Stephen Hawking and the Universe
a fascinating story
Spiral Galaxy
Dr. Mounib Eid
AUB, Physics Department
meid@aub.edu.lb
CVSP204:
Fall-2015
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Spiral Galaxy like ours
‫كالم جميل‬
The scientists does not study nature because it is useful;
he/she studies it because he/she delights in it, and he/she
delights in it because it is beautiful. If nature were not beautiful,
it would not be worth knowing, and if nature were not worth
knowing, life would not be worth living.
Who? Henry Poincare
My translation
‫العالم اليدرس الطبيعة ألنها مفيدة بل ألنه يجد متعة بجمالها‬
‫لو كانت الطبيعة غير جميلة ليس من متعة في معرفتها‬
‫ومعنى الجمال‬, ‫وأن لم نكن نتوق لمعرفة الطبيعة فما هو معنى الحياة‬2
Timeline of the universe
The Universe was not in hurry to form the first stars:
about 400 million years after creation
380,000 years First atoms formed
Microwave Background radiation escaped
13.7 billion years
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Short Outline
Part I: Picture of the Universe
Interesting History
Space-Time & Relativity & and Reality
Part II: Universe
Big Bang
Inflation
Cosmic History
Anthropic Principle
Part III: Relativity & Black Hole (if time available)
My aim in this lecture is three-fold:
not to tell trivial things
Avoid Equations
Initiate interests and thinking
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Part I: Picture of the Universe
1. Interesting history
 Aristotle (340 B.C.) :
Earth spherical. He realized that the Moon is eclipsed by the Earth and
Earth’s shadow was round. He thought: Earth stationary, Sun and Moon
circle the Earth- he believed in the perfection of the circles. This was a
philosophical bias
 Eratosthenes (276-194 B.C.) : (eratostenes)
has determined the diameter of the Earth rather accurate (see details at
the end of the file}. Human beings were always intelligent.
This measurement was done 200 B.C. by this Greek Philosopher
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 Ptolemy: (100-170 A.D.) “Almagest”
Geocentric model of the universe
Seven of
these
turtles
Epicycles and Planetary Motion
This model was suggested to resolve the
retrograde motion
tortis
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Copernicus:
Polish (1473-1543):
Heliocentric model of the Universe, 1514
removed Earth from the center, put the Sun
instead. He was afraid of the church. Hundred
years passed before the idea was take seriously.
Great achievement:
Sun at center (not the Earth) and at rest.
 Only the Moon orbits the Earth.
 Retrograde motion of planets a consequence of the
Earth’s motion relative to a planet.
 All planets revolve around the sun.
Copernicus model represents an exceptional turning point
in the history of science and human thought. He once for
all displaced us from our centric view of ourselves that
Wait for animation
our planet is a focal point.
Copernicus writings on the heliocentric universe
were placed on the Church’s index of prohibited
books in 1616. They have been published after 73
years, scientific truth wins always. Well,
meanwhile Vatican has observatory! It is a new
relation between science and religion
Mars
Sun
Earth
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GALILEO GALILEI:
Italian (1564- 1642)
Has used a self-made telescope in 1604
and his observation led to a breakthrough.
Galileo found:
 Moon shows mountains and craters,
 Dark spots on the surface of the sun
(sunspots). He found that the Sun is
rotating
 Galileo also observed Jupiter. He saw
FOUR small moons orbiting Jupiter
(invisible to the naked eye). This was
another conflict with the Greek’s model:
The Earth is not the only center of rotation
Amazing
Galileo published his results, and this was a play with fire. The Church
(Inquisition) put him under house arrest in 1633 until his death.
Firstly, in 1992, Galileo was rehabilitated by the pope in Rome. A new era
began having a healthy relation between science and religion.
But why we have problems now???
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JOHANNES KEPLER:
German: (1571-1630)
Found a simple striking structure of the solar system
He formulated three laws
1. he orbits of the planets are ellipses with the Sun at
one focus
2. An imaginary line connecting
the sun to any planet sweeps out
equal areas of the ellipse in equal
intervals of time.
slow
fast
3.The square of planets orbital period (p)
is proportional to the cube of its semimajor axis (a).
P a
2
3
Low of harmony
P in Earth's years
A in unit of 150 million km ,
Earth-Sun distance
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Isaac Newton :
British, (1642-1727)
Philosophiae Naturalis Principia Mathematica
Most important single work in physical science
He invented three laws:
1. Every object remains at rest, or moves at
constant speed in a straight line, unless an
unbalanced force acts on it.
2. The acceleration (a) produced by an applied force
F on an object of mass m is: a = F/m or F = ma
3. If an object exerts a force on another object, the
second objects exerts an equal and opposite force on
the first: Action = Reaction
He invented the force of gravity
Newton was a great physicist
M PM S
FG  G
r2
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Newton” laws in action
The Sun attracts the planet due to the
force of gravity at each position. It forces
into a curved path
The Sun is by far more massive, the
planet tries to go straight (First law),
but it becomes accelerated (Second
law)
Conclusions (so far):
The insight through the works by Copernicus, Galileo, Kepler and
Newton’s laws helped to understand the solar system in a simple way .
The motion of the planets is related to basic principles. The planets are
not independent of each other as the Greeks thought.
Reality is now subject to experience
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Stimulation to relativity
https://www.youtube.com/watch?v=DL1nepxj2F8.
5:45 min
Brian Green, Colombia University, will be talking
Play 4 min
In short: Green says we have to deal with enormous range in lengths, mass or
speed. Our experience is so restricted to a very small region in this 3-dim world
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1. Space-Time and Special Relativity
Aristotle and Newton both believed in absolute time and that time is separated
from space. This is also usually our common sense
But our common sense fails when dealing with high speeds if it is a significant
fraction of the speed of light, or when dealing with subatomic particles.
We need the theory of relativity and quantum mechanics in the hope
to understand the Universe
Why relativity?
It is a striking fact in our universe that the speed of light is universal and given by
c=2.99792458x108 m/s
in vacuum, independent of the motion of the source or the observer. This is
experimentally verified by the famous experiment by Michelson and Morley
The theory of special relativity (Einstein 1905) is based on two fundamental
postulates:
1. Universality of the speed of light
2. The description of physical reality is the same
regardless of the constant velocity at which the observer s moving
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Before Einstein, the believe was among physicists that there existed a
frame of reference absolutely at rest, in which the aether sat motionless.
Only in this frame, the speed of light was exactly c.
Then, an observer in a frame can measure the velocity of light. If it is equal c,
then her frame was absolutely at rest 😃. Otherwise her velocity is with respect
to the aether.
Exactly this was the aim of the Michelson-Morley experiment, which has
failed to detect any motion relative o the eather.
This negative result opened the door to theory of relativity
All motion is relative
No frame of reference is preferred
Einstein’s principle of relativity removed the validity of absolute truth.
Instead of one frame of reference at rest, any not accelerating frame may be
considered at rest.
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What about space
According to relativity, space does not exist per se, it is rather a relationship
between physical objects. If there were no objects, there will be no space.
Outstretch your arms, put them down at your side. What happens to the space
between them?
That is really amazing: for Newton, space is still there, but not for Einstein.
What about time
Einstein's view: it is the relation between events. If there were no
events in the universe, there would be no time. When nothing is happening
to us, we loose our sense of time.
Let us summarize:
Newton: time has an absolute status
Einstein: relationship between events.
For Newton, time is what a clock tells. But how was the concept of time before
the invention of a clock?
We follow the order of events without necessary agreeing on their durations:
Whistle, stamp your feet, clamp your hands. Order is clear, but no agreement on
the durations of the events.
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The constancy of the speed of light has
destroyed the Newtonian view of absolute time
consequences
1. Time dilation
2. Length contraction
3. E=mc2 (equivalence between mass and energy)
Focusing on time dilation:
A moving clock runs slower
Suppose a clock is moving at a velocity v=0.866 c Then one hour on the
moving clock will be on the stationary (Earth) clock equal to:
1 hr
1 v / c
2
2
 2 hr
This means: moving clocks appear to loose time. If the moving observer reaches
the speed of light, time comes to standstill for the Earth’s observer.
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This sounds like science fiction
Reflection
Although, the time in the moving frame appears to slow down for the stationary
observer, the moving observer does not experience that slowing down.
In fact, from the point view of this observe, the stationary frame is in motion. She
thinks the time is slowing down in the other frame
Strange: each observer believes that the time is dilated in the other
frame. How can this be possible? Which clock is telling the correct time?
Asking this question means assuming absolute time. But it is so that two
observers moving relative to each other do not observe the same things.
Example:
The meson is an unstable elementary particle, it lives 10-8 sec in it
is frame at rest. But it lives longer when moving with high velocity
by
8
10 sec
1  v 2 / c2
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This means, the meson projects its self into the future.
We reach the point to talk about
space travel and time machine
Space travel
Closest star, alpha Centauri is about 4 light years away (light year is a
distance covered by the light traveling one year at a speed of 3x108 m/s
and covers about 1013 km. There are very few stars it in a distance of 50
light years, that is 100 light years for a round trip. Hopeless situation for us.
But he time dilation helps. Suppose an astronaut can travel at 0.999 c
to a star 99.9 light years away Then, in his frame of reference, he will
need only:
100 yrs
1  0.999
2
 4.47 yr
Or he will be aged about 9 yrs when he returned to Earth, where 100 yrs
have elapsed
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In other words, the astronaut has projected himself 91 yrs into the future
So, we have a time machine, but not of the type imagined
by H.G wells, traveling back an forth. Why not?
You cannot travel into the past unless you travel faster than light. If you could,
overtake light signals, you could watch the Egyptians building the pyramids. No
one can do that !!
Do you want to tell me that you have watched your father
falling in love with your mother, before they married
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Relativity for me is so beautiful, because
it challenges my imagination 
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Space-Time Diagram:
If the Sun would disappear you cannot
know it immediately
Time [min]
x=ct
Presence
Not accessible
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
Future
But wait for 8.33 min
Earth
enters the
future
cone of
the Sun
0 0.2 0.4 0.6 0.8 0.9 1.0 1.2 1.4 1.6
x/(1011 m) Distance
Earth
Sun
150 million km
Past
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Light cone in 2 dimensions
time
In case of 2 space
dimension, we hav a
light cone
y
x
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Message to the audience
Science has the freedom of thoughts,
but also the responsibility of proofs
Religion is a believe, does not need a proof
There is no terror in a religion, but
there are terrorists in each religion
‫في العلم حرية التفكير لكن مسؤولية البرهان‬
‫الدين عقيدة التحتاج إلى برهان‬
‫اليوجد دين إرهابي بل إرهابين في الدين‬
In Paris on Jan 2015, one Moslem killed the other, before he died he
shouted: I am Ahmed, even this did not help
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Part II: Universe
This part is challenging. But challenging is the best way for learning
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Expanding Universe
Expansion of space does not affect the size of the material objects such as
galaxies and even apples.
Circle a galaxy on a balloon, the circle will not expand as the balloon is inflating
Important to know this:
We can detect the expansion only if our measuring instruments have fixed sizes.
Otherwise, if everything changes size, we would not notice the difference
interesting
Even before Hubble’s discovery of the expanding universe, the Russian Alexander
Friedman, solving the Einstein's equations, proposed that the galaxies are moving
away from each other, He proposed:
The universe looks the same in each direction and for all observers wherever they
are.
How do we understand this?
Analogy:
If you look at a forest. Nearby, you see the space between the trees. But from far
away every square meter is similar to another.
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Friedman’s model begins with zero size followed by expansion. But with what
fate? Expanding forever, or crushing?
The idea of the Big Bang was born, but not many believed it
Fred Hoyle (Scottish) proposed a steady-state universe.
But in 1965, a great discovery was made, the discovery of the cosmic
Cosmic Microwave Background Radiation (CMBR)
It is like in the microwave in your kitchen, but less powerful, and it is partially on
your TV when no picture appears
It was an accidental discovery by Penzias and Wilson (1965), It is a relic
of a hot Big Bang (BB),
But, why do we say “hot”? How do we know?
Fingerprint
At about 3 min after BB, the universe was a fusion reactor. The percentage of
Helium in the Sun is 28%, But Helium cannot be produced in stars except of
(3-4)%. Thus most of the helium was made during the first minutes and this
needs at least 100 million degrees, really hot.
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Interesting
The relativity theory of Einstein does not provide a true picture about the origin of
the Universe.
Why?
Einstein’s theory of general relativity predicts infinite temperature, density and
curvature, or singularity in mathematical sense. It is like dividing by zero.
Reflection
We are not saying that the general relativity is wrong, on the opposite it is
beautiful. However, it cannot be applied to the Big Bang. It is after all a
classical field theory.
First Phase of expansion
It is called “cosmological Inflation:
The Universe expanded by 50 order of magnitude between a time 10-32 and 10-30
seconds
See Figure next page
Project for you: can you talk about MCBR with single temperature without inflation?
This is called horiozon problem
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We have a trouble!
It looks as the Universe was
expanding faster than light
We calm down:
The expansion of the
universe is the expansion of
space itself not the motion of
objects through space.
Concept of limited speed to
the speed of light does not
apply here
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Three indicators in favor of the Big Bang
1. Cosmic microwave background radiation (CMBR)
2. Helium-production
3. Hubble Expansion (receding Galaxies)
Now, the theory of inflation is not well understood, it needs quantum gravity. But
the inflation was not completely uniform. This means very small irregularities in
temperature may exist.
Indeed, the COBE (Cosmic Background Explorer) satellite, and also the
WMAP (Wilkinson Microwave Anisotropy Probe) both discover this fluctuation
So it is experimentally verified that the CMBR temperature is
T=(2.726  0.0013) K
So accurate
This will not heat your food in the microwave more than -270 K
The Universe is really cool in every respect
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Reflection:
A uniform Universe would be boring.
We are lucky to have these irregularities.
The Universe having some irregularities mean that some regions would
have slightly higher densities. Gravity would help the regions to collapse
to form galaxies, stars and planets and finally we came out!
If you think about this you could say we are product of quantum fluctuations in
the very early Universe.
WE ARE a PRODUCT OF a GREAT DESIGN
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Most Challenging:
The universe was a cataclysmic quantum event, because of the extremely
small scales.
To have the ambition of understanding the beginning, a combination of general
relativity and quantum mechanics is needed
But how does this work?
Gravity wraps space and time
Relativity easy to wrap space, but how to wrap time?
Let us think:
If you speak about space and time separately, you can do that in case of
low speed and weak gravity
But in general, space and time are intertwined. In case of space, you have no
problem to go around like around the Earth, since it is not flat. But time is like a
railway track, isn’t ?
If it has a beginning, then someone has to set the train going. But does this go on
forever?
When quantum mechanics is added, time becomes like another dimension of
space, which means that the early inverse is at least four-dimensional.
When we say “beginning”, we look backward to a time beyond our
experience, but not beyond our imagination, it is still subject to
Mathematics (this sounds very nice indeed )
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Project:
what is Mathematics? More imagination than experience? The physicists have to add
to their imagination their experience
Very strange
If time is another direction of space (say: spacetime), then we get rid of talking
about the beginning of time. Simply, you cannot bring it into our experience!
Question:
Why religions can speak of beginning? Is it because the are not asked to bring it
into the physical experience?
Back to physics
If we can combine general relativity with quantum mechanics, we stop asking
about what was before the beginning
Asking St Augustine: what was God doing before creating the Universe
His Answer: he was preparing hell for you when asking this question
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Historical Remarks
Many believed including Aristotle: Universe must have always existed in order to
avoid the problem of how it was set up
Others believed: Universe had a beginning and used that to indicate
existence of God
Any alternative?
Hawking: Universe governed by the laws of sciences and does not need to
be set in motion by some God. This touches your and my believe !
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Comments
Einstein’s general relativity predicts that space-time begins at Big Bang singularity.
But this is a classical field theory. Then, what is the role of quantum mechanics?
The problem is due to the very strong gravitational field
Questions:
 Why the Early Universe so hot?
 Why the Universe so uniform on large scale?
 Why does it look the same in all directions and having the same temperature?
 we have Galaxies and stars owing to density fluctuations. What is their origin?
Challenging thoughts
How were the initial conditions been chosen? By God the omnipotent?
But why God did choose to let the Universe evolve according to laws we understand?
There is a clear order in the Universe.
Is that order divinely inspired?
diveinly
We arrive to talk about the Anthropic principle
We see the Universe the way it is, because we exist- a problematic statement
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Two versions of the anthropic principle: weak version and strong version
Weak version:
In a Universe that is large or infinite in space and/or time the conditions
necessary for the development of intelligent life will occur in certain regions
limited in space & time. No surprise
It is like a rich person living in a wealthy area not seeing poverty
Strong version:
Either many different universes, or many different regions in a single universe.
Our own universe would have the right conditions for the development of
intelligent life.
The strong statement is:
Asking: Why the universe is the way we see it?
Answer: if it were different, we would not exist.
Well, we know that the fundamental constants (strength of the forces) are fine-tuned.
Still there are objections to the strong version.
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In what sense can many universes exist? We cannot know what happened in
them. We tend to remove them from the theory
If there are different regions of the same universe, the law of sciences would be
the same in each region, so that we can move from one region to another. But
the initial conditions would be different, and we are back to the weak version of
the anthropic principle
Another argument against the strong version is:
It runs against the tide of the history of science. All what we have discovered
from the geocentric model to the modern picture and all what is existing,
hundred billion of galaxies, all this is for our sake?
I hope you enjoyed the lecture. We are ready for your questions
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Implication of the first postulate:
Imagine you are in a spaceship moving toward a flashlight. You could be
moving at 90% of the peed of light, you will measure the speed of light for the
flashlight, as your spaceship were motionless.
Implication of the second postulate:
Suppose you were inside a railroad car moving due north in a straight line at 100
km/h . Any measurement you make inside the car, for example how long you can
stand on one leg , will be the same as if the car is moving in any other direction
or any other speed or not moving at all.
Striking consequence of relativity
1
Equivalence of energy and mass: E=mc2: energy of a moving object adds to
its mass. This means it becomes harder and harder to increase its speed.
Any normal object is forever confined by relativity to move at a speed
less than the speed of light. Only light can travel at that speed
2
Our ideas of space and time received a shock. Relativity put an end to the
idea absolute time, every observer has own clock carried with him. And
moving clocks run slower: Time dilation
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Part III: Relativity and Black Holes
The most dramatic prediction of general theory of relativity is the existence of black
holes.
As the matter is compressed to extreme densities the strength of gravity at the
surface of the sphere increases dramatically.
Consequence .
Gravitational bending of light (experimentally tested)
How come that this bending happens at all? Newtonian description of gravity
cannot be applied to light, because it has no mass (it is pure energy). But
Einstein’s description of gravity gives the answer. (see next page)
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Curved spacetime around a black hole: no escape even for light.
Gravity in the Einstein’s theory is affecting the geometry of space, it
corves the space in the vicinity of the mass
Black hole in general relativity
Singularity
The event horizon:
or
the entrance to the
end of time. Doest the
time flow in the hell?
Schwarzschld’s radius
Rsch
2GM

c2
12:00 p.m.
Cross sections
In a collapsing star
you
astronaut
Observer A: Never sees
the light emitted at 12:00
p.m., when observer B
enters the event horizon.
His light signal propagates
along the edge of the
cylinder
B
A
Observer B: his time runs normal.
However he/she disappears for ever
beyond the event horizon
Psart I: details
At the time one assumed that the
distance Aswan-Alexandria is 5000
stadia, where one stadium is 0.16
km, or 800 km=5000x0.16 km.
So, 800x50 km=40000 km very close
to the diameter of the Earth
These people had no electric lights
and Iphones, but they were smart.
To be smart, you should have a
functioning brain not only a
functioning cell phone.
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Cosmic History
1
2
3
You see 3 symmetry breakings to separate the forces
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Some references:
1. A Brief History of Time
S. Hawking 1988
Check chap. 1,2,3 and 8 (not easy to read)
2. The Grand Design
S. Hawking and L. Mlodinow, 2010
3. Universe
Freedman, Keller and Kaufmann
9th edition, Freeman 2011
end chapters in the book
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All Israel 200 atomic bombs and more, cannot stop the desire for life
of these young Palestinians
Watch and wonder
https://www.youtube.com/watch?v=8l_AqyHWxYM.
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