by Animesh Kumar
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Science Behind Time Travel : Part 1

Chapter 1 of 8

About this guide
Time travel is as little understood as it is popular. Pop culture has shown humans (and dogs) getting into a cabin (or car) like machine with dials and knobs that ship you through time in a few seconds. 
But is it really possible to travel through time or is it all just science fiction?

This guide is an effort to explain how time travel to the future could really work from a layman's point of view. Here's a list of areas we'll cover -
  • Chapter 1 : The three dimensions
  • Chapter 2 : Time, the fourth dimension
  • Chapter 3 : Travelling through time
  • Chapter 4 : Time travel to the future - time dilation
  • Chapter 5 : Time dilation caused by relative velocity 
  • Chapter 6 : Time dilation caused by higher gravity 
Let's dive in...

Chapter 2 of 8

The three dimensions
It's no surprise that we live in a three dimensional world. You can move -
  • Forward and backward (x axis)
  • Left or right (y axis)
  • Upwards and downwards (z axis)
Of course, when you climb the stairs, you might say that you are moving forward and upwards. But you are simply moving in the x-y direction which is just a part of our 3 dimensional world.

You may have heard that time is the fourth dimension. But what does it exactly mean?

Chapter 3 of 8

Time, the fourth dimension
We often hear time is the fourth dimension. But what does that really mean?

Let's consider an example - 

Jason asks Alex if they can meet up at the club. Alex agrees and Jason tells him the way : 
Head straight (x axis) from your home for 1 mile and then take a right (y axis) at Bo's Deli. Keep moving until you reach Trinity building and once there, come up (z-axis) to the third level. I'll be there.
Once Alex makes it to the club, Jason is nowhere to be found. What went wrong!

While there may be several reasons, the one critical for it to all make sense is that when Jason told Alex the direction, he mentioned every detail except the time at which he'll be there to meet up.
And so, while Alex had information on all physical coordinates - x, y and z, he didn't have any information on the time coordinate

Chapter 4 of 8

Travelling through time
At this point, I hope you understand how we need information on all four coordinates (x, y, z, time) for meeting anyone. While it's quite clear, that we can travel in x-direction (forward/backward), y-direction (left/right) and z-direction (upward/downward), can we also move in the time direction?

The answer is yes! In fact we're always travelling through time at the speed of one hour per hour. But only in the future time direction.
That said, if you could simply find a way to travel less than one hour per hour, time will pass slowly for you than it would for others and you'd age slower than others.

That's literally travelling to the future. But travelling to the past is much more complex even in theory and interesting paradoxes start to surface.

So what's the science behind all of this?

Chapter 5 of 8

Time travel to the future - time dilation
It's important to understand at this point that time is relative. In fact the idea that my time could be faster or slower compared to your time is the very basis of time travel to the future.

Example to break this down - 

To better understand this, let's consider Jason and Alex, who are both 25 years old. Alex enters a room where time passes so slowly that every hour spent in that room is equivalent to 10 years passed outside of it. This means that if Alex steps out of the room after 3 hours, Jason and the entire world would have lived through 30 years. So while Alex is still 25, Jason would be 55. A corollary to that is - Alex travelled 30 years into the future.

Whatever is in that room, caused something known as time dilation which slowed down Alex's time, as compared to the rest of the world.

Can such a room exist? While there's no guarantee for that, the results can likely be achieved. There are two ways to trigger time dilation - 
  1. By moving at a higher velocity compared to everyone else in the world.
  2. By experiencing stronger gravity than that experienced by everyone else in the world.
Let me try and explain how...

Chapter 6 of 8

Time dilation caused by relative velocity
According to Einstein, there's a profound link between passage of time and motion through the 3-dimensional space. Simply put, the more you have of one, the less you have of the other.

To understand this, let's consider an example -

Let's say you are driving northward at 40 kmph. All your motion is concentrated in the north direction (Y-axis). 
But let's say you change your direction and start heading north-east at the same speed. Once you do that, all your motion will no longer be concentrated in the north direction (x-axis) and even though you are travelling at the same speed you will not make the same amount of progress towards north as earlier.
Simply put, if you move north-east, you loose speed in the north direction (y-axis) because some of it gets passed on to the east direction (x-axis).

Similarly, when you are not moving in the 3-dimensional space, you are only travelling in the time direction.

But the moment you start moving in the 3-dimensional space, some of your motion gets passed on to the x, y, z directions and you move slowly through the time direction (compared to someone standing still and not moving).

And so, time passes more slowly for you the faster you move compared to others. Now this difference is negligible to show an effect in our day to day life but becomes more observable as you near higher speeds (starting about 30,000 km/second).

So taking a ride on a spaceship which travels at 100,000 km/second is one way to travel to the future.


Chapter 7 of 8

Time dilation caused by higher gravity
If you go to Planet X which has 10 times more gravitational pull than Earth, your time would pass slower compared to the rest of us on Earth and once you come back, we all would have aged much more than you would have. And so, you'd have travelled to the future without having aged as much. 

But how does time slow down in regions of high gravity?

To understand this, it's relevant to know three facts - 
  1. Speed of light is a universal constant. It does not change, no matter what.
  2. Strong gravitational regions bend the path of light.
  3. A beam of light reaches from point A to B in a lower gravity region in the same time as it would in a high gravitational region.

Now let's imagine A and B are two points on Earth. Also, C and D are another two equidistant points on Planet X, whose gravity is 10x of Earth's gravity.

Let's shoot two beams of lights simultaneously from A and C -
Since the gravity on Planet X is higher, the light beam from C to D (situated on planet X) will bend and take a longer path.

Since the distance travelled by the beam from C to D is longer (refer fact 2 above), it would take this beam more time than the one shot from A to B. But according to fact 3 above, they must reach at the same time.

Using elementary level math, we know that 
speed = distance/time
Since speed of light is a constant, for the two beams to reach at the same time, time must slow down for the one travelling through a bent (and longer) path between C to D

And so, time slows down for you if you're in a region of high gravity compared to someone else present in a region of low gravity.

So another way to time travel to the future? - Hang round a region of high gravity (notably close to a blackhole - just not too close).

By the way, this is how time slowed down for Mathew McConaughey and his crew in the movie Interstellar. The planet they go to 'Miller' was near a blackhole 'Gargantua' which makes there one hour equivalent to 7 years on Earth! 

Chapter 8 of 8

Conclusion
Now that you have some idea about how time travel to the future may work, you must be wondering how time travel to the past works! 

I'll publish another guide this week to give you an idea of how that might work.

Stay curious.
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