Time travel...the idea itself is much fascinating when one think of it.With this idea comes various thoughts regarding what we will do after being travelled through time?Most plan(actually dream of) to go to future and see their future life and so that later after returning back to present they can prevent any miss happening yet to occur.
But many think to go back in time,to prevent and rectify whatever wrong has happened with them,may be to save their loved one or to correct grieved mistakes committed by them.
Everyone will have their own plans...but what physics say about,what you can do after travelling through time?
Since nobody has actually traveled through time yet so there are two open possibilities....
Either we will be able to change the past or we won't be able to change the past.
The idea of being able to change the past seems fantastic but by making even minute changes in the past you can drastically change the future not only your...but of whole universe.
If you have watched the movie Men In Black 3 then you will understand what i mean.The movie shows clearly that how an evil alien change the future by killing the agent K in past.
The ideas picturised in science fiction are not the imagination of directors or producers but are the concepts adopted from physics,for example teleportation you may think that it is not true but teleportation origin from the equations of quantum mechanics.Teleportation is based on the "spooky" action at a distance (as Einstein called it) between the entangled particles(for detail reading search for Quantum entanglement and Quantum teleportation).Scientist have successfully carried out teleportation with photons.Changing the past can not only change the future drastically but it can also give rise certain paradoxes..for example the most famous Grandfather Paradox.
The Grandfather Paradox:-
The grandfather paradox is a proposed paradox of time travel first described (in this exact form) by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent (Future Times Three). The paradox is described as following: the time traveler went back in time to the time when his or her grandfather had not married yet. At that time, the time traveler kills his or her grandfather, and therefore, the time traveler is never born when he or she was meant to be.Now,if his grandfather died before having children then the time travelers parents would never been born and in turn there will never born the child which will go back in time to kill his grandfather...thus the time travelers grandfather must be alive.The situation demands that the grandfather should be dead and alive at the same time...hence a paradox.
Here's a video as well to understand more clearly..you will find many like this on youtube.
It is due to these paradoxes that the idea of time travel is always been controversial topic among the physicists.
Now,the second possibility-which restricts us to make any change in past.
A logical question may arise in your mind that how can it be possible that i travel to past and cannot change it by anyway?
It can be possible because by travelling to past you will only b full filling the need of the past.
Let us consider a hypothetical situation to understand this..
When you were 12 years old you met with an accident,however you survived,having some scratches here and there... because someone saves you but the accident had an psychological effect on you and you clearly don't remember what actually happened that day except this that you went to pick the ball from the road.
Later,You grew up to become an eminent scientist and you built a time machine.Now you decide to travel back in time to figure out what actually happened THAT day of accident.You set the date in time machine to the date when that accident occurred.As you reach there(back in time) you find your-young-self playing in the open park near to your house,you remember up to this clearly.You stare yourself for sometime and started to wait for the accident to occur.You wait for considerably long time and started to loose temper then suddenly a ball hits you,you angrily throw the ball aimlessly which landed on the road.Soon you see your young-self going to retrieve the ball in hurry.You notice a truck coming in speed towards the boy(childhood you) which the boy didn't noticed.You abruptly run to save yourself being hit by the truck,just in time you forcefully pulled the boy aside of the truck..resulting in a roll down to the ground.You and your young self got some scratches here and there..and your young self has been fainted during all this.
So now you understand what actually happened that day,it was you who threw the ball thus causing the accident but it was also you who saved yourself from the accident and since you didn't observe the truck coming towards you when you went to pick up the ball and also had been fainted during all this that is why you don't remember what happened at that day in your childhood.
It can also be realized by again consider the grandfather paradox.In the paradox ,you plan to go back in time and kill your grandfather in past.The act of killing is associated to the past...things which have already happened.That when you in your present is planning to kill grandfather the attempt of killing the grandfather has already happened because it happened in past and since you are alive it means that you have already failed in past.
No matter how,maybe you miss the shot or forgot to fill gun's magazine...it can be anyway in which you can fail to kill.Your existence in future showed that your grandfather had survived the past.
This is first proposed by David Lewis in essay The Paradoxes Of Time Travel.
And i strongly believe in this picture of time travel.
However this second possibility also gives rise to a very interesting paradox,this paradox i read in Michio Kaku's book HYPERSPACE,which was originally from Robert Heinlein classic story "All You Zombies" :-
A baby girl is mysteriously dropped off at an orphanage in Cleveland in 1945."Jane" grows up lonely and dejected,not knowing who her parents are,until one day in 1963 she is strangely attracted to a drifter.She falls in love with him.But just when things are finally looking up for Jane,a series of disaster strike.First,she becomes pregnant by the drifter,who then disappears.Second,during the complicated delivery,doctors find that Jane has both sets of sex organs,and to save her life,they are forced to surgically convert"her" to a "him".Finally,a mysterious stranger kidnaps her baby from the delivery room.
Reeling from these disaster,rejected by society,scorned by fate,"he" becomes a drunkard and drifter.Not only has Jane lost her parents and her lover,but he has lost his child as well.Years later,in 1970,he stumbles into a lonely bar,called Pop's Place,and spills out his pathetic story to an elderly bartender.The sympathetic bartender offers the drifter the chance to avenge the stranger who left her pregnant and abandoned,on the condition that he join"time travelers corps".
Both of them enter a time machine ,and the bartender drops off the drifter in 1963.The drifter is strangely attracted to a young orphan woman,who subsequently becomes pregnant.
The bartender then goes forward 9 months,kidnaps the baby girl from the hospital and drops off the baby in an orphanage back in 1945.Then the bartender drops off the thoroughly confused drifter in 1985,to enlist in the time travelers corps.The drifter eventually gets his life together ,becomes a respected and elderly member of the time travelers corps,and then disguise himself as a bartender and has his most difficult missions:a date with destiny,meeting a certain drifter at Pop's Place in 1970.
Now, the question is-who is Jane's mother,father,grandfather,grandmother,son,daughter??
the girl,the drifter and the bartender are the same persons,if you observed.
This is the most mind boggling and cleverly formulated paradox on time travel.
However the mystery yet remains the same whether we can change the past or not? and it can only be solved after traveling through time.