Curiosity is welcome here!

Our universe is a beautiful place, it contains everything that we know about and everything that we aspire to know about. It gave us birth and one day, unfortunately, will bestow upon us death. There are an infinite number of secrets hidden inside the cosmos and this blog is entirely dedicated to uncovering as many that are humanly possible.

## Time Travel

Humans have always wondered if they could travel through time to observe the future and change the past. While travelling to the future might be possible, travelling to the past might prove to be one of the hardest tasks faced by mankind since the beginning of its existence.

### Arrow of Time:

The ‘arrow of time’ basically says that time travels in a specific direction, and that direction remains constant (Not taking into account the possibility of parallel universes – Will be discussed in another blog post). The direction here is forward (The direction of time is towards the future). Things that happen in real time can’t be reversed and due to this **asymmetry,** these events are called** irreversible.**

There are actually 3-5 other arrows of time but the only other arrows of time we will mainly discuss are the ‘Thermodynamic arrow of time’ and the ‘Cosmological arrow of time’.

**The Thermodynamic arrow of time:**

The thermodynamic arrow of time is based around the ‘second law of thermodynamics’, it states that the **entropy** of our entire universe increases positively over time. Now, what does this mean? Entropy in layman terms would be the **measure of disorderliness.** When our universe began it was packed into a small area which was compact and organised, hence if we were to reverse the evolution of the universe, its entropy would decrease. Our universe keeps on expanding and grows ‘disordered’ showing a sign of an increase in entropy.

**The cosmological arrow of time:**

The cosmological arrow of time says that as time passes our universe is consistently expanding. This shows that after a long period of time that the entropy of the universe will reach its largest point, resulting in stopping of the expansion.

Now, to go deeper into the topic of ‘time travel’ we will learn about a **4-dimensional continuum called ‘spacetime’.**

### Spacetime:

We all would’ve heard the name ‘Albert Einstein’ at least once in our lifetime, he was a remarkable scientist and has made numerous contributions to modern physics, but one of his most famous and most valuable contribution is his **‘theory of special relativity’.**

This theory shows that space and time are actually intertwined with each other as a 4-dimensional continuum with time as the fourth dimension called spacetime.

Our universe is made up of this **fabric of spacetime. **Now, Why is it called a fabric? Spacetime has some weird properties which have got it classified as fabric-like. Spacetime is really flexible, like a fabric, it can be bent, warped and torn too, this bending and warping might cause changes in the speed with which time travels.

How can spacetime be bent? Matter bends spacetime. Even our bodies are bending spacetime, not as much as the earth but a little bit. **When spacetime bends it causes gravity. Hence, gravity is caused by the bending of spacetime. **

Every single thing that exists physically in our universe has a place in this fabric of spacetime. **It would have three space coordinates and one time coordinate.**

How is this information relevant to time travel? With the concept of spacetime, concepts like **time dilation** came in too. What is time dilation? The Wikipedia definition describes it best- **” According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other or by being differently situated relative to a gravitational field”.**

#### Time Dilation:

Time dilation basically says that the faster you go the slower time travels for you compared to other people who are travelling at a lesser velocity, it also says that the stronger the gravitational field around you the slower time travels compared to people in a weaker gravitational field.

Gravity is the cause of the warping of spacetime. In simple terms the more gravity an object has, the more it warps spacetime and the more spacetime is warped the slower time is for the objects in that gravitational field. This is called **gravitational time dilation**.

Now, we shall discuss **time dilation due to the difference in velocity. **This effect can be explained with a very famous analogy. (Introduced to me by the youtube channel-Fermilab)

Imagine three objects travelling at different speeds:

Object ‘a’- This object is stationary, hence, it is only travelling through time in the graph above. Time should move comparatively faster for this object as it is only moving through time and not space.

Object ‘b’- This object is moving at a speed at which we all travel every day, somewhere between 40-150kmph. For this object, time travels slower when compared to object ‘a’, as it is moving through time and space simultaneously. Hence, moving lesser through time than object ‘a’.

Object ‘c’- This object is travelling at light speed or **186,282 miles per second.** As you start nearing the speed of light you start travelling lesser and lesser through time, resulting in the slowing of time for you. When you reach light speed, time is said to stop.

This is why the faster an object travels the slower time passes for it.

**Application of time dilation for time travel:**

Using the concept of time dilation for time travel would only allow us to go to the future and not the past, but something is better than nothing, isn’t it? To go substantially into the future you would need to travel at 85-95% the speed of light for a few hours and when you return to earth who knows what you would find.

You can use the formulae for time dilation to see for yourself:

Gravitational time dilation:Here,

1. t’ – Change in time in the gravitationally influenced reference frame.

2. t – We can look at it as a “standard” hour. Not influenced by any gravity.

3. G – The gravitational constant – 6.673 * 10^-11 Nm^2/kg^2

4. M – Mass of the object being approached

5. r – Distance from the object being approached

6. c – Speed of light

Time dilation due to the difference in velocity: Here,

1. v – Velocity (In terms of light speed- 99% speed of light- **0.99c**)

2. c – Speed of light

3. T A – Time passed for a person on earth

4. T B – Time passed for the person in a spaceship travelling at ‘v’

That was pretty much everything about time dilation, a way to travel into the future. Now, let’s talk about something that might take us into the past – **Wormholes.**

### Wormholes:

Wormholes or **Einstein-Rosen bridges **are pathways to different places in spacetime.

The concept of a wormhole says that it bends spacetime so much that it brings two very distant points together.

This would allow the traveller to reach a point in spacetime that is light years away in a matter of minutes, without the need to travel faster than light (Will be discussed in another post).

As wormholes connect two points in spacetime, the destination we reach after travelling through the wormhole would have 3 space co-ordinates and 1 ‘time’ co-ordinate. This means that a wormhole can not only help us travel to a distant location in space but also a distant location in time. This could be the future or also the **past.**

This is the reason why wormholes are so essential to the concept of time travel. It is also said that if two distant singularities are brought close enough they both might join and create a wormhole.

We have completed discussing ‘travelling’ through time. Now, let us talk about looking through time.

### Looking at the past:

The concept discussed here will only allow us to see the past and not the future, but again, something is better than nothing.

Currently, the galaxy Andromeda is about 2.537 million light-years away. This would mean that the light from Andromeda takes 2.537 million years to reach earth and vice-versa.

So, if some advanced civilization flourishing in Andromeda would use their technology to look at the Earth from their planet they would see the Earth 2.537 million years in the past. So they would see the homo habilis roaming our planet.

Due to this effect, the stars we see at night might not even be there today, we might only be seeing the light of those stars which takes millions of years to reach our eyes.

Time travel is a fascinating topic and it is one my favourites if any of you guys want to hear more about it you can visit youtube channels like – **TedEd**, **National Geographic** and **Vsauce**. Other youtube channels I would recommend are **Kurzgesagt-In a Nutshell** and **Fermilab**.

Thank you for reading!

**Pictures:**

No copyright was intended for all the pictures that have been used in this post.

Fantastic! So do you believe time-travel can lead to an event which disrupts space time continuum?e.g. the popular ‘back to the future’ concern of a future person running into their past version causing the universe to implode or something? ðŸ™‚

Yes I do, the first part. I will look into the second part

I’ve always found the idea of time travel fascinating and this blog explains it so simply that it is a very interesting read!