Haripriya Latheeshan/ Bahrain
“Mankind was born on Earth, it was never meant to die
here”-Joseph Cooper
Joseph Cooper exists only in a fictional universe created
by the genius movie director and screenwriter
Christopher Nolan.
The person reading this article is
probably familiar with this iconic director. So must be
the movie from which the phrase mentioned above has
been quoted, Interstellar, an epic science fiction movie with deep roots in scientific
facts. Then again, that’s a classic Nolan movie trait. This movie is all about space
travel, time travel, higher dimensions and much more. But what closely intertwines
these things in the movie is something that is known to probably every 5th grade
student. A concept or phenomenon that is ,at a point in the movie, referred to as a
ghost and as a powerful force at another point. What is it, you might ask? Well, it is
gravity. What is gravity? Is gravity a phenomenon well understood? Do we have
enough information about its workings and associated intricacies? Or is it still an
esoteric, abstract concept that is yet to unfold itself?
One of the earliest attempts to give a scientific explanation for gravity was made by
the Greek philosopher Aristotle, who described gravity as the tendency of the
material bodies to go to their “natural location”. Terrestrial objects belong to the
Earth, and they return to the Earth whenever displaced from their natural location.
Other objects have other natural locations. For example, fire belongs to heaven, and
its natural tendency is to rise towards the heavens. However, he advocated a
geocentric model of the universe, where the Earth is at the centre of the universe.
An Italian astronomer and physicist Galileo Galilei, showed that gravity causes
objects of different masses to fall with the same acceleration. He is said to have
demonstrated this (between the year 1589 and 1592) by dropping two spheres of
different masses from the leaning tower of Pisa, with both of them reaching the
bottom of the tower at the same time. A few physicists, however, believe this
experiment to have been just a thought experiment rather than a physical one. In
recent years, researchers have replicated this so-called “test” by dropping atoms of
two different isotopes in a tube kept under vacuum. Both the isotopes accelerated at
the same rate to an accuracy far greater than 0.0001%. Yet, there was no explanation
that could unify the mechanics of how objects fell on earth with those of how objects
moved in the sky.
Then came the legendary, and most influential, scientist and mathematician, Sir Issac
Newton. In his book “Philosophiæ Naturalis Principia Mathematica”(Mathematical
Principles of Natural Philosophy), published in the year 1687, he made a
comprehensive conceptual and mathematical description of his perception of gravity,
a lot of which is still valid to this date. He showed that the force that makes the apple
fall and that holds us on the ground is the same as the force that keeps the moon and
the planets in their orbits. He demonstrated that terrestrial and celestial gravity were
one and the same. In his law of universal gravitation, he states that every object in the
universe attracts every other object with a force which is proportional to the product
of the masses of the two objects and inversely proportional to the square of the
distance between them. The greater the mass of the objects involved, the greater
would be the gravitational force of attraction between them, and the greater the
separation between the objects, the smaller would be the force of attraction between
them. It’s only the law of gravitation that we need to get to the moon and back. This
theory could accurately predict much of the observable celestial phenomena within
our solar system.
Nevertheless, over time, many phenomena have been proven to behave in a manner
that defies this law and the foundations that it was built on. An example of such a
phenomenon is the precession of the planet Mercury. Newton’s law of motion can
clearly describe the motion of any two-body system in our solar system, but the
precession of the planet Mercury did not match the predictions of Newtonian
physics. Several theories were proposed to explain this deviation, but none of them
were proven right. Also, Newton inherently assumed that gravitation is an action-at-adistance force. That is, even without any physical contact, a change in the position of
one mass is instantly communicated to all other masses. Newton’s theory didn’t
address the reason for the nature of gravitational force being this way. In fact, in his
letter addressed to Richard Bentley-a fellow of the Royal society of London, he
rejected the possibility of remote action .In the letter he wrote,“gravity must be
caused by an agent acting constantly according to certain laws but whether this agent
is material or immaterial,I have left to the consideration of my readers.”
Centuries passed by without any significant contributions to the concept of gravity.
Then, in the year 1915, German-born theoretical physicist and genius Albert Einstein,
in his “Theory of General Relativity”, came up with a startling revelation that gravity
is not the result of a force, rather it emerges out of a geometric property of space and
time, or four-dimensional space-time fabric. Gravity, at least according to the theory
of relativity, is just a consequence of the existence of mass in space. Massive,
energetic objects, merely by virtue of their presence, warp the space surrounding
them, and this is manifested as gravity. This theory compares space to a trampoline,
and any object placed on it can warp and curve space, forming depressions in a field
called the Space-time fabric. This distortion in space-time is what makes planets,
moons, and other celestial bodies move in a curved path around another massive
celestial body, such as a star. The greater the mass of the objects placed on the spacetime fabric, the greater the distortion it causes. Surprisingly “the space-time tells
matter how to move and matter tells space-time how to curve” Moreover, according
to this theory, whenever masses accelerate, changing the distortion of space-time,
ripples are created in the fabric. These ripples are referred to as gravitational waves.
The theory of general relativity could accurately account for the precession of
mercury. It also predicted the existence of regions of space in which space and time
are distorted in such a way that nothing, not even light, escapes. These regions are
called black holes, a term that has garnered much fame and attention due to the
concept of black holes being at the center of the plots of several sci-fi movies,
“Interstellar” being one of them. Over the past few decades, several predictions of the
theory of general relativity have been proven to be accurate. One of those predictions
is what we call gravitational lensing. This is a term used to describe a phenomenon in
which light takes a curved path due to the gravitational pull exerted by a star or
celestial body. So, as light travels towards an observer, it will bend and give an
incorrect indication of the position of the source of light, i.e., the position observed
will not be the actual position of the body in space. In the year 1919, Arthur
Eddington and his team, during a total solar eclipse observed that light from stars
passing close to the Sun was slightly bent and the stars appear slightly out of position
and hence proved the predictions of the theory of relativity.
Another such prediction of general relativity is gravitational time dilation, a term
describing the effect that gravity has on time. For the speed of light to remain
constant, as required by the special theory of relativity, time itself must pass slower in
stronger gravitational fields where light bends more. Time dilates in a stronger field,
so time slows down or speeds up according to the way in which the mass wraps
space-time. This dilation of time has been verified multiple times by experiments.
Remarkably, time dilation due to gravity is significant enough that GPS satellites
must adjust their internal clocks to take into account their faster time so as to
accurately determine the location of GPS receivers on the ground.
Now coming to the last prediction of theory of general relativity-gravitational waves.
All the existing telescopes are completely blind to gravitational waves.So a different
set of instrument is required to detect the gravitational waves.Two towns, separated
from each other by thousands of miles and located on the North American continent,
are home to two very advanced instruments that might have accomplished the task of
detecting gravitational waves. There are two L shaped buildings one in Hanford
Washington and another in Livingstone Luisiana.Two identical instruments called
aLIGO, or advanced Laser Interferometer Gravitational Wave Observatory, a device
that uses interference of light to detect anomalies in the energy radiated by the
universe. The concept is simple, yet the mechanics and mathematics may not be. The
physical world, which includes the aLIGO apparatus, can be stretched and squeezed
by gravitational waves as it pass through the instrument. Therefore, when light is split
and directed to the two tubes of this L-shaped instrument and later merged together, a
difference in pattern or interference can be observed. What remains after all noise or
random interference has been removed, is the proof of gravitational waves. This is the
reason for having two identical instruments, kept so far apart and running the same
test; to cancel out all the irrelevant information through comparison of data at both
locations.
In 2015, aLIGO was able to detect gravitational waves (it was awarded 2017 Nobel
prize in Physics) that might have originated from a collision of two massive black
holes 1.38 billion light years away, (1.38 billion years ago) one with 29 times the
mass of the sun and the other with 36 times the mass of the sun. When this celestial
event occurred, our earth was nothing but a barren waste land with no plants or
animals, but primitive multicellular creatures in the sea. This discovery is a great
vindication of Einstein’s theory of general relativity, one of the many predictions of
the theory that have been proven to be accurate.
The detection of gravitational waves might be able to give us a more accurate
measure of how fast the universe is expanding and how much dark energy there is in
the universe. It may also be able to help us understand the nature of space-time fabric.
To be honest, we might not even be able to imagine what discoveries we will make
by unlocking the secrets of gravity. In fact, what we find may even be beyond our
understanding or comprehension.
So, is general relativity an adequate theory of gravitation which can explain every
existing phenomena in the gravitational realm? Even though it well passed many
observational and experimental tests, it fails to work in situations like blackhole
singularities (also initial singularity predicted by some models of bing-bang). Within
the blackhole lies something that seems impossible , a mass concentrated in a small
point with infinite density-this is called singularity .The perplexing aspect of
singularities is that General relativity seems to break down here. To figure out what
happens at this really smalls scales ,we need Quantum mechanics.Right now, the two
theories – general relativity and quantum mechanics remain incompatible.
If the two theories -general relativity and quantum mechanics -could be brought
together, we could truly understand how gravity behaves at the tiniest scale and what
gravity actually is, or what the essence of it is. This combined theory is called
Quantum Gravity. It may open up possibilities that we can’t comprehend right now
and answers to questions that have never even been asked. Are black holes gateways
to wormholes leading to other universes or other places in our universe? Can we
harness and control gravity? Can we reduce gravity to the extent of making rocket
propulsion easier? And most importantly, how will all these affect us? We simply
won’t know until we have unlocked the theory of quantum gravity. All that we can
say is that ,what we find will definitely cause a paradigm shift in our understanding
of the world we live in and the universe that it belongs to.
