybe looking at the freefaller diagram here will help with an illustration.
If you are a very distant free-falling observer, then stuff that falls through the event horizon far ahead of you in your own coordinates leaves a trace at the event horizon. You will eventually connect with that trace when you pass through the event horizon yourself: you'll pass through light-like worldlines connecting you to all the stuff that fell through the event horizon ahead of you. In principle, then, you are seeing images of things that were very distant from you in your own coordinates when the light was emitted. In practice, of course, these worldlines will contain vanishingly few photons. This business of seeing very distant stuff very belatedly continues below the event horizon. Up ahead, you "see" very distant objects (stuff that fell across the event horizon long before you did) very faintly and very redshifted. So there's a practical limit (relating to redshift and faintness) beyond which you can't see, but there are "effectively empty" light-like worldlines connecting you to much more distant objects.
Grant Hutchison
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