After entering the event horizon, no signal ever reaches

QUESTION

Q1. Which of the following statements about the journey into theblack hole is not correct?(A) After entering the event horizon, no signal ever reaches to a receiver at1mm above the Schwarzschild radius.(B) As approaching to the black hole, the traveler can see the back side ofthe black hole.(C) Because the time stops at the Schwarzschild radius, nothing can go intothe event horizon.(D) The light signal that the traveller periodically sent out to the Earth willarrive with increasing time intervals as the traveller is close the black hole.(E) The light signal that the traveller periodically sent out to the Earth willstretch to the redder side of the spectrum as the traveller is close the blackhole.Q2. The Schwarzschild radius of the black hole is proportional tothe total mass of the black hole. Then, how does the minimumdensity of black holes scale as the mass increases/decreases?(A) M(B) 1/M(C) M22(D) 1/M(E) The density of black holes are constant.Q3. (Select all to get the full credit)
The Schwarzschild black hole is[A] non-rotating[B] positively charged[C] spherically symmetric[D] static[E] warping space negativelyQ4. According to the waterfall model of black holes, a person whofree-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 thehorizon.(E) experiences nothing special at the horizon.Q5. According to Thorne Ch 3, Einstein rejected black holesbecause:(A) There was no observational evidence for black holes.(B) It was absurd to think that nature could actually make such anextreme object.(C) A black hole has a singularity at its horizon, and singularities shouldnot exist in nature.(D) A black hole has a central singularity, where spacetime comes to anend, which is impossible.(E) A black hole would curve space so much that it would swallow itselfand disappear.Q1. The massive white dwarfs and neutron stars are smaller andthe lighter white dwarfs and neutron stars are larger, because(A) they must have the same density(B) there are minimum sizes for white dwarfs and neutron stars(C) the massive stars need higher pressure support(D) the massive stars have weaker gravity(E) the massive stars have higher luminosityQ2. Which of the following will be most probably a black hole?(A) A star born with 1.4 Msun(B) A star born with 3 Msun(C) A star born with 10 Msun(D) A stellar core with 2Msun(E) A stellar core with 4MsunQ3. The Sun has been stable for a long time, because(A) it has more pressure at the surface(B) thermonuclear fusion keeps its interior hot enough to hold againstgravity(C) the core is denser than the envelope(D) the free-fall time is long enough(E) the Sun has a Helium core burning into CarbonQ4. The masses of neutron stars observed in pulsar binary systemsare all consistent with 1.4 solar masses. What is the significance ofthis 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.Q5. When a star (the Sun, say, to be specific) runs out of a nuclearfuel of some kind (Hydrogen, say, to be specific), what will happenthen?(A) The star will cool down.(B) The star’s core will contract, and heat up to the point that it canignite some other fuel.(C) The star will contract until it becomes supported by electrondegeneracy pressure.(D) The star will collapse to a black hole.(E) The star will explode as a supernova.Q6. What kind of pressure holds up a white dwarf star against itsown 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.Q7. Which of the following statements about radiation processes instars is not correct?(A) Neutron stars transfer rotation energy to radiation.(B) The main sequence stars are powered by thermonuclear fusion ofhydrogen.(C) White dwarfs emit thermal radiation with constant luminositythroughout its lifetime.(D) There is a maximum luminosity that a given mass of the star canemit.(E) Neutron stars emit periodic radio signal with milli to a few secondsperiods.Q1. Looking from a far away observer, collapsing stars are frozenat the Schwarzschild radius, which means that(A) black holes in fact never form in reality.(B) a light emitted from the star at the Schwarzschild radius cannotescape.(C) gravity is so strong at the Schwarzschild radius that nothing canimplode inside.(D) The actual collapsing of stars gets slower and takes infinite time tocollapse.(E) event horizon is an illusion.Q2. When a highly irregular, non-spherical shape star with 5 solarmass collapses without any mass loss, it would eventually be(A) a spherically symmetric black hole(B) a black hole whose event horizon is the same shape as the star atthe Schwarzschild radius(C) an irregular-shape neutron star(D) an irregular-shape white dwarf(E) a brown dwarfQ3. What is so special about 1.5 Schwarzschild radius ofSchwarzschild black holes?(A) It defines the size of event horizon(B) It defines the size of singularity surface(C) It is the radius at which tidal field becomes infinity(D) It is the radius at which you can see your back(E) It is the radius below which no photon can escapeQ4. Choose the black hole that can not exist (even briefly) innature(A) A spherical black hole with M=5Msun(B) A elliptical 10Msun black hole rotating with a half of speed of light(C) A cigar shaped black hole with 10km long, 2km thick whoseSchwarzschild radius is 5km(D) A spherical black hole with M=5Msun, and electric charge of 10Coulomb(E) None of the aboveQ5. What does no hair conjecture state?(A) A black hole is completely characterized by mass, electric charge,and spin.(B) A black hole is completely characterized by mass, electricresistance, and spin.(C) A black hole is completely characterized by mass, electricconductivity, and ellipticity.(D) A black hole is completely characterized by mass, magneticmoment, and spin.(E) A black hole is completely characterized by mass, magneticmoment, and ellipticity.Q6. If a star collapsing into a black hole has a tall mountain as wellas magnetic field anchored on the surface, what would happen?(A) The star will radiate gravitational waves, then leave a sphericalevent horizon anchoring magnetic field on the surface.(B) The star will radiate gravitational waves and electromagnetic waves,then leave only a spherical event horizon.(C) The star will radiate electromagnetic waves, then leave an eventhorizon with the mountain.(D) The star will leave an event horizon with both the mountain and themagnetic field.(E) The star will be electrically charged black hole with the mountain.Q7. Which of the following descriptions about Kerr black hole isnot correct?(A) It is a rotating black hole.(B) The event horizon bulges out at equatorial plane.(C) Space drag effect forces all particles nearby co-rotate with the blackhole.(D) The black hole has a singularity (a sphere with radius of Plancklength) at the center.(E) One can extract maximum 29% of the rest mass energy by Penroseprocess.Q2. Which of the following information was essential forastronomers when they estimated that about 0.1 percent of totalstars are black holes in the Milky way?(A) the observed density of black holes.(B) the observed initial mass function of stars in the open clusters(C) the theoretical estimation of the lifetime of the main sequence stars(D) the total number of supernovae in the Galaxy(E) the age of the UniverseQ3. According to your teacher, which of the following phrasecorrectly describes what astronomers nowadays think about theobservational evidence of the black holes?(A) “If not a black hole, we don’t know what else could it be!”(B) “They are definitely black holes.”(C) “They are definitely not black holes.”(D) “All evidences are not conclusive at all, and we have no idea as ofknow what they are.“(E) “Black holes are so weird, and there must be a law of nature thatprevents its existence.”Q4. Which of the following will be the strongest, unequivocalproof of the existence of a black hole?(a) A test body that survives when passing through the event horizon ofa black hole.(B) There must be a region from which no light can escape.(C) A stellar object that shows strong X-ray luminosity.(D) A stellar object that shows prominent jet in radio band.(E) A micro-lensing event from an object massive than 3 solar masses.Q5. In order to observe an event horizon of Sagittarius A* (a 4million-solar-mass object, 8kpc away from the Earth), we needs atelescope with minimum 10 micro arc-seconds resolution. Whatshould be the minimum resolution of the telescope which isrequired to observe an event horizon of the central object (4billion solar masses) at M87 that is 16 Mpc away from the Earth?(A) 1 micro arc-second(B) 2.5 micro arc-second(C) 5 micro arc-second(D) 10 micro arc-second(E) 20 micro arc-secondQ6. Why did astronomers build the VLBI (very long baselineinterferometry) telescope whose effective aperture is as big as thesize of the Earth?(A) To enhance an international friendship(B) To increase the light collecting area(C) To increase the angular resolution(D) To enhance the effective observing time(E) To minimize the effect of observing condition that varies site by siteQ7. In the figure below, what is the significance of the sharpcircular edge?(A) The edge shows the event horizon.(B) The edge shows the singularity surface.(C) The edge shows the photon sphere.(D) The edge shows the surface of the starcollapsed into the black hole.(E) The edge shows the inner-most stablecircular orbit.Q8. Which of the following is not a stellar mass black holecandidate?(A) An 1.4 solar masses companion in the binary system which isoptically dark, but X-ray bright.(B) A 3 solar mass isolated object that makes the star behind at theexact line of sight brighter for a hundred of days.(C) An object which emits gamma-ray burst for 10 seconds.(D) A 5 solar-mass object which is dark in optical but bright in X-ray, alsoit ejects strong radio jet.(E) All are black hole candidates.Q9. Which of the following statements about accretion disk is notcorrect?(A) Accretion disk converts gravitational potential energy first to the heatenergy, then to the radiation.(B) Because energy is transferred to outward, inner part of the accretiondisk is cooler than the outer part.(C) Because angular momentum is transferred to outward, outer part ofthe accretion disk rotates faster than what it would rotate otherwise.(D) There is a gap between innermost radius of he accretion disk andthe event horizon of the central black hole.(E) An object falling into a Kerr black hole can convert up to about 40percent of its rest mass energy into the radiation.Q10. What evidence most convincingly suggests that there is asupermassive 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 avortex.(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 MilkyWay indicate a large mass in a small space.Q11. Where do the x-rays in an x-ray binary like Cyg X-1 comefrom?(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 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.Q12. If x-rays from an AGN vary on a timescale of one day, thenthe 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.Q13. What is the source of energy for the x-ray emission from anx-ray binary or AGN?(A) Chemical energy.(B) Nuclear fission.(C) Nuclear fusion.(D) Gravity.(E) Alternative energy.Q14. What do astronomers consider to be the best currentobservational 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 keepspassing observational tests.Q15. How are GRBs (Gamma-Ray Bursts) related to black holes?(A) The long GRBs are associated with core-collapsing supernovaefrom which black holes may be formed.(B) The short GRBs are the signature of black holes formation.(C) Isolated black holes emit strong gamma-rays.(D) Distribution of GRBs are uniform over the sky.(E) Because its afterglow show clear optical signature, GRBs cannot beblack holes.Q16. According to Blandford-Znajek process, where is the energyof the jet coming from?(A) Accretion disk(B) Magnetic field(C) Black hole(D) Accreting material(E) Thermonuclear reaction around the event horizon

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