how far away from a black hole will light distortion be visible?

[bwanaaa]

gravitational lensing of light is how we currently detect and measure black holes. So a black hole is basically a convex lens with the middle 'blacked out' (because it swallows light in the middle) How far away from a black hole (as a multiple of the blacked out center) can we still see light distortion?

Theoretically, this depends on the black hole density. since gravity is proportional to the inverse square of distance, the 'refractive index' of a black hole decreases with distance.

has anyone written a small app that uses the current powerful gpu cards avaiable to model this kind of light distortion?

Does any knowledgable person here know experimental data in terms of radius of light distortion of a black hole?

 

[silverpig]

That sort of has a lot of answers. First, in order to know you're looking at a distortion, you have to know where the star you're looking for really is. At intergalactic distances, things don't move across the sky at all, so you have no basis for comparison in a "before" vs. "after" way. Second, there are some fairly simple computer models which have been used to solve this problem with up to seven black holes and a light source (last I checked anyways). Third, we don't actually ever see a blacked out center. A black hole has a large accretion disk of matter rapidly falling into it and rotating. This heats and compresses the gases which emit primarily x-rays. We can't resolve the disk or the hole in the center, so what we see is a large powerful x-ray source.

 

[Locut0s]

Originally posted by: bwanaaa
gravitational lensing of light is how we currently detect and measure black holes. So a black hole is basically a convex lens with the middle 'blacked out' (because it swallows light in the middle) How far away from a black hole (as a multiple of the blacked out center) can we still see light distortion?

Theoretically, this depends on the black hole density. since gravity is proportional to the inverse square of distance, the 'refractive index' of a black hole decreases with distance.

has anyone written a small app that uses the current powerful gpu cards avaiable to model this kind of light distortion?

Does any knowledgable person here know experimental data in terms of radius of light distortion of a black hole?

This depends a lot more on the equipment being used to observe the black hole than anything else. We have yet to build telescopes or any instrument for that matter with the resolving power necessary to resolve a black hole's event horizon from the accretion disk surrounding it. Remember we are just now starting to be able to capture images of Jupiter sized objects orbiting nearby stars. If the sun were to become a black hole its event horizon would be on the order of 2 miles across!

 

[bsobel]

Outside the event horizon, the gravitational lensing effect is identical to any other body with mass.

 

[bwanaaa]

tnx for the informative posts. but please explain what you mean that the sun as a black hole would have a 2 mi event horizon

1) to make an object with the sun's mass into a black hole, the necessary density would result in a 2 mi diameter object

OR

2) a black hole the size of the sun would distort light out to a distance of 2 mi from its surface

 

[bwanaaa]

Originally posted by: silverpig
... A black hole has a large accretion disk of matter rapidly falling into it and rotating. This heats and compresses the gases which emit primarily x-rays. We can't resolve the disk or the hole in the center, so what we see is a large powerful x-ray source....

so the accretion disk is inferred because of the xray jets? After all, if the matter falling into a black hole was just a massive cloud of crap, why would it spin? The tidal forces of gravity would overwhelm whatever little kinetic energy in the cloud crap-sort of like the reason the moon doesnt rotate.

 

[bsobel]

Originally posted by: bwanaaa
tnx for the informative posts. but please explain what you mean that the sun as a black hole would have a 2 mi event horizon

1) to make an object with the sun's mass into a black hole, the necessary density would result in a 2 mi diameter object

OR

2) a black hole the size of the sun would distort light out to a distance of 2 mi from its surface

He's saying that if the sun collpased to a black hole, the event horizon ould be 2 miles in diameter. "Mathematically, the size of the black hole is given by GM/c2 where G is the gravitational constant, M is the mass of the black hole and c is the speed of light. So, when one says that the black hole has a size of 5 km, it means that the event horizon is at a distance of 5 km from the center of the black hole. If the Sun were to become a black hole, then its size would be about 3 km."

To the distoring light question, the black hole (outside the event horizon) would distort light the same way as the sun currently does.

 

[bsobel]

a black hole was just a massive cloud of crap, why would it spin? The tidal forces of gravity would overwhelm whatever little kinetic energy in the cloud crap-sort of like the reason the moon doesnt rotate.

The moon rotates and see http://curious.astro.cornell.e...uestion.php?number=725

 

[Biftheunderstudy]

Actually, the lensing from the sun as a black hole would be stronger than non-black hole sun. This is only true for light that passes through the region between the old radius and the event horizon, otherwise it is the same. Lensing from the sun deflects something like 2" near its radius as demonstrated by the (in)-famous experiment by Eddington, in between the normal radius and the event horizon would get more and more dramatic.

Originally posted by: bwanaaa

Originally posted by: silverpig

so the accretion disk is inferred because of the xray jets? After all, if the matter falling into a black hole was just a massive cloud of crap, why would it spin? The tidal forces of gravity would overwhelm whatever little kinetic energy in the cloud crap-sort of like the reason the moon doesnt rotate.

The primary reason why things coalesce into discs and rotate is because they have some initial velocity relative to the central object and will then fall into an orbit, if you can follow the orbital dynamics, things tend to naturally form discs since it is the lowest stable energy configuration.

 

[bwanaaa]

Originally posted by: silverpig
..... At intergalactic distances, things don't move across the sky at all, so you have no basis for comparison in a "before" vs. "after" way. ....

yes, but they wobble due to the gravitational pull of bodies near them. Isnt that how the existence of Gliese 581c was inferred? ( the first low mass extrasolar planet found to be near its star's habitable zone, discovered in April 2007) So, if a black hole's position oscillates because of the solar systems orbiting it, then its position relative to the light from other farther galaxies will appear to wobble for us. Cant this wobbling be used to measure the bending of light from farther galaxies just barely visible past the rim of the event horizon?

 

[fornax]

Originally posted by: bwanaaa
gravitational lensing of light is how we currently detect and measure black holes. So a black hole is basically a convex lens with the middle 'blacked out' (because it swallows light in the middle) How far away from a black hole (as a multiple of the blacked out center) can we still see light distortion?

Theoretically, this depends on the black hole density. since gravity is proportional to the inverse square of distance, the 'refractive index' of a black hole decreases with distance.

has anyone written a small app that uses the current powerful gpu cards avaiable to model this kind of light distortion?

Does any knowledgable person here know experimental data in terms of radius of light distortion of a black hole?

That's not quite true. We mostly detect black holes by their gravitational influence on neighboring objects (stars and gas). Most gravitational lensing that we see today is caused by big, massive galaxies.

 

[Locut0s]

I'm not sure if there is any examples of the detection of the gravitational lensing from any blackhole in the literature, I could be wrong. I would think we just don't have the resolving power yet. The gravitational lensing that we have seen largely comes from entire galaxy clusters acting as a gravitational lens. Most black holes have been inferred based on the momentum of material observed to be orbiting near the centre of galaxies (or other compact volumes). Based on the amount of material orbiting the central several light years of a galaxies core and given the speed this material is orbiting at one can easily compute the amount of matter required to exert these gravitational forces. Given how much matter we are talking about in these cases and how compact the region of space being observed the most likely candidate are large black holes. Gravitational lensing never comes into play.

 

[bsobel]

Originally posted by: Biftheunderstudy
Actually, the lensing from the sun as a black hole would be stronger than non-black hole sun.

Why, the mass is the same...?

 

[Locut0s]

Originally posted by: bsobel

Originally posted by: Biftheunderstudy
Actually, the lensing from the sun as a black hole would be stronger than non-black hole sun.

Why, the mass is the same...?

It wouldn't be any different at the same radius as the suns current surface. But remember the blackhole would have the same mass compacted into a vastly smaller volume so you could get MUCH closer. Closer in the gravitational force would be higher and the lensing effect would be greater.