Chapter 9 Flashcards
What is the Swartzchild’s Radius
The event horizon of a black hole
What is the event horizon of a black hole?
The radius of the part of the Black hole where nothing, not even light, can escape.
How large does a star have to be to collapse into a black hole?
More than 40 solar masses
What is an escape velocity?
The velocity required to escape the gravitational pull from any large mass
What is the escape velocity of Earth?
7 miles per second
11 kilometers per second
What is the escape velocity of a black hole?
There is no possible escape velocity, the gravitational pull of it is too strong
What happens to something that is ejected upwards from something with a gravitational pull?
It goes up, but if it has not exceeded the escape velocity then it will come back down.
What can travel faster than the speed of light?
Nothing
What is the relationship between a black hole and the Swartzchild radius?
They are directly proportional; the larger the mass, the larger the radius
What happens to time with something approaching a black hole and why?
They start to appear slower due to the curvature in space time
What happens to distances with something approaching a black hole and why?
They start to appear shorter due to the curvature in space time
Would you feel the affect of black holes on time and distance if you were in the black hole?
No. It is only the observer who would see these affects looking into something in a black hole
What happens to light before you entrant the event horizon of a black hole?
It undergoes gravitational redshift
What is gravitational redshift?
It is the lowering of the frequency of light that leaves the black hole. So, lights will appear more red when they are coming from out of a black hole
What happens to light once inside the event horizon?
It cannot leave as the escape velocity is faster than the speed of light in a vacuum
What happens to light when it crosses the event horizon?
It is frozen into the event horizon in an extremely redshifted image
What is a true singularity?
A location at which physical entities become infinite
What is coordinate singularity?
A location at which a particular coordinate system fails, such as the Schwarzschild metric coordinates at the centre of a black hole
What would happen in a spaceship going near to the speed of light (inside and outside the ship)
Inside: Nothing, it would feel completely normal. This is special relativity
Outside: time would appear to slow, and distances appear to shrink
When are tidal forces created?
When the gravitational force varies over a body
What are tidal forces like near a black hole?
Gravitational pull grows so quickly near the event horizon that a part of the probe may experience much more force than another part of a probe approaching the hole.
What is a naked singularity?
A singularity that has no event horizon around it
what is the cosmic censorship hypothesis?
The idea that there are no naked singularities
What is the relationship between the radius and the mass of a black hole?
They are proportional; the bigger the radius, the bigger the mass
What is the average density of a black hole?
1/mass squared
How strong is the gravitational pull of an black hole?
No stronger than any other object of the same mass
What is Birkhoff’s theorem?
That the gravitational field outside any spherical object cannot be affected by purely radial changes in the object
What would happen if the Sun turned to a black hole?
We would notice the absence of light, but we would continue our orbit normally
at what point does the gravity become so intense that it is impossible to resist downfall?
3 times the orbital radius
At what point does the light curve so much that it traces out a circular orbit around a black hole?
What is this called?
1.5 times the Schwarzschild radius
photon orbit
What happens within the photon orbit?
Light cannot remain in orbit. It moves either radially inward or downward
What is this bending and focusing of light called?
gravitational lensing
What is the difference between a black hole and a white hole?
Whereas in a black hole, things go in and do not come out, in a white hole things can only go out
How are white holes formed?
We do not know?
What is a wormhole?
this is a curvature produced by a black hole in the space time continuum that forms a kind of funnel
How can one pass through a Schwarzschild wormhole?
It is impossible; you have to be going greater than the speed of light
What is the condition of wormholes?
They are often unstable, doing things like pinching off
When is a black hole formed?
When a star of over 40 solar masses consumes its nuclear fuel and is left with a core greater than the Chandrasekhar limit, it collapses
How can we see black holes if they emit no light and absorb none either?
We can infer their existence from what we can see (e.g. light distortion, gravitational attraction to on another matter)
How can a black hole be detected using binary stars?
The black hole and the star may be orbiting one another and the hole’s gravitational pull may cause the stars behaviour to alter (e.g. by wiggles in the movement)
What happened with Cygnus X-1?
- star was orbiting something we couldn’t see
- Kepler’s Third Law said that it was orbiting something very large (10x that of the Sun)
- The only possibility was a BH
What is an accretion disk?
it is the gas pulled from a star that orbits the black hole in a flattened spinning disk
Why is the gas in an accretion disk flattened
It possesses some angular momentum
what happens as the gas falls into the black hole?
Some of the gravitational potential energy is converted into heat energy
What emits x-rays in black holes?
The gas falling into the hole collides with the matter already occupying the area near the hole, causing the gas to be compressed and heated
Sufficiently hot gas will emit x rays
What will be emitted by a cooler gas?
Light waves
