astronomy exam 3 chapter 14

how are we able to see most black holes?

We see the accretion disks around the black holes.

Schwarzschild radius

The distance from a black-hole singularity where the escape velocity equals the speed of light Not a physical boundary; purely mathematical. Defines a point of no return for all objects falling into a black hole, because nothing can travel faster than c.

Black Holes how it's formed.

- Mass greater than 20-30 M sun -The star collapses on itself. With nothing to restrain the inward crush of gravity, it collapses further toward an ever-smaller radius. -Because a zero radius gives a zero volume, the stellar density shoots toward infinity as the radius shrinks to nothing.

supermassive black holes

10 to the 6th - 10 to the 9th

• stellar-mass black holes

2.7 - 100 MASS

When the sister on the rocket returns to Earth, she is 21 years old. How old is the sister who stayed behind on Earth?

30 years old

The escape velocity of the Earth is about 11 km/s. The sun has an escape velocity of 600 km/s. A neutron star can have an escape velocity of 150,000 km/s. What is the escape velocity at the event horizon of a black hole?

300,000 km/s (the speed of light)

Sagittarious A has a mass of

4 million

It wont happen on its own, but in order to form a black hole with a star the mass of our sun, it would need to be compressed to a diameter of only

6km

Cosmic Ray

A cosmic ray is a high-speed subatomic particle (atomic nucleus) from space.

Muon Experiments

A muon is an elementary, extremely short-lived particle that is heavier than an electron.

A singularity

A singularity is the central point of a black hole, where density and space- time curvature are infinite. It is essentially a geometric point!

What is Sagittarius A*?

A supermassive black hole located in the center of the Milky Way

Twin Paradox

A twin gets on the rocket moving near the speed of light and a twin stays on earth - the twin on the rocket takes one year to get back while the twin on the earth ages 10 years.

how do we know there's a supermassive black hole at the center of the Milky Way

All stars have crazy orbits around the center of the milky way.

existence of gravity waves

Astronomers used the binary pulsar PSR 1913+16 to infer the existence of gravity waves, ripples in the fabric of space-time.

GPS includes the effects of

BOTH special relativity and general relativity.

What will happen to the Earth when the sun becomes a black hole?

Bad question. The sun will never be able to form a black hole

twin paradox contined...

During the trip, when the rocket is moving with a constant velocity, it is not possible to say which frame of reference is moving. This is the paradox! The paradox is resolved because only one twin feels acceleration.

A muon is a type of subatomic particle. If a muon is at rest in the laboratory, it will decay into an electron after about 2 microseconds. Suppose an observer watches a muon travel through the atmosphere at 99 percent of the speed of light. How does the lifetime of the moving muon compare to the muon at rest?

It is longer.. According to the ideas of Special Relativity, observing the moving reference frame, we would see the clock tick slower in the moving reference frame (time dilation). In addition, we would measure the "mass" of the object to increase. If we could ride along with the muon, we would see the distance as shorter than in the laboratory reference frame, whcih is length-contraction.

Jets

Jets of material can move at velocities of 0.99 c and are seen hurtling away from microquasars.

Tidal Force of a black hole

Near a black hole, the force fo gravity on your feet is significantly larger than on your head. Spaget

Why is the event horizon the edgy of the black hole

Point of no return If you get any closer, nothing can escape, it is considered the edge

The Planck length

The Planck length is the size scale at which space-time should begin showing its quantum nature, about 10-44meters (a hundred million billion billion billion billionth of a meter); smaller than the nuclei of atoms.

Why is the Schwarzschild radius significant?

The escape velocity is the speed of light which means that nothing, not even light can escape from within this radius.

Curved Space-Time

The general theory of relativity states that matter-energy tells space-time how to bend, and space-time tells matter-energy how to move.

Special theory of relativity Two postulates

The laws of physics are the same for all observers in inertial reference frames**. The speed of light in a vacuum is the same for all observers in inertial reference frames**. **An inertial frame reference is moving with a constant velocity relative to another reference frame.

General Theory of Relativity

The theory to replace Newton's Law of Gravity. It DOES explain why gravity is felt. Deals with large gravitational fields OR large accelerations (non-inertial reference frames).

what would we see if someone fell into a blackhole with a laser

You will see the laser change: the blinks of the laser will get longer as their "clock" slows down until they are infinitely slow - the color will become redshifted - ex: changing from violet to red/green etc.

Gravitational redshift -

a photon escaping moving away from a gravitational body will change frequency as it moves away. The greater the gravitational field, the greater the redshift will be.

a black hole is

a region of space-time whose strong gravitational distortion prevents anything, including light, from escaping. These stars collapse into apparently infinite densities and zero radii, a singularity!

Lorentz Factor

a term that designates how much an object's time, space, & mass are altered as a result of its motion. The factor is very close to 1 except at speeds approaching the speed of light.

According to Albert Einstein, there is no perceptible difference between a gravitational field and ___________________________. This is known as the Equivalence Principle.

an accelerating reference frame

Microquasars

are binaries in which one of the companions is a black hole or a neutron star.

GPS satellite system -

at launch engineers did not believe that relativity would be needed for doing the precise positioning calculations. One day of relativistic effects made it 10 km off! When the relativistic corrections were included, it worked perfectly!

Two very precise clocks are sychronized together at sea level. Clock A is kept at sea level, and clock B is taken to the top of Mount Everest for a year. Clock B is returned to sea level and the time on the two clocks is compared. Which of the following would be expected as a result of general relativity

clock A will be behind clock B because it has been running slower.

In March 2011, a star which wandered in too close to a black hole was

completely ripped apart and briefly flared to be as bright as a trillion suns

In 1783, British geologist John Mitchell

discovered that if the mass M of a central object was large enough, or its radius R was small enough, then the escape velocity would be greater than the speed of light: ve > c.

When light escapes from a black hole, it appears to lose energy. This is known as

gravitational redshift

When light escapes from outside the event horizon of a black hole, it appears to lose energy. This is known as

gravitational redshift

Gravitational lensing

is the change in the path of light passing a massive object produced by the curvature of space-time.

Gravitational lensing -

light that passes through a gravitational field is bent.

The temperature of the accretion disk

may be 1 million K and so it emits most strongly in the x-ray part of the spectrum.

Muon flux at the surface -

muons created in the upper atmosphere by cosmic rays (particles from distant stars or galaxies) should not be able to make it to the Earth's surface. They do because they are travelling near the speed of light.

gravitational free-fall

must also be equivalent to an inertial state, even though the capsule and its inhabitant are accelerating (changing direction and speed).

Einstein said that the equivalence principle is...

no perceptible difference between a gravitational field and an accelerating frame of reference, so they must be the same.

If a supernova remnant core has a mass greater than 2.8 solar masses, then what force will be able to stop gravity from collapsing the core?

nothing; it is not possible to stop gravity in this case

James Clerk Maxwell

provided an explanation of light as an electromagnetic wave before Einstein was born.

Which of the following is an effect of Special Relativity?

relativistic mass, time dilation and length contraction

If you were to fall feet first into a 10 solar mass black hole, the tidal forces would

spaghettify you.

A black hole with a mass between 3 solar masses and 12 (it could be more than this though) solar masses is considered to be a

stellar black hole

At the core of nearly every galaxy is higher mass black hole. The first one that was conclusively observed is the one at the center of the Milky Way with a mass of more than 4 million solar masses. These black holes at the centers of glaxies are known as a

supermassive black hole

At the centers of all galaxies

supermassive black holes.

Cosmic censorship is the conjecture

that the Universe will not allow the existence of a singularity without an event horizon. - The cosmologist Stephen Hawking proposed the principle of cosmic censorship in which naked singularities (singularities lacking an event horizon) would never occur.

The no-hair theorem

the description of a black hole as having only three properties—mass, spin, and electric charge—that never get erased when matter crosses the event horizon.

Length Contraction -

the distance measured by person on the moving rocket (L) will be shorter than the distance measured by the stationary observer (L ).

relativistic mass

the mass of a moving object measured in the frame of reference of the object is called the proper mass (m ) and it will be smaller than the mass measured by a stationary observer (m).

Physicists Michaelson and Morley set up experiments with an aether (like the river) and light (like the boat) but could not find an aether current

the relative flow of the aether past a moving observer. -Einstein refuted the aether problem and assumed that the speed of light is always constant!

According to general relativity, the stronger the gravitational field (or the greater the acceleration),

the slower the clock will appear to tick.

The strange thing is that time will not pass the same for people in different gravitational fields, according to an outside observer. The greater the warping of spacetime (the stronger the gravitational field)

the slower the clocks will appear to tick

Imagine that a spaceship is traveling at half the speed of light relative to a stationary observer. The spaceship shoots a laser in the direction of its motion. How fast is the laser light traveling relative to the stationary observer?

the speed of light.

Gravitational Time Dilation -

the stronger the gravitational field strength, the slower a clock will tick. This means that clocks will tick a little faster at the top of a mountain than they will at sea level and even faster in space.

time dilation

the time for the observer on the ground is longer than the time for the person on the rocket! The light travelled a longer path, which must take more time. This is called time dilation.

Time Dilation

the time measured by a stationary observer (t) will be longer than the time measured by the person on the moving rocket

Proper time is found by the formula

time= distance over speed

Hafele-Keating experiment -

two atomic clocks compared after one is flown in a plane. The clock on the ground measures MORE time passing.