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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
huishbDate: Monday, 07.12.2015, 01:51 | Message # 376
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Just wondering. Also once you cross the event horizon of a black hole and watch the universe collapse behind you would you be engulfed in brightness, or would you be in darkness? If so, is the inside an infinitely big area of space? What if you get lost and cant find your way out? wacko




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HarbingerDawnDate: Monday, 07.12.2015, 02:51 | Message # 377
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Quote huishb ()
would you be engulfed in brightness, or would you be in darkness?

Dark all around you, with the universe being an extremely bright point in the "up" direction.





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WatsisnameDate: Monday, 07.12.2015, 05:02 | Message # 378
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Quote HarbingerDawn ()
Dark all around you, with the universe being an extremely bright point in the "up" direction.


This is a very common misconception, perpetuated by years of documentaries and films that keep getting it wrong. The view of the universe collapses to a bright point above you only if you are trying to accelerate upwards to try to avoid falling in (and we're talking accelerations that are enormously impractical). The reason for that collapsed view has little to do with the black hole, and more to do with relativistic beaming, the effect seen in the video parameciumkid posted previously.

But if you fall freely into the hole, then the view looks like this. This is much less intuitive. Notice you can't tell when you cross the horizon, and the dark void of the black hole still takes up less than half of your field of view even when you are deep inside the horizon and about to meet the singularity.

Furthermore, as you get closer to the singularity, the view gets darker "above", and brighter "around", due to the tidal forces. This strength of tidal force is incredibly lethal.

Quote
If so, is the inside an infinitely big area of space?


This is weird, but there is no clear way to define the volume of space within the black hole. The answer you get depends on your choice of coordinates. You can even choose a coordinate system where the volume of the interior is zero!

The surface area of the event horizon, however, has a very clear definition which does not depend on your coordinate system.

Quote
What if you get lost and cant find your way out?


It is impossible to get lost inside, for there is only one direction you can go: closer to the singularity toward the center. The singularity actively pulls you closer to it faster than the speed of light. There is nothing you can do to avoid it.

Even more bizarre fact: you might consider trying to accelerate upwards, or sideways, to try to at least delay your fate of being crushed at the center. Surprisingly, this has the opposite effect -- it makes you meet the singularity sooner. (Yes, this can be proven.) Black holes are like quicksand made of space -- the more you struggle, the shorter you live. smile





 
LookAtDatDakkaDate: Monday, 07.12.2015, 05:11 | Message # 379
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Quote Watsisname ()
This is a very common misconception, perpetuated by years of documentaries and films that keep getting it wrong. The view of the universe collapses to a bright point above you only if you are trying to accelerate upwards to try to avoid falling in (and we're talking accelerations that are enormously impractical). The reason for that collapsed view has little to do with the black hole, and more to do with relativistic beaming, the effect seen in the video parameciumkid posted previously.


I agree.





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parameciumkidDate: Monday, 07.12.2015, 09:10 | Message # 380
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Whoa o_o
I think you're right! I mean I know I'm just going off my gut feeling again, but that actually makes a lot of logical sense. I think... well at least the guy sounds like he knows what he's talking about xD

And regarding the "volume inside a black hole":
- The volume within the event horizon is easily calculable using the formula for the volume of a sphere. Use the Schwarzchild radius and pi. It's that simple.
- Inside black holes, paths through space and time often switch "roles", insofar as a trajectory through space becomes more like a trajectory through time or vice versa. It's all a bit cerebral, but the upshot is that when you're deep inside the event horizon, the singularity isn't so much in front of you as in your future. You can try to fly about any which way you like, but all possible actions you take will still result, in the end, in you reaching the center (or at least your constituent particles reaching it. Your body would get shredded by tidal forces). Basically, just the way that, when you stand on the North pole, all horizontal directions are South, all directions inside a black hole's event horizon are toward the center.





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Edited by parameciumkid - Monday, 07.12.2015, 09:12
 
JackDoleDate: Monday, 07.12.2015, 09:52 | Message # 381
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Quote parameciumkid ()
The volume within the event horizon is easily calculable using the formula for the volume of a sphere. Use the Schwarzchild radius and pi. It's that simple.

Quote parameciumkid ()
all directions inside a black hole's event horizon are toward the center

And already alone therefore, the volume calculation, as you are suggesting, can not be correct. In a normal, calculable sphere, not all roads lead to Rome, er, I mean, to the center. If you have a something in which space and time are warped so that all directions go to the center, a normal volume calculation can not be right.





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WatsisnameDate: Monday, 07.12.2015, 09:59 | Message # 382
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Sorry, but there is no clear meaning for volume enclosed within a black hole. It really does depend on your coordinate system. I can measure the volume in standard Schwarzschild coordinates, and the result will be zero. :)

http://arxiv.org/pdf/0801.1734v1.pdf

Quote parameciumkid ()
The volume within the event horizon is easily calculable using the formula for the volume of a sphere. Use the Schwarzchild radius and pi. It's that simple.


The formula 4/3*pi*R^3 for the volume enclosed by a sphere only applies in flat Euclidean geometry. The space near or inside a black hole is not flat!

Consider trying to measure the circumference of the black hole by looping some (super awesomely strong) string around its horizon. Do you think you will need 2*pi times the Schwarzschild radius to fully wrap around it? You would be wrong. smile You need significantly less.

Kip Thorne describes this very elegantly in his book "The Science of Interstellar", using an analogy of an ant on a warped rubber sheet. Since the sheet is warped like a funnel, the ratio of circumference to radius diameter (oops) is not pi. It is less than pi.

Quote parameciumkid ()
Inside black holes, paths through space and time often switch "roles", insofar as a trajectory through space becomes more like a trajectory through time or vice versa.


Yes, the metric signature of the space-time switches inside the horizon. The radial coordinate becomes time-like and the time coordinate becomes space-like. This means that you are forced further inward, in much the same way that here on Earth, you are forced towards tomorrow. "The singularity lies in the direction of your future." Or, "all possible paths lead toward the singularity." Different, very cool descriptions of the same physical phenomenon: once inside the horizon, you are destined to meet the singularity. Its gravitational attraction is too strong.





 
HarbingerDawnDate: Monday, 07.12.2015, 11:17 | Message # 383
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Quote Watsisname ()
This is a very common misconception

I was referring to what would be seen in SE with the current implementation, not what would be seen in reality smile





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huishbDate: Monday, 07.12.2015, 13:28 | Message # 384
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Hi SpaceEngineer, I was also wondering: if you fall into the black hole and see the universe collapse behind you and dissapear, what if you got lost and cant get out? is the space in a black hole infinite?




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JackDoleDate: Monday, 07.12.2015, 13:34 | Message # 385
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Quote Watsisname ()
Sorry

When the 'Sorry' relates to what I said, then oops? Is my English so bad? I said nothing of a clear meaning for the volume of a black hole. I just said that it is not to be calculated by normal volume calculations. Or are you saying that standard calculations can indeed be applied, however, lead to stupid results? I must agree then probably. biggrin





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WatsisnameDate: Monday, 07.12.2015, 19:50 | Message # 386
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JackDole, I think I refreshed and made my post before your post appeared. I was referring to parameciumkid. smile You were correct that the usual formula for volume does not apply to the interior of a black hole.




 
sinsforealDate: Monday, 07.12.2015, 20:09 | Message # 387
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Quote SpaceEngineer ()
Yes. But it have no physical sense. Terra with life can exist only in only some artifical system with weak and stable accreation disk and a planet on circular orbit around BH. Like in Interstellar.



What do you mean by but it have no physical sense?





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JackDoleDate: Monday, 07.12.2015, 20:10 | Message # 388
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Quote Watsisname ()
I think I refreshed and made my post before your post appeared.

Okay. wink





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SpaceEngineerDate: Monday, 07.12.2015, 22:49 | Message # 389
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I repeat my reply here:

Quote DenebStarFTW ()
So accretion disk emit heat? Can we technically have terras around them now?

Yes. But it have no physical sense. Terra with life can exist only in only some artifical system with weak and stable accreation disk and a planet on circular orbit around BH. Like in Interstellar.

Quote Watsisname ()
This is a very common misconception, perpetuated by years of documentaries and films that keep getting it wrong.

I know. SE renders black hole for stationary observer. And this have sense: you can lock camera at any distance from the horizon. For ships falling into BH, another rendering is needed. It may be implemented in future, or may be not: any ship will be disrupted by tidal forces or evaporated by accretion disk emission a long time before reaching the horizon. Even billion solar mass BH with no accretion disk will heat up interstellar gas falling to it by 10-20 thousand Kelvins.

BTW, I didn't saw any documentary showing optical effects near black holes. Up to "Interstellar", all documentaries depicted black holes as black spheres with no lensing effects.

Also, this conversation if off-topic here.





 
SpaceEngineerDate: Monday, 07.12.2015, 22:50 | Message # 390
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Quote huishb ()
Hi SpaceEngineer, I was also wondering: if you fall into the black hole and see the universe collapse behind you and dissapear, what if you got lost and cant get out? is the space in a black hole infinite?

Camera is stopped at the horizon, so you can't fall through it.

Quote sinsforeal ()
What do you mean by but it have no physical sense?

Terra with life cannot exist near black hole in reality.





 
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