Spring 2012 ASTR 2030 Clicker Questions

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Spring 2012 ASTR 2030 Black Holes: Clicker Questions

Tue 2012 Jan 17 (not graded):

Will you get an A in this class?
A. Yes, I am at least 90% sure of it.
B. I think I have a better than 50% chance.
C. Probably not.
A black hole is:
A. A collapsed star.
B. A quasar.
C. An object whose gravity is so strong that not even light can escape.
D. A region of spacetime where space is falling faster than light.
E. A mathematical object predicted by general relativity, which does not exist in reality.
The event horizon of a black hole is:
A. The physical (gaseous/liquid/solid) surface of a black hole.
B. The surface from within which light cannot escape.
C. A place where you will be torn apart by tidal forces.
D. A place where space and time as we know them come to an end.
E. The entrance to a wormhole, which will transport you to another place and time.
The singularity of a black hole is:
A. The surface from within which light cannot escape.
B. A place where you will be torn apart by tidal forces.
C. A place where the spacetime curvature becomes infinite.
D. A place where space and time as we know them come to an end.
E. The entrance to a wormhole, which will transport you to another place and time.
Th 2012 Jan 19 (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.
Why does the spaceship not go any closer to the black hole?
A. Because the black hole would suck in the spaceship regardless of how hard the spaceship fired its rockets.
B. Because it would take too much rocket fuel to go closer.
C. Because the acceleration needed to stay outside the event horizon would squash a human.
D. Because the x-rays would kill a human.
E. Because the tidal forces would tear a human apart.
Will the probe fall into the black hole?
A. Yes.
B. No.
How long does Thorne say that our journey to the center of the Milky Way, 30,000 lightyears distant, will take, from our point of view? We accelerate at one gee (one Earth gravity) for the first half of the journey, then decelerate at one gee for the second half.
A. Almost no time.
B. 20 years.
C. 30,000 years.
D. 60,000 years.
E. Much longer than any of the above.
Tue 2012 Jan 24:
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.
If the postulates of special relativity are correct, would the light have moved differently if Cerulean, not Vermilion, had emitted the light?
A. Yes.
B. No.
On this spacetime diagram, how fast is Cerulean moving relative to Vermilion, in units of the speed of light?
A. 0 .
B. 1/2 .
C. 1 .
D. 2 .
E. The information on the diagram is insufficient to answer the question.
Th 2012 Jan 26:
According to Thorne, what influence did the Michelson-Morley (1887) experiment have on Einstein's thinking?
A. Michelson and Morley's failure to detect any difference in the speed of light in different directions suggested to Einstein that the speed of light is a universal constant;
B. Michelson and Morley's failure to detect the motion of the Earth through the aether suggested to Einstein that there is no aether, no absolute space or time;
C. It caused Einstein to think that motion through the aether would cause clocks to run slow and rulers to contract;
D. It provoked Einstein to think about Maxwell's equations of electromagnetism, which showed that light is electromagnetic waves;
E. Very little.
Which of the following is an inertial frame?
A. A frame that carries inertia.
B. A frame with respect to which unaccelerated objects move in straight lines at constant velocity.
C. A rotating frame.
D. An accelerating frame.
E. The frame in which you are sitting right now.
If Cerulean were moving as shown relative to Vermilion, which line would be a “now line” for Cerulean?
A.
B.
C.
D.
E.

Tue 2012 Jan 31:
On a spacetime diagram, the worldline of a person must point:
A. vertically upward;
B. at less than 45° from vertical;
C. at 45° from vertical;
D. horizontally;
E. in any direction.
On your way out to Alpha Cen, you appear to me, watching you through a telescope on Earth, to move at what speed?
A. 0;
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?
A. 0;
B. ½ c;
C. Near c;
D. 2 c;
E. Near infinite speed.
If round-trip time is 8 years from my (Earth) perspective, then the round-trip time from your (traveler's) perspective is:
A. less than 8 years;
B. 8 years;
C. more than 8 years.
Th 2012 Feb 2:
What “breaks the symmetry” between you and your twin, that allows the twin to be younger than you on return?
A. On the outward journey, the velocity at which the twin sees you recede is different from the velocity at which you see the twin recede; and then on the return journey, the velocity at which the twin sees you return is different from the velocity at which you see the twin return.
B. There is no difference, so in fact you must both age the same.
C. Twin moved through space, whereas you did not.
D. Twin accelerated (at Alpha), whereas you did not.
E. Twin experienced a sudden loss of time at Alpha.
You on Earth watch your twin through a telescope, and the twin watches you through a telescope. Which of the following is true?
A. You both see each each other's clocks slowed down, on both the outward and return journeys.
B. You both see each other's clocks slowed down on the outward journey, speeded up on the return journey.
C. You both see each other's clocks speeded up on the outward journey, slowed down on the return journey.
D. You see the twin's clock slowed down on both the outward and return journeys, whereas the twin sees your clock speeded up on both the outward and return journeys.
E. You see the twin's clock slowed down on the outward journey and speeded up on the return journey, whereas the twin sees your clock speeded up on the outward journey and slowed down on the return journey.
Your twin watches you on Earth through a telescope. At the moment that the twin accelerates at Alpha, the twin sees your clock:
A. Run at the same slowed-down rate both before and after accelerating.
B. Suddenly change from 2 years to 8 years.
C. Suddenly speed up.
D. Suddenly slow down.
E. Change from running forward to runing backward.
How fast does your twin appear, as seen through a telescope, to recede from you?
A. 3/8 c;
B. 3/5 c;
C. c;
D. 3/2 c;
E. Near infinite speed.
Tue 2012 Feb 7:
Not graded: Do you understand time dilation?
A. I think so!
B. I understand the diagram we filled in two weeks ago, but not much more.
C. I think I could maybe understand if I thought more about it.
D. I have no clue.
Not graded: Do you understand Lorentz contraction?
A. I think so!
B. I understand the diagram we filled in two weeks ago, but not much more.
C. I think I could maybe understand if I thought more about it.
D. I have no clue.
When you accelerate to relativistic velocities, the view ahead appears to shrink. Why?
A. Because you are moving away from the scene ahead.
B. Because of Lorentz contraction.
C. Because of special relativistic aberration.
D. Because acceleration curves space.
Very Long Baseline Interferometry (VLBI) observations of the quasar 3C273 show blobs emerging at 8c. This indicates that:
A. The jet is moving faster than c.
B. Special Relativity is wrong.
C. The blob is not a parcel of matter, but rather a wave of brightness passing along the jet.
D. The jet is pointed almost towards us, and is moving at close to c.
E. The jet is pointed away from us, and is moving at close to c.
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.
Th 2012 Feb 9:
In George Gamow's classic book “Mr. Tomkins in Wonderland,” Mr. Tomkins has a dream that the speed of light is 30mph. In the dream, passing bicyclists appear Lorentz contracted along the direction of their motion. Does Mr. Tomkins' dream portray correctly the appearance of objects moving relativistically?
A. Yes.
B. No.
You pass through a scene at near the speed of light. Clocks on objects ahead of you appear speeded by exactly the same amount as the frequencies of photons of light from those objects appear increased. True or false?
A. True.
B. False.
Tue 2012 Feb 14:
Suppose you watch this scene while falling freely. According to the Principle of Equivalence, what kind of trajectory will the cannonball follow from your perspective (relative to you)?
A. A vertical line.
B. A straight line.
C. A curved (parabolic) line.
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.
According to Special Relativity, does B see the light from A to be redshifted or blueshifted?
A. B sees the light to be redshifted (lower energy) compared to A.
B. B sees the light to have the same energy as A.
C. B sees the light to be blueshifted (higher energy) compared to A.
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:
A. Slow;
B. At the same rate as your own clock;
C. Fast;
D. None of the above.
In the situation of the previous question, the object therefore appears to you (according to Special Relativity):
A. Redshifted;
B. Neither redshifted nor blueshifted;
C. Blueshifted.
Th 2012 Feb 16:
According to Thorne Ch 2, Einstein came up with the Principle of Equivalence:
A. In 1905, the same year he published his theory of Special Relativity;
B. In 1907, while preparing an invited review on Special Relativity;
C. In 1915, after 10 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 Poincaré, not Einstein, who first proposed the Principle of Equivalence.
According to the river model of black holes, a person who free-falls through the horizon of a black hole appears to an outside observer to freeze and become highly redshifted at the horizon because:
A. the black hole freezes at its horizon, and never actually collapses;
B. the person freezes at the horizon, and never actually falls through;
C. the curvature of space becomes infinite at the horizon;
D. the closer to the horizon a photon is emitted, the longer the photon takes to forge against the inrushing torrent of space to reach an outside observer;
E. it's not true: an outside observer sees the infaller become highly speeded up and blueshifted.
Tue 2012 Feb 21:
Vote for the best scene.
A.
B.
C.
D.
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 (difference in gravitational force between two points a small distance apart);
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.
Why is the x-ray sky dominated by objects containing black holes or neutron stars?
A. Because x-rays are a particularly energetic form of electromagnetic radiation.
B. Because x-rays are absorbed by the Earth's atmosphere.
C. Because x-rays can penetrate through a large amount of matter.
D. Because the radiation from such objects is gravitationally redshifted.
E. Because black holes and neutron stars create a lot of radioactive elements in their vicinity.
Th 2012 Feb 23:
According to the river model of black holes, a person who free-falls through the horizon of a black hole:
A. experiences time to come to a halt at the horizon;
B. experiences an infinite curvature, a singularity, at the horizon;
C. is tidally torn apart at the horizon;
D. sees an infinitely bright shaft of light directly above them at the horizon;
E. experiences nothing special at the horizon.
The curve of enclosed mass versus radius has a flat part, then an upturn at larger radius? What mass causes these?
A. All the mass is produced by stars.
B. All the mass is produced by a black hole.
C. A point mass causes the flat part; stars cause the upturn.
D. A point mass causes the flat part; an accretion disk causes the upturn.
E. A star causes the flat part; a black hole cause the upturn.
Tue 2012 Feb 28:
What evidence most convincingly suggests that there is a supermassive black hole at the center of our own Milky Way?
A. The black hole causes gravitational lensing.
B. The supermassive black hole is in an x-ray binary star system.
C. Hubble Space Telescope observations show gas swirling down a vortex.
D. A relativistic jet is seen coming out of the core of the Milky Way.
E. Measurements of the velocities of stars at the center of the Milky Way indicate a large mass in a small space.
The relation v » ( G M / R )^1/2 between velocity v, mass M, radius R, and Newton's gravitational constant G is not central to:
A. Setting the radius of the horizon of a black hole;
B. Determining the escape velocity from the surface of a planet or star;
C. Measuring the mass of the neutron star or black hole in an x-ray binary;
D. Measuring the mass of a supermassive black hole;
E. Measuring the redshift of light.
Does the Schwarzschild geometry describe the geometry of empty space around the Sun?
A. Yes, to a good approximation.
B. No, not at all.
Not graded: How to get to the Parallel Universe?
A. Accelerate towards the Parallel Universe.
B. Accelerate away from the Parallel Universe.
C. A, then go backwards in time.
D. B, then go backwards in time.
E. It's impossible.
From a safe distance, we watch a spherical, pressureless star collapse to a black hole. What will we see?
A. The star will appear to freeze when at the horizon radius, and never collapse.
B. The star will appear to branch into a wormhole and white hole, before becoming a black hole.
C. The star will appear to collapse to a singularity.
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.
Th 2012 Mar 1:
Oppenheimer and Snyder carried out the first calculation of the collapse of an idealized, spherical, uniform, pressureless star in 1939. They found that:
A. All observers would see the star collapse to a singularity;
B. All observers would see the star freeze at its horizon;
C. An observer collapsing with the star would find that it collapsed, whereas a distant observer would see the star freeze at its horizon.
What does the arrowed blue line in the Penrose diagram of the Schwarzschild geometry represent?
A. A possible worldline of a person (timelike line);
B. A possible worldline of light (lightlike line);
C. A possible “now” line, a line of simultaneity (spacelike line).
The horizon lines in the Penrose diagram are:
A. timelike;
B. lightlike;
C. spacelike.
From the point of view of an observer outside the outer horizon of a white hole, is the white hole gravitationally attractive or gravitationally repulsive?
A. Attractive.
B. Repulsive.
C. Neither.
Tue 2012 Mar 6:
According to Thorne Ch 3, Einstein rejected black holes because:
A. There was no observational evidence for black holes;
B. It was absurd to think that nature could actually make such an extreme object;
C. A black hole has a singularity at its horizon, and singularities should not exist in nature;
D. A black hole has a central singularity, where spacetime comes to an end, which is impossible;
E. A black hole would curve space so much that it would swallow itself and disappear.
According to Thorne Ch 6, what breakthrough persuaded physicists that the Schwarzschild solution might represent an object that could actually exist in physical reality (in Thorne's words, “What converted Wheeler from a skeptic of black holes to a believer and advocate?”)?
A. Gullstrand and Painlevé's discovery in 1921 of coordinates that showed space flowing like a river into the black hole.
B. Zwicky's discovery in the 1930s of supernovae, which might represent the creation of a black hole.
C. Oppenheimer and Snyder's 1939 calculation that a collapsing star would appear to an outside observer to freeze at its horizon, but would appear to an observer on the star's surface to collapse through the horizon to zero radius.
D. Finkelstein's 1958 discovery of a coordinate system that showed that light could fall through the horizon.
E. Observations around 1970 that seemed to establish that Cygnus X-1 contains a black hole.
A Penrose diagram is a kind of spacetime diagram, in which time proceeds upwards, space fills out horizontal dimensions, and light goes at 45°. What is a Penrose diagram useful for?
A. Illustrating and resolving paradoxes in special relativity;
B. Depicting the curvature of spacetime around a gravitating object;
C. Depicting the causal structure of spacetimes, especially complicated spacetimes like black holes;
D. Depicting interactions between particles in high energy physics;
E. Showing off your IQ.
In the BHFS visualization of a black hole, which object is the antihorizon?
A. The gravitationally lensed image of the outside Universe.
B. The Einstein ring.
C. The dark red grid.
D. The blue-white grid.
E. It's not visible in the visualization.
Th 2012 Mar 8:
Which of the following did the Andromeda movie get right?
A. The Andromeda had normal brightness.
B. The spectrum of light from Andromeda was normal.
C. The Andromeda was not torn apart by the tidal force from the stellar-mass black hole;
D. When the Andromeda came back up off the horizon, it had lost 300 years;
E. The Andromeda was pulled by the salvage ship off its frozen position on the horizon.
Tue 2012 Mar 13:
The inflationary instability at the inner horizon of a rotating black hole causes an exponentially huge growth in the density and interior mass of the black hole. According to the river model, should the inflationary instability have any effect observable to an outside observer?
A. Yes, the mass and gravity of the black hole should appear to increase.
B. Yes, the inflationary instability would create gravitational waves which the black hole would radiate away.
C. Yes, the inflationary instability would be able to drive jets emerging from the black hole.
D. Only if the instability creates a baby universe, in which case the baby universe would engulf our Universe.
E. No.
Where do the x-rays in an x-ray binary like Cyg X-1 come from?
A. Light from the companion star, gravitationally lensed and blueshifted by the black hole.
B. The black hole.
C. Hot gas in an accretion disk spiralling on to the black hole.
D. A jet emerging from near the black hole.
E. Decay of radioactive elements synthesized near the black hole.
If x-rays from an AGN vary on a timescale of one day, then the AGN is probably:
A. Smaller than one lightday across.
B. Larger than one lightday across.
C. Variability places no constraint on the size of the AGN.
D. A black hole.
E. In a binary system.
What is the source of energy for the x-ray emission from an x-ray binary or AGN?
A. Chemical energy.
B. Nuclear fission.
C. Nuclear fusion.
D. Gravity.
E. Alternative energy.
You are in orbit in a spacecraft around the Sun. To go into a faster orbit, with a shorter orbital period, you should:
A. Accelerate. in the direction of your orbit;
B. Decelerate, in the direction opposite to your orbit.
Th 2012 Mar 15:
From outside the horizon, you watch your friend free-fall into a black hole. You talk to each other constantly via a direct radio link.
A. You both hear each other's voice continue almost normally.
B. You both hear each other's voice become much deeper.
C. You both hear each other's voice become much higher.
D. You hear your friend's voice become much deeper, while she hears your voice continue almost normally.
E. You hear your friend's voice become much deeper, while she hears your voice become much higher.
You free-fall into the rotating, supermassive black hole at the center of the Galaxy. What kills you?
A. The gravitational acceleration crushes you.
B. The tidal force tears you apart near the horizon.
C. High energy radiation from gas in an accretion disk kills you.
D. You are vaporized at the inflationary instability at the inner horizon.
E. You die at the central singularity.
According to Thorne Ch 8, how were astronomers, in 1971, able to identify Cyg X-1 definitely with the star HDE 226868?
A. Because HDE 226868 had a peculiar spectrum.
B. Because both objects varied with same 5.6 day binary period.
C. Because a radio flare from the position of HDE 226868 coincided with an x-ray flare from Cyg X-1.
D. Because an occultation of Cyg X-1 by the moon pinpointed its location at that of HDE 226868.
E. With the advent of high resolution x-ray telescopes, the positions of Cyg X-1 and HDE 226868 were found to coincide precisely within one arcsecond.
According to Thorne Ch 9, what is the brightest radio source in the sky?
A. The Sun.
B. Jupiter.
C. The x-ray binary Sco X-1.
D. The central regions of the Milky Way.
E. The quasar 3C 273.
Tue 2012 Mar 20:
Besides visible light, what form of electromagnetic radiation from the cosmos can be seen from the ground on Earth?
A. Radio.
B. Microwaves.
C. Ultraviolet.
D. X-rays.
E. Gamma-rays.
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. The gravity does not escape, but the tidal force does escape.
C. Gravity does not escape from a black hole: a person outside the BH experiences the gravity of the matter that long ago collapsed to, or fell into, the BH.
D. Gravity travels faster than light.
Th 2012 Mar 22:
Andrea Ghez was one of the astronomers featured on the Nova program. What has Andrea Ghez and her group been doing?
A. Observing large numbers of stellar-sized black holes in the Milky Way.
B. Discovering that almost every large galaxy has a supermassive black hole at its center.
C. Following the orbits of stars around the black hole at the center of the Milky Way.
D. Observing gigantic jets from supermassive black holes, and their effect on the surrounding galaxy.
E. Observing gamma-ray bursts and afterglows.
What observational evidence did the Nova program show that suggests that supermassive black holes affect the galaxies they live in?
A. Infrared observations show flares from supermassive black holes.
B. Hubble Space Telescope observations show that almost every large galaxy harbors a supermassive black hole.
C. X-ray observations reveal that jets from supermassive black holes blow huge bubbles in the hot gas enveloping a galaxy.
D. Gamma-ray observations detect gigantic bursts from supermassive black holes.
E. Gravitational wave observations reveal merging supermassive black holes.
Tue 2012 Apr 3:
What does the silence (in the first scene of “Contact”) mean?
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. The Universe is large.
What will Ellie Arroway's boyfriend Palmer Joss turn out to be, professionally? (What would generate most conflict?)
A. A scientist.
B. An artist.
C. A journalist/writer.
D. A religious person.
E. A multi-billionaire.
Who is “the leader of the scientific team that made this remarkable discovery” who will take the podium?
A. Ellie Arroway.
B. David Drumlin.
Who does Ellie meet on the beach? (What would be the best climax?)
A. Herself as a young girl.
B. Drumlin.
C. Her father.
D. An alien.
E. God.
Th 2012 Apr 5:
Who is clearly the hero of this movie?
A. Captain Dan Holland (Robert Forster).
B. Dr. Kate McCrae (Yvette Mimieux).
C. The robot, Vincent.
D. The black hole.
E. There isn't a clear hero.
Why does the dialog suck?
A. Because the script writer cannot write.
B. Because the actors cannot act.
C. Because the story is boring.
D. Because it's a Walt Disney movie aimed at kids.
E. Because there is no emotional connection or conflict between the characters.
What is the best way to avoid 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.
Who will the villain turn out to be?
A. Journalist Harry Booth (Ernest Borgnine).
B. Dr. Alex Durant (Anthony Perkins, the Psycho guy).
C. Maximillian, an evil robot.
D. A mad German scientist who wears a white coat and who has turned all his crew into mindless robots.
E. There will not be a discernible villain.
Not graded: The climax of the movie is: The orange jello knocks the ship into the black hole, and:
A. The villains fall into the black hole, killing them, but the heros manage to escape to safety just in time.
B. The heros and villains have an all-out battle as they fall into the black hole (with an intense one-on-one between Vince and Maximilian), all the villains are killed, and all the heros escape through a wormhole.
C. The crew who had been turned into mindless robots are magically restored to life inside the black hole, and all the good guys escape through a wormhole.
D. Reinhardt is crushed by a beam, and goes to “hell” inside the black hole, while our heros fly through a “heavenly” wormhole to safety.
E. All die inside the black hole, excepting that Vince rescues Bob the Texan robot.
Tue 2012 Apr 10:
When you, a human, try to compress ordinary solid or liquid matter, what happens to the electrons orbiting nuclei in atoms?
A. They orbit much slower.
B. They hardly change their orbits.
C. They orbit much faster.
D. They move so fast that they escape the nuclei.
E. They jiggle around randomly.
Planets are made of solid/liquid matter, which is almost incompressible. Their density (mass/volume) equals atomic density, which is constant. What does the imply about the relation between mass M and radius R of planets?
A. The radius R is constant, independent of mass M.
B. M µ R.
C. M µ R².
D. M µ R³.
E. M µ R^1/3.
Eddington's paradox: If the charactistic velocity v of atoms inside a star obeys v » (GM/R)^1/2, and if the temperature T of atoms increases with their velocity as T µ v², then as a star decreases in temperature T (at fixed mass M), then its radius R should:
A. Decrease.
B. Remain the same.
C. Increase.
Suppose you put an electron in a box and removed as much energy as possible from the electron. 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.
Th 2012 Apr 12:
What kind of pressure holds up a white dwarf star against its own gravity?
A. The pressure of a liquid or solid of atoms.
B. The pressure of a gas of atoms.
C. The pressure of a hot gas of nuclei and electrons (a plasma).
D. Electron degeneracy pressure.
E. Neutron degeneracy pressure.
According to Thorne Ch 4, a white dwarf becomes highly compressible when it reaches the Chandrasekhar limit of 1.4 solar masses, at which point the white dwarf collapses. Why does the gas become so compressible?
A. The matter ceases to be solid or liquid.
B. Electron velocities exceed the atomic velocity, and are ionized out of atoms.
C. Electrons become relativistic (move at almost the speed of light), and can move no faster.
D. The temperature increases enormously.
E. A supernova explosion occurs.
Tue 2012 Apr 17:
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.
When a star (the Sun, say, to be specific) runs out of a nuclear fuel of some kind (Hydrogen, say, to be specific), what will happen then?
A. The star will cool down.
B. The star's core will contract, and heat up to the point that it can ignite some other fuel.
C. The star will contract until it becomes supported by electron degeneracy pressure.
D. The star will collapse to a black hole.
E. The star will explode as a supernova.
What will happen when the electron-degenerate iron core of a massive star builds up to 1.4 solar masses?
A. The iron will undergo fission into lighter elements.
B. The iron will fuse to heavier elements.
C. The iron core will collapse to a neutron star.
D. The iron core will collapse to a black hole.
E. The iron core will explode as a supernova.
A core collapse supernova is powered by:
A. Gravity.
B. Electromagnetism.
C. Nuclear fusion.
D. Nuclear fission.
E. Radioactivity.
Th 2012 Apr 19:
The masses of neutron stars observed in pulsar binary systems are all consistent with 1.4 solar masses. What is the significance of this mass?
A. It is the maximum mass of a white dwarf.
B. It is the maximum mass of a neutron star.
C. It is the minimum mass of a black hole.
D. It is the maximum mass of a black hole.
E. It is probably just a coincidence.
For discussion (not graded): If a gravitational wave is a wave of spacetime curvature, then doesn't the wave also affect the lengths of rulers (and rates of clocks), so how can LIGO detect gravitational waves?
Hint: what is spacetime curvature?
The energy that produces a thermonuclear supernova explosion is:
A. Gravity.
B. Electromagnetism.
C. Nuclear fusion.
D. Nuclear fission.
E. Radioactivity.
Of the two kinds of supernova observed by astronomers, which could possibly produce a black hole?
A. Thermonuclear supernova (Type I), from the explosion of a white dwarf.
B. Core-collapse supernova (Type II), from the explosion of a massive star.
C. Either of the above.
D. Neither of the above.
Tue 2012 Apr 24:
In the future, which of the following should provide the most powerful test of general relativity?
A. Larger and more sensitive x-ray telescopes.
B. More precise measurements of the mass and size of stellar-sized and supermassive black holes.
C. High resolution imaging of black holes such as the one at the center of the Galaxy.
D. A definitive measurement of the spin of black holes.
E. The detection of gravitational waves from merging black hole binaries.
When two black holes spiral together and merge, emitting gravitational waves, the mass of the merged black hole is:
A. More than the sum of the masses of the two individual black holes.
B. Exactly equal to the sum of the masses of the two individual black holes.
C. Less than the sum of the masses of the two individual black holes.
D. Could be either more or less than the sum of the masses of the two individual black holes.
Could the x-ray emission observed from x-ray binaries containing a black hole be Hawking radiation?
A. Yes.
B. No.
A more massive black hole produces Hawking radiation with longer wavelength. Therefore a more massive black hole produces Hawking radiation that is _____ energetic, and therefore has a _____ Hawking temperature.
A. Equally, equal.
B. Less, lower.
C. More, lower.
D. Less, higher.
E. More, higher.
Th 2012 Apr 26:
For discussion (not graded): Scientifically, what's wrong with the story?
Suppose that the two black holes in the binary had different masses. Would you get a faster slingshot by shooting nearer the smaller or the larger black hole?
A. The smaller one.
B. The larger one.
C. Both will give the same velocity.
Tue 2012 May 1:
Would you vote in favor of making a baby Universe?
8 A. I think that it is immoral to attempt to make a baby Universe. I vote no.
13 B. If this is the only way that our Universe can reproduce, then I think society has a moral duty to make it happen. I vote yes.
23 C. I don't think society should waste resources attempting to make a baby Universe. I vote no.
54 D. I don't have a strong moral opinion, but I support the notion that society should attempt to make a baby Universe. I vote yes.
5 E. I don't really care either way. I probably won't bother to vote.
If society were presented with the one-time opportunity to make a baby Universe, what do you think the eventual outcome would be?
23 A. All out war between the yes and no factions.
18 B. Fierce political discussion, resolved by the democractic process.
4 C. Fierce political discussion, leading to deadlock.
1 D. Nothing. Society would not care, and would just go about its business.
You are the leader of the “No-baby-Universe” faction. You believe deeply that your cause is right. You strive for right. But your faction has lost the vote. What do you do?
17 A. Continue to argue non-violently for your cause.
39 B. Fight for right. Start a guerilla war.
16 C. Spread disinformation about your opponents and their theory of making baby Universes.
1 D. Something else.
How should the movie end?
33 A. As the science says: the people outside the black hole never know whether a baby Universe was made, or what might be the nature of that baby Universe.
16 B. The baby Universe should expand out into the old Universe, destroying it, and starting afresh.
20 C. Somehow there is an unexpected line of communication from inside the black hole to outside, that allows people outside the black hole to discover what happened.
26 D. Postpone the conclusion to a sequel.
7 E. Something else.
Th 2012 May 3:
What aspects of these observations allows the mass of the black holes to be measured?
A. The brightness of the central region of the galaxy.
B. The velocity of the stars.
C. The size of the region containing the stars.
D. The combination of the stars' velocity and the size of the region containing them.
E. The fact that the velocity is constant at small radius.
What do astronomers consider to be the best current observational evidence for black holes?
A. Evidence for relativistic motion, such as superluminal jets.
B. Evidence for very high energy phenomena, such as quasars.
C. A lot of mass in a small space.
D. Gravitational waves.
E. Einstein's theory of general relativity predicts black holes, and keeps passing observational tests.

Black Hole silhouetted against the Milky Way Spring 2012 ASTR 2030 Homepage

Updated 2012 May 7