What is Gravity?
Gravity
holds answers to so many questions that physicists have pondered and stumbled
upon over a long period of time. From the falling of an apple, the Earth
orbiting the Sun, the Solar System orbiting the center of the Galaxy to the
outward expansion from the Big Bang, all explanations have deep roots in the understanding of Gravity.
Gravity is most accurately described by the General Theory of relativity, also called the Geometric Theory of relativity, published by Albert Einstein in 1915, which describes gravity not as a force! But as a consequence of masses moving "straight ahead" in a curved space time caused by the uneven distribution of mass. Gravitational force is kind of an illusion. But is this true? Lets first look into what is the General Theory of Relativity!
General Theory of Relativity
Let’s imagine two situations. In the first condition, you have jumped from a height and you are experiencing free fall. When you fall, you do not feel your own weight. So it seems like there is no force acting on you (neglect air resistance). Your weight would only become apparent when you hit the ground and break your bones. Now imagine if your friend is to jump along with you, he would remain stationary to your frame of reference.
The second condition is to imagine yourself in deep space with no large masses of any kind near you. You would be floating around in your spaceship. You would have constant velocity and no acceleration just like everything in your spaceship.
Don’t these two situations seem to be the same? According to Einstein, they are! And this is what is known as Einstein’s General Relativity. This would mean to say a man falling from a roof is not accelerating and is not under the influence of any kind of force
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General Theory of Relativity
Let’s imagine two situations. In the first condition, you have jumped from a height and you are experiencing free fall. When you fall, you do not feel your own weight. So it seems like there is no force acting on you (neglect air resistance). Your weight would only become apparent when you hit the ground and break your bones. Now imagine if your friend is to jump along with you, he would remain stationary to your frame of reference.
The second condition is to imagine yourself in deep space with no large masses of any kind near you. You would be floating around in your spaceship. You would have constant velocity and no acceleration just like everything in your spaceship.
Don’t these two situations seem to be the same? According to Einstein, they are! And this is what is known as Einstein’s General Relativity. This would mean to say a man falling from a roof is not accelerating and is not under the influence of any kind of force!
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Are these two situations physically the same?
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The important point to realize here is the inertial frame of reference. Note that in the inertial frame of reference of the two friends who jumped together, they see each other just floating together in one direction at a constant velocity with no acceleration.
Where is the acceleration!?
Then the question rises, why does a third person standing on the ground, see an object falling with acceleration of 9.81m/s^2 if there is no force acting on him? And why are the two friends falling towards the ground and not just floating away at constant velocity to outer space?
According to Einstein and his relativity theory, the acceleration that is seeming to exist and its direction is what it is because space and time are not flat like a sheet of paper, but curved near large bodies. This curvature is due to the mass and energy of matter and its movement through space. Matter in turn is affected by this curvature. You can think of this like a ball in a trampoline. The ball curves the trampoline due to its energy. A smaller matter (let’s say a smaller ball) placed near the curvature formed by this ball is pulled towards this ball, which is called the gravitational well.
The potential well
One of the aspects to be realized here is that there is no force involved when two bodies come together due to the curve in space time. Well, unfortunately our minds are not conditioned to recognise or understand this easily. One good way to understand this is to think of a geodesic! So what is a geodesic?
Consider the 3d spherical shape of the Earth. Let us say two friends are standing at the equator at a particularly large distance from each other, laterally. If they start walking on a straight path to the North Pole, it would seem to the both of them that they are moving parallel to each other. But as and when they reach the North Pole they would eventually meet each other! It would seem as if a certain unnatural force has pulled them together, while in reality, we know it’s not a force but just the spherical shape of the Earth that brought them together. Gravity is the same thing, but in a different dimension that we cannot perceive.
Now to come back to the initial question, what is the value 9.81m/s^2 all about? According to Einstein, in a flat space, you do not need any acceleration to stay still. But in a curved space time, you need some acceleration to just remain still due to the gravitational well. This acceleration that is required for matter to stay still is provided by the Earth and is equal to 9.81m/s^2. This value varies based on the matter that causes the space time warp.
How does this explanation of gravity affect time?
According to Einstein, matter does not just bend space. It bends space time! According to general relativity, clocks tick slower in stronger gravitational fields. This effect is known as "gravitational time dilation". We should note here, however, that an observer in the strong gravity experiences his time as running normal. It is only relative to a reference frame with weaker gravity that his time runs slow. A person in strong gravity therefore sees his clock run normal and sees the clock in weak gravity run fast, while the person in weak gravity sees his clock run normal and the other clock run slow. There is nothing wrong with the clocks. Time itself is slowing down and speeding up because of the relativistic way in which mass warps space and time. So next time you plan a visit to the moon, remember you are not just reducing your weight, but also time because moon’s gravitational potential is lesser than Earth’s. But don’t be too excited. This is only relative! What does this mean?
Let us consider you have a lifespan of 50 years. If you choose to be born on the moon, sadly you will only live 50 years. But to your relative on Earth, on an exaggerated level, it would seem like you lived till 52!
Doubts & Tests:
In 1915 when Einstein first developed his theory of relativity, the biggest challenge he faced was to prove it. Many people found the theory to be outlandish and absurd. But not very long after in 1919, his theory was proved by discovering the bending of light around the Sun. Scientists have over the years tried to find alternatives or debunk General relativity. Fortunately or unfortunately, the theory was able to withstand all the tests that were put to it. One such test is mentioned below.
On August 22nd, 2014 the European Space Agency launched 2 satellites. They were intended to become part of the Global Navigational Satellite System or GNSS. The satellites were to be injected into a circular orbit approximately 23000 kms above the surface of the Earth. But due to a thermal leak between a line of cold helium and propellant, the propellant froze causing a failure to the altitude control thrusters. This sent the satellites into highly elliptical, random and useless orbits. On one extreme, the satellite was very close to the Earth while on the other it was so far away that it experienced significant radiation from the Van Allen belts.
The 2 ESA satellites (from Wikipedia)
This misfortunate incident was made use of by Dr. Sven Herrmann & Dr. Pacome Delva to test the gravitational redshift of a clock, which is one of the three original tests proposed by Einstein. They realized that these satellites in elliptical orbits are moving from higher gravitational potential to lower gravitational potential. This means that by measuring the passage of time when the satellite is near the Earth (Perigee) and farthest from Earth (Apogee), it can be found that time goes slower in the stronger gravitational field. If you compare the clock on the ground and on the satellite, there will be a periodic variation of time. This experiment is all the more accurate because the same instrument measures time in both the extreme conditions and hence, there is no instrument noise. This test was performed for three years to collect the required information, and proved the space time curvature of general relativity. It is arguably the best test yet of general relativity!
The conclusion:
The General relativity theory helps solve some of the mysteries that were otherwise absurdly explained using the Newton’s Universal Law of Gravitation. For e.g. why do all objects fall at the same rate? According to Newton, this can be explained using the following equation:
But the mystery is in why would gravitational mass and inertial mass of an object cancel each other out? Similarly, according to Newton, any two masses will have a force of attraction between them. This force is so negligible even between a big and a small object that it cannot be measured and hence, no proof! Whereas Einstein’s theory makes it all easy because everything falls at the same time as all the objects are not accelerating at all!
And that is about it. The basic difference between Newton’s and Einstein’s theories is that Newton would say, if you are to jump off the roof, “The force of gravity is pulling you down.” Einstein would say, “Nope! If you jump of a roof, you take away the force that is pushing you up!”
In almost all educational institutions across the world, gravitational force is an essential component of today’s physics class. So is all that is being taught wrong? Well, the truth is it does not matter. Newtonian physics and Newton’s law of gravitation are correct and valid in almost all practical cases in consideration. Its only when you look at the bigger picture, outside our planet into the vast universe, maybe if you plan to pay a visit to a black hole someday that Einstein’s complex theory plays a part.
References:
Wikipedia:
https://en.wikipedia.org/wiki/General_relativity
https://en.wikipedia.org/wiki/Gravity#Tests
The Galileo test:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.231101
You Tube (the trampoline):
https://www.youtube.com/watch?v=MTY1Kje0yLg
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