Time travel has always fascinated us. It is perhaps one of the most discussed topics among school students and scientists alike. From our everyday experiences we believe that time travel is next to impossible. But, is this true or is there any chance by which we can move forward to backward in time at least for a few seconds?
Figure 2: An illustrative flowchart to represent Grandfather paradox.
Figure 4: Schematic representation of space-time curvature
Figure 5: Einstein-Rosen Bridge Model of a wormhole
Is time travel possible? When you ask this question to a child, he might, out of his curiosity say that it is possible. He may visualise in his mind all those moments when the dinosaurs ruled over our planet and how things came into being, the way we see it today. In the language of science, time travel is an impossible event (as of now!) and many thinkers have proposed certain postulates which clearly pertains to the fact that time travelling is impossible. We are about to look into these 'concepts' and for knowing them the most important thing that you must possess is 'visualisation'.
Many concepts were put forward by humans and let's discuss about two widely accepted concepts which may sound alike but are different.
''If time travel is possible, then where are the tourists from the future?''
-Stephen Hawking
Bootstrap Paradox:
For explaining 'Bootstrap Paradox', let us take a simple example.
Since Einstein's Theory of Relativity is one of the most substantial works in science, let us take that into our consideration.
You are sitting in your Physics class and your teacher is teaching you about Theory of Relativity. You are science enthusiast and therefore decided to go back in time with the help of a time machine to meet Einstein in person to talk about the theory. You finally meet him walking down a lane.
You ask him about the theory of relativity and he says he knows nothing about it. This is because Einstein hasn't yet started his work on 'relativity'. Thus you decide to tell him the entire theory and bids him farewell. You then time travel to future to sit in your class with the teacher teaching you the Theory of Relativity. During the period of your travel from Einstein's time to the present, Einstein published his Theory of Relativity.
Now my question is who is the author of the principle of relativity? Is it you or is it Einstein?
From what we know, Einstein was the author of the Theory of Relativity. But according to the situation described above, you are the real author!
How? Because you were the one who went back in time, met Einstein and lectured him about the Theory of Relativity, which was followed by the publication of theory by Einstein. It was because of you that Einstein knew about it and he published it.
But another interesting and ridiculous fact pops up at this moment. If Einstein didn't publish his Theory of Relativity then how could the teacher teach you Einstein's work and how could you teach Einstein about the theory.
I know this is little complicated. Let us use letters to explain the same idea.
Let consider three letters A, B and C, each representing a particular event in your life. This can be schematically represented as follows...
Figure 1: A representative 'idea' to explain Bootstrap Paradox.
EXPLANATION:
In figure 1, as already mentioned, time is moving in the way we normally observe i.e., from one instance to another instance moving only in the forward direction. Here the letters A,B and C signify three instances in your life.
An instance A which happened in the past resulted in an instance B, which happened in the present and this instance B will be the cause of an instance C which will happen in the future. This means that what happens in future is determined by what you do today.
Now this 'forward movement' of time is easily relatable as we are living in a world were time moves only in the forward direction, but is that the case? Can time move backwards?
This question was answered by the Bootstrap Paradox. According to it the event C is the cause of event A, this means that event C (or future) has already happened.
Now let us try to relate it with our Einstein example.
Exploration 1 :
Event A would be the instance when Einstein was working on his theory and event B would be the instance when Einstein published his theory. And event C would be the instance when you were sitting in your class learning Einstein's theory. So far so good because it is according to our so called 'forward movement of time'.
Exploration 2 :
Now let us consider the second event that we discussed in our example. You learnt the Einstein's theory in school and made a time machine which happens to be event C, then you went back in time that is from C→A, to meet Einstein in person and narrated the theory to him which you learnt in the school. Event B represents the time when Einstein publishes the theory which is followed by instance A when you are at school learning the theory.
The second exploration totally defies the 'fundamental notions of time', because it states that you went back in time to teach Einstein a theory which you learnt because of Einstein!
Hence, Bootstrap Paradox provides us with an excellent 'contradiction' to the concept of going back in time.
Still has doubts regarding this theory? The following theory will, for sure, explain the fact that going back in time is near to impossible.
Grandfather Paradox:
This theory may sound similar to the Bootstrap paradox but this one became even more famous than the former, due to its simple illustration which anyone can easily digest.
I guess there is no need for an explanation to this theory. But let's do it. Sam doesn't like his grandfather and held a grudge towards him for some reason which we need not consider here. Sam was a science enthusiast and had a very sharp and imaginative mind. One day he decided to make a time machine. He completed that mammoth task in just an year and wanted to use it for a noble cause, he calculated and re-calculated and finally decided what he would do with it. He made up his mind to go back in time to the mid-nineteenth century, the period in history when his grandfather was born. The reader should also be aware of the fact that Sam is an excellent shooter.
He managed to reach the exact location were his grandfather was born and saw him playing with a toy car. Sam took aim and was about to shoot when his 'sharp' mind gave him a little hindrance.
Sam reasoned that if he killed his grandfather, then his father will not be born. And if his father was not born then there would be no one named Sam holding a gun to kill his grandfather!
So this in turn means going back in time is just not possible under any circumstances.
But don't worry he still didn't consider another possibility,
Can we time travel to future?
Time travel to future
Speaking frankly, we are all travelling in time at approximate the same speed, that is 1 second per second, there exists a 1 to 1 correspondence which we usually represent as 1:1.
There are some interesting experiments that we undertook to understand more about time and its relation with speed.
All these experiments were conducted to verify 'a bold proposition' given by a patent clerk. His papers revealed several startling facts, like, there is no entity in the entire universe which can travel at the speed of light, when we travel at greater speeds, time slows down.
The second prediction put forward by the patent clerk- when we travel at greater speeds, time slows down, was verified by scientists in a very enthralling experiment.
Hafele-Keating Experiment
Two clocks were set to an exact time, one stayed on Earth, while the other was on-board an aeroplane travelling in the direction of rotation of Earth. After one complete cycle by the aeroplane around the Earth, the time in the two clocks were recorded. Just as the patent clerk had predicted the time recorded on the clock (on-board the aeroplane) ticked slower/showed a time slower than the one kept on Earth, thus the clock 'on-board' deviated slightly from 1:1 ratio discussed earlier.
→Initial time (say at time, t seconds)
→Time recorded after t + t' seconds (say)
→ Time recorded after t + t'' (say)
Figure 3: A set of representative diagrams to explain the Hafele-Keating Experiment.
If our patent clerk's prediction was correct, then will it be applicable to the GPS (Global Positioning System) satellites moving at mammoth speeds above the Earth?
Our patent clerk said, 'greater the speed with which you travel, greater is the decrease in time'. This means that time in the GPS satellites moves slower than the time on Earth. If this disparity exist then the navigation aids that a GPS satellite provides us should be incorrect, but GPS satellites have an efficiency of about 95 percent, this efficiency is solely because of the corrections made in our devices according to the 'Special and General Theory of Relativity'.
Worm Holes
A worm hole is a theoretical concept which predicts the existence of a region in space which acts like 'tube of space', connecting two distant locations in space.
It is like a 'tunnel' in space through which you can get in, and you come out through the other end (of the space tube), by travelling billions of light years separating those distant locations.
An interesting fact associated with a worm hole is that you can travel through large distances in a short span of time.
Figure 4: Schematic representation of space-time curvature
This follows from Einstein's theory of Relativity which states that space and time are curved, as a result you can get through on of the tube (say A) and exit out through the other end (say A'), in reality the distance AA' is very large (measured in light years, 1 light year = 9.46 × 10¹⁵ metres)
So you can imagine how much it takes to traverse such a long distance but due to the space-time curvature, this distance becomes easily traversable.
Figure 5: Einstein-Rosen Bridge Model of a wormhole
We now know that large distances between two distant locations in space can be easily traversed with the help of a wormhole and this is so because of space-time curvature. Figure 5 shows exactly the same scenario. Look carefully at the figure, the points 1 and 2 represents the two distant locations in space. Here Path (a) is the actual distance that one must traverse to move from 1 to 2. But with a worm hole, we can traverse that large distance more easily, this is represented as Path (b) in the figure. From the figure you can clearly see that a worm hole, is like 'tube of space' connecting two distant locations.
An important aspect associated with worm holes is that when we come out of the tube, we may be in a region in space which is running behind in time, that is you have actually travelled from present to past. In a way you can visit your own past and come back to the present through the same worm hole. But this is theoretical concept.
Conclusion:
Though time travel is still there in science fictions and movies, in practice it is impossible, as of now. But humanity is continuously acquiring knowledge to know more about the mysterious and exciting phenomena of time travelling, it may be possible in future. I am reminded of Leonardo da Vinci's once a great dream, which became reality 400 years later with the Wright Brothers who flew da Vinci's dream which we now know as an aeroplane.
We have all dreamt about time travelling and optimistically speaking, some day, in the future this dream will surely turn into a reality when our successors would come back in time to meet us and we would see our 'once a dream' come true. This sounds like another perfect example of a Bootstrap Paradox isn't it?
REFERNCES:
1. How to time travel?- National Geographic
2. Time travel - Cinemagic YouTube Channel
3. Principles of Physics - book by Jearl Walker, David Halliday and Robert Resnick
4. Time travel is possible, but it's a one way ticket - blog.scienceborealis.ca
5. Is Time Travel Possible? - spaceplace.nasa.gov
6. Images: Figure-3 (spaceplace.nasa.gov-NASA), figure 4 and 5 (Wikimedia Commons), figure-2 (iFunny.com)
*All the media published in this blog belongs to their original creators and 'Knowledge Through Science' does not claim any right over it.
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