D. Cowen
Professor of Physics and Astronomy
University of Pennsylvania
The regions around black holes often make lots of high energy particles which have enough energy to escape the region around the black hole and make it all the way to us on earth, where they can be detected. Also, black holes can act like lenses, focusing starlight from stars far behind the black hole, and we can detect this temporary increase in star brightness.

Astronomer*
Black holes were initially a theoretical consideration of stellar evolution. Detecting
them comes from an indirect means. Velocities of a star in orbit around another star are
frequently measured. Black holes are suspected if we find a star orbiting at great speeds
around an unseen companion. The object must be massive because of the great speed at which the star orbits. Normally something that massive would be a star and should be seen as well. Gas from the normal star can also fall into or around this potential black hole and heat up so much that it radiates X-rays. All the observations tell us that there is definitely something non-luminous, massive, and compact that the star is orbiting. The trick is then to determine what the mass of this unseen companion is so that we can use what we know about atomic physics to determine if it is a neutron star or a likely black hole.

Douglas Scott
Professor
University of British Columbia
The matter interacting near the black hole tends to be very hot, and hence luminous in x-rays.

Dr. Michael M. Davis
Astronomer
SETI Institute
By their gravitational effects. Even though light cannot get out of the enormously deep gravitational well surrounding the black hole, if you stand back from the hole far enough the gravitational pull is the same as for any other astronomical body of the same mass. In particular, matter orbiting close to but not inside the boundary where light can no longer escape can be circling the black hole at very high speed. The Doppler shift from this high speed matter can be measured with the Hubble space telescope. This has been done for some of the black holes which are now generally accepted as being real. The speed of this matter in its orbit is a measure of the amount of matter inside the orbit (Newton actually figured this out), and the size of the orbit tells how small a volume is occupied by this matter. If the amount of matter per unit volume is above a critical value, the only valid physical model is a black hole.

Yervant Terzian
Professor
Cornell University
We see the black holes influence in its surroundings, such as very high velocity of
nearby stars, and high energy emission from high velocity gas that is being pulled by the BH.

Mordecai-Mark Mac Low
Curator/Professor of Astrophysics
American Museum Natural History
By looking for the effects of their gravity, in two main ways:

1. Interstellar gas falling into them tends to whirl around in a disk as it goes in (think of the whirlpool in a bathtub drain). Since that gas is falling in extraordinarily fast (0.1 speed of light or more), when it hits this disk, it heats up to tens of millions of degrees, and glows not just white-hot, but even hotter: X-ray hot. These X-rays can be seen from satellite telescopes like the Chandra X-ray Observatory.

2. Stars can be seen orbiting an invisible black hole like the one in the center of our galaxy. Their motions can actually be measured, and used to infer that there must be a million solar masses within a cubic light year at the center of their orbits, where only a radio source can be seen behind the obscuring dust. Nothing other than a black hole could be so massive, and yet shine so dimly.

Prof. Wayne G. Roberge
Theoretical Astrophysicist
Rensselaer Polytechnic Inst.
There are several ways. For example, General Relativity says that whenever matter
is packed into a small enough region, it MUST become a black hole. If the Sun were
packed into a sphere just 3 kilometers in radius, it would become a black hole. So
one of the ways we search for black holes is to look for regions with lots of matter
packed into a small region. An example is the center of our Milky Way galaxy, which contains a few million sun's worth of mass in a region about the size of our solar system. This is called a supermassive black hole. Supermassive black holes have now been discovered in many other galaxies. We suspect that every galaxy has one lurking in its center.

Another way to find black holes is to use the fact that gravity bends light. When a black hole passes in front of an ordinary star, the light from the star gets bent (an effect called "gravitational lensing"). Several black holes were discovered this way during the last year.

Andrew Liddle
Astronomer
University of Sussex, UK
Astrophysical black holes are usually eating up their surroundings and this creates a lot of energy some of which is released. In fact some of the brightest objects in the Universe, eg quasars, are probably powered by black holes.

Karen Vanlandingham
Assistant Professor
Columbia University's Biosphere 2 Center
Anything with mass will affect other things with mass. You are affected by the Earth (you are bound to it), the Earth is affected by the Sun (it orbits the Sun), and so on. A black hole doesn't shine but it has a LOT of mass. We can observe black holes by noticing how other objects behave near them. This is also how we find planets around very bright stars - the star's light hides the planet from view but the mass of the planet makes the star wobble.

Astronomer*
They see them due to the gravitational effects the black hole has on nearby material.

Eilat Glikman
Graduate Student
Columbia University
A black hole may be black, but it is VERY massive for its size. If a star revolved around it, and we could see the star's motion, we could not only infer the presence of a black hole, but we could also calculate the mass of the black hole from the motion of the star. Another way to detect black holes (MUCH HARDER) is by gravitational lensing. Because they are so massive, black holes *BEND* space. When we look through space that is bent by a black hole, its like looking through a store window that has a bent piece of glass (a lens) instead of a flat window. The shapes behind the glass will be distorted by the lens. Sure. No problem ... as long as the system is set up carefully. The most stable way to do it is to let the stars be very close together and the planet be much farther out than the separation of the stars. Then the planet orbits both stars almost as if they were one.

Professor*
The cleanest way is to look at the motions of objects (stars or gas) that orbit close enough to the black hole to feel its gravity. The chain of reasoning is slightly complicated but not really difficult: the aim is first to measure the gravitating mass and then to figure out if it can be anything more mundane than a black hole.

Walter Harris
Astronomer
University of Wisconsin-Madison
Two ways.

1) They may not be bright, but they are massive. We can detect black holes by looking at the motions of nearby stars/gas and determining the mass of the attracting source. We have found evidence of massive black holes in other galaxies with this technique.

2) As material drops onto a black hole it becomes very hot and begins to emit light like coals in a fire pit. Eventually the gas gets SO hot that it emits x-rays. So long as the gas is *falling* onto the black hole but is not *in* it, we can see this light. We discovered our first black hole (cygnus X-1) this way.

Graduate Student*
Astronomers have a few tricks for finding black holes. One of the easiest ones is to watch how other objects appear to be moving around black holes. If a star or many stars or some blobs of gas appear to be moving really fast in a circle around something that does not produce any light, then they have to be moving around something really massive but really small. The only possibility would be a black hole. The second way astronomers find black holes (or at least assume that black holes are present) is when they see certain kinds of electromagnetic radiation, such as lots of X-ray or radio waves, from something very small. Black holes are thought to be the only objects that can compress gas in space enough to make it so hot that it produces so much of these kinds of radiation. A third trick astronomers use is to look for plumes of gas that emit radio waves. Astronomers think these plumes (called jets) come out of the gas that is being compressed around black holes, although how the gas jets out is not well understood.

Ed Churchwell
Professor/Astronomer
University of Wisconsin
As matter (atoms) fall into the black hole, they emit a lot of X-rays and
ultraviolet light before they go beyond the point where light cannot escape.
They are also detectable by their gravitational pull on objects around them.

Graduate Student*
The existence of black holes in the universe is inferred through indirect observation.
Our theories (General Relativity -- Einstein's theory of gravity) predict that these objects
should exist. If astronomers observe some phenomenon which doesn't fit with any known type of object (ie. stars, gas, etc.), they will then try to use more exotic explanations. One of these is a black hole. One example of how a black hole could be indirectly observed is by looking at a binary system of a black hole and an ordinary star. We can calculate the masses of both objects, and if the star's companion is more than about 1.4 times the mass of the sun -- which is the maximum mass of a neutron star -- and we can't see it directly, there is a good chance it is a black hole. Looking at high-energy radiation (eg. x-rays) from an unknown object is another way in which they are observed.

Steven Novotny
Air Force Officer/Graduate Student
University of Florida Astronomy Department
Events occur around a black hole that can be seen. As particles spiral into a black hole, they emit particular forms of radiation which we can detect. Also, we can observe behavior of other stars near the black hole.

Bob Mathieu
Professor of Astronomy
Department of Astronomy, University of Wisconsin - Madison
While black holes do not allow light to escape, they still have mass and thus cause and respond to gravity. One way we find black holes is by seeing a star orbiting something "invisible". If that something turns out to be a few times more massive than the Sun but is emitting no light, then we conclude it is a likely black hole! Similarly, if we see stars orbiting something very massive (millions of solar masses) in the center of a galaxy that is not emitting light, we conclude there is a black hole there too.

Actually, even though black holes themselves don't emit light, they are very often surrounded by disks of gas which are orbiting them. This material is orbiting so fast that it gets very hot and emits X-rays that we can see. Eventually the material will find its way into the black holes, but until then their X-rays are a beacon by which we find black holes.

Professor Rex A. Saffer
Physicist, Astronomer, Educator
Dept. of Astronomy & Astrophysics, Villanova University
We detect black holes by the influence of their gravity on matter that orbits around them.
Most black holes suck in material from around them, and this material rubs against it self and heats up by friction. It is the hot, glowing material being sucked in that we see, not the black hole itself.

David Batuski
Astronomer
University of Maine
They 'mess up' the view of stuff behind them by bending space.

Eric McKenzie
Graduate student
University of Florida
There are two ways that I know of. If a black hole is big enough and lots of material is nearby, then all the material which gets pulled into the black hole gets compressed and heated up (a lot!), so that before it actually enters the black hole it gives off lots of light. This is what we think is happening when we see quasars: that they are huge black holes at the centers of galaxies that are sucking in tons of material, which glows extremely brightly before it enters the hole.

Another method for detecting black holes is looking for a gravitational pull on nearby objects that is so strong that only a black hole could be responsible.

Astronomy Professor*
Through their gravitational influence on nearby objects such as stars or by matter falling
into the black hole being heated up and emitting x-rays before that material crosses the event horizon.

John Huchra
Astronomer/Professor
Harvard-Smithsonian
We see the stuff falling into them, sometimes in x-rays and sometimes in optical light.

*Respondents opted for anonymity and we respect their wishes.