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AstronomyAnswerBook: Black Holes


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1. Do Black Holes Exist? ... 2. Black Holes ... 3. Gravity Attracts Light ... 4. The Size of a Black Hole ... 5. The Formation of a Black Hole ... 6. Diving into a black hole ... 7. Combining Black Holes ... 8. Worm holes ... 9. Characteristics of Black Holes ... 10. Schwarzschild Singularities ... 11. The Inside of a Black Hole ... 12. Black Holes are No Threat to Earth ... 13. The Black Hole in the Center of the Milky Way ... 14. Bending of Space-Time Near a Black Hole ... 15. Why Study Black Holes If They Are No Threat To Earth?

This page answers questions about black holes. The questions are:

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1. Do Black Holes Exist?

It is not easy to find black holes because they are black and you can't see them. We can only detect black holes through the influence they have on their surroundings. Hard proof is if there is so much mass in a small area that it can't be anything except a black hole. If you think there is so much mass in a small area that it has to be a black hole but you aren't sure, then it is a "candidate black hole". Even things that seem to fit the rules for a black hole are often still called candidates because there might in principle still be ways that we haven't yet discovered by which there may be that much mass in a small area and yet not be a black hole, and also because it isn't easy to measure the mass and size of such candidates and strange effects may make the mass appear greater or the size smaller than they really are. We just don't know enough yet about such very massive and dense things to be completely sure.

However, so many black hole candidates have been discovered already that it seems very unlikely to me that all of them are "normal" things that just happen to resemble black holes in all measurements that we've made of them. So, I am convinced that black holes do really exist.

2. Black Holes

One of the things that the theories of Newton and Einstein predict is that in exceptional circumstances gravity can get so strong that not even light can escape from it. To create such a case you must compress matter very very hard, much harder than we can do on Earth. A body that has such strong gravity that not even light can escape from it had to look black, so it has been called a "black hole" for about the last 40 years. Calculations of the life of stars show that only the heaviest stars will end their life as a black hole. The Sun is not nearly massive enough to turn into a black hole.

Nothing at all (whether it be material, radiation, or information) can escape from inside a black hole. The outer boundary from inside which nothing can escape is called the event horizon. You cannot see, feel, or measure the event horizon. If you fall into a black hole then you won't notice it when you pass the event horizon, but once you pass that boundary then you cannot escape from the black hole anymore.

On Earth we also have invisible, unmeasurable boundaries that separate areas where you can and cannot do some things. For instance, there is such an invisible and unmeasurable boundary above your head that indicates how high you can reach if you jump as high as you can. If you fall into a dry well with smooth sides, then you can get out by yourself if the rim of the well is below that invisible boundary: then you can jump up and grab a hold of the rim and climb out. If the rim of the well is above the invisible boundary, then you're stuck. The same goes for climbing trees: if the tree is thick and smooth and if there are no tree limbs below your jump boundary, then you cannot climb that tree.

The difference between the well and the black hole is that if you are stuck in a well that is too deep then you can still get out if someone else helps you (maybe with a rope), but if you are stuck in a black hole then nobody can get you out. If your friend throws one end of a rope to you in the black hole, then the rope would become so heavy because of the strong gravity of the black hole that nobody and nothing can lift it up (unless it breaks outside the event horizon, in which case the bottom part would drop completely into the hole and you'd still be stuck).

3. Gravity Attracts Light

Gravity attracts light as well as material, so if light passes through the event horizon into the black hole, then it is doomed, just like anything else. Nobody knows what the mass inside a black hole looks like, but even if it were a perfect mirror and reflected the light right back up, the light would not be able to escape the gravity of the black hole. It would go up and up, but slower and slower all the time (in some sense - time itself behaves strangely in very strong gravity), until finally it turned around (still below the event horizon) and fell back down, just like when you throw a stone up into the air.

This may sound strange, because we never see light bend around and come back again, but that is because the gravity of the Earth and the Sun and everything else in our solar system is not nearly strong enough to affect light much. If you squeezed the Sun together until it became a black hole, then the gravity at its event horizon would be about a million million times as strong as the gravity on the Earth's surface. Then it could prevent light from escaping.

Any object can turn into a black hole if it is squeezed small enough. The limiting diameter is called the Schwarzschild diameter, and in the simplest case it is proportional to the mass of the object. For instance, the Sun would turn into a black hole if its diameter were squeezed smaller than its Schwarzschild diameter of 3.7 miles (5.9 km). The Earth would have to be squeezed smaller than 0.7 inches (1.8 cm), and a human would have to be squeezed much smaller than an atom. The Schwarzschild diameter is also the diameter of the event horizon, so an object turns into a black hole if it is squeezed to within its own event horizon.

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4. The Size of a Black Hole

Because we don't know the shape of the matter after it has disappeared into a black hole (or even if it is matter at all anymore), we cannot say how large that shape is. So, the best measure for the size of a black hole is the size of the horizon of the black hole.

For an ordinary object with a particular shape there are fixed relationships between the different measures for the size of the object: If, for example, you know the diameter of a ball, then you can easily calculate the radius, the circumference, the surface area, and the volume of that ball. It really doesn't make much difference which measure you use, because you can calculate the other ones from that one.

Because space and time are strongly distorted near a black hole, such relationships do not necessarily hold there. For example, a black hole doesn't have a diameter of which you can measure the length with something like a ruler, because if you pass through the horizon to start using your ruler, then you can never get out again.

When scientists talk about the diameter of a black hole, then they mean the diameter that an ordinary object has (that is not a black hole) with the same circumference or surface area as the horizon of the black hole. In the simplest case, this is the Schwarzschild diameter, which is proportional to the mass of the black hole. The Schwarzschild diameter is also a reasonable estimate for more complicated black holes. If \(M\) is the mass, then the Schwarzschild diameter \(d_\text{s}\) is equal to

\begin{equation} d_\text{s} = \frac{4 G M}{c^2} \end{equation}

where \(G\) is the universal constant of gravity (6.672 × 10−11 m³ s⁻² kg⁻¹) and \(c\) is the speed of light (299792458 m/s). The following table shows the Schwarzschild diameter for various masses.

Table 1: Schwarzschild Diameters

Mass Schwarzschild Diameter
1 Sun 5908 m
1 Earth 1.775 cm
1 kg 2.969 × 10−27 m

5. The Formation of a Black Hole

There appear to be different kinds of black holes: there are "ordinary" black holes, with about the same mass as a star, but there are also supermassive black holes that have millions of times more mass than a star. The supermassive black holes tend to reside in the center of galaxies.

The only process we know of that can form ordinary black holes is the evolution of very massive stars (that start out being more than about ten times heavier than the Sun).

It isn't clear yet how the supermassive black holes in the centers of galaxies form. There are various theories:

  1. slow growth of an ordinary black hole that formed from a (giant) star.
  2. formation from a giant cloud of gas that collapses into a supermassive "star" that turns into a black hole without a supernova explosion. Such stars don't exist today, so apparently very special circumstances are needed to prevent such a cloud of gas from falling apart into a large number of small clouds of gas that each turn into an ordinary star
  3. collapse of a dense group of stars. When a star escapes from such a group then the remaining stars on average get a bit closer together. When they all get too close to each other, then perhaps they meld together, and then they turn into a black hole.
  4. perhaps those supermassive black holes formed out of density waves in clouds of gas during the Big Bang.

In our region of the Universe the conditions don't appear good today for the formation of such supermassive black holes, so it is difficult to test these different theories.

6. Diving into a black hole

Since no information can escape from a black hole, we do not know what conditions are like inside. We can figure out what the conditions are like just outside the event horizon of the hole, and they are bad for your health.

Suppose you dive toward a black hole with your head first. Because your head is closest to the black hole, it is attracted more strongly by the black hole than your legs are. Relative to the center of your body, your legs feel a force away from the black hole, and your head feels a force toward the black hole. These forces are tidal forces and act in any kind of gravity, but are generally so small that you don't notice them. For example, in a low orbit around the Earth, the tidal forces (due to the Earth) on an astronaut are equivalent to a weight of about one millionth of a pound (1/2000 of a gram), so they are completely negligible there.

Because the gravity around a black hole is so strong, tidal forces are appreciable there. For a black hole with a mass equal to that of the Sun, the tidal forces on you are comparable to your weight on Earth when you are still a few thousand kilometers away from the center of the black hole. Far from the black hole, the tidal forces increase eight times whenever your distance to the center of the black hole is cut in half, and close to the black hole they increase even faster. You will be pulled apart by the tidal forces long before you reach the event horizon.

Black holes emit radiation at a temperature which seems to increase the closer you get to it. If you are close enough to the black hole, then you'll be bombarded by unhealthy X-rays and gamma rays.

Your friend, who is circling around the black hole inside your spaceship at a safe distance, would see you fall toward the invisible event horizon and the closer you seem to get to that, the slower your movements become and the dimmer your image (because the light coming from closer and closer to the event horizon takes a longer and longer time to escape the powerful gravity near the event horizon, and loses more and more energy while doing that). Your friend could wait forever, but would never actually see you cross the event horizon, while you'd have disappeared into the black hole after only a short time on your clock.

So, material falling into a black hole is torn apart thoroughly and is also fried by X-rays and gamma rays, so no living thing is expected to survive a fall to a black hole, let alone a fall into a black hole.

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7. Combining Black Holes

If two black holes come together, then they form a single black hole with the combined mass, electrical charge, and rotation of the two black holes. That new black hole behaves itself just as any other black hole with the same amount of mass, charge, and rotation, regardless of how it was formed. A black hole does not show evidence of how it was formed (but in some cases its surroundings do).

If two black holes get very close together then they'll emit strong gravity waves, which we may be able to detect in the near future. If there is some ordinary material orbiting around either or both black holes, then that material will probably emit very strong X rays and gamma rays while the black holes coalesce, and we can measure such radiation with satellites that orbit around the Earth.

8. Worm holes

Some scientists and sci-fi writers speculate that perhaps through some weird physics a black hole may be connected with a so-called "white hole" at some other point in space and time, and that material falling into a black hole would reappear at this other point in space and time. Such a connection to another point in time and space is called a "worm hole". We have no evidence for the existence of such worm holes, and, as described above, you'd be torn apart and fried long before your remains disappeared into a black hole, so even if worm holes do exist, they do not appear to be very useful means of transportation.

We don't know what it looks like inside a black hole. Some theories of how a black hole works allow that a black hole is some sort of passage to a very far location, perhaps very far away in the same Universe or perhaps even in a separate universe. Unfortunately, in all of those theories the black hole looks practically the same from the outside, so we cannot tell from observations from Earth which one of those theories is the right one. It might also be that the situation inside the black hole is very different than those theories say, because perhaps other things become very important there that are hardly or not at all noticeable outside of the black hole. If we don't know about those things, then we cannot put them in our theories.

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9. Characteristics of Black Holes

Black holes have only three characteristics: their mass, their electric charge, and their amount of rotation. The mass is connected with gravity, and the electric charge with the electromagnetic force. There is no place for the nuclear forces in black holes, and it is thought that the electric charge of most black holes is so small that it can be ignored, so gravity is effectively the only one of the fundamental forces that is important to a black hole.

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10. Schwarzschild Singularities

Something that most definitely does not follow from the theory of Newton, but that does follow from the General Theory of Relativity of Einstein, is that gravity distorts the structure of space and time. Here again, this is only measurable in very extreme circumstances or with very accurate measurements. Karl Schwarzschild from Germany applied, around 1916, the General Theory of Relativity to the inside of a simple black hole and found that the theory predicts that the distortion of space and time at the very center of such a black hole becomes infinitely great. Such a point at which the predicted distortion of space and time is infinite is nowadays called a singularity, and the predicted singularity at the heart of a simple black hole is called, after its discoverer, a Schwarzschild singularity.

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11. The Inside of a Black Hole

Nobody knows what the mass inside a black hole looks like. From measurements of the gravity field outside the black hole you cannot determine how the mass inside the black hole is distributed. One can invent many models that each have a different distribution of matter but yet yield practically the same gravity field outside the black hole. One of those models is one where all of the mass is concentrated in a single point.

In the same way, measurements of the gravity field outside the Earth are not sufficient to be able to determine the precise distribution of matter inside the Earth. We can yet say something useful about that mass distribution because we can reasonably assume that the material inside the Earth is of the same kind as the material that we can investigate in our labs, and we can calculate how such material ought to be distributed to give the observed gravity field.

That same trick won't work as well for black holes, because we don't know what the characteristics are of the matter inside a black hole.

It seems reasonable to me to assume that a black hole is mostly empty, but we just don't know whether the matter is all concentrated in a single point of size zero, or if it is perhaps concentrated into a small sphere, or perhaps in a weird cloud in which "our" laws of nature don't hold anymore.

The important point is that you cannot tell from the gravity field outside of an object what the precise distribution of material inside that object is. At 100,000 kilometers or miles from the center, the gravity of Earth is just as strong as the gravity of a black hole with the same mass as the Earth. Measurements of the gravity field at large distances can't tell if the object is an Earth or an Earth-mass black hole.

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12. Black Holes are No Threat to Earth

It is very unlikely that a black hole will cause problems for the Earth in the near future, because the Earth and the Solar System have existed for five thousand million years and have not been troubled by a black hole during all that time, or at least not in such a way that we can find traces of it today, and there is no reason to expect a much greater chance of black hole trouble in the future.

The most important thing to know about black holes is that they do not have special sucking powers just because they are black holes. The gravity of a black hole is of the same kind as the gravity of any other thing. If you replace the Sun by a black hole with the same mass as the Sun, then the planets will continue in their orbits just like they did around the Sun, and with the same orbital periods. The gravity at 1 million kilometers from the center of the Sun is just as strong as the gravity at 1 million kilometers from the center of a black hole with the same mass as the Sun.

If you get so close to the black hole that for an ordinary thing with the same mass you'd already be below its surface, only then can you expect forces of gravity around the black hole that you'd not find around the ordinary thing.

So if a black hole were to come towards our Solar System, then the results would be mostly the same as if a star or other ordinary thing with the same mass came to our Solar System. The extreme circumstances that you can find around a black hole only become important if you get really close to the black hole, and the chances of that happening are very small. Also read the answer to question 10.

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13. The Black Hole in the Center of the Milky Way

There is probably a giant black hole at the very center of the Milky Way. That center is some 26,000 lightyears away from us in the direction of the constellation of the Archer. (For comparison: the nearest star beyond the Sun is about 4 lightyears away.) That black hole has no important influence on us.

Whatever important effect you choose (like gravity), the rest of the Milky Way has enormously more of that effect on the Sun than the black hole does. The (approximate) location of the center of the Milky Way wasn't even discovered until 1918, and strong proof that there is a black hole there wasn't found until 2002, after much work specifically designed for that. If the black hole at the center of the Milky Way had important effects on Earth, then it would have been detected a lot sooner.

See //en.wikipedia.org/wiki/Sagittarius_A%2A, //en.wikipedia.org/wiki/Galactic_center, and //en.wikipedia.org/wiki/Milky_way for more information about the center of the Milky Way.

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14. Bending of Space-Time Near a Black Hole

Bending of space-time near and inside a black hole works the same as it does in and around "normal" things (see question 557), but more strongly. Orbits are bent more strongly, speeds are greater, and time is slowed down more strongly. As far as we know, for the effects of gravity it does not matter whether the mass that causes the gravity comes in the shape of a black hole or in the shape of something more ordinary.

For a spherically symmetric object, the strength of the force of gravity at a particular distance from the center of the object depends on that distance and on how much of the mass of the object is closer to the center than the chosen point is at which you want to know the strength of the force of gravity. All mass of the object that is further away from the center than the point of interest is is balanced exactly and therefore makes no net contribution to the gravity at that point. If there were a hollow sphere exactly in the center of the Earth, then gravity would be zero everywhere in that sphere, because then there would be no mass closer to the center than for any point in that sphere.

This explains why gravity just outside, and presumably also inside, a black hole can be far stronger than around or inside an ordinary object. Let's compare the Sun with a black hole with the same mass as the Sun. The radius of that black hole would then be only 1.5 km (0.9 mi). If you are at a distance of 2.0 km (1.2 mi) from the center of the black hole, then all of the mass of the black hole is closer to the center of the black hole than you are, so you feel the gravity of all of that mass. If instead you are at a distance of 2.0 km from the center of the Sun, then only a very small fraction of the mass of the Sun is closer to the center of the Sun than you are. The radius of the Sun is 696,000 km (433,000 mi), so the gravity of all of the mass that is between 2 and 696,000 km from the center of the Sun cancels out to zero, as far as the force of gravity at 2 km from the center goes. That explains why the force of gravity at 2 km from the center of the Sun is far less strong than the force of gravity at 2 km from the center of a black hole with the same mass as the Sun, and that is why space-time can be bent far more strongly around a black hole than around the Sun.

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15. Why Study Black Holes If They Are No Threat To Earth?

How can you know that something is not really a threat to Earth if you don't study it first? Leaving that aside, things do not have to be important in order to be worthy of investigation. There are many different possible reasons why someone may want to investigate something, perhaps because it is curious, or strange, or difficult to study, or even just "because it is there" (as some mountain climbers are supposed to have said when asked why they wanted to try the dangerous climb to the top of a very tall mountain). And things in the Universe do not come much stranger than black holes. Also see question 258.



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