# Complete Guide For Time Travelling

### Don't you wanna meet you dinosaurs ancestors ? If yes then read this full article .

Traveling through time may be theoretically possible, but it is beyond our present technological capabilities.

But first of all what is time? Most of us think that it is constant, but Albert Einstein has clearly showed us that time is an illusion, or is relative and it depends on different observers depending on your speed through space.

### TRAVELING THROUGH TIME.

Is it possible?

Well the answer is a big yes!

But only in one direction! Time is linear and it moves in strictly one direction only. That means that we can only travel to the future and we cannot travel to our past (physically). We can travel to future and at present we are actually traveling to our future. According to Albert Einstein's theory of relativity, traveling hundreds or even thousands of years to future is possible!

Let's assume that you and I are wearing identical watches and we synchronize them right before I blast off into space.

In the spaceshuttle, I’m travelling at roughly 28,000 km/hr relative to the Earth.If I make a few orbits around the Earth before I return,when we compare watches.We’ll see that less time has passed for me.A very , very tiny amount of time, but is there nonetheless.This is called Time Dilation, and it simply means that according to Einstein’s Theory of Relativity.

“Time measured along different trajectories is affected by differences in either gravity or velocity - each of which affects time in different ways.”

One example of time dilation is when we compare the passage of time on the ISS(satellite) versus on Earth.

After spending six months on the space station, astronauts have aged approximately .005 seconds less than those of us here on Earth.

Of course, this is a very insignificant amount of time,but it would be much more apparent if the astronauts were able to travel closer to the speed of light.

Interestingly, the effects of time dilation are fairly minor even upto around 70% of the speed of light. However,once we reach 75% or so, the effects become dramatic.

Now, bear with me here,because this is where things get a bit weird. Possibly,the strangest aspect of special relativity is the distances shrink in the direction of motion.

When we think of traveling to a point 10 light years away at 90% of light speed ,you would expect it to take 11 years, right?That’s not exactly the case.

To a stationary observer, it would indeed seem to take your vessel 11 years but to the people inside the vessel, not only time,but distance would dilate and you would reach that point in only 4.4 years.

Okay,that’s already fascinating, but is it really time travel? Perhaps,not in the sense that we usually think of it,but according to the nature of space-time, it is.

If you were to return home from 90% light speed trip,you would be almost 9 years older.But everyone back on Earth would have aged 22 years. So in essence,you’ve travelled 13 years into the future relative to your time in space.

But this is possible only if we attain astonishing speeds (at least close to speed of light). But what will happen to our body if we travel in that speed!? Forget attain speed of light because we need infinite amount of energy to do that.

Don't worry only speed is not the key to travel to future even gravity can help us. Gravity controls time, stronger the gravity gets slower passes the time.

For example in Interstellar, time on a planet close to black hole passes slowly and hours spent in that planet is like years on Earth.

This is because gravity bends space time.

So now these jumps us to point that time travel is possible.

### Ways to Time Travel

### 1. Through the Wormhole

Wormhole is one the many fascinating topic among our scientist. It is theoretically possible to time travel through wormhole. Wormhole can take us in 1 minute past. It is not much but still it is time travel and the fact is it is the only way to travel in past. Though there are many problem associated with worm hole.

1. Its small size , even word small is too big . Wormhole is found (in theory) at the centre of nucleus.

2. Even if somehow we manage to magnify the wormhole , even then it will not be possible to pass through it due to its very very high resistance.

3. And What if we travel back in 1 minute past and shot ourselves . Then what ? We are dead 1 minute before. But then who killed us ? It creates a paradox.

And this paradox is the best reason which state that nature will never allow us to travel in past.

We will dicuss it further.

### 2. Blackhole

We have heard that Gravitational force attract object but the fact is even attract time. Like time near our Earth is dense. What does dense time means ? It means time moves slow near Earth and yes it is proved by a experiment conducted by International Space station. In the experiment two quantum watch were taken with same time. (Quantum watch are Highly accurate watch ) One watch was sent to Space station away from gravity while other stayed on earth. After a year , it was found that Space’s quantum Watch was 1 Nanosecond ahead of Earth's watch which proved that Yes, Time really moves slow near Gravitational Field.

Now what if we Revolve around Black hole which has infinite Gravitational force. Yes, it will really work if somehow we can manage to revolve around Black hole. Time near the orbit of blackhole move half of that on earth i.e. if we revolve around it for 20 years , 40 years will pass on Earth. Though it will be impossible since to revolve around black hole we will need speed greater than speed of light since even light cannot escape the gravitational pull of black hole and but hopes are still alive.

### 3. Speed of Light

One of the many interesting topic in physics is Speed of Light. Speed of light is topic of discussion since the era of Newton and it is still a controversial topic in Modern science. Let's get back to the point. Theoretically, if an Object travel at speed of light , time for it will stop completely.

To understand it more perfectly let us take an example. We say that Light of stars take around millions of years to reach us. It is our perspective. But for photon (Light particle) , it take zero second to travel millions of light years i.e. for photon , it is emitted by the star and next moment it reaches to earth. Why ? Because it is travelling at speed of light and time for it becomes zero.

Now if somehow our train or spaceships are able to travel at speed of light, time for the person sitting inside that Train or spaceship will be zero. But it is not possible to travel at speed of light since it is universal speed limit. But the beautiful point in this theory is even if we are able to travel at 99% of speed of light we can still time travel at rate of 1 second on Train equal to 1 year on Earth. So currently it is only possible way to travel in future.

Main problem in this theory is energy and work is going on in this field and latest discovery in the field is of Anti-Matter. Mixture of Antimatter amd matter creates tremendeous amount of energy. It has a drawback though. Once we travel in future we can never return back to present.

### Mathematical Investigation .

Time is 4th Dimension but its not like the other 3 -spatial Dimensions (x,y,z) .

Let me give you a simple Mathematical Concept.

Consider a particle moving in space-time. Let

v = Relative velocity of particle w.r.t photon

c = Velocity of light =3×108m/sec

then

If v<c => Particle moves Forward in Time.

If v=c => Time will stop altogether for him.

If v>c => Particle moves Backward in Time. eg Tachyons

So our target should be to build a space-ship with speed > 3×108m/sec

But fastest speed of present day’s Space-ship ~ 15.73×103m/sec.

According to Special Theory of Relativity, “Nothing can travel faster than the speed of light.”

Reason for that is:-

According to Relativity Eqn, E=mc2

From above eqn we can conclude => Energy ‘ E ’ is directly propotional to mass ‘ m ’

As particles tries to increase its velocity ‘ v ’, Kinetic-Energy ‘ E ’ increses. Hence according to eqn mass ‘ m ’ must also increse. When velocity of particle approaches to ‘ c ’ mass of particle increses enormously almost Infinity. At very large velocity of the order of ‘ c ’, mass of object m -> Infinty and hence becomes very difficult to reach that velocity.

According to Thermodynamics, Entropy of the system always Increases naturally, so is the reason Time always ticks in forward direction naturally.

*Matter - Travels forward in Time.
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Light Particle called Photon - Independent of Time. It means light particle at the same moment can be at n different positions.

Actually Photons travel perpendicular to Space-Time axis( 5th -Dimension).

Darkmatter - Travels back in Time.

**Time is Tensor Quantity**. Tensor Quantity has Different magnitude in different direction. i.e., Magnitude of Time along X, Y and Z axis is not equal.

Running of Time = function of [Gravitational waves, Velocity of particle ]

Gravitational Waves-

Time runs slowly where gravity is strong. i.e, Time runs comparatively fast at Moon than at Earth because of less Gravity.

Relative Velocity of Body -

Time runs slow when speed of body increases. i.e., If A and B are two friends. A is travelling in Train and B is waiting for him at next station, then due to motion(i.e speed) of A, time running for A will be less as compared to B who is standstill. In other words A is still in past w.r.t B and this is actual basis of Time Travel.

A myth of having a mechanical machine = called Time Machine which will take us to future or past is Just Only a Concept. Same gets applied for Wormholes (though they may be Theoritically correct )

Suppose you are traveling in a rocket near speed of light say v=0.99c and your friend also measures the time for your journey was 20 years (time measured by u from take off to landing). Also you have a friend who measured time of your journey(i.e dT ’). Now we will calculate how much time he measured or how much he waited for U to come back.

Substitute in above eqn then we will get => dT′=141.77 years i.e., he waited 141 years for you to come back while according to you, you only took 20 years. Well this is Time Dilation. So you have come 121 years in the future.

above diagram is called Space-time region. With two lines AOB and A'OB' (each of the eqn x=ct ) these lines are called world lines of light wave.

Now,in four dimensional space they form cone, called light cone, because only light-like vectors can form it.

For understanding why actually time travel is impossible we have one point (P2) Now,if you want to go to point (P1) situated in affective past region A'OB’ one have to move faster than light in order to reach the point,and same process to come back,

But STR says it is impossible, because that intervel (P3P1) would be space-liked.

Now for future,there are two things

If you only just sit in a chair nearby to you for a while,we know time is passing,you can go two seconds further if you sits on the chair for two seconds.But can't exceed that time,( means you can't go 4 seconds further in just 1 or 2 seconds).This is not really time travel,that we talk about.

Now if we want to go a point (P2) one have to move faster then light ( that would be liked to go 4 seconds in just 1 or 2 seconds).i.e. the actual time travel.Also intervel (P3P2) would be space-liked, which doesn't exist.

So conclusion is that time travel that we think is like to be impossible.For past is is impossible,for future isn't possible in that manner.

For understanding what are space,time and light like vectors and intervel you can go Durgesh Dinesh Tinker's answer to What are time-like, space-like and light-like vectors?

(For more information you can read Feynman lectures in physics vol 1 ch 17.)

Hence travelling into the future is possible! We do it all the time ! While I'm trying to write this , I'm travelling to the future at the rate of 1 second per second. No machine required. However, if we want to get to the future faster, we just have to travel faster. Special Relativity tells us that as you go faster, for an external observer at rest, time for you slows down.

### Traveling back to past.

Imagine you own a watch that lets you travel through time .One day you get the bright idea to travel back to the 1920’s to mess with your grandparents.You press a few buttons, and suddenly you’re standing beside old Pappy(Grandpa) who is taking in the scenery at the precipice of the Grand Canyon.Since you’ve never been the biggest fan of your grandfather,you decided to push him over the edge.Gravity does its thing, and a few seconds later,no more Pappy.

But wait if you killed your grandfather before he ever met your grandmother, you wouldn’t have been born to be able to go back in time and kill him!

Here’s the problem. If you didn’t exist to kill your grandfather,you couldn’t have killed him….because you didn’t kill him,he’s still alive.But that would mean that you yourself would be alive and now you can go back in time to kill him.And so the endless loop goes on.This is called Grandfather Paradox.

Traveling to future is as easy but traveling to past is much more complicated because time is linear.

It is simply may be impossible!

It may be possible but it’s confirmed that we cannot change it.

Because what happens if we kill our own grandparents?

Traveling back to time and changing things is tricky, it will lead to an unexplainable paradox.

If it’s possible and a man travels to his past and change things or meet himself, he must have already seen himself in his past when he was young. It’s easy to say that you will not invent a time machine and meet yourself in past, if you've not seen yourself in your past. How is it possible? And if we began to change things in past, we would spend a lot of time there and will our body stop its biological actions that makes us old, if we stay in past we will definitely get old at the same rate.

*But there is a twist called muons !*

*Yes. Muons.
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*A muon is a subatomic particle. They are generally created during particle collisons. Also known as heavy electrons, these muons have a charge equal to that of electron but are 206 times heavier than the mass of electrons.
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But, muons are short-lived particles. Their half-life is somewhat close to 2.2 microseconds. That means, if there are 100 muons, after a 2 microsecond time span, we will be left with only 50(approx.). They decay into electrons and neutrinos.
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*Now, Muons are also created at high altitudes where the top of the atmosphere is bombarded with solar and cosmic protons. The generated muons rain down at high speed, some of them decaying partway down and others making it all the way to the ground at sea level.
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*The distance these muons will cover will depend on their initial speed. Let’s say a muon is travelling at a speed half the speed of light(c/2). We know the average life span of muon is 2.2 microseconds. So, we would expect that the muon will travel for only 300 metres before decaying.
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Based on this we should expect the number of muons detected at that 300m distance to be half of the original. And at 600m down we'd expect a quarter of the original to remain, at 900m one eighth would remain, etc. We can continue this down to sea level and determine what percentage would survive based on their original velocity.
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But, the numbers tell a different story. It is experimentally verified that the number of muons that reach the ground is much larger than the theoretical number. Somehow, these muons lifespan has increased helping them cover more distance. So, from the muons frame of reference, their viewpoints, they are travelling into the future. (If you have seen Interstellar, remember Cooper had aged only a few months, while back on Earth his Children had lived 20 years since his departure.)
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The reason for this is Special Relativity. When the muons are travelling at a velocity comparable to speed of light, a clock in their frame of reference starts ticking slowly. This phenomenon is called Time Dilation. The time slows down by a factor of 1(√1−v2c2) .
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That’s how muons can cover a larger distance within their short lifespan. They are time travelling!!!!*

*But some expirements found them travelling in past .*