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Forum » SpaceEngine » Space Journeys » Fear of Black Holes
Fear of Black Holes
WatsisnameDate: Thursday, 07.01.2016, 11:32 | Message # 76
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Much harder, it is, to tell when you pass through the horizon of a black hole on a freefall trajectory. It happens right at the 34 second mark in this video. Maybe a good cue is the moment where the Einstein ring becomes a great circle on the sky, though I'm not sure if that's precise or always the case.




 
DoctorOfSpaceDate: Thursday, 07.01.2016, 12:02 | Message # 77
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Watsisname you can embed vimeo smile







Intel Core i7-5820K 4.2GHz 6-Core Processor
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n0b0dyDate: Thursday, 07.01.2016, 12:21 | Message # 78
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Thanks guys! But I am puzzled Watsisname, . You say that EventHorizon is 4 in SE i.e. when FOV of universe and light becomes a dot but in the YT video it is when FOV of BH = FOV of universe? I prefer SEs rendering more smile but which of the two is more accurate scientifically?

Edited by n0b0dy - Thursday, 07.01.2016, 12:21
 
Kamil_CaderDate: Thursday, 07.01.2016, 14:32 | Message # 79
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Honestly I doubt a human could even survive crossing the moment where gravity starts to significantly bend the light, long before approaching the event horizon, no matter how big the black hole is. Since photons can barely resist the gravitational acceleration already, I cant imagine human body "not feeling a thing" at that moment. Dont know how much G-forces humans can take before dying, though. Ironically, black holes are the least lethal stellar mass objects for humans imo, unless it has an accretion disc. Even a brown dwarf is more dangerous because of radiation of whatever kind.
 
WatsisnameDate: Thursday, 07.01.2016, 23:45 | Message # 80
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Quote n0b0dy ()
Thanks guys! But I am puzzled Watsisname, . You say that EventHorizon is 4 in SE i.e. when FOV of universe and light becomes a dot but in the YT video it is when FOV of BH = FOV of universe? I prefer SEs rendering more smile but which of the two is more accurate scientifically?


They are both accurate views, as seen by different types of observers. SE always shows the view as if from a stationary observer -- a camera which is fixed at that particular point in space. (So this doesn't quite work while you are moving the camera.) In the video linked (and then embedded, thanks Doc!), the view is from an observer making the plunge on a freefall trajectory. This is a computationally more difficult view to show, SE can't do it yet.

Also, for the freefalling observer, the FOV of the black hole is less than the FOV of the outside universe, even when inside the event horizon. It equals the FOV of the outside universe only at the instant you hit the singularity, which is the final frame of the video.





 
WatsisnameDate: Friday, 08.01.2016, 00:15 | Message # 81
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Quote Kamil_Cader ()
Honestly I doubt a human could even survive crossing the moment where gravity starts to significantly bend the light, long before approaching the event horizon, no matter how big the black hole is.


This is not correct. The strength of the bending of light (the gravitational lensing) depends only on distance to the horizon in units of the horizon radius (Schwarzschild radii). The Schwarzschild radius grows in direct proportion to the mass. But what kills you are the tidal forces, not the bending of light. The range of tidal forces grows with the cube root of mass (much more slowly). So the more massive the hole, the weaker the tidal forces at its horizon. Tidal forces at the horizon of a supermassive black hole are very weak. You can see this in SE: compare the tidal force (in units of g per meter) near a stellar mass black hole to a supermassive black hole. BIG difference!

Quote Kamil_Cader ()
Since photons can barely resist the gravitational acceleration already, I cant imagine human body "not feeling a thing" at that moment.


You do not feel gravitational acceleration. Do you feel any gravitational force on you when you are in freefall? Do astronauts feel any forces on them when they are in orbit?

This is the equivalence principle. You cannot tell the difference between freefall in a gravitational field versus floating freely in space with no gravitational field at all. What you feel are forces that prevent you from being in freefall. (Here on Earth's surface, you feel the Earth pushing you upward, preventing you from falling through it.) So if you plunge into a black hole, never do you feel pulled toward the black hole. The only way the gravitational force is felt is in the way it changes in strength and direction from one location to another, and that's what the tidal forces are.





 
n0b0dyDate: Friday, 08.01.2016, 10:39 | Message # 82
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Quote Watsisname ()
They are both accurate views, as seen by different types of observers. SE always shows the view as if from a stationary observer -- a camera which is fixed at that particular point in space. (So this doesn't quite work while you are moving the camera.) In the video linked (and then embedded, thanks Doc!), the view is from an observer making the plunge on a freefall trajectory. This is a computationally more difficult view to show, SE can't do it yet.

Also, for the freefalling observer, the FOV of the black hole is less than the FOV of the outside universe, even when inside the event horizon. It equals the FOV of the outside universe only at the instant you hit the singularity, which is the final frame of the video.


Ok got it. So SEs previous versions were showing the FOVs from a freefalling observers point of view. So the most accurate view would be for the engine to constantly adjust the FOV depending on both a) velocity of observer/camera/spacecraft in relation to the black hole and b)distance from it. Which is very computationally demanding with current hardware/software.

But I can't imagine how cool it would look cry . JackDole / DoctorOfSpace if you are reading this maybe you can do your magic with shader programming wink . Just kidding - I have become more and more excited with SEs development in the recent years..


Edited by n0b0dy - Friday, 08.01.2016, 16:45
 
Kamil_CaderDate: Friday, 08.01.2016, 14:44 | Message # 83
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Watsisname, Aren't tidal forces something that happens when two bodies influence each other like earth and moon?
I think, non influenced by other massive enough bodies singularity would create a constant, not distorted into "waves", "forcefield" of gravity. A perfect "sphere" in which the deeper you go the faster you accelerate. At some point acceleration grows rapidly enough that the difference between the accel at, let's say your feets and your head grow more and more rapidly and it makes you a spagetti. Dont know if i'm clear enough here... or maybe that's what you call tidal force, that difference in acceleration?
 
WatsisnameDate: Saturday, 09.01.2016, 03:44 | Message # 84
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Quote n0b0dy ()
So SEs previous versions were showing the FOVs from a freefalling observers point of view.


Not exactly. SE's previous versions simulated the distortion effect of the black hole with shaders, but the shaders did not actually use the Schwarzschild metric. So it wasn't a relativistically correct representation (hence the weird fractal artifacts when you got close, and even the lensing effect from a distance wasn't completely accurate.) Qualitatively, I guess you can say it was closer to what a free-falling observer would see, but still, it wasn't doing relativistic calculations and so it wasn't showing any observer's view in a true sense.

Now that SE is using the Schwarzschild metric, the visuals are way better, even if only for stationary observers. Showing a non-stationary view is just really, really, hard. On my machine it takes about half a second per frame to render the view near a black hole. Making the free-fall video required a super-computer, and each frame probably took even longer to render.

Quote Kamil_Cader ()
...or maybe that's what you call tidal force, that difference in acceleration?


Yes!

I've been calling it the difference in strength and direction of the force of gravity between two points in space. However, since force causes acceleration, you can indeed use the relative acceleration of nearby test particles, and determine the tidal force that way. In fact, that's exactly what we do in general relativity to define the space-time curvature. In Einstein's view, tidal force is a direct manifestation of curved space-time.

As for it being caused by massive objects influencing each other? I think you may be confusing its effects for the cause. The tidal force is a property of the field produced by any mass. It's just how that field changes from one place to another. It even works the same way for the field of a planet as it does for a black hole singularity (if we're talking the non-rotating Schwarzchild one). If you map it out (the relative change in length and direction of the gravitational force vectors, subtracted from the vector at whatever reference point you choose), you get an image like this. This shows the tidal forces of the Moon, across the Earth. The stretch radially and squeeze the other way -- exactly as how spaghettification works if you fall into a black hole.

I hope that's clearer? Feel free to ask more questions. smile

Edit: Oops, fixed link.





 
Kamil_CaderDate: Saturday, 09.01.2016, 14:43 | Message # 85
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Watsisname, Now I see, thank You. You're a smart guy.
I thought that if theres a point in space where gravity is strong enough to curve the paths of photons significantly, it would be much easier for it to trap heavier&slower particles we are made of. But now I rememberd, even massive clusters of galaxies can create gravitational lensing also... Now I understand the tidal forces but "einstein's rings" nature are still uncertain.
 
WatsisnameDate: Saturday, 09.01.2016, 21:10 | Message # 86
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Yeah, it is a bit counter-intuitive that the gravitational lensing of the light can be seen even where the gravitational field isn't that strong. The key here is the angle of deflection. When we look at a galaxy, it is very far away and the angular size is very small. It only takes a tiny angle for the light to be bent through so that an object far behind that galaxy is seen as a ring. So, if seen from a great distance, we can see Einstein Rings produced by weak gravitational fields. (Weak in the sense that the space-time curvature is very small.)

Near a black hole that angle becomes very significant, so you can see the Einstein ring from up close. At 1.5 times the Schwarzschild radius the bending occurs at exactly the right rate to make the photons travel in a circle around the hole, and we get the "photon sphere".

But now think of what the curvature of that circle (how much angle it turns through after traveling some distance along it) is for black holes of different size. If it's a small, stellar-mass black hole only a few kilometers across, that turn must be very sharp to form a circle! That's very strong-space time curvature, which implies very strong tidal forces. Indeed, this strength of tidal force is lethal. But for a supermassive black hole, the turn is much more gentle to form a much larger circle. This is weaker curvature, and the tidal forces would hardly be felt.





 
PHIJKCHU_SnivyDate: Tuesday, 15.03.2016, 16:53 | Message # 87
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Gas giants always scare me more than black holes.
 
lolcats12345Date: Thursday, 17.03.2016, 05:04 | Message # 88
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I definitely know the feeling, i used to be scared of black holes in space engine as well. After playing for a while i got over it though :P
 
maxitoarchangelDate: Tuesday, 29.03.2016, 17:04 | Message # 89
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Haha, its feels weird when a black hole appears suddenly in front of you. Eveb worse if you have a 60" 4k tv (sitting at 1.5m) that kind of fear is strange Looks like an innate fear to that distorsion of space, not just darkness. Looks like its out of screen. I show to my mom and she thoug it was a flaw on tv.
 
keegs611Date: Tuesday, 12.04.2016, 03:03 | Message # 90
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In honor of all the black hole jump scares I have had in Space Engine, I made this video:
https://www.youtube.com/watch?v=i2NLvcDZ8-E
:D
 
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