What Is General Theory Of Relativity?
After 10 years of racking his brains, Einstein finally resolved this and came up with his General Theory of Relativity, that also considers the effect of gravity on space-time. What this theory says is that gravity not only pulls at massive objects like the moon and small objects like a football, but affects space-time. But for gravity to pull or act on space-time, it has to be a real quantity like a stone. How can gravitational force pull space-time that has no mass or structure? Until now, space-time was not treated as a real entity. If space or time were some kind of real quantities, we could have moved them around like a stone. But Einstein's theory now gave space-time some identify and structure. We could imagine the space-time fabric as some kind of invisible, colourless and stretched piece of rubber membrane.
How Did Einstein Proved His General Theory Of Relativity?
Einstein's theory of space and time was proved experimentally on a number of occasions. One of them was carried out during the solar eclipse of 29 May 1919, when the sun was masked by the moon. Normally during daytime, we can't see stars due to the sun's brightness in the sky. However, during a solar eclipse, the moon temporarily blocks this light, dimming the sun's brightness so we can observe bright stars even during the day.
Being an astronomy buff, I occasionally carry around my 8" Newtonian reflective telescope during dark, starry nights for a sky-watch. It is great fun to point the telescope towards stars like Sirius (the brightest star in the sky), that is 8.6 light-years away (86 trillion kilometers), which means the light coming from Sirius started its journey about 8.6 years ago. So what we see now is the 8.6-year-old image of the star. The light from the star comes straight to us without bending. If you were to watch this star during a solar eclipse (assume that the sky configuration of the sun and Sirius is such that they are visually close to each other with Sirius lurking right behind the sun), the light beam coming from Sirius will be seen slightly bent towards the sun. The reason for this is that the space around the sun is warped or curved due to the influence of the sun's massive body. We learnt earlier from Einstein's General Relativity theory, that massive objects warp the space around them and result in the bending of light passing through the warped space.
A light ray always takes a straight path while travelling. But if the space through which it passes is itself curved due to the effects of a nearby massive object, the light follows the slope of the curved space, thus suggesting that the straight path for light is not a straight line but a curved path.
Henry Cavendish in 1784, and Johann Georg von Soldnier In 1801, had pointed out that Newtonian gravity predicated the bending of light around a massive object. They even calculated the amount the light bent. But there was no foundational theory to support the prediction. The experiment carried out by Arthur Eddington (1882-1944), and his collaborators in 1919, established beyond doubt the effects of gravity on light and their results accurately matched the theoretical values predicted by Einstein's theory This spectacular news made headlines in newspapers, making Einstein an instant celebrity. When asked what if the experiment had proved him wrong, Einstein famously quipped, "Then I would feel sorry for the dear Lord. The theory is correct anyway".
Visualizing the curving of empty space-time, is hard. Let me provide an example of how the warped or curved space-time under the influence of massive objects such a star or a black hole, can be visualized. Assume that four friends have gone on space travel and are now orbiting a massive star. They get excited to measure the curvature of space-time using a simple technique. Three of them had a laser gun each and took positions in space to act like the three tips of a triangle. Next, each one pointed the laser beam at the person on their right that they formed a triangle with the laser beams as the three sides. The fourth person, the one with a protractor, quickly visited each vertex of the laser beam triangle and measured the three interior angles.
He then added them all up to find the total angle. Obviously, one would have expected the total to be 180 degrees. But to everyone's surprise, they found it to be more than 180 degrees, suggesting that the triangle was not flat but slightly curved outward. It was like drawing a triangle on a football and measuring the interior angles, which would obviously be more than 180 degrees. This clearly violated the property of the triangle we studied in high school using Euclidian geometry. Of course, you could as well have got the total to be less than 180 degrees, if the space-time curvature was inward, like drawing a triangle inside a soup bowl. What this suggests is that, space-time is like a rubber membrane that can be warped and twisted under the influence of a massive body.
Conclusion
Obviously, the above story is a bit exaggerated. The curvature of space can never be so prominent as to allow three friends to draw a triangle and measure its angles. The space-time warp would not be so easy to measure. Even with massive objects like our sun, the impact on space-time curving would be very small.
One of the key inferences of Einstein's theories was to equate the speed of the gravitational force (resulting from the curved space), to the speed of light, thus putting to rest the myth that gravity travels instantly and faster than light. One of the basic premises on which Einstein's Special Relativity thrives, is that it puts a limit on the speed of light to about 300,000 km per second. Nothing can travel faster than light. But recall from the previous chapter on QM that, Quantum Entanglement allowed the photon pair to communicate instantly, even though they were separated by a galaxy. Einstein could never come to terms with the Quantum Entanglement paradox and struggled in the latter part of his life to resolve this inconsistency through a series of thought experiments.
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