Explaing and higlighting the recent researches about the black hole , in recent times..

1. Physics Assignnment
Submitted By : Jaspreet Singh
Ruchit Porwal
Submitted To : Hukmi
suthar sir

2. Topic: What Have we learned about black holes in the
universe in recent times.

3. What Is a Black Hole .. ?
John Mitchell (1973) :
a star that was sufficiently massive and compact, would have such a strong
gravitational field that light could not escape.
Such objects are what we now call black holes, because that is what they are,
black voids in space
Marquis de La~plass :
Both Michell and La~plass thought of light as consisting of particles, rather like
cannon balls, that could be slowed down by gravity, and made to fall back on the
star.
But a famous experiment, carried out by two Americans, Michelson and Morley
in 1887.
Showed that light always traveled at a speed of one hundred and eighty six
thousand miles a second, no matter where it came from. How then could gravity
slow down light, and make it fall back. This was impossible, according to the then
accepted ideas of space and time.

4. Sir Albert Einstein (1915):
General Theory of Relativity
Space and time single quantity (not separate and independent)
This spacetime was not flat, but was warped and curved by the matter and
energy in it.
In order to understand this, considered a sheet of rubber, with a weight placed
on it, to represent a star. The weight will form a depression in the rubber, and
will cause the sheet near the star to be curved, rather than flat. If one now rolls
marbles on the rubber sheet, their
paths will be curved, rather than being straight lines
Consider now placing heavier and heavier, and more and more concentrated
weights on the rubber sheet. They will depress the sheet more and more.
Eventually, at a critical weight and size, they will make a bottomless hole in
the sheet, that particles can fall into, but nothing can get out of.

5. Structure and Propertise:
Physical properties:
The simplest static black holes have mass but neither electric charge nor angular
momentum. These black holes are often referred to asSchwarzschild black holes after Karl
Schwarzschild who discovered this solution in 1916.According to Birkhoff’s theorem, it is
the only vacuum solution that is spherically symmetric .This means that there is no
observable difference between the gravitational field of such a black hole and that of any
other spherical object of the same mass. The popular notion of a black hole "sucking in
everything" in its surroundings is therefore only correct near a black hole's horizon; far
away, the external gravitational field is identical to that of any other body of the same
mass.
Black hole classifications
Class Mass Size
Supermassive black hole ~105–1010 MSun ~0.001–400 AU
Intermediate-mass black hole ~103 MSun ~103 km ≈ REarth
Stellar black hole ~10 MSun ~30 km
Micro black hole up to ~MMoon up to ~0.1 mm

6. Event Horizon :
• Falling through the event horizon, is a bit like going over Niagra Falls in a canoe. If
you are above the falls, you can get away if you paddle fast enough, but once you are
over the edge, you are lost.There's no way back. As you get nearer the falls, the current
gets faster. This means it pulls harder on the front of the canoe, than the back. there's a
danger that the canoe will be pulled apart. It is the same with black holes.
• If you fall towards a black hole feet first, gravity will pull harder on your feet than
your head, because they are nearer the black hole. The result is, you will be stretched
out longwise, and squashed in sideways..
• If the black hole has a mass of a few times our sun, you would be torn apart, and made
into spaghetti, before you reached the horizon. However, if you fell into a much larger
black hole, with a mass of a million times the sun, you would reach the horizon without
difficulty.

7. • As you fell into a black hole, someone watching you from a distance, would never see
you cross the event horizon. Instead, you would appear to slow down, and hover just
outside. You would get dimmer and dimmer, and redder and redder, until you were
effectively lost from sight.
• As far as the outside world is concerned, you would be lost for ever. Because black
holes have no hair, in Wheeler's phrase, one can't tell from the outside what is inside a
black hole, apart from its mass and rotation. This means that a black hole contains a lot
of information that is hidden from the outside world.
• Information requires energy, and energy has mass, by Einstein's famous equation, E =
m c squared. So if there's too much information in a region of space, it will collapse into
a black hole, and the size of the black hole will reflect the amount of information.
• It is like piling more and more books into a library. Eventually, the shelves will give
way, and the library will collapse into a black hole.

8. Particles can leak out of a black hole :
The reason is, that on a very small scale, things are a bit fuzzy. This is summed up in the
uncertainty relation, discovered by Werner Heisenberg in 1923, which says that the more
precisely you know the position of a particle, the less precisely you can know its speed, and
vice versa. This means that if a particle is in a small black hole, you know its position fairly
accurately. Its speed therefore will be rather uncertain, and can be more than the peed of
light, which would allow the particle to escape from the black hole.
Mini black holes :
A black hole of the mass of a mountain, would give off x-rays and gamma rays, at a rate
of about ten million Megawatts, enough to power the world's electricity supply. It wouldn't be
easy however, to harnass a mini black hole. You couldn't keep it in a power station, because it
would drop through the floor, and end up at the center of the Earth. About the only way,
would be to have the black hole in orbit around the Earth.

9. • As particles escape from a black hole the hole will lose mass, and shrink.This will
increase the rate of emission of particles. Eventually, the black hole will lose all its
mass, and disappear.
•If information were lost in black holes, we wouldn't be able to predict the future,
because a black hole could emit any collection of particles. It could emit a working
television set, or a leather bound volume of the complete works of Shakespeare,
though the chance of such exotic emissions is very low
• The point is, that from the outside, one can't be certain whether there is a black
hole, or not. So there is always a chance that there isn't a black hole. This possibility
is enough to preserve the information, but the information is not returned in a very
useful form. It is like burning an encyclopedia..
•Information is not lost if you keep all the smoke and ashes, but it is difficult to
read. Kip Thorne and Stephen hawking had a bet with John Preskill, that
information would be lost in black holes. When Stephen hawking discovered
how information could be preserved, He conceded the bet. He gave John Preskill
an encyclopedia. Maybe given him the ashes.

10. Singularity :
•At the center of a black hole, as described by general relativity, lies
a gravitational singularity, a region where the spacetime curvature becomes
infinite.
•For a non-rotating black hole, this region takes the shape of a single point and for
a rotating black hole, it is smeared out to form a ring singularity that lies in the
plane of rotation .
•In both cases, the singular region has zero volume. It can also be shown that the
singular region contains all the mass of the black hole solution.The singular
region can thus be thought of as having infinite density.
• Observers falling into a Schwarzschild black hole (i.e., non-rotating and not
charged) cannot avoid being carried into the singularity, once they cross the event
horizon.
•They can prolong the experience by accelerating away to slow their descent, but
only up to a limit; after attaining a certain ideal velocity, it is best to free fall the
rest of the way.

11. •. When they reach the singularity, they are crushed to infinite density and their
mass is added to the total of the black hole. Before that happens, they will have
been torn apart by the growing tidal forces in a process sometimes referred to
as spaghettification or the "noodle effect".
• In the case of a charged (Reissner–Nordström) or rotating (Kerr) black hole, it is
possible to avoid the singularity. Extending these solutions as far as possible
reveals the hypothetical possibility of exiting the black hole into a different
spacetime with the black hole acting as a wormhole .
•The possibility of traveling to another universe is however only theoretical.

12. •What does this tell us about whether it is possible to fall in a black hole, and
come out in another universe.
• The existence of alternative histories with black holes, suggests this might be
possible. The hole would need to be large, and if it was rotating, it might have
a passage to another universe.
•But you couldn't come back to our universe.

13. Formation and evolution
Gravitational collapse
•Gravitational collapse 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 through stellar nucleosynthesis, or
because a star that would have been stable receives extra matter in a way that does not
raise its core temperature.
•In either case the star's temperature is no longer high enough to prevent it from
collapsing under its own weight . The collapse may be stopped by thedegeneracy
pressure of the star's constituents, allowing the condensation of matter into an
exotic denser state. The result is one of the various types of compact star.
•The type of compact star formed depends on the mass of the remnant of the original
star left after the outer layers have been blown away. Such explosions, from
a supernova explosion or by pulsations, leads to planetary nebula. Note that this mass
can be substantially less than the original star

14. High Energy Collisions :
•A simulated event in the CMS detector, a collision in which a micro black hole
may be created.
•Gravitational collapse is not the only process that could create black holes. In
principle, black holes could be formed in high-energy collisions that achieve
sufficient density. As of 2002, no such events have been detected, either directly or
indirectly as a deficiency of the mass balance in particle accelerator experiments.
• This suggests that there must be a lower limit for the mass of black holes.
Theoretically, this boundary is expected to lie around the Planck
mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are
expected to invalidate the predictions of general relativity.
•This would put the creation of black holes firmly out of reach of any high-energy
process occurring on or near the Earth.

15. 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. Although not necessary for
growth, this is thought to have been important, especially for the early
development of supermassive black holes, which could have formed from the
coagulation of many smaller objects. The process has also been proposed as the
origin of some intermediate-mass black holes.

16. Observatinal evidence
•Black holes of stellar mass are expected to form when very massive stars collapse
at the end of their life cycle. After a black hole has formed, it can continue to grow
by absorbing mass from its surroundings. By absorbing other stars and merging
with other black holes, supermassive black holes of millions of solar masses (M☉)
may form. There is general consensus that supermassive black holes exist in the
centers of most galaxies.
•In this way, astronomers have identified numerous stellar black hole candidates
in binary systems, and established that the radio source known as Sagittarius A*, at
the core of our own Milky Way galaxy, contains a supermassive black hole of about
4.3 million solar masses.

17. Proper motions of stars orbiting Sagittarius A* :
The proper motions of stars near the center of our own Milky Way provide strong
observational evidence that these stars are orbiting a supermassive black hole .
Since 1995, astronomers have tracked the motions of 90 stars orbiting an invisible
object coincident with the radio source Sagittarius A*.

18. • By fitting their motions to Keplerian orbits, the astronomers were able
to infer, in 1998, that a 2.6 million M☉ object must be contained in a
volume with a radius of 0.02 light-years to cause the motions of those
stars. Since then, one of the stars—called S2—has completed a full orbit.
: Simulation of gas cloud after close approach to the black hole at the centre of
the Milky Way.

19. • From the orbital data, astronomers were able to make refine the calculations of
the mass to 4.3 million M☉ and a radius of less than 0.002 light years for the
object causing the orbital motion of those stars.
•The upper limit on the object's size is still too large to test whether it is smaller
than its Schwarzschild radius; nevertheless, these observations strongly suggest
that the central object is a supermassive black hole as there are no other plausible
scenarios for confining so much invisible mass into such a small volume.
• Additionally, there is some observational evidence that this object might
possess an event horizon, a feature unique to black holes.

20. Conclusion :
•The message of this , is, that black holes ain't as black as they are
painted. They are not the eternal prisons they were once thought.
•Things can get out of a black hole, both to the outside, and possibly,
to another universe. So, if you feel you are in a black hole, don't give
up. There's a way out.