In the year 1915, a revolutionary theory named General relativity was published by a scientist who was also known for his contributions in the Photoelectric effect, Albert Einstein. Unlike the theories of classical physics, this theory explained that Gravity is not a force, but a result of acceleration of a body, following the curvature of the fabric of space, known as the spacetime, which combines the time with the other three spatial dimensions.
Einstein’s general theory of Relativity states that Time is a relativistic concept, and it does not remain constant in all of the regions of the universe, as opposed to Newtonian Physics. This theory also states that the Time can be affected using various factors such as,
Kinetic Time dilation- Kinetic time dilation is a phenomenon where an object, approaching the velocity close to the Light Speed ‘c’ has time pass slower through the accelerating body than an object in a location, which has little to no velocity, or the body which does not accelerate.
Gravitational time Dilation- In Gravitational Time dilation, an object passing through a celestial body which has more density, has its time pass slower compared to an object orbiting a low-density celestial body. This is because the denser object makes a bigger curvature into spacetime, leading to a longer path covered by the object closer to it in the time dimension.
The theory is totally based on Einstein's Field Equations, which are said to be one of the most complicated and the equations with one of the groundbreaking solutions of all time. These equations explain how mass and energy influence the curvature of spacetime. Shortly after Einstein published his theory, a German Mathematician named Karl Schwarzschild found out the solutions of the Field Equations. These solutions are nowadays used to understand about Black Holes, which have a gravitational pull so intense, the escape velocity exceeds the speed of light. These objects warp the spacetime up to such a limit that the time passes the slowest in the region. These equations state that Gravity is not a force. If you were to drop three balls from 1 million Kilometres above the surface of the earth, the balls would follow different trajectories, influenced by the curvature of spacetime, before falling down to the earth’s surface.
A picture of One of Einstein’s Field Equations. Sources:-https://en.wikipedia.org/wiki/Einstein_field_equations
This theory also encompasses the topic of Equivalence principle, a fundamental concept of the theory that bridges the gap between Gravitation and acceleration. The Equivalence Principle states that, “gravitational acceleration is locally indistinguishable from acceleration caused by other forces.” This means that the effects of Gravity can be replicated in non- inertial frames such as Acceleration.
A common experiment is the Elevator thought Experiment. Imagine as if you’re in an elevator and it is accelerating upwards. You will feel the same effect of being pulled down by a body, which is somewhat indistinguishable to the effect of Gravity you feel on the surface of the Earth. This shows that locally there is no observable difference between being in a gravitational field or being in Acceleration, a frame of reference.
Equivalence principle also states that no matter how massive an object is, it will fall in the same rate or velocity towards a celestial body with a gravitational field, regardless of air resistance. This was tested by Galileo Galilei and confirmed by further experiments.
A drawing representing Einstein’s Equivalence Principle with a rocket as an accelerating body. Sources:-https://en.wikipedia.org/wiki/Equivalence_principle
A Diagrammatic Representation of Gravitational Lensing between two Galaxies. Sources:-https://www.eso.org/public/images/eso1313b/
General Relativity also explains about Gravitational Lensing. This is a phenomenon where Light gets warped by passing by massive objects due to spacetime curvature. This effect has been in appearance with various galaxies and stars and is used in astrophysics to find and study distant objects.
The General Relativity also solves the “43 arcseconds per century excess” problem of the “Perihelion Precession of Mercury”. During the Perihelion Precession, the Perihelion, the closest point where a planet orbits around its star, changes with time and does not remain at the same location in space. This causes the pattern of a certain number of orbits of a planet around a star to look the same like a Rosette, which is triggered due to the Gravitational Influence of other celestial bodies present in the star system. This phenomenon happens with every planet in the solar system due to The Gravitational Influence of other planets.But in mercury’s case, The Newtonian Physics could explain most of the precession, but there was a small unexplained excess of 43 arcseconds per a century. According to Einstein’s General Relativity, the Sun’s massive Gravitation warps Spacetime around it, due to which, Mercury gets the most exposure of Sun’s curved spacetime out of all planets. The curvature causes Mercury's orbit to shift more than Newton's Laws Predict.
Einstein’s equations precisely accounted for the 43 arcseconds per century, providing one of the first major conformations of General Relativity.
A Diagram Representing The Perihelion Precession of Mercury due to Sun’s Curved Spacetime. Sources:-https://testtubegames.com/gsim101/?p=mercury_precession
Gravitational Waves, being an important topic of the theory, are ripples in the spacetime caused to the high- velocity acceleration of two massive objects of the universe, such as Black Holes and Neutron Stars, later on, triggering these monstrously massive and dense objects to collide with each other. These were predicted by Einstein and later discovered by LIGO (Laser interferometer gravitational wave observatory) in 2015.
Picture of Gravitational Waves Caused due to Merging Neutron stars. Sources:-https://aeon.co/ideas/gravitational-waves-will-bring-the-extreme-universe-into-view
While Einstein’s General Theory of Relativity has been successful in explaining Gravity and the Large-scale universe, The theory also has some Limitations that make this theory present with Major Flaws,
The General Theory of Relativity is based on classical Physics and remains incompatible with Quantum Mechanics. It breaks down at very minute (Planck) stages, where quantum effects, such as Quantization, Superposition, Quantum Entanglement and The Uncertainty Principle (Heisenburg) dominate. Till date, consistent quantum theories, such as The String Theory and the Quantum Loop Theory still feel incompatible or lacking to explain the fundamentals of the Quantum stages.
The Theory also feels short to explain the value of the Cosmological Constant () , where the theory includes the constant in Einstein’s Field Equations. Recent Observations suggest a very small but non-zero (related to Dark Matter), which is vastly smaller than the predictions from the Quantum Field theory- a problem known as the Cosmological Constant problem.
When attempting to quantize General relativity using Standard Quantum Field theory techniques, it Turns out to be Non-renormalizable, meaning infinite quantities appear which cannot be systematically canceled out, where the consequence on non-renormalization is, Breakdown at High Energies, where the GR cannot be reliably used at energies near the Planck Scale (~ 10^-19 GeV), due to which Quantum corrections become Infinite and impossible to tame.This makes GR incomplete — it doesn’t work when both quantum mechanics and gravity are important (e.g., black hole singularities, Big Bang.)
In conclusion, Einstein’s General Theory of Relativity describes the 4 dimensional spacetime as the intermingling of Time with three Visual space Dimensions. It is totally based on Einstein’s Field Equations, which describe the curvature of spacetime. Later on, the first solutions of the Field Equations were published by Karl Schwarzchild in 1916. This Theory also covers the topic of Time Dilation, where Einstein stated that Time is not Constant, but relative across the various regions of the universe, depending upon the Acceleration and the Gravitation ( spacetime curvature ) of a body. One of the groundbreaking statements made by the theory was that Gravity is not a force but the result of following the curved paths through the curved spacetime around caused by the mass of the object. The theory also tells us about the equivalence principle, which states that the effects caused by Gravitation can also be formed in Non- inertial (not resistant to change of any state, such as rest) cases such as Acceleration. The General Theory of Relativity also predicts the existence of Gravitational Waves, which are ripples in spacetime caused due to high-speed acceleration of two massive and dense objects such as black holes, neutron stars, forcing them to collide. One of the major pieces of evidence of these waves is recorded by the LIGO (Laser interferometer Gravitational Wave observatory) in Caltech, USA.This Theory also explains about Gravitational lensing, where Massive Bodies warp Light around them due to their curved Spacetime. This Theory also completes the explanation of the reason behind the Precession of Perihelion of Mercury, which had a small unexplained excess of 43 arcseconds per a century, which was more than Newton’s laws Predicted. Although This theory is successful in explaining the Relativistic Nature of Time, Gravitation and Large-Scale universe, This Theory Has Limitations, such as Quantum Mechanics Incompatibility, Non-Renormalization and The disability of Explaining the Value of The Cosmological Constant. Despite these limitations, The General Relativity is remarked as one of the Most Groundbreaking theories of classical Physics By Nobel Prize Winner, Albert Einstein.
By Aayush Sachin Surve.