Relativity

Albert Einstein was a renowned scientist, in the 20th century.

In the early 20th century, he propounded the theories of relativity,

which revolutionised science.

Einstein was a humble clerk, when he published, the theory of special relativity, in 1905.

Ten years later, he expanded the theory to include gravity,

to propose the theory of general relativity.

Einstein's theory of relativity, led to the formulation of many more modern theories.

His theories of relativity, shook the foundation of science,

and led to the birth of many more revolutionary theories.

Even after a century, after its publication,

it is still a part of the tapestry of today's leading edge scientific research.

In 1905 light was considered as a wave.

Einstein wrote a paper, which proposed that light comprised of particles, called photons.

This is the beginning of quantum mechanics.

He then went on to publish the special theory of relativity.

This theory proposed the distance, speed, and time are relative,depending on the observer.

As a consequence of special relativity, he derived the now world famous equation,

E=mc squared.

According to Newton's theory gravity exerts its influence across space instantaneously .

Einstein sought to rewrite this fundamental belief.

In 1915 Einstein announced the theory of general relativity.

We can understand the concept of relativity, from several perspectives.

We will discuss it from a few perspectives.

Einstein did not use laboratory equipment to formulate his theories.

He used "thought experiments", to visualise and formulate his ideas.

We will also use in our discussion "thought experiments", to present some concepts of relativity.

According to the theory of relativity, if we have two objects and nothing else,

then it is not possible to tell, which object is moving,

and which object is standing still.

This is true even if one object, is an entire planet.

We will illustrate this concept with some thought experiments.

Let us imagine that Malli, is flying in an aircraft.

Ashok is standing on earth, and observing the aircraft.

Malli perceives, that the aircraft is standing still, and the earth is moving under it.

Ashok feels that Malli is moving.

Both the perceptions are equally correct.

There motion is relative.

We on earth perceive the sun as rising and setting,

we feel that the sun rises and moves across the sky.

Let us imagine an distant observer, standing well outside our solar system.

He will feel that the sun is standing still, and planet earth, and the aircraft with Malli,

is revolving around it.

Let us imagine an even more distant observer standing, outside our galaxy, the milky way.

He will perceive, that the sun is moving around, in the milky way,

revolving around an invisible black hole.

Any observer can say, that they are standing still,

and the rest of the universe is moving.

If we watch the night sky, through the night,

we will say, while we stand still, the stars are moving across the sky.

An observer in a planet, of a distant star, would say the same thing,

of all the stars, including our sun.

All the observers would be equally correct,

because all motion is relative.

The scientist Maxwell unified the theory of electricity and magnetism.

Electro magnetic theory was a major breakthrough in science.

All electromagnetic waves travel through empty space, at a fixed speed.

This speed is the cosmic speed limit, denoted by 'c'.

The value of 'c', the speed of electromagnetic wave propagation,

is about 300 thousand kilometers per second.

Nothing can travel faster than this cosmic speed limit.

Light is one type of an electromagnetic wave.

The speed of light is also about 300 thousand kilometers per second.

It is common practice, the speed of light as the cosmic speed limit.

According to classical relativity, speed is relative.

This raises the question, "What is the speed of light relative to?"

Einstein restated classical relativity.

He proposed that the speed of light is the same for all observers.

This means that speed of light is not relative to the speed of the observer.

It is an absolute value.

Any person moving at a constant velocity, will observe the same laws of physics.

For example, force is equal to mass into acceleration.

Momentum is equal to mass into velocity.

Velocity is equal to acceleration into time.

In the same manner, for a person moving at constant velocity,

the speed of light is an absolute value of 300 thousand kilometers per second.

This leads to some interesting implications.

This means that time and distance, are not absolute, but relative.

The concept of time being relative, is called time dilation.

We will again use thought experiments, to illustrate this concept.

We will imagine that Malli is in a moving spaceship.

Ashok is standing still on earth.

Malli fires a laser to the ground.

The beam of light strikes earth and returns to the spaceship.

Malli is able to record the time taken, to return to the spaceship.

Ashok is standing on earth, and observing this beam of light.

Since the spaceship is moving, he observes that the beam of light,

traces a 'V' shaped path, from the spaceship to earth, and back to the spaceship.

From Ashok perspective the beam of light has travelled a longer distance,

to bounce back to the spaceship.

The speed of light however should be constant to all observers.

Malli observes the beam of light, as travelling straight down and up.

Malli's clock will read less time between the firing and return,

compared to the stationery Ashok's clock.

This means that Malli's clock will run relatively slower,

compared to Ashok's clock.

This might sound incredible.

We have always perceived time to be constant, regardless of the observer.

One of the remarkable concepts that comes as a by-product of relativity,

is that time itself is relative.

Malli's clock will record less time, for the laser beam to bounce back from earth.

Malli's clock is perceived as slower than Ashok's clock.

The faster Malli's spaceship travels, the slower the time will flow, in the spaceship.

If the Malli's spaceship, manages to travel close to the speed of light,

time will almost come to a stand still.

Malli will never notice that the time is flowing slower.

Everything in the spaceship will be slower, by exactly the same amount.

Even Malli's speed of thought, will flow slower.

So from Malli's perspective time is flowing normally.

Malli perceives that the spaceship is standing still,

and the rest of the universe is moving.

Malli will think that everyone else's time is moving slowly.

Let us imagine that there are three spaceships,

moving in the same direction, and with the same speed.

Since they are moving at the same speed, Malli perceives that the other ships are stationery.

Malli is the commander of these spaceships,

and is travelling in the spaceship in the centre.

Malli fires a laser to the other two ships.

The two other ships will receive the laser beam at the same time.

Ashok sees the light from both lasers, move at the same speed.

Ashok will see the left spaceship receive the light,

before the right spaceship.

The clocks on both the spaceships, read the same time,

when they receive the laser light.

Ashok will perceive the clock on the left spaceship,

to be running ahead of the clock on the right spaceship.

From Malli's point of view, time on both spaceships is the same.

The perception of time is relative to the observer.

Malli sends a command for all the ships,

to fire their engines at the same time.

From Malli's point of view, all spaceships, will accelerate together.

The distance between them remains the same,

From Ashok's point of view, the time on the left spaceship,

is running ahead of the time in the right spaceship.

Ashok will see the left spaceship accelerate first,

and the right spaceship accelerate last.

The distance between the spaceships will shrink.

Each spaceship can be thought of, as being made of smaller spaceship.

The distance between them shrinks as they move faster.

The closer the spaceship, approaches the speed of light,

the shorter it becomes.

Malli will never notice that the spaceship is getting shorter.

Everything inside the spaceship, will get shorter by exactly the same amount.

For example, a ruler on the spaceship, will also shrink in proportion.

Measured with this ruler, the spaceship will be of the same size.

From Malli's point of view her spaceship is standing still,

and the rest of the universe is moving.

From her perspective the rest of the universe is getting shorter.

If Malli is moving at almost the speed of light,

Then from Ashok's point of view,

the flow of time inside the spaceship will almost stop completely.

Ashok will perceive Malli travelling along the universe,

while the clock moves only a few seconds.

Malli feels that time is running normally.

The length of the universe passes in a few seconds,

because the length of the universe has shrunk to almost zero.

This illustrates that all observations are relative to the observer.

The only exception is the speed of light, which is the same for all observers.

Einstein proposed that gravity and acceleration are equivalent.

This helped him generalise the concept of special relativity.

This lead to the formulation of general relativity.

Let us imagine an elevator in which the cables and other connections have been disconnected.

We can imagine the elevator chamber in such a condition will freely fall towards Earth.

Now let us imagine a person standing inside a freely falling elevator.

He will feel weightless, as if he was floating.

If he drops his keys, it will float along with him.

The person has no way of knowing, if the elevator chamber was falling at an accelerated rate,

or whether he was floating, in a zero gravity region or space.

Now let us imagine, that the same chamber is in a zero gravity region of space.

Now the chamber and himself are gravity acceleration free.

Let us say, a force is applied on the chamber, so as to pull it up at an accelerated rate.

Now the person, would feel his feet pressed to the ground.

This is similar to a feeling experience, when a car is accelerating,

and driver experiences himself being pressed to the back of his seat.

Now the accelerating chamber, acts as if gravity is acting on it.

If the person drops the key now, it will fall to the floor, just like it would on Earth.

There is no way, to distinguish the effects of gravity, and the effects of being accelerated.

This is called the equivalence principle.

The effects of gravity and acceleration are the same.

Both are the same manifestation of the same phenomenon.

Some cosmic field accounts for both gravity and acceleration.

Imagine a chamber that is accelerating upward.

A light beam comes in through a pin hole in one wall.

By the time it reaches the opposite wall, the light is closer to the floor.

This is because the chamber is accelerating upwards.

We can also state it as the chamber is resting still, in a gravitational field.

This implies that light should bend, when passing through gravitational field.

Einstein was interested in how a gravitational field acts on matter,

telling it how to move,

and how matter generates gravitational fields, in spacetime,

telling spacetime how to curve.

Finally he managed to find the equation to describe special relativity.

The left side of the equation is known as Einstein tensor.

It describes how the geometry of spacetime is warped and curved by massive objects.

The right side describes the movement of matter in the gravitational field.

The interplay between the two sides,

shows how objects curve spacetime, and in turn,

this curvature affects the motion of objects.

With his special theory of relativity,

Einstein showed that space and time did not have independent existences,

but instead formed a fabric of spacetime.

This fabric of spacetime was not merely a container for objects and events.

It had its own dynamics, that were determined by,

and in turn help to determine, the motion of objects within it.

This like the way, the fabric of a trampoline will curve,

as a heavy ball moves across it,

and in turn the dynamic curving of the trampoline fabric,

will determine the path of the rolling ball.

The curving and rippling fabric of spacetime explained gravity,

its equivalence to acceleration, and the general relativity,

of all forms of motion.

This was probably the greatest scientific discovery ever made.

We can take the analogy of Malli in the spaceship.

Things change dramatically, if Malli fires the rocket,

to accelerate the spaceship.

Malli will get thrown back in her seat.

She will feel a force acting on her.

She will perceive that the universe is accelerating.

She feels a gravitational field is acting as a force to push her back.

The gravitational force we can see is a perception caused by acceleration.

Gravity and acceleration are equivalent.

Gravity is not a force, but a curvature in spacetime.

Spacetime can be visualised as a two dimensional fabric.

Objects with mass, cause a curvature in spacetime.

The larger the mass, the larger the dent, that it will make,

in the spacetime fabric.

We can visualise the solar system, from a spacetime view point.

The sun is much heavier than the planets.

It will make a large dent, in the centre of the spacetime fabric.

This causes a steep curvature, in the fabric.

This curvature, attracts other planets, towards the sun.

This is interpreted as a force of gravity, by conventional thinking.

Gravity only appears to be a force.

It is actually, the curvature in spacetime, which is responsible for this perception,

of the gravitational pull of the planet, towards the sun.

The planets themselves cause a curvature in spacetime.

The magnitude of the curvature is proportional to the mass of the planet.

More the mass, more will be the curvature.

The curvature caused by Earth, for example, causes the moon to orbit it.

Relativity theory proposes that gravity was a warp in space and time.

This was a startling idea, even to the best of scientists.

All of them had been grounded, in Newton's theory of gravity.

In the traditional theory, a planet such as Earth,

attract and pulls towards itself other objects.

For example, an apple falls to the ground, due to the gravitational attraction,

of the Earth.

According to Einstein's theory, the Earth dents the surrounding environment,

causing the apple to slide along a spacetime chute, and directs the apple,

to fall to the ground.

At this time, it was only a theory.

In 1919 scientists established that the position of stars,

was slightly different from what Newton's theory predicted.

Then stars position was in conformity, with Einstein's theory.

Einstein was relatively unknown till this time, suddenly shot into fame.

Charlie Chaplin once remarked to Einstein:

"People applaud me, because every one understands me,

people applaud you, because no one understands you".

This was true to a large extent, because Einstein's theory was so radically different ,

that it was very difficult to grasp, even by scientists.

General relativity went on to trigger many other dramatic development in science.

Modern cosmology which involves the origin and evolution of the entire universe,

is one of them.

Scientists used Einstein's equation to show that space should be expanding.

Einstein initially did not subscribe to this idea.

He even went on to insert the infamous "cosmological constant",

hoping to prove, that the universe is static.

Even great minds can be briefly wrong.

Observations from Hubble's telescope, provide convincing evidence,

that distant galaxies were moving further and further away, from each other.

Einstein was forced to accept that the universe was expanding.

An expanding universe, meant that the universe was much smaller in the past.

Extending this line of thought, it implied that the universe,

emerged from a single primordial speck.

This led to the formulation of the Big Bang theory.

The Big Bang theory, has been subsequently refined and developed,

and is now widely accepted by scientists.

Interestingly the revised version, of the discarded "Cosmological constant",

was resurrected, in the Big Bang theory.

Sometimes Einstein's mistake, itself could be correct.

Another scientist went on to derive an exact solution to Einstein's equation.

This would give a precise description of the warped spacetime,

produced by a spherical body like the sun.

This revealed something, even more interesting.

If we compress any object, to a sufficient small size, strange things happen.

For example, if a sun was compressed to an object 5 km across,

the resulting spacetime wrap, will be so severe,

that anything approaching too close, including light will be trapped.

This led to another new concept in science, the "Black hole".

We now know that Black holes are very real, and plentiful in the universe.

Scientist like Stephen Hawking have made further contributions, in this field.

The outer edge of a black hole, is called the event horizon.

Quantum processes can effect the understanding of the event horizon,

and even the interior of the black hole.

Scientist are trying to blend general relativity, with quantum mechanics.

Einstein's theories seeded a number of new fields of science.

His theories are relevant even today.

They have opened up an entirely new vision of scientific thinking.

This has shaken up all our earlier beliefs of reality.

This new reality is not yet fully understood,

and is the subject of intensive research.