A guide to temporal mechanics. You're going to need aspirin.

This guide deals only with temporal mechanics by assuming that time-travel is possible. For a more in depth look at the ins and outs of time travel, check out Time-Travel - the Possibilities and Consequences

Temporal mechanics is the study of time-travel and its consequences. It is a real science and many a PhD thesis has been written on the subject. This entry uses science fiction to illustrate points - as the name implies; it is science fiction - ie, fiction based on science.

The Timeline

All time travellers talk about the timeline and a basic understanding of this is required to understand this entry.

Take a piece of blank paper and draw a horizontal line on it. Mark the left edge 'Past', the right edge 'Future' and the middle 'Present.' This is a timeline, and it represents time as it should be. It can represent all of the universe, or one individual's lifetime.

Now, draw a curve from the future to the past above the time line. This represents a time journey from the future to the past. Mark the point it intersects the past with a cross: X.

Draw a diagonal line moving down and right from the cross. Stop moving diagonally when you are under 'Present'. Continue the line in a horizontal fashion to run parallel with the time line. This represents an alternate timeline, created by making a change to the past. It may be minor, such as one daisy not growing, or major, such as the Nazis winning World War II (a favourite among time-travel stories).

Now, here's the important distinction. The timeline to the left of the cross is the same for both the original timeline and the alternate time line. They do not have 'identical' pasts, they are the same past.

Temporal Contamination

Star Trek IV: The Voyage Home provides a good example of temporal contamination. This occurs when people or objects from the future are left in the past. Examples:

• Mr Scott gave the formula for transparent aluminium to Dr Nichols.
• McCoy extended the life of an elderly dialysis patient.
• Chekov has left a non-functional phaser and a communicator in the hands of the United States Navy.
• One marine biologist disappeared from the face of the earth for several hundred years.

These are events that individually could have profound effects on the timeline.

Temporal Attack

This is one of the simplest principles in temporal mechanics. Time-travel is used to go back in time to attack an enemy when they are unprepared or not as technologically-advanced. Star Trek: First Contact featured this concept, when the Borg travelled back in time to destroy Earth's first warp capable ship. This would prevent the Vulcans making contact and ultimately mean the Federation and Starfleet would never be formed. As a result, the assimilation of Earth would be much easier. It is similar to the grandfather paradox (see below).

Alternate Timeline

This is when a change to the past changes the present that the time-traveller comes from. This was most famously demonstrated in Back To The Future Part II, when Marty's purchase of a sports almanac in the future (2015) leads to Biff going to the past (1955) and giving it to his younger self. This changes history for everyone except Marty, Doc Brown and Einstein (the dog) who are protected from the change by the time-travel journey back to 1985. They arrive in an alternate 1985 where Biff is rich and corrupt and life is lawless. Marty suggests going back to 2015 to stop himself buying the almanac, but he can't because he would be travelling to the alternate 2015 where his purchase of the almanac never took place, as the Doc never built the time machine in 1985. This leaves them with a problem, as they do not know when old Biff gave the almanac to young Biff.

Temporal Displacement

This is the effect felt by an individual or object that is moved through time.

Temporal Displacement Wave

Upon making a change to the timeline, a temporal displacement wave would ripple out from the point of change. This wave was shown in the Star Trek: Voyager episode 'Year of Hell, Part One'. When the wave hit Voyager, the alien delegate was erased and the ship was changed to a much more damaged version of itself, as the attacking vessel was far stronger in armaments. The propagation speed of the wave is unknown - it may be limited to light speed (c), or may be much faster, possibly travelling instantaneously.

In the Terminator films, the displacement effect is the sphere that destroys everything out to a radius of about a metre from the object being displaced.

Temporal Radiation

This is the principle that an operating or damaged time machine will leak temporal radiation, as shown on Enterprise. The radiation can cause time to move slower or faster than normal, stop time altogether or cause it to loop.

Temporal Echoes

These arise from idea that time travel¹ can cause people to see their past or future. Babylon 5 and Red Dwarf featured this. The echoes showed people what their future held. Lister, from Red Dwarf is told by Rimmer that he is going to die in a computer explosion. He tries to prevent it, but eventually discovers that the echo was of his grandson being killed.

Temporal Paradox

All time-travellers should beware of the temporal paradox. Put simply, it is an error or complication that can only have involved time-travel. Temporal paradoxes are the main cause of headaches for the prospective time-traveller or the temporal mechanics student. These paradoxes usually follow the same line of thinking as the paradox: 'I always lie', or 'I have always told the truth, but I didn't yesterday'. The statement cannot be true, and yet also cannot be false.

Grandfather Paradox

This is the most famous temporal paradox there is. Let's say you go back in time and assassinate your grandfather when he was a child - or, perhaps we should say you killed him accidentally by landing your time machine on him. Now, if he's dead, your father was never born and consequently, neither were you. But, if you were never born, then what inconsiderate idiot just killed your grandfather? It's sometimes called the infinite loop paradox. The theory is that there are two loops that repeat forever, one after the other;

• Loop 1: You build a time machine go back and accidentally kill gramps.
• Loop 2: Gramps is dead, so you never built the time machine, so he lives. Therefore, so do you. Proceed to loop 1.

This looping then continues forever until someone or something intervenes to stop it. There are several likely candidates;

1. God.
2. Another time-traveller - possibly the time police.
3. The Universe prevents you from going back - no time-travel is allowed.
4. Chaos / Quantum Uncertainty. Each trip is slighly different from the last, so that eventually the loop breaks itself and the time machine lands a few feet from gramps, or he wanders out of the way.

There is another possibility. Assuming you kill your grandfather and then return to your own time, your father will not exist, but you still do. Instead of returning to your 'present' you have in fact returned to an alternate timeline (or alternate universe, if you prefer), where everything is identical except that your grandfather's life and all consequences of his life are missing.

Self-fulfilling Paradox

In Babylon 5, Sheridan sees the future of Centauri Prime² devastated by the Shadow's allies. Whole cities go up in flames and he is told by Delenn, his future wife: 'Do not go to Zahadoom.' This event is the very event that persuades him to go to Zahadoom, to try and prevent the fall of Centauri Prime. Had he not seen this future, he would never have considered going at all.

Temporal Duplication

This concept is demonstrated in Star Trek IV: The Voyage Home. Kirk sells his antique eyeglasses, a gift from McCoy, for a hundred dollars. Now, these glasses are actually about four hundred years old, but were made only a hundred years before (on the buyer's timeline). For their age, they are in remarkable condition; yet we must wonder whether they are in very good condition for the (apparent) time elapsed since their creation. What is more significant is the following exchange between Spock and Kirk;

Spock: Pardon me Admiral, but weren't those a present from Dr. McCoy?'
Kirk: And will be again, that's the beauty of it.'

There are two possibilities:

1. Kirk expects to receive the pair he pawned. These glasses are a temporal duplicate; they already exist somewhere else in this time. Dr. McCoy will buy the other pair, and give them to Kirk, who will bring them back to the 20^th century and sell them. Returning to the future, he will no longer own them. This duplicate set might survive to the 23^rd century, but he will not know where they are.
2. The pair he pawns are the same pair as the ones that McCoy will buy three hundred years from now. McCoy buys them and gives them to Kirk, who breaks the lenses and takes them back to the 20^th century. He then pawns them for $100. In this case, the glasses were originally made in the 19^th century, survive to the 23^rd, then get taken back in time to 20^th and continue to loop from there. The original pair are only given to Kirk the first time - each subsequent pair he receives from Dr McCoy are a copy. The copy does not exist at all as they are caught and indeed created by their loop through time.

The problem with the second option is that the glasses continue getting older each time they loop round. Eventually they will be older than the universe they are in, and since, from the point of view of someone who exists after the point where the loop starts to return to the past - ie, to the right of the loop on the timeline - there are anywhere between 0 - ∞ (infinity) iterations of the loop.

There's another way of looking at this problem, demonstrated in the Babylon 5 episode War Without End.

The Minbari have three sacred relics called tri-luminaires³ that they've had for about 1000 years. In the episode, Jeffrey Sinclair takes one of the tri-luminaires back 1000 years into the past, where he uses it to transform from human into the minbari's greatest leader; Valen.

The consequences: After Sinclair goes back to the past, the Minbari only have two tri-luminaires, but the Minbari have only ever had two tri-luminaires. The third one is a temporal copy of the second. There were originally only two tri-luminaires. Sinclair takes one back and then there are three. Every time round the loop, there are three tri-luminaires but one is perpetually being copied.

Self-Creating Paradox

This concept is a real migraine-causer. This is the opposite of the grandfather paradox. Instead of the time-travel preventing something happening in the past, the time-travel actually causes something to happen in the past. A prime example of how this might occur is demonstrated by the story told in the Terminator movies. The computer SkyNet, sends a terminator back in time to kill John's mother, Sarah, before he was born. In doing so, SkyNet accidentally arranges for John to be conceived, as the future soldier sent back to save Sarah is in fact John's father.

As if this wasn't bad enough, the partially-destroyed neural net chip (the brain of the terminator) finds its way to Cyberdyne, a computer research company. They use it to make thinking computers and their research leads them directly to create SkyNet.

SkyNet's creation of the time machine leads directly to its own creation and the creation of its arch-nemesis, John Connor⁴.

To sum up: The time-travel technology creates itself (through SkyNet). This has been a popular theory among students of temporal mechanics. There has been some serious research done (and many science fiction stories written) relating to the idea that time-travel can only be created through a paradox.

More information is available in the Computers In Science Fiction: Movies - The Terminator Movies entry.

Temporal Loop

This is often called the Groundhog Day loop. It is when time repeats itself over and over. There are many TV series that have done the temporal loop, but one of the very best was done by Star Trek in the temporal causality loop, see below.

Temporal Causality Loop

This concept is rarely explored by film or television, but is shown in the Star Trek: The Next Generation episode 'Cause and Effect.' The Temporal Causality Loop occurs when a person or people are caught in a loop of time. They continue to repeat the same segment of time over and over. In 'Cause and Effect' the loop is created when the Enterprise strikes another ship and explodes, then loops back three days and starts all over again. The crew are only subliminally aware of the loop, but experience repeated feelings of déjà vu.

The name comes from the fact that the temporal loop is self-causing - ie, it created itself through the explosion of the Enterprise. In short, effect preceded cause⁵.

Many critics of the episode have asked 'If the crew knew they were in a temporal causality loop, why didn't they call Starfleet for help?' The answer is that as the crew had no way to tell how long they had been in the loop, Starfleet may no longer exist. Even if they have only been in the loop for a few days, calling Starfleet may be what caused them to create the temporal loop. In addition, Starfleet would probably not be able to reach them in time before the loop reset itself and the message was never sent. In temporal mechanics, second-guessing yourself is never a good idea.

An alternative way of looking at it is this: For all they knew, the loop was universal (encompassing the entire Universe); but they could neither know that, nor act on that possibility. Indeed, they later found that they had been (strangely) out-of-the-loop for a period of some days.

If they had called Starfleet, Starfleet could have relayed the results of each attempt at a solution at the beginning of the next loop. Being out of their loop, they would still be moving forward so that the 'Enterprise loop' was actually more like a revolving wheel travelling along in time with repeating events represented by the spokes of the wheel.

Temporal Static Bubble

This is an unusual type of temporal anomaly. It is an area of space where time is 'stuck' and not flowing correctly. It was demonstrated by the Star Trek: The Next Generation episode 'Timescape'. There, Picard and his team arrive to find the Enterprise and a romulan warbird stuck in the time bubble after experiencing a fractured time stream where parts of the surrounding space are running either too fast or too slow. Time inside the Enterprise is not stopped but is running so slowly that the crew are effectively frozen.

Unavoidable Paradox

In the Star Trek: The Next Generation episode 'Time's Arrow', Starfleet discovers Data's 300-year-old severed head in a cavern. Picard does everything in his power to avoid sending Data on the away mission that will certainly involve his 'death', but events conspire to mean that Data's presence is essential to the success of the mission. This also involved temporal recovery, the act of leaving something behind and then going forward in time to recover it years later. For the time-traveller, the trip is instant, but the object will have been there for years. As a result of this, Data's head is now 300 years older than his body.

Temporal Cold War⁶

This is a very odd principle indeed. Much touted on Enterprise, this is where people or persons in the future communicate with people in the past to change history. They can feed them dates and places of historical events that, if prevented, will change history. They can supply technology and tactical data about races, weapons and places. By making subtle changes, rather than one or two dramatic changes, the temporal cold war time-traveller can change his own time and thus, change his status in the galactic struggle by instantly strengthening his forces, or weakening his enemies.

Worthy of mention is HG Wells's 'The Time Machine'. Instead of going back to his present and trying to prevent that future, he gathers up 'stuff' and takes it to the future in hopes of improving that period. This is a kind of reverse Temporal Cold War.