Time and Time Travel 

Introduction to Time

Time is undefined. According to Sir Albert Einstien, time is what we see in our clocks, and to be more specific, in our calendars too. This concept requires a lot of “thought experiments”. 

Time is like a river with a level that lets the events flow through it. That level is “present”. The events behind the level are the “past” that can never be back and the events beyond it are the “future” which is completely unknown.

The formula of time?

L.P Hartley once wrote, “why do we remember the past but not the future?” The answer to this question is that Present-happening. 

Happening or present is a line between past and future but science doesn’t know define happening. 

We can say that time is just a summation of past and future events. Adding a present won’t make any difference because the present moment occurs for the shortest period of time, it is zero. 

The set of Past events is finite as the current age of the universe is also finite but the set of future events is infinite. Anything plus infinite is infinite and infinite does not exist in the physical universe. 

So, “time does not exist.”

An Attempt to explain time

4 Dimensional Space

To understand the concept of time better, scientists have come up with the idea of 4-d space. Treating time along with space as coordinate .i.e time as the fourth dimension.

Parallel Universes

Scientists say that spacetime repeats itself in finite ways and infinite times. More universes are likely to exist. 

They might be a mirror image of us and might be behaving in the same ways. 

There may be some earth and the same people with different outcomes. 

E.g- You are wearing a white shirt today and “another you” in that universe is wearing a black shirt today. 

These types of universes are known as parallel universes. The concept of a parallel universe gives an idea of the coexistence of many states and another idea that “everything happens”. 

These ideas are widely accepted by many scientists. 

Time Travel

Time travel is the biggest question in the world of science. Time travel means going back in time, maybe to undo something. 

For this, we should be mentally prepared for thinking of “unspilling a glass of milk”. 

Time travel simply does not mean reversing actions because particles are moving at the quantum level and that thing has to be reversed. 

Laws of physics are combinations of “CPT”

C – Changing particles into antiparticles

P – Reversing motion of particles

T – Taking mirror images so left and right are swapped.

But the second law of thermodynamics forbids time travel because entropy cannot decrease. Entropy is now known as the Arrow of time which is moving forward (increasing). 

In 1949, Feynman prepared a set of diagrams known as Feynman diagrams. In those diagrams, the description of a positron moving forward in time is exactly that of an electron moving backward in time. 

The amount of matter and antimatter in this universe is the same, however, but in the observable universe, we can only see the matter. Scientists consider there to be a universe that is a mirror image of us made up of antimatter having time moving backward in it.

Another concept is that light bends around massive objects (massive stars) due to their immense gravitational field. The relationship between the speed of light and time is much deeper than we think. 

Twin Paradox

According to the theory of general relativity and special relativity, time runs slower at higher speeds and faster for stationary objects. This can be expanded by the twin paradox – two brothers of the same age live on earth. One day, twin A takes a spaceship and travels to a distant planet at the speed of light. He comes back to earth after many years. He finds that his brother is older than him and he is much younger. This is due to the time dilation concept. 

Another phenomenon related to this is that black holes have immense gravity and time means nothing there because time is generally an effect of gravity. That means People living on the second floor will have time passing slower for them than those living on the ground floor. Time will pass negligibly slower for them. However, this difference is extremely less and cannot be measured or observed. This has been observed for astronauts as well.

How to Travel in Time?

Scientists have stated three ways for time traveling –

1)Wormhole

It is a shortcut in space. Mark a point at the bottom of the paper and mark another point at the top of the paper. Mark the distance between them. It would be the longest distance. If we fold the paper, there will be the shortest distance to cover. These are loops in space to travel large distances to find parallel timelines.

2) Reaching 100% of the speed of light

Traveling at the speed of light. Moving into distant space may find the past.

3) Infinite Speed

Rotating an infinitely long rod would generate holes in space but this is not possible to build.

4) Light itself

Light from distant galaxies takes a finite time to reach further, considering a hypothetical mirror to be placed in space and light from our earth to get reflected by the mirror to only see the images.

What if you can really travel in time?

If we make time travel possible somehow, it would confuse us deeper. Suppose, if make time travel is possible, somehow, the travelers go into the past. 

Shakespeare Paradox

Traveler makes changes there. The traveler takes a Shakespeare book from his timeline and gave it to Shakespeare in the past before he was not even thinking about that. Shakespeare writes the book later on and becomes famous, time passes by and the traveler reads it and goes back to Shakespeare to give him the book. 

The question now arises – who wrote the book? 

Grandfather Paradox

Another famous paradox is that a time traveler goes back in time, killing his grandfather before the birth of his father I.e grandfather in his childhood. The upshot is that the time traveler ceases to exist. If the time traveler doesn’t exist, how could he have traveled back in time to kill his grandfather?

The answers to Grandfather Paradox are

1- The universe will not permit these changes.

2-some Mixed states will get generated.

3- Some loops will get generated

4-Parallel universes with that timeline as their current timeline would be there which will have different futures.

Conclusion

All these are thought experiments and are having mathematical proofs. Small-scale experiments are carried out to create antimatter and the testing of the Higgs field is carried out by the Large Hadron collider in CERN. 

The god particle has been confirmed with help of LHC in CERN. This is a revolution that will help us to understand many things but for now, time travel is hypothetical. We are expecting to reach the speed of light someday by having a great accelerator. 

Realistic velocity is not possible because mass becomes infinite but still we are expecting to achieve 50-70% in the coming centuries.

We, humans, are slaves of time and it is good to be. The self-made god is complicated stuff.