I am off to visit a fiend this weekend, and even though he is somewhat technical and a bit of a geek, he has never really watched/read any SciFi! So I thought I would write up one of the common SciFi themes for him: time travel.
First off, some basics. SciFi is fiction, and to watch it you have to suspend disbelief and imagine, for a short while, that the universe in which the story is set could exist. For science fiction this works by taking the normal universe in which we live, and tweaking it a bit to create something that is a bit different. Obviously, you also have the fact that you need a good story, and characters and a plot, and so on. Indeed, the science itself, and the changes that make this universe different to our own, do not actually have to be key to the plot - they could simply be utilities that make the plot even possible. An example would be the “faster than light” travel in Star Trek - without that the entire 5 year mission would be the crew getting to Jupiter, maybe, and the drama would be the sonic showers breaking down. Some science fiction revolves around the key scientific difference that the depicted universe has from our own. Time travel is rarely simply a utility for the plot but usually a key plot element. Annoyingly I know I saw a film where the time travel was totally incidental to the story but cannot for the life of me remember what film it was!
Some of the most plausible science fiction is set in a universe where the laws of physics are as they are here, but that some current leading edge theories are assumed to have been proven and practical innovations and technology created from them.
One of the things that puzzles my friends is that I will watch SciFi and criticise the science some times. They find it odd and point out that it is science fiction. I have to point out that it is science fiction. It is worth explaining what I mean. Basically, the universe in which the story is set must be one where science would work. Science is not the laws of physics of our universe, but is in fact a toolkit of methods by which we have test and understand the universe in which we live. The universe of a SciFi story can be very different to ours, but science in that universe would work to allow the rules of that universe to be understood.
Of course, as an audience, we are unable to perform tests on the fictional universe, so the author has to do those for us - part of the story will have to be some demonstration of the key difference between that universe and ours, and provide some framework by which we can understand the rules by which that universe operates.
What causes annoyance, and makes the fiction break down is when the rules are not consistent. This is especially difficult for time travel as making any consistent set of rules for time travel is actually quite tricky - there are many ways it could work.
Ideally the author makes a proper set of consistent rules in their own mind, even if they are not always revealed in the story or done so over time. This avoids inconsistencies. Sadly this is often not the case and you realise that the social structure would not exist if that technology existed. An example if Star Trek transporters. They conveniently gloss over the details as much as possible but it is not clear if the device somehow moves you from A to B (via some energy conversion or something) or, as suggested and even necessary in some story lines, it somehow digitises you, converting you in to information and sending that to create a new instance of you at the destination. The latter is extremely problematic as it has huge moral/ethical issues, killing one instance to make another. It would also mean nobody could die as information can be backed up so you could've a backup made every day. It would also allow an army of soldiers to be created all the same from your best individual examples. Society would not be the same if that is how it worked.
Even so, many science fiction stories are set in a world that is meant to be based on ours. A common theme is to set the story in our future. The key scientific differences being the result of discoveries and subsequent technology developed in the future. Star Trek is another good example of this. Another common idea is to set the story in the here and now, but where only a select set of people are aware of these scientific revolutions - Stargate is a good example of that. A lot of alien based science fiction assumes the here and now, but with the availability of alien technology and understanding that we do not yet have. In either case the “normal” laws of physics have to still apply except where modified by these specific key innovations - and this is where you can end up shouting at the screen - when something stupid and wrong is included in the story for no good reason. One simple example is the idea that cold air (still is gaseous form) can flash freeze someone solid in seconds if it is cold enough, simply as an example of extreme bad weather. That is not how it works - gaseous air does not have enough thermal mass and conductivity for that. For science fiction to work, the science has to work even if the laws of physics are different.
Forward in time
The concept of going forward in time is not difficult. We are all doing all the time, it is simply a matter of going faster. Indeed, even going far ahead in time is not difficult to conceive as it is functionally the same as simply being in some sort of suspended animation for some long period.
Time travel stories rarely have just forward travel though :-) And even when they do, there is the transporter problem (see below).
Backwards in time
This is where it really becomes a problem. Basically you have to resolve the paradox caused by backwards time travel. The classic being the grandfather paradox where you go back and kill your grandfather before your parents are even born. But any travel back in time, or even simply being able to send some information back in time creates a paradox. There are a number of ways this can be addressed (or ignored) in fiction…
A classic method of addressing the paradox is to basically make the story such that the time traveller did not actually change history at all. Everything he does is something that already happen. In such a story he could not in fact kill his grandfather - not due to a law of physics, but because the author does not make that happen. Typically the exact nature of what the time traveller does is not obvious until the end, when it becomes clear that he has done what happened anyway. Indeed, some stories work on the time traveller trying to make changes, and ending up being the cause of the things he meant to change in the first place due to misfortune. Some stories have the characters deliberately try to reproduce what they recall from history for fear of some catastrophic effect due to a paradox. Of course, such stories can work well, but they do not actually address the issues, and you are left wondering why the character did not at least make some small attempt to create a paradox of some sort. 12 Monkeys worked like this.
First law of time
There are some stories where there are “laws” of time travel which the characters have to obey. These usually involve avoiding a paradox very carefully, and again the issues are not really resolved. In some cases these are rules and regulations of some time travel agency our authority from the future. Being rules, the characters can be compelled to stop people breaking them and hence avoid the author having to address the issues.
It all turns out right in the end
Another common theme is that the changes to the past end up changing the future as a consequence. This can leave the characters never needing to back in time even, and is usually the last scene of a story where everything turns out all right. Off course, this is a paradox anyway, as who actually fixed the past if not those that are no longer going back to fix it. Stargate had a fun one like this with a tiny twist that there are fish in the pond this time around! This sort of story can be a bit unsatisfying as you end up with the whole story you just watched being erased from history and not having happened.
Ripples in time
Another idea is that the consequences of the change happen, but take time (!) to happen. Somehow the traveller is protected from them - finding himself back in a new future with his old memories - or in some cases with both sets of memory (confusing). These sorts of stories can involve multiple attempts to go back and fix things. Again, at the end of it all, the story you watched is erased from history to give a reasonably happy ending.
Changes in the past having “real-time” impact
This is where things get very strange, and Back to the Future sort of did this. The idea that as you change the past, and as the chance of you fixing that change diminish, you impact yourself in some way. Marty sees his siblings vanish slowly from a picture, and even starts to fade out himself until he eventually manages to fix his mistake. I have to say that this sort of thing is really messy. The whole thing is very inconsistent and hard to derive any sort of laws of physics from it.
You also get this where someone manages to meet themselves, and perhaps injure themselves and in doing so magically they get a scar from that injury. Again, very inconsistent logic.
One of my favourites comes from a Dr Who episode. Dr Who stories try very hard not to create a nasty paradox, but they do happen, and the TARDIS has technology to handle a paradox even. But interfering in your own timeline is seen as something you must not do. But rather than this being some law of people (or time lords) or a law of physics, it is done as a law of nature, sort of. The idea is that by creating the paradox, such as saving the life of your father, you create cracks in time that allow some nasty creatures in to reality. So the law is there to protect you, not something that there are police to enforce. Fix the paradox and seal the cracks!
One way that you can create a consistent set of rules for time travel is to use the concept of the multiverse. The concept is that at each point in time, a branching set of subtly different parallel universes are created.
In this situation, going backwards in time is easier than going forwards! Going forwards is steering a branch through that multiverse - the decisions and random events of our daily life steering as we go. Gong backwards is simply going back up the tree - only one path to take. However, when you arrive at a point in history, your arrival creates a new branch of history with you in it.
This has the advantage of removing any paradox. You can change what you like in your new branch of history and not impact the branch from which you came.
Even this gets abused, as in Back to the Future, where they are again inconsistent in the rules of time travel. It also has several issues with time travel duplicates and getting crowded.
Time travel duplicates
A consequence of multiverse logic is that you are immediately a duplicate of yourself when you go back in time. Go back 10 seconds and tell yourself not to go back, and bingo, there are now two of you. One branch of the multiverse has lost you and another has gained you. That breaks ordinary laws of physics in some ways, as it means matter/energy within a single universe being created or lost, but that could be seen as that law of physics being too narrowly scoped and not catering for the multiverse!
Futurama has a special twist on this, using basketball mathematics, they deduce that a time travel duplicate is always doomed - having a high probability of demise to leave only one of you in any time line… But that is sort of science comedy!
Sideways in time
A consequence of multiverse logic is also the concept of travelling sideways in time. The idea that you can go to the same time in an alternative parallel universe in the multiverse. This was the main theme in Sliders and is explored in many science fiction stories, including Stargate and even Star Trek.
In Stargate they created an extra twist on this, that somehow you could not exist in the wrong universe for very long and would eventually collapse at a sub atomic level in some horrid way, so you have to return. They also explored the issue of addressing (numbering) the multiple universes - a key plot issue in Sliders.
One of the problems that is rarely addressed in any of these stories is the fact that you end up with matter appearing or disappearing. This is the same issue with a Star Trek transporter. When you have someone pop in to or out of existence, how do you ensure that the borders are correct and that they do not take half the pavement with them. And how do you target things to avoid appearing in a wall. For that matter what happens to the air that was where they are when you appear. Even forward time travel has this issue. Terminator tried to address this by putting the traveler in a sphere and that would actually cut a hole in the ground when the traveller appears - it solves the question of how do you get a skin tight enclosure for the traveller to be cut out of one space and put in another.
You also have the issue in a multiverse scenario - whilst there may be many futures all the same in most aspects there is only one past - surely if one person goes back in time to a specific point, a million others have done the same and are trying to occupy the same space at the same time in the same universe at that point in the past? Well that could be solved by saying that the new branch of the multiverse they create by their arrival is the one that has them in it, and all of the others are in another one. That does not really handle things very well, or cater for the versions of themselves that decided to arrive from that new future and arrive a second later in to their timeline. It gets very messy and should get very crowded the second someone invents time travel!
Winning the lottery
One common concept of time travel, of even just sending yourself information in to the past, is that you send winning lottery tickets. This is where I have an opinion which I have not seen in any film. The issue here is how that time line goes forward. It is fair to say that you would follow the same path forward with all of the quantum level decisions being the same. Maybe lottery balls are actually random enough not to come out the same? Perhaps a better idea is to bet on sporting events where the outcome may be much more Newtonian and following the skills of the players rather than quantum level random luck?
Another twist is the idea that you do not physically travel in time, but your mind travels. In the case of About Time the traveller is travelling to an earlier version of himself. This does not quite eliminate any paradox but does avoid many questions. He returns back to a later version of himself but having followed a new future because of his actions in the past. The issue is that random things have changed making it somewhat pot luck, and meaning that he realises he could never go back to before his child was born for fear of changing that event. In some ways Ground Hog Day is like this but the travel back to his own body at the start of the day is involuntary and even works if he dies.