I What Would it Look Like to Fall into a Black Hole?

[Hornbein]

There are a number of videos of simulations of this. They all end with complete blackness. This seems wrong to me because light is concentrated by the black hole. There should be more light closer to the center.

On second thought I guess it is OK. While the observer would encounter more light, it would all be from behind or the side while the perspective in the videos is straight ahead.

It would be better to approach the black hole obliquely. Not only is this a lot more likely than a direct hit, but then a forward view would show the strong light that is inside the object. Or so I suppose.

Or maybe both a front and rear view. The rear view would shrink into a very bright dot, I would think.

 

[PeterDonis]

light is concentrated by the black hole. There should be more light closer to the center.

Inside the hole this viewpoint does not work. There is no "center" as regards a point in space. The locus $r = 0$, the singularity, is a moment of time, not a place in space, and light is not "concentrated" at this moment of time.

 

[Hornbein]

Inside the hole this viewpoint does not work. There is no "center" as regards a point in space. The locus $r = 0$, the singularity, is a moment of time, not a place in space, and light is not "concentrated" at this moment of time.

OK, then what do you think would be "seen" by our imaginary pointlike observer? What light would intersect that point from which direction?

 

[PeterDonis]

what do you think would be "seen" by our imaginary pointlike observer?

It would depend on what light sources were present and how they were moving. The hole itself is vacuum so it is not a source of light.

If you mean as the observer approaches the singularity, light sources would be expected to disappear from the observer's field of vision (because the region of space that is in the past light cone of the event where the observer hits the singularity gets smaller and smaller).

 

[Ibix]

If you look backwards you would see the sky outside shrinking towards a point, I think. It would be bright due to the concentration of the light into a small angular subtense, but dimmed due to redshift. It's the wrong side of midnight at the moment for me to figure out which effect wins...

 

[Hornbein]

If you look backwards you would see the sky outside shrinking towards a point, I think. It would be bright due to the concentration of the light into a small angular subtense, but dimmed due to redshift. It's the wrong side of midnight at the moment for me to figure out which effect wins...

Both the light and the observer are falling into the black hole so your relative motion is unchanged and no shift is seen, such is my guess.

 

[Hornbein]

It would depend on what light sources were present and how they were moving. The hole itself is vacuum so it is not a source of light.

If you mean as the observer approaches the singularity, light sources would be expected to disappear from the observer's field of vision (because the region of space that is in the past light cone of the event where the observer hits the singularity gets smaller and smaller).

I'm thinking the sights would be fixed stars.

So the observer crosses the event horizon. Everything closer to the singularity than is the falling observer would be invisible and black. But light that is further from the singularity would be visible even though it is inside the event horizon. I'm interested in light that is originally from a star but has been so curved by the black hole that its essentially of random origin and forms a sort of haze. I would think that the closer one got to the singularity the more concentrated and brighter this haze would be. You seem to be saying that this isn't correct, which certainly could be.

I was also thinking that dust and junk would be cofalling, scattering light, making it possible to see what is going on and making the light more random, and that said dust would become more concentrated closer to the center. But perhaps I am wrong about that.

 

[PeterDonis]

Both the light and the observer are falling into the black hole so your relative motion is unchanged and no shift is seen, such is my guess.

You should not guess. You should do the math. When you do, you will find that, to an observer free-falling into the black hole, light from the outside becomes increasingly more redshifted as the singularity is approached, with the redshift diverging to infinite as the singularity is reached.

Everything closer to the singularity than is the falling observer would be invisible and black.

Once again, the singularity is not a place in space. It's a moment of time. You are thinking of it as a place in space, which is wrong; and any reasoning you do based on that premise will also be wrong.

As far as "space" inside the hole is concerned, for the purposes of the scenario we are considering here, it can be considered to be infinite. It is certainly not the case that "space gets smaller" as the singularity is approached. So any reasoning you do based on that premise (which appears to be a premise that is implicit in a number of the things you say) will also be wrong.

 

[Hornbein]

You should not guess. You should do the math.

Well duh. I have tried and I cannot do it. Thank you for helping a layman out, which I believe to be one of the main purposes of physicsforums.

 

[PeroK]

Well duh. I have tried and I cannot do it. Thank you for helping a layman out, which I believe to be one of the main purposes of physicsforums.

There's a calculation here of the last light signals to reach an infalling object:

https://www.physicsforums.com/threa...into-a-black-hole.1012103/page-3#post-6599762

And a calculation of the redshift:

https://www.physicsforums.com/threads/black-hole.104577/#post-861282

 

[Ibix]

Both the light and the observer are falling into the black hole so your relative motion is unchanged and no shift is seen, such is my guess.

No, because you and a light pulse that passes you at some instant took different paths through spacetime to get where you are, so the cumulative effects are different.

 

[Hornbein]

No, because you and a light pulse that passes you at some instant took different paths through spacetime to get where you are, so the cumulative effects are different.

Aha. But then wouldn't the shift be randomly either blue or red?

 

[Ibix]

Aha. But then wouldn't the shift be randomly either blue or red?

No. Where would randomness come from?

 

[Hornbein]

No. Where would randomness come from?

From the different paths.

 

[Ibix]

From the different paths.

Why would they be random? GR is a deterministic theory.

 

[Hornbein]

Why would they be random? GR is a deterministic theory.

The light from different sources -- or even the same source -- would take different paths.

 

[Ibix]

The light from different sources -- or even the same source -- would take different paths.

So? Why would that randomly induce red or blue shift? It's a bit like thinking that if you throw balls in different directions some of them will fly upwards.

 

[Hornbein]

So? Why would that randomly induce red or blue shift? It's a bit like thinking that if you throw balls in different directions some of them will fly upwards.

OK, I'll take your word for it, red shift only.

 

[Ibix]

OK, I'll take your word for it, red shift only.

You don't have to take my word for it - PeroK linked to a calculation.

 

[Hornbein]

You don't have to take my word for it - PeroK linked to a calculation.

There's no way in h that I would ever be able to understand that calculation. I am perfectly willing to take your word for it.