The Grandfather Paradox

If you could travel back in time to a period before your father was born, and there you were intrigued with just how old their ways were, and you went about being an appropriate teenager, all curious and excited. You decided that you must find out how your grandparents lived so off you go to the location history told you they had inhabited. What you did not know was that you looked exactly like your grandfather's little brother that had died at your current age during the civil war. Poor guy! As soon as your grandfather set his eyes on you, he had a heart attack and died! Hold up, hold up! But your father was not even born yet.

So what does this mean to you? Would you disappear from reality forever? Or would you somehow still have been born? If we follow this logically, you should not have existed in the future since you have terminated your grandfather prematurely from ultimately bearing your father and then you. And if you lived in the future, you should not have been able to kill your grandfather in the past. This is what is called the Grandfather Paradox in physics, and it forms the foundation for why philosophers believe that travelling back in time is not possible because it has no logical basis.

But What is Time?

Even though we all use time every day and we have continued to depend on it to measure the progress of existence, defining time is surprisingly tricky and the true nature of it largely unknown by the general populace. One definition of time is as:

a measure of duration of events or the intervals between them

Most definitions try to link up the past, present and the future. For instance, the World English Dictionary defined time as:

the continuous passage of existence in which events pass from a state of potentiality in the future, through the present, to a state of finality in the past

This definition has all the elements of Absolute or Newtonian Time which is independent of the observer: it just flows from the past to the present and to the future.

Albert Einstein, in a letter he wrote to the family of his deceased friend rejected this distinction:

According to Einstein, time is relative and the observation of the passage of time would depend on the relative speed of the observer.

Relativistic Time

It was Einstein's work on Special Relativity(1905) which made us see time as a fourth dimension inextricably bound to space as space-time and his General Relativity(1916) which showed time as one dimension in a dynamically curved space-time has changed our view of time as absolute time.

He figured that the only thing that is absolute is the speed of light. If this is so, he realised that space and time must be flexible to accommodate the invariability of the speed of light. Einstein's theories have always been considered counter-intuitive and perhaps a little too tricky for the average person to understand. For that reason, it may be necessary to examine Hermann Minkowski's explanation of the concept of relativistic time.

Minkowski was Einstein's teacher at some point before they became colleagues. To explain relativistic time, Minkowski introduced the concept of proper time and this, according to him, was the elapsed time between two events as a clock passes through both events. However, if a distant observer used his method of assigning time to measure the apparent time between two events, that time is called coordinate time. So not worry if those definitions do not make any sense to you. It is from the description of event that Minkowski's point starts to become clear and he defined an event as both a place and a time which can be represented by a point in space-time. Recall that Einstein had already told us that space and time are inextricably bound into the fabric of the Universe.

If an event is a point in space-time, then space-time itself must be an infinite number of events. This means that time, like space, does not "pass" or "flow" from the past, present, and to the future, it simply just is. Just there, like space. I read a book, "The Power of Now" and it said that the past and future does not exist, only Now. While I understand the attraction for living in the present, if Minkowski and Einstein are right, it means that time is just like space which is just there whether or not we are there to experience it. In other words, the past, present and future are there even if we are not there to witness it. This line of thought implies that there is no "now" which gradually progresses into the future. According to proponents of relativity, seeing time as flowing is an illusion that was developed by our brain as a way of coping with reality.

So if time is relative and is the fourth dimension of space forming space-time, what else do we know about time?

Well, time dilates. What this means is that time passes differently for two observers depending on their relative speeds. In order words, two synchronised clocks would not remain so if they move relative to each other. This occurrence is almost imperceptible at low speeds. But as the speed approaches the speed of light (which is called relativistic speeds), the difference in time becomes noticeable. According to the theory, a spaceship travelling at 99% the speed of light, the time inside the spaceship (proper time) would be half as fast as the time from a hypothetical stationary observer (coordinate time). This time difference would rise geometrically as the spaceship approaches the speed of light such that at a speed of 99.9999%, the proper time would be more than 700 times slower than the coordinate time. If the spaceship could move at the speed of light, then time would stop in the spaceship completely.

Even smart people admit that this is a difficult concept to comprehend. However, one may begin to understand it if she realises that space and time are inextricably bound in space-time and that the nature of the speed of light is invariant, absolute and inviolable. If an object moves through the variable space-time, the invariable speed of light forces the object to share its spatial movement with movement in time, so that the faster the object moves, the slower time progresses in the object. This is much like the direction of a man travelling South-east could be described by the South and East coordinates. The closer to South he gets, the lower the value of the Eastern co-ordinate because the sum of the two cannot exceed 90 degrees. If the coordinate of South reaches 90 degrees, then the East must be zero, and in this case, the man is travelling due South. In the case of a moving object, when the speed of the object reaches the speed of light, time stops. Of course, the speed of light cannot be exceeded.

What this means for those that want to travel forward time is that if you somehow manage to find a spaceship that can travel close to the speed of light to a star about 150,000 light years away, for those of us here counting the years for you, it would take you 150,000 years to get there, which means we would have died by the time you return but you would hardly have aged at all, and of course, by your reckoning, you would have travelled to the future. Your problem would begin when you want to go back.

The Twin Paradox

This gives birth to another paradox, called the Twin Paradox in which a twin that made the space trip could return home after, say twenty years to find his twin much older than him. But according to Einstein, there is no preferred point of reference, so we may choose to see it that it was the earth and everyone in it that shot away from the spaceship at near the speed of light. In this case, the astronaut would come back to meet his twin and everyone else younger that he left them since time on Earth almost stopped when he travelled. But according to Mach's Principle, the former would be the case since it is the spaceship that accelerated away from the rest of the universe and the Earth did not.

Time Travel

Before we began to think of time as relative, all we knew about time was time as Newton saw it and this was absolute time. It is arguable that we are always travelling through time from present to the future, but this journey is always in one direction. However, when people talk about time travel, it is usually regarding how we may travel forward in time or backwards in time by manipulating the laws of physics due to our knowledge of relativity. General Relativity offers some insights into how we may begin to achieve time travel as we have seen earlier in this post. The prospect is limited by the ability to travel at relativistic speeds as was demonstrated and still what happens is that the whole time actually passes, only that the time in the spaceship was too slow for him to notice it. And then there is the issue of not being able to go back in time.

Many scientists from the time of General Relativity have tried to find ways in which time travel is possible under the theory, but the conditions necessary to achieve it are either impossible or impractical.

According to General Relativity, gravity is a feature of space-time that occurs when a massive body exists on it. In this case, space-time is warped much like a bowling ball dropped on a trampoline. Theoretical physicists, Kip Thorne and Paul Davies showed that space-time could be so warped that it literally bends onto itself, thereby providing instantaneous gateways for travel from one time to another much like General Relativity provided for travel through wormholes from one spatial location to the other. The problem with this theory is that the amount of energy that would be required to create such a wormhole would be enormous and studies at CERN seems to indicate that such a wormhole, if created, would collapse almost immediately. There are many other theories of time-travel as you can imagine but the conclusion is the same every time - we are stuck in this time for now.

Conclusion

It turns out that there is more about time than meets the eye. We owe it to Albert Einstein, the knowledge that time is not absolute. Instead, time is relative to the motion of the observer. While this is not suggestive that time is undefined and ungoverned by the subsisting laws of physics but to suggest that even though these laws govern it, is predictable and regular, it is not absolute as was believed by Newton. Furthermore, he showed that time was not independent but is a fourth dimension of space-time which is a fabric of the Universe. These observations led to the formulation of the Special and General Theories of Relativity. Even though these have helped us understand our Universe better than we did before Einstein, we cannot still jump from one time to another as we can do with space. So for now, we are stuck in 2018.

Thank you for reading.

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References

1. Space | Theory of General Relativity
2. Space | Time Travel - Science Fiction or Reality
3. Popular Mechanics | Building a Time Machine
4. Physics.org | Is Time Travel Possible
5. Birmingham Research | Time Travel

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