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15 Sep 2020 06:23

JackDole wrote:
To be honest, I don't know how that is possible either. But fact is, the bigger a black hole, the lower the gravity on the 'surface'.
For example, there was the theory that the whole universe is a black hole! https://en.wikipedia.org/wiki/Black_hole_cosmology

That's a different concept.  Poplawski and Smolin made the conjecture that the universe exists inside a black hole inside a larger universe which is much different from the universe actually being a black hole.  It's an elegant possible solution to the chasm which separates relativity and quantum mechanics.
And if you were "inside" a supermassive black hole, you probably wouldn't even realize it (nor would you be torn apart).
 
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15 Sep 2020 06:48

A-L-E-X wrote:
Source of the post But what about black hole cosmology, which states that the universe isn't a black hole but exists inside a black hole inside a larger universe?

¯\_(ツ)_/¯

It's a cool idea, but it doesn't yet provide us with a good way to find out if it is correct. The practical falsifiability is lacking, which is typical of many conjectures.

A-L-E-X wrote:
Source of the post And if you were "inside" a supermassive black hole, you probably wouldn't even realize it (nor would you be torn apart).

The more massive the black hole then the more time you may have to enjoy in the interior (yes, you meaningfully experience a black hole's interior) before being torn apart near the singularity, but your view of the universe would be profoundly distorted.
 
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15 Sep 2020 06:54

Wat you gave me an idea!  Is there a way to simulate a voyage into the interior of a supermassive black hole using SE?  I made a video traveling through the central black holes of our galaxy, M31 and M33 but it just showed the stars on the "other side" rather than what I would see from the interior.  I dont know if I did it correctly though, I just centered the black hole and traveled straight ahead.  The accretion disk kept getting larger and then all I saw was black and then I popped out on the other side and saw lots of stars.

And about the falsifiability of Poplawski's conjecture, I think something that might help us figure that out is the new generation of high power particle colliders?  When they have the ability to collide particles at high enough intensities they might be able to simulate early universe conditions.  Meanwhile we shall have to satisfy ourselves with supercomputer simulations....I posted a Quanta magazine article on a supercomputer simulation that showed the viability of the Big Bounce idea, perhaps you could give a commentary on the article at some point.
 
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15 Sep 2020 07:07

A-L-E-X wrote:
Source of the post Is there a way to simulate a voyage into the interior of a supermassive black hole using SE?

Not currently. It would take a more complicated relativistic rendering system, which would also likely be very slow and computationally expensive. For now, SE calculates the view of a black hole according to an observer who is always at rest at that particular location when the frame is rendered. Because no observer can be at rest on or within the horizon, you actually cannot enter a black hole in SE. When you fly toward one and see the blackness surround you, it's the effect of relativistic aberration, since you are accelerating upward like mad against a waterfall of space in order to stay in place there, which concentrates and blueshifts your field of view above you.

While SE can't yet give you a perspective of such a journey into a black hole, this youtube video shows it accurately (rendered by Andrew Hamilton with website and explanation here.) You cross the horizon 34 seconds into the video (not obvious at all), and hit the singularity in the final frame.



One thing to note though is that the video is slowed down dramatically as you get closer (not because of time dilation, but just to make the journey easier to perceive). In reality, the time you experience inside the hole is very short compared to the time you experience approaching it. Some more detail about the counter-intuitive effects of time experienced inside a black hole in my post about falling into SgrA* here.
 
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15 Sep 2020 11:50

Everything goes red in that video just before the singularity, but are the colours accurate?  Since you would reach crazy speeds relative to the background stars before the event horizon is crossed, why don't we see any redshift?
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15 Sep 2020 20:12

midtskogen wrote:
Source of the post Since you would reach crazy speeds relative to the background stars before the event horizon is crossed, why don't we see any redshift?

Great question! Your speed relative to the stars would indeed be enormous when falling into a black hole. In fact, it is equal to the speed of light at the moment of crossing the horizon (as measured by observers stationed arbitrarily close to the horizon), and this is true no matter how you might try to enter the black hole (drop in from rest just above it, or from far away above it, or on a rocket ship accelerating into it, or anything else.) So because you cross the horizon at the speed of light, you'd think the light from distant stars should be infinitely redshifted.

But wait. At the event horizon, the time dilation is infinite. So your view of the distant universe is infinitely sped up, and therefore shouldn't the light be infinitely blueshifted? 

Which of these arguments is right?

Both are right! 

Not just sort of right, but exactly right, and because both are exactly right, they exactly cancel and there is no shift at all. :P If you freely fall into a black hole, then you observe no time dilation or shift in wavelength of distant starlight. 

One way to reason why this is true is that those photons are making the same journey that you are. If you were hovering at rest close to the black hole, then you would observe them blueshifted, because they gain energy falling down the gravitational well. But if you're in freefall, then they are not gaining energy relative to you. Or in terms of time dilation, the special relativistic effect of your speed exactly cancels the general relativistic effect of the gravity.

A perhaps better way (more in line with how we like to teach general relativity) to reason it out is that freefall is the most natural frame of reference to be in. It is an inertial frame, in which physics behaves exactly the same as if you were freely floating in space with no gravitational field at all. Therefore by the equivalence principle you measure no effects on the light reaching you. Whereas if you hover near the black hole, then you are actually accelerating against the inflow of space, equivalent to being on an accelerating rocket ship far from any gravitational field, in which case you observe the time dilation and relativistic aberration effects, with the sky compressed and blueshifted in the direction you are accelerating.

midtskogen wrote:
Source of the post Everything goes red in that video just before the singularity, but are the colours accurate?

Yes, this was a surprise to me as well. The colors are indeed accurate. The effect here is not caused by gravitational redshift, but rather by the extreme tidal forces near the singularity, which become infinite at the singularity itself. These tidal forces are literally ripping the wave fronts of light apart in the same way that they would rip your body apart -- pulling them apart in the radial direction, while compressing them in the perpendicular directions. If you fall in feet first, this causes a redshift of the light from above and below, and a blueshift of the light about your middle.

In reality you would not have much time to notice this effect, because it is only significant just before you hit the singularity, and you would be getting destroyed by those tidal forces in that same moment. That span of time between when the tidal forces become violent and your meeting the singularity is a small fraction of a second, and this turns out to be independent of the mass of the black hole. (Whereas the time you experience from the horizon to destruction by tidal forces and the singularity does depend on the mass, and is greater for a more massive black hole.)
 
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15 Sep 2020 23:47

Yes, I wondered if that would cancel each other out, but then I saw that the colours still were shifted, so I thought it wasn't that simple.

If you were already approaching the black hole close to the speed of light, so stuff in front of you should be blue shifted and stuff behind you should be red shifted, what then when diving into the black hole?
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16 Sep 2020 00:52

midtskogen wrote:
Source of the post If you were already approaching the black hole close to the speed of light, so stuff in front of you should be blue shifted and stuff behind you should be red shifted, what then when diving into the black hole?

In this case your journey to the center will be shorter (no surprise), and the black disk of the hole will appear smaller than it otherwise would. You can think of your motion as swooping in even faster than the "waterfall of space", so you would encounter light rays coming from even steeper angles in front of you, and therefore the outside universe takes up more of your sky. Yes, the sky behind you will be redshifted, and the sky in front will be blueshifted.

In the limit that your speed (before getting close to the hole) goes to c, I expect the duration of your experience both inside and out of the black hole goes to zero, and the apparent size of the black hole also goes to zero.
 
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16 Sep 2020 02:31

Watsisname wrote:
Source of the post Yes, the sky behind you will be redshifted, and the sky in front will be blueshifted.

But approaching the black hole behind and in front get somewhat smeared out, so would there rather be bands of red and blue?

The short duration could be compensated by making the hole larger.
Maybe this is something best answered by simulations, as it's easy too overlook some effects.
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17 Sep 2020 01:47

midtskogen wrote:
Source of the post But approaching the black hole behind and in front get somewhat smeared out, so would there rather be bands of red and blue?

Objects behind you will be aberrated into view ahead, but there won't be bands of color. Just a smooth transition from redshift behind to blueshift ahead, as illustrated here.

The reason is that gravitation is a conservative force, even in general relativity with black holes. This means the change in energy, and thus color, of a photon is path independent, and only depends on the change in radius in the gravitational field. Since all starlight comes from very far away from the black hole, where its gravitation is negligible, the change in color of starlight due the gravity is the same regardless of the direction it came from. It won't even matter if a photon made multiple orbits near the photon sphere before reaching you vs. came in from directly behind you.

So, the effects of the black hole's gravitation and your extra velocity do not entangle. We can treat them separately. If you fall in with some additional velocity, then you introduce the special relativistic aberration, but there's still no change in color across the sky from the gravitation because of the path independence.

The special relativistic aberration effect also appears in the above video. The camera didn't drop straight in, but started off with a little sideways velocity, and spirals inward. That sideways velocity increases closer to the black hole's center, blueshifting and brightening the view in that sideways direction of motion. (Another way to think of this effect is that the camera is plowing sideways into a rain of photons -- causing them to appear to come more from that direction and with greater numbers and energy, just like driving in rain and having the raindrops hit your front windshield harder.) You can really begin to notice it at around the 35 second mark, as an intensifying glow directly ahead and above the black disk.

Here's another useful visual (source, again Andrew Hamilton's work) showing what it would look like to be orbiting at close to the speed of light, just above the photon sphere. Special relativistic aberration concentrates, brightens, and blueshifts the view ahead. The aberration also warps the black disk, making it appear larger and take up almost half the sky. (Aberration preserves the shape of a circle, but not its size.)
Image
 
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17 Sep 2020 09:52

Watsisname wrote:
midtskogen wrote:
Source of the post Since you would reach crazy speeds relative to the background stars before the event horizon is crossed, why don't we see any redshift?

Great question! Your speed relative to the stars would indeed be enormous when falling into a black hole. In fact, it is equal to the speed of light at the moment of crossing the horizon (as measured by observers stationed arbitrarily close to the horizon), and this is true no matter how you might try to enter the black hole (drop in from rest just above it, or from far away above it, or on a rocket ship accelerating into it, or anything else.) So because you cross the horizon at the speed of light, you'd think the light from distant stars should be infinitely redshifted.

But wait. At the event horizon, the time dilation is infinite. So your view of the distant universe is infinitely sped up, and therefore shouldn't the light be infinitely blueshifted? 

Which of these arguments is right?

Both are right! 

Not just sort of right, but exactly right, and because both are exactly right, they exactly cancel and there is no shift at all. :P If you freely fall into a black hole, then you observe no time dilation or shift in wavelength of distant starlight. 

One way to reason why this is true is that those photons are making the same journey that you are. If you were hovering at rest close to the black hole, then you would observe them blueshifted, because they gain energy falling down the gravitational well. But if you're in freefall, then they are not gaining energy relative to you. Or in terms of time dilation, the special relativistic effect of your speed exactly cancels the general relativistic effect of the gravity.

A perhaps better way (more in line with how we like to teach general relativity) to reason it out is that freefall is the most natural frame of reference to be in. It is an inertial frame, in which physics behaves exactly the same as if you were freely floating in space with no gravitational field at all. Therefore by the equivalence principle you measure no effects on the light reaching you. Whereas if you hover near the black hole, then you are actually accelerating against the inflow of space, equivalent to being on an accelerating rocket ship far from any gravitational field, in which case you observe the time dilation and relativistic aberration effects, with the sky compressed and blueshifted in the direction you are accelerating.

midtskogen wrote:
Source of the post Everything goes red in that video just before the singularity, but are the colours accurate?

Yes, this was a surprise to me as well. The colors are indeed accurate. The effect here is not caused by gravitational redshift, but rather by the extreme tidal forces near the singularity, which become infinite at the singularity itself. These tidal forces are literally ripping the wave fronts of light apart in the same way that they would rip your body apart -- pulling them apart in the radial direction, while compressing them in the perpendicular directions. If you fall in feet first, this causes a redshift of the light from above and below, and a blueshift of the light about your middle.

In reality you would not have much time to notice this effect, because it is only significant just before you hit the singularity, and you would be getting destroyed by those tidal forces in that same moment. That span of time between when the tidal forces become violent and your meeting the singularity is a small fraction of a second, and this turns out to be independent of the mass of the black hole. (Whereas the time you experience from the horizon to destruction by tidal forces and the singularity does depend on the mass, and is greater for a more massive black hole.)

If we were approaching a ring singularity in a spinning black hole would the tidal forces be as strong near the center?  I remember we talked about certain theories that they might not be.
 
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17 Sep 2020 11:35

A-L-E-X wrote:
Source of the post If we were approaching a ring singularity in a spinning black hole would the tidal forces be as strong near the center?

I believe that depends on the mass and the spin of the black hole. One could imagine a combination such that the ring singularity may be passed through with negligible tidal effects.

However, I again feel the need to emphasize that a ring singularity does not exist in nature. It's a feature of the Kerr metric, but the Kerr metric doesn't accurately describe the deep interior of a real rotating black hole, because it assumed all the mass was contained at a central location from the beginning.
 
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17 Sep 2020 12:41

Watsisname wrote:
Source of the post Objects behind you will be aberrated into view ahead, but there won't be bands of color. Just a smooth transition from redshift behind to blueshift ahead, as illustrated here.

The reason is that gravitation is a conservative force, even in general relativity with black holes. This means the change in energy, and thus color, of a photon is path independent, and only depends on the change in radius in the gravitational field.

What I don't quite catch here is that the black hole could make me see the redshifted things behind me in two different directions, once directly (behind) and once bent around the black hole (in front).  Therefore I would expect bands.
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17 Sep 2020 13:16

Watsisname wrote:
A-L-E-X wrote:
Source of the post If we were approaching a ring singularity in a spinning black hole would the tidal forces be as strong near the center?

I believe that depends on the mass and the spin of the black hole. One could imagine a combination such that the ring singularity may be passed through with negligible tidal effects.

However, I again feel the need to emphasize that a ring singularity does not exist in nature. It's a feature of the Kerr metric, but the Kerr metric doesn't accurately describe the deep interior of a real rotating black hole, because it assumed all the mass was contained at a central location from the beginning.

Yes, I remember, everything would have to be perfect for that to happen; I think you compared it to a waterfall flowing upward lol.  I did love the Penrose diagrams.

Here's a different take on it though- given that it doesn't exist in nature- do you think it's possible that humanity (or some other technologically advanced species) could create such a black hole in the (even if distant) future?
 
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18 Sep 2020 01:49

midtskogen wrote:
Source of the post What I don't quite catch here is that the black hole could make me see the redshifted things behind me in two different directions, once directly (behind) and once bent around the black hole (in front).  Therefore I would expect bands.

Aye, I think I see your confusion, and it's my fault for not explaining the special relativistic aberration effect very well. Yes, the black hole generates many images of the same object, because light rays may circulate around many times near the photon orbit before spiraling back to reach you. But the special relativistic red or blueshift of the image due to your velocity does not depend on whether the source of that light was behind or in front of you, but rather only on the direction from which the light ray ultimately intersects you, and aberrated (appearing to come more from the direction you are flying to). Normally in special relativity this distinction wouldn't matter, but with gravity bending the paths of light rays, it is surely confusing.

So an image that was lensed by the black hole to appear in front of you -- of an object that is actually behind you -- will be seen to be blueshifted, not redshifted (if you're hurtling toward the black hole with a great initial velocity). This is because the change in the photon's energy did not depend on the path it took around the black hole (that's what I mean earlier by the path independence due to gravity being a conservative force), but you are "driving into it" (invoking the driving in rain analogy for aberration), and because you can't measure a faster speed of light you instead see it have a higher energy (bluer color). You'll get multiple images, but they'll progressively go from redder appearing behind you, to bluer appearing more directly ahead.

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