Time travel is a concept that has been depicted in various types of media. In comic books, there are some series that center stories around the idea on a constant basis. Being able to change the past or future is something superheroes could use to their advantage. It could make saving the world an easier task.
Unfortunately in the Marvel Universe, time travel doesn't quite work the way the travelers might like. It was discovered or theorized that actual time travel wasn't possible. Traveling back in time actually resulted in the creation of an alternate reality. Going back and suddenly appearing would cause a divergence and there would be a butterfly affect. Each tiny change would lead to a another and another which would often result in catastrophic results.
Traveling into the future could have the same affect. The person traveling is making a change by simply being there. The simple act of drinking a cup of coffee means one less cup for someone else. That could result in the person seeking coffee elsewhere and that journey could lead to other things. And so on and so on. In THE AMAZING SPIDER-MAN #678, we do see a little trip to the future in a way that hasn't really been shown before. How much change could traveling 24 hours into the future have? (Minor spoilers below).
== TEASER ==
In the issue, a colleague of Peter Parker's at Horizon Labs manages to construct the Break Room of Tomorrow. You walk in and are transported 24 hours into the future. For example, when Grady Scraps demonstrates to Peter, he emerges with the next day's copy of the Daily Bugle. When Peter gives it a try, the instant he steps foot in the room, everything changes. New York City has been destroyed. Countless lives have been lost. All of this is because Peter traveled 24 hours into the future.
Returning to the present didn't change anything. They could still see the destruction through the open doorway. The reason the change and destruction occurred is a simple one that is pretty much always ignored in time travel stories. The moment Peter stepped through and traveled 24 hours into the future, he was no longer part of the current timestream. He basically disappeared. He no longer existed in the timestream until the time in the future in which he arrived. The world around them would continue on its path but Peter Parker (and Spider-Man) would not be around in that 24 hours.
The issue goes on to deal with Spider-Man trying to figure out what changed. What small event failed to occur with him not existing during the next 24 hours?
This is where the different theories come along. What we have here is one timeline and no alternate realities. If a person traveled into the future, they are jumping out of the present timestream. The lives of those around would continue but suddenly the time traveller would be missing.
This is what bothered me about the movie Back to the Future II (which I still love). Marty McFly and his girlfriend, Jennifer Parker, go into the future with Doc Brown to deal with an issue with their kids. By making the journey, they no longer will be around to actually have kids. Biff was seen outside the house and witnesses the DeLorean time machine make its departure. Time goes on without them. As soon as they arrive in the future, their kids wouldn't exist because they had been missing and out of the timestream for those 30 years.
Of course it could be argued that because they would eventually return to their present, everything would be righted and they would go on to have their kids, etc, etc. That's too easy of an explanation and time travel is anything but easy.
Time travel stories shouldn't be taken lightly. Going back in time will change things even if no major action is taken. It all goes back to the butterfly affect. Once a person went back, their present would immediately change around them, unless, as in the Marvel Universe, going back in time creates an alternate reality. Traveling into the future could be possible but the traveler would witness the intended future since whatever role they were meant to take is no longer possible with them disappearing from the timestream.
It's all a confusing mess. It could easily give one a headache trying to sort it out. It takes clever writing (and Dan Slott deservers praise for focusing on this often ignored angle of time travel). The only way one should travel through time is by living life normally. We're all traveling into the future one second at a time. Only when that second comes, the future immediately is transformed into the present. Time travel stories shouldn't be overused. There should be certain rules set in place in each comic universe. Creating an alternate reality by going into the past makes sense. Visiting your future shouldn't be possible since it's always changing and by going to the future, it changes even more.
After the experience Spider-Man goes through, it'll probably be a long time until he tries messing with time travel again.