Chief of Research at the Time Travel Institute (and fictional character) Dr. Gunthar VonSchnelling presents some fundamental theories and concepts in time travel. Don't go any further until you've read what Dr. VonSchnelling has to say.
The question everyone wants answered is this: How do you travel in time? It is a simple question, but the answer is not quite as simple.
Where to begin? There are so many aspects. Let's begin with how time is calculated.
To travel in time you will require computer hardware and software that is probably beyond the technology presently available to us. But all the silicon in the world won't help you overcome the biggest hurdle for the hardware. Dr. VonSchnelling puts it best:
As I told my friend Albert, time is relative. A zecond is a zecond because vee zay it is a zecond. A day is a day because zat is the time it takes for our vorld to rotate once on its axis. On another vorld, it would be different. A machine has no concept of time. Certainly you can make a machine count forvard with quartz and crystal and mechanisms zat zimulate time, but zat is all it does... zimulate. How do you make a machine count... backwards? How am I to tell my computer "take me to June 16th, 1927"? And does the machine require me to input a precise time? Should I be precise to the minute? the zecond? the nanozecond?
A commonly ignored obstacle to time travel is the simple fact that the universe can only hold so much matter. This is commonly referred to as the "Bucket Factor" by our engineers. Again we refer you to Dr. VonSchnelling's notes:
Imagine zat zee universe is a bucket. zis bucket is full of water to zee brim. Zuppose now that a time traveller from zee future arrives in zis universe. Pretend zat your fist is zee time traveller. Plunge your fist into zee bucket. Water spills from zee bucket. You have just filled the universe with more mass zan it can zupport. You have just exploded zee universe. Zat is not good.
And what of the universe zat the time traveller has just left? Zere is now less mass in that universe zan zere should be. Zee inevitable conclusion is zat zuch an action would cause and immediate black hole zat would swallow zee entire universe. Zat is not good, either.
To understand the repercussions of time travel, you must understand quantum mechanics and quantum realities. Dr. VonSchnelling simplifies quantum realities for us:
Many of us understand the gist of quantum realities. Zimply put, every action creates an alternate universe. Examples of zis are numerous. Turn left down zis street and not right down zat street, and you will not have a car accident. You zimply need to ask yourself "vat if" to understand the repercussions of zuch a zeory. Vat if Napoleon had decided to get up one night for a znack and had tripped on his robe and broken his neck? What if zee cave man who discovered fire decided to zleep in zat day?
But quantum mechanics operates on a much smaller scale zan zat. The zeory proposes zat every minuscule action and event has an opposite. Entvine zis with zee chaoz zeory and time travel becomes quite dangerous.
Zuppoze your time traveller arrives in a grassy field. Every blade of grass crushed underfoot, every particle of oxygen he inhales and carbon dioxide he exhales, every cell of dead skin he sheds; it all has an effect on zee universe around him. Zpend one minute in zee past, and return to discover zat the President of the United States is not who you remember.
Logic would suggest that if time travel were ever to exist, then it already does. If time travel is possible, then the methods required will eventually be devised. Maybe it will take another 10 thousand years to discover the secrets of time travel, but if it's possible, then it's inevitable. Which suggests that time travellers are already visiting us... and visiting our past. What does that mean? To comprehend this, we must look at the many possible repercussions of time travel.
Travel back in time to save someone's life only to discover that it cannot be avoided, or worse yet, you were in fact the cause of the person's death in the first place. This is amongst the most plausible theories.
Travel back in time to save someone's life, succeed, return to your time to discover that nothing has changed... you've only changed the timeline of an alternate quantum reality. This theory is also amongst the most plausible.
Travel back in time to kill your great-grandfather and succeed. This theory is very unlikely since if you were to successfully kill your great-grandfather, you would inevitably never be born, and therefore never go back in time to kill your great-grandfather. Thus the paradox and the implausibility.
Travel back in time to alter history and succeed, but the only persons capable of differentiating between the reality left behind and the new reality are those directly associated with the time travel... the time traveller. The extent of the paradox rests in how the time traveller is affected. Existing "out of time", he may not be affected by whatever changes he inflicts on the timeline, thus the time traveller himself becomes a stranger in this "new" present. He may, in fact, go back in time, kill his great-grandfather, and return to the present to discover that there are no records of his own existance.
Dr. VonSchnelling has addressed these hurdles in his computations and he believes he has overcome them. We suppose only time will tell.