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Welcome to my course on physics I'm gonna be giving you four video lectures
on physics. I'm just making power point presentations.
So, You don't have to watch this in HD. Physics and Philosophy is the first part of
my lectures. There are four lectures actually.
Physics and Philosophy . It's quite funny when we think about it but
Physics was a part of philosophy a few hundred years back.
Physics was considered to be a sacred subject and only some were allowed to study it.
Imagine what would happen if we relate science and religion.
Disaster. That's how the human brains functioned.
Even great people like Aristotle didn't have that scientific mind to find reason in happenings.
We can't blame them for their ignorance. There was no technology, no research centers
or laboratories. It was just the humans making up stuff by
observing with their eyes. That's how astronomy was born.
As far as real science is concerned, the scientific revolution was started by Newton.
Yes, the guy who gave a theory on gravity after getting hit by an apple.
Imagine someone throws a ball up, what will happen next?
The obvious. The ball comes back down.
This is something we see every day but do we ask strange questions like the ones Newton
asked? Why does the ball come down?
Why did the apple fall down? These are revolutionary questions indeed.
But questions can't be left unanswered. Newton's curiosity and love for the subject
helped him achieve great heights. Newton started observing how objects fall.
He noted how fast something fell when its mass is changed, when it is too far from the
ground. He found something remarkable.
The relation between the mass, distance and acceleration show us that there is a constant
that remains constant even if something is changed.
That's what a constant is actually. This constant later came to be known as the
Gravitational constant. Newton made a lot of experiments and proved
that his theory of gravity was right. But there was a problem.
Newton himself has written in his book that he didn't know how gravity worked or what
makes gravity work. It is a question that was partially answered
by Albert Einstein when he discovered the theory of general relativity.
We'll later discuss about the theory and some answers that string theory gives us.
Newton also used calculus to develop a set of equations that give meaning to motion.
A funny fact about calculus: It is believed that Archimedes was the first to write about
calculus before Newton. But as his writings got damaged, Newton is
the one who is given the honors. But Newton's scientific knowledge was not
up to the mark! Newton had a wrong belief.
He believed that time and space were both fixed.
He told that time and space look the same from every point in space.
He also believed that the universe was small and limited.
A lot of scientists had the same belief until Albert Einstein (again) gave answers.
Did anything make sense actually? Probably no!
Let me clear this out. Take time and space.
Time just runs at some rate calculated in units of seconds, hours, minutes, days and
years. Newton believed that time does the same job
for a person living in another planet. And space, Newton had a weird belief that
is not worth explaining. Back to the story.
After decades, there came scientists like Maxwell, Faraday, Gauss and many more.
These people gave us equations that gave value for forces and fields with accuracy.
Some of their equations gave a result that made people mad. We know that a graph sheet
has 2D structures drawn on it and that we live in world with 3 Dimensions (x,y,z).
The equations yielded a result that showed that there were 4 dimensions, 3 Spatial and
1 Time. This gave scientists a hard time and only
some accepted the fact. Those who believed gave a new type of graph
known as the space-time. This graph is a beautiful one and no explanation
is necessary as the picture is self explanatory. Next comes Minkowski space diagrams. Minkowski
space diagrams were developed and they gave a 3D representation of the space time graph.
I guess it is on the screen now. This is a complicated diagram and needs an
explanation. The two cones represent the past and the future.
We are in between the two cones at the point where the two cones meet.
We can see the past but never see the future. This can be explained by explaining light.
What is light actually? It is information, not literally.
Skin color, Television display and more give out light that can be viewed by our eyes.
This gives us information on what is sending it.
But wait, if light travels and comes to us, do we receive information that was sent by
an object some time back (as light takes time to reach us)? Yes
Light travels so fast that it is impossible to note the time gaps on earth but these gaps
can be seen when observing planetary objects. Galaxies that are too distant from us (like
light years away) send us signals and we receive it after years. That indeed means that we
see the past happenings of the galaxy. The Minkowski space diagram represents a 3D
structure of space time and as recent discoveries have shown us that the Universe is so big,
A lot of space is cramped up in a small part of the diagram.
We are in between the cones. What we see in the universe can be plotted
in the diagram by drawing lines (path of light) that are at 45 degrees to us.
The lines go down in all directions and form a region of the shape of a cone.
The cone is everything that can be seen by us.
There will be a picture on the screen which will make you understand better.
This diagram has been used for a lot of purposes from studying black holes to constructing
few equations. I'm going to use this to give an introduction
to Einstein's theory of Special Relativity. To be clear, Einstein gave us two theories
on relativity. Special relativity is constrained to non-accelerating
stuff and General Relativity which applies to all.
Einstein conducted thought experiments about relative motion.
How will world look according to people moving at a constant speed was what he was wondering
about. Scientists conducted experiments that Einstein
earlier thought of. They used basic equations to calculate the
time 500 Muons take to fall down from a mountain. They used this in the half life equation and
calculated that there will be 60 Muons on the ground when they fall down.
But the result was unexpected. There were about 290 Muons when the Muons
hit the ground. The experiment was done again and again and
again and they got the same result. The only explanation was that time was playing
a game. Einstein developed a few equations which had
a term called the 'Lorentz Factor' , it will be on the screen, and he got a result that
no one expected. The equations showed that people who travel
fast have a clock that runs slower than the people at rest.
He showed that time runs slow for people who travel fast.
He also got a result that showed that mass increases as something travels faster.
He also showed that moving objects appear shorter than objects at rest.
The main result was that all the equations showed that the speed of light is the speed
limit set for all objects in space and that if something goes faster than the speed of
light, which are known as tachyons, they get negative mass which is impossible.
So, it was proven that light has no mass. This theory that there are different clocks
for different people makes less sense and I plan to use the Minkowski Space to make
it clear. Time is always heading up in the graph.
Space to the right, left, front and back. Someone idle is moving up in time at the rate
time is moving. That is, if I sit idle for 10000 years, I'll
be of the same age as the Earth. That's how the diagram works.
Moving means moving right or back in the graph.
I made a mistake in that! I have to tell this.
A person at rest takes time that is the normal one.
And the person at motion; his motion diagram is represented in the right.
If you can compare the diagrams, the first person takes less time to reach some point
in space whereas the second person takes a longer time to reach the same point of time.
(He ages slower.. check out the spin paradox). Which means that, if the first person reaches
10 years in 10 years, the second person will reach 10 years in 15 years!
I don't know if it made any sense but that is relativity.
I'll just move on the general theory of relativity. Explaining the special relativity alone will
not give you closure. So, here is my explanation of the General
Theory of Relativity. At Einstein's time, only two of the four forces
were known. Einstein wanted some nice explanation to what
the forces actually meant. So he developed the general theory of relativity.
Einstein was not satisfied with Newton's description of Gravity.
It just calculated gravity but it didn't explain gravity.
Einstein was confused with how gravity works. How will the Sun which is so far from our
Planet, affect the solar system? He had another question in mind, what will
happen when the sun suddenly disappears? Will it have an immediate impact on Earth?
To find answers, Einstein went on a journey. A journey that led him to the Field Equations
a.k.a Theory of General Relativity. Look at these beautiful equations below.
I guess only one equation is there on the screen.
Actually there are sixteen equations hidden in there.
You don't have to split anything to find the equations.
If you notice the indexes 'mu' and 'nu' near 'R' and 'g', those are the dimensions.
It was believed that the universe is made up of four dimensions.
Three spatial and one time. So count the number of combinations.
Four plus four plus four plus four. Which gives sixteen. So there are sixteen equations
we get with one set of inputs. Filled with tensors, values, the cosmological
constant, Energy and the Gravitational constant, this beautiful equation develops a rather
mathematical approach on gravity. The left side of the equation contains Curvature
tensors that are mathematical functions that define curved surfaces.
The right side has elements like Gravity, Mass, Tension and Energy that we deal with
in classical mechanics. This equation has given us an understanding
of what Gravity actually is. Gravity is just a set of interacting curves
in space and time. It might be ridiculous to tell that Time bends
but it does. Interacting curves in space and time.
Which means time bends. It will be ridiculous but
To make a better sense out of that, imagine a black hole.
(nah.. I'll explain about black holes sometime later.. it is just that time becomes 0 at
the horizon because of relativistic effects). Space bends and that is the source of Gravity.
Imagine that the universe is a trampoline. Place a heavy ball in the center and it pushes
the material downwards. Place another small ball on the edge and it
goes towards the big ball and revolves around it.
The sun is the big ball and earth, the small. It is difficult to draw a diagram that showed
curves in every direction possible but using one curve to represent a certain part of space
might be useful and it will help you understand better.
This theory predicts the existence of what are called the Gravitational Waves.
Ripples sent out in space by a heavy object. Just like light waves, they can be detected
by motions in space. I recommend you to search for 'Gravitational
wave detectors' on Google for more information. This sensing technology is done by placing
some lasers out in space and they are separated by huge distances.
When there are gravitational waves sent out by a huge object, there will be movement of
the lasers. If you just calculate the delay in receiving
the information from the laser, you can calculate how big the ripple is.
You can use it to calculate the size of the planetary object.
If it is too big, we can find the direction from which the big *** started (probably!).
The theory also tells us about how gravity contributes to the bending of light.
Wait! Did I say light?
Light is mass less right? How can it bend?
This is very easy to explain. Gravity, Mass and Energy bend space itself.
That is what the general theory of relativity says.
Even if light travels straight in space, space is bent and so light bends along with space.
Oh! We started with psychology and ended up here.
Wait for me to post the lecture two which will be on the topic Particles and Astronomy.
Thank you for watching this lecture