We'll continue our LEARNING series
during this quarantine period
Today's topic is GRAVITY
So, first of all, what is gravity? In
the most basic terms, gravity is the
force by which the earth pulls
everything towards it. So, for example, if
an apple- the famous example- if an apple
falls from a tree, it doesn't fly in the
sky! It actually falls down towards the
earth. But actually, it's not just that!
Now, let me ask you a basic question. What
happens when something is dropped?
And this is a very simple question. the answer is- things fall down towards the earth.
And people have been (long) fascinated by this simple question. So, Aristotle was among
the people who pondered over this
question. And he said "(For example) If you
have a ball made of iron and it weighs
10 kilograms; and there's another ball
made of iron,
but it weighs only 5 kilograms;
If you drop them from the same height, they will fall at different speeds."
And people believed in this for a long long time.
Now enters our next hero of the story of
gravity; and his name is Galileo Galilei.
The year is 1583. Galileo is 19 years old;
and, he lives in this city of Pisa.
Galileo is supposed to listen to the
sermons. So, there's a lot of chandeliers
on the ceilings, to give light to people
who are gathered to listen to the sermons.
But Galileo has no interest in
the sermons. Instead, he just starts
looking up. And he starts noticing- well,
there are big chandeliers and small
chandeliers- all kind of; all different
sizes of chandeliers, are just spread
across the ceiling. So, he starts
measuring the pendulum movement of these
chandeliers. Today we take things like
watches for granted, but in Galileo's time-
you'll be surprised to know that- such a
great scientist didn't have a reliable
way of measuring time. So, he uses his
pulse to time the pendulum movement
of the chandeliers. And to his surprise, he
discovered that the time it takes for
the pendulum to complete one cycle does
NOT depend on the WEIGHT (mass) of the
chandelier; but it actually depends on
the LENGTH of the rope or the string by
which it is hung to the ceiling. And that
was an eye-opener for young Galileo!
Because this challenges Aristotle's idea!!!
People had been believing Aristotle's theory.
To Galileo's surprise, MASS DOESN'T MATTER! So, what does he do? So, he then
took the leap in his reasoning; and said,"
I can then deduce that when things are
in free-fall, then also it doesn't matter
what the weights of the free-falling
things are. As long as they are dropped
from the SAME HEIGHT, they should reach
the ground at the SAME TIME. And so, in
1589, Galileo is about 25 years old.
He does this famous experiment. He drops
balls of different weights from the top
of the Tower of Pisa. And, he does a public display of his
discovery- that actually, WEIGHT DOESN'T MATTER !!!
Galileo then reasoned,"If you take
away the air; if you create a vacuum and
then if you drop a feather and a ball of
iron, they will fall to the ground at the
same time. They'll take the same time to
reach the ground! So, Galileo was
interested in knowing whether the speed
of a falling thing remains the same as
it reaches the ground? Or does it
increase? Or does it decrease? He didn't
know! So what he did is, he had a slope.
And from the top of the slope he would
drop a ball. And then, he would time the
ball on how much distance does it travel in
how much time. He finds that as the ball
keeps on rolling the speed increases!
He figures out that in the first second the
ball rolls- let's say- 1 meter. After 2
seconds, the ball has covered a distance
of 4 meters. After 3 seconds, the ball has
covered a distance of 9 meters. After 4
seconds, the ball has covered a distance
of 16 meters. After 5 seconds, the ball
has covered a distance of 25 meters.
Now, can you see the correlation here? This
law that Galileo discovered is called
the TIME SQUARE LAW; which means, the
distance traveled by a ball on a slope
can be measured as T Square. And mind you, Galileo did these experiments; and he
deduced this Time Square Law; and the law
of pendulum, when there were no reliable
sources of measuring time, the system of
equations in mathematics had not evolved,
the notations of mathematics had not
come about, people didn't use mathematics
the way we use! So, he was still using the
old mathematics rules of finding the
ratios and proportions. But, for the first
time, Galileo discovered ACCELERATION.
So, he reasoned- If you just make the slope
higher and higher, you know, more and more
towards the vertical line,
it will still remain; it will still keep
the relationship of acceleration. So he said-
Yes, as things fall down, they accelerate!
So, this idea of things gaining speed-
accelerating- was verified by Galileo.
But that's all Galileo could contribute to
our story of gravity. And now, let's move
on to the next hero of our story of
gravity; and that is, Sir Isaac Newton!
So, the year is 1665. Newton, 22 years old,
is studying at the University of
Cambridge. And in the nearby city of
London, the infamous bubonic plague raises
its ugly head. The scare is so much that
the University of Cambridge decides to
close the university; and all students
were then asked to go back to their
homes until further notice! So, Newton
starts a journey to his home about 100
km away. There, is quarantined for 18 months.
And in that solitude of his quarantine
period of 18 months is when he discovers
the Law of Gravitation. So, just like
today we have COVID-19; people have been
living in quarantine for 3-4 months now!
Well, if you use your quarantine properly-
-like Newton did- maybe you can end up
discovering a new theory of quantum
gravity! Who knows?
What is the Law of Gravity? Well, he summarized everything
with a very simple equation which is
this! This is a very simple equation-
'm1' and 'm2' are the weights (or the
masses) of things that we are considering
gravitation between; and 'r' is the
distance between them. And so Newton says-
"This is the relationship; and they must
be a gravitational constant (G). And Newton
doesn't know the value of that
gravitational constant (G) at that time but
he says -well, there must be.... we have to
find this gravitational constant (G) and
once we find it, we can actually find the
gravitational force between two things.
Now, that's super amazing! A simple
equation! And people worked on the equation
and they find that actually Newton's
Law works perfectly on things that, for
example, fall towards the earth.
Newton's genius was thinking- Okay, so
apples fall to the ground because
earth attracts them with the force -and
he named the force, gravity. So according
to Newton, gravity is a FORCE. Now keep
this in your mind. So, when things fall
down, they fall down because earth is
attracting them towards itself with the
FORCE of gravity!
And then Newton looked up in the sky; and
he thought- Wait a second!
So, what about the moon? Isn't it
possible that maybe, the earth also
attracts the moon around it; and keeps it in
the orbit because of its gravitational pull!
And that was the leap! And, by the
way, that's also the reason why earth
moves around the Sun! The Sun attracts
earth with its gravity! Not only earth, but
it also attracts other planets that we knew
at that time! And then people made
calculations, and they found- yes, that's true! But...
 
Very soon, people started finding the
problems with Newton's equation! It's a
simple equation; and it's true for most
things that we observe in our everyday life.
And, of course, for Moon and Sun and
the Earth's rotation around the Sun.
Things work very well with this simple
equation! But some scientists, some people,
also found that this equation is good
for a set standard- a minimum and a
maximum weight of things- but beyond it,
it doesn't work very well. So, there were
two problems with Newton's idea of
gravity as a force. First, when people
asked Newton "How does the moon stay
around the earth?", Newton says "Well, just
take a stone and tie a string around it
and move it around you. What happens?
What happens is, because of this (rotary) movement, the stone wants to go away but it doesn't go away!
Because the string keeps the stone
in balance moving it in the air so it
doesn't go away! Similarly, gravity is
like a string. It's an invisible string. I
can't find it, but maybe, in the future
people will find this string!" But people
said, "Well that's...that's like, you're
talking about... voodoo.... an invisible string
that keeps things in in balance in the
universe!" When people asked more,
Newton just said- well, these are the things that are created by a super power! because in
Newton's time even scientists believed
in God! He also believed in alchemy!
But anyway, despite these shortcomings, he was a legitimate genius of this time!
So, that's the first problem with this
theory of gravity offered by Newton
because there's no string!
The second problem was- when people measured motion
of different planets around Sun, they
discovered that yes, of course, Earth's
motion and Venus' and Mars' is defined
very well by Newton's equation.
But Mercury, the nearest planet to Sun it
deviates from the path predicted by Newton!
So, it doesn't follow Newton's equation!
Also, a new planet was found called Uranus.
And Uranus also doesn't
follow the path that's predicted by
Newton's equation! So, Newton's law of
gravity works BUT for a narrow spectrum
of things. It works for things that are
at a certain distance from the Sun; and
up to a certain distance in the solar
system.
There were limits to how far we can use
this equation. So, for things that are
very near to the Sun or that are very
far away from the Sun, this equation
didn't work very well.
Now enters our next hero in this story of gravity.
And his name is Albert Einstein!
Let's now move to 1915- the year Albert
Einstein devised the Theory of General
Relativity! But the work had already started-
he's already started thinking about it-
in 1907, when he was working at a patent
office! One day, he saw a painter who was
painting the walls of a neighboring
building. Some accident happened, and he
started falling down towards the Earth
and Einstein started thinking about it-
What does gravity have to do with a
falling person? Well, according to Newton
gravity is the force that is pulling the
person down towards the ground.
But Einstein thought- Imagine you are riding a roller coaster.
You're going up; and then, you start
falling down. When you are falling down
you feel very lightheaded; you feel as if
you will fly in the air;
you feel weightless!
Similarly, experiments have been done
with people on elevators. So, if you're
standing on a weighing machine inside an
elevator; and let's imagine the elevator
starts accelerating towards the Earth
you will observe that the weight- your
weight- actually decreases. If you
accelerate to the ground in an elevator
at 9.8 meters per second squared
actually the weighing machine will show
the weight as 0 kilograms
Isn't that interesting! Now if the
painter starts falling towards the
ground, does he 'feel' his weight? He
doesn't feel his weight! His weight is zero!
When something is in free-fall, it
doesn't have any weight! it has
ZERO weight. It's as if you're falling in
space! So Einstein said- a person floating
in the space is the same as a person
falling down from a building! Both of them
have zero weight. So both of them have an
equivalence. And that principle is called the
Principle of Equivalence. Now, why is that
important? Because Newton's idea of
thinking of gravity as a force doesn't
hold much weight! So if gravity is not a
force then what is it? And then Einstein
comes with a new explanation of gravity!
The speed of light (always) remains constant; and
using the speed of light he proves that
time and space are expandable. Space can
expand or contract
Similarly time can expand and contract.
And so, he proposed this idea of
SPACE TIME FABRIC. What is space time?
Well, space we understand. We have
three-dimensional space but Einstein
said space and time are interwoven into
each other.
Imagine a fabric made of elastic rubber.
Now that is space time. So space is the
three-dimensional space that you have
and time is interwoven into this fabric
of space. And so Einstein proposes that a
massive object like Earth
when it falls on that space-time fabric
it actually bends the fabric of
space-time and when it bends the fabric
of space-time it actually creates curves
in the space-time (fabric). Sun is much much more
massive than Earth so it warps the
fabric of space-time with a greater
curve. When earth creates the warp in the
space-time the moon for example revolves
around it not because of the
gravitational force but rather because
earth has warped the space-time. So the
moon is revolving just because of the
warping of space-time due to Earth.
Similarly we feel that the earth is
revolving around the Sun but in reality
the Sun keeps on moving; and the earth
keeps on falling down a space-time
curvature created by the Sun. So
Einstein's idea of space-time actually
expanded from understanding and after it
was discovered by Hubble that our space-
our universe- is expanding, actually
Einstein's equations predicted many more
things, which actually have proven to be
true. For example, Einstein's equations
predicted gravitational waves. In
September of 2015 for the first time we
were able to measure gravitational waves
which were generated 14 billion years
ago by two neutron stars (massive things)
revolving each other and smashing
into each other. I think it was September 14
2015. On that day, our Earth actually warped a little.
A mini teeny tiny amount; but it did press
a little. That's super amazing!
Einstein also predicts black holes which, now, astrophysicists have actually observed in the space.
Einstein also applied his equations on
the path of Mercury (planet). Now remember, if you
apply Newton's equations, you cannot
define with accuracy the part of Mercury
and Uranus. But if you use Einstein's
equations, Yes!
Einstein's equations are applicable on
everything you can measure with Newton's
equation but they go beyond Newton's
equation if you apply Einstein's
equations on Mercury and Uranus you can
actually measure- predict- their paths around the Sun with much more accuracy.
So, now the idea of gravity- because of
space-time- actually has many more things
than just the planets around the Sun. So
gravity is NOT A FORCE but rather it's a
FIELD. What is a field? A field is just
like a matrix that's interwoven all around
you; and it's when a massive thing- like
Earth- interacts with space-time it warps.
And that curving of the space-time-
warping of the space-time- creates
gravity. So, what we call Gravity is
nothing but warping of space-time.
So, gravity is a field; it's not a force. And
that is a big massive leap. So, we are
coming all the way from Aristotle
thousands of years ago, saying that-
yeah, things just fall down to the ground; and
their speed depends on their bus the
Galileo & Newton proved that that's not
true. And Einstein proved that actually
it's not a force. Gravity is not a force
it's a field. It's a matrix, which
interacts with massive objects- things
with mass. BUT...
Einstein's space-time idea of gravity
has also its own limitations. First of
all, it doesn't define massive black holes. So, first let's talk about what a black hole is.
What happens when the star dies?
And let's say the star is as massive as Sun.
When a star dies it enters into
different phases. Finally, it reaches the
stage of a neutron star.
What is a neutron star? A neutron star
has the same weight as- let's say- our Sun
but the diameter of a neutron star is
around 20 kilometers.
And when this thing interacts with the fabric of space-time, it actually makes a very big warp
into it. That's what a neutron star does! A
neutron star creates a huge warp because
of its extremely high mass but limited
space. So, the space-time that it interacts
with has a very small horizon. So, that's
what neutron stars do! Now, what is a black hole?
A black hole is a neutron star's BIG BROTHER. What do you mean by that?
A black hole is- first of all it's
not a thing; like, for example, a neutron
star is a neutron star, Sun is Sun, but a
black hole is not a thing. It's a PLACE.
where things happen! So imagine the
weight of Sun, but in the dimensions of a
small marble ball. So now when this kind
of thing interacts with space-time it
makes such a huge warp into it that the
space-time breaks in those places!
So, in a black hole, the idea of space-time
doesn't exist any longer! So Einstein's
equations don't hold in a black hole. That's the
upper limitation of Einstein's space-time
equations. Now, there's also a lower limit-
and that is Quantum Mechanics. At the
smallest of the smallest level, small
particles- they don't follow Einsteins
space-time equations. A proton or neutron
or electron- they have some mass. So as long as you have mass, you should be able to
interact with the space-time around you!
But they don't follow Einstein's
space-time equations; they don't warp the
space-time as they should- as predicted
by Einstein's equations. So very small
things work on the mechanism of quantum-mechanics.
Quantum mechanics doesn't follow Einstein's space-time equations!
So, WHAT IS GRAVITY?
We started with gravity as simply a
force. Then we said well Gravity is not
just a force, it's a field! But even that
breaks down at very small- at sub atomic
level, AND, at very big- at black hole
level- Einstein's equations don't hold!
So, today scientists are trying to find
what you can call Quantum Gravity, wherein
everything from subatomic to the
black hole and beyond them is defined by
a single equation! Scientists today are
trying to find the common ground between
quantum mechanics and Einstein's field
equations. And when these things match
that would be a uniform theory of Gravity. So that was our video for today. Thank you for
watching; and as always, please Like,
Subscribe and spread this video as much
as possible! And I'll see you in the next
one. Until then, bye bye!!!
 
