This has been bugging me lately, so I felt I needed to get it off my mind. So listen well, sirs and madams, while I explain the mechanics and feasibility of time travel that doesn't have to interfere with physics or logic as does much of the time travel we see in popular media.
Parallel Universes
First thing's first. For paradox-free time travel to be possible, we need to assume that there are parallel universes or timelines. Many prominent physicists agree upon this, that our universe is simply one of many floating in the 11th dimension. Where do these universes come from? They split off every time a sapient being makes a decision, thus making all possible choices for them real.
Now, we also have to assume that humans have free will. You can argue the philosophy of this until the cows come home, but if humans don't have free will, they can't make choices, and then we can't have our parallel universes, and now we have some angry physicists.
But wait, couldn't random events trigger these timeline splits? Like what if the sun randomly blew up one day?
Nope. And to explain why, here's a quote from Stargate:
"According to Newtonian physics [...] if you could know the position and velocity of every particle in the universe at any given moment, you could accurately predict all of their interactions for the rest of time."
Now, assuming we have free will, that means that apart from the choices we make, all things that happen in the universe are inevitable and governed by fate: a domino effect throughout the universe that began since the dawn of time. Therefore, unless something with the ability to make conscious decisions intervenes, everything will happen in a predetermined way that will not alter from timeline to timeline.
In conclusion: human decisions make parallel universes.
The Nature of Time
"Time isn't made out of lines! It is made out of circles. That is why clocks are round!"
This is absolutely wrong. Time is made out of neither lines nor circles, it is more like the branches of a tree. We'll call the trunk of the tree the original timeline, the one that existed when everything began. Then come along us clever humans who can make conscious decisions. Each decision we make, the tree branches off, each branch representing the various different choices we could have made. This is obviously quite a large tree.
Now, don't get me wrong, universes aren't shaped like trees (physicists can't agree on the shape, but it really relevant at the moment), rather, the conceptual functioning of the splitting of timelines looks like a tree.
The Time Machine
Right, we've got our parallel universes and we've got our time tree, now for the fun part: our time machine. Before we begin, we have to imagine that cost and technology are not an issue as this deals with things that may take eons for humanity to develop.
The machine itself doesn't need to be anything fancy. Just something big enough for you and anyone you intend to bring with you to fit in. Here's what it needs to do, though:
1) Escape the fourth dimension
According to Einstein (who seemed to know what he was talking about), we live in four dimensions of space-time (length, height, depth, and time). As we are all moving forward through the fourth dimension at more or less the same speed (except for those of us who can move at the speed of light), if we want to move backwards through time, we will need to escape its clutches. Now, since dimensions 5-10 are somehow wrapped up in superstrings, we'll need to journey to the 11th dimension, or the plane that contains all the various universes on membranes (just go with me on this, it's what physicists are saying).
Going forward through time is much easier (just freeze yourself and wait), but if ever you want to get back to where you started, you're still going to need to go back.
2) Make you feel at home
As you depart the fourth dimension, you'll find that you sorely miss the laws of physics as the atoms that form your body begin folding in on themselves. So, the interior cabin of your time machine will need to retain or simulate a pocket of four dimensional space-time so that you don't crumble into dust.
3) Be able to navigate the "time tree"
Since your eyes probably aren't adjusted to working in the 11th dimension, you'll need your time machine to have the capability to detect your point of origin on the time tree and then calculate where it is you would like to re-enter the fourth dimension. Once again, problematic in that the concept of "where" would be quite different in the 11th dimension, but let's just say it's doable.
Changing the Past
The main issues that people have with time travel is that if you mess with the past you could alter the future so that you would never have travelled back in time in the first place, thus never having changed the past...
This is a paradox. Logically, an action cannot cause or prevent itself from occurring.
Therefore, as your time machine enters the past, you will instantly create a new time branch, given that you did not exist in the original series of events. Now, you can do whatever you like, even kill your past self with no adverse effects because the future that your actions create is a different one than the one you came from.
Similarly, if I were to go into the future and bring back a Playstation 4, I would create a different timeline and thus a new future than the one the device originated from.
Benefiting from Changing the Past
Unless you wanted to watch the Egyptians build the pyramids or see what colour dinosaurs were, chances are you travelled back in time to change something in order to improve the life you presently live.
Let's say you're a broken-hearted inventor with a time machine. Five years ago, your wife got in a car accident and now you're all alone. The accident was preventable, though, and you could have saved her. So you go back in time and save your wife's life. She happily returns home to your past self and it looks like all is well.
Feeling rather pleased with yourself, you return to your own time only to find that nothing has changed. "That's right," you say. "I need to go to a future that stemmed from the change I made." So you hop back in your machine and jump one universe over, to the exact same time, where you find your alternate universe self still happily married.
Now you're left with a difficult decision:
A) Let them live together happily and get on with your life.
B) Convince your alternate universe self to trade lives with you (this shouldn't be too hard, heck, in your life, you own a working time machine!).
C) Kill your alternate universe self, dispose of the body, and then pretend to be him.
Sticky, yes, but paradox-free.
Well, I hope you budding geniuses out there will be able to use this lesson to work out the kinks in your time machines. If this does help you, you can repay me by getting me a pet Compsognathus.
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