Newton’s theory of gravity explains the effect of this force on matter but doesn’t explain its cause. It was the genius of Einstein who solved this mystery when he presented his theories of special and general relativity. Theory that is reaching its 100 years of being proven.
The well-known Theory of Gravity that Sir. Isaac Newton presented in his most relevant work, “Philosophiae Naturalis Principia Mathematica” or simply “Principia”, allows us to accurately describe how this force acts and its effect on the interaction of two bodies or masses. This theory remains fundamental to understanding the mechanics of the Moon’s trajectory revolving around the Earth, or to calculate trajectories of space probes traveling to other planets; such work is still founded on calculations using Newton’s laws of gravity and motion.
But Newton’s theory, although it is indisputably precise, does not describe the cause of gravity; that is, it clearly describes its effect, but its cause remained unknown. And in certain cases, it could not fully explain certain observations and measurements. This condition was maintained for more than 220 years.
With the first precise astronomical observations, some inconsistencies arose in this theory; for example, in the nineteenth century, it was noted that certain gravitational effects in planetary orbits did not matched at all with calculations based on Newton’s theory; an example is an effect in the Mercury’s orbit, observed by Le Verrier, a French scientist, who in 1856 noticed that Mercury’s elliptical trajectory was not static, but instead it had a shift or precession every time the planet reached its perihelion ( its closest point to the Sun); marking a route that is slightly different from the previous orbit; Given this effect, and based on Newton’s theory, Le Verrier made adjustments with which he predicted the existence of another planet causing this precession, due to its gravitational influence, planet that he called Vulcan.
Mercury’s Precession – physics.aps.org
Einstein’s breakthrough
One of Einstein’s brilliant abilities was undoubtedly his capability for abstraction and deep thinking about a problem, with this skill he took as his mission to fix a major inconsistency in physics, between Newton’s theories and the recent results of James Clerk Maxwell’s work (presented in 1861). The discrepancy was due to the consideration of Newton, who took as a basis for his theory the “Galileo’s Invariance”, establishing that the attributes of a body do not change with speed; in simple terms, a one meter object is still a meter long regardless of the speed at which it moves. This assertion was conflicted with Maxwell’s demonstration that the speed of light is constant. And at extreme speeds this rule cannot be met; these two premises were contradictory, and something that Einstein did not tolerate were inconsistencies.
Einstein’s genius
To solve this type of discrepancies Einstein imagined these effects in extreme conditions, making use of imaginary or so-called thought experiments about what happened in such circumstances; so, then he can formulate a mathematical model to describe it.
Special relativity
First, he imagined what happens with the constant movement of light. In his experiment, I imagined how a ray of light would be seen by a person at rest and another in motion, moving at extremely high speed. To do this, he imagined a passenger, Bob, waiting at a train station; and another passenger, Alice, traveling in a train at a speed close to light-speed. At the train station a lamp is turned on; Bob, waiting at the station, perceives that the light moves at the known constant speed described by Maxwell, but for Alice, the beam of light seems to move more slowly, since she also moves at a speed close to that of light. In this condition Einstein concluded, if the speed of light seems to decrease for Alice, but in reality it is constant, then this variation should be in time.
Thought experiment – Light beam
A person, moving at extreme speeds, like Alice, would experience a time dilation, and her watch (and time), would move more slowly, relative to Bob’s who is at rest in the train station, in this way the constant speed of light is not affected, even when Alice sees a different and slower displacement. This conclusion was what Einstein presented as Special Relativity.
To properly describe what happen, he imagined an experiment where, at the precise moment that Alice and Bob are aligned, two lightning bolts fall at equal distance from them. In this situation Bob, waiting in the station, observes that the two lightning bolts fall simultaneously, since the light travels the same distance towards him; but in Alice’s case, who is in motion, given that the train is in motion relatively in the direction of one of the lightnings and moves away from the other, she would notice that one lightning bolt falls before the other; the lightning towards which she’s moving would seem to fall first.
Thought experiment – Double lightning bolt.
General relativity
The inconsistency with Maxwell’s laws and Newton’s laws of motion were resolved, but there was still the question about what the origin of gravity really was. To analyze this Einstein again made use of a “thought experiment”. This second imaginary experiment was based on a simple question, what happens when something is in free fall?
To exemplify this, he imagined a person enclosed in a cube or a box, floating in space, in such condition this person would be floating freely in the box; but then considered the following; If this box is subjected to an acceleration, this situation would cause the person to experience the sensation of gravity while its being subjected to this acceleration. Similarly, if the box approaches a massive object such as a planet, the person would, in the same way, experience the same sensation of gravity, in this case provoked by the gravitational attraction of the planet.
On the opposite; If this box were floating in empty space the person inside would experience weightlessness and would be floating in the box. This would be the same sensation that this person would have if the box were in free fall towards the earth, given the action of earth’s gravity. In both cases the feeling of weightlessness is the same.
Einstein’s proposal was that, in zero gravity or free fall, there’s no force acting on a body; and the acceleration or presence of a massive body would cause the feeling of gravity. He called this duality “The equivalence principle“.
The equivalence principle
To explain this situation, Einstein concluded that gravity does not act on an object; and instead, he presented a totally different model. Einstein introduced the concept of the geometry of space-time, which is the environment in which we are immersed; and concluded that gravity is an effect of the alteration or distortion of this space-temporary geometry.
Einstein proposed that gravity is not an invisible force acting between two bodies; but the result of a change in the space-time geometry around them. Newton always considered space and time as two separate entities; and Einstein proposed that both are mutually intertwined (space-time) and that they can be distorted by the effect of mass or acceleration.
Newton’s description of the invisible force that mutually affects objects’ movement, even when it correctly describes this effect, ceased to be a clear description of how gravity works.
Einstein’s theories, both that of special relativity and that of general relativity, presented in 1905 and 1915 respectively, involve many paradigm shifts, such as time dilation, the equivalence between mass and energy (the famous equation E=MC2 ), and extreme gravity phenomenon such as black holes and neutron stars were some of the conclusions that his theory brought to light.
The case of Mercury’s precession, presented by Le Verrier was one of the calculations with which Einstein himself proved that his model worked, as this movement was clearly explained using his new theory.
100 years of being a proven theory.
Einstein’s theory, although it was generally accepted in the scientific community, was not yet proven, and was still subject to criticism by some detractors; it was evident that an experimental test was necessary to prove once and for all this theory.
With this purpose, two astronomers, Frank Dyson and Arthur Eddington, presented proposals to test Einstein’s theory by measuring the gravitational influence that the sun exerted on a light ray coming from a star. Considering for this purpose the eclipse predicted for May 29 of 1919 as the ideal moment to make this measurement; since in the eclipse it is possible to observe the stars and it was possible to measure the influence that the solar mass would have on the light these stars, this influence would cause a deviation of its light in an estimated 1.74 seconds of arc, these measurements could then be compared with the previous measurements of the Oxford telescope with these stars in their real position. The difference in position would show the effect described by Einstein’s theory on the distortion of light due to a mass.
Frank Dyson organized an expedition to make this measurement, which was led by Charles Davidson, Dyson’s assistant, making this measurement in Sobra, a city located in the northern area of Brazil, near Earth’s equator. The eclipse lasted 6 minutes and 51 seconds, a very long eclipse, which gave them the opportunity to acquire precise photos of the solar corona and stars around them.
Arthur Eddington also made an expedition to Prince’s Island in the Gulf of Guinea to make the same shots and measurements as well to verify the change in the relative position of the stars around the sun.
Light’s relativistic effect during solar eclipse – openmind.com
Both Dyson and Eddington presented their results later that year of 1919, both corroborating that Einstein’s theory correctly predicted the effects in the star’s light, measured during this eclipse. This information did not took long to become known in the scientific community and in the news media of that time, making Einstein the most know and famous scientist of that time, and maybe in history.
The theory of relativity today
Since then Einstein’s theory has been successfully tested on multiple occasions; both in various astronomical observations, as well as through different experiments, the last one being the first photo of a black hole in the galaxy M87, revealed a bit over a month ago, on April 10, 2019 by the Horizon Event Telescope experiment. Photo taken from the combined signal from 7 telescopes, including the Afonso Serrano Millimeter Telescope in Mexico, image processed and composed in supercomputers at the Max Planck Institute and the MIT’s Haystack Observatory.
Black hole in M87 galaxy – EventHorizon.org
Einstein’s theory of relativity was the greatest reconciliation of how the cosmos works and the force of gravity. Theory that in this month and year reach its one-hundredth anniversary of being proven.
Happy hundredth anniversary, Einstein’s theory of relativity.
Regards, Alex – ScienceKindle!
