Black holes

ByJAHNAVI

A Brief History Of Black Holes.

First proposed by John Mitchell in 1783

Later in 1916 Karl Schwarzschild solves Einstein’s field equation for space time

In 1887, MichelSon and Morley ‘s experiment said that light travels at a speed of 186,282 miles per second.

General RelativityPublished in 1915Describes the

relationship between mass and space_time.

Space_time was not flat but it is wrapped and curved by matter and energy in it.

Confirmed in 1919 by Arthur Eddington.

Spacetime:

• Apparent vs. real trajectory of a light beam

Event horizonA boundary in space time through which only

matter and light only can pass through inwards.Not even light can escape from inside a event

horizon.To a distant observer, clocks near a black hole

appear to tick more slowly than those further away from the black hole.

Due to gravitational time dilation, an object falling into a black hole appears to slow as it approaches the event horizon.

any light emitted by the object to appear redder and dimmer, an effect known as gravitational redshift.[55] Eventually, the falling object becomes so dim that it can no longer be seen.

FormationGravitational collapseThe gravitational collapse of heavy stars is

assumed to be one of the reasons for the formation of stellar mass black holes.

Occurs when an object's internal pressure is insufficient to resist the object's own gravity.

For stars this usually occurs either because a star has too little "fuel" left to maintain its temperature .

• Black holes could also be formed in high-energy collisions that achieve sufficient density .micro black holes can be created in the high-energy collisions that occur when cosmic rays hit the Earth's atmosphere

• They could also possibly be created in the Large Hadron Collider at CERN. These theories are very speculative, and the creation of black holes in these processes is deemed unlikely by many specialists.Even if micro black holes could be formed, it is expected that they would evaporate in about 10−25 seconds, posing no threat to the Earth.

On the basis of mass :three classifications on blackholes

• Stellar mass: 3 to 20 times the mass of the sun.

• Super massive: Black holes with millions to billions of the mass of our sun.

• Mid-Mass:In between stellar-mass and super massive.

Growth• Once a black hole has formed, it can continue to

grow by absorbing additional matter. • Any black hole will continually absorb gas and

interstellar dust from its surroundings and omnipresent cosmic background radiation.

• This is the primary process through which supermassive black holes seem to have grown.

• A similar process has been suggested for the formation of intermediate-mass black holes found in globular clusters. Another possibility for black hole growth, is for a black hole to merge with other objects such as stars or even other black holes.

singularity• Gravitational blackhole is present in the

centre of a blackhole, which is one dimensional and laws of physics cease to operate.

• A black hole’s singularity remains hidden behind its event horizon.

In 1939 , Oppenheimer shows massive stars collapse to a singular point.

Singularity surrounded by event horizon, point of no return.

A black hole can essentially be described by just three quantities: how much mass is entering into it,how fast it is spinning and its electrical charge.

Classic black holesIn 1960s, Roger Penrose prooved a theorem which showed that the gravitational collapse of a large dying star might result in a singularity. • The simplest type of blackhole in which

the core doesn’t rotate and just has a singularity and an event horizon is known as schwarzchild blackhole

• Another type of blackholes are the spinning ones discovered by Kerr.

2 main typesSchwarzschild• Static, no rotation• Point-like singularity• Event Horizon

Kerr• Rotates• Ring shaped

singularity• Inner Horizon• Outer Horizon• Ergosphere

2 main typesSchwarzschild Kerr

Can Black Holes Radiate?Maybe so…• 1970 : Jacob Berkenstein suggests area

of event horizon is a measurement of black holes entropy.

• 1970 : Stephen Hawking shows that area of event horizon always increases.

• 1971 : Borisovitch Zel’dovitch claims rotating black holes radiate until they stop spinning .

Hawking radiation:• In 1974, Hawking predicted that black holes are not entirely black but emit small amounts of thermal radiation which is known as Hawking radiation.• By applying quantum field theory to a static black hole background, he determined that a black hole should emit particles that display a perfect black body spectrum. Since Hawking's publication, many others have verified the result through various

approaches.

If Hawking's theory of black hole radiation is correct, then black holes are expected to shrink and evaporate over time as they lose mass by the emission of photons and other particles.The temperature of this thermal spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which, for a Schwarzschild black hole, is inversely proportional to the mass. Hence, large black holes emit less radiation than small black holes.If a black hole is very small, the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant.

So What’s Left?• Resolution of the “Information paradox”

• Development of an adequate quantum theory of gravity, Relativity is no longer a valid approximation on scales smaller than the Planck length, lp = (hG/2πc3)1/2 .

So Why study Black holes?• Black holes push the limits of physical

theories• Among the most extreme phenomena

in the universe• I think they’re cool.• That’s it• Thanks for listening• The End• (applause…. please?)

References :• Kip Thorne, Black Holes And Timewarps (Norton, 1994)

• Eric Poisson, A Relativist's Toolkit, The mathematics of black hole mechanics (Cambridge University Press, 2004).

• Alessandro Fabbri and Jos\'{e} Navarro-Salas,Modeling Black Hole Evaporation (Imperial College Press, 2005)

• John A. Wheeler, E.F. Taylor, Exploring Black Holes, Introduction to General Relativity (Addison Wesley, 2000)

• Stephen Hawking, The Universe In A Nutshell (Bantam, 2001)

Images :• http://chandra.harvard.edu/resources• http://www.belmontnc.4dw.net/DWFNEU5.gif• www.tqnyc.org/NYC040808/ neutron_star.jpg• http://en.wikipedia.org• www.scienceweek.at