Spring 2005 ASTR 2030 Homepage
Spring 2005 ASTR 2030 Black Holes: Clicker Questions
Wed 2005 Jan 12 (not graded):
A black hole is:
A. A pulsar.
B. A quasar.
C. A collapsed star.
D. An object whose gravity is so strong that not even light can escape.
E. Science fiction.
The event horizon of a black hole is:
The singularity of a black hole is:
If you fall into a black hole:
A. Time will appear to slow to a halt as you enter the event horizon.
B. You will die, torn apart by tidal forces.
C. You will die, incinerated by intensely hot radiation.
D. You will die, crushed at the central singularity.
E. You will pass through a wormhole into another part of the Universe.
Fri 2005 Jan 14 (not graded):
For what kinds of black holes do astronomers see evidence?
A. Exploding mini-black holes.
B. Stellar-sized black holes.
C. Supermassive black holes at the centers of galaxies.
D. The Universe as a whole is a black hole.
E. Astronomers do not observe black holes at all.
The radius of the event horizon of a black hole is called:
A. The Schwarzschild radius.
B. The Einstein ring.
C. The tidal radius, because it is the point where an infaller will be tidally torn apart.
D. The singularity.
E. It does not have a special name.
Thorne names the black hole at the center of our Galaxy:
E. He does not give it a name.
You press the hyperlight button. What happens?
Will the probe fall into the black hole?
When you fall through the horizon of a black hole:
A. The view of the outside world disappears, and you find yourself in darkness.
B. The view of the outside world changes only slightly, and there is no clue that you have actually fallen through the horizon.
Wed 2005 Jan 19:
What is the subject of Thorne's Prologue?
A. Describes the contents of the rest of the book.
B. A short history of Black Holes.
C. A brief introduction to Relativity for the layman.
D. Warns the reader of the dangers of Black Holes.
E. Takes the reader on an imaginary voyage to several Black Holes.
Which of these Black Holes is a real known Black Hole:
A. Hades (10 Msun, near Vega);
B. Sagittario (4 x 106 Msun, at the center of our Galaxy);
C. Gargantua (15 x 1012 Msun, far, far away in the vicinity of the quasar 3C273;
D. Orion binary (a pair of 24 Msun BHs, in the Orion nebula in our Galaxy);
E. An exploding mini-black hole.
The tidal force is the difference in the gravitational force between two parts of an object (e.g. between your head and your toes).
The tidal force of a black hole will tear you apart:
A. at the event horizon;
B. somewhere inside the event horizon;
C. at the singularity;
D. inside the event horizon if the black hole has a stellar-size mass, or outside the horizon if the black hole is supermassive;
E. outside the event horizon if the black hole has a stellar-size mass, or inside the horizon if the black hole is supermassive.
Fri 2005 Jan 21:
Which of the following is an inertial frame?
A. A frame with respect to which unaccelerated objects move in straight lines at constant velocity.
B. A rotating frame.
C. An accelerating frame.
D. A frame with some weird Dahlian labeling of coordinates of space and time.
E. The frame in which you are sitting right now.
Which of the following is not one of the 4 postulates of Special Relativity?
A. Spacetime forms a 4-dimensional continuum.
B. Globally inertial frames exist.
C. The speed of light is a constant, the same in any inertial frame.
D. The Principle of Relativity: spacetime has no absolute existence.
E. Spacetime can be curved.
What makes the Black Hole Flight Simulator science as opposed to art?
Why is the lightcone on a spacetime diagram unchanged by a Lorentz transformation?
Mon 2005 Jan 24:
Lorentz invented Lorentz transformations in 1904,
and Poincaré invented the Principle of Relativity in 1904/5,
both before Einstein 1905.
Why then does Einstein get the credit for Special Relativity?
A. Because history is unfair.
B. Because Lorentz's and Poincaré's work contained errors.
C. Because Einstein made the connection to the Michelson-Morley 1887 experiment.
D. Because it was Einstein who said that the speed of light is constant.
E. Because Einstein abolished the aether.
If light is a wave, what is waving?
D. Electromagnetic fields.
E. The speed of light.
Would the light have moved differently if Cerulean had emitted the light? Why?
If an object moves at LESS than the speed of light, then on a spacetime diagram its worldline must:
A. point vertically upward;
B. point in some direction < 45° from vertically up;
C. point at 45° from vertically up;
D. point in some direction > 45° from vertically up;
E. any of the above.
Wed 2005 Jan 26:
Lorentz, Poincaré, and Larmor
all independently discovered, before Einstein 1905,
that Maxwell's equations of electromagnetism were more beautiful
if they transformed according to Lorentz transformations
rather than classical Galilean transformations.
What example does Thorne give of Maxwell's equations being more beautiful?
A. There were fewer equations.
B. They predicted that electric fields are produced by static charges,
while magnetic fields are produced by moving charges.
C. They predicted that light is electromagnetic waves.
D. They predicted that the speed of light is constant.
E. Magnetic fields lines have no ends, under any circumstances.
If Earth time is 4 years, then the traveler's time is:
A. 4 years;
B. less than 4 years;
C. more than 4 years.
On your way out to Alpha Cen, you appear to me, watching you through a telescope on Earth, to move at what speed?
B. ½ c;
C. Near c;
D. 2 c;
E. Near infinite speed.
On your way back from Alpha Cen, you appear to me, watching you through a telescope on Earth, to move at what speed?
B. ½ c;
C. Near c;
D. 2 c;
E. Near infinite speed.
As you move through a scene at near the speed of light, the view is changed in 4 ways compared to the view at rest. Which one of the following is not one of those 4 effects:
A. Aberration: the view ahead appears fiasheyed, the view behind zoomed;
B. Color: blueshifted (higher energy photons) ahead, redshift (lower energy photons) behind;
C. Brightness: brighter ahead, dimmer behind;
D. Time: faster ahead, slower behind.
Fri 2005 Jan 28:
In the animation showing a Lorentz transformation between Vermilion's and Cerulean's frames,
what is changing as the animation proceeds?
A. Space and time are changing.
B. The coordinate system labeling space and time is changing.
C. Vermilion and/or Cerulean are moving.
D. We are moving.
Mon 2005 Jan 31:
What "breaks the symmetry" between you and your twin,
that allows the twin to be younger than you on return?
A. There is no difference, so in fact you must both age the same.
B. Twin moved through space, whereas you did not.
C. Twin accelerated (at Kant), whereas you did not.
D. Twin experienced a sudden loss of time at Kant.
Why is a second jet not observed in 3C273?
A. There is no second jet.
B. Aberration bends the second jet out of view.
C. The second jet is relativistically dimmed out of view.
D. The second jet is relativistically redshifted out of view.
E. The second jet is behind the quasar, which obscures it.
A magazine is doing an article on the appearance of a scene seen at near the speed of light.
The editor assigns you to take a photograph, which the magazine's special effects people will distort specially relativistically.
What would you photograph? What kind of lens would you use? How would you compose the scene in the viewfinder?
Mon 2005 Feb 7:
For this stellar-sized black hole (30 solar masses),
the gravitational tidal force will tear you apart:
A. Well outside the horizon of the black hole;
B. At the horizon of the black hole;
C. At the Schwarzschild radius of the black hole;
D. At a non-zero radius inside the horizon of the black hole;
E. At the central singularity.
If the probe falls through the horizon thinking that time goes by normally,
then you, watching from outside, see the probe clock:
A. tick at the same rate as your clock;
B. slow to a halt at the horizon;
C. speed up to an enormous rate at the horizon.
As the probe approaches the horizon, photons emitted by the probe appear to you:
A. slowed in frequency;
B. speeded up in frequency;
C. neither slowed nor speeded up in frequency.
Wed 2005 Feb 9:
After inventing SR, Einstein invented GR:
A. In 1905, only a few months later;
B. In 1907, after two years of concentrated labor;
C. In 1915, after ten years of intermittent work;
D. Over a period of several decades, up to nearly the end of his life in 1955;
E. Actually it was Hilbert who first invented GR.
The "No Hair" Theorem says that isolated Black Holes are characterized by:
A. Luminosity and Temperature;
B. Mass and Chemical Composition;
C. Mass, Electric Charge, and Spin;
D. Radius and Circumference;
E. The number and type of singularities.
If nothing can escape from a Black Hole, how can its gravity escape?
A. Gravity is a curvature of space, and does not need to escape.
B. A person outside the BH experiences the gravity of the matter
that long ago collapsed to, or fell into, the BH.
C. Gravity travels faster than light.
The tidal force from a spherical Black Hole:
A. Accelerates you towards the black hole;
B. Allows you to remain in a stable or unstable orbit around the black hole;
C. Crushes you in the radial direction, and stretches you in the transverse direction;
D. Stretches you in the radial direction, and crushes you in the transverse direction;
E. Pushes and pulls you in randomly varying directions.
Fri 2005 Feb 11:
According to Thorne Ch 2, an astronaut in outer space can detect that spacetime is curved from:
A. Measuring the speed of light (with ruler, mirror, and clock);
B. The presence of a gravitational force;
C. The presence of a tidal force;
D. Outer space is empty, so spacetime there must be flat, not curved;
E. The Principle of Equivalence implies that the astronaut cannot detect that spacetime is curved.
Mon 2005 Feb 14:
According to Thorne Ch 3, Einstein rejected black holes because:
A. Light could not escape from them;
B. They would be redshifted out of existence;
C. The curvature around them would be so strong that space would close up around them, and they would disappear;
D. The black hole would not be able to resist the force of gravity, and would collapse;
E. A black hole is a star somehow frozen at its Schwarzshild radius, which is impossible.
The worldline of an object that moves faster than light (a hypothetical "tachyon") would be:
D. Could be any of the above
If a star, when it collapses to a black hole,
appears to all outside observers to freeze forever at its horizon,
does it ever actually collapse?
Wed 2005 Feb 16:
In GR, an embedding diagram is:
A. A kind of spacetime diagram.
B. A diagram that illustrates the curvature of space by embedding the space inside a fictional higher-dimensional space.
C. A diagram that illustrates that curved space must exist physically inside a higher-dimensional space.
D. A diagram that illustrates how objects move in curved space.
E. A diagram that illustrates the depth of the gravitational potential.
Does the Schwarzschild geometry describe the geometry of empty space around the Sun?
A. Yes, to a good approximation.
B. No, not at all.
When the mass of a black hole increases, its horizon expands.
Does the horizon appear to engulf stuff that previously fell through the horizon?
A. Yes, stuff that previously fell into the black hole disappears.
B. No, stuff that previously fell into the black hole remains frozen at the horizon,
appearing to expand with the horizon.
Fri 2005 Feb 18:
You are in flat space.
You swing a clock on a rope in a circle around you,
so that the clock is moving at near the speed of light relative to you.
According to Special Relativity, the clock will appear to you to tick:
B. At the same rate as your own clock;
D. None of the above.
In the situation of the previous question,
the object therefore appears to you (according to Special Relativity):
B. Neither redshifted nor blueshifted;
Standing on the surface of the Earth,
you hold a negative mass object in your hand.
According to the Principle of Equivalence,
which way does the negative mass object fall when you drop it?
A. Down to the floor;
B. It justs hangs there in mid-air, falling neither down nor up;
C. Up to the ceiling;
D. None of the above.
An observer and an emitter are at at rest in a uniform gravitational field,
with the emitter lower in the field than the observer.
According to the Principle of Equivalence,
the situation is equivalent to one in which there is no gravitational field and the observer is:
A. Accelerating away from the emitter;
B. Not accelerating;
C. Accelerating towards the emitter.
In the situation of the previous question,
the emitter therefore appears to the observer (according to Special Relativity):
B. Neither redshifted nor blueshifted;
Mon 2005 Feb 21:
The Principle of Equivalence is:
A. E = m c2.
B. Gravitational mass = inertial mass.
C. Go to the free-fall frame; then the laws of physics are the same as for inertial frames.
D. Gravitating frame = accelerating frame.
E. Gravity = curvature of spacetime.
Mon 2005 Feb 28:
What lies the other side of the horizon of a
(as opposed to idealized mathematical) black hole?
A. A parallel universe.
B. A wormhole.
C. A white hole.
D. Empty space surrounding a singularity.
E. The remnant of the star that collapsed to the black hole.
What does the silence mean? (Referring to the first scene in "Contact".)
A. Sound waves cannot go through space.
B. Radio waves cannot go through space.
C. Radio waves from Earth have not yet reached this distance.
D. Radio waves from Earth are too faint to reach this distance.
E. Radio waves from Earth are too redshifted to reach this distance.
Wed 2005 Mar 2:
What is wrong scientifically with this scene? (Referring to the first scene from WD's "The Black Hole".)
A. The astronauts should be weightless.
B. The astronauts should not be able to talk to each other, because sound cannot travel in space.
C. The gravity should not shake the astronauts.
D. The descriptions of black holes by the science officer are wrong.
E. The dialog sucks.
What is the best way to stop being pulled into the black hole?
A. Fire rockets continuously.
B. Invent anti-gravity (the movie solution).
C. Go into orbit, rely on centrifugal force.
D. Reduce the mass of the ship by throwing almost everything overboard.
E. Fall into the black hole and get frozen at its horizon.
Why does the dialog suck?
Mon 2005 Mar 7:
What kind of pressure holds up a white dwarf star against its gravity?
A. Atomic pressrure.
B. Nuclear pressure.
C. Gas pressure of a hot plasma.
D. Electron degeneracy pressure.
E. Neutron degeneracy pressure.
Suppose you put an electron in a box and cooled the box to absolute zero temperature. Would the electron:
A. Stop moving?
B. Have a small but finite velocity?
Suppose you made the box smaller (and kept it at absolute zero temperature). Would the electron:
A. Remain at rest?
B. Go slower?
C. Go faster?
D. Stay the same velocity?
E. You could not make the box smaller.
Wed 2005 Mar 9:
Fri 2005 Mar 11:
Heisenberg's Uncertainty Principle of quantum mechanics relates:
A. The momentum (mass times velocity) and wavelength of an electron (or other particle);
B. The energy and frequency of an electron (or other particle);
C. The velocity and energy of an electron (or other particle);
D. The wavelength and frequency of a photon (or other particle);
E. The mass and radius of a black hole.
The transition between atomic pressure and electron degeneracy pressure in cold matter at very high density occurs when:
A. The matter is cooled to absolute zero temperature;
B. The matter is heated to the point where the electrons are ionized;
C. The electrons are compressed to the point where their velocities exceed the atomic velocity c/137;
D. The electrons are compressed to the point where their velocities approach the speed of light c;
E. The electrons and protons fuse into neutrons.
If you add 0.02 solar masses to a 1.39 solar mass white dwarf, then its radius will:
A. decrease slightly;
B. increase slightly;
C. collapse to a neutron star;
D. collapse to a black hole;
Mon 2005 Mar 14:
The fact that pulsars were observed to pulse at several different radio frequencies ruled out the possibility that pulsars were LGM
(Little Green Men). Why?
A. Because any signal from LGM would be too faint to be detectable.
B. Because LGM would probably not use radio waves.
C. Because LGM would emit only at a single narrow frequency.
D. Because LGM would emit a complicated signal, not just pulses.
E. Because the probability of LGM existing is tiny.
A core collapse supernova is powered by:
C. Nuclear fusion.
D. Nuclear fission.
You and a friend are in a race to orbit planet Sram.
You start together in circular orbit.
The rules of the race allow you to fire your rockets just once, in a burst at the beginning of the race.
The race ends after one orbit.
The winning strategy is:
A. Accelerate forwards to increase your orbital velocity as much as possible;
B. Accelerate forwards just enough to reach escape velocity;
C. Do nothing: just remain in circular orbit;
D. Decelerate, reducing your orbital velocity as much as possible, though not so much as to crash into Sram;
E. Decelerate hard, so going into a retrograde orbit at maximum velocity.
Wed 2005 Mar 16:
How were astronomers in 1971 able to identify unequivocably that the blue main sequence star HDE 226868 was the optical counterpart to Cygnus X1?
A. Both were in exactly the same spot on the sky.
B. The blue star had the same spectrum as the x-ray object.
C. Both varied with precisely the same 5.6 day period.
D. The orbital dynamics indicated that both star and x-ray object had the same mass.
E. The blue star was seen to lose mass on to the x-ray object.
The x-rays emitted from Cygnus X-1 come from:
A. The main sequence star.
B. A neutron star.
C. A black hole.
D. An accretion disk around the compact object.
E. A jet.
How do astronomers conclude that a compact object in an x-ray binary is a black hole, as opposed to a neutron star?
A. More than 3 solar masses in a space smaller than a white dwarf.
B. The compact object is black.
C. The compact object shows gravitational lensing.
D. There is no sign of a pulsar or of pulsations.
E. The x-ray brightness varies chaotically.
What is the source of energy of an Active Galactic Nucleus?
A. Chemical energy.
B. Nuclear fission energy.
C. Nuclear fusion energy.
D. Electromagnetic energy.
E. Gravitational energy.
Wed 2005 Mar 30:
What is the best evidence that an Active Galactic Nucleus contains a black hole?
A. A lot of mass in a small space.
B. Prodigious luminosity.
C. Non-stellar spectrum.
D. Relativistic jets.
E. Gravitational lensing.
For which of the following is gravity not the source of energy?
A. A main sequence star.
B. A core-collapse supernova.
C. X-ray emission from an accretion disk around a black hole.
D. A jet from an Active Galactic Nucleus.
Fri 2005 Apr 1:
Gravity is the "perpetual motion machine" that drives the Universe. How so?
A. Because gravity binds together the Earth, the solar system, the Milky Way, and other such systems.
B. Because gravity is the source of energy for the Sun and other stars.
C. Because the gravitational force is attractive, whereas the electromagnetic force is repulsive.
D. Because gravity is the most powerful of the forces.
E. If you remove energy from a gravitating system, instead of cooling down, it (contracts and) heats up.
If the enclosed mass at radius r (the total mass inside a sphere of radius r) is constant, it probably means that:
A. The stars within radius r are all tidally torn apart.
B. There are stars uniformly distributed out to radius r.
C. Some of the mass is at the center, and some is distrbuted out to radius r.
D. Almost all the mass is concentrated at the center.
E. Most of the mass is at radii much larger than r.
The increase in enclosed mass at larger radii is probably caused by:
A. The change in velocity of stars further from Sagittario.
B. Tidal forces.
D. The mass contributed by many stars.
E. The increase in radius of stars further from Sagittario.
Fri 2005 Apr 8:
At the Planetarium last week, you saw that the (brighter) x-ray sources on they sky consist mainly of:
A. Stars like those we see with our eyes.
B. Short-lived objects, such as supernovae in the process of exploding, or neutron stars and/or black holes merging.
C. X-ray binary systems concentrated in the plane of, and especially towards the center of, the Milky Way.
D. Active Galactic Nuclei away from the plane of the Milky Way.
E. Interstellar gas.
The theme of Webster Cash's lecture on MAXIM (Micro-Arcsecond X-ray Imaging Mission)
and the NWI (New Worlds Imager) was:
A. NASA should devote more of its resources to astronomy, less on the space station.
B. Space telescopes having mirrors covering many square kilometers of area.
C. Detecting gravitational waves from colliding black holes.
D. We will see new worlds (through telescopes) before we visit them (with spacecraft).
E. We better start preparing for the killer asteroid that will hit the Earth some day.
Why is the Reissner-Nordström solution for the interior of an electrically charged black hole,
which assumes that the black hole is empty of matter except at the singularity, inconsistent?
A. Because a charged black hole would quickly neutralize itself.
B. Because it contains a wormhole and white hole to a new universe.
C. Because it predicts that a huge density of mass-energy collects at the inner horizon.
D. Because it contains a naked singularity.
E. Because it is gravitationally repulsive in its core, so that infalling matter and charge would not fall to the singularity, and the black hole would not be empty.
Mon 2005 Apr 11:
What black hole has the higher Hawking temperature?
A. The more massive black hole.
B. The less massive black hole.
C. All black holes have the same Hawking temperature.
What is the relation between the Hawking radiation and the observed radiation from x-ray binaries?
A. The observed radiation (e.g. in x-rays) is Hawking radiation.
B. Hawking radiation probably dominates the observed radio emission.
C. Hawking radiation probably dominates the observed gamma ray emission.
D. Hawking radiation makes a negligible contribution at all wavelengths.
Wed 2005 Apr 13:
Which of these gravity assists past the moon will slingshot the spacecraft furthest out in the solar system?
Mon 2005 Apr 18:
As regards their Hawking radiation, more massive (as opposed to less massive) black holes are:
A. Hotter and brighter.
B. Hotter and fainter.
C. Colder and brighter.
D. Colder and fainter.
E. Black holes are black, and they do not emit any kind of radiation.
A person on Earth watching you slingshot sees you apparently accelerate by 1000 km/s in 1 second,
equivalent to about 105 gees. This enormous acceleration kills you.
Wed 2005 Apr 20:
Hubble's law, v = H0 d, states that the recession velocity v of a galaxy equals Hubble's constant H0 times the distance d to the galaxy. This observation is evidence that:
A. Our Milky Way galaxy is at the spatial center of the Universe.
B. The Universe is expanding.
C. There was a Big Bang.
D. It takes light time to travel from distant galaxies, so we are seeing galaxies as they used to be.
E. Gravity is the perpetual motion machine that drives the Universe.
Hubble's constant H0 is an important parameter of cosmology because:
A. It determines the age t of the Universe, through t = 1/H0.
B. It determines size of the observable Universe.
C. It determines the curvature of the Universe.
D. It determines the fate of the Universe.
E. It determines what kind of mass-energy the Universe is made of.
Einstein's equations of general relativity relate the curvature of the Universe to its mass-energy density. Astronomers quantify the mass-energy density with the parameter
W = (actual density of the Universe) / (critical density of Universe).
A Universe with W = 1 is:
E. Finite in size.
Fri 2005 Apr 22:
Thorne shows that generically, wormholes permit travel forwards and backwards in time. He concludes that;
A. Wormholes cannot exist.
B. Maybe wormholes exist.
C. Wormholes probably do exist.
If the Universe is expanding, what is it expanding into?
A. A higher dimensional space.
B. The future.
C. It is not expanding into anything: expanding means that the distances between objects (galaxies) in the Universe are increasing with time.
D. The Universe is not expanding: it's just that our rulers (distance measuring devices) are contracting.
If the observable Universe is almost flat, W = 1 ± 0.01, then it probably means that the Universe as a whole
(including the parts beyond our horizon) is:
D. Could be any of the above.
Mon 2005 Apr 25:
Which of the following statements about the Cosmic Microwave Background (CMB) is false?
A. It has an exquisite black body spectrum with a temperature of 2.726 Kelvin.
B. It shows a dipole distortion on the sky that indicates that the solar system is moving through the CMB at about 365 kilometers per second.
C. After subtraction of the dipole distortion, it is amazingly uniform, to a few parts in 105.
D. The CMB that we see today comes to us from the time of Inflation, when the Universe was about 10-33 seconds old.
E. The small temperature fluctuations in the CMB show a characteristic size of order 1°.
The Theory of Inflation postulates that:
A. The Universe began with a Big Bang.
C. The Universe is powered by gravity.
C. The Universe lives in 10 or 11 dimensions.
D. The very early Universe (age less than 10-33 seconds or so) was dominated by vacuum energy.
E. The Universe began at the singularity of a black hole.
Which of the following problems does the Theory of Inflation not solve?
A. The Horizon Problem: How come regions of the CMB more than 2° apart have the same temperature, even though they were causally disconnected at the time of Recombination?
B. Why the Universe is expanding.
C. Why the Universe appears to be so flat (W = 1 ± 0.01).
D. Where matter and radiation came from.
E. Why the Universe began.
Spring 2005 ASTR 2030 Homepage
Updated 2005 Apr 25