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Gravity
The Fundamental Forces of Nature
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There exist only four fundamental forces
Electromagnetism
Weak force
Strong force
Gravity
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The Hierarchy Problem
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Gravity is far weaker than any of the forces!
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Why?!?
NO ONE KNOWS!!!
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Newton’s Law of Gravity
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Newton first told us about gravity
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Newton’s gravitational force
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This force holds together the BIGGEST
objects in the Universe!
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Stars
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This force holds together the BIGGEST
objects in the Universe!
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Nebulae
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This force holds together the BIGGEST
objects in the Universe!
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Galaxies
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This force holds together the BIGGEST
objects in the Universe!
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Clusters of Galaxies
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This force holds together the BIGGEST
objects in the Universe!
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Filamentary structure
GRAVITY CONTAINS THE
BIGGEST MYSTERIES IN THE
UNIVERSE!
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Einstein’s Theory of Gravity
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Einstein changed Newton’s
theory of gravity.
Newton said that gravity was a
force
Careful experiments showed
problems with Newton’s theory!
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Einstein’s Theory of Gravity
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Gravitational lensing
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Einstein’s Theory of Gravity
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Einstein said space is curved!
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Einstein’s Theory of Gravity
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Einstein’s said gravity is much more complicated!
What else does Einstein say?
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Remember the Escape Velocity
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Throw a ball into the
air
By conservation of
energy
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Remember the Escape Velocity
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This is the escape
velocity of the Earth
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Gravity
For Earth it’s about
11 km/sec!
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How High Can the Escape Velocity Go?
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We can make the
velocity higher by
increasing the mass.
For the sun, vesc ~ 618 km/s
Or, we could keep the mass fixed, and squeeze it
into a ball of smaller radius!
The biggest the escape velocity can get is fixed by the
cosmic speed limit, the speed of light c!
Suppose we want to squeeze a body of mass M
into such a small ball that its escape velocity is
equal to the speed of light.
The Schwarzschild Radius
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Even Light Can’t Escape!
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If the radius of the star is less than
the Schwarzschild radius, then not
even light can escape its gravitational
pull! The event horizon!
All of this has been done assuming Newtonian gravity!
Einstein said Newton was wrong!
If you do it right using the correct theory of gravity, you
find exactly the same answer! This was shown by Karl
Schwarzschild not long after Einstein discovered General
Relativity.
So, we really do have a solution describing a region that’s so
dense that not even light can escape -
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Even Light Can’t Escape!
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By looking at Einstein’s theory we can learn so much more!
So, we really do have a solution describing a region that’s so
dense that not even light can escape - a black hole!
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Black Holes
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Let’s look at black holes a little more closely.
What happens at the Schwarzschild
radius?
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What happens at the center?
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The black hole contains a singularity in its core!
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To completely understand the singularity we need a quantum
theory of gravity, which is still completely unknown!
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Properties of Black Holes
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Black holes are not cosmic vacuum cleaners!
You can have a stable orbit around a black hole (as long as
you’re further than 3 times the Schwarzschild radius)!
Suppose we watch a helpless astronaut falling down into the
black hole. What would we see?
Time stops at the event horizon! We would see the
astronaut frozen on the horizon forever! (That’s not quite
right…)
The astronaut would see nothing special as he passed
through the horizon!
BOTH ARE RIGHT!
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Black Holes Aren’t Completely Black!
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According to Einstein’s General
Relativity, black holes are one-way
tickets - things enter, but never
leave.
Einstein never believed in quantum
mechanics.
Quantum mechanics allows for
particle pair creation!
Hawking reconsidered black holes
using quantum mechanics.
Einstein was wrong!
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Black Holes Evaporate!
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Particle pairs are created in the
strong gravitational field around the
event horizon.
Once in a while one of the particles
falls into the black hole while the
other escapes. The hole looks like a
hot object with a certain
temperature!
To conserve energy, the hole must
lose mass! It has a finite lifetime!
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The Expansion of the Universe
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There is one more very interesting aspect of gravity to discuss,
dealing with the expansion of the Universe.
If we look out at distant galaxies they are almost all moving away
from us!
The BIG BANG!
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The Cosmological Constant
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If gravity is always attractive, then the expansion of the
Universe should be slowing down.
There are other possibilities...
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The Cosmological Constant,
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λ
Einstein added this to his equations to get a static Universe.
After it was found that the Universe was expanding he banished
it from his equations, calling it his “biggest blunder!”
For ~ 60 years it was believed to be zero
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The Cosmological Constant ,

Observations found it!
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λ
Observations differ from theory…
THE BIGGEST MYSTERY IN PHYSICS!!!
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