ASTRO 130 FINAL
The typical frequency of gravitational waves is given by the gravitational frequency of the system, which is inversely proportional to the orbital period of an imaginary planet at the Schwarzschild radius. How should the frequency scale differently for gravitational wave sources with different mass? The circular velocity is given by v = (GM/r)1/2
1/M
The Luminosity of the Hawking radiation decreases as 1/M2. If the evaporation time for the Planck-mass (1019GeV~0.01mg) black hole is 10-39 second, what would be the evaporation time for the black hole with mass 1TeV=(103)GeV?
10^-87 sec
The Luminosity of the Hawking radiation decreases as 1/M2. If the evaporation time for the Planck-mass (1019GeV~0.01mg) black hole is 10-39 second, what would be the mass of the black hole whose evaporation time is about the age of the Universe (13.7 billion years)?
10^11 kg
In our Milky Way galaxy, there are roughly 1011 stars, and 0.1% of these stars are black holes. If the density of black holes is uniform over the Universe, there are 10^( ) black holes contained in 1011galaxies in our observable Universe.
19
The velocity of a satellite orbiting with orbital radius r from the center of the Earth is given by v=sqrt(Gm/r) where G is Newton's gravitational constant, and ME is the mass of the Earth, When the orbital radius of the GPS satellites (which orbits around the globe every 12 hours) is about 27000km, which of the following is closest to the orbital radius of the geosynchronous satellites that orbits around the globe every 24 hours.
42,000
Building a time machine is essentially the same as constructing [A] trajectory in a spacetime. Although this is strictly prohibited in [B], one might have a chance to build it in [C]. If we have a wormhole, we can make a time machine via [D] effect; either by placing it near the black hole or by moving to vega with it.
A Close Time-like Loop B Special Relativity C General Relativity D Time Dilation
Which of the following system outside of the Milky Way do NOT emit the gravitational waves that can possibly be detected by gravitational wave detecters with current technology? A supernova explosion A black hole - neutron star binary A stellar mass blackhole merging onto a super-massive black hole A neutron star - neutron star binary A black hole - black hole binary
A supernova explosion
Order following celestrial objects from top (time runs fastest) to bottom (time runs slowest). A) Black hole (M=solar mass, R=3km) B) Sirius B (white dwarf) (M=1 solar mass, R=5000km) C) Neutron star (M=1.4 solar mass, R=10km) D) Red Giant star (M=10 solar mass, R=630,000,000km) E) Sun (M=1 solar mass, R = 700,000km)
A) 5 B) 3 C) 4 D) 1 E) 2
Which of the following is not a stellar mass black hole candidate? A) An 1.4 solar masses companion in the binary system which is optically dark, but X-ray bright. B) An object which emits strong gamma-ray burst for 10 seconds. C) All are black hole candidates. D) A 3 solar mass isolated dark object that makes the star behind at the exact line of sight brighter for a hundred of days. E) A 5 solar-mass object which is dark in optical but bright in X-ray, also it ejects strong radio jet.
A) An 1.4 solar masses companion in the binary system which is optically dark, but X-ray bright.
Which of the following statements about Newtonian gravity are incompatible with the special theory of relativity, and motivate Einstein to build up the general theory of relativity? Select all that applies. No partial credit will be given for this problem. A) Distance between two objects that appears in the formula of Newton's gravitational force depend on the motion of observer. B) The weight of a free-falling body vanishes because of the inertial force which exactly cancels the force of gravity. C) The escaping velocity of a body can exceeds the speed of light. D) There are two different kinds of scales: one measures mass and another measures weight. E) The force of gravity applies immediately and extends to infinite.
A) Distance between two objects that appears in the formula of Newton's gravitational force depend on the motion of observer. E) The force of gravity applies immediately and extends to infinite.
What is the source of energy for the x-ray emission from an x-ray binary or AGN? A) Gravity. B) Chemical energy. C) Alternative energy. D) Nuclear fusion. E) Nuclear fission.
A) Gravity.
Choose all statement that correctly describes general relativity. Select all that applies. A) It takes about eight years (including the time that he worked on 'realm of the small') for Einstein to complete the general theory of relativity from his moment of 'happiest thought'. B) The path that free-falling particles follow in gravitational field is geodesic of spacetime with warped time and flat space. C) Spacetime tells mass how to move, and mass tells spacetime how to curve. D) Time-direction is flat and Einstein's equation relates space curvature to distribution of mass, energy and their flow. E) Einstein had to learn non-Euclidean geometry from his old classmate Grossman.
A) It takes about eight years (including the time that he worked on 'realm of the small') for Einstein to complete the general theory of relativity from his moment of 'happiest thought'. C) Spacetime tells mass how to move, and mass tells spacetime how to curve. E) Einstein had to learn non-Euclidean geometry from his old classmate Grossman.
Which of the following person's reference frame is close to the local inertia frame? A) Lisa who is bungee-jumping off a bridge. B) Marge who is watching Lisa under the bridge. C) Maggie who is crawling next to Marge. D) Bart who is in the artificial gravity device make 1g gravity in the international space station. E) Homer who is riding an elevator car for going up to the top of the bridge.
A) Lisa who is bungee-jumping off a bridge.
Which of the following descriptions about Kerr black hole is NOT correct? A) The black hole has a singularity at the center as a ball with radius of Planck length. B) The event horizon bulges out at equatorial plane. C) One can extract maximum 29% of the rest mass energy (of the black hole) by Penrose process. D) Space drag effect forces all particles nearby co-rotate with the black hole. E) It is a rotating black hole.
A) The black hole has a singularity at the center as a ball with radius of Planck length.
As you travel near the speed of light, you will see the things from the backwards of you to be (choose all that applies.) A) larger B) the same color, the same size C) bluer D) redder E) smaller
A) larger D) redder
Consider following thought experiment involving two clocks on the ground (of a planet whose gravity at the surface is pretty strong). You hold two clocks A and B that are synchronized, then throw the clock B with high speed at noon. Suppose that it takes two hours for the clock B to come back to the ground. When it come back, the time the clock B is pointing is A) later than the clock A: 3pm. B) earlier than the clock A: 1pm. C) the same as the clock A: 2pm.
A) later than the clock A: 3pm.
Which of the following is NOT relative in special theory of relativity? Choose all that applies. A) spacetime B) time C) speed of light D) space E) motion
A) spacetime C) speed of light
List following celestial objects in the distance-from-the-Earth order (from closer to further): A) GRB090423 at redshift z=8.2 B) The star, Sirius C) M87, an elliptical galaxy with AGN D) A radio source Sagittarius A* E) M31, andromeda galaxy
A)5 B)1 C)4 D)2 E)3
Which of the following statements about the singularity is NOT correct? In order to understand what happens at the singularity, we need the quantum theory of gravity. Singularity is the place where classical general relativity breaks down. For a Schwarzschild black hole, singularities happens inside of a ball whose radius is the Planck length (~10-35m). All singularities are hidden behind event horizons. When the impulsion of star happens without bound, evolution to a singularity is inevitable.
All singularities are hidden behind event horizons.
Which of the following statements about Hawking radiation is NOT correct? Microscopically, it can be explained by pair-creation and pair-annihilation of particles near the event horizon.As a result of Hawking radiation, all black holes in the Universe must be already evaporated. As a result of Hawking radiation, all black holes in the Universe must be already evaporated. Evaporating primordial black holes would imprint the signature in the gamma-ray sky. Hawking radiation temperature is inversely proportional to the mass of black hole. Macroscopically, it is the result from that the black hole is a thermodynamical object which has an entropy.
As a result of Hawking radiation, all black holes in the Universe must be already evaporated.
The figure shows the orbits of stars in the center of the Milky Way Galaxy. The different symbols (square, triangle, circle, etc) each orbit mark the position of the orbiting stars at 1 year intervals. In this plot, 0.1'' corresponds to 800AU.Let's focus on the star S0-20. Measuring in terms of the keynote pixel, your instructor find that the length of the red line is 67 pts, and the length of the two yellow lines is 290 pts. Based on this measurement, what can you conclude about the size of this star's semi major axis in AU? A) 1500 AU B) 1750 AU C) 2000 AU D) 1000 AU E) 1250 AU
B) 1750 AU
The figure shows the orbits of stars in the center of the Milky Way Galaxy. The different symbols (square, triangle, circle, etc) each orbit mark the position of the orbiting stars at 1 year intervals. In this plot, 0.1'' corresponds to 800AU. Let's focus on the star S0-20. Which of the following is the closest to the orbital period of this star in years? A) 16 years B) 32 years C) 24 years D) 8 years E) 40 years
B) 32 years
According to the Hoop conjecture and ignoring the rotation, which of the following black holes cannot be formed (even briefly) in nature? Here, Msun stands for the mass of the Sun. A) A spherical black hole with M=5Msun B) A cigar shaped black hole with 10km long, 2km thick whose Schwarzschild radius is 5km C) All black holes can be formed. D) A elliptical 10Msun black hole rotating with a half of speed of light E) A spherical black hole with M=5Msun, and electric charge of 10 Coulomb
B) A cigar shaped black hole with 10km long, 2km thick whose Schwarzschild radius is 5km
Which of the following statements about accretion disk is NOT correct? A) Accretion disk converts gravitational potential energy first to the heat energy, then to the radiation. B) Because energy is transferred to outward, inner part of the accretion disk is cooler than the outer part. C) Because angular momentum is transferred to outward, outer part of the accretion disk rotates faster than what it would rotate otherwise. D) An object falling into a Kerr black hole can convert up to about 40 percent of its rest mass energy into the radiation. E) There is a gap between innermost radius of he accretion disk and the event horizon of the central black hole.
B) Because energy is transferred to outward, inner part of the accretion disk is cooler than the outer part.
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 neutron star. B) It is the maximum mass of a white dwarf. C) It is the maximum mass of a black hole. D) It is probably just a coincidence. E) It is the minimum mass of a black hole.
B) It is the maximum mass of a white dwarf.
Consider a car and a garage that perfectly fits the car when it is at rest within the garage. Now, suppose that the car is passing the garage with 86.6 percent of the speed of light (that makes the Lorentz factor γ=2). When the garage doorman who sits at the center of the garage sees the car, the length of the car is contracted along the direction of its motion and the car is well within the garage. So, the doorman closes both front (left) and rear (right) doors for a short time and open them again so that the car can pass through the garage. What is happening here? Choose all statements that correctly describe the situation. A) No damage is done to the car because the garage stands still and not moving when looking from the car; there is no length contraction of garage. B) No damage is done to the car because, although the doorman shut the front and rear doors simultaneously in his frame of reference, the two doors are not shut at the same time in the car's frame of reference. C) No damage is done to the car because the car must be perfectly fit into the garage like the case when it is at rest. D) For car's point of view, the garage is moving backward. Therefore, relativistic length contraction dictate that the doors hit the car because the size of the garage must be shrunk along the direction of motion. E) The car passes the front (left) door after it shuts/re-opens, and passes the rear (right) door before it shuts/re-opens. right in the makeup
B) No damage is done to the car because, although the doorman shut the front and rear doors simultaneously in his frame of reference, the two doors are not shut at the same time in the car's frame of reference. E) The car passes the front (left) door after it shuts/re-opens, and passes the rear (right) door before it shuts/re-opens. right in the makeup
Which of the following statement is true for black holes? Select all right descriptions to get the full credit. A) The concept of the dark stars (e.g. black holes) did not exist before Einstein. B) Some black holes have accretion disk around it, but other black holes are isolated. C) Because light travels with the fastest speed, only light can escape from black holes. D) Black holes are black, and, therefore, do not emit any radiation. E) General theory of relativity predicts the existence of black holes.
B) Some black holes have accretion disk around it, but other black holes are isolated. E) General theory of relativity predicts the existence of black holes.
Which of the following is NOT correct for the gravitational redshift? A) The gravitational redshift can be explained by gravitational time dilation. B) The gravitational redshift violates energy conservation principle. C) Photons loose energy as it travels outside of the gravitational pulling. D) The Pound-Reba experiment measures the gravitational redshift in the 22.6m height building despite the tiny size of the effect. E) The gravitational redshift of the light coming out of the Sun was measured by James Brault in 1962.
B) The gravitational redshift violates energy conservation principle.
Which of the following information was essential for astronomers when they estimated that about 0.1 percent of total stars are black holes in the Milky way? A) The theoretical estimation of the lifetime of the main sequence stars. B) The observed mass fraction of stars (with given mass range) in the open clusters. C) The age of the Universe D) The total number of supernovae in the Galaxy. E) The observed density of black holes.
B) The observed mass fraction of stars (with given mass range) in the open clusters.
If an isolated star collapsing into a black hole has a tall mountain as well as magnetic field anchored on the surface, what would happen? A) The star will leave an event horizon with both the mountain and the magnetic field. B) The star will radiate gravitational waves and electromagnetic waves, then leave only a spherical event horizon. C) The star will radiate gravitational waves, then leave a spherical event horizon anchoring magnetic field on the surface. D) The star will radiate electromagnetic waves, then leave an event horizon with the mountain. E) The star will be electrically charged black hole with the mountain.
B) The star will radiate gravitational waves and electromagnetic waves, then leave only a spherical event horizon.
Which of following is NOT a major observational test that supports general relativity? A) A clock on top of Mount Everest runs faster than a clock at sea level. B) Uranus's orbit is slightly different from the Newtonian calculation when adding gravitational forces from the Sun and the six inner planets. C) Astronomers see arc patterns in images of galaxies behind galaxy clusters. D) Mercury's orbit precesses around the Sun more than expected by Newton's law of gravity, because when Mercury gets closer to the Sun, spacetime curves more allowing Mercury to travel a little further than if spacetime around the Sun were not curved. E) Stars behind the Sun appear to be in a different place than it would if the Sun were not in the path, since the Sun warps spacetime.
B) Uranus's orbit is slightly different from the Newtonian calculation when adding gravitational forces from the Sun and the six inner planets.
Which of the following will be the strongest, unequivocal proof of the existence of a black hole? A) A test body that survives when passing through the event horizon of a black hole. B) We must see a event horizon. C) A stellar object that shows strong X-ray luminosity. D) A micro-lensing event from an object massive than 3 solar masses. E) A stellar object that shows prominent jet in radio band.
B) We must see a event horizon.
Select all to get the full credit. No partial credit will be given for this problem. The Schwarzschild black hole is A) collapsing B) spherically symmetric C) non-rotating D) positively charged E) warping space positively
B) spherically symmetric C) non-rotating E) warping space positively
Why can we hear the sound of gravitational waves from merging black holes? Because gravitational waves vibrate the air to generate sound waves. Because scientists converted the gravitational wave signal into the sound signal with the same frequency. Because stretching/squeezing spacetime would generate sound waves. Because LASER interferometry makes audible noise. Because that is the way that we detect the gravitational waves.
Because scientists converted the gravitational wave signal into the sound signal with the same frequency.
If you are falling into a Schwarzschild black hole, far inside the horizon, what would happen? The tidal spaghettification effect will decrease as you approach the singularity. Because the spacetime itself is deformed inside of black hole, you would be spaghettificated along the random directions in a very chaotic manner. Because the black hole is spherically symmetric, you would be spaghettificated only along the radial direction. Due to equivalence principle, you, as a freely-falling observer, would not feel any force until you hit the singularity.
Because the spacetime itself is deformed inside of black hole, you would be spaghettificated along the random directions in a very chaotic manner.
According to Blandford-Znajek process, where is the energy of the jet coming from? Accretion disk Thermonuclear reaction around the event horizon Accreting material Magnetic field Black hole
Black hole
What is the difference between a Radio galaxy and a Blazar? Both are AGN, but Blazar's jets are oriented 45 degrees from head-on. Both are AGN, but our line of sight is perpendicular to the jet when looking at Blazars. Blazars are AGN, while Radio galaxies just have jets and radio lobes and are not really active. Both are AGN, but Blazar's jets are oriented nearly head-on. Radio galaxies are AGN, while Blazars are just what we call a jet of an AGN.
Both are AGN, but Blazar's jets are oriented nearly head-on.
One day, your twin sister Jackie decides to travel to Vega which is 25 light-years away from the Earth. She says that her spaceship will travel with 99.98 percent of speed of light, which corresponds to the Lorentz factor of γ=50. Having lots of homework to do, however, you decide to stay at home, here on the Earth. While Jackie is traveling to Vega, for your point of view, it will take approximately 50 years for Jackie to reach Vega and to come back home. But, what will be surprising to you is that Jackie will be (1)____ years older than now! What about Jackie's point of view? From Jackie's point of view, as the Earth and Vega both will move with 99.98 percent of speed of light, the distance from the Earth to Vega will become (2)____ light year long, and she arrives at Vega after (3)____ months. But, when Jackie turns around, because she need to slow down, Jackie's clock runs (4)____ (faster/slower) than the clocks on the Earth due to the acceleration (5)____ (gravitational time dilation/time dilation in special relativity).
Box 1: 1; Box 2: 0.5; Box 3: 6; Box 4: slower; Box 5: gravitational time dilation
According to your teacher, which of the following phrase correctly describes what astronomers nowadays think about the observational evidence of the black holes? A) "They are definitely not black holes!" B) "They are definitely black holes!" C) "If not a black hole, we don't know what else it could be!" D) "Black holes are so weird, and there must be a law of nature that prevents its existence!" E) "All evidences are not conclusive at all, and we have no idea as of know what they are!"
C) "If not a black hole, we don't know what else it could be!"
Each of the following lists two statements. Which two are the basic premises for the special theory of relativity? A) 1. Everything is relative. 2. You can never really tell who is moving, and who is at rest. B) 1. You can't go faster than the speed of light. 2. Time is different for different people. C) 1. The laws of nature are the same for every inertial observers. 2. The speed of light is the same for every inertial observers. D) 1. The speed of light is the same for inertial observers. 2. You can't go faster than the speed of light. E) 1. The laws of nature are the same for inertial observers. 2. Everything is relative.
C) 1. The laws of nature are the same for every inertial observers. 2. The speed of light is the same for every inertial observers.
Five minutes before his dinner appointment with Lois, the Superman decides to do some exercise and flies around the Earth with 99.92% of the speed of light (γ=25). Then he comes back after 50,000 revolutions. How long does it take for the Superman to complete 50,000 revolutions for Lois and for himself? Choose the closest answer. Speed of light is 300,000km/s, and the radius of the Earth is 6400km. For this problem, ignore the effect from gravity. A) 4 minutes 30 seconds for Lois, one hour and 51 minutes for the Superman B) 4 minutes 30 seconds for both the Superman and Lois C) 4 minutes 30 seconds for the Superman, one hour and 51 minutes for Lois D) one hour and 51 minutes for both the Superman and Lois
C) 4 minutes 30 seconds for the Superman, one hour and 51 minutes for Lois
What kind of pressure holds up a white dwarf star against its own gravity? A) The pressure of a hot gas of nuclei and electrons (a plasma). B) The pressure of a gas of atoms. C) Electron degeneracy pressure. D) The pressure of a liquid or solid of atoms. E) Neutron degeneracy pressure.
C) Electron degeneracy pressure.
Where do the x-rays in an x-ray binary like Cygnus X-1 come from? A) Light from the companion star, gravitationally lensed and blue-shifted by the black hole. B) The black hole. C) Hot gas in an accretion disk spiraling on to the black hole. D) A jet emerging from near the black hole. E) Decay of radioactive elements synthesized near the black hole.
C) Hot gas in an accretion disk spiraling on to the black hole.
Which of the following statements about radiation processes in stars is NOT correct? A) Neutron stars emit periodic radio signal with milli to a few seconds periods. B) As they age, white dwarfs emit thermal radiation with less and less luminosity. C) Neutron stars transfer their rotation energy to radiation, with constant luminosity throughout their lifetime. D) The main sequence stars are powered by thermonuclear fusion of hydrogen. E) There is a maximum luminosity that a given mass of the star can emit.
C) Neutron stars transfer their rotation energy to radiation, with constant luminosity throughout their lifetime.
How are GRBs (Gamma-Ray Bursts) related to black holes? A) Distribution of GRBs are uniform over the sky. B) Because its afterglow show clear optical signature, GRBs cannot be black holes. C) The long GRBs are associated with core-collapsing supernovae from which black holes may be formed. D) Isolated black holes emit strong gamma-rays. E) The short GRBs are the signature of black holes formation.
C) The long GRBs are associated with core-collapsing supernovae from which black holes may be formed.
Which of the following statements about geodesic is NOT correct? A) For given two points, geodesic is the shortest route connecting them. B) In a sufficiently small region of curved space, geodesic appears to be straight in a usual flat, Euclidean sense. C) Two geodesic curves parallel to each other never meet together in the positively curved space. D) Geodesic in curved spacetime is a route that gives rise to the maximum age of the clock following that route. E) Geodesic is a natural extension of the straight line in Euclidean (flat) geometry to non-Euclidean (curved) geometry.
C) Two geodesic curves parallel to each other never meet together in the positively curved space.
Which of the following statements is NOT correct about the cannon shoot parallel to the ground? Here, parallel and perpendicular directions are defined with respect to the ground, and consider only the case near the ground where gravitational acceleration is constant. A) Without gravity, the cannon ball would move straight line along the parallel direction. B) At a given time, the perpendicular directional displacement is independent of the mass of cannon ball. C) When shooting the same cannon ball on the moon (at the same height), it takes a shorter time for the ball reaches to the ground. D) The perpendicular directional acceleration of cannon balls is the same for all cannon balls regardless of their shooting altitude. E) When shooting with the same initial velocity, a cannon ball shot from higher altitudes reaches further along the parallel direction compared to one shot from lower altitudes.
C) When shooting the same cannon ball on the moon (at the same height), it takes a shorter time for the ball reaches to the ground.
Consider a 2000km-long man who are free-falling from the Moon to the Earth. Then, this man A) feels only a constant gravitational force throughout. B) is in an inertial frame, and he cannot tell the different between his situation now and weightless state. C) feels the tidal force that squeezes him along the left-right direction and elongates him along up-down direction. D) feels the tidal force that elongates him along the left-right direction and squeezes him along up-down direction. E) is not in an inertial frame because he is affected by gravity from both the Earth and the moon.
C) feels the tidal force that squeezes him along the left-right direction and elongates him along up-down direction.
The massive white dwarfs and neutron stars are smaller and the lighter white dwarfs and neutron stars are larger, because A) they must have the same density B) the massive stars have weaker gravity C) the massive stars need higher pressure support D) there are minimum sizes for white dwarfs and neutron stars E) the massive stars have higher luminosity
C) the massive stars need higher pressure support
The Sun has been stable for a long time, because A) it has more pressure at the surface than the center B) the core is denser than the envelope C) thermonuclear fusion keeps its interior hot enough to hold against gravity D) the Sun has a Helium core burning into Carbon E) the free-fall time is long enough
C) thermonuclear fusion keeps its interior hot enough to hold against gravity
The Schwarzschild radius of the black hole is proportional to the total mass of the black hole. Then, how does the minimum density of black holes scale as the mass increases/decreases? A) 1/M B) M^2 C) M D) 1/M^2 E) The density of black holes is constant.
D) 1/M^2
Suppose that the muon is moving with 99.98 percent of speed of light (the Lorentz factor of γ=50). How far can muon travel within its lifetime of 2.2 micro-seconds (2.2x10-6 second) as observed from the ground? The speed of light is 300,000km/s. A) 3.3 km B) 660 m C) 1.65 km D) 33 km E) 16.5 km
D) 33 km
What evidence most convincingly suggests that there is a supermassive black hole at the center of our own Milky Way? A) A relativistic jet is seen coming out of the core of the Milky Way. B) Hubble Space Telescope observations show gas swirling down a vortex. C) The supermassive black hole is in an x-ray binary star system. D) Measurements of the velocities of stars at the center of the Milky Way indicate a large mass in a small space. E) The black hole causes gravitational lensing.
D) Measurements of the velocities of stars at the center of the Milky Way indicate a large mass in a small space.
Which of the following statements about the journey into the black hole is NOT correct? A) The light signal that the traveller periodically sent out to the Earth will arrive with increasing time intervals as the traveller is close the black hole. B) The traveller can cross the event horizon without any trouble. C) After entering the event horizon, no signal ever reaches to a receiver even at 1mm above the Schwarzschild radius. D) The light signal that the traveller periodically sent out to the Earth will stretch to the bluer side of the spectrum as the traveller is close the black hole. E) As approaching to the black hole, the traveler can see the back side of the black hole.
D) The light signal that the traveller periodically sent out to the Earth will stretch to the bluer side of the spectrum as the traveller is close the black hole.
Let's suppose that somehow the Sun suddenly turns into a black hole (while maintaining its mass, without any violent event) one day. Which of the following correctly describes its impact on the Earth? A) The orbit of the Earth won't be changed, and the sunlight will suddenly shut off at the moment that the Sun turns into a black hole. B) The strong gravity of the black hole will pull the Earth closer, and the sunlight will suddenly shut off at the moment that the Sun turns into a black hole. C) The strong gravity of the black hole will pull the Earth closer, and the last sunlight come to the Earth about 8 minutes and 20 seconds after the Sun turns into a black hole. D) The orbit of the Earth won't be changed, and the last sunlight come to the Earth about 8 minutes and 20 seconds after the Sun turns into a black hole.
D) The orbit of the Earth won't be changed, and the last sunlight come to the Earth about 8 minutes and 20 seconds after the Sun turns into a black hole.
A spaceship is moving past us at a constant speed close to the speed of light. Choose all correct statements. A) Clocks in the spaceship are going faster (measuring more time) than ours. B) The passengers on the spaceship would see our clocks are going faster (measuring more time) than theirs. C) The clock runs at the same rate for everyone (us, passengers). D) The passengers on the spaceship would see our clocks are going slower (measuring less time) than theirs. E) Clocks in the spaceship are going slower (measuring less time) than ours.
D) The passengers on the spaceship would see our clocks are going slower (measuring less time) than theirs. E) Clocks in the spaceship are going slower (measuring less time) than ours.
A spaceship is moving past us at a speed close to the speed of light. How would we measure the width of the spaceship (3m along the direction perpendicular to the motion) and the length of the spaceship (10m along the direction of the motion) as it goes by? A) The width of the spaceship becomes shorter than 3m, but the length of the spaceship stays the same (10m). B) Both the width and length of the spaceship becomes shorter. C) The width and the length of the spaceship becomes exactly half as long as their size. D) The width of the spaceship is the same (3m), but the length of the spaceship becomes shorter than 10m. E) Both the width and length of the spaceship becomes longer.
D) The width of the spaceship is the same (3m), but the length of the spaceship becomes shorter than 10m.
Why did astronomers build the VLBI (very long baseline interferometry) telescope whose effective aperture is as big as the size of the Earth? A) To enhance the effective observing time of the telescope. B) To minimize the effect of observing condition that varies site by site. C) To increase the light collecting area of the telescope. D) To increase the angular resolution of the telescope. E) To enhance a spectrum resolution of the telescope.
D) To increase the angular resolution of the telescope.
According to no hair conjecture, when a highly irregular, non-spherical shape star with 5 solar mass collapses without any mass loss, it would eventually be A) an irregular-shape white dwarf B) an irregular-shape neutron star C) a black hole whose event horizon is the same shape as the star at the Schwarzschild radius D) a spherically symmetric black hole E) a brown dwarf
D) a spherically symmetric black hole
Which of the following statements about merging binary black holes is NOT correct? Astronomers think that the black holes at the center of galaxies also merge when two host galaxies merge. It emits gravitational waves throughout the process until the two horizons merge to make one horizon. Due to the kick during the merger, the final black hole (event horizon) is kicked out of the accretion disk. Black holes first in-spiral, then merge, and finally ring down. The orbit of two spinning black holes can be very different from a Keplerian, co-planar orbits.
Due to the kick during the merger, the final black hole (event horizon) is kicked out of the accretion disk.
In order to observe an event horizon of Sagittarius A* (a 4 million-solar-mass object, 8kpc away from the Earth), we needs a telescope with minimum 10 micro arc-seconds resolution. What should be the minimum resolution of the telescope which is required to observe an event horizon of the central object (4 billion solar masses) at M87 that is 16 Mpc away from the Earth? A) 10 micro arc-second B) 2.5 micro arc-second C) 1 micro arc-second D) 20 micro arc-second E) 5 micro arc-second
E) 5 micro arc-second
Based on your answer to the above two questions, what is the closest to the mass of the black hole at the center of the Milky Way (Sagittarius A*) estimated from the star S0-20? A) 7.8 million Solar mass B) 1.9 million Solar mass C) 31 million Solar mass D) 1.9 million Solar mass E) 5.2 million Solar mass
E) 5.2 million Solar mass
The total luminosity of the Sun is 3.9 x 1026 (Joule/second), and the hydrogen fusion mass-energy efficiency of the Sun is 0.4%. In the homework, you have estimated the total mass of hydrogen that is burnt per second in the Sun. Using the result of that calculation and that the mass of the Sun is 2x1030kg, you can estimate the lifetime of the Sun. If the Sun was born with 100% hydrogen, how long would the Sun live. That is, how long would it take for the Sun to burn all hydrogen? A) 80 Billion years B) 100 Billion years C) 20 Billion years D) 40 Billion years E) 60 Billion years
E) 60 Billion years
What does no hair conjecture state? A) A black hole is completely characterized by mass, electric resistance, and spin. B) A black hole is completely characterized by mass, magnetic moment, and ellipticity. C) A black hole is completely characterized by mass, electric conductivity, and ellipticity. D) A black hole is completely characterized by mass, magnetic moment, and spin. E) A black hole is completely characterized by mass, electric charge, and spin.
E) A black hole is completely characterized by mass, electric charge, and spin.
Which one of the following is closest to the inertial reference frame? A) A frame of reference of Mark who is stepping on an accelerator in a racing car toward the maximum speed. B) A frame of reference of John who is sitting on a bench in front of the Old Main. C) A frame of reference of Robin who is landing on Mars. D) A frame of reference of Jane who is riding a roller coaster. E) A frame of reference of Ana who is traveling to the star Vega with a constant velocity.
E) A frame of reference of Ana who is traveling to the star Vega with a constant velocity.
Which of the following statements is NOT a major prediction of special theory of relativity? A) All other statements are correct. B) Time runs more slowly for a person traveling at relativistic speeds compared to a stationary person. C) No information can travel faster than the speed of light, and no material object can ever move at the speed of light. D) Objects moving at relativistic speeds will appear shorter along the direction of their motion. E) All events are simultaneous for two observers moving relative to each other at a constant velocity.
E) All events are simultaneous for two observers moving relative to each other at a constant velocity.
Which of the following statements about gravitational lensing is incorrect? A) Gravitational lensing can distort the shape of background galaxies. B) Gravitational lensing can generate multiple images of background galaxies. C) Gravitational lensing effect from general relativity is twice as big as that of Newtonian prediction. D) Gravitational lensing can magnify the image of background galaxies. E) Gravitational lensing can distort the color of background galaxies.
E) Gravitational lensing can distort the color of background galaxies.
What is so special about 1.5 Schwarzschild radius of Schwarzschild black holes? A) It defines the size of singularity surface. B) It is the radius below which no photon can escape. C) It is the radius at which tidal field becomes infinity. D) It defines the size of event horizon. E) It is the radius at which a person can see her/his own back.
E) It is the radius at which a person can see her/his own back.
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 contract until it becomes supported by electron degeneracy pressure. B) The star will explode as a supernova. C) The star will cool down. D) The star will collapse to a black hole. E) The star's core will contract, and heat up to the point that it can ignite some other fuel.
E) The star's core will contract, and heat up to the point that it can ignite some other fuel.
Looking from a far away observer, collapsing stars are frozen at the Schwarzschild radius, which means that A) gravity is so strong at the Schwarzschild radius that nothing can implode inside. B) event horizon is an illusion. C) black holes in fact never form in reality. D) the actual collapsing of stars gets slower and takes infinite time to collapse. E) a light emitted from the star at the Schwarzschild radius cannot escape.
E) a light emitted from the star at the Schwarzschild radius cannot escape.
7From where did the primordial black hole forms? From the first galaxies in early Universe From the particle accelerators From the first generation of stars in early Universe From the quantum fluctuations in early Universe From the cosmic microwave background radiation in early Universe
From the quantum fluctuations in early Universe
According to Newton's law of gravity, gravitational force between two masses m1 and m2 separated at a distance r is given by (m1+m2)/r^2 Combining this equation to the Newton's second law of motion, you can find the unit of gravitational constant G in terms of meter (m), kilogram (kg) and second (s). Which one of following is the correct unit of the gravitational constant G?
G = 6.67 x 10-11 m3kg-1 s-2
Which one of the following is NOT true about the relationship between the supermassive black holes (SMBHs) and their host galaxies? The gravitational binding energy of SMBH dominates over the gravitational binding energy of the bulge, which is a hundred million times bigger than the black hole in size. More massive the SMBH, more luminous the host galaxy's bulge. Galaxies with SMBHs in their centers contain less gas as most of the gas was consumed by the black hole. More massive the SMBH, larger the host galaxy's bulge. A SMBH can influence properties of its host galaxy by feedback - jets and high-energy radiation from the accretion disk can suppress star formation in other parts of the galaxy.
Galaxies with SMBHs in their centers contain less gas as most of the gas was consumed by the black hole.
What happens to the entropy of the Universe when John Wheeler throws his teapot into the black hole? He decreases the black hole's entropy. Because the entropy of the black hole is already infinity, he cannot change the entropy of the black hole. The entropy of the Universe is not chanced. The entropy of the Universe decreases as the black hole absorb the entropy of the teapot. He increase the black hole's entropy.
He increase the black hole's entropy.
Which of the following statements about the detection of gravitational waves is NOT correct? eLISA is a planned gravitational-wave detector in the space. LIGO, VIRGO, GEO600 are ground-based gravitational-wave detectors using laser interferometry. LIGO can detect a million solar-mass black holes while eLISA can detect a solar-mass black holes. Pulsar Timing Array will detect very-low frequency (about nano-Hertz) gravitational waves. The Pulsar Timing Array project has been monitoring many milliseconds pulsars over 10s of years.
LIGO can detect a million solar-mass black holes while eLISA can detect a solar-mass black holes.
Match the observatories to the major target black holes PTA : LIGO-Virgo: LISA:
PTA : BH binaries with billion solar mass SMBH LIGO-Virgo: Stellar Mas BH, neutron star binaries LISA: BH binaries with million solar mass BH
Question 2 If x-rays from an AGN vary on a timescale of one day, then the AGN is probably: Smaller than one lightday across. In a binary system. A black hole. Variability places no constraint on the size of the AGN. Larger than one lightday across.
Smaller than one lightday across.
Which of the following description of quantum mechanics and relativity theory is NOT correct? Special relativity cannot be combined with quantum mechanics. Loop quantum gravity is developing quantum mechanical description of spacetime. String theory predicts the existence of six extra spatial dimension. It is essential to combine general relativity and quantum mechanics in order to figure out what is happening at the singularities. Quantum mechanics is the physics of atomic world.
Special relativity cannot be combined with quantum mechanics.
Which of the following does NOT imply that there is a supermassive black hole in the center of a galaxy? Velocities of stars increase very close to the center of the galaxy. We see a positive and a negative spike in velocity (the "zigzag" in the plot) of the gas close to the center of the galaxy. We sometimes see an X-ray flare. The center of the galaxy appears completely dark. The number of stars close to the center of the galaxy increases.
The center of the galaxy appears completely dark.
If a pulse of gravitational wave passes through the Earth, what will happen? The radius of the earth shrinks in all direction by a tiny amount. The radius of the earth expands in all direction by a tiny amount. The earth squeezes in one direction and expands in another direction. Nothing happens. The same effect as tidal field from the Moon.
The earth squeezes in one direction and expands in another direction.
Why is it that we often see an asymmetric jets (and sometimes even only on one side) of an AGN? The jet on the other side of the accretion disk is not as fast. The jet on the other side is there, it is just emitting light at a different wavelength. The magnetic field lines of accretion disks around SMBHs are so twisted that they only point a jet in one direction. The jets that are pointed more directly toward us, appear brighter and we see them more easily. The hosting galaxy as we well as AGN are moving to the perpendicular direction.
The jets that are pointed more directly toward us, appear brighter and we see them more easily.
Why do jets produce such large lobes, larger than the host galaxy of the SMBH that they come from? The material carried by the jet is so hot that it has to travel very far from the SMBH in order to finally cool and expand. Because the jet materials are dispersed while traveling such a long distance. Because they carry a huge amount of gas from the vicinity of the black hole. The material from the jet encounters the interstellar or intergalactic medium. Lobes and jets are actually not related and are a completely separate phenomenon.
The material from the jet encounters the interstellar or intergalactic medium.
Which is NOT a characteristic of an AGN? Existence of a "star-like" object at the center that dominates the galaxy's luminosity. They contain large proportion of high-energy photons. Their spectra show radiation over a wide range of wavelengths. All are much more luminous than normal galaxies Their host galaxy is always elliptical.
Their host galaxy is always elliptical.
When observing with the same VLBI system, how are the images taken by 7mm-radio wave compared with images taken by other wave bands? They have about twenty times worse resolution than images from 18cm-radio wave They have about twice better resolution than images from 1.3cm-radio wave They have about twice better resolution than images from 18cm-radio wave They have about a half of resolution of images from 18cm-radio wave They have about a half of resolution of images from 1.3cm-radio wave
They have about twice better resolution than images from 1.3cm-radio wave
What will be the major benefit of building a third gravitational wave observatory in the other part of the globe? To achieve the better frequency resolutions To enhances the international friendship To locate the position of the gravitational wave source with better accuracy To enhances the image quality of the gravitational wave sources To enhance the sensitivity of the detection of gravitational waves
To locate the position of the gravitational wave source with better accuracy
The escape velocity at the surface of the Sun is about 600 km/s. If there should exist a black hole (or, John Mitchell's dark star) whose density is the same as the density of the Sun, how large should the mass of that black hole be? The escape velocity is given in terms of mass M and radius r as V=sqrt(2GM/r) and the speed of light is 300,000 km/s.
at least 500 times as heavy as sun
Why does the orbital period of the binary pulsar PSR B1913+16 decrease in time? because gravitational waves are continuously passing the system. because the orbital radius expands. because the pulsar period changes. because the accuracy of measuring the period increases. because the system is inspiring and emitting gravitational waves.
because the system is inspiring and emitting gravitational waves.
According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of Gases expelled from super-Eddington luminosity of the black hole. the combined light of thousands of young, high-mass stars that orbit the black hole. radio waves emitted from radio lobes found on either side of the galaxy we see in visible light. intense radiation emitted by the black hole itself. light emitted by hot gas in an accretion disk that swirls around the black hole.
light emitted by hot gas in an accretion disk that swirls around the black hole.
If a gravitational wave of strain strength 10-9 pass through the earth, what how much is the size of the earth changes? radius expands/shrinks by 64 micro-meter radius expands/shrinks by 6.4 mm radius expands/shrinks by 6.4 m radius expands/shrinks by 6.4 cm radius expands/shrinks by 0.64 mm
radius expands/shrinks by 6.4 mm
