Theory:Time travel

Hiro travels through time while riding a Tokyo subway train.

In Heroes, Hiro Nakamura & Peter Petrelli have the ability to travel forwards and backwards in time, and have (to some degree) the ability to alter historical events. However, Hiro's power still obeys some underlying rules.

Temporal Mechanics Primer

Space-Time, Entropy, and the Arrow of Time

In 1905, Albert Einstein proposed a theory that unified mass, energy, motion through space, the relativity of time, and formed a new mathematical concept called spacetime. All objects in the universe move through spacial dimensions while time passes, but the rate at which time passes is relative to the velocity that you are traveling. We humans experience three dimensions on a daily basis (our frame of reference), and within our frame of reference, time "flows" at a rate of 1 second per second. Observers from a different frame of reference will see our frame moving faster or slower than theirs, but they will still measure time in their frame at 1 second per second.

Some physicists have hypothesized that the reason time "flows" is because of a property of matter called entropy defined in the Second Law of Thermodynamics. Entropy is the measurement of how energy will disperse in a given system, such that energy will always move from a higher state to a lower equilibrium state. It is proposed that the entire universe is "unwinding" from its initial high energy state (the "Big Bang") creating the dimension of "time" as the universe expands.

In physics, the mathematics behind the known laws of space-time are symmetric; in other words, the formulas are the same for an object moving forward in the time as an object moving backwards in time. Knowing an object's current position and velocity lets you determine where an object was (backwards in time) and where an object is going (forwards in time).

However, we humans do not perceive time flowing in both directions. For us, the arrow of time points "forward". We have a memory of where we have been, but no perception of what we are going to do; we can only perceive the probability of events that will occur as derived from our current conditions.

Cause and Effect

In the direction of forward flowing time, an action (cause) will have a predictable outcome (effect) according to the laws of physics, where the cause always precedes effect.

Paradox

If alteration of a sequence of events (via time travel) were to occur where an effect no longer had a cause, the situation would result in a paradox. Perhaps the most common thought experiment is the grandfather paradox, where a time traveler goes back in time and inadvertently causes the death of his own grandfather (effect) before his father's conception. If his father were never born, then the traveler himself would never be born, and thus he could never go back in time, and thus never cause the death of his grandparent. If the grandfather is not dead, then he has a child, who has a child who becomes the time traveler, and the cycle loops.

The results of encountering a paradox are unknown. Science fiction writers envision temporal paradoxes having catastrophic effects, ranging from violent explosions to "winking out of existence".

Principle of Self-Consistency

It is assumed that if time-travel were possible in the future, then statistically someone is traveling into their past (our present) with the intent to change it, yet somehow, paradoxes are not being observed (in the present). The Principle of Self-Consistency states that the probability of events that result in a paradox is zero.

This essentially states that logical paradoxes that result from time traveling are self-correcting, such that a time traveler cannot alter history in such a way that would be inconsistent with the reasons for time traveling in the first place. Either the traveler is prevented from making their intended change to the past through an unexpected sequence of events, or any change made to the past was always meant to happen and is incorporated in the history of the time traveler.

The self-consistency principle is often controversial due to the implications that the time traveler has no free will while traveling in the past, yet does have free will when in the present and future.

Mechanics of Time Travel

Fixed Time Theory

In a universe with a single timeline, all events that occur at every point in history (past, present and future) must not only be self-consistent, but immutable. A time traveler moving in the past is simply acting out a role; they cannot change anything that would alter the chain of events that eventually lead to their decision to go back in time. Every action that occurs was predestined to occur.

Multiple Universe Theory

The Multiple Universe theory allows that every probable outcome of any event spawns a new universe with its own timeline, and that a time traveler simply moves up and down the timelines, taking different branches along the way. A time traveler moves from the present back in time to undo a great injustice. They succeed and return to the present, however, are they really returning to "their" present?

Multiverser Theory

Contrary to the Fixed Time Theory, the Multiverser theory states that the universe's timeline acts more like a record head on a VCR; jumping forward in time allows all events in the universe to play out as if the traveler were not there, and jumping backwards "overwrites" any events that occurred in between. The theory allows for free will in every timeline, and multiple timeline rewrites are allowed to occur, but all events in every timeline adjustment must be causally self-consistent or a "time loop" will occur.

References by Episode

• Multiverser: In Don't Look Back, Hiro jumps forward in time to November 8th. When Hiro is arrested and the detectives call Japan, Ando replies he has not seen Hiro in 5 weeks. However, when Hiro returns to the past, he immediately contacts Ando, invalidating that future statement.
• Multiverser: In Seven Minutes to Midnight, a photo of Charlie's birthday party is seen containing just Charlie. After Hiro travels to the past, Ando sees Hiro and Charlie standing in the photo.
• Fixed Time / Multiple Universes (Same Universe): In Six Months Ago, Hiro travels into the past to save Charlie. He attempts to call Ando in Tokyo, Japan, but instead gets "himself" on the phone, and quickly hangs up. Hiro is exactly where he would be (at work at Yamagato Industries) six months prior to Homecoming, long before he realized he had the ability to manipulate time.
• Fixed Time / Multiple Universes (Same Universe): In Saving Charlie, Hiro makes several trips back and forth in time in his efforts to romance Charlie and to save her. Every time he comes close to physical intimacy with Charlie, he is forcibly teleported back to where he should be at that time (i.e. his cubicle in Japan) or to someplace relevant to his destined mission (i.e. the morgue where James Walker's body is being held). Eventually, he and Charlie spend a final night together before she goes into work, knowing full well that she is going to die. Her thought processes are not described in detail, but it is suggested that she knows her fate and that she pretends not to recognize Hiro when he meets her for the first time. Hiro then waits until after he originally left to save Charlie and then "returns" to meet-up with Ando, thus catching-up with the alternative timeline.
• Paradox: Hiro travels to the past to save Charlie from being murdered, and offers a trip to Japan to Charlie. If Charlie goes with Hiro, then she will not be in the restaurant when Sylar kills her. If she is not killed, Hiro will not travel back into the past to save her. This paradox is resolved by having Hiro forcefully return to the future without saving Charlie.
• Multiverser: In Five Years Gone, Hiro travels five years into the future. Because of this, he does not stab Sylar and stop the explosion, which causes the dystopian future to occur. By going back to his present time, Hiro overwrites all the events between his time and five years later, allowing him to change the future.
• Fixed Time: In Five Years Gone, Future Hiro reveals that he traveled back in time to visit Peter to create a chain of events that would stop the explosion. However, when he returned to his own time, he found that nothing had changed; his actions had not altered the outcome of events.
• Multiple Universes: In Five Years Gone, despite Hiro traveling into the future and negating his presence for the past five years, his future self has had a marked effect on events leading from the explosion to its fifth anniversary. Also, he has no memory of any of the alterations to his past, particularly that Claire's life was saved and that he once traveled five years into the future. This seems to suggest that the dystopian future presented in Five Years Gone lies on an alternate timeline than that of present-day Hiro Nakamura.
• Paradox: Hiro's journey to feudal Japan in Four Months Later seemingly causes a paradox when Hiro finds that his actions have lead toward the destruction of a village that Kensei was meant to save.
• Fixed Time: Hiro's actions throughout the first half of Season Two and the eventual resolution of his conflict with Kensei suggest that rather than causing a paradox with his time-travel, Hiro was predestined to become the Kensei of legend.
• Paradox: Hiro is forced to abandon Yaeko, in order to prevent the paradoxes that would be caused by his absence in the present, his continued presence in feudal Japan and by the stories of Kensei ending with the real Kensei (Hiro) living happily ever after with his princess.
• Multiple Universes: In The Second Coming, Hiro travels five months into the future and sees Ando kill a future version of Hiro, indicating that Hiro has travelled to a universe in which Hiro didn't go to the future, but instead stayed in the present during the five months. This notably differs from Hiro time-traveling in Don't Look Back in which Hiro travels into the future, but Ando reports that he has been missing in the interim.
• Multiple Universe: Peter travels to four years in the future where everyone has abilities in I Am Become Death. This future is the same future that Hiro witnessed in The Second Coming in which the formula becomes public. He returns into a timeline in which the world is not informed on the special abilities.
• Paradox: In Our Father, Hiro and Claire travel to 1991 to attempt to change the course of the catalyst. If the catalyst had originally been given to baby Claire, Hiro and Claire’s history changing actions should have caused a massive paradox resulting in a very different timeline.
• Fixed Time: All events in Our Father happened in the original timeline. Claire was never the host of the catalyst, as she had thought, but instead it was Hiro, then Arthur. All events of 1991 played out as they originally had.

External Links

• Novikov Self-Consistency Principle on Wikipedia.
• Multiverser Rules by M J Young