THE VIOLENT UNIVERSE PHYS006 TEST
QUESTIONS AND ANSWERS
You are sitting in an extremely powerful spaceship, 100 km away from a black
hole that has a mass of 30 MSun. Is there any hope that you can escape from the
gravitational pull of the black hole? Explain your answer. - ANSWER The
distance is bigger than the schwarschild radius, therefore you can escape.
If black holes cannot be seen, how can we detect them (in general)? -
ANSWER • Black holes cannot be "seen" directly, but their gravitational effects
can be detected.• Easiest to find if:• They are in close binaries (stellar-mass
black holes)• They are in centers of galaxies (supermassive black holes)
How do we detect stellar-mass black holes? - ANSWER • Detected by
observing stars that are orbiting an invisible companion that is heavier than 3
solar masses• If the visible star is close enough to the black hole, its outer layers
are attracted to black hole and form an accretion disk• Accretion disk is very
hot; emits X-rays that can be detected
How do we detect super massive black holes? - ANSWER Detected by
observing the motions ofmaterial around SMBH
What is an active galactic nucleus (AGN)? - ANSWER • Bright, accreting
SMBHs in centers of galaxies are called active galactic nuclei• The brightest of
them are called quasars or quasi-stellar objects: they are so bright, it's hard to
see the galaxies where they live
Last year, a historical announcement was made about the first detection of
gravitational waves. Explain what it was that scientists observed and why it is
important. - ANSWER They found 2 colliding black holes, we can now
observe the universe in new ways, other than just using light waves.
According to the theory of General Relativity, how does mass affect spacetime?
- ANSWER • A black hole's mass strongly warps space and time in vicinity of
, event horizon• Black hole forms a bottomless pit in spacetime• Event horizon
mark "the point of no return."
Explain a black hole according to the theory of General Relativity. - ANSWER
Einstein's theory of general relativity tells us thatgravity is caused by the
curvature of space and time (spacetime)• A massive object curves the spacetime
around it.• Far from the object, spacetime is nearly "flat"; close to the object, the
curvature forms a "well."• In Einstein's picture of gravity other objects sense the
curvature and are drawn into the "Well."
What would happen to you if you fell into a black hole? - ANSWER • As you
get closer, you feel tidal gravitational forces. You are eventually ripped apart.•
As you approach, images of far away objects may be distorted and brighter.• As
you cross event horizon, you can still see out.• Nothing can stop you; you hit
singularity within seconds
If you were to look in the direction of a black hole, images of objects behind it
would look distorted due to gravitational lensing. What does that mean? -
ANSWER Gravitational lens: a massive body that bends light passing near it, in
this case, a black hole.• A gravitational lens distorts our view of things behind
it. As I look toward the direction of a black hole, the black hole acts as a
gravitational lens, and so the images of the objects behind it appear distorted.
1. You are standing on the Earth and your friend Pete is standing on a white
dwarf. A Martian is in space, observing both of you. According to the Martian,
whose watch ticks slower, yours or Pete's? Why? - ANSWER Einstein's theory
of General Relativity tells us that time runs slow in gravitational fields. The
stronger the gravity, the slower time runs.• This is called gravitational time
dilation.• Clocks near the surface of Earth runslightly more slowly than those in
orbit.
2. If you fell into a black hole and Romily watched you from a distance, he
would never see you cross the event horizon. Why? - ANSWER • You move
more and more slowly as you approach the horizon. You never do cross the
horizon.• You become redder and redder (meaning longer and longer
wavelength) -- infrared, then radio...until he can't see you anymore.
QUESTIONS AND ANSWERS
You are sitting in an extremely powerful spaceship, 100 km away from a black
hole that has a mass of 30 MSun. Is there any hope that you can escape from the
gravitational pull of the black hole? Explain your answer. - ANSWER The
distance is bigger than the schwarschild radius, therefore you can escape.
If black holes cannot be seen, how can we detect them (in general)? -
ANSWER • Black holes cannot be "seen" directly, but their gravitational effects
can be detected.• Easiest to find if:• They are in close binaries (stellar-mass
black holes)• They are in centers of galaxies (supermassive black holes)
How do we detect stellar-mass black holes? - ANSWER • Detected by
observing stars that are orbiting an invisible companion that is heavier than 3
solar masses• If the visible star is close enough to the black hole, its outer layers
are attracted to black hole and form an accretion disk• Accretion disk is very
hot; emits X-rays that can be detected
How do we detect super massive black holes? - ANSWER Detected by
observing the motions ofmaterial around SMBH
What is an active galactic nucleus (AGN)? - ANSWER • Bright, accreting
SMBHs in centers of galaxies are called active galactic nuclei• The brightest of
them are called quasars or quasi-stellar objects: they are so bright, it's hard to
see the galaxies where they live
Last year, a historical announcement was made about the first detection of
gravitational waves. Explain what it was that scientists observed and why it is
important. - ANSWER They found 2 colliding black holes, we can now
observe the universe in new ways, other than just using light waves.
According to the theory of General Relativity, how does mass affect spacetime?
- ANSWER • A black hole's mass strongly warps space and time in vicinity of
, event horizon• Black hole forms a bottomless pit in spacetime• Event horizon
mark "the point of no return."
Explain a black hole according to the theory of General Relativity. - ANSWER
Einstein's theory of general relativity tells us thatgravity is caused by the
curvature of space and time (spacetime)• A massive object curves the spacetime
around it.• Far from the object, spacetime is nearly "flat"; close to the object, the
curvature forms a "well."• In Einstein's picture of gravity other objects sense the
curvature and are drawn into the "Well."
What would happen to you if you fell into a black hole? - ANSWER • As you
get closer, you feel tidal gravitational forces. You are eventually ripped apart.•
As you approach, images of far away objects may be distorted and brighter.• As
you cross event horizon, you can still see out.• Nothing can stop you; you hit
singularity within seconds
If you were to look in the direction of a black hole, images of objects behind it
would look distorted due to gravitational lensing. What does that mean? -
ANSWER Gravitational lens: a massive body that bends light passing near it, in
this case, a black hole.• A gravitational lens distorts our view of things behind
it. As I look toward the direction of a black hole, the black hole acts as a
gravitational lens, and so the images of the objects behind it appear distorted.
1. You are standing on the Earth and your friend Pete is standing on a white
dwarf. A Martian is in space, observing both of you. According to the Martian,
whose watch ticks slower, yours or Pete's? Why? - ANSWER Einstein's theory
of General Relativity tells us that time runs slow in gravitational fields. The
stronger the gravity, the slower time runs.• This is called gravitational time
dilation.• Clocks near the surface of Earth runslightly more slowly than those in
orbit.
2. If you fell into a black hole and Romily watched you from a distance, he
would never see you cross the event horizon. Why? - ANSWER • You move
more and more slowly as you approach the horizon. You never do cross the
horizon.• You become redder and redder (meaning longer and longer
wavelength) -- infrared, then radio...until he can't see you anymore.
