Even if theoretically possible, time travel would likely complicate our lives-what would your life be like if time travelers were constantly changing history?
With that in mind, I thought I might summarize what TT might do to us:
TT from the past visits the present: No effect
TT from the past visits the future: no effect
TT from the present visits the past: major disruptions possible; history changes
TT from the present visits the future: no effect
TT from the future visits the past: major disruptions possible, history changes
TT from the future visits the present: major disruption possible
TT possible, but changes to history caused are not perceptible
Have I covered everything?
Also, can the time traveller only bring themselves (e.g. Terminator) or can they bring unlimited tech with them (e.g. The Philadelphia Experiment)? It would be a heck of a lot easier to disrupt history with some 2013 weaponry then just with little ole me.
If you travel into the past and have sex with your grandmother and get her pregnant, and that baby grows up to give birth to you one day… congratulations! You’re your own grandpa
You’ve basically covered time travel but you should also consider information travel which basically works the opposite way.
viewing the past from the current time or from the future: no problem
viewing the future from the current time: contradictions possible
The real problem with time travel is free will. Its easy to imagine a 4th dimensional static universe which includes some fixed time loops. The problem is that if you add free will then you can choose certain actions that will inevitably lead to contradictions. I eat an apple and then go back in time to tell myself that I ate an apple so decide to eat an orange instead.
I actually spent some time trying to see if I could get Dr. Who style time travel to work, and came up with an idea that seemed to allow it.
Basically you enter your time travel device at time T_0 (Gallifrey) and then can travel back an forth through time so long as you never went back before T_0 plus the amount of time you spent time traveling. The way it worked was by having
a very accurate prediction of what the universe would likely be like at all times in the future.
a simulation (ala the matrix) that the time traveler would enter to manipulate this prediction.
a matter projector that going forward would make alterations described in part two to reality based on the current configuration of the simulation at the current time.
So if the tim traveler went into the simulation stepped forward 1000 years, met Fred and then took Fred back in time 50 years to kill his grandfather, the simulation would wait 950 years then create Fred and have Fred kill his grandfather, and let time carry on.
I don’t think such a device violates causality but I welcome attempts to poke holes in it.
I recently read Isaac Asimov’s “The End of Eternity” and was hugely impressed. He describes an entire society (a damn huge one!) built around time travel and history manipulation. “Changing the Past” is as fundamental to that world as “arresting and imprisoning criminals” is to ours. It’s a titanic four-dimensional machine, constantly self-correcting. It’s brilliant!
(And the darn cluck went and trashed it at the end, with a “Captain Kirk” kind of rationalization that we shouldn’t live in paradise, but should have to struggle for progress. Sheesh.)
That story, like Fritz Leiber’s “The Big Time” depends on the existence of a super-time or meta-time – a higher time dimension that even time travellers are subject to.
(In a time-travel board-game, you might call these the “turns” of the game itself. There is a classic old paper-and-pencil time-travel game called “Assassin” where the various assassins move around from city to city, and year to year – but the game is played in turns.)
Asimov embraced paradox as a part of the structure of his civilization. (He also rationalized it by claiming that changes “damp out” over the centuries, so, for instance, killing Julius Caesar won’t change the eventual rise of a steam-and-electricity Industrial Age. It just means that it isn’t James Watt and Thomas Edison who invent it.)
Could someone please go back in time and bring us Isaac Asimov again?
11)TT to the past is possible, but doing so creates an alternate universe
Time travel is technically possible, but attempting to actually build a time machine will lead to your annihilation by a sort of universal reflex. Niven’s Rotating Cylinders and The Possibility of Global Causality Violation.
13)Time travel to the past is possible, but changes to the timeline only progress forward in time at the normal rate so they never reach the time the traveler arrived from. Crawford Kilian IIRC wrote some series where it worked like that.
14)TT from the past is possible and can change that future, and the past if they return. Lightning, Koontz.
15)TT to the past is possible, small changes are damped by the universe but larger ones cause something else. A timeline split in Simon Hawke’s Time Wars series; universal destruction in Spider Robinson’s Callahan’s Place series.
“The Encyclopedia Galactica has much to say on the theory and practice of time travel, most of which is incomprehensible to anyone who hasn’t spent at least four lifetimes studying advanced hypermathematics, and since it was impossible to do this before time travel was invented, there is a certain amount of confusion as to how the idea was arrived at in the first place. One rationalization of this problem states that time travel was, by its very nature, discovered simultaneously at all periods of history, but this is clearly bunk. The trouble is that a lot of history is now quite clearly bunk as well.”
Time travel machines have a habit of inadvertently keeping itself from working the instant it worked…
As soon as you go back in time, you find yourself there pressing the button to go back in time…pressing the button to go back in time…the button to go back in time…to go back in time…back in time…time… … .
How does the Journeyman Project time-travel universe fit into this? If I remember correctly, there was some sort of disk that had all of recorded history on it, sent way way into the past (like during the dinosaur era). The machine that allowed you to travel in time could somehow detect when a change in the timeline was coming, and warn you. When the timeline changed, you would go back and get the disk that was sent way into the past (to a time where no one was likely to ever travel back to, and even if they did, they probably wouldn’t change anything important anyway). You would put the disk in the machine and the machine would reconcile reality with the information on the disk, and locate probable points in time that were altered to create the new reality. The disk did not change because evidently it was safe from alterations in the past, that is to say, the change in time only propogated forward in time, and items from the Old Reality that had been sent to the past before any manipulations of the timeline were safe from being altered into the New Reality.
I don’t know if this matches up or makes sense. It might be paradoxical itself. Anyone play the Journeyman Project games?
That’s one of my two theories - the other being that backward time travel is impossible because you can’t “jump” back in time, but have to pass through all points in time between where you are now and where you are headed, just as you can’t jump between two points in space - and when you go back a sufficiently small enough amount of time, you will be blocked - by yourself occupying that space in that time.
If you can go back in time, then what happens is:
(a) As far as you are concerned, “history” from the point where you arrived onward can change;
(b) As far as everyone else from where you left is concerned, you have disappeared, and nothing in history changed.
Of course, for all we know, some alien race gave us time travel technology, but we abused it, so they took it back retroactively in such a way that we never realized we ever had it. (Er, all of you have read “Scherzo with Tyrannosaur,” right?)
A wormhole goes through different dimensions of space; you are still creating a path of travel. There is only one dimension of time, so any path would have to be through what we understand to be “forward” and “backward” in time.
