Introduction to Theory of General Relativity
The theory of general relativity was founded by Albert Einstein in 1905 while formulating his famous formula E=mc2. It revolutionized the field of astrophysics and solved the mystery of speed of light and gravity. Before Albert Einstein, speed of light was considered to be varying with the medium and it was governed by Maxwell’s electromagnetic law and the gravitational force was formulated according to Newton’s laws of gravitation. But Einstein theorized that the speed of light is constant everywhere in the universe and speed of a body traveling in the universe depends on the mass and energy of that body. In his general relativity theory, he proposed space-time, a four-dimensional continuum formed by clubbing three-dimensional spatial coordinates and time. Space-time in the universe is woven into continuous vertical sheets called space-time fabric. All the objects present in the universe that are having mass will bend the space-time fabric. The bending of the space-time creates gravity and the strength of the gravity depends on the mass of the body. If the body has more mass, it bends the space-time more and has greater gravity. Massive objects like sun bend the space-time to such an extent that it distorts space-time of the surrounding.
Mercury Overview
Mercury is the closest planet to the sun and the smallest planet in the solar system. Mercury has a diameter of 4879 km. It has a thin atmosphere and therefore the temperature can reach as high as 430 degrees centigrade and falls as low as -180 degrees centigrade which is lower than its neighboring planet Venus. Mercury takes 88 earth days to complete one orbit around the sun and it takes 57 earth days to spin around its axis. Mercury is a terrestrial planet with rocky surface and mountains and also has impact craters. Mercury has a weaker gravity due to its small mass.
A Discrepancy in Mercury’s Orbit-Early Observations
All the planets in the solar system move in systematic orbits around the sun and these are governed by two laws; mainly Kepler’s laws of planetary motion and Newton’s laws of gravity. These laws predict the shape of the orbit of a particular planet around a sun-like star depending on the mass of the planet, the distance between the planet and the host star, and also the distance between the planet and the neighboring planets. According to Newton’s laws of Gravitation, a planet closer to the sun has near elliptical orbit than the planets orbiting at a farther distance. Even the nearby massive planets can also cause a shift in its orbital structure. Mercury also falls under this category. As mercury is the closest planet to the sun, it has near elliptical orbit with an eccentricity of 0.21; the most eccentric orbit of all planets in the solar system. Mercury has a perihelion shift or increase in its perihelion of 531 arcseconds per century. But researchers who observed its orbital shift observed that there was an error of +43 arcseconds which led to the discrepancy in the Newtonian calculations. The experimental observations led to a perihelion shift of 574 arcseconds per century. Also the orbit of the mercury is wobbly; that means it has a slight shift in its course from the plane of rotation. These discrepancies made scientists assume that there might be an inner planet orbiting the sun that is causing these discrepancies and they even named the planet as “Vulcan”. But they were not able to spot any of them.
The Solution to the Wobbling Orbit of Mercury
Albert Einstein proposed the theory of general relativity that stated “Objects with mass can bend the space-time.” Objects with heavier mass can also distort space-time. Mercury is a practical proof of the above statement. Sun is a massive object which occupies 99.8% of the entire solar system. So, it bends the space-time to such an extent that it distorts the space-time in the surrounding region. In this region, the space-time would be stretched than the original. Mercury also falls in this region of distorted space-time. Therefore its orbit is wobbled and stretched in space-time. Venus won’t experience this wobble as it does not fall under the region of distorted space-time. Therefore the additional error of +43 arcseconds to the original of 531 arcseconds calculated by Newton is due to this distorted space-time. Thus Einstein proved the discrepancy of Mercury’s orbit by his theory of general relativity.
Conclusion
Thus the long term dispute regarding Mercury’s peculiar orbit was proved with the theory of relativity by Einstein. For objects with lower mass, Newton’s gravitational law and Einstein’s theory of relativity gives the same result. Only for the objects with heavy mass, we get the accurate result with relativity theory due to the addition of space-time distortion error.
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