Researcher Q&A FAQ-Black Holes

These questions have been answered by the scientists who are part of the Ask a High-Energy Astronmer program.

Evidence for Black Holes

QUESTION:
Who was the first person to discover a black hole and what was the date?

ANSWER:
Black holes cannot be observed directly and therefore cannot be 'discovered'.

However, as we explain at our website, the indirect evidence for two kinds of black hole is now overwhelming - those of a few solar masses produced by supernovae and much larger ones at the center of some galaxies.

The existence of bodies with gravitational fields strong enough to allow nothing to escape has been a topic of speculation for hundreds of years. Einstein's general theory of relativity (published in 1916) predicts just the kinds of object we are now inferring.

Perhaps the first object to be generally recognized as a black hole is the X-ray binary star Cygnus X-1. Its effect on its companion star suggested as early as 1971 that it must be a compact object with a mass too high for it to be a neutron star. (That was 2 years after the American astronomer John Wheeler coined the term 'black hole').

Paul Butterworth
for the Ask a High-Energy Astronomer Team

QUESTION:
How has it been demonstrated that black holes are real? What are the various positions that astrophysicists take regarding black holes? Are black holes related to the big bang? How or in what way?

ANSWER:
Astrophysicists generally agree that black holes exist. There is good observational evidence from X-ray observations and from the Hubble Space Telescope that there are massive black holes (with masses more than a million times that of the Sun) exist in the centers of some galaxies. For further information, check the Active Galaxies topic of the Level 1 or Level 2 sections of our web site. (http://imagine.gsfc.nasa.gov )

In addition, there is also evidence for black holes which result from the final stage of a star's life. These "galactic" black holes (so called because they are in our Galaxy), are usually 3 - 10 times the mass of the Sun. They also often orbit a companion star, and by observing the X-rays emitted from the region near the black hole and/or the visible light from the companion, a mass can be determined for the black hole. If it is larger than the accepted mass for a neutron star (about 1.5 times the mass of the Sun), then astronomers generally agree the object is a black hole. There are quite a number of these objects known in our galaxy.

For further info, check the Black Holes topics on our web site. You might also enjoy

http://www.owlnet.rice.edu/~spac250/steve/index.html

No, black holes are not related to the big bang.

Jim Lochner
for Ask a High-Energy Astronomer

QUESTION:
I've been for a long time trying to get a catalog where I could find the coordinates of most (if not all) of the black hole candidates so far discovered... Does this catalog exist ???

ANSWER:
There was a recent paper in the Astrophysical Journal by Chen, Shrader, and Livio (1997 ApJ volume 491, page 312, Dec 10 issue) which lists a number of the black hole systems (see table 4). It lists 7 firm systems, and 10 black hole candidates. Any reasonable college physics library should have this journal.

Thanks for your question.

Eric Christian and John Cannizzo
for Ask a High-Energy Astronomer

QUESTION:
It is my understanding that observation of x-rays have been used in the past in the search for black holes. I have been trying to find out whether there has been any reasonable evidence found for their existence. Having glanced through numerous out of date texts on the subject, it seems people are unwilling to say one way or another. If there is any generally accepted evidence I would be most grateful if you could let me know.

ANSWER:
Here are WWW pages that show some X-ray evidence for black holes:

http://lheawww.gsfc.nasa.gov/users/nandra/pubs/paper2/figure.html
http://lheawww.gsfc.nasa.gov/users/turner/agn_reproc.html
http://lheawww.gsfc.nasa.gov/users/ptak/agn/liner_llagn.html

You may also want to revisit Imagine the Universe! at:

http://imagine.gsfc.nasa.gov

In addition to the discussions of black holes and x-ray astronomy you will find there, check out the archive of past 'Ask a High-Energy Astronomer' questions and answers. Many of those under 'black holes' are relevant to your question.

There is now a strong consensus that black holes are present in the Universe, both small ones associated with stars in binary systems (Cygnus X-1 is a convincing candidate, for example) and much larger ones in the centers of many galaxies (including our own). The best lines of evidence are currently coming from determinations of the masses of non-stellar perturbing objects from their effects on the motions of stars (measured from Doppler shifts in their spectra). In many cases the unseen perturbing object is so massive that it is hard not to conclude that it is a black hole.

As an indication of how rapidly the evidence is accumulating, only a couple of hours ago NASA issued a Press Release on recent results from the New Hubble Instruments. Here's an extract:

BEAM FROM A BLACK HOLE

The imaging spectrograph, which just last month demonstrated its efficiency as a black hole hunter, now shows the results when the power of a black hole is unleashed into its surrounding environment. In a single observation, the spectrograph measured the velocities of hundreds of gas blobs caught up in a twin-cone beam of radiation emanating from a supermassive black hole at the core of galaxy NGC 4151. Follow-up observations reveal hot gas emanating from deep within the throat of the beam, near the vicinity of the black hole. These observations also allow scientists to map the mass outflows near the black hole. The surprisingly complex motion may offer clues to the galaxy's stellar population, the orientation of the beam in the past, or evidence of some kind of backflow of gas into the central cone regions.

For pictures see:

http://oposite.stsci.edu/pubinfo/Pictures.html

Best wishes,

Paul Butterworth and Andy Ptak
for the 'Ask a High-Energy Astronomer' team

QUESTION:
I am an undergraduate in Astrophysics at the University of Calgary. I am doing a small research project on the evidence for and against a black hole at the center of the milky way. I found your email address on the StarChild page dealing with this topic. I was wondering if you had any suggestions of articles or books discussing this subject. Thank you for your time.

ANSWER:
It is generally believed that a black hole does exist at the center of the Milky Way galaxy. The latest value we have seen is that it has a mass of about 2,000,000 that of the Sun. In fact, it is believed that this may be common for most galaxies. Observational evidence supports these ideas more and more. However, you must keep in mind that due to the large absorption and source confusion when trying to look into the center of a galaxy, it is very, very hard to see what's there! So we have to be clever about the observations we make and the interpretations of these observations. This is one reason that X-rays and gamma-rays are powerful probes in trying to answer such questions; they are much more likely to "get out" of the central region of the galaxy than other wavelengths.

Some references you may find useful (and which give many more references) are:

A more general Milky Way reference is Blitz, Binney, Lo, Bally & Ho 1993, Nature 361, 417.

QUESTION:
What is known about Sagittarius A*, the center of our galaxy?

ANSWER:
Sgr A* at the center of Milky Way is probably a massive black hole of about a million solar masses (the mass of the Sun). The mass is estimated from the motion of gas and stars in the region. Although Sgr A* is gathering mass from its vicinity at a rate of about 10E-4 solar masses per year, a rather high rate, it is not as bright as would be expected. Therefore, Sgr A* is extremely inefficient (one part in 100,000) in converting the gravitational energy of the gathered material into radiation.

Koji Mukai
with helps from Drs. Chen, Loewenstein and Snowden

Black Hole Appearance

QUESTION:
We are deeply indebted to you if you can help us in obtaining two representative images about: 1)the real image (picture) of a "black hole" (photographed) 2)the most distant part of the Universe ever photographed.

ANSWER:
1) There are no "real" pictures of a black hole. This is because black holes themselves do not emit of reflect any light (that's why they are called black holes), and they are too small and too far away to be imaged. There are images of binary star systems consisting of one normal star and one black hole, and of the central regions of Galaxies that are believed to contain black holes. There are some examples of the latter, taken with the Hubble Space Telescope, at: http://oposite.stsci.edu/pubinfo/PR/97/01.html

But these pictures don't actually show a black hole, you need to study the motion of stars to infer that there must be a black hole.

2) Again, you may want to look at some Hubble pictures (with explanations): http://oposite.stsci.edu/pubinfo/PR/94/52.html

These are some of the most distant galaxies ever photographed; although some quasars are believed to be more distant, they make boring photographs (they just look like a point of light).

Best wishes,

Koji Mukai

QUESTION:
Hi, I am intersted in black holes and have taken a Astronomy 100 college class. My question is: if you were to take a picture of a star-field and a black hole were to come between you and the center of the same star-field, and it is relatively near to you, how would the appearance of the star-field change? Thanks!

ANSWER:
The black hole would look black, since any stars behind it would be shadowed by it. Around the black hole, outside its event horizon, though, you would see a rim of light from stars behind it. Some of the light from these stars that started out at an angle from you will be bent by the strong gravitational field and would reach your eye. In a very real sense, the black hole is a "gravitational lens."

For a pictorial explanation look at

http://www.mpifr-bonn.mpg.de/staff/hfalcke/bh/sld11.html

We hope this helps!

Enectali Figueroa and John Cannizzo
for "Ask a NASA Scientist"

QUESTION:
Is a black hole planar? Like, if you went UNDER a hole, what would happen?

ANSWER:
Black holes are really 4-dimensional (3 dimensions of space and 1 of time). Sometimes we see drawings of a black hole that may make it look like it's 2 dimensional, but that's because we don't know how to draw 4 dimensional objects on a piece of paper.

Best wishes,
Koji Mukai

QUESTION:
What is the volume of a black hole?

ANSWER:
Our intuitive sense of volume breaks down in the strong gravitational region in a black hole. So while the "size" of a black hole is given by the radius of its event horizon, it's volume is not determined by the usual 4/3*pi*r3. Instead, relativity makes it more complicated than that. As you pass the event horizon, the spatial direction 'inwards' becomes 'towards the future'-- you WILL reach the center, it's as inevitable as next Monday. The direction outsiders think of as their future becomes a spatial dimension once you are inside. The volume of a black hole, therefore, is its surface area times the length of time the hole exists (using the speed of light to convert from seconds to meters). Since a black hole last practically forever, the black hole's volume is almost infinite. (This is also a way of explaining the fact that you can pour stuff into a black hole forever and never fill it up. Another reason why black holes never fill up is that the radius of the event horizon increases as the mass of the black hole increases.)

David Palmer and Jim Lochner
for Ask a High-Energy Astronomer

QUESTION:
Are there two types of blackhole? One is like the Cygnus X-1, and the other is a super massive blackhole in the active galactic nuclei?

ANSWER:
Thank you for contacting our Ask a High-Energy Astronomer service. The only real difference between these two kinds of black hole is size. A black hole in an X-ray binary will only be a few times as massive as the sun. A black hole at the center of an active galaxy can be millions of times as massive as the sun. It can get so big because the density of matter in a galactic center is high so there is lots of matter for it to accrete.

Both kinds of black hole can have an accretion disk. Usually most of the radiation we detect comes from the accretion disk. The accretion disk of a super-massive black hole will be much larger than that of one in an X-ray binary. This means that the radiation that we detect varies on a much longer timescale (days instead of milliseconds).

Damian Audley
for Ask a High-Energy Astronomer

QUESTION:
I am a junior and have a passion for space. I just read in a Popular Science about someone doing an experiment with a satellite. Inside the satellite, will be a vacuum ten times stronger then that of space. It is being built to test Einstein's theory that a body such as the earth, drags space\time along with it. I had asked an astrophysicist at some collage via E-mail if a black hole was planer. She told me that a black hole has four dimensions. Three of space and one of time. Now, since all objects in the Universe revolve around something, that means its moving through space time. What I'm asking you is when the hole moves through space, does it drag time along with it, or does it tear it temporarily? What is you theory of what happens?

ANSWER:
You've got the right idea. Yes, a black hole does move through "space-time". It certainly moves through space, just like any other star in the galaxy. And everything moves through time. Since Einstein's theory predicts a dragging of space-time by massive bodies, a black hole could also be expected to drag space-time.

This dragging of space-time, however, is usually discussed in terms of rotating bodies. The experiment you read about aims to measure the dragging near the Earth due to the Earth's rotation, not due to its motion around the Sun. But this "frame dragging" (as it's called) also occurs around rotating black holes. In fact, frame dragging becomes so extreme as you approach the event horizon of a rotating black hole that at a certain distance (the "static limit"), all bodies **must** orbit a rotating black hole. (That is, no amount of rocket power could keep you from orbiting, as seen from a great distance). The effect intensifies until at the horizon itself, there's no place to go but into the hole.

However, the dragging is smooth, just as the motion through the galaxy would be smooth. As far as I know, the "tearing" of space-time (or just time) is more of a science fiction idea. The notion of tearing may come from taking the analogy of the "fabric of space time" too seriously.

Jim Lochner

QUESTION:
I read in a book about black holes that their singularities could be circular if the black hole had spin, if it rotated. Besides that book, everything I've read said that singularities are points of infinite density etc., but none of them said it had spin. In the book it said black holes can be defined by charge, spin, and something else. Is this false information?

Sincerely,

a mixed up 7th grader.

ANSWER:
Properties of black holes are indeed be defined by its mass, charge, and spin.

The simplest type of black holes are the known as Schwarzschild black holes, named after the scientist who discovered this solution of General Relativity. Schwarzschild black holes have 0 spin (they are not rotating) and 0 charge. It is possible that some books describe only this type of black holes, without saying so.

Rotating black holes are known as Kerr black holes, which can indeed have ring-shaped singularity.

Hope this has cleared things up a bit.

Koji Mukai
for Ask a High-Energy Astronomer

QUESTION:
How could I model a black hole for a science project? I am in the third grade.

ANSWER:
If you can get hold of a sheet of rubber, or some other fabric that stretches. You can use that as an analogy of space: Put something heavy on it, that would be like a normal star. Roll a little marble around it.

Now make a hole in the sheet, and pull the edges of the hole down. That would be like a black hole.

Best Wishes,

Koji Mukai
for "Ask a High-Energy Astronomer"

Pulled in by a Black Hole?

QUESTION:
What would happen to Earth's orbit if the Sun became a black hole?

ANSWER:
Thanks for your question about the orbit of the Earth if the Sun became a black hole. The Sun is not massive enough to ever evolve into a black hole; it will end its life in about 4.5 billion years as a white dwarf star. But, perhaps you are asking what would happen to the Earth's orbit if the Sun's mass was suddenly collapsed into a black hole (not plausible,given our understanding of stellar evolution, but an interesting theoretical question).

If a one solar mass black hole were to suddenly replace the Sun at the center of our solar system, the orbits of the planets would not change. This is because the physical laws that determine the orbital motion of the Earth depend only on the actual mass of the Sun, and not on whether it is distributed within a sphere (like the Sun) or at a point (like a black hole). I hope that answers your question.

Regards,
Padi Boyd,
for the Ask a High-Energy Astronomer

QUESTION:
I want to know how a black hole that is so small that it is only 2 to 3 kilometers big can pull an entire star into its crevice.

ANSWER:
How does a mouse eat an elephant? One bite at a time.

A black hole in a close orbit around a star can pull the top layers of the star off the surface and down its own gravity well. Once the material passes beyond the black hole's event horizon, it is gone, and more stuff can be consumed by the black hole. You are left with a slightly larger black hole, and a slightly less massive star, so the black hole can pull a little more material off the star. This continues until the star is gone, and the black hole's hunger is yet unabated.

David Palmer
for Ask a High-Energy Astronomer

QUESTION:
Before you reach the event horizon of a black hole and are pulled in, do you first reach a period where you ORBIT and can't break out of it? Let's say a heavy body does drag time around with it. Fine. On a planar model, the dragging time would represent a well of sorts around the star or whatever. Let's say that someone found a massive enough body that has a big enough well, and kind of slipped something into that well, what would happen?

ANSWER:
Yes, if the black hole is rotating, you first orbit around it before passing through the event horizon.

In answer to your question about what would happen if you slipped into a black hole, you'd get stretched by the tidal forces and your time would slow down as observed by a distant observer. To see what would happen to a body falling into a black hole (or a neutron star), take a look at

http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html

You can also learn more about black holes by taking a look at the Imagine the Universe! page on the basic high energy astrophysics of black holes. You can also look in the advanced section, where there's a list of books you might find interesting.

Jim Lochner

QUESTION:
I was wondering, what would happen if a black hole passed THROUGH a star, lets say a B-O type star?

ANSWER:
This would depend on the mass of the black hole. Astrophysicists are fairly confident that there are supermassive black holes at the centers of many galaxies. These are millions of times more massive than the Sun, and are bigger than most stars. One of these supermassive black holes would tear the star apart and then accrete the gas. (See the Imagine the Universe! discussion on Active Galactic Nuclei for more information on these types of massive black holes.) We are also confident that very massive stars would end up as black holes, with masses 5-10 times that of the Sun. Collision of such a black hole and a normal star would be very violent, and may completely disrupt the normal star.

More speculatively, much lighter black holes may have been created shortly after the big bang. If a tiny black hole passed through the star, then the star may continue without much disruption.

Best wishes,
Koji Mukai

QUESTION:
Admittedly, I am very ignorant of the practical study of astrophysics based on relativity and quantum mechanics. I do enjoy studying astrophysics though, and would like to ask a few questions concerning a black hole and a possible neighboring star. I read the article on X-ray emission from a star into a black hole and wanted to know if there is any speculation that goes a step further; what would happen if a star were to fall or be launched into a black hole? Question two: would a star continue to fusion just outside and or inside the event horizon of a black hole? If a star could continue fusion, would the change in surface area of the star (due to the gravitational increase as it is closer to the singularity) allow for a greater consumption of fuel leading to an increase in temperature? I would truly appreciate any and all insight you can provide for me.

ANSWER:
One thing you need to consider is the relative sizes of a star and that of (the event horizon of) a black hole. For example, the Sun has a radius of ~700,000 km; a black hole with the same mass would have a radius of ~3km.

(See, http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html for the formula that describes the Schwarzschild radius.)

So if the black hole in question is a few times more massive than our Sun (astrophysicists believe massive stars go supernova and leave such black holes behind), then it's much smaller than the star. In X-ray binaries, a black hole and an ordinary star are in orbit around each other. The star is distorted by the tidal force of the black hole, but otherwise normal, and only slowly (over many millions of years) loses gas onto the black hole. A direct, head-on collision is rare but if this happens, it would be very violent and the star would be torn apart very quickly. So quickly, in fact, that the core of the star (where the fusion is happening) probably won't have time to respond to the changing conditions before it gets torn apart. A completely torn-apart remnant of the star may form a disk around the black hole.

What tears the star apart is the tidal force, or the gradient of the gravitational field. The gradient is less for more massive black holes, so if the black hole is about a billion times more massive than the Sun, then normal stars may be able to fall inside the event horizon of a black hole without being disrupted by the tidal force. Many galaxies are believed to contain Such supermassive black holes at their respective centers, although the typical inferred mass is rather less than a billion times solar (10 to 100 million may be more typical).

Best wishes,

Koji Mukai and Tim Kallman

QUESTION:
Do black holes ever collide?

ANSWER:
Yes --- although direct collisions of black holes are rare because they are small for their mass.

Best Wishes,

Koji Mukai
for Ask a High-Energy Astronomer

Inside a Black Hole

QUESTION:
I am a senior in High School and was wondering if you could take a couple minutes from your very busy day to answer some questions concerning black holes for my Astronomy class.
1. What exactly happens to the material absorbed into black holes ?
2. Wouldn't the black hole finally fill up ?
3. Can life be maintained within a black hole ?
4. Can you see the back part of a black hole ?

ANSWER:
1. You probably have heard about Einstein's famous equation, E = mc2, which gives the energy associated with material of a given mass m. When material falls into black hole, a process called accretion, usually about 10% of the mc2 energy gets radiated away as the material approaches the black hole. The other 90% gets absorbed into the blackhole and simply adds to its mass. In some cases, the material won't have a chance to radiate much energy and essentially all of the mass goes right into the blackhole.

2. Actually, a black hole is already essentially a geometric point, with effectively infinite density. There is no inherent limit to the mass of a black hole. There is a region around black holes called the event horizon. Once anything, including light, crosses the event horizon, it can never escape. This is what gives the black hole its name. The size of the event horizon gets bigger as the black hole gets more massive. This allows the black hole to "grow", in a sense, as more mass falls in. There is very strong evidence that some galaxies have black holes as massive as a billion Suns at their centers (one example is the Sombrero galaxy... you can see a picture of this galaxy at http://lheawww.gsfc.nasa.gov/users/ptak/agn/liner_llagn.html.

3. No, anything that falls into a black hole will get heated to very high temperatures (this is how the 10% of the energy gets radiated away... the material gets very hot, in a process similar to how meteors and space debris burn up due to friction as they enter the Earth's atmosphere). Also, once the material gets very close to the blackhole, tidal forces will stretch it very thin (just think about the effect that a Moon has on the Earth's oceans, and a typical blackhole is likely to be much more massive than the Moon).

4. By definition, you can't see a black hole at all... again not even light can escape from within the event horizon. Interestingly, though, black holes warp space so much that if you could orbit a black hole close to the event horizon, you could see the back of your own head... light reflecting from the back of your head would get bent around the black hole so that you could see it. You can see some movies that demonstrate this and similar effects for neutron stars at: http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html

Andy Ptak
for the Ask a High-Energy Astronomer team

QUESTION:
This might sound like a stupid question, but I want to know what the name for the center of a black hole is. My Dad told me it would be a cool name for my rock band, but he couldn't remember what it was. If there are several applicable terms please include them all, as my Dad's memory can't be trusted.

ANSWER:
At the center of a black hole is a point where the laws of physics break down. This point is called a 'singularity'. These singularities are hidden, or 'clothed' by the black hole, so that the effects of the breakdown cannot be observed by people outside.

However, small black holes are known to decay by the process known as 'Hawking Radiation' http://imagine.gsfc.nasa.gov/docs/dict_ei.html#H. It is currently a matter of debate as to whether the singularity disappears along with the black hole that clothes it, or whether you are left with what is called a 'naked singularity'.

I will not suggest that you call your band 'Naked Singularity', as the term might be misunderstood by your audience and lead to a decline in public morals, which is not part of NASA's charter.

David Palmer
for Ask a High-Energy Astronomer

It is currently a matter of debate as to whether the singularity disappears along with the black hole that clothes it, or whether you are left with what is called a 'naked singularity'.

I will not suggest that you call your band 'Naked Singularity', as the term might be misunderstood by your audience and lead to a decline in public morals, which is not part of NASA's charter.

David Palmer
for Ask a High_Energy Astronomer

QUESTION:
Understanding that a singularity is the theoretical remnant of a supernova, and that the singularity has mass, the question I have is in regards to the accretion disk that is formed when the singularity is near a star (Cygnus X-1 as an example). If matter is being drawn into the singularity from the exterior source, what happens to the accreted matter once it has collapsed into the singularity? Linear inference dictates that the singularity must be accruing mass from this exterior source. Is it possible that the singularity would eventually acquire enough "donor" matter to rebuild a physical structure, eventually reversing the object's status from a singularity to a supermassive body (analogous to a neutron star)? The question is academic, but this is a phenomenon which has puzzled me for a very long time. Thanks!

ANSWER:
Black holes can be produced by supernovae, but other production mechanisms are possible. Many galaxies for instance, including our own, may have super-massive black holes at their centers, which have grown by accretion where the galactic densities were highest. Wherever sufficient mass is crammed into a sufficiently small space a black hole will result. If matter is added to a neutron star for example, at some point (somewhere between 1.4 and 3 solar masses) the internal pressure within the star cannot resist gravity and a black hole is formed. Isolated black holes will be almost impossible to detect. There are a number of binary stars however, where one of the pair is a compact object (white dwarf or neutron star or black hole) accreting material from its companion (and generating X-rays and gamma-rays in the process) and studies of the binary system motion (using the Doppler shifts of spectral lines) suggest that the compact object is too massive to be a neutron star. Cygnus X-1 is just such a binary, where the likely mass of the compact object appears to be considerably more than 3 solar masses.

Adding mass to a black hole just makes it more massive. It doesn't fill it up. Quasars may represent instances where black holes have swallowed significant fractions of entire galaxies - billions of solar masses! Once matter has entered a black hole, it is not accessible to observation. All we can know about that black hole is its mass, charge and angular momentum. Everything else is open to untestable speculation.

In 1974 Stephen Hawking made the surprising discovery that quantum mechanics permits black holes to emit particles, an effect entirely forbidden under classical mechanics. (There are many situations in nuclear physics where quantum particles can similarly 'tunnel' through what would otherwise be impenetrable barriers.) For massive black holes the rate of particle escape is very low. A singularity with the mass of the Sun, for example, would lose an utterly insignificant fraction of its mass over many billions of solar lifetimes. It's still an interesting effect though!

Hawking has some fine discussions of black holes in his two popular books 'A Brief History of Time' and 'Black Holes and Baby Universes'. Imagine the Universe! includes other good references in its Black Holes section. The X-ray Binaries section is also relevant. I hope this answer has been helpful.

Paul Butterworth

QUESTION:
When light enters a black hole does it curve or is it bent?

ANSWER:
Light entering a black hole can curve. However, the light thinks that it is going straight, and from its point of view it is. (Consider walking around the Earth on the Equator. You think that you are always going in the same direction, but to somebody watching you from space, you are going in a circle.) It just goes as straight as it can through curved space.

David Palmer
for Ask a High-Energy Astronomer

QUESTION:
If time comes to a standstill in a black hole, where does time began again out side the black hole?

ANSWER:
It is a bit misleading to say that 'time stands still' inside a black hole. Actually, if you could survive a trip into a black hole (which you couldn't) you would not be aware of any slowing down of any clock you carried as you fell in. However, if you could compare the speed of your clock with that of a reference clock kept far away, then then the clock falling into the black hole would appear to slow down relative to the clock far from the hole. The place where the falling in clock would appear to 'stop' is the spherical surface called the event horizon. This is also regarded as the boundary of the black hole, since nothing crossing this surface can escape. For more information, look at the entries for event horizon on our web page:

http://imagine.gsfc.nasa.gov/docs/dictionary.html

I hope this helps.

Tim Kallman
for the Ask a High-Energy Astronomer Team

Light and Matter from Black Holes

QUESTION:
How do black holes emit X-rays?

ANSWER:
The first place we suggest you look are the black hole pages in Imagine the Universe! under the Basic and Advanced sections. There you can find out information on Black Holes and more specifically X-ray Binaries (which, we believe, is what you are asking about).

Many stars are observed to be in binary systems, where two stars are orbiting each other (as the Earth orbits the sun). Another thing to know is that, the more massive a star is the faster it uses up its nuclear fuel (mostly hydrogen); therefore the sooner it "dies".

If we happen to have a binary star system, and the more massive of the two stars explodes as a supernova and it leaves behind a neutron star or a black hole, then it will result in a binary star system with a normal star and a compact object orbiting each other. All these things working out is rare, but there are over a billion stars in the galaxy, so even rare things happen fairly often.

Now, imagine that the "normal star" then runs out of its fuel. The first thing it will do is expand as it enters its "red giant phase", as our Sun will about 4,000,000,000 years from now. Then, some of the star's outer atmosphere will spill over onto the black hole. It will eventually fall in, and in the process become very hot. We can observe this hot gas with X-ray telescopes, so we call this an X-ray binary.

As far as the significance of the X-ray emission, it is to let us observe the effects of the black hole, and therefore learn something about it. Black holes do not emit light, in fact they are so dense that they trap it. Therefore, the best way to learn about them is by observing the material they effect. Observing X-rays from an X-ray binary is one effective way of doing this.

QUESTION:
Why does accretion of matter onto black holes make them luminous?

ANSWER:
As the matter falls into the black hole, and before it enters the event horizon, it turns gravitational potential energy into kinetic energy and so picks up a lot of speed. Some of this energy is then converted into light and other electromagnetic energy. But the most luminous part of black holes are the jets of accelerated matter that are emitted from the polar regions. These are probably generated because of the very strong rotating magnetic field that is usually associated with Black Holes.

You should also look at: http://imagine.gsfc.nasa.gov/docs/science/know_l2/black_holes.html for more information.

Thanks for your question.

Eric Christian
for Ask a High-Energy Astronomer

QUESTION:
Is it possible that high speed (close to c) particles are flying around a neutron star/black hole?

ANSWER:
That's an interesting question. Matter accreting onto a black hole can reach relativistic speeds. I think the question you are asking is if the energy density in the accretion disk is high enough to cause significant light bending. The answer is no. The gravitational field of the black hole or neutron star dominates.

If we are looking at the disk almost edge-on and the disk is flared (i.e. gets thicker towards its outside edge) it will obscure the central X-ray source. If the disk is warped or is precessing (wobbling) we may see a periodic modulation of the X-ray flux.

Damian Audley
For the Ask a High-Energy Astronomer Team

QUESTION:
I teach a 10th grade astronomy class and am unclear on how the strong rotating magnetic field of a black hole creates the jets of accelerated matter that is emitted from the polar regions. How is it that this matter/energy escapes the event horizon? Thank you for your time.

ANSWER:
This is a very good question, which the scientists are still trying to work out.

However, it is not that jets escape from inside the event horizon. Nothing can escape from an event horizon, by definition! Instead, it is thought that jets originate in the accretion disk that surrounds the event horizon.

A fluid falling onto a small object usually cannot fall directly onto it --- most of the matter will miss the object initially, rotate around it, and only gradually be able to hit the central object. Think of draining your bathtub: Accretion disks are the celestial equivalent of this phenomenon, and can be found around black holes, neutron stars, white dwarfs, or around ordinary stars (planets are believed to form from an accretion disk around a protostar).

Accretion disks around a variety of objects seem to be able to produce jets (protostars certainly do, in addition to accreting black holes). It is just that the ones from an accreting black hole tend to be the fastest and the most spectacular. A general rule of thumb is that the speed of a jet is about the same as the escape velocity of the central object --- so the jets from accreting black holes are at near the speed of light, while protostar jets are much more leisurely.

Although accretion disks have sufficient energy to eject a small fraction of the infalling material as jets, it is not clear exactly how. The accretion disks are thought to generate tangled-up magnetic field, which is probably what collimates the jets. However, astrophysicists are still working out the details of how material is lifted up from the accretion disk in the first place, and how exactly it is that the accelerating force (probably radiation pressure) can overcome the gravity of the central object.

Some theorists do think that there is something about jets from black holes, and that an intrinsic property of black holes helps create such powerful jets. Other theorists disagree.

Hope this helps.

Koji Mukai and Maggie Masetti
for Ask a High-Energy Astronomer

QUESTION:
Can black holes be used as a power source?

ANSWER:
Black holes are like 40 ton gorillas, they sit wherever they want to. And that will be a major problem with using them as a source of power. How do you get one to where you can use it, and how would you keep it there once you have it. Black holes are sources of extreme amounts of energy and are responsible for some of the most luminous objects in our Universe (AGN, etc.) as well as some pretty hefty beasts in our own Galaxy. If you haven't already, you can read about them in Imagine the Universe! The article on black holes also suggests several books to read on the subject.

While most of what has been written about deriving useful energy from black holes is in science fiction, here are a few good references. One way of extracting energy from a black hole is known as the Penrose process. The energy comes from reducing the angular momentum of a black hole (or any massive body). A popular-level description is given in Nigel Calder's "Einstein's Universe" in the "Ultimate Waterfall" chapter and a graduate-level description is given in Bernard Schutz' "A First Course in General Relativity", pg 304-305. The Hawking process also results in a "free" energy source since the black hole is "evaporating" quantum mechanically, with photons carrying away energy (also discussed in Schutz' book).

QUESTION:
Are there any sub-atomic particles that are able to escape a hole's event horizon?

ANSWER:
No. It is believed that once a particle is inside the event horizon, it can never escape, regardless of what kind it is. There is a phenomenon predicted by Dr. Hawking where energetic particles are emitted from just outside the event horizon (see Hawking radiation in the Imagine the Universe! dictionary), but even then the particles are not coming from inside the event horizon.

Best wishes,
Koji Mukai

QUESTION:
How is it possible for black holes to emit matter even if their gravity field is so intense?

ANSWER:
Black holes emit matter BECAUSE their gravity field is so intense.

Specifically, black holes emit particles by a process known as Hawking radiation, described in

http://imagine.gsfc.nasa.gov/docs/dict_ei.html#H

What is ordinarily considered empty space is full of 'virtual pairs' which are particle-antiparticle pairs which pop into existence, separate a very short distance, come back together, and disappear a very short time later. This happens so quickly that the Universe doesn't notice that for a short while there was extra mass-energy. The law of conservation of energy only holds over sufficiently long periods of time, and can be briefly violated.

In the neighborhood of a black hole, the virtual pair can pop into existence, and when they separate, one can go so deeply into the black hole that its falling releases enough energy that the other particle can continue to exist, outside the hole, with the total energy of the virtual pair being zero.

It takes a huge gravitational field to release such a large amount of energy when the particle falls such a very short distance. Such huge fields are found only around black holes.

David Palmer
for Ask a High-Energy Astronomer

What is ordinarily considered empty space is full of 'virtual pairs' which are particle_antiparticle pairs which pop into existence, separate a very short distance, come back together, and disappear a very short time later. This happens so quickly that the Universe doesn't notice that for a short while there was extra mass_energy. The law of conservation of energy only holds over sufficiently long periods of time, and can be briefly violated.

In the neighborhood of a black hole, the virtual pair can pop into existence, and when they separate, one can go so deeply into the black hole that its falling releases enough energy that the other particle can continue to exist, outside the hole, with the total energy of the virtual pair being zero.

It takes a huge gravitational field to release such a large amount of energy when the particle falls such a very short distance. Such huge fields are found only around black holes.

David Palmer
for Ask a High_Energy Astronomer

QUESTION:
What is the temperature of a black hole?

ANSWER:
The temperature of a black hole is determined by the 'black body radiation temperature' of the radiation which comes from it. (e.g., If something is hot enough to give off bright blue light, it is hotter than something that is merely a dim red hot.)

For black holes the mass of our Sun, the radiation coming from it is so weak and so cool that the temperature is only one ten-millionth of a degree above absolute zero. This is colder than scientists could make things on Earth up until just a few years ago (and the invention of of a way to get things that cold won the Nobel prize this year). Some black holes are thought to weigh a billion times as much as the Sun, and they would be a billion times colder, far colder than what scientists have achieved on Earth.

However, even though these things are very cold, they can be surrounded by extremely hot material. As they pull gas and stars down into their gravity wells, the material rubs against itself at a good fraction of the speed of light. This heats it up to hundreds of millions of degrees. The radiation from this hot, infalling material is what high-energy astronomers study.

David Palmer
for Ask a High-Energy Astronomer

Formation of Black Holes

QUESTION:
How are black holes formed by supernovae?

ANSWER:
Yes, in the formation of a neutron star the infall is so great that it is compressed to up to 50 % greater than its normal density. In a neutron star, the nuclear force is strong enough to cause a rebound from this compression, which gives an outward push to the remaining outer layers of the stellar interior.

However, the rebound is only part of the mechanism by which the supernova generates its energy. The binding energy of a neutron star is much less than that of a non-collapsed stellar core. The tremendous amount of energy generated by the neutron star formation drives the supernova. The same is true with the formation of a black hole, save that the binding energy of the black hole is even less than a neutron star and hence the explosion would be somewhat more energetic. So the formation to a black hole still includes an explosion.

Another way of thinking of the same issue: if the rebound was the only source of energy driving the supernova, the surface layers of the star would only bounce back up to their original radius prior to the stellar collapse if no energy was generated by the formation of the neutron star, rather like dropping a superball onto the floor and letting it bounce back. The rebound effect is rather like having the floor jump up at the superball, so it bounces somewhat higher. But in this case, the binding energy released in the explosion is even greater: the superball is thrown off into outer space instead of just bouncing higher.

Jesse Allen and Jim Lochner
for "Ask a High-Energy Astronomer"

QUESTION:
What force causes the compression of matter to a singularity?

ANSWER:
Gravity does the work. When you have enough material together, gravity can be very strong. And the more mass you have, the lower the density needs to be in order to make a black hole.

If all nuclear burning in a star greater than about 1.4 solar masses were to stop, and the star allowed to cool and solidify, the solidified material would not be strong enough to support its own weight, and it would collapse as the electrons were pushed into the protons to make neutrons. This neutron star material is stronger, but the star would be only about 20 km in diameter. If you piled on more material, you would eventually get to the point where there is so much gravity that not even the neutrons could hold it up, and the star would collapse into a black hole.

David Palmer and Samar Safi-Harb
for Ask A High-Energy Astronomer

QUESTION:
I just finished reading Kip Thorne's book "Black Holes and Time Warps". I seem to recall an article in the last 6 months or so, I thought from the NY Times Science pages, about a new theory on collapsing stars that allowed for black holes to form from stars much smaller than previously thought. The mechanism as I recall was for a short burst of radiation release such that the neutron pressure that normally keeps the star from collapsing is released. I recall that one of the authors was in his 80's and the other in his 90's. Could you give me a reference to this work?

ANSWER:
I believe you are referring to a paper by Gerald Brown with Hans Bethe called "A Scenario for a Large Number of Low-Mass Black Holes in the Galaxy". The paper was published in the Astrophysical Journal, volume 423, page 659, in March 1994. The mechanism they propose "softens" the equation governing the collapse of a neutron star so that stars with lower mass that previously thought can produce black holes after a supernova explosion. Hans Bethe turned 90 years old this past July, and was presented by the APS with a prize that will be set up in his name. The prize will be awarded annually to an astronomer or nuclear physicist (or someone who is both!). Hans considers himself both an astronomer and a nuclear physicist, and is the person who explained the generation of energy in the Sun by the fusion of hydrogen to helium. He won the Nobel prize for this research in 1967. He is a national treasure, and as you will see at http://mocha.phys.washington.edu/dap/hans_1.html he is still a vibrant and happy member of the physics community. In fact, he is still publishing. His latest paper in the "Ap. J" was in December, on Supernova Shocks. I'm sorry I don't know the age of the other author of the paper.

Here's the blurb about the project that appeared in Physics News Update:

August 25, 1993

WHY DON'T MORE SUPERNOVAS LEAVE BEHIND PULSARS? The venerable Hans Bethe, pioneer in the study of stellar nuclear physics for decades, and Gerald Brown of Stony Brook, propose that some collapsing stars---with masses as low as 18 solar masses---refrain temporarily from vanishing into a black hole and instead pause at the neutron-star level of compactification long enough to trigger a supernova explosion. The residual core would thereafter shrink without a trace into a black hole. Bethe and Brown invoke a hypothesis about neutron stars introduced in the last few years by Brown and others. According to this model, as the density at the core of a collapsing star exceeds three times the density of ordinary matter, electrons might not necessarily combine with protons to form neutrons, as was thought, but might instead spawn K mesons and neutrinos. In place of the traditional pure neutron matter, the collapsing core would then be a more compressible mixture of protons and neutrons, buoyed up for a time by the restless motions of the neutrinos. When they depart the collapse would continue, leaving behind either a "nucleon star" or a black hole. (Science, 13 August 1993.)

Regards,
Padi Boyd,
for the Ask a High-Energy Astronomer team

Black Holes and Gravity

QUESTION:
What is gravity's effect on itself?

ANSWER:
Thank you for your question about gravitational radiation from black holes. It is clear that you have given this quite a bit of thought! Your question has touched upon a number of other very interesting questions concerning gravity, such as what is meant by the term gravitational energy and where is it located. However, in a system of two colliding black holes the issues are not necessarily difficult to understand, at least at a basic level. As you can probably guess, a complete treatment can rapidly become incomprehensible and can be left to those people who do this kind of work for living. The short answer therefore, hopefully appropriate for Science readers, is the following:

The gravitational radiation associated with this system comes from the time dependent nature of spacetime outside the black holes as these objects move towards each other and accelerate. This is, to some extent, equivalent to the electromagnetic radiation of two charges attracted to each other. You need not know the precise structure of the charges or whatever singularities they enclose. All you care is that they accelerate as they approach each other, and that produces time dependent electromagnetic fields. At large distances these time dependent electromagnetic fields appear as radiation. The just replace the charges with black holes and electromagnetic radiation with gravitational radiation.

There are qualitative differences of course, the charges have to have opposite signs to attract while the black holes attract each other anyway. There is also some of the energy (whatever the term means in this case) which is lost down the black holes while this is not in the charge case. There are additional issues which the experts worry about and presumably treat correctly.

Cheers,
Steve Snowden, Laura Whitlock, and Demos Kazanas

QUESTION:
Just curious about Black holes, and I wanted to know if the gravitational field of a black hole would pull an object in faster than the speed of light. If I understand correctly objects cannot go faster than the speed of light our they would be going back in time. If the acceleration of a black hole is constant, would an object that got sucked into a black holes velocity increase beyond the speed of light the closer it got to the black hole?

ANSWER:
The answer to your question is that the motion of a particle near a black hole is not governed by Newton's laws of motion in the familiar sense. The correct equations for motion near a black hole predict that an object on a radial path into the hole will have a velocity which approaches the speed of light as the object approaches the event horizon. For more information, I can only refer you to a textbook on general relativity, such as the one by Steven Weinberg ("Gravitation and Cosmology..." 1972 (Wiley: New York).

I hope this is of some help.

Tim Kallman

QUESTION:
What is Quantum Gravity and how does it relate to Black Holes?

ANSWER:
Current theories of gravity are based on the geometric curvature of space. Current theories of other fundamental forces in the universe are 'quantum field theories', where particles pass other particles back and forth among themselves to interact.

We know that geometric gravity theories conflict with quantum field theories, and that this conflict means that we don't know what happens under extreme conditions.

A quantum theory of gravity would involve particles passing 'gravitons' back and forth among themselves. This quantum theory would probably be a more accurate description of gravity, and might be accurate enough to describe the extreme conditions found at the center of a black hole.

David Palmer
for Ask a High-Energy Astronomer

QUESTION:
It is believed that gravity, like other forces, has a counterpart in the particle world, usually called the "graviton". How come the graviton can escape the inner side of a black hole ? Since the space-time fabric inside a black hole makes everything move toward the center, gravitons should never escape, therefore black holes should not have a gravity field outside the horizon. I'm an engineer in information technology, and I have a good knowledge of what a black hole is. Don't hesitate to show me formulas !

ANSWER:
The following website deals with issues such as you have raised:

http://sciastro.astronomy.net/sci.astro.4.FAQ

In fact, it contains an answer to your question:


Subject: D.09 How can gravity escape from a black hole? Author: Matthew P Wiener , Steve Carlip

In a classical point of view, this question is based on an incorrect picture of gravity. Gravity is just the manifestation of spacetime curvature, and a black hole is just a certain very steep puckering that captures anything that comes too closely. Ripples in the curvature travel along in small undulatory packs (radiation---see D.05), but these are an optional addition to the gravitation that is already around. In particular, black holes don't need to radiate to have the fields that they do. Once formed, they and their gravity just are.

In a quantum point of view, though, it's a good question. We don't yet have a good quantum theory of gravity, and it's risky to predict what such a theory will look like. But we do have a good theory of quantum electrodynamics, so let's ask the same question for a charged black hole: how can a such an object attract or repel other charged objects if photons can't escape from the event horizon?

The key point is that electromagnetic interactions (and gravity, if quantum gravity ends up looking like quantum electrodynamics) are mediated by the exchange of *virtual* particles. This allows a standard loophole: virtual particles can pretty much "do" whatever they like, including travelling faster than light, so long as they disappear before they violate the Heisenberg uncertainty principle.

The black hole event horizon is where normal matter (and forces) must exceed the speed of light in order to escape, and thus are trapped. The horizon is meaningless to a virtual particle with enough speed. In particular, a charged black hole is a source of virtual photons that can then do their usual virtual business with the rest of the universe. Once again, we don't know for sure that quantum gravity will have a description in terms of gravitons, but if it does, the same loophole will apply---gravitational attraction will be mediated by virtual gravitons, which are free to ignore a black hole event horizon.


J.K. Cannizzo
(for "Ask a High-Energy Astronomer")

Advanced Concepts

QUESTION:
What are the meaning of the letters in the "BKZS Limit"?

ANSWER:
Thank you very much for your interesting, very high level, question. We could not answer your question ourselves, but we have managed to find an expert, Dr. Charles Dermer of Naval Research Laboratory, who could. His answer is given here.

The questioner is quite right about what he has heard. There is a fundamental limiting temperature above which steady thermal plasmas cannot exist. The limit is named after the authors of the paper which points out this limit, and the complete reference is:
Bisnovatyi-Kogan, G. S., Zel'dovich, Ya. B., and Sunyaev, R. A., 1971, Soviet Astronomy, AJ, vol. 15, p. 17.

The question outlines the essential reason for this limit: at sufficiently high temperatures, there is a competition between two-body processes. On the one hand, collisions of electrons with other particles (such as electrons, positrons, or protons), makes electron-positron pairs through the process:

  1. particle 1 + particle 2 --> particle 1 + particle 2 + electron + positron. The electrons and positrons are made at the expense of the kinetic energy of particles 1 and 2.
  2. On the other hand, pair production is balanced by the pair annihilation process: electron + positron --> two gamma-ray photons.
At sufficiently high temperatures, the addition of electron-positron pairs through process (1) makes additional electron-positron pairs through process (1) and so on, and this cannot be balanced by the pair annihilation rate. The result is unlimited production of pairs if one requires the system to remain at a fixed temperature. In reality, of course, energy cannot be continuously injected and the system cools, so that the runaway pair production is quenched.

BKSZ calculated a maximum temperature of about 20 MeV; subsequent research revised that maximum to about 12 MeV (see, for example, A. A. Zdziarski, 1982, Astronomy and Astrophysics Letters, vol. 110, p. L7). This is for a completely transparent medium, and the maximum temperature is even less if the system is opaque (i.e., has finite optical depth).

The discovery of time-variable sources of 0.511 MeV annihilation radiation was the impetus for this work, although the reality of black hole sources of annihilation radiation is now in dispute (although diffuse annihilation radiation in the galaxy and on the Sun from radioactive beta-emitters is beyond question). A Scientific American article by Gehrels, et al. (December 1993, page 68) discusses cosmic annihilation radiation, though not in great detail.

Hypothetical Questions

QUESTION:
If white holes eject matter so quickly, why do they exist ? Wouldn't they destroy themselves? Maybe they are at the other end of a black hole ?

ANSWER:
White holes are VERY hypothetical. They are, in fact, predicted as a possible "other end" of a black hole that has punctured a "worm hole" through space, but black holes are most likely just a point in space without an other side. The matter/energy coming out of white holes is supposedly the matter falling into a black hole. I have only seen them discussed in theoretical physics talks. At one point scientists speculated that quasars may be white holes, but now we are fairly certain that quasars are powered by supermassive black holes, in which case the light we see comes from matter as it falls into the black hole. After it falls in, we assume the matter just becomes part of the black hole and does not come out anywhere (see the Basic or Advanced discussions of active galactic nuclei in Imagine the Universe! for more on this.)

Cheers,
Andy Ptak

QUESTION:
I have a question about Black Holes, can Black Holes transport you to other galaxies, like I see in Star Trek or in Star Wars movies, or is it just show biz? Are Black Holes transports to other worlds, other universes?

ANSWER:
In principle, the mathematics of black holes show that they might be able to transport you to another region of space or possibly another universe. However, the mathematics also shows that the connection only lasts for a fraction of a second at a time, and that the link to other universes depends on a specific set of conditions which scientists do not believe to have existed. Even if it were likely, however, the strong gravity field of the black hole tears apart any material falling into the hole. So it would not come out the other end looking anything like how it went in.

Jim Lochner