This page contains answers to questions Mr Sunspot received about Black Holes. The questions are:
[136] In your answer to question 133 you say that "not even black holes can store received energy indefinitely". Then what happens to that energy? edited question from Dr. A.N. Dharmawansa of Colombo, Sri Lanka. 22 April 1998[135] How do you know that black holes exist? paraphrased question from Fabian May (15) of Berlin, Germany 21 April 1998
[122] The news reports are that Britain's Nuffield Radio Astronomy Laboratory has found a black hole in the center of GR1915, micro-quasar and "the images show two streams of bullets of ultra-hot gas being shot out in opposite directions with an apparent speed more than double that of light." How is this possible? Asked by David Frei. 3 December 1997
[114] Is it possible that moving black holes exist? edited question from Warsaw, Poland 13 November 1997
[61] My friend thinks that there are no such things as black holes because, she thinks that light can bounce off and escape. Is this true? asked by Kristin Altrock of Cloudcroft, NM. 6 March 1997
[47] Is it true that there is a gigantic black hole in about the centre of our galaxy that "sucks" everything around it? How do we know it? asked by DJ.Kool of Thessaloniki, Greece. 20 November 1996.
[46] Are black holes really a doorway to the past or the future? Are they stars? asked by DJ.Kool of Thessaloniki, Greece. 20 November 1996.
[13] How close is the nearest Black Hole to the Earth? asked by Sarah Beth Jones (10) of Middleton, Wisconsin.
You might also want to read about gravity.
Black holes
A black hole is an object that is so dense that the gravity at its outer boundary is too strong for even light to escape from it. Nothing at all (whether it be material, radiation, or information) can escape from inside a black hole (but see below at black hole radiation). The outer boundary from inside which nothing can escape is called the event horizon. You cannot see, feel, or measure the event horizon, and if you fall into a black hole you won't notice it when you pass the event horizon, but once you pass that boundary you cannot escape from the black hole anymore.
That this event horizon cannot be seen, felt, or measured is not as strange as it sounds. 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 tree limbs: 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 be gone forever).
Gravity attracts light
Gravity attracts light as well as material, so if light passes through the event horizon 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 into 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 radius is called the Schwarzschild radius, 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 radius is also the radius of the event horizon, so an object turns into a black hole if it is squeezed to within its own event horizon.
The only process we know of that can form black holes is the evolution of very massive stars (that start out being more than about ten times heavier than the Sun). Once a black hole has formed, it can grow by merging with other black holes or by capturing more material. There are very strong indications that very massive black holes (millions of times more massive than the Sun) exist in the centers of many galaxies, including our own Galaxy. Maybe such black holes were formed by the merging of large numbers of star-derived black holes, or perhaps they were formed in some other way unknown to me.
Black hole radiation
Black holes are not invulnerable: they can lose energy. From far away a black hole in a vacuum is seen to act as if it has a temperature of 6.17e-8 / M kelvin, where the mass M of the black hole is measured in units of one solar mass. Only black holes much less massive than the Sun have an appreciable temperature: Temperatures greater than or equal to room temperature are only associated with black holes that are less massive than 4e20 kg, which is 1/15000th of the mass of the Earth. The event horizon of a black hole with a mass of 4e20 kg has a diameter of 1/800,000th of a meter (1/20,000th of an inch). If the temperature of a black hole is greater than the ambient temperature, then the black hole loses energy (= mass) and its event horizon (whose radius is proportional to the black hole's mass) shrinks: then the black hole evaporates.
However, the bigger the mass of the black hole is, the longer it takes for it to evaporate. For black holes with a mass greater than 1e14 kg (1/60,000,000,000th of the mass of the Earth; about as heavy as a round stone with a radius of 2 km, 1.2 miles), which have a diameter greater than 3e-13 m (about 1/3000th of the diameter of the smallest atom), the evaporation time (in vacuum) is approximately equal to 1.5e66 M^3 years. For black holes lighter than 1e14 kg, the proportionality constant is greater. Such black holes evaporate in less time than the lifetime of the universe. All heavier black holes that were ever formed (including all of those that astronomers think they have found) are still around and evaporate so slowly that you can't tell the difference even after 10 thousand million years.
The energy loss from a black hole (in a vacuum) that has a mass of at least 1e14 kg (M at least equal to 5e-17) is equal to only 1.8e-5 / M^2 watt. Only about 1/8th of this energy is emitted in the form of photons (i.e., electromagnetic radiation): 1/70th part is emitted as gravitons, and the rest (i.e., the main part) as neutrinos. Both gravitons and neutrinos are exceedingly difficult to detect.
So, even though black holes can evaporate, the black holes that are formed from very heavy stars that run out of fuel as well as those very heavy black holes that reside in the centers of some galaxies evaporate so slowly that it is to all intents and purposes impossible to detect.
[LS 22 April 1998]
Diving into a black hole
Since no information can escape 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 of the difference in distance to the black hole, your legs are attracted a bit less strongly by the black hole than your head. This means that 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 called tidal forces and act in any kind of gravity, but are generally so small that you don't notice them. In a low orbit around the Earth, the tidal forces on an astronaut due to the Earth are equivalent to a weight of about one millionth of a pound (1/2000 of a gram), so they are completely negligible there.
Because a black hole is so dense, its gravity is very high and tidal forces are appreciable. For a black hole with a mass equal to that of the Sun, the tidal forces on you if you are 5 ft long (1.5 m) are equivalent to your own weight on Earth when you are still 2100 miles (3400 km) 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.
If you were made from some remarkably strong material that could withstand the enormous tidal forces, then you might still be alive when you cross the event boundary, but you would not notice the boundary. You'd just keep falling lower and lower until you'd finally slam into the material at the very center of the black hole.
- 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. You'd be long dead inside the black hole, but a dim image of you would still linger for travelers outside the black hole to see. Of course, after a short while your image would become so dim that no instrument could actually see it anymore. 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 into a black hole.
See the Black Hole Science Page for more information on how the possibility and properties of black holes were discovered.
Worm holes
There is speculation that perhaps through a weird kind of curvature of space-time a black hole may be connected with a so-called white hole or worm 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. 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.
[LS 20 November 1996 - 6 March 1997]
Apparent Speeds Faster than the Speed of Light
Since about 1970 astronomers have observed things near quasars (and other active galaxies that are suspected of having black holes in their centers) moving at speeds that seem greater than the speed of light. During the last few years such apparently superluminal motion has also been seen around a few objects in our own galaxy. The key word here is "apparent". The apparent superluminal speeds are explained as effects of geometry coupled with speeds close to that of light. In this case it makes things moving at close to the speed of light in a direction which makes only a small angle with the line of sight appear to move across the line of sight at much greater speeds. For example, if the object moves at 0.995 times the speed of light at an angle of 6 degrees with the line of sight, then to us on Earth it would seem to move across the line of sight at ten times the speed of light.
The formula is:
beta_app = beta sin alpha / (1 - beta cos alpha)
where beta is the actual speed divided by the speed of light, beta_app is the apparent speed across the sky as seen from Earth, and alpha is the angle between the velocity of the object and the line of sight. For a derivation, see Explanation Page 8.
[LS 7 December 1997 - 23 January 1998]
The nearest black holes
It is not easy to find black holes, because they are black and you can't see them. We can only find a black hole by detecting the influence it has on nearby stars or gas clouds and by checking very carefully that whatever is causing this influence is so heavy that it can only be a black hole. If this object is too small and faint to be a star and the mass of this object may be large enough to be a black hole but we don't know for sure, then the object is called a "black hole candidate". Even objects that cannot be explained as anything else and that appear to be massive enough to be black holes (i.e., with masses of about 3 or more times that of the Sun) are usually called black hole candidates rather than black holes, because there may be ways we have not yet discovered in which strange objects can have such large masses and yet not be a black hole, and because measuring the masses of black-hole candidates is tricky and perhaps there are some effects we don't know about that might make it appear as if the object is very massive even if it is not. We just do not know enough (yet) about things as massive and compact as black-hole candidates to be sure that they can only be black holes. For more information on why astronomers think there are black holes at all, see the Black Hole Science page.
If the black hole candidate orbits around a visible star, then we can get fair estimates of its mass from observations of the rotation of the visible star and the black-hole candidate around each other. Masses and other estimates for at least ten black hole candidates have been determined in this way. These are listed below. In addition, fourteen other black hole candidates show characteristics in observations that are assumed to indicate the presence of a black hole, though no mass estimates are available for those systems. All in all, then, twenty-four black hole candidates are presently known.
"V" is the visual magnitude of the black hole system (including the visible companion star). MXRB stands for "Massive X-Ray Binary" which means that the black hole candidate orbits around a visible star which is much more massive than the Sun and that the whole system emits a lot of X-rays. SXT stands for "Soft X-Ray Transient" which means that the black hole candidate orbits around a visible star which is at most about as heavy as the Sun and that the whole system occasionally emits soft X-rays. "dist" is the system's distance to us, measured in thousands of lightyears. The entries with a star (*) behind them are uncertain. "P" is the orbital period of the system, in days. "mass" is the estimate of the mass of the black-hole candidate. The "companion" column lists the spectral type and size class of the visible companion star: "d" is a dwarf star (like the Sun), "g" is a giant star, and "sg" is a supergiant.
Black Hole Candidates Name V type dist P mass companion mag kly d sun Cyg X-1 9 MXRB 8 5.6 10 - 15 O sg LMC X-3 17 MXRB 175 1.7 4 - 11 B d LMC X-1 14 MXRB 175 4.2 4 - 10 O g V616 Mon 18 SXT 3 0.32 3.3 - 4.2 K d V404 Cyg 18 SXT 11 6.47 8 - 15 K d Nova Mus 1991 20 SXT 10 0.43 4 - 6 K d Nova Oph 1977 21 SXT 10* 0.70 >4.1 K J0422+32 22 SXT 8* 0.21 4.5 M d J1655-40 17 SXT 10 2.61 4 - 5.2 F-G GS2000+25 22 SXT 8 0.34 5.3- 8.2 K d
The black-hole candidate closest to us is V616 Mon in the constellation of the Unicorn, which is still about 3,000 lightyears away from us.
Besides the "small" black holes that are described above, astronomers think there are also very large black holes in the center of many galaxies, including our own. These black holes are thought to have masses of millions of times that of the Sun. The X-ray source Sgr A* in the constellation of the Archer is thought to represent the black hole in the center of our own Galaxy, at about 30,000 lightyears distance from us.
Don't be afraid of the Big, Bad, Black Hole
The only two real differences between a black hole and an ordinary star are that the black hole is black so you cannot see it, and that it is very small so you can get much closer to it than to a star. For the rest, a black hole affects stars and planets just the same way as an ordinary star does. A black hole moves around just like a star with the same mass would in the same place: if no forces act on the black hole, then it will move in a straight line. A black hole is not a vacuum cleaner: It does not "suck in" planets or stars orbiting it, just like the Sun does not suck in the Earth and other planets. You can only have something disappear into a black hole if you aim it exactly at the black hole, or if you embed it in a cloud of gas or other particles that generate enough friction to slowly get rid of the speed of the object around the black hole. Therefore, you do not have to be afraid of black holes unless you are very close to them.
If a sizable black hole were travelling through space near our Solar System, then we'd notice it long before it got here because of its gravitational influence on the nearby stars that it passes on its way.
[LS 26 April 1996 - 7 December 1997]
Suggested Reading
- "Black Holes in Binary Stars: Weighing the Evidence" by Philip A. Charles and R. Mark Wagner, in "Sky and Telescope" magazine, May 1996, pages 38-42.
- "How Do We Know It's a Black Hole?" by Harry L. Shipman, in "Sky and Telescope" magazine, May 1996, pages 42-43.