One of the most important theories in the history of Astrophysics which has changed our views to see the universe entirely. One of the most brilliant minds the world has ever seen, Albert Einstein has proposed this theory of general relativity. The General theory of relativity and Time Dilation go hand in hand. This theory further expands the views of time dilation and combines it with space expansion. Let’s get into it. Again, this might shatter some of our preoccupied notions and might be thought as sci-fi but, it has been accepted by many scientists and its time we do too.
Let’s again start with a hypothetical experiment. We are sitting in the car which is at rest and the headlights are blinked. The speed of light emitted from headlights is measured and is recorded accurately as speed of light. Now, if the car is moved at a speed of say 80 and the headlights are again blinked, and the speed is recorded, the speed remains constant. Yes, as the speed of car is negligible as compared to that of light but still there is not even negligible increase in the speed of light when measured accurately enough. The reason for this as we know now is Time Dilation. The time moves slowly so as to keep the speed of light constant. But, Einstein says that it’s not only time that moves slowly but, the space (or in case of car ‘Distance’) expands. So, basically, we know Speed= distance(Space)/Time. Einstein says not only time slows down but also the space expands and both of them thus keep resultant speed of light constant. This led to a new revolutionary theory that space and time are not different entities at all. They are related and linked to each other. This new entity, Einstein called it as Spacetime.
Einstein thought of this at just age of 16. He did a thought experiment that what if I chased a light beam? If you chase a light beam at speed of light, according to rules of Newtonian relativity, the light must appear stationary to you as your relative speeds are same. So basically light seems to be stopped which is against the property of light. Light never ceases to halt. It means that some other things must change. He then realized that the space the light was travelling in expands and time dilates as we approach speed of light. This led him to the observation that time and space are relative which in turn led to his Special theory of relativity.
Okay, so time and space are linked so what does it exactly mean? In his general theory of relativity Einstein suggested the Spacetime as a kind of Fabric. He says that everything around us is surrounded by this fabric. The stars, planets and other massive bodies cause the fabric to warp and cause a ripple which leads other planets move around sun in orbits and which leads moon revolve around the earth. In Einstein’s own words he describes Spacetime as entity that is everywhere. He said we are not aware of it just as the fish are not aware that they are surrounded by water.
I know, this is getting a bit too much to get. Now, let’s make it simple. Consider a Trampoline. This is the space-time (We are considering it in 2D only for simplification). Now put a bowling ball in the center of that trampoline. You don’t even have to do the experiment, you can easily imagine the picture that the ball will stretch the trampoline fabric and form a curvature. This ball is the Sun. Now, take a marble and throw it along the curvature formed by the bowling ball. It will move around the curvature a couple of times before falling into the curvature. Consider this marble to be earth just that there is no friction to decrease the revolution speed so, it will continue to be in the circular path along the curvature. This is Einstein,s General theory of relativity. It simply states that the gravitation can warp the Spacetime fabric which causes planets and other bodies to revolve.
Einstein also suggested that the warp in space-time caused by the gravitation also warps light around it. To prove this, he conducted an experiment. He made an observation on a total solar eclipse. When the moon comes in between sun and earth casting its shadow, he recorded an observation of stars that are visible just around the sun they seem to be a bit expanded. In reality it’s not the distant stars that expand but, the light coming from them is curved due to the Sun’s gravitation. This could only be observed during total solar eclipse as any other time stars are not visible due to sunlight. This fact led to prove that light does bend due to the Gravitation which further explained the existence of black holes and why even light cannot escape them.
This theory was one of the most controversial theories and was rejected to be true by many other scientists but, the theory does answer some of significant questions about the Universe like black holes and worm holes. The general theory of relativity can tell us what exactly Black holes are. If the enormous mass (millions times more than sun) causes the space-time fabric to bend, it will cause a curvature so deep that nothing (not even light) will be able to sustain itself from falling into it thus causing the black hole.