How to perform time travel (in theory)
I know how hilarious it sounds given our present state of technology. And given the huge amount of science fiction novels and movies about time traveling, we always like to believe that traveling through time is a fantasy thing which can't really be accomplished by humans. And yet, humans once dreamed of flying chariots during the middle ages and those dreams actually came true centuries later in the form of aeroplanes.
Coming back to time traveling, there is no easy way to travel into future as the variables are too complex, its an entirely different universe which hasn't yet formed but will be formed by your actions and mine, there isn't any physical access or window to it. But the past, on the other hand, is an entirely different story. It is not only accessible but looking at the past also feels very natural.
For example, when you look into a mirror, you are actually looking at your past self (because of the few milliseconds it takes for light to reflect and reach your eyes and then your brain to process that information). A more interesting example is when you look at the stars:
Do you know that the star you may be looking at on a clear night sky may not even exist (or be entirely different) in reality? It actually takes several years for even light to travel that kind of distance and reach your eyes. The nearest star from planet earth is Alpha Centauri which is located at about 4.37 light years from us. That's a massive and huge distance spanning millions of kilometers. So, when you look at the Alpha Centauri star, you are in fact seeing how it looked 4.37 years ago! The moment you look at Alpha Centauri star, you are already performing time travel into the past!
So, we base our thought experiment of time travel on the assumption that we can only travel (watch) the past, not the future. We also make few more assumptions:
1. Traveling faster than light is possible: I know Einstein said otherwise but let's assume that by year 3000, we would have proven him wrong and the humans of that age would have successfully built a spaceship that can travel 4.37 light years in just 4.37 minutes.
2. Communications faster than light is also possible: Let's also assume that our telecom engineers of circa 3000 AD have the ability to transmit inter-spatial radio signals too, so that the astronaut sitting in a spaceship in the Alpha-Centauri's solar system can send an instant message to back home on earth and that message will also take only 4.37 minutes to reach!
3. Innovations in technology have enabled us to store almost unlimited data (zillions of terabytes) on small devices.
4. We are going to watch the past in "read-only" mode, we are not changing any past events.
With these assumptions holding true, what happens one day is that our astronaut dude working at ISRO (Indian Space Research Organization) gets this idea of taking his spaceship for a brief walk to our neighboring solar system!
Now, Alpha-Centauri is a whole solar system, let's assume that there is a planet in that solar system also known as Alpha-Centauri (for simplicity). Its the blue cylindrical shape in above diagram and that's where our astronaut dude is heading on his spaceship named Alpha-Yaan.
Now, our astronaut dude lands the Alpha-Yaan on the AC planet and takes out a small voyager (which can be controlled directly through the ISRO station on earth). The voyager is programmed to patrol the surface of the planet and keep sending live video footage of everything it captures.
Back home at ISRO, the data sent by voyager is recorded and stored on disks which can be viewed any time in future. It means, from that time onward (let's call it Epoch X), everything that happens on the AC planet can be seen on earth at any point of time! From our present ability to watch a stream of light broadcast from another star 4.37 years ago, we have graduated to the ability to watch whatever is happening there at any point of time (starting from Epoch X). For example, in the year 3100, we can "travel" back a fifty years to year 3050 and see what happened on the planet at that time by watching the stored footage!
Now, fast forward to year 4000 when humans have a space station on AC planet too! By this time, our satellites have also become so powerful that they can scan the earth's surface close enough to make out the size of a ballpoint pen (just like an eagle does) and send that footage to data-centers situated light years away. Just as we started watching the AC's events while sitting on earth, we can now watch earth's events while sitting in AC planet's space center!