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Black holes are created by the gravitational collapse of massive stars.

But in theory, black holes can also be created by any mass at all that is confined within a small enough volume of space, defined by the Schwarzschild radius.

This question got me thinking:
If you take matter such as gold and add enough of it, eventually it would become dense enough to be gravitational distorted. This is called gravitational lensing.

If you keep adding more and more mass, eventually a black hole is formed, where you cannot see the matter anymore (although you can still detect it in various ways)

So, this made me realize that there must be some kind of intermediate step, between a visible object with high gravitational lensing, and a black hole.

Would the object appear to slowly get darker and darker as more mass gets added, as the escape velocity gets closer and closer to the speed of light?

This would be an interesting object to place in my universe, and an interesting discovery. For bonus points, what would be easier to detect? This theoretical object, or a black hole? (since this object is not a star, would it not radiate?)


If this theory is completely wrong, please put that in the answer and explain it. I am not an expert in this field of science.

Worldbuilding context: It's just an interesting celestial object whose discovery I would put in my story and its history books and whatnot.

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  • $\begingroup$ Gravitational lensing has nothing to do with the shape, distorted or otherwise, of an object. Gravitational lensing is the bending of light as it passes near a large mass. And an object with enough mass to produce significant gravitational lensing of light is a star. $\endgroup$ – AlexP Nov 11 '19 at 15:30
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Yes an object would appear to get darker as its mass grows.

A high mass object distorts spacetime and causes light to be stretched out or "red shifted" as it tries to escape. An object on he cusp of collapsing into a black hole would be so massive, that the light would be red shifted outside the range of human perception.

This video explains it better than I can: https://www.youtube.com/watch?v=ljyDoxl-ybc A photon leaving a massive object is literally losing energy as it struggles to move to a "higher" spacetime region.

Fig. 1 - A photon losing energy and redshifting its energy away as it tries to leave a gravitational well.

Source: https://en.wikipedia.org/wiki/Gravitational_redshift

However, the accretion disk around the growing black hole could still be seen.

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    $\begingroup$ This only applies if the object in question is the source of that light or doesn’t reflect higher frequencies: otherwise the light shining on the object blueshifts on its way down and redshifts on its way back up. $\endgroup$ – Joe Bloggs Nov 11 '19 at 18:34
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Short answer? Not really.

Longer answer:

The intensity of light isn’t altered by gravitational forces. If I shone light onto this hypothetical object the same number of photons would bounce back off it and climb up the gravity well. As the speed of light is the same for all photons (locally, at least!) there won’t be a proportion of photons lost, so the intensity of light will be the same.

What will happen however is an effect called gravitational redshift. This is very similar to regular redshift, luckily! Basically the wavelength of the light will shift towards red/infrared as the photons climb.

But wait... your object isn’t radiating. Any light that’s climbing out of the gravity well will be redshifted by the same amount that it was blueshifted as the photons fell in the first place. Net result: the light you see reflected from the object is the same as the light you fired at the object.

Now: some oddities may occur, since there will be temporal dilation going on near the object’s surface (light can seem to go slower if you look at it from a different frame of reference), there will be contraction going on (space will be shorter to make sure that the speed of light is still locally correct), and you’ll be dealing with degenerate matter (whose optical properties are unclear), but there won’t be a gradual ‘darkening’ like you might think.

It would be a very interesting thing though. How the heck did that much matter get together without heating to the point where it’s radiant??!?

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  • $\begingroup$ Wait, so the light doesn't "slow down" when the escape velocity is near the speed of light? How does that make sense? Light has mass. $\endgroup$ – overlord Nov 11 '19 at 15:26
  • $\begingroup$ @overlord-ReinstateMonica if you shine a light onto your object, the light will gain energy as it gets closer, and lose it as it gets further. For any normal object, these two would cancel out perfectly. For a black hole, frame-dragging actually allows you to sap angular momentum from the black hole with photons leaving with more energy than when they arrived, $\endgroup$ – John Dvorak Nov 11 '19 at 15:30
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    $\begingroup$ @overlord-ReinstateMonica Photons are massless particles. As they try to leave a black hole they lose energy due to red shifting. $\endgroup$ – SurpriseDog Nov 11 '19 at 15:31
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    $\begingroup$ @SurprisedDog: I’m talking locally. The speed of light is the speed of light. The amount of time I see light taking to get from A to B is subject to the whims of good old GR. $\endgroup$ – Joe Bloggs Nov 11 '19 at 15:35
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    $\begingroup$ Consider: You have a 97% black hole. Light coming in is blue shifted into hard x-ray. X-rays don't reflect well. Instead they heat the surface to a nice red glow, which gets shifted to microwave on the way out. So, yes, it gets darker as it gets heavier. $\endgroup$ – Sherwood Botsford Nov 11 '19 at 16:13
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First i want to make something clear. Basically EVERYTHING produces gravitational lensing, as everything interacts gravitationally with everything. Even light.
Only problem is, most masses are to small to have an measurable effect, so only bigger and more massiv objects are interesting.

Now, there are objects massive and dense enough, that they visibly bend the light. Neutron stars are leftovers of dead stars. The Name already says it, but they are so dense, that the electrons are pushed into the core of the atom and into every proton, they they turn into neutrons. So what every element you had before, now you have that much of theoretical neutron matter.

Secondly, light always travels at the speed of light. You cannot reduce the speed of light, but a lightwave can loose energy by extending its wavelength. Basically, if light looses energy, it gets red. That`s why it is called redshifting. So when light tries to escape from an dense object, it will not get dimmer and dimmer (as the amount of photons will stay the same) but it will get red and more red, until it is no longer visible to the human eye and then even further.

Since for now we are only able to reliable detect electromagnetic waves, only the neutron star would be directly observable. A black hole is just black and can only be detected by its effect on its surrounding.

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  • $\begingroup$ The speed of light is always c locally, but this question was written from the point of view of a distant observer who will see light taking a long time to climb out of the gravity well: physics.stackexchange.com/questions/77227/… $\endgroup$ – SurpriseDog Nov 11 '19 at 15:53
  • $\begingroup$ @SurprisedDog Sorry, i know you are right, but that does not matter for the answer, or does it? The effect stays the same from the outside view. $\endgroup$ – PSquall Nov 11 '19 at 16:07
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Nope. Sorry.

The problem is, by the time you start getting significant gravitational effects you have already gotten to well beyond what matter can support. This is one lesson of the Chandrashekhar limit. If an object is large enough to gravitationally crush its own matter enough to produce significant red-shift, then it is strong enough to disappear behind its horizon.

Think of it another way: The nature of the forces that hold particles apart (photons, atoms, nuclei, electrons, protons, neutorns, quarks, etc.) are all speed-of-light based. So if something starts turning that back on itself, it will do it for all of the forces involved. The electromagnetic force, the weak nuclear force, the strong nuclear force, all get curved back on themselves when gravity gets that insistent. So the forces that previously held those particles apart start to fail to do so at exactly the place that light does.

So strong red-shift for light means strong crushing means collapse.

Nope. No almost black holes.

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