General relativity and quantum physics, are two distinct theories.
Both of them work very well, in their own distinct domains.
It has been the dream of scientists, to unify them into a single fundamental theory.
The quest is still on.
Scientists are still struggling to find a unified theory.
The quest for unification is not new.
It has happened several times, in the history of science.
Each time the answers required as to break away completely,
from old ways of thinking, and understand it in an entirely new way.
At one point we thought, that the sun was a great fire, created by God.
The stars were pin holes, in the celestial sphere, to let in the light of heaven.
At this point it was impossible to believe, that the stars are actually suns.
Early scientists had great difficulty, in making people believe this.
It went against basic religious beliefs.
Some of pioneers of this thought, were persecuted, for their advanced thinking.
Today however we believe in it, without any difficulty.
This is a powerful illustration, how new insights for unification,
can be very difficult to conceive, and accept.
Humans, plants, animals, fungi and bacteria seem like very different things.
Could it be possible, that they all have a common ancestor?
Impossible to believe a few centuries ago,
but now commonly understood, even by school children.
The chemical processes, that construct and power the cells,
are the same, across the whole spectrum of life.
This unified understanding, was a great leap forward.
In the history of science there have been many theories of unification,
which was not correct.
For example, it was once thought that sound and light were essentially the same thing.
Sound was vibrations in air.
Light was vibrations in something proposed as aether.
Of course, it turned out there is nothing called aether,
and sound and light are very different concepts.
This teaches us to be wary about proposed theories of unification.
Today there are several theories of unification,
of special relativity and quantum physics.
We do not know, which one of them is true.
Aristotle proposed that Earth was at the centre of the universe.
The planets were unified with the sun and the moon.
This unified theory was believed by everyone, including scientists,
for more than thousand years.
Copernicus proposed a new theory, where the sun was at the centre of the universe,
and Earth was unified with other planets.
This broke down the entrenched and created a new model of reality.
In the frontier of science, sometimes we need to breakaway from old beliefs.
One of the outcomes of the sun centred solar system,
is that the Earth is moving around the sun.
This was difficult for early scientist to believe.
If the Earth was moving, how come we don't feel it?.
We do understand now that motion is relative to the observer.
Sometimes when a new theory is propounded,
it makes some assumptions which cannot be tested.
Copernicus was the first to propose that the Earth moved around the sun.
He needed to explain why the relative position of the stars do not change,
even though the Earth moved.
Copernicus had to make the assumption,
that the stars were at a great distance from Earth.
At that point there was no way, that the distance of the stars could be measured.
Though it could not be tested at that time, the theory turned out to be very true.
If stars were at a great distance, how come we are able to see it?
They must be very bright, perhaps as bright as the sun.
This imaginative proposal, also turned out to be true, though it could not be tested.
Later experiments however proved all these assumptions to be true.
When propounding the new theory, we may not be able to test some assumptions.
But, eventually the technology for experiments improves sufficiently to test the assumptions.
Einstein proposed the existence of gravitational waves.
It took about 100 years to set up a laboratory, which could detect gravitational waves.
In the frontiers of science, it is possible that sometimes theory precedes experimental proof.
Sometimes different proposals for unification go together.
The proposal that stars are unified with the sun,
goes with the proposal, the planets are unified with the Earth.
Both of them require that motion and rest be unified.
Particle physicists sometimes need to invent an unseen particle.
They need this particle to make sense of certain theoretical or mathematical results.
One example is the neutrino.
The neutrino needed to interact very weakly, to satisfy the equations.
It was difficult to detect, because they interacted very weakly.
Much later technology improved sufficiently, to build a lab which could detect neutrinos.
Neutrinos were eventually discovered.
They also found that it interacted very weakly.
In this case, experiment was able to prove the existence of a imaginary particle.
Kepler discovered the laws relating to planetary motion.
This took the Copernicus unification theory to its logical conclusion.
Copernicus had proposed that the sun was at the centre of the universe.
But he did not know the role it played in the motion of the planets.
Kepler wondered why the sun was at the centre of each planet's orbit.
Could the sun be exerting some force which influenced the motion of the planets?
To answer this question, Kepler had to predict the position of the sun,
in each planet's orbit.
He made a break through discovery that the orbits were elliptical.
He discovered that the sun was exactly at the focus of the ellipse.
This was the first law.
The second law was that the speed of the planet, increased when it moved closer to the sun,
and decreased when it moved farther from the sun.
He also discovered the third law, which governed how the speeds of the planets were related.
If a new planet is discovered, we can predict its path, using Kepler's laws.
Newton took unification to an even higher level.
He had the great insight, to realise that the force exerted by the sun on the planets,
is the same force of gravity that holds us on Earth.
Now the Earth was unified with the planets.
The sun was unified with the stars.
Rest and uniform motion was unified.
Gravity on Earth was unified with gravity of the sun.
The result was a Newtonian revolution, that transformed our understanding of nature.
The next significant unification, was the unification of electricity and magnetism.
Maxwell achieved this in the 1860's.
He built upon the discoveries made by Faraday.
In the 1840's Faraday had proposed a powerful idea called a "Field".
This explained how a force could be conveyed through empty space,
from one body to another.
Faraday studied the electric field.
We can visualise it as an arrow which can vary its direction and length.
We need to imagine such an arrow in each point of space.
The arrows arrange themselves so that they point to nearby negative charges,
and away from nearby positive charges.
Faraday studied magnetism.
He invented the magnetic field.
This was also a collection of arrows, which point to poles of a magnet.
His laws explained how electric and magnetised field arrows are influenced,
by nearby charges and magnetic poles,
and also by arrows of nearby fields.
Maxwell unified both the concepts of electric and magnetic fields.
This allowed the electric and magnetic fields, to turn into each other.
This led to the concept of the electromagnetic wave.
Maxwell was able to compute the speed of the electromagnetic wave, from his theory.
Amazingly it turned out to be the same as the speed of light.
Maxwell was able to achieve something far greater,
than unification of electric and magnetic fields.
He discovered that the waves passing through electrical and magnetic fields, are light.
Maxwell realised that electromagnetic waves was there at different frequencies.
Light was a electromagnetic wave, in a certain range of frequencies.
This led to the discovery of radio, infrared, ultraviolet,
and entire spectrum of electromagnetic radiation.
Maxwell's discoveries led to a conflict between two unifications.
The unification of everything as matter obeying Newton's laws.
The unification of motion and rest.
Young Einstein meditated on this puzzle for 10 years,
starting at the age of 16.
In 1905 he realised that the resolution required,
a complete revision of our understanding of space and time.
He realised that distinction between electric and magnetic effects,
depends on the motion of the observer.
Not only were the electric and magnetic fields, different aspects of the same phenomena,
but different observers who draw the distinction differently.
One observer might explain a particular phenomena in terms of electricity,
while another observer, moving relative to the first,
who would explain the same phenomena in terms of magnetism.
The two would however agree of what was happening.
Einstein's special theory of relativity was born.
It unified Galileo's unification of rest and motion,
with Maxwell's unification of electricity and magnetism.
Many things followed from this.
One consequence is that light must have a universal speed.
This must be independent of the motion of the observer.
Another consequence was the unification of space and time.
Einstein's theory of special relativity was profound.
But Einstein himself was moving ahead,
and was thinking of something more deeper and more profound.
This eventually led to the theory of general relativity.