This week, I want to talk about one of my all-time favourite sci-fi topics, 'Time-Travel'. I will try not to go into the super-confusing physics part and only discuss Time-Travel as a general idea. So, head past the break and dive into my world of sci-fi fantasy!

I love this quote from the sci-fi series 'Doctor Who' and it aptly explains our general understanding of (or lack of... ) 'Time' as a concept. But as far as I can remember, I've always been fascinated by the idea of Time-Travel. To be able to re-live the past or to foresee the future. The first time I encountered this idea was in H. G. Wells' 'The Time Machine' (the first story that had a proper time machine if you don't count 'The Clock That Went Backward' by Edward Page Mitchell) when i was a little boy. The story of a man from the 19th century travelling to a distant post-apocalyptic future was fantastic enough to capture the imagination of a young boy. I even started dreaming of becoming such a Time-Traveler someday. As I'm some of you did too. And it is no surprise that this idea has intrigued many others in the past and will also continue to do so in the future.

Instances of Time-Travel can be found in various works of fiction even from the very early days of literature (Even in the Mahabharata!). Though in those earlier works, the method of Time-Travel was mostly based on magic or was left unexplained (so, they can't really be termed as 'science fiction'). Some of them, however, had 'deep slumber' as the method of time travel (which is quite ingenious as we will find out later in this post). In the later works, specifically in the late 19th century, the concept gradually started tipping over a little to the 'science-y' side.

So, lets start the discussion with 'Time' as a concept. As we all know, the theory of general relativity says that our universe is made up of 4 dimensions (Believe me, It was in High School Science!). Out of which, 3 are spatial dimensions, i.e. length, width and height, and 1 is a temporal dimension, i.e. time. Together, they constitute something called the 'Spacetime Continuum'. We can easily travel along the 3 spatial dimensions as we do so in our everyday lives. So, theoretically, it should also be possible to travel both ways in the temporal dimension as we can do in the other dimensions. Alas, its not as simple as it sounds. The first problem is that we can see these 3 spatial dimensions but we can't actually see the temporal dimension. So, traveling along this temporal dimension is obviously going to be trickier than traveling in the others.

So, can we go backwards in time? Well, there are a few ways in which it could be possible. The first one is traveling faster than the speed of light. Faster than light (FTL) travel actually reverses the time because traveling at such fast speed distorts the spacetime continuum and you start moving backwards in time. It works in the following way. First, as we move slower than time, time runs forward (relatively for us). But, if we start moving close to the speed of light, then the time will slow down and as we reach 'light-speed', the time will come to a halt. From there, if we go any faster, then the time will start moving backwards and we will start travelling backwards in time. It all sounds so good but the problem is traveling FTL is totally impossible. Our universe has a 'Cosmic Speed Limit' (unfortunately.. :( ) which is the speed of light. Nothing can reach it so, it is quite impossible to cross it.

However, there's another way of traveling to the past. As we've seen in sci-fi movies, time machines are devices capable of bridging the gap between two places distant in time through some sort of a time-tunnel. In reality, such time-tunnels can actually exist (yay!). For example, if you look at a very flat surface (3-dimensional), then it might seem super-smooth to you, but put it under an powerful electron microscope and you'll see tiny crevices and wrinkles riddled all over it. This is also true for the fourth dimension. There are tiny cracks in time too! So, creating a time machine, much like the one in H. G. Wells' novel, can be possible! In physics, these time-tunnels are called 'Wormholes'. A wormhole is basically a crevice or a void in the spacetime continuum through which we can jump in and pop out in a different time and place. Down at the smallest of scales, smaller even than molecules, smaller than atoms, we get to a place called the 'Quantum Foam'. This is where wormholes exist. Tiny tunnels or shortcuts through space and time constantly form, disappear, and reform within this quantum world. And they actually link two separate places and two different times! But, these are way too tiny for a human to pass through (darn!). Though some scientists claim that it can be possible to capture it and enlarge it many trillions of times to make it big enough for a human or even a spaceship! A giant wormhole created in space can actually transport people across the universe or even across time, provided that enough energy and power are fed to it.

Now, the first problem (of course! there has to be a problem.) in creating wormholes is the 'feedback'. We are all familiar with feedback in music concerts (that screeching sound the loudspeakers make). Sound enters the mic and gets amplified through the loudspeakers but when the amplified sound again enters the mic, it gets further amplified. Thus, creating a loop in which the intensity increases over time till it destroys the sound system. It is the same with wormholes, except the sound is replaced by natural radiation. The natural radiation feedback causes the wormholes to collapse. Hence, they are pretty unstable and it would probably require a zillion-zillion volts of energy to stabilize it long enough to be able to enter it. Therefore, using the energy of a single lightning bolt to power a time-machine (remember Marty Mcfly?!) is like hoping to move a truck full of cargo by simply blowing at it (unless of course if you're a Kryptonian stranded on Earth, in which case you can!).

Okay, lets assume that in the future they actually do make a time machine by stabilizing a giant wormhole. Now, we can do a little experiment suggested by the great physicist Stephen Hawking. Right now, at this very moment when you are reading this, plan a welcome party only for time-travelers from the future. Make a mental note that you will pass this invitation down the generations, so that someone from the future, after the time machine has been invented, decides to drop by to pay you a visit. Now wait... 1.. 2.. 3.. Did anyone appear? No? Maybe because they didn't want to cause any 'Temporal Paradoxes'.

Now, what the heck is a temporal paradox? Well, temporal paradoxes are the mind-twisting philosophical problems which arise due to backwards time-travel. The most famous is called the 'Grandfather Paradox'. Lets assume that you have successfully invented a time machine and you decide to go to the past. In the past, you accidentally kill your own grandfather before he can meet your grandmother. That means, your father or mother (depends on who you'll kill) was never born and hence, you were never born! So, who killed your grandfather?? Confusing, right? Maybe that's why we don't see many time-travelers among us.

What about going forward in time? Well, fortunately forwards is more plausible than backwards. There could be three methods for such travels. But first, lets talk some more about the 4 dimensions. We move in every dimension, including time. But the movement in time is constant and everlasting. And as everything can be measured along the spatial dimensions (length, width & height), everything can also be measured along time. For instance, the average length of a human being in time is approx. 80 years and the length of the Earth in time is about 4.5 billion years and increasing. So, what happens if we move very slowly along the time while the rest of the universe keeps moving at normal pace? That way we will stay young while others age. In other words, we'll travel to the future. How do we do that?? The answer is 'Deep Sleep' (didn't I say the early writers were lucky with this one? :p). Or more correctly 'Cryogenic Sleep'. In a cryogenic sleep, one's body is cooled to such levels that it stays completely preserved. And when the time is right, the body is revived and brought back to life. The person will have no memories of the time in between and for him the time would have literally stopped. So he will experience no gaps between going to sleep and waking up. For example, if he goes to sleep in 2050 and is woken up 100 years later, then he will experience time travel of 100 years in a moment. But this involves raising the dead (Necromancy?! well... sorta.) much like the Frankenstein's monster and the current medical science isn't actually capable of such a feat. So, this method is out of question for now.

The second method uses the same principle but in a different way. In a stream, the water flows in the same direction but the speed of water varies at different places. Similarly. according to the theory of general relativity, the speed of time also varies at different places in the space. General relativity says that the time generally moves slower near massive objects compared to the deep space. For example, the GPS satellites in orbit around the Earth have a very precise clock built in them. In spite of their accuracy, the clocks gain around a third of a billionth of a second every day! (Its a fact.) The clocks aren't defective. Its just that being closer to the Earth, (a massive object) time moves slower for us than the satellites in orbit.

About 26,000 light-years away in the centre of the milky way galaxy is a super-massive black-hole which is almost 4 million times the mass of our sun. If someday, we could go near the black-hole and move in a circle around it just beyond it's extraordinary gravitational pull, then we will actually move at half the speed than everyone else back in Earth. So, If such a spaceship spends about 5 years circling the black-hole then about 10 years would have passed back on Earth. This method however, isn't as effective since we can only go a few years in the future. Although, the black-hole is more stable than the wormholes since there's no question of feedback here, but going near one wouldn't really be a good idea (not even light can escape it, what chances does a spaceship have?)

The last method of forward time-travel brings us back to the topic of traveling near the light speed. If you recall, we have already established that moving at near light-speed will actually slow down the time. Now we can use that phenomenon to go to the future. If we make a train in the orbit of the Earth to go around it at near light speed, then approaching the light speed (almost 7 times around the earth in a second) will slow down the time inside the train. The passengers will experience less time than the people on earth and when they'll slow down after some time, they will be out of sync with the Earth-time. Although, the idea is very appealing (we don't even need to go near a black-hole), its not quite possible for the present technology to build such a high-speed train.

In closing, I'd like to say that the concept of time travel is indeed one of the most intriguing one in the sci-fi history and someday, we might even actually invent a time-machine or a time-train but that is so distant in the future that we need not worry about that now. But, for now, all I want to tell you is to keep looking for any time-travelers hiding in our time and if you do find one then steal his time machine and come back to this time and comment here. I'll contact you and we can go on a tempral voyage! ;)

Cheers! And stay tuned (or in our case 'connected') for next Sci-Fi Sunday!