Hawking Presentation2012

8/2/2019 Hawking Presentation2012

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From very early in human civilization, humans have tried to explain

the universe.

An early Babylonian idea was that Earth was a flat stationary plate,

and the sky above was like a moving dome, or a roof enclosing earth

as a half-circle.

Later, the ancient Greeks figured out that Earth could not be flat.

As travelers, the Greeks were navigating using the stars for

orientation. One orientation point was the North Star. They noticedthat starting out from Athens, the North star would hover just above

the horizon, but the farther they traveled north, the further it would

raise above the horizon. This could only be explained if the Earth was

round and not flat.

They also experimented with sticks of equal length placed on

different locations on earth, for example, one in Athens and one in

Alexandria. They would place them standing in right angles to theEarth, and measure the shadows they were throwing at one

particular date and time. They now noticed that when one stick at

one date and time threw no shadow in Alexandria, the stick in

Athens at the same date and time would have a shadow. If the earth

were flat, they should throw the same shadow; only if it was curved,

the shadows would be different.

From a Flat to a Spherical EarthBelow: a man, living on the flat earth, under a dome

of sun, moon, and stars, try to break out of the

dome and take a look at the other side. If he goes

any further he falls down.That the earth was flat was obvious from sense

experience: earth is experienced as flat and we

dont fall off. That the sky was moving was also

obvious from experience, since all the objects in the

sky seem to be moving around us in a half-circle

from morning to dusk: the sun, the moon, the stars.


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The Ptolemaic conception of the universe.

To different renditions, First, a Medieval: Earth is not only

in the center of the universe, it is also huge compared to

the sun and the other planets orbiting it. Second, a more

formal model illustrating the centered earth and the eightspheres.

The Greeks had the idea that the most perfect movement had to be

circular.

The circle was the most perfect geometrical form, so if sun, moon,and star revolved around earth, they would do so in perfect circles.

Ptolemy elaborated on Aristotles ideas, and came up with a model

of the universe, that would last for nearly two thousand years.

In the middle we have Earth, and revolving around earth, we have

eight different spheres, that each of them control the movement of

different bodies in the sky. The sphere closest to earth would thus

account for the movement of the moon; the fourth sphere would be

the sphere of the sun; the eighth sphere, farthest away, would be the

sphere of the fixed stars.

The universe was like an onion. In the middle, the earth, from there

you can go out layer by layer, until the eighth sphere. What was

outside the onion, nobody knew or asked about. One assumed that

this was the sphere of God and his heaven.


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Below we still have Ptolemys model. Earth is in

the center, and various planets and stars are

orbiting around it in perfect circles. But we see a

modification of the model, because one has

added to the perfect circles so-called epi-circles.

Observation had shown the early physicist that

the sky was filled with so-called wanderingstars(Planetos), that were wandering forth and back

on the sky. How to solve the problem, without

destroying the idea of perfect circles? By creating

epi-circles!

There was problems with Ptolemys model. Not all bodies on

the sky seemed to move in perfect circles.

Some bodies seemed to wander around in strange patterns,

one therefore gave these bodies the name, planetos, the Greek

for wanderer.

One tried to account for these strange movements by adding

epi-circles to the original circles; one added a circle to the

original circle, such that the second circle had a center moving

with the original circle. Strange movements could now be

explained by epi-circles; and if not by one epi-circle, then by

adding an extra epi-circle, creating an epi-epicircle, etc.

Ptolemys model survived. And when Christianity became the

official religion in Europe, the theologians adopted the model

too, because of its simplicity and perfection. The circle was still regarded as perfect; and could God have

created the universe other than perfect? That the eighth

sphere was a natural boundary of the universe also fit into

Christian thinking. One had created enough space for heaven

and hell. The church liked the model, and regarded every attack

on it as heretic.

Problems with the

Perfect Circles


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The problem with the Ptolemaic model was thepeculiar epi-circular movements. One problem was as

Copernicus understood that every movement could be

explained by means of epi-circles, depending on how bigone made the circle and how many of them one invented

to do the job. Therefore, epi-circles were applied

dogmatically, and did not correspond to observable facts.

Secondly, because of all these epi-cicles, and epi-epi-

circles, etc., the Ptolemaic model had become extremely

complex.

In physics you always seek the simplest explanation.And Copernicus realized that placing the sun in the

center of the universe, and the planets orbiting this

center, would both be simpler, and would explain

observable fact, that before could not be explained.

In this new helio-centric universe, earth was no longerstationary, and it was no longer the center of the

universe.Later Kepler took up the model, and refined it. but itwas not before Galileo, the idea was supported with

observable facts.

At this point, the new model began to concern thechurch, and they deemed the idea heretic, and forced

Galileo to retract his observations.

The Copernican Revolution The two models below look the same, buteverything has changed from one to the other.

The first is the Ptolemaic; it is GEO-CENTRIC:

the Earth in the center, the Sun orbiting

around. The second is the Copernican; it is

HELIO-CENTRIC: Sun in the center, and Earth is

orbiting around


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Although the trinity, Copernicus, Kepler, and Galileo had suggested an

entirely new picture of the universe, much remained unresolved. One for

example did not know why the planets were forced into these orbits around

the sun, and believed it had to be a kind of magnetic forces attracting them.

It was not before Newton, one understood the law of gravity. According to

Newtons law, two bodies will attract each other with a force that is

proportional to their mass, and inverse proportional to their distance. If we

have the bodies, Earth and an apple, the two bodies attract each other, but

because the Earth is enormous compared to an able, we only experience a

pull in one direction, the able falls downwards. Newton extrapolated, the

moon also fallsdownwards, but because of the centrifugal force pulling it in

the other direction, it would be held in a stable orbit around earth.

Since the gravitational force is a weak force, we dont perceive two bodies

of similar mass attracting each other on earth. Two apples on a plate attract

each other, but not visibly.

However, if we talk about massive objects, like moon, planets, sun, then

the attraction is significant. The pull of the gravitational force keep them in

place. Otherwise, they would just be moving in straight lines through space.

In one bodys orbit around another body, there is a balance between the

gravitational pull in one direction, and the centrifugal force in the other.

Thanks to this balance, planets orbit around stars, and moons around

planets.

Newton and Gravity Newtons law for Gravitational

attraction: each body in the

universe is attracted toward

every other body by a force thatis stronger the more massive the

bodies and the closer they are to

each other. Hawking, p. 5

Below: two bodies with the

masses M and m. The force of

the gravitational attraction onefinds by multiplying M and m,

and the gravitational constant, G,

and thereupon divide the result

by the square of the distance

between the bodies.


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During the history of Cosmology our image of the universe has continuously been expanding. The universe not only

expends in actuality, it expands in our imagination. Historically, it was first nothing but a flat earth with a dome on top; then

it became a solar system; then a galaxy; then astronomers started to speculate on the existence of other galaxies; today we

know that our visible universe consists of 1-200 billion galaxies. We also know that this is only our observable universe and

that it must be bigger. Cutting edge cosmology speculates that there may exist an infinite number of alternative universes;that we consequently live in a multiverse.

When we measure distances in the universe, we no longer measure in kilometers, but in light-years. The distance traveled

by light in one year is a light-year. It takes the light of the sun eight minutes to travel to earth, so the sun is eight light

minutes away from us. It takes light from the star closest to our own (Alpha Centauri) about four years to travel to us, so, it

is four light-years away from us.

If we adopt a scale where the Sun is an orange and Earth is a pinhead, the distance between the orange and pinhead is around15 meters.

Distances in the Universe: Planets, Stars, and Galaxies


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Our Address in the Universe

In our galaxy, the Milky Way, there are billions of stars like our sun (approximately 100 billions). We assume that there

are billions of planets orbiting these stars, but we cant detect them easily, because they are small and they dont emit light.

Since there are billions of galaxies in the universe, and these billions of galaxies are composed of billions of stars, around

which trillions of planets must be orbiting, astronomers seriously believe given the huge numberthat life must exist on

some of these other planets. Our location in the Milky Way is somewhat in the outskirts of the galaxy. It is fortunate that we are not too close to the

center, because it consists of a super-massive black hole, that ribs everything apart coming too close.


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Einstein discovered that light has a speed, and it is invariable. It takes time for light

to travel. A thousand kilometers takes around 0.0033356 second, and a million

kilometers 3.3356 seconds. In other words, light travels at a speed of approximately

300,000 kilometers per second. Since light has an invariable speed, and we know its value, we can measure

distances by measuring the time it takes for light to travel from one event to another

(therefore, we no longer measure distances in a metrical system). In Hawkings

illustration to the right, one sends a pulse of radio-waves (traveling with the same

speed as light) out to an object, and measure the time it takes for it to be reflected

back. Time is measured on the vertical y-axis; while distance is measured on the

horizontal x-axis. To know the distance to the object, we take the time for the pulse

to be reflected back to us, divide it by two (since it took a round-trip), and multiply

time with the speed of light. Consequently, we have the distance to the object.

We have said that the distance from the sun to Earth is about 8 Light-minutes. This

means that the sunlight we see now is eight minutes old, or, it was emitted eight

minutes ago. It also implies that we can know nothing about the sunlight that is

being emitted in our present now. And since nothing travels faster than light, there is

no way we can know the present

On Hawkings illustration to the left, we have the Sun and the Earth lined up attime 0 on the x-axis. Time 0 represents the absolute present. At time 0, we imagine

that the sun implodes and disappears; however, Earth is still unaffected, because it is

outside the light cone of the sun; it is in what Hawking calls the Elsewhere. As the

clock ticks, we move up along the vertical time axis, and after 8 minutes, we enter

the light cone of the sun, and experience what happened 8 minutes ago, that the

sun has vanished.

Speed of Light


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When we enter the suns light-cone, we experience the implosion, although it iseight minutes old. It is in the so-called absolute past that is, from our point ofview. There are light cones for as well absolute futureand absolute past. They

are relative to what we decide is the present observation point. The light cone inthe figure illustrates the relations between absolute past, present, and absolutefuture.

The Light Cone

The Future andPast Light Cone

When we observegalaxies and galaxyclusters, we only seewhat is past. The fatheraway the object is, thelonger it takes for its lightto reach us, and the

further we look back intothe Past Light Cone. What happens in ourpresent, we cannot know;we cannot know whatspace is on the so-calledHyper-Surface of thePresent. Relative to the FutureLight Cone of an object,we are in the Elsewhere;only as time passes, wemove into the light

emitted from the object.


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In order to understand the next discovery about our universe, we

must understand the doppler effect.

The doppler effect is well-known in our experience of sound.When for example a car comes toward us, the sound of the car

increases in pitch (the sound waves are compressed), and when it

moves away, the sound decreases in pitch (the sound waves are

stretched out). As with the police-car in the example. When the

sound moves toward us (when sound-waves are compressed), the

waves are blue-shifted. When the sound moves away from us, the

waves are red-shifted.

The same is the case with light. When observing light emitted by a

star, one breaks it up into a spectrum spanning from the deep red

to the deep blue. Depending of the chemical composition of the

star, the spectrum reveal patterns of absorptions lines (i.e., patternsof dark lines in the spectrum that indicates the presence of various

elements, such as Helium, Hydrogen, Carbon, etc.) From the

laboratory we know these patterns well enough, and from looking

at them the astronomer can quickly determine the stars chemical

composition.

However, as in sound, there is a difference in the light spectrum

according to whether the object moves away from us, or toward

us. If it is moving toward us, the pattern of absorption lines is

shiftedtoward blue. If it is moving away from us, the pattern is

shiftedtoward red.


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In 1929, the astronomer Edwin Hubble applied the notion ofthe Doppler effect to his observations of galaxies. He

expected to see a random distribution of blue-shifted and

red-shifted galaxies, but observed instead that distant

galaxies are moving rapidly away from us; they were all red-

shifted.

Furthermore, not only were galaxies moving away, but there

was a correlation between their distance from us and their

velocity. In other words, the father away they are from us, the

faster they move away.

In the model to the right, the x-axis represents the distance

from the observer; the y-axis represents the velocity (speed)

of the galaxy. Galaxies close to us, moves away from us with

slower speeds. Distant galaxies moves away with higherspeeds. The father away a galaxy is, the faster does it also

move away. The relation is a constant (diagonal line; Hubbles

Constant: H0 = 72)


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This observation could only imply that the universe is expanding, and more

rapidly so, the deeper we go into the universe.

The universe expands, like if you blow up a balloon, or set a raisin bread to

rise. In the form of the raisins, you have certain points representing stars or

galaxies. We choose one of the raisins to be our observation point, and in the

vicinity of that point, we follow the expansion of other raisins. When the

dough starts to raise, the raisins start to expand away from our observations

point, and the further away they are, the more they expand. On the model,

the point nearest us expands from 5 cm to 10 cm; the point farthest away

from us expands from 10 cm to 20 cm.

Understanding this logic, it did not much thinking to figure out that if galaxies

are expanding, they must at some point have been closer together than they

are now. If they are expanding today, they must have been closer together

yesterday, and still closer the day before yesterday, and so on until we find a

beginning of the expansion.

Hubble was able to calculate the rate by which they expand, that is, the

velocity of expanding galaxies. This made it possible to estimate the time it

has taken the universe to expand this far, and estimate the distant beginning

of the universe; a point called a singularity, or better known as the big bang.

The universe was no longer eternal and unchanging. It had a beginning, and it

was, and is still, constantly changing. The universe had a birth, a creation

from where it came into being.

The Universe is Expanding

from a Singularity


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A Model of the Universe Expanding from an

Inflationary Hot Big Bang Event

A model of the expandinguniverse. It starts in a super-hot Big

Bang explosion in the first fractionof a second (T = 10-43). At this point,there are no particle formation andno physical forces; consequently nophysical laws.

At T = 10-32 seconds, this bigbang singularity starts to inflate(with a doubling time of 1picosecond). The model illustrateshow the different forces starts toform, first strong, then weak, theelectromagnetic, finallygravitational, and how the firstparticles start to form.

Not before at around one secondafter the explosion, light elementsstart to form like He, D, Li. Theexploding universe continues toproduce clouds of mass, until thegravitational effect takes over, andmass starts to collapse into starsand galaxies. After around 13.7

billions years, the universe reachesis current state.


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A Model of the Universe Expanding from an

Inflationary Hot Big Bang Event

A model of the expandinguniverse. It starts in a super-hot Big

Bang explosion in the first fractionof a second (T = 10-43). At this point,there are no particle formation andno physical forces; consequently nophysical laws.

At T = 10-32 seconds, this bigbang singularity starts to inflate(with a doubling time of 1picosecond). The model illustrateshow the different forces starts toform, first strong, then weak, theelectromagnetic, finallygravitational, and how the firstparticles start to form.

Not before at around one secondafter the explosion, light elementsstart to form like He, D, Li. Theexploding universe continues toproduce clouds of mass, until thegravitational effect takes over, andmass starts to collapse into starsand galaxies. After around 13.7

billions years, the universe reachesis current state.


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Cosmic Expansion:

Look-back Distance versus Real DistanceEleven Billion years ago, a

photon of light departs froma distant galaxy toward the

Milky Way. Lets say that the

two galaxies are separated by

4 billion light years

However, the universe keeps

expanding, and the photon

does not reach the Milky Wayafter 4 billion years as one

would suppose. After 8, 9,

and 10 billion years, it is still

traveling.

First after 11 billion years, it

reaches the Milky Way, and

we see it as it was emitted 11

billions years ago; this is theso-called look-back distance.

The real distance to the other

galaxy is different, because

with the expansion of space it

has kept expanding away

from the original emission

point. It is now 18 billion

light-years away from us.


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To the left, we have a model that depicts us in the middle, as observing the universe. The universe is

consequently all around us, and the deeper we look into the universe with advanced telescopes like Hubble,

the earlier an universe we see. With the current telescopes with can see until the blue line, the so-called

Hubble Ultra Deep Field, which approximately corresponds to the formation of the first galaxies. Further outwe have the radiation era, the Cosmic Microwave Background, and the outer periphery would represent the

Big Bang, 13.7 billion years ago. In this perspective the Big Bang is all around us. We notice that the universe

expands from the inner core, i.e., from the center of the circle, pressing the periphery outwards

To the right, we have a model that depicts the Big Bang in the middle, therefore a realistic model where

the universe starts in the Big Bang, then expands up till the outer periphery, which represents the present

state of the universe.


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The Cosmic Microwave BackgroundThe white noise one picks up in radios and television sets is coming from the Cosmic Microwave Background,the universe as it had formed 400.000 thousand years after the big bang. The Microwave Background is aplasma of high-density matter. Stars and galaxies have as yet not formed. The minute differences temperatures(in the order of 1/10,000) can be picked up, and be depicted as in the colored map as below. These differencewill eventually end up as the differences in galaxy concentrations in our universe. The Microwave Background is

like the embryo of the universe. The existence of the Microwave Background is proof of the Big Bang theory andthe expansion of the universe.

iff ibl f h i


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Different Possible Fates of the Universe:

Old View: Decelerating Expansion was accepted as the behavior of the universe, out of

three possibilities: the universe re-collapses in a big crunch; the expansion of the

universe flattens out; the universe keeps expanding, but with decelerating velocity.New View: The universe keeps expanding, but with accelerating velocity.


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Fine Tuning and Anthropic Principle


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if a star before it starts to collapse on itself, is sufficiently

heavy (about 2-3 times the mass of our sun), then the

exclusion principle can no longer support it. The repulsive

forces from nuclear reactions in its core cannot counteract its

collapse. That is, without anything to stop the process, itcontinues to collapse. It collapses into a singularity, or a black

hole.

A black hole is a star that has collapsed into a mass that is so

dense that light can no longer escape its gravitation. It is

relatively easy to escape the gravitational field of earth. If one

can make an object travel with a certain speed, one can send

it out in space. The speed it needs in order to escape thegravitation of Earth is called its escape velocity. On a planet

with higher mass than earth, it would be harder to escape,

and one would need a higher escape velocity for the object

to escape. On a neutron star, an object would need an

incredible high speed.

Now, since we know that there is a limit to speed, since

nothing can travel faster than light, it can now be calculatedthat some objects would have masses so dense that not even

light would have an escape velocity sufficiently high to

escape from the objects gravitation. Such objects would be

black holes. They must obviously appear black, since they

emit no light. Light would be dragged back into the black

hole. And if light can not escape, nothing else can

everything is pulled back into the black hole.

A star with large mass has eventually burned out

of fuel. Without sufficient repulsion of nuclear

reactions, it continues to collapse into

a singularity. This singularity is infinitely dense,

and everything that comes within a certain

boundary is pulled in and destroyed. This

boundary, which is called the event horizon, is

the distance within which everything, also light,

is pulled in.


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The two giants of modern physics: Newton and Einstein.

Newton with his fixed and flat space, and Einstein with his

dynamic and curved.


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Hawking Presentation2012