﻿ Time Travel - A Journey Through Time - Go Physics

# Time Travel – A Journey Through Time

(Last Updated On: August 6, 2019)

#### Time travel and its origins

The concept of time travel, as we know it, was born in the past centuries. There are events of time travel in our ancient relics, but they are more of a fantasy and a fairy tale than science. There were many books that used dreams and magic for time travel. But H.G. Wells novel, “The Time Machine”, was the first to feature time travel as a science, through science-fiction, which later led its way to the theories of science which tried to make it possible.

#### What does travelling in time actually mean?

Time travel, as we know it, means jumping from one point in time to another. We are actually travelling to the future even now, you’re going one second per second into the future while reading this. We can also travel to the past through our memories, even if they’re memories, you’re still able to experience the past. Also, we can see the past in the night sky. We see stars as if they were in their past.

Consider a star, which is 100 light-years away, you would see it as if it was 100 years ago. And you’re seeing the sun as if it was eight minutes ago. This is because light takes time to travel. It travels at a constant speed of approx 3*10^8 m/s. It takes around eight minutes for the light from the sun to reach our eyes. So yeah, you can see the past.
Many people from ages have theorized time travel in their own ways. Some treat it as your consciousness travelling Back to your younger self. But that wouldn’t satisfy our needs, as we might want to go back to the time before we were born.

So time travel, as we all accept it, is the physical ‘you’ jumping to another point in time.
Einstein’s theory of relativity treats time and space as a unique space-time. So, according to Einstein, as you move through time, you move through space too. And surprisingly, Einstein’s equations say that time travel is possible! Eureka!

Read Also: Space-Time – Simple Explanation

#### Is time travel really possible?

To answer this, let us break it down into two parts, travelling to the past and travelling to the future. I’m sure there is nothing special about the present travel to discuss (laughs).

#### 1. Travelling to the future

Travelling to the future seems highly possible because you don’t need to cross the speed of light for it. You don’t even need to reach it. And that’s explained by the theory of relativity.

Let’s start with this strange stuff called time dilation, which is the difference in the elapsed time measured by two observers, either due to relative velocity difference or the effect of gravity. Let us take the Einstein example, consider Mr Albert Einstein is sitting in a tram car and was intended to travel away from the big ben which has a big clock, and he’s observing the motion of the clock’s hands during his motion.

While he is stationary, the second hand (second hand of the clock) moves at a definite speed. As he starts his journey with a certain velocity, he observes the speed of the second hand as it was before, no change. Even if he goes on increasing the speeds until the highest speeds that the tram car is capable of, he would observe the speed of the second hand with no change. But, that’s wrong. There is actually a very minute negligible difference in its speed which cannot be sensed at the car’s speed. Since, the car’s speed is negligible compared to the speed of light, which is travelling from the clock to his eyes.

But, let us consider that the car can travel at very higher speeds, closer to the speed of light but less than it. Now, as the speed of his car gets closer to the speed of light, the light from the clock takes even more time to reach his eyes. Due to this, he observes the second hand ticking at a slower speed. Imagine, if the car travels at the speed of light, then the light from the clock would never reach his eyes, and only the light which hit his eyes would travel along, making the clock freeze. That is, he experiences no time. Let’s stop this “reached the speed of light” topic here. We’ll go through it again.

So, as he’s moving closer to the speed of light, time around him slows down, time slows down for him. Thus, when he reaches back to the clock, all the people around the clock would be aged more than him.
And as I mentioned above, you’re travelling to the future even now, at the speed of one second per second.

So, travelling to the future is possible.

#### 2. Travelling to the past

Achieving future travel is quite simple when compared to past travel because things get really wild here. Even if the possibilities of future travel are the most possible, the ones of past travel are minimum, or maybe absolute zero. Because mass increases with increase in speed. If you reach the speed of light, your mass would be infinite. Only the particles with zero rest mass like photons could achieve it.

But if somehow you reached light speed, consider the “if” case, things get even weirder. Recall our previous concept of time dilation, which was explained above. We’ve understood that time slows down with increasing speeds, it tends to zero. You go to the future. On reaching the light speed, time becomes zero. So what’s beyond that? Time becomes negative. That is, it flows backwards and you travel to the past.

But, crossing light speed is not possible. Some scientists feel that it may be possible in future, but still, there are a lot of problems with past travel. A lot of paradoxes arise with past travel. Let me explain the most famous grandfather paradox and the bootstrap paradox.

Consider you are angry on your grandfather for no reason or some ugly reason and just wanted to kill him. Yeah, I know. I know that you would never do it, but for time being, just imagine. So you planned to kill him even before his marriage by travelling to the past. And then you go to the past and kill him before his marriage. Now that he’s dead before his marriage, your father wouldn’t have born and then you wouldn’t have born. So you would’ve never travelled to the past to kill your grandfather, and he’ll be alive. As he’s alive, your father and you would’ve born. And then you would’ve again travelled back to kill him. This continues as an infinite loop giving two possibilities, that’s you and you’re grandfather is both dead and alive. This is not possible. So, it questions past travel.

Consider that someone from the future came to you and told you that he was you, but from the future, and handed you the formula to build a time machine. You build the time machine with the help of the formula and then travel back in time to give the formula to your younger self. And he too has to do the same thing. Then, where did the actual formula for time travel come from? Out of nowhere and stuck in the time loop, which is not possible. This again questions past travel.

Read Also: Time – An Illusion

#### Multiverse – Possible Solution

So as to resolve these paradoxes, we can consider the existence of the multiverse. That is, we don’t live in a single, one and only, ultimate universe. Instead, we live in a system of universes, called the multiverse. So, when you travel back in time to kill your grandfather, you don’t travel to your past. Instead, you travel to a parallel universe, with its time corresponding to your grandfather’s childhood. Thus, you would kill the grandfather of some other one just like you in a parallel universe, and your grandfather would still be alive in your universe.

Similarly, in the bootstrap paradox, the formula might have been created in some random universe and is circulated through some other universes. This doesn’t mean that you travelled to the past. You just travel to another universe with a different time. There is no evidence for the existence of a multiverse. String theory explains that we live in a multiverse.

#### Theories which may make their way in far future

Many theories have been hypothesized to explain the possibility of time travel, which includes, Wormholes, Alcubierre warp-drives, Tipler cylinder and etc. There is a theory about the possibility of a faster than light particle called Tachyon, which is hypothetical and cannot exist in this universe. Let me explain a few points about wormholes and Alcubierre warp drives.

#### Wormholes

Einstein- Rosen bridges, popularly called as wormholes, are a possible solution of Einstein’s equations. They work by bending space-time.

Consider two points, each on the opposite ends of a paper. So what’s the shortest distance between them? A straight line, that’s what our math taught us. But what if we bend the paper and make both the ends meet? That’s how a wormhole works. This connects two points in space-time by bending it. It doesn’t violate the law of physics that nothing can cross the speed of light. So if you could open a wormhole, you would’ve created a shortcut through space and also through time.

But the problem is we didn’t find any wormhole till now and even if they exist, they may exist at the subatomic level, which would be of no use for us. And for a wormhole large enough to let us travel through it, it needs a very high amount of negative matter to stabilize it, whereas we haven’t found at least an atom of it. There are theories that say a black hole may be the sucking end of a wormhole whereas a white hole would be its releasing end. But we haven’t found a white hole.

#### Alcubierre warp drives

It is an idea based on the solutions of Einstein’s equations. It was proposed by a Mexican theoretical physicist, “Miguel Alcubierre”.

Instead of exceeding the speed of light in the local reference frame of a spacecraft, it would travel by contracting space in front of it and expanding space behind it. Thus the Alcubierre warp drive shifts space around the spacecraft, that it reaches the destination faster than light, without breaking the laws of physics.

Read More About Alcubierre Warp Drives: The Alcubierre Warp Drives – Faster Than Light

#### So, what next?

Thus, travelling to the future is highly possible but to the past seems a bit difficult due to its paradoxes. But still travelling to a parallel universe with a different time may be possible, if the multiverse exists.

Let us wait for the future to unfold. Let’s wait to see if the hypothesis today could be the reality tomorrow.

Another Article On Time Travel For Better Understanding: Is Time Travel Possible? – Check

References –

1. Time Travel: A History By James Gleick
2. How To Build A Time Machine The real science of time travel By Brian Clegg

#### Tarang Garlapally

He is an astrophysics enthusiast. He is also a science fiction writer and a poet. He applies all the science he learned into his stories too. He always tries to learn new concepts in astrophysics.