So, in this blog I am gonna continue from where I had left in my previous blog. As I had posted that we can answer the question whether the laws of physics allow spacetime to be so warped that a macroscopic body such as a spaceship can return to its own past; in many levels.
According to Einstein's Theory, a spaceship necessarily travels at less than the local speed of light and follows what is called a timelike path through spacetime. Thus one can formulate the question in technical terms : Does Spacetime admit timelike curves that are closed--that is, that return to their starting point again and again? These paths are referred to as "time loops" by the famous physicist Stephen Hawking.
There are three levels on which we can try to answer this question. The first is Einstein's General theory of relativity, which assumes that the universe has a well-defined history without any uncertainty. For this classical theory we have a fairly complete picture. However, this theory can't be quite right, because we observe that matter is subject to uncertainty and quantum fluctuations.
We can therefore ask the question about time-travel on a second level, that of semiclassical theory. In this, we consider matter to behave according to quantum theory, with uncertainty and quantum fluctuations, but spacetime to be well defined and classical. Here the picture is less complete, but at least we have some idea of how to proceed.
Finally, there is a full quantum theory of gravity, whatever that may be. In this theory, where not just matter but also time and space themselves are uncertain and fluctuate, it is not even clear how to pose the question whether time travel is possible. Maybe the best we can do is to ask how people in regions where spacetime is nearly classical and free from uncertainty would interpret their measurements.
To start with classical theory : the flat spacetime of special relativity (relativity without gravity) doesn't allow time travel, nor do curved spacetimes that were known early on.
Time travel "horizons" (the boundary separating the region of time loops from the region without them) are rather like black hole horizons. While a black hole horizon is formed by light rays that just miss falling into the black hole, a time travel horizon is formed by light rays on the verge of meeting up with themselves.
Even if it turns out that time travel is impossible, it is important to understand why it is impossible. But, mathematically it may be possible. The probability that one can go back in time and kill his/her grandfather is 1/10^[(10)^60] or in other words its less than 1 in 10 with a trillion trillion trillion trillion trillion zeroes after it but its not zero; that means its not an impossible event. Thus, whatever may be the correct answer for time travel, one thing is certain that it is one of the toughest and exiting questions in modern science. May be we can know about that in the coming centuries!! Till then.......Good Luck!
According to Einstein's Theory, a spaceship necessarily travels at less than the local speed of light and follows what is called a timelike path through spacetime. Thus one can formulate the question in technical terms : Does Spacetime admit timelike curves that are closed--that is, that return to their starting point again and again? These paths are referred to as "time loops" by the famous physicist Stephen Hawking.
There are three levels on which we can try to answer this question. The first is Einstein's General theory of relativity, which assumes that the universe has a well-defined history without any uncertainty. For this classical theory we have a fairly complete picture. However, this theory can't be quite right, because we observe that matter is subject to uncertainty and quantum fluctuations.
We can therefore ask the question about time-travel on a second level, that of semiclassical theory. In this, we consider matter to behave according to quantum theory, with uncertainty and quantum fluctuations, but spacetime to be well defined and classical. Here the picture is less complete, but at least we have some idea of how to proceed.
Finally, there is a full quantum theory of gravity, whatever that may be. In this theory, where not just matter but also time and space themselves are uncertain and fluctuate, it is not even clear how to pose the question whether time travel is possible. Maybe the best we can do is to ask how people in regions where spacetime is nearly classical and free from uncertainty would interpret their measurements.
To start with classical theory : the flat spacetime of special relativity (relativity without gravity) doesn't allow time travel, nor do curved spacetimes that were known early on.
Time travel "horizons" (the boundary separating the region of time loops from the region without them) are rather like black hole horizons. While a black hole horizon is formed by light rays that just miss falling into the black hole, a time travel horizon is formed by light rays on the verge of meeting up with themselves.
Even if it turns out that time travel is impossible, it is important to understand why it is impossible. But, mathematically it may be possible. The probability that one can go back in time and kill his/her grandfather is 1/10^[(10)^60] or in other words its less than 1 in 10 with a trillion trillion trillion trillion trillion zeroes after it but its not zero; that means its not an impossible event. Thus, whatever may be the correct answer for time travel, one thing is certain that it is one of the toughest and exiting questions in modern science. May be we can know about that in the coming centuries!! Till then.......Good Luck!
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