time travel is currently deemed as fiction only, at least to me.

As einstein stated in his theory of special relativity, there is no object can travel faster than the speed of light.

Most theories suggest that you can do time travelling when you can exceed the speed of light. But if you cannot exceed the speed of light, then any theory beyond it will be falsified automatically.

A speed near to speed of light will have the effect of time dilation but not time travelling.

Perhaps we can have some new theory in the future that suggest some new models of space time. Let's wait and see.

How it is possible to go back to the past? Even if you can go faster than the speed of light, you would probably be able to delay time.

time travel isn't possible if it's believed in its conventional forms. There are theories that time in essence is constant. Its the universe that is changing, moving or so goes of the theories based along that line.
There's also parallel universe kind of thing and also the sting theory whereas other dimensions existed until the 10th. Time itself is a dimension whereas we lived or perceived time in a linear fashion like 2D.

Why not? I am pretty sure we or something from us could go back to the past. I am not sure about the future though...

Consider a space ship traveling from Earth to the nearest star system outside of our solar system: a distance d = 4.45 light years away, at a speed v = 0.866c (i.e., 86.6 percent of the speed of light). The Earth-based mission control reasons about the journey this way (for convenience in this thought experiment the ship is assumed to immediately attain its full speed upon departure): the round trip will take t = 2d / v = 10.28 years in Earth time (i.e. everybody on earth will be 10.28 years older when the ship returns). The amount of time as measured on the ship's clocks and the aging of the travelers during their trip will be reduced by the factor \epsilon = \sqrt{1 - v^2/c^2}, the reciprocal of the Lorentz factor. In this case \epsilon = 0.500 \, and the travelers will have aged only 0.500×10.28 = 5.14 years when they return.

The ship's crew members also calculate the particulars of their trip from their perspective. They know that the distant star system and the Earth are moving relative to the ship at speed v during the trip. In their rest frame the distance between the Earth and the star system is εd = 0.5d = 2.23 light years (length contraction), for both the outward and return journeys. Each half of the journey takes 2.23 / v = 2.57 years, and the round trip takes 2×2.57 = 5.14 years. Their calculations show that they will arrive home having aged 5.14 years. The travelers' final calculation is in complete agreement with the calculations of those on Earth, though they experience the trip quite differently.

If a pair of twins are born on the day the ship leaves, and one goes on the journey while the other stays on Earth, they will meet again when the traveler is 5.14 years old and the stay-at-home twin is 10.28 years old. The calculation illustrates the usage of the phenomenon of length contraction and the experimentally verified phenomenon of time dilation to describe and calculate consequences and predictions of Einstein's special theory of relativity.

Source

Interesting topic.

1. If time travel is possible, why hasn't any future human come by and visit us? Given the infinite amount of time available in "future", surely humans would have invented a time travel machine if it is at all a possible feat.

2. The paradox of what would happen if you travel to your past and changed something to yourself. What would happen to your current self then? I do not agree the concept of journeying to a parallel universe counts as time traveling.

I think they have. Think of events in history, like the accuracy of the alignment of the pyramids and why astronomers today still use Inca ruins to correct their computer calculation programmes. Probably this explains why there are sightings of UFO sometimes. They could be future humans instead of aliens. I think once time travelling is discovered, those in authority create some laws to govern time-travelling, to ensure that we do not go back and screw the past, which might lead to the extinction of humans.

In my opinion, it's the other way around. I think one can travel to the future, but not the past. Here's my justification:

Using the twin paradox, I present an example from wikipedia:

In this case, in my opinion, you can say that the twin which travelled to space has gone into the future. But you cannot say that the twin on earth has gone to the past just by seeing his younger twin. This is because everything else has changed normally except the shape ship crew. And this is why I don't see how one can go to the past.

Interesting topic.

1. If time travel is possible, why hasn't any future human come by and visit us? Given the infinite amount of time available in "future", surely humans would have invented a time travel machine if it is at all a possible feat.

2. The paradox of what would happen if you travel to your past and changed something to yourself. What would happen to your current self then? I do not agree the concept of journeying to a parallel universe counts as time traveling.

Your kidding right? I mean none of those explanations constitute hard evidence for the existence of time travelling.
And I usually believe in simpler explanations rather than thinking of the many fantastic possibilities.

Your arguments about time dilation merits some thoughts though.

time dilation has nothing to do with time travelling at all.

The twin paradox which is based on time dilation is just an effect observed from special relativity.

Those who studied special relativity should be able to understand twin paradox easily.

For those who don't, it's good for you to learn some.

Actually, anyone who travel on bike or car or plane would live a longer life compare to your friend standing on earth (the initial frame). However, the effect is too little (perhaps the range of less than few nano seconds) thus having no impact at all.

However, a number like 0.8c will have such effect by then.

Those who are interested you can study length contraction as well.

To TS, what you mention is wormhole, another theoretical proposal which still haven't been proven until today. However, i do believe wormhole exists as in GR but doesn't promote time travelling. GR is not really my field so my statement is not really correct. It is just a believe rather than a physics research.

1. How do you know they hasn't came back? Probably they still look like us. Walk like us. Talk like us. And most likely time travelling in the future will be limited to certain groups of people only (like the military or scientists). It wouldn't be a public thing to do. Imagine the consequences if everyone can travel backwards. There will be chaos. Paradoxes everywhere when everyone alters history.

2. Well, the grandfather paradox. Like if you go back in time and kill your grandfather then you will cease to exist. Maybe that's what exactly will happen, you simply cease to exist. Maybe history can be altered...

How do I know, no I don't know. I'm simply questioning the odds of it happening, when given infinite amount of time.
And you're fantasizing la. Explain how a human being would simply "cease to exist". You think a bunch of molecules would just disappear without a trace like there's some kind of universe fabric with self-correction mechanism somewhere?
I'm not disagreeing with your ideas, just that there has to be some more "reasonable" explanations for them.

No, the grandfather paradox would mean that the bunch of molecules will never end up turning into your present self. Sure they'll exist, but will you?

Well TS just said I simply ceased to exists without elaborating further.

Taking your point of view, say if I'm talking to you face to face right now while someone goes back in time and killed my previous self, the current universe that contains me would have to be unravelled and rebuild in a new fashion without me and everything that I have done or changed. The question is what physics law governs that to happen? You said "never". However I am "already" here so something has to make the change. I'm questioning what forces of nature governs or controls this self-correcting mechanism. For all the molecules that's going to be restructured, where are all those energies going to come from?

The grandfather paradox scenario is I go back in time and killed or change something (killing my grandfather) that prevents my creation in the future so I couldn't have gone back in time in the first place thus creating a paradox.