Re: They\'re baaack?!? Are they?
Also, what makes it easier for a putative time traveller to "change" "their" "past" than for them to change their own time, or for someone from this time to change this time?
Thermodynamic systems and systems involving sentient creatures evolve differently.
In a strictly thermodynamic system, for example one where you have billions of individual gas molecules floating about in a closed vessel, there is no definite observable arrow of time. Watching the thermodynamic process proceed forward in time would appear the same as watching it evolve backwards in time, at least to some limit involving time.
Obviously if the system started with the gas contained in a compressed gas bottle and the gas was released into the vessel you'd be correct in assuming that time was probably running backwards if all the gas eventually flowed back through the valve against the pressure gradient and re-compressed itself in the bottle. But during the mid-life of the system in the vessel you couldn't determine the direction of the flow of time. Knocking a few molecules off course, no matter which direction in time the system was evolving, wouldn't significantly alter the description of the system or give any clue about what direction time was flowing.
The same can't really be said of systems involving creatures that are sentient because unlike gas molecules sentient beings make choices rather than reacting as a strictly thermodynamic system.
If you are in the "here and now" and have knowledge about how a human system has evolved, especially if you have done a really cool GANTT Chart on the system and identified critical pathways and critical events, and you have the ability to time travel you can seriously affect the evolution of the system through otherwise minor interactions with it. All you need to do is affect a change at one of the critical events on the chart.
I've used this example before so I'll use it again:
The time traveler doesn't have to kill anyone or take any grossly obvious action to affect the entire history of the planet. All the TT has to do is post to a forum such as this knowing that in the history known to him/her that didn't occur.
In that history Doghead meets hs future spouse on Wednesday 11-FEB-2009 at 1900 hrs. He is walking out of a market with the groceries and accidentally bumps into this stunning woman. Both their grocery bags are now on the parking lot surface. They laugh, pick up the groceries, exchange pleasantries and phone numbers. They rest is history. They have two wonderful children, who in turn marry and have four children, who in turn marry and have eight children, and so goes the evolution of the history of their extended family through the generations.
This is the history of Family Doghead that the TT would have the ability to have knowledge of.
But...
Our TT posts something very interesting on the board this afternoon. Doghead, as he's getting ready to go to the store, takes a few extra moments to read and respond to the post. Now, instead of exiting the store at 1900 hrs he exits at 1905 hrs. He misses "Ms. Perfect" by 5 minutes. They never meet. At some later time he meets "Ms. Equally Perfect" and they marry, have their children, etc.
Doghead's extended family is now completely different. The people who evolved from his "original" family never exist. Whatever those people did in the original history never occurs.
Ms. Perfect meets and marries someone else and she has an entirely new previously non-existent family.
Ms. Perfect's new husband's family history is completely changed because he doesn't meet and marry the same person as in the original history.
The same is true of Ms. Equally Perfect and her original husband.
This cascading chain of events, over a rather short period of time, no more than six generations (six degrees of seperation), alters the entire history of the planet. At six degrees of seperation from the critical event it is highly probable that not a single human being on the planet who existed in the TT's original history now exists.
In this case we can, as TT's, most definitely determine that there were events flowing against the forward arrow of time. Bumping "a few molecules" off course results in a system that is readily identified as evolving differently than the original system.
If the TT came from, say, 100 years into the future from today, upon returning home he'd no longer recognize that world or its history. His original knowledge of events would be of a world that no longer exists.
I'm not suggesting that this is how physics actualy works. Hawking has proposed his Chronology Protection Conjecture (CPC) that says that this can't occur. Not because some
ad hoc hand reaches down and prevents the events from being altered but because the law of physics "conspire" against the creation of closed timelike curves thus preventing time travel. Li-Xin Lu also submitted a paper responding to Hawking that proposed an Anti-Chronology Protection Conjecture. We don't know which paper is correct. But if time travel is possible the possible consequences are not something to be taken lightly. Though I stated above that human evolution doesn't proceed as a thermodynamic system it does have some similarities. The indeterministic processes are such that we can't alter history in a way where we can craft it to our liking with deterministic results. We can set a new set of probabilities into motion but it would be in a scenario where we no longer know what the future will be because the future that we, as a TT, came from no longer applies. We'd be on equal footing with the people whom we visit in the past as to the uncertainty of how the system evolves - how the future will unfold. The only difference is that we'd have memories of a history that no longer exists and left wondering, paradoxically, just how it is that we persist in existing if our parents no longer exist.
Note: Now someone is going to toss in the Many Worlds Interpretation and suggest that its just another universe. Maybe that's true but MWI refers to interactions at the sub-atomic level and really only addresses single isolated particles. But in the real world single isolated particles don't exist nor do infinite numbers of qantum states exist. With ~10^90 particles in the universe the number of quantum states possible is theoretically countable. The particles interact and the interactions eliminate most of the possible evolutions of the system. Large numbers of particles and large, observable, massive structures don't split into infinite alternate universes - if they split at all. MWI, as we know it on forums like this, is a sci-fi theory that has little in common with the actual physics described by Everett and Wheeler,
et al.
