# How much starlight do black holes absorb?

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Can it be calculated how much light from other stars a black hole of certain sizes would absorb? And how long will it take for the number of active stars to decrease to the point where Hawking radiation would surpass it?

tldr; extremely little. Space is big, even biggest black holes are ridiculously tiny in physical size

The amount of starlight absorbed by black holes is really minuscule. A black hole is not going to be sucking light in: it can be fairly well approximated to be an absolutely black object around the size of its event horizon (or photon sphere, does not really matter for the purpose for this thought experiment, since photon sphere is 1.5x EH).

Now, let's think of occlusions by more familiar objects: the Moon and the Sun. Now let's think our Moon would be a black hole. Retaining its current size, it would need to have mass in range of about 500 solar masses - pretty big for a BH. It would be also orbiting really close to our Sun; so close, it would probably get eaten by the Moon-BH relatively soon. Also, we would be observing this system really close.

But even in this case, we would still see the Sun most of the time unobstructed. Obstruction means absorption of light, thus there's really not much of this happening.

The amount of absorption of light from a star can be calculated by figuring out the ratio of black hole surface area to the surface area of a sphere with the radius equal to distance between the star and the black hole. In most cases you will be using square kilometers for the BH area, and square light years for the sphere. Which is almost exactly the same as comparing single atom mass to grams.

For the second question, star shine does not really contribute much to the temperature of space, so CMB will dominate this until very very far in the future, when all the stars are already gone

## How much starlight do black holes absorb? - Astronomy

I have only one question about black holes, why are we studying them and how are they going to benifit mankind in any way. Looking at asteroids, its possible for one to hit earth, so we should be considering them, but why black holes?

I guess it is true that there is unlikely to be any event in the near future endangering human life that involves Black Holes. However, if that were the criteria upon which we based what we should study there would be no computers for me to tell you this! There are many things which benefit humankind, and I believe strongly that one of them is the search for knowledge. I think most people would agree that art for art's sake benefits humankind, so why not science for science's sake!

Black Holes may also be fundamental to properly understanding gravity which is very useful. In fact it is necessary to include corrections from General Relativity (which has been much better understood through the study of Black Holes) to the Global Positioning Satellites in order to make them accurate to more than a few metres. This may ultimately help blind people have a better standard of living and I believe has already been used to rescue lost people.

### About the Author

#### Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

## Black holes bend light the 'wrong' way

Refraction effect may be distorting astronomers' results.

Astronomers could be misinterpreting their observations of distant stars, suggest mathematicians.

Starlight may be bent in odd directions when it passes close to a rotating black hole, the researchers say, unexpectedly shifting its source's apparent position in the sky. The cause is a recently discovered phenomenon called negative refraction, which physicists are still struggling to understand.

Astronomers already adjust their observations to account for the fact that light is bent by massive objects such as black holes, an effect called gravitational lensing. But Akhlesh Lakhtakia, a mathematician at Pennsylvania State University in University Park, has studied what happens when a black hole rotates. In this case, light is bent in the direction opposite to that predicted by conventional theory.

"Astronomical measurements, particularly those relating to black holes and other massive stellar bodies, need careful reinterpretation," says Tom Mackay of the University of Edinburgh, UK, who worked with Lakhtakia on the analysis, published online in Physics Letters A 1 .

Negative refraction is new to astronomy, but has been causing a stir in materials science in recent years. When light crosses a boundary, it is bent in a characteristic way this is why an oar dipped in water looks as though the submerged part is angled towards the surface.

But in 2001, US researchers showed that certain artificial materials bend light in the opposite direction 2 . If water had this property, the submerged oar would appear to angle away from the surface.

The revelation prompted a flurry of research, most of which has focused on understanding and developing negative refracting materials. "But this is exactly the same phenomena," Mackay points out.

Last year, Mackay and Lakhtakia demonstrated that negative refraction could occur in a vacuum, provided that the gravitational field in the region had the right properties. Now, they have identified something that meets these requirements: a rotating black hole. Very large rotating stars would have the same effect, adds Mackay.

This might force astronomers to rethink some of their observations. "The deflection of light could be significant," says Mackay. In theory, starlight could even turn through a 90° angle, apparently putting the star in a completely different part of the sky. "And the further away the object is, the more likely it is that these effects are interfering with observations," adds Mackay.

However, some researchers question how much influence the effect will have in practice. Matthias Bartelmann, a theoretical astrophysicist at the University of Heidelberg in Germany, describes Mackay and Lakhtakia's paper as very interesting. "But I'm in doubt as to the astronomical relevance," he says. Bartelmann points out that the effect will be limited to small regions of space, as it can only occur in regions where the gravitational field is extremely strong.

The effect could find other uses, however. Theoretical astronomers are currently debating whether the cosmological constant, a key number in the equations that describe the evolution and growth of the universe, is positive or negative. Mackay says that negative refraction can only take place if the constant is positive, so experimental verification of such refraction could help to settle the debate.

## Visualizing black holes

There is a pretty simple way you can imagine this scenario. Pretend that you are floating above a flat surface. Place a golf ball on it. The surface bends a little under the weight. Now place a bowling ball nearby. They will start to roll towards each other. That is gravity!

Place the heaviest golf ball you can imagine on the surface. This ball is so heavy that you cannot see how far down the bend under the ball goes. That is a black hole?

## How can black holes shine?

Artistic view of a radiating black hole. Credit: NASA

We hear that black holes absorb all the light that falls into them. And yet, we hear of black holes shining so brightly we can see them halfway across the Universe. What's going on? Which is it?

I remember back to a classic episode of the Guide to Space, where I provided an extremely fascinating and concise explanation for what a quasar is. Don't recall that episode? Well, it was super. Just super. Alright slackers, let's recap.

Quasars are the brightest objects in the Universe, visible across billions of light years. Likely blanching life from everything in the path of the radiation beam from its lighthouse of death. They occur when a supermassive black hole is actively feeding on material, pouring out a mountain of radiation. Black holes, of course, are regions of space with such intense gravity where nothing, not even light itself, can escape.

But wait, not so fast "recap" Fraser Cain. I call shenanigans. If black holes absorb all the radiation that falls into them, how can they be bright?

You, Fraser Cain of days of yore, cannot have it both ways. It's either a vortex of total destruction gobbling all the matter and light that fall into them OR alternately light can escape, which still sounds good. I mean, it could be WHERE NO STUFF CAN ESCAPE, except light.

If you'll admit that you of the past was wrong, we'll put you in the temporal cone of shame and move on with the episode. Right? Right? Wrong.

Let's review. Black holes are freaky complicated beasts, with many layers. And I don't mean that in some abstract Choprian "many connections on many different levels". They're a gobstopper from a Sam Neill Event Horizon style hellscape. Let's take a look at the anatomy of a black hole, and everything should fall into place, including the terror.

At the very heart of the black hole is the singularity. This is the region of compressed matter that used to be a star, or in the case of a supermassive black hole, millions or billions of times the mass of a star. Astronomers have no idea what the singularity looks like or behaves, because our understanding of physics completely breaks down, along with the rest of our brains.

It's possible that the singularity is a sphere of exotic matter, or maybe it's constantly compressing down into an infinitely small size. It could also be a pork pie. We'll never know, because nothing goes fast enough to escape from a black hole, not even light.

Maybe you'd need to be going 10 times the speed of light to escape. Or maybe a trillion times the speed of light. Which makes it easy as far as we can tell, nothing can go faster than the speed of light, and so nothing is escaping.

As you get further from the singularity, the force of gravity decreases. Initially, it'll still requires that you go faster than light. You'll finally reach a very specific point where the escape velocity is exactly the speed of light. This is the event horizon, and it's a different distance from the singularity with every black hole. That's the line. Within the event horizon, the light is doomed, outside the event horizon, it can escape. This is the hard candy shell surrounding the chocolately unimaginable nightmare of physics.

Illustration of Cygnus X-1, another stellar-mass black hole located 6070 ly away. Credit: NASA/CXC/M.Weiss

So when see bright black holes, like a quasar, we're not actually seeing light coming from inside the black hole itself or reflected of its surface. What we're seeing is the material that's piling up just outside the event horizon. For all its voracious hunger, a black hole's gravitational eyes are much bigger than its stomach, and it can only feed so quickly. Excess stuff piles up around the black hole's face and forms a vast disk of material, just like me at a Pizza Hut's \$5 all you can eat buffet. This pizza heats up until it's like the core of a star, and starts blasting out radiation into space.

Everything I've said is for non-spinning black holes, by the way. Physicists will always make this point with great emphasis. Stay your angry comments astrophysicists, for I have said the magic stone-cutter appeasement code-word, "Non-rotating".

A WFPC2 image of a spiral-shaped disk of hot gas in the core of active galaxy M87. HST measurements show the disk is rotating so rapidly it contains a massive black hole at its hub.

Of course, black holes do rotate, and can rotate at nearly the speed of light. And this rotation changes the nature of the black hole's event horizon in ways that make difficult math even harder. All this spinning generates powerful magnetic fields around the black hole, which focuses jets of material that blast out for hundreds of thousands of light-years. When we see these bright quasars, we're staring right at these jets with our delicate little eyeballs.

So how can we see light coming from black holes when black holes absorb all light? It's not coming from black holes. It's coming from the super-heated region of junk all around the black hole. And still, anything that falls through the event horizon, whether it be light, junk, you, me or Grumpy Cat it will never been seen again.

## Black Holes: Everything You Think You Know Is Wrong

If most people know one thing about black holes, they probably know that nothing can escape from them, not even light.

Yet this most basic tenet about black holes has actually been disproven by the theory of quantum mechanics, explains theoretical physicist Edward Witten of the Institute for Advanced Study in Princeton, NJ, in an essay published online today (Aug. 2) in the journal Science.

Black holes, in the classical picture of physics, are incredibly dense objects where space and time are so warped that nothing can escape from their gravitational grasp. In another essay in the same issue of Science, theoretical physicist Kip Thorne of Caltech describes them as "objects made wholly and solely from curved spacetime."

Yet this basic picture appears to contradict the laws of quantum mechanics, which govern the universe's tiniest elements.

"What you get from classical general relativity, and also what everyone understands about a black hole, is that it can absorb anything that comes near, but it can't emit anything. But quantum mechanics doesn't allow such an object to exist," Witten said in this week's Science podcast.

In quantum mechanics, if a reaction is possible, the opposite reaction is also possible, Witten explained. Processes should be reversible. Thus, if a person can be swallowed by a black hole to create a slightly heavier black hole, a heavy black hole should be able to spit out a person and become a slightly lighter black hole. Yet nothing is supposed to escape from black holes. [Photos: Black Holes of the Universe]

To solve the dilemma, physicists looked to the idea of entropy, a measurement of disorder or randomness. The laws of thermodynamics state that in the macroscopic world, it's impossible to reduce the entropy of the universe &mdash it can only increase. If a person were to fall into a black hole, entropy would increase. If the person were to pop back out of it, the universal entropy tally would go down. For the same reason, water can spill out of a cup onto the floor, but it won't flow from the floor into a cup.

This principle seems to explain why the process of matter falling into a black hole cannot be reversed, yet it only applies on a macroscopic level.

Physicist Stephen Hawking famously realized that on the microscopic, quantum mechanical level, things can escape from black holes. He predicted that black holes will spontaneously emit particles in a process he dubbed Hawking radiation. Thus, quantum mechanics refuted one of the basic tenets of black holes: that nothing can escape.

"Although a black hole will never emit an astronaut or a table or a chair, in practice, it can definitely emit an ordinary elementary particle or an atom," Witten explained.

However, scientists have yet to observe Hawking radiation.

"Unfortunately, the usual astrophysical black holes, formed from stellar collapse or in the centers of galaxies, are much too big and too far away for their microscopic details to be relevant," Witten wrote.

Witten's essay is one of five new papers in Science this week summarizing the state of black hole research.

## Astronomers Find Three Supermassive Black Holes in NGC 6240

NGC 6240, a well-studied nearby galaxy system in the process of merging, contains three supermassive black holes at its core, two of which are active and each with a mass of about 90 million solar masses, according to a study published in the journal Astronomy & Astrophysics.

NGC 6240 harbors three supermassive black holes at its core: the northern black hole (N) is active and was known before the zoomed-in new high-spatial resolution image shows that the southern component consists of two supermassive black holes (S1 and S2) the green color indicates the distribution of gas ionized by radiation surrounding the black holes the red lines show the contours of the starlight from the galaxy and the length of the white bar corresponds to 1,000 light years. Image credit: P. Weilbacher, Leibniz Institute for Astrophysics Potsdam / NASA / ESA / Hubble Heritage / STScI / AURA / Hubble Collaboration / A. Evans, University of Virginia, Charlottesville / NRAO / Stony Brook University.

NGC 6240 is a system of merging galaxies approximately 400 million light-years away in the constellation Ophiuchus.

It spans 300,000 light-years and has an elongated shape with branching wisps, loops and tails.

Until now, astronomers have assumed that it was formed by the collision of two galaxies and therefore contains two black holes in its core.

“Through our observations with extremely high spatial resolution we were able to show that NGC 6240 hosts not two but three supermassive black holes in its center,” said Professor Wolfram Kollatschny, an astronomer at the University of Göttingen.

“Each of the three heavyweights has a mass of more than 90 million Suns.”

“They are located in a region of space less than 3,000 light-years across, i.e. in less than one hundredth of the total size of the galaxy.”

Professor Kollatschny and colleagues observed NGC 6240 using the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s Very Large Telescope (VLT).

“Up until now, such a concentration of three supermassive black holes had never been discovered in the Universe,” said Dr. Peter Weilbacher, a researcher in the Leibniz Institute for Astrophysics Potsdam.

“The present case provides evidence of a simultaneous merging process of three galaxies along with their central black holes.”

According to the team, the discovery of the triple supermassive black hole system is of fundamental importance for understanding the evolution of galaxies over time.

“Until now it has not been possible to explain how the largest and most massive galaxies, which we know from our cosmic environment in the ‘present time,’ were formed just by normal galaxy interaction and merging processes over the course of the previous 14 billion years,” the scientists said.

“If, however, simultaneous merging processes of several galaxies took place, then the largest galaxies with their central supermassive black holes were able to evolve much faster. Our observations provide the first indication of this scenario,” Dr. Weilbacher said.

W. Kollatschny et al. 2019. NGC 6240: A triple nucleus system in the advanced or final state of merging. A&A, in press doi: 10.1051/0004-6361/201936540

## How Do Black Holes Really Work?

Science fiction has often relied on the concept of black holes as a plot device, painting them as portals to other universes or as vehicles for time travel. But what happens when we take the fiction out of it? What’s really going on inside those terrifying faraway entities?

In short, black holes are massive pits of gravity that bend space-time because of their incredibly dense centers, or singularities.. When a star dies, it collapses inward rapidly. As it collapses, the star explodes into a supernova—a catastrophic expulsion of its outer material. The dying star continues to collapse until it becomes a singularity—something consisting of zero volume and infinite density. It is this seemingly impossible contradiction that causes a black hole to form.

The extreme density of the new singularity pulls everything toward it, including space-time. Space-time, in a very basic sense, is the union of space and time as one four-dimensional continuum. So, what happens if you bend it? Well, if you were to experience a black hole up close, time would definitely move much differently from the way it does here on Earth. If you imagine space-time as a suspended flat plane of Silly Putty, then creating a singularity would be like putting a marble in the center. The marble would bend the plane downward dramatically, which would elongate any interaction with the plane toward the marble. The same thing happens with black holes, though the distortion you would experience would be a bit more severe than anything Silly Putty could generate.

At the edge of a black hole, or the event horizon, time begins to slow astronomically. The farther into a black hole you venture, the more distorted time becomes. Some theories even propose that if you could survive the initial entry into a black hole, the inside would produce images of the future and the past all at once—an idea consistent with the multiverse theory of the universe. While this is an interesting concept—and no doubt the origin of many sci-fi favorites—because of the inaccessibility of black holes, there is no known way to test it. What is commonly accepted, however, is that, because of a black hole’s distortion of the space-time continuum, time at the base of its event horizon passes far slower than time on Earth.

Black holes are hard to find, but if you not only did find one but also went inside it, you would discover that it is fatal. The intense gravitational force from the singularity pulls at different rates, depending on location relative to the center, which can produce a “spaghettification” effect on any object unfortunate enough to be caught inside. Just as the word suggests, spaghettification elongates the object in question so that it resembles spaghetti.

We may never be able to prove exactly what happens inside black holes, although many scientists are making the connection between singularities and the big bang theory, which proposes that our universe exploded into existence from what could have been a singularity.

## How does black absorb all the wavelengths?

Something that looks black may only be absorbing the visual spectrum. Fire some radiowaves at it and they might just go straight through it, making it transparent.

In other words if our sight was adapted to see radio waves instead of the visible, the world would look a very different place, many non-metalic objects would appear transparent, and the sky would be constantly lit up with radio and TV signals bouncing around.

Something that looks black may only be absorbing the visual spectrum. Fire some radiowaves at it and they might just go straight through it, making it transparent.

In other words if our sight was adapted to see radio waves instead of the visible, the world would look a very different place, many non-metalic objects would appear transparent, and the sky would be constantly lit up with radio and TV signals bouncing around.

Here is an interesting article on parakeets vision and how they use it

A weird question.. I know that we can use equipment to see in infrared, but the image just uses our perceptible spectrum to give an image of what that would look like. Meaning that the equipment shows us a picture using visible red which isn't the actual color of infrared. Are those waves on the non-visible part of the spectrum colors that we can't comprehend? Do other animals see colors that we don't (and can't) have in our box of Crayolas?

To the original poster: I think you need to look at the problem the other way around: Black appears to us as black because it absorbs all wavelengths. There's not really a reason for 'black' things to do this, it is a fact that they do, and that makes them appear to us as black.

The reflection of the glass is not increased if you place a black object behind it, this is merely a visual illusion (which I believe works equally well with all other solid colors). If there is no black object but a variety of objects behind the glass, it is hard for our eyes to focus on the reflection in the glass and the objects behind the glass simultaneously. If you try really hard however you can focus on the reflection and you will see that it is just as visible as it would be with a black object behind it.
With the black object behind it, there is nothing for your eyes to focus upon behind the glass, so our natural reaction is to focus on the reflection, making it seem more visible.

To 'blipped':
The waves on the non-visible part of the spectrum can be regarded as colors, but I would like to see a definition of 'color' first. I think color is just what appears to us as the visible spectrum. We cannot see infrared or ultraviolet for example, but if we could see it there is no telling how it would look. We might have called infrared red and ultraviolet violet.

Yes, many animals can see more wavelengths than us as far as I know.

To 'blipped':
The waves on the non-visible part of the spectrum can be regarded as colors, but I would like to see a definition of 'color' first. I think color is just what appears to us as the visible spectrum. We cannot see infrared or ultraviolet for example, but if we could see it there is no telling how it would look. We might have called infrared red and ultraviolet violet.

Yes, many animals can see more wavelengths than us as far as I know.

Thought experiment. not really a physics questions though.

We see color in relation to the object it is on, like a leaf or dirt, etc. Our brain has the ability to process these colors because our eye is able to take them in. If we bypass the eye and send signals of an image in ultraviolet light how would our brain handle it?

Thought experiment. not really a physics questions though.

We see color in relation to the object it is on, like a leaf or dirt, etc. Our brain has the ability to process these colors because our eye is able to take them in. If we bypass the eye and send signals of an image in ultraviolet light how would our brain handle it?

I'm not really an expert in brains lol, but I would guess that that depends fully on which type of signals we send. Red colors send a different signal than blue colors for example. If you send the UV light as blue color signals, we see blue light.

You might want to send the UV light as 'UV color signals' but what would these be? I think it would be possible to send red light signals since we know what red is and we can see if the signals actually produce a red light. Since we don't know what UV light looks like I don't think we can send signals and then expect to see UV light.