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## Is Time Travel Possible?

Time, as simple as it seems, is a very complex phenomenon to understand. Every time in our life, we think of a possibility that what if we can go back in time and change a few events, or could just leave this awful time and move to a **utopian future**.

We, the millennials grew up watching heroes going backward in time and saving the world, the latest being The Flash in Justice League. But the question that one common person can ask is, what exactly is time? Is it more than just a physical or mental construct and something metaphysical?

Let’s understand the arrow of time in this post. Everybody has heard of something like Time is a dimension according to Relativity theory and we move in time just like we move in space. But most of the time, we just memorize these words and get away with them.

## What is meant by the time is a dimension?

We can move in space- left, right, up, and down but hardly anyone notices about this movement in the dimension of time. Albert Einstein, who started all this, also gave a simple and precise definition of time as “Time is what a clock reads” and this is just how we understand it too.

Materialist philosophers have their own interpretation and they say that time is invented by clock companies to sell more clocks. Now, this is a bit absurd but let’s understand this phenomenon from a scientific point of view.

Relativity as we know it as of now, goes far back to Galileo who gave the first Principles of Relativity and on whose shoulders, classical and **Newtonian mechanics** further developed. The motion of objects is relative to the motion of other objects, adding the velocities when they approach each other and subtracting when they move away from each other.

Newton theorized the absolute model of the universe in which space and time are both absolute entities. Space is the background on which objects move and time is a universal clock being the same for all observers inside the universe.

Every observer present in the universe, despite the distances between, will agree on the measurement of time and distances measured from a standard ruler. Around the same time, Danish astronomer Ole Christensen Rømer measured the speed of light by studying the eclipses of the moons of Jupiter.

In the first half of the 19th century, Michael Faraday proposed that electric and magnetic fields are one, and together they combine to form a single unified Electro-magnetic wave.

JC Maxwell gave his world-famous Maxwell’s Equation and showed that light is also an EM wave having wavelengths of 300-700 nanometer on the whole spectrum of different waves having different wavelengths.

Using the equation he calculated that the speed of light is a constant value denoted as “c” and its mathematical value was later fixed to be 299,792,458 m/s. The question that arises here is that if the speed of light is constant, then with respect to what?

The proposal was that there is a uniform omnipresent element spread across the universe known as the Luminiferous Aether and light is moving at a constant speed with respect to this Aether.

This theory of ether was discarded in 1887 when the famous Michelson-Morley experiment was performed which measured the speed of light coming to the Earth at various angles.

It was assumed that since the Earth is rotating, it will also make the ether field around it rotate, and hence light from the sun coming to Earth will get tangled in the ether drag produced around the planet thus making the light travel a longer distance as compared to light coming perpendicular to the surface.

To explain this phenomenon, many attempts were made by genius mathematicians like **Hendrik Lorentz** and Poincare before Einstein came into the picture.

Lorentz proposed a model that the ether is present but when light moves through the ether-drag, space gets contracted and hence we always get to measure the adjusted speed of light.

This method is called **Lorentz Contraction** which Einstein later used after he proposed his special theory of Relativity. There’s a story about young Einstein that when he was young he thought of racing a light beam in his dream.

When he reached the speed of light, he got to see the stationary wave of light. But after he learned about Maxwell’s theory, it seemed irrelevant to think of light as a stationary wave because electromagnetic disturbance always traveled at a constant speed.

Light can never be stationary and hence we can never be able to reach the speed of light because something else is changing and preventing us to do so.

Let’s do a thought experiment. Suppose a car with its headlight on, is moving towards you. Using the velocity addition formula of Galileo, you can calculate the speed of light coming towards you. If the speed of light is C and the speed of the car is V, then the total speed of light is C+V. But how is that possible?

Light has constant speed C and using the Galilean relativity we get to measure the value more than C. Using this thought experiment, he gave two postulates:

- Laws of physics should be the same for all observers in an inertial frame of reference.
- The speed of light is constant for all observers.

### Special Theory of Relativity

Inertial frame of reference means that the object in that frame is moving at constant velocity with no acceleration and hence it is called Special Theory of Relativity because it is only applicable to special cases when acceleration is absent. Remember the most used formula of classical mechanics, Speed = Distance/Time.

Using this formula, we can understand the concept of Special Theory of Relativity in very simple terms. Since speed is constant, then the other two parameters present i.e. Distance and Time must adjust themselves to keep the value of speed constant.

This means that space and time are not absolute features but are fluid and can change their structure. In relativity, every observer in the universe will agree that the speed of light is constant but they disagree on the point that how much distance the light has traveled and how much time it has taken to span that distance.

Time is not a universal clock and it can have multiple values for different observers. Space and time are relative and are adjustable parameters. They are not the background on which we do classical mechanics, in fact, due to the occurrence of events we get to measure the values of space and time with respect to the speed and position of the observer observing the occurrence.

In 1908, Hermann Minkowski, one of the math professors of a young Einstein, fused the two parameters space and time in what is now called Minkowski spacetime and we get to know that space and time together produce the 4-dimensional structure of the cosmos.

Space and time can change places as they do inside the black hole. We move in space and time simultaneously, and as the speed of an object increases in the spatial dimension, its speed in the temporal dimension decreases, and vice versa.

If we are sitting still, we are not moving in space but we are moving in time at a constant speed. As the speed of an object reaches the speed of light, the speed in the temporal dimension approaches zero. Hence as soon as we acquire the light speed, time stops.

This of course is fancy to say so but at the light speed, there are infinite possibilities of what could happen. Now if time and space are interchangeable, we can move in time both forward and backward just like we move in a spatial dimension.

### Arrow of Time

But practically that is not the possibility we see in real life and why is that? There is a concept called the arrow of time. We always perceive time moving forward, never backward, and therefore the arrow of time is only future-oriented.

**Thermodynamic arrow of time:**Among several arrows of time, the first one is called the Thermodynamic arrow of time and as the name suggests, this shows the tendency of the flow of time according to the laws of thermodynamics.- Now the second law of thermodynamics says that the entropy of the universe always increases, and hence the universe always expands, and it is always more disoriented than before. Thus moving from present time to some past event, we have to go to a state of the universe that has lower entropy than the present, and that is not allowed by the second law of thermodynamics.

Time always moves in the direction in which the disorder of the universe increases. Hence, space-like movement in time is restricted by a very basic law.

### Psychological Arrow of Time

The second arrow is called the Psychological arrow of time. Now pay attention to this very part. Since every point in space is true, that should be applicable for time too, hence just like every point in space exists all the time, every point in time also exists all the time.

Every moment in time either in the past or future exists eternally, and this is what Einstein meant when he said that “Time is but a stubborn illusion”. There is no meaning of the future and past because events exist even at this very point.

Confused already? Wait, there’s more. According to the psychological arrow of time, time moves only in that direction in which we remember the past but not the future. Just think about it, we know that the past exists because we have memories of the past events in our mind.

If one could remove this memory, there is no meaning that the past ever happened. If our mind does not store any memory, we can live one moment at a time. Every moment will be like one frame among the thousands in a video.

Storing the memory in our mind increases the entropy of the brain and just like the first arrow, time moves forward in the direction of increasing entropy.

### Cosmological Arrow of Time

The last arrow is the Cosmological arrow of time that says time will move in that direction in which the universe is expanding. The universe started with the Big Bang and started expanding from then, this expanding universe is increasing the entropy of the universe, thus again the direction in which time moves forward is in the direction of increasing entropy.

If you want to go back in time to some good old days but you can not, blame the entropy. That simple second law of thermodynamics which we study in elementary physics is the biggest roadblock for time-traveling enthusiasts.

But nothing is impossible, and there is a catch to this problem. Actually, not one but many, which we will discuss some times later. Can entropy be reversed? And can we finally be able to travel to the past? Wormholes are a suitable choice for that but we need a lot of negative energy for that, and that’s another major issue altogether.

Or we can move faster than light but that too brings us to the field of exotic particles. Remember, traveling in the future is physically and practically possible, we do it every time, and it also has been proven experimentally but it’s the past that is still haunting us.

Maybe the Universe is saying, it’s better to leave the past behind, or we can hope, someday, someone among us will solve this puzzle and can take us beyond time. What are your thoughts about time? Let us know in the comments. If you found this article interesting, please share it with your friends.