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The idea of anything crossing the event horizon of a black hole from any observer's v 
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#1
Oct3111, 09:55 PM

P: 28

I don't like the idea of anything crossing the event horizon of a black hole from any observer's view point. The closer something gets to the event horizon, it either appears to slow down, or the event horizon appears to stretch and move further away. How can anything cross it?
For this reason, i like the idea that all the energy in a black hole is concentrated in a shell on the surface. This, in turn, leads me to believe that all matter is energy condensed around localized coordinates. Unfortunately, I don't have any citations or math to back me up. I'm curious how much legitimacy there is to these thoughts. 


#2
Oct3111, 10:08 PM

P: 28

Also, it seems like this would imply that there is no point where escape is impossible, as nothing crosses the event horizon. This would make it easier to fit black holes into accepted theories of thermodynamics, as information never really becomes inaccessible, just highly fixed.
Now, about their formation. Theoretically, the original "event horizon" would form as a collapsing star compresses two bundles of energy into a small area. Actually, this is likely to happen in many locations near simultaneously. As additional matter approaches these bundles of gravitational energy, the event horizons would stretch and merge, coalescing into the celestial bodies we can "see". Now a question: suppose an object passes between two of the forming event horizons, would its timeframe be slowed by both gravitational fields or would they cancel out? I am assuming the former, as time dilation is independent of spacial velocity direction. 


#3
Oct3111, 10:27 PM

P: 15,319

It is only from the point of a distant observer that an object doesn't appear to cross the EH. That's because of the fallible nature of observations by light. An observer falling into an EH will cross the EH with no untoward effects  afterall, the EH is not a thing at all, nothing actually happens there. It is a derived point at a radius, nearer than which light cannot escape. That means nothing to an infalling observer, and they neither know nor experience anything about it. They could calculate when they'd crossed the EH, just like you could calculate when a trip to the cottage was exactly half over. But nothing changes at the halfway point to the cottage  there's no physical significance to this point. And after all that, rest assured, the infalling observer will proceed to the singularity in an extremely finite time. 


#4
Oct3111, 10:47 PM

P: 28

The idea of anything crossing the event horizon of a black hole from any observer's v
So what exactly is time dilation? Doesn't gravity slow each particle's 'clock' as they approach the source? I have been under the impression that as a particle's movement through space increases, its worldline is skewed so that it takes a path almost perpendicular to the time axis. I noticed that the lorentz factor is just a form of pythagoras's theorem stating the hypotenuse as the speed of light and spacial velocity as one side ofthe triangle. I figured spacetime velocity was constant.
Do you have any recommended reading for me? 


#5
Oct3111, 11:19 PM

Sci Advisor
P: 2,851

The time dilation due to gravity is always an Observer 1 observering observer 2 (in a stronger gravitational field) to have time dilation. To any observer observing his own time, his time MUST click by normally (he cannot, by definition, notice the "time dilation" on himself).
You can see this easily if you think about Einstein's Equivalence principle. No local experiment done in freefall can distinguish whether you are in a gravitational field or not (e.g. you can't tell the difference between just floating out in space, and freefalling in some gravitational field). This means, for the observer freefalling into the black hole, he can't actually tell if he fell through the EH or not at all (assuming he doesn't get "spaghettified"). You can't actually locally measure the curvature of spacetime. You can only measure curvature through tidal forces (this is what causes "spaghettification"). This is particularly apparent if you transform to EddingtonFinklestein coordinates or KruskalSzekeres coordinates (pardon the spelling...), where no singularity appears at the Event Horizon. For this reason, we call the event horizon a "coordinate singularity", it's not a real physical singularity. 


#6
Oct3111, 11:53 PM

P: 28

But relative to the particles ahead of the observer near an event horizon, the distance between the two would be streched. And this would apply to each subsequent particle closer to the horizon. So from any real viewpoint, nothing ever reaches the boundary. Is this different than general relativity? I guess this thinking comes from the idea that space is not a coordinate plane but a measurement of the 'time' it would take a particle to travel to any other particle.
Hope I'm making sense. 


#7
Nov111, 12:01 AM

P: 28

I'm sorry, it's late. The measurement i mentioned was energy, not time.



#8
Nov111, 06:45 AM

Mentor
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#9
Nov111, 08:19 AM

P: 15,319




#10
Nov111, 06:23 PM

Emeritus
Sci Advisor
P: 7,634

Note that an accelerated observer has an event horizon which is mathematically very similar to the event horizon of a black hole.
It is quite possible to imagine the Earth falling into the Rindler horizon of such an accelerating observer. The accelerating observer will never see the earth cross the horizon, but the people on the Earth will see it happen at some specific date, say New Years day in 2012. The accelerated observer watching the Earth fall into the horizon will never see any event beyond this date, i.e. he will only see events before New Years day 2012. However, it would be quite a mistake for the accelerated observer to conclude from this that "the world ends in 2012", or that "the Earth never falls beyond the horizon". It seems to me that this is just the mistake you are making, though. 


#11
Nov111, 06:57 PM

P: 700

The same applies to a black holes event horizon. 


#12
Nov111, 07:44 PM

P: 95

It also doesn't have to be "from the point of a distant observer". Any observer outside the event horizon will do just fine, no matter how close. That means all observers that exist for us. 


#13
Nov111, 07:55 PM

P: 95

If you keep your ideas about black holes simple, and only consider what an outside observer would be able to see, you'll be spared a lot of headaches. 


#14
Nov111, 08:00 PM

Emeritus
Sci Advisor
P: 7,634

("Statements about observer independent facts" is my operational defintion of "statements about reality" by the way). If you qualify the statements, such as "The external observer never sees the object cross the black hole", or "Using the particular notion of simultaneity used by static observers, the time of the event that's simultaneous with the horizon crossing approaches infinity", you'll be OK. Note that the notion of simultaneity used by moving observers is different from the notion of simultaneity used by static observers. If this isn't obvious, perhaps a different discussion on the "relativity of simultaneity" is in order. So saying "it never happens" is false, because for some observers it does happen. 


#15
Nov111, 08:17 PM

P: 15,319

This gets worse the farther apart they are. However, if one spaceship followed immediately behind another into the BH, the apparent discrepancy in events would be correspondingly reduced. To point where, if the observer is pulls up parallel to the initial object (i.e. distance reduced to zero) as it enters the EH, they will not observe the object experiencing this halting at the EH. 


#16
Nov311, 05:22 AM

PF Gold
P: 1,376

So your idea about "shell black holes" seems contradictory. 


#17
Nov311, 06:10 AM

P: 95

You can consider that nothing passes through the event horizon, and still have reason to believe that there are black holes. The fact that nothing passes through the event horizon can be a simplification by considering only what an observer can see, and not bothering with what happens but we can't see. The existence of black holes is not about personal belief, as many were discovered. In the case of Sagittarius A, situated in the center of our galaxy, the evidence is very strong for it being a supermassive black hole. 


#18
Nov311, 08:22 AM

PF Gold
P: 1,376

What specific evidence we can have to claim that this very massive object has something like event horizon? 


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