# Time Travel(Part 2)

The post is about the progress in our understanding of time and the possibilities of time travel in our modern world.The effort is to provide the basic idea without the rigorous mathematics and terms.

So,in previous post we came to know that the space which we see as nothing is actually effected by the presence of  mass or energy.Greater mass means greater will be the warping of space and since gravity is nothing but this curvature of space so greater will be the gravity...it is an effect originated from the curved geometry of space.
In short
Heavy mass--->more warping of space--->strong gravity.
It can be easily realized..among planets,Jupiter is the heaviest planet in solar system..so it warps more space than other planets and hence it has the strongest gravity.Similarly Sun is more heavier than Jupiter thus it will warp more space around it therefore strong gravitational pull.

Since both space and time are linked with each other(known as space-time)the distortion in space also results in the distortion of time.Greater the warping of space slower will the passage of time.
Now,
Heavy mass--->more warping of space--->strong gravity--->slower passage of time.

While discussing time dilation of special relativity we understand that the faster we move,slower will be passage of time and also the space will be contracted.These effects(effects discussed in SPECIAL RELATIVITY) are relative i.e you,moving in car,will say people on ground are moving slowly and are contracted(since things are moving for you) while on the other hand for people on ground you are acting slowly and you along with your car have been contracted(as for them you are in motion).
Whereas  the effects of general relativity are in agreement from both the observers.

For example,suppose you travel to some other region,which has a much more strong gravity than that of earth.Now,when you again look at the grand clock on earth,you will notice that the grand clock is ticking fast while your wrist watch is ticking at the normal rate.And people on earth,observing your wrist watch will say that your wrist watch have been slowed down and their grand clock is ticking normally.
Here both are saying the same thing but the other way round.
It is because when you moved to the other region with strong gravity(i.e region of more warped space).This curved space also effect your senses and since our senses along with the space have also been distorted that is why we feel everything normal in our local region...and obscure in other,non-local regions.
Such dilation in time due to the curved space is generally called gravitational time dilation.

Einstein's theory of general relativity not only tells us about the curved space-time but it also predicts two highly fascinating objects which should exist according to the theory.
First are Black holes,which have been successfully detected and the other are wormholes.
BLACK HOLES are formed after a star dies crushing down completely under its own gravitational pull.They are highly dense..the neutrons and other subatomic particles are almost touching each other having no space between them.
To get an idea of the density of black holes...a ONE SPOON FULL mass of a super massive black hole will be much heavier than the mass of whole landmass of  Delhi.

Since Black holes are much more massive and dense... they warp the space around them to a greater extent and hence have a very strong gravitational pull.Even light can not escape the gravitational pull of black holes if it enters the event horizon.
Event horizon is the limit beyond which if anything goes,into the black hole,then it can never came back and will be crushed by black holes immense gravity.
The first image below shows the different stages a dying star passes through to become a black hole

Now since even light can not escape their gravitational pull,they appear black.

BLACK HOLES CAN BE USED TO TRAVEL FORWARD IN TIME...
If you have understand gravitational time dilation(discussed above) then it will be easy to understand.
Suppose you enter a space shuttle and travel close to a Black Hole...since you are now in a region of high gravity then time will pass more slowly for you than the time here on earth.After spending just few hours near to the black when you will return back to the earth then you will find that many years have been passed on earth.
That is you traveled to the future spending just few hours near to black hole.
However, there are no black holes present near our solar system.The nearest black hole is several light years away from earth..so just to get there requires a lot of fuel first and to spend few hours near it,without being pulled inside will require immensely powerful exhaust system..to overcome their gravity.
So going to the future is possible however it may take several years to built such highly efficient ships.

Now,what about going to the past??
This is achieved by the wormholes described by general relativity.

Wormholes are just like the tunnels connecting two different places with each other...wormholes also connect different regions of space in our universe.A wormhole simply cut short the distance between two regions of space.Since they are bridges connecting two regions so there can be different kinds of wormholes,some connecting two regions in same universe or some connecting two different universe(if multiverse exist). Wormholes are also supposed to connect different times as well.
That is if you enter a wormhole you not only be transferred to an other place but also to either to future or to the past as well.

All this seems exciting right?
But sad part is that no macro wormholes(macro here denotes big enough so that spaceships can enter it) have never been detected so far.And if they are detected in future then the physics underlying them states that they will be very unstable.So much sensitive that if anything will cause any disturbance,they will collapse..closing forever.

But in June 1988,three physicist Kip Thorne,Michael Morris and Ulvi Yurtsever 'solve' for a womhole
(solve because wormholes are predicted mathematically by general relativity,they don't have any physical existence yet but they are supposed to exist),one which is stable and can be used to travel.

So it may be possible that one day these wormholes were detected and our technology become so advance that we can construct efficient ships to travel close to a black hole and to pass through wormholes.

So whatever you see in science fiction movies are often valid theoretical possibilities off physics which are yet to be verified experimentally.
Now  it's all folk regarding the time travel..Time travel is not possible with today's technology but may be in a distant future it may becomes the reality.

So,if we happened to travel back in time..can we change the past?overturned history completely?rectify our mistakes done in past?....in next post.