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You can scroll through the questions yourself or click below:
  1. What is antimatter?
  2. When and by whom were black holes first identified-- either physically or theoretically?
  3. What happens to the indivdual atoms of matter that is sucked in to a black hole?
  4. Can a Black Hole suck up the whole universe?
  5. Can a Black Hole come through Earth's atmosphere and hit land?
  6. What occupys the space between a singularity and the event horizon? Is there light inside the black hole?
  7. What is the Strong Force and how does it relate to neutron stars?
  8. How long can the cycle of stardeath and star birth go on?
  9. What's the difference between black holes and wormholes?
  10. How can the gravity of a white dwarf be greater than that of a red giant?
  11. Don't black holes and neutron stars undergo a supernova explosion as they form?
  12. What are quasars?
  13. How do we detect black holes that aquire matter from nearby stars?
  14. What are some books dealing with black holes?
  15. Why is it immpossible to travel faster than the speed of light?
  16. I read that a gigantic black hole is in the galaxy M87. Where is M87?
Q JTA writes, "What is antimatter?"

A Antimatter is made of atoms containing oppositely charged particles. Antimatter atoms have negatively charged nuclei and positively charged electrons. The electron antiparticle is a positron. When a particle and its antiparticle counterpart meet, they annihilate each other with a release of energy. Luckily (for us matter beings), there seems to be more matter than antimatter in the Universe.

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Q Steve Deacon writes, "When and by whom were black holes first identified--either physically or theoretically?"

Q Jeff Conry asks, "Who made the first discovery of black holes? I need to know for a school project."

A Many things in physics and astronomy are predicted by theory before they are actually observed. This is true with black holes. According to Stephen Hawking, a man named John Michell proposed the concept. In 1783, Michell suggested in Philosophical Thoughts of the Royal Society of London that a sufficiently massive star would not allow light to escape. In 1928, Subrahmanyan Chandrasekhar, an Indian grad student at the time, figured out that a star over a certain mass must collapse after it stops burning. American scientist John Wheeler coined the term "black hole" way back in 1969. Of course, these scientists weren't working in a vacuum; theory about black holes is the product of many, including Albert Einstein and Robert Oppenheimer.

Since black holes cannot be observed directly, their existence must be inferred. As I discuss in Black Holes and Neutron Stars, astronomers look for x-rays, gravitational lensing events, and effects of gravity on visible object. Early black hole candidates, Cygnus X-1 and Centaurus X-3, were identified in the early 1970s. Cygnus X-1 not only emits x-rays, but it also sends out radio waves. In 1973, radio astronomers were able to locate Cygnus X-1 and its binary partner, a blue supergiant star (HDE 226868). Because HDE 226868 is orbiting around something that is unseen, yet emitting x-rays, the astronomers concluded it is a black hole. Centaurus X-3 was identified as a black hole in a similar way. In 1973, Polish astronomer Wojciech Kreminski determined that Centaurus X-3 is a black hole orbiting with a blue giant star. In 1995, a team of American and Japanese scientists used radio telescopes to calculate the velocities of gas and dust in the center of the spiral galaxy NGC 4258. Their measurements strongly suggest a black hole with the mass of 40 million solar masses sits at the center of NGC 4258.

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Q Michael McCaffrey writes, "I have searched (to no avail) for information regarding what happens to an individual molecule or atom when it enters a Black Hole. Can an individual atom be compressed in some way by the intense gravity of a Black Hole? I have looked at various other web pages and have seen conflicting reports. Some say that a Black Hole reaches a point of 0 diameter and infinite density, but how can this be? If it has 0 diameter what happens to individual atoms?"

A I don't think anybody understands (or at least can visualize) the state of matter in a black hole singularity. But atoms don't even survive the collapse into a neutron star, much less into a black hole.

When a star collapses to form a neutron star, the tremendous pressure from the gravity causes something called inverse beta decay. Inverse beta decay occurs when an electron and a proton combine to make a neutron and a neutrino. So the atoms are inonized (stripped of their electrons) and the protons in the nucleus are converted to neutrons. As the pressure continues to increase, the atomic nuclei fall apart to form a gas of mostly neutrons (hence, the name neutron star). Then, the increasing pressure causes the neutron gas to pack into a crystalline solid. That's the core of the neutron star--no atoms, mostly just neutrons.

When a star of 3 or more solar masses dies and collapses, all its matter is squeezed into a singularity--zero diameter, yet it has mass. A paradox. This paradoxical black hole singularity is reminiscent of the state of the Universe before the Big Bang (Hawking, Stephen, A Brief History of Time: From the Big Bang to Black Holes. New York: Bantam Books. p 88). According to Zeilik and Gaustad, where a black hole singularity starts, "present knowledge of physics ends" (Zeilik and Gaustad, Astronomy: The Cosmic Perspective. New York: John Wiley & Sons, 1990. p 566). At the center of a black hole, the laws of physics do not apply.

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Q balls asks: "What if a blackhole or neutron-star for that matter sucks up dust from nearby planets or meteors and slowly raises its mass this way? Wouldn't gravity raise with it? And, after a long, long time wouldn't it be able to swallow, for example, a relatively nearby twin-star? And then, of course, the circle would be complete and in a very hypothetical case, the whole universe could be swallowed..."

Q Anita Lobo asks: "Can a Black hole suck up the whole universe?"

Q Miranda Michanek asks: "Could you explain more in detail how a black hole would evaporate --two kinda-particles would all of a sudden become more than virtual and , well, somehow the hole would loose this mass. Kind of like sort of something like that and so on. Also, have there been any recent advances in quantum gravity that can be understood by an undergrad student?"

A Physicist Richard Feynman believed that even particle physicists do not understand advances in particle physics. What a smart guy.

It's easy to imagine a black hole growing until it comprises all the matter in the Universe. That even sounds like what a pre- or post- Big Bang Universe is like. And astronomers are finding huge black holes at the center of galaxies that have taken on mass after forming (there are mass limits on stars and, therefore, mass limits on new black holes). A black hole at the center of the spiral galaxy NGC 4258, for example, has the mass of 40 million suns. There was not a star of that mass that collapsed--the black hole grew to that mass by sucking in stuff around it.

One of the things that could prevent a black hole from swallowing up the whole universe is distance--the fact that galaxies are so far apart. Even if a black hole was to swallow up a whole galaxy, it would contain all the locally available mass and could grow no more. The event horizon of the black hole would be very large, but it would not span the enormous distance that exists between galaxies. The gravitational effect of the black hole on neighboring galaxies would be the same as when it was a galaxy.

Another scenario that suggests black holes will not consume everything is proposed by Stephen Hawking. In "empty" space, fluctuations of electromagnetic and gravitational fields produce a pair of "virtual" particles--particle pairs that exist only briefly. They appear, move apart and then back together, annihilating each other. One particle has positive energy, the other has negative energy; added together, their energies equal 0.

When virtual particle pairs appear near a black hole's event horizon, one of the particles may fall into the black hole, leaving the other particle free. The particles with negative energy that fall into the black hole contribute negative energy, thus reducing the total energy of the black hole. This decreases its mass (remember e=mc^2--energy and mass are equated), and the black hole eventually evaporates away.

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Q Brian Witten asks: "Can a Black Hole come through Earth's atmosphere and hit land?"

A I don't think a black hole would make it through to crash on land. Although the singularity is a single point, it affects far away stuff just as if it were a star. So the Earth would probably either go into orbit around the black hole, as it orbits around the Sun, or the Earth would fall into the singularity.

Actually, the sun would probably orbit the black hole, because the sun's mass is so much larger than the Earth's mass. In the presence of the Sun and the black hole, the Earth's mass really doesn't count for much. So the black hole and the Sun might form a double "star" system, orbiting eachother, with the Earth continuing to orbit the Sun.

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Q Cerraeh asks: "Supposing you have a blackhole (singularity) with a 6 kilometer radius for the event horizon, what is occupying the space between the singularity and the event horizon? Also, if a black hole attracts light particles but does not allow them to escape, then is there light inside the black hole?"

A The event horizon is not a physical boundary but a gravitational one. It is the distance at which nothing can escape the singularity's gravitational influence--the point of no return. So between the event horizon and the singularity is space (space really warped by gravity, but still space). Nothing, not even light, hangs around for very long in the space between the singularity and the event horizon. Everything that enters through the event horozon becomes part of the singularity.

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Q Kate Schuetze writes: "I am doing a visual presentation on neutron stars and their relation to nuclear physics. Could you please tell me about Strong Nuclear Force and how so many neutrons can be condensed with such a strong force? Thank-you."

A Currently, physicists say there are four forces of Nature: gravitational, electromagnetic, weak, and strong. Gravitational force is the attraction between two or more masses that we know as gravity. The electromagnetic force holds the electrically charged particles of the atom--the nucleus and the electrons--together. The weak force is responsible for radioactivity. The strong nuclear force binds together quarks, which make up protons and neutrons, and it also holds together the protons and neutrons to form the atomic nucleus.

When a star collapses to form a neutron star, the tremendous pressure from the gravity causes something called inverse beta decay. Inverse beta decay occurs when an electron and a proton combine to make a neutron and a neutrino. So the atoms are ionized (stripped of their electrons) and the protons in the nucleus are converted to neutrons. As the pressure continues to increase, the atomic nuclei fall apart to form a gas of mostly neutrons (hence, the name neutron star). Then, the increasing pressure causes the neutron gas to pack into a crystalline solid. That's the core of the neutron star--no atoms, mostly just neutrons. Since there are no positive or negative charges, there is no electromagnetic force at work on the sub-atomic particles. The strong nuclear force is what holds the neutrons together.

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Q Badri asks: "I inderstand that a massive star dies a spectacular death in the form of a supernova and the the remnants can re-group to form another star . This is how the solar system was formed. How long can such a re-cycling process go on? Is it possible that in an open universe, star birth/death/re-birth cycle can go on eternally? And where does entropy come in here? "

A There are three commonly accepted scenarios for the fate of the Big Bang Universe: closed, flat, and open. A closed Universe means that the present expansion resulting from the Big Bang will not overcome internal gravitational attraction. Eventually, the expansion will stop and the Universe will collapse back on itself. A flat Universe has just enough expansion to balance out its gravity; it will expand to a certain point and then stop. An open Universe continues to expand forever.

The closed Universe scenario suggests the recycling of matter on a big scale. Recycling of matter from supernovae explosions into new stars could probably go on indefinitely in a flat Universe. Physicists theorize that eventually an open universe will cool off and die.

Entropy is how disordered a system is. The second law of thermodynamics says that entropy always increases with time unless work is done to it. An organized system always becomes disorganized. Certainly an open Universe's entropy would always increase. The flat and closed Universe scenarios suggest that entropy may not continue to increase. Perhaps increasing entropy is a function of an expanding Universe.

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Q bterry writes: "I am a gifted teacher in our high school. My students and I are working in the Future Problem Solving Program. Our topic this time is Extraterrestrial Life. We are interested in knowing the difference in Black Holes and Worm Holes. They have found these topics to be very interesting in solving problems with time and space."

A A black hole, as described in my Website, is matter that has collapsed under its own gravity to form a singularity. A wormhole is a theoretical shortcut from one place in space-time to another. This shortcut can be accomplished by traveling in a "straight" line to connect two points of curved space-time. Let's say you and a friend are roaming around in space and you happen to come upon a wormhole, at point A. If you entered the wormhole and your friend took off for point B going through regular space-time, you would arrive at point B before your friend (assuming you're both going the same speed). You would have taken a shortcut through both space and time.

picture of wormhole joining spacetime

Astrophysicist Lawrence M. Krauss points out the unlikeliness of this scenario: what the wormhole goes through is not 4-dimensional space-time. 4-dimensional space-time is what the wormhole connects. If space-time is all there is to the Universe; there is nothing for the wormhole to exist in. Krauss has more to say about wormholes in his book, The Physics of Star Treck.

Krauss, Lawrence M. The Physics of Star Treck. New York: HarperCollins, 1995. (ISBN 0-465-00559-4).

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Q Jake Kramer asks: "How is the gravity of the White Dwarf greater than a larger star, like a red super giant, even though it's much smaller?"

A It's possible for "regular" stars to have greater surface gravity than red giants. That's because the amount of gravitational force that an object exerts depends not only on the mass of the object, but also on the distance from that object.

A red giant star is a "giant" because of its size and not because of its mass. Red giants form from "regular" stars, like our sun, that are very old. When old stars have used up the hydrogen in the core, the core stops burning and contracts. This contraction ignites the remaining hydrogen surrounding the core, which causes the outer layers of gaseous material to expand outward. So a red giant is composed of a very massive core surrounded by a bunch of relatively thin gas. (When the Sun becomes a red giant, long from now, it will be larger than Mercury's orbit, but no more massive than it is now.) Because most of the mass is in the core, and the red giant's surface is so far away from the massive core, its surface gravity is less than the surface gravity of ''regular" stars. The gravitational influence of a red giant on object farther out (planets, other stars), however, depends on mass and distance, just like any other star.

Red giants that are not massive enough to become neutron stars or black holes go on to become white dwarfs. When all nuclear reactions in the star stop, it blows off all the material around the core in an expanding shell of gas that looks like a giant smoke ring. This giant smoke ring is called a "planetary nebula." What is left is the core of the star, glowing with leftover heat. This glowing core is a white dwarf. Eventually, the core cools to where it stops glowing, and it is called a black dwarf.

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Q Almeida asks: "I understand that a star collapses (due to gravity) until it becomes a Black Hole, a Neutron Star, or a white dwarf. Yet does it not undergo a supernovae and THEN become a black hole or neutron star?"

A You are right: black holes and neutron stars (including pulsars) undergo supernovae explosions as they form. Young stars are hydrogen, and the nuclear reaction converts hydrogen to helium with energy left over. The left over energy is the star's radiation--heat and light. When most of the hydrogen has been converted to helium, a new nuclear reaction begins that converts the helium to carbon, with the left over energy released as radiation. This process continues converting the carbon to oxygen to silicon to iron. Nuclear fusion stops at iron. If you could slice a very old star in half, it may look (sort of) like this:

cross-section of star core


The star now has layers of different elements. The outer layers of hydrogen, helium, carbon, and silicon are still burning around the iron core, building it up. Eventually, the massive iron core succumbs to gravity and it collapses to form a neutron core. The outer layers of the star fall in and bounce off the neutron core which creates a shock wave that blows the outer layer outward. This is the supernovae explosion.

When a white dwarf forms, however, a lot of gas is blown off (but not in a supernova explosion) to form a "planetary nebula," which looks like a giant smoke ring. The still-glowing core that is left over is a white dwarf. When it cools and no longer glows, it is a black dwarf.

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Q Almeida asks: "How are quasars formed and how are they detected?"

A Quasars (or quasi-stellar objects) are point-like sources of energy (light, x-rays) that are very red-shifted. The red-shifts suggest that quasars are moving away at speeds greater than 90% the speed of light. Astronomers now think that quasars are distant galaxies as they appeared 15.5 billion years ago, when the universe was very young.

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Q Bruno Navert asks: "I'd like to have more information on how to detect black holes. I know it's possible to detect them when they acquire matter (accretion disk), particulary in binary star systems. How exactly does this process work?"

A Astronomers suspect a black hole in a binary system when they observe a rapidly varying x-ray source that may stop and start back up periodically. A star circling a black hole will lose matter which forms a disk (accretion disk) around the black hole as it falls in. The matter heats up as it falls and emits x-rays. Since the amount of matter falling into the black hole varies, so do the x-rays.

infalling material causes x-rays

If the orbital plane of the binary system is in our line of sight, the x-ray source appears to shut off periodically when the star eclipses the black hole.

Since X-rays do not come through the Earth's atmosphere, x-ray astronomy must be done from space. The Uhuru satellite, launched in 1970, was designed to observe x-ray sources. Some binary x-ray sources, which are thought to be black holes, that were found by Uhuru include:

  • Cygnus X-1
  • Centaurus X-3
  • Small Magellanic Cloud X-1
  • Vela X-1
  • Hercules X-1
  • Scorpius X-1


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Q Greg Crivello asks: "What are some of the best books concerning just black holes (and perhaps similar celestial objects such as neutron stars)?"

A Here are some books about black holes:

  • Black Holes: A Traveler's Guide by Clifford A. Pickover
  • Black Holes: The Membrane Paradigm by Kip S. Thorne
  • Black Holes and Baby Universes and Other Essays by Stephen Hawking
  • Black Holes and Other Space Phenomena by Philip Steele
  • Black Holes and Time Warps: Einstein's Outrageous Legacy by Kip S. Thorne
  • Black Holes in Spacetime by Kitty Ferguson
  • A Brief History of Time: From the Big Bang to Black Holes by Stephen W. Hawking
  • From Black Clouds to Black Holes by Jayant V. Narlikar
  • Gravity's Fatal Attraction: Black Holes in the Universe by Mitchell Begelman
  • Mysteries of Deep Space: Black Holes, Pulsars, and Quasars by Isaac Asimov
  • Prisons of Light: Black Holes by Kitty Ferguson
  • Relatively Speaking: Relativity, Black Holes, and the Fate of the Universe by Eric Chaisson
  • Space, Time, and Gravity: The Theory of the Big Bang and Black Holes by Robert M. Wald
  • The Collapsing Universe: The Story of the Black Holes by Isaac Asimov
  • Gaps in Space: A Book About Black Holes by Jill C. Wheeler


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Q Greg Crivello asks: "Also, why is it a physical impossibility to travel faster than the speed of light, creating a situation in which nothing can escape a black hole?"

A Many science fiction stories have spaceships warping around faster than the speed of light. The speed of light seems to be a limit for how fast you can go in this universe. Electromagnetic radiation, such as light, goes that fast, but you would have a problem getting enough energy to accelerate yourself to even near-light speeds. That's because objects become more massive the faster they go. Particle physicists observe the masses of the particles in their accelerators increase thousands of times as the particles go faster and faster. Einstein's famous equation, E=mc2, tells us that energy is directly related to mass--the more energy there is, the more mass there is. As you approach near-light speeds, you would need increasingly larger amounts of energy to go faster. At light speed, your mass would be infinite, which would require an infinite amount of energy.

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Q Greg Crivello asks: "Lastly, I read in a book of a gigantic black hole (10 billion times more massive than our sun) sucking in the galaxy M87. Can you provide more information, or tell me where to find it?"

A M87 is an elliptical galaxy of about 5 trillion stars, which is about 50 million light years away. It is in the constellation Virgo.

Using NASA's Hubble Space Telescope, astronomers measured the velocity of hot gas as it orbits around an unseen object. They concluded that only a black hole can produce enough gravity to explain the tremendous speed of the gas.

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