My thoughts on time travel (Can I possibly be the first to think of this?) (long)

First, IANAP.

Getting that out of the way, my fundamental assumption is that the amount of “stuff” in the Universe remains constant. By stuff, I mean energy (including mass, which is a form of energy).

Okay, when we push something forward in time, universal energy remains constant. If we are talking about the twin paradox (one is sent off to space and travels near light speed, one stays home) then the traveling twin will appear to slow down, shrink, and gain mass in relation to the homebody twin. In other words, the traveling twin won’t be able to get a date.

Now, note that the traveling twin does not blink out of existence. I believe this is because the amount of energy in the universe must remain constant. Instead, the various “properties” of the twin are (relatively) changed to compensate for the (relative) time slowing.

For reverse time travel, I would think the same must hold true. You can’t just blink out of existence, totally removing the mass/energy from one frame of reference and add that mass/energy to another. Each frame of reference must have a consistent amount of energy. So, although a reverse time traveler would look really weird (with effect preceding cause), the reverse time traveler must still “exist” at every “moment” for every frame of reference.

Therefore, assuming every frame of reference must have a constant energy, reversing time travel must be impossible. Otherwise, you could keep on increasing the energy in one frame of reference and decreasing the energy in another.

Think about it. Since we already said you couldn’t blink in and out of existence, you must be present in the universe at every “moment.” So you wake up one day and see someone (who looks exactly like you) going in reverse, with effect preceding cause. As you are watching that, you come up to you and say “that must have been us while we were going in reverse. I switched directions a couple of minutes ago (or “from now,” depending on your reference). So, in the “normal” reference point, there are 3 of you. But, according to my premise, you can’t change mass/energy in a frame of reference. Therefore such a scenario is impossible.

Of course, the same prohibition holds true regardless of how far in the past you go. You will always be adding mass/energy into the universe

So, my conclusion is that once you are going in one direction, you cannot reverse directions.

I should mention that I see one loophole with my theory. Once again, IANAP, so I’m not sure if the loophole is relevant or not. My premise is that you cannot reverse directions. However, what if you find a whole bunch of matter that is already going in the backwards direction? Anything you did in your future to the reverse stream would effect your present reverse stream. Once you start sending yourself messages, you can run into the grandfather paradox all over again.


Would that I could time travel back to the beginning of this thread prior to adquiring the massive headache that ensued upon reading it.

Please let me know when you work out the kinks.

I don’t believe time travel is possible. There are way too many things that could happen. Sometimes my brain gets into a mode and all I do is think, for days on end, I think. time Travel was one of those thoughts, one reason I didn’t believe it to be possible is that you could kill your father, or postpone the conceivment date, or…hell, even have sex with your own mother! Would life in its entirety be so oblivious to these variables? I don’t think so. But only time can tell. Maybe someone will be lucky.

Ah yes, there was one other variable(thought kind of dumb). Suppose a time Travelling machine was set up somewhere in, lets say New York. Now they decide to go 2 days in the future, would the ship end up in exactly the same spot? Chances are you would be floating in space by that time. That is one huge variable I hope our future time travel engineers don’t leave out!

Good point about the spacial problem with time travel, although defining “the same spot” would be tricky, sinc ethere is no absolute frame of reference. Would the time machine in the same spot relative to the earth? The sun? The center of our galaxy? The great attractor?

By necessity, any working time machine would have to be able to travel to an arbitrary point in space and time.

Of course, were time travel possible, wouldn’t we know by now? Wouldn’t someone from the future have come back and told us about this great experiment? Or are we the “now”…Meaning: Until we find a way to travel in time, there is no future?
Or worse yet: The nuclear holocaust has already happened, and there IS no future?
OH WAIT…WAIT…Could it be that I’ve been listening to too much late night AM radio???

In Asimov’s “The End of Eternity”, he proposed the interesting idea that Time Travel devices could not travel before the invention of time travel devices.

If there were time travel there would be lethal inconsistencies, even if you don’t go much of anywhere in time or do anything while doing it. Example: We invent a warp engine that goes forward in space while going backward in time. You can now travel to anywhere in teh universe a few moments versus either on-board clocks or external observers - and it doesn’t make a difference how slowly you move in space because you only have to go back in time faster, preserving Special Relativity. We send a 1000 tonne ship to the moon in 1 second apparently elapsed (129% of lightspeed net if it weren’t also traveling backwards in time). A distant external observer sees local reality suddenly mass 1000 tonnes more (ship traveling and at its destination - information cannot propagate faster than lightspeed). 1 second later 1000 tonnes disappears. The universe hates contradictions.

Any universe in which time travel is brought into existence will alter its future and past until time travel no longer exists. Given the way things work, one expects the time machine to persist and the universe to vanish. Uncle Al bets that will hurt.

Uncle Al
(Do something naughty to physics)

I don’t see how that alone creates a problem. Sure, an observer sees an inconsistency. This may violate some of their assumptions about the universe, but I can’t see how this observation leads to the destruction of the universe.

I’ll mention a couple of points on this:

First of all, there is a form of two-way time travel that I’ve seen proposed that seems fairly plausible, and I’ve yet to see anyone specifically refute it as impossible (though whether or not it’s reasonable to actually perform is up to debate). The way it goes is: Create a wormhole, anchor each end to one point/object. Take one end, have it move away at near c. By special relativity, the moving end will age more slowly than the stationary end. When it comes to a rest, you know have one point that has aged more than the other, ie, it is further forward in time. Traveling back in time is thus accomplished by moving from the younger end (the one that moved) to the older end. Now, this presupposes several things: First, that wormholes exist. Second, that they can be made large enough and stable enough to pass through. And third, that once formed, the ends of a wormhole can be moved without the wormhole being destroyed.

Second, assuming this works, does it violate causality? Does it create paradoxes? Nope. Why not? Light cones.

(Disclaimer: I’ve never read about the following anywhere; it’s my own dumb idea. Take it for what it’s worth, but it makes sense to me, anyway. :slight_smile: )

To those who don’t know what they are (if you already know what a light cone is, you can skip this paragraph):

An object’s light cone is an imaginary construct in space time that defines what other points in space time that object can communicate with, or travel to, or in some way interact with. A point in space time that is outside an object’s light cone can never be reached, no matter how fast the object moves. An example: Say that at 8:00 AM I’m sitting here in my office. Those coordinates - 8:00 AM in my office - define my location in space-time. Say I want to be at the sun by 8:05 AM. Is this possible? No. It takes 8 minutes (roughly) for light to travel from here to the sun. Thus, even if I moved at the speed of light, I couldn’t be at the sun any sooner than 8:08 AM (relative the earth’s time frame, not mine). The point in space time (8:05 AM, the sun) falls outside my light cone, therefore I can never be there. I could be there at 8:10, though, if I travel fast enough. Therefore, (8:10 AM, the sun) is within my light cone. And the point (8:08 AM, the sun) lies right at the edge of my light cone.

So, how do light cones make time travel - even time travel backwards - okay? Well, time travel is allowed, provided your destination in space-time lies outside the light cone of your current position. In layman’s terms, time travel is okay as long as it takes you to a place where you can never interact with anything in such a way as to violate causality. If I time travel to 5 minutes ago on the opposite side of the universe, that’s fine. If I time travel to 5 minutes ago right here, that’s a problem, and thus is not allowed. Basically, the farther back in time I go, the farther away I need to travel spacially.

This fits in nicely with the time-travel method mentioned above, as well. To go back farther in time, you need to take the end of the wormhole farther away. This insures that every time you go back in time, you go far enough away that you’re safely outside your prior light cone. And thus, you’ve safely insured that you won’t wind up your own grandpa. :slight_smile:
And of course, there’s always the HHGTTG explanation for time travel: You can’t violate causality, because everything is already predetermined to work out in the end. If you’re going to wind up your own grandfather, so be it. And if you end up killing your father, well, the universe will make sure that there’s a perfectly good explanation for it. Hey, works for me.