What’s the Matter?
So the end of 2015, we brought the end of the centenary year to the year Einstein’s General Theory of Relativity was fully proved, mathematically and experimentally both. So generally there has been much fuss about this between people and everyone’s talking about Einstein’s great achievement and remarkably most of the talk has been on Einstein’s miss on, maybe embarking on the best discovery in cosmology in all time, arguably of all time in physics. But what I’m going to describe you about in this is what invaluable contribution in physics was made by Einstein in this tremendous theory and will also try to see and understand what caused his famous mistake and how important was it .
The Concept of Space :-
Space has always been an idea of much interest for human kind. People have wondered from the time of renaissance of what actually is Space? Interestingly when Newton wrote his famous book Principia Mathematica , he once wrote that “ Space and Time are such entities which can easily be understood by everyone and I feel no need to define them “. But Newton might as well have felt the need later on in his life when he finally defined space in his ideology. Newton said that space is like a stage, where actors perform their roles and go away, events keep on occurring on the stage while it itself doesn’t changes. Simply, Newton described space as an absolute quantity, unchanging and not influenced by events occurring in it. Also Newton’s contemporary and another independent creator of Calculus, the branch of mathematics for which Newton is acclaimed, Gottfried Wilhelm Leibniz said that space is meaningless if it’s empty, pretty much like an alphabet is without the words. His and Newton’s ideology was pretty similar. But this ideology of space was seriously challenged by an Austrian fellow named Ernst Mach. His ideology was that in an empty universe or more precisely in empty space if you spin, you’ll feel nothing. So in Mach’s way of thinking, acceleration is only felt when you accelerate relative to the average distribution of material around in the cosmos. This insight generated excitement because it pointed towards a view of space in which it is not absolute as Newton said. But the idea of space alone would not be enough to know the secrets of the cosmos as humankind came to know at the start of 1900’s.
A Revolutionary unification ;-
At start of 20th century the concept of space was well into question and was not alone in list of big questions, light was amongst them. Questions concerning the nature of light, its medium independence were all into question. And then arose Albert Einstein from the shadows of being a patent clerk in an office in Burn, Switzerland. He gave in 4 astoundingly brilliant papers on the light and it’s relativeness and what happens when you move at speeds close to that of light and his insights were stunning for not only the physics community but for science as whole. Now roll 2 years from 1905 and in 1907 , Einstein’s former teacher , at Polytechnic university in Zurich where he had studied , Hermann Minkowski showed that Space and Time as individual entities could not be treated properly and he said that the special theory of relativity , the accumulation of all the ideas of the 4 papers in 1905 presented by Einstein , could be better understaood by a unification of Space and Time as Space-Time. Take a common experience to understand it , while you may sitting in front of your computer reading this article , you’re at rest and so by your frame of reference you’re not moving. But time is still going on as you read this and so you are moving through time. And so Minkowski said that to understand the cosmos treating space and time together is the best option. Einstein took this thing one step further; he asked what would happen if Space-Time is forced to change shape? And so we embark on understanding the general theory of relativity.
Einstein’s idea of Space-Time :-
Now , Einstein’s view of space and time was quite radical from what we have seen so far. And when worked on the question I have just mentioned above he got some groundbreaking results, results which were to change our view about the cosmos. Einstein came to a conclusion that space can be twisted, curved, warped and is pretty elastic. Space is not absolute pretty much Like Newton and Leibniz remarked and to Mach’s point, his (Mach’s) idea had proved to be one of the key factors in constructing general relativity as a whole. Einstein had found a principle which was the basis and the soul of general relativity, that concept is called the equivalence principle. He found out that the events which are happening in gravitational fields will be equivalent to events happening when a body accelerates. And so Mach’s statement with a bit of Einstein’s extension proved to be vital mathematically for the theory. And we had earlier noted that Einstein concluded that space can be curved, so what would be the consequences of this curvature? And this brings us to the laurels of Einstein’s theory.
Brilliant New Realizations of General Relativity
The Cause of Gravity:-
When Einstein was child he wondered, what causes gravity? He wondered about it but got no clue. Until around this time when he had a natural realization that space-time bending and gravity could be related. But what he got was tantalizingly brilliant. He found out that bending of space-time causes Gravity. Consider a person sitting on a trampoline, so the trampoline will get bended. Now roll a ball on the trampoline and you will see that the ball will start rotating around the part which is bended. It is pretty much the same; just consider the trampoline to be space-time. Actually, the theory explains that when a massive object is in space-time he bends the space-time around and any object coming in that bent space-time revolves around the massive body in a manner in which it looks like some attractive force is being applied on the body. And this bending causes our good old force gravity. Also , it’s not only the mass which is a factor for gravity , we’ll see to it later. But is this it to the bending thing? Isn’t there anymore consequences, well gladly there are.
Gravity and Time ;-
Einstein now had shown that massive bodies can bend space-time and cause gravity. The more massive the body is the more space-time it bends. The bending of Space & TIME clearly shows that the body is influencing time and so Einstein said and described that the body will slow the passage according to its bending of Space-time. And so it means the more close you are to any massive the body, the more slow time moves for you. And now Einstein provided one concrete idea to support the unification of space-time and if there were any doubts left, those were too cleared about uniting space and time. This idea is perfectly applicable to stars, planets and anywhere. For a common experience, a person on the bottom of an apartment or a building will have time moving slowly for him by a miniscule amount to the person who is living at the top of the building. We have so far discussed how our good old force of gravity acts but Einstein, gravity is not just what we see it to be.
Repulsive Gravity :-
I once asked one of my physics teachers, why is gravity always attractive? He, seeming to not have much idea, said it is because of our observation, we have observed that is always attractive and so it is. Fair Enough. It is a common observation that gravity is attractive, why? More precisely because you can’t have negative mass and from Newton’s laws, you can’t have negative gravity either. Now Einstein while making his theory was on a roll of stunning realizations and in this context he made another. He found out by his equations, that the age old belief that mass is only responsible for gravity is wrong. Not that it doesn’t but simply that it’s not the only one that should. He found out that Energy and pressure are equally important factors in determining a body’s gravity as Mass is and so as there could be negative energy or pressure, he also came to the conclusion that Gravity could be repulsive too. For our normal experiences, the contribution of energy and pressure is negligible but at the large scales in the cosmos it is equally valid as mass is. But can there be nothing in the theory which will influence the whole universe at once and not just certain areas for some time. Well to our delight there was but it was not observed by Einstein or more precisely misinterpreted by Einstein. And so we come to Einstein’s famous mistake.
Universal Expansion :-
When Albert Einstein was making general relativity, there was a conception in physics or more precisely a misconception, that the universe is static on a large scale, unchanging. That means to say that a galaxy which is here today will be here 10 years into the future too. But Einstein didn’t believe it he thought that the universe should be contracting because of the matter in the universe because there was another misconception that the universe is dominated by matter. And when he got to know it is not contracting, so he thought that there must be something which is balancing the gravity of the matter. And he installed in his equations a term describing the mysterious force and that term was the cosmological constant. And he said that this is the constant which balances gravity and keeps the universe static. But soon as Edwin Hubble in 1928 proved through his observations of galaxies and nebulas proved that the universe is expanding and the equations of Georges Lemaitre, Alexander Friedmann , Howard Robertson and Arthur Walker proved by concrete mathematics and with some solutions of general relativity that the Universe was expanding, Einstein was on the regret. Because with the help of Cosmological Constant he could have also predicted the expansion of the universe way before it was experimentally discovered and that could have been the biggest discovery in the history of cosmology. But that’s not the end of the story; in 1998 a team of researchers found out that this expansion is because of a mysterious form of energy they don’t know about. And so this form of energy was named Dark Energy. And the measurement of its magnitude was the cosmological constant. Personally, I don’t like it to call Einstein’s mistake but rather his prescience.
Consequences Of General Relativity :-
Black Holes :-
Einstein’s theory was experimentally proven in 1915 and Einstein became a world star in physics community and in the world very fast. And as fast as he rose to stardom, equally rapid was the coming of mathematical solutions to Einstein’s theory. And one of the earlier ones was from Karl Schwarzschild, who showed that general relativity shows that there could be objects in the cosmos which are so dense that objects near them will get torn apart and even light cannot escape them, he predicted the existence of black holes. While many were not convinced by Schwarzshild’s radical claim, his claim was pretty good going with ideas of 18th century physicist John Mitchell. Einstein once wrote a letter around the 1940’s, describing why doesn’t believe in black holes and that was the stand of many physicists for a long time. Until astronomers made a momentous breakthrough in 1972. They observed that an x ray source named Cygnus X-1 was pretty convincingly a black hole. And so experimentally it was also proved that black holes exist. Earlier we had seen how gravity and time are interrelated, the more gravity the body has the more it bends space-time and so time near a black hole is very slow. And so if you take some rounds of a black hole for say 2 hours and return back to Earth, for you only 2 hours have gone by but for others at earth nearly half a century has gone by! But an even astounding discovery was in the wake and it was related to black holes.
Singular Space-Time :-
We have talked about black holes but not questioned how is it formed in the first place? That was the question a mathematician named Roger Penrose was working upon. And he found out through some solutions of General Relativity, that when a supernova occurs on a star which satisfies the mass specified by the Chandrasekhar limit, wait what’s the Chandrasekhar Limit? The Chandrasekhar limit is a mass limit gave by Indian physicist Subramanyam Chandrasekhar in 1928 which said that a star whose core is 2.5 or more times massive than our sun can only form a black hole. So when the supernova occurs, theouter parts of the star are expelled violently into space, while the core completely collapses under its own weight. If the core remaining after the supernova is massive enough, no known repulsive force inside, a star can push back hard enough to prevent gravity from completely collapsing it , the core leads to a black hole. From the perspective of the collapsing star, the core compacts into a mathematical point with virtually zero volume, where it is said to have infinitedensity. This is called a singularity.The singularity has infinite density and infinitely small and this where space and time come to a stop, isn’t that brilliant. But this isn’t the end of remarkable consequences of General Theory of Relativity.
Big Bang Singularity :-
We have earlier come to know that Roger Penrose gave the theory about singular space times. So naturally he had went to give some talks about it on a road trip. And once he gave some graduate students a talk about his theory and one student amongst them was particularly much interested in it. He wondered, what would happen if Penrose’s equations are applied to the whole universe? That student was Stephen Hawking. Penrose’s equations had also shown that a singularity can destroy the universe. And Hawking wondered that if he reverses his equations, should the universe have arisen from a singularity. Because according to Robertson-Friedmann-Walker-Lemaitre equations we earlier mentioned, the farther back you go in time, the smaller the universe will be. And so Hawking concluded that the Universe had come into being from a Singularity. The thing which banged in the big bang was a Singularity, which he named as the Big Bang Singularity.
What can we Say Finally on General Relativity ? :-
Great, Magnificent, Tantalizing, Astounding, simply words can’t describe the beauty and greatness of General Relativity and it’s great significance and invaluable contribution to Cosmology and Physics and to the World. General Relativity has sure reserved its place in human history and the remarkable contributions it has made are really great. Einstein’s mistake was a secondary to his other great discoveries by this theory and I sincerely think that it was not a mistake rather anyone would have committed it when they would be in the same Mental and relationship condition Einstein was when he gave his theory, but still these contributions are excellent and I would take this moment to express my gratitude on behalf of all physics community to the great, Albert Einstein.