h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There are quite a number of possible answers to the paradoxes of time travel. There are also plenty of answers to the how to time travel problem. Remember, just because you don't agree with those theories because they sound wacky, impossible or not even thought up yet, doesn't mean time travel isn't possible. Technology and theories are always advancing. Eventually we might discover something just under our noses that makes time travel easy. Now, if it is possible then we have to consider the complications.

One answer to the "destroying the mere possibility of your existence before your creation" (aka "killing your own mother in the past), is destiny. If there is such a thing as destiny, the guiding force that says all things are planned from the beginning, then when you try to travel into the past to kill your mom, something would stop you. This is a very mysterious theory (although it's just as mysterious as the heisenberg uncertainty principle). The theory would mean that in your trip to the past, somehow you will end up doing nothing that could change the possibility of your trip into the past, because "you have already happened and cannot do anything to destroy your existence or your journey to the past".

Another possibility is that if you did succeed in killing your own mom, perhaps time would sort out from there. Ie... at some point in your mom's life a man that looks remarkably like a possible future son your mom appears out of nowhere and kills your mom. The events of time after that point in history would not longer exist. So time would just continue on from the point in which your mom died. This would mean though that matter perservation (creating nothing out of something) is possible if the universe needs it to be. That's because the atoms that you were made of were never born in your mother's womb anymore. They were artificially created by the universe itself the moment you arrived in the "past" so that life, the universe and everything didn't explode in a puff of logic. This is almost as valid as saying another universe was born to handle the outcome of this timeline.

What if there were an infinite (or very very large) number of parallel universes that all started out the same? And what if a man does choose to go back in time to erase his own existence not knowing that he was actually going into the past of a parallel universe. Yes this would happen in parallel universes. But it is more than likely then that they would all end up going to the same universe because they all choose the same calculations and configuration. They would either destroy that universe, merge into one "man in time machine" seamlessly, or you'd have a massive bloody mechanical mess. Assuming they somehow all merege into one man, bent on killing his own mum, in time machine in universe #423. One man though would probably be from the universe he's travelling into the past of. The man who originates from universe #423 would have the thrill of killing his own mom in the past for just once before his existence is obliterated from that universe's timeline. But universe #423 would remain intact logically. Then, the murderer returns to his time machine and travels back to the future of the current universe, #423 and discovers that his mom, and himself, have supposedly never existed. But since he's from any of the other universes, it's fine. However, now universe #423 is different from all the rest. If someone were to travel back in time to destroy thier own mother later on in the future of universe of #423 (uniquely to the different timeline of #423), then that universe would be screwed. That is unless whenever something unique happens to a universe, it automatically has a new parallel universe created to handle the other outcome. This might mean that instead of an infinite number of universes to start with. Instead we would have 2 to start with. Eventually someone goes back in time and destroy's thier own existence before they were born. We'd end up with 4 universes then. Here's how it works...


What happens in present time of Universe 1 and 2:

Man (aka psychotic genious) decides to travel back in time to kill his own mom before he was born, to taunt god, time, and logic or maybe just to test this theory.


What happens in Universe 1 (past):

Two men in time machines arrive in the past, merged into one man in a time machine (because they arrive in the same place at the same time, and are made of the same atoms). The man then kills his own mom. And then travels to the future/present of universe 1.


What happens in Universe 1 (present - changed by the act of time travelling):

Man's mom actually didn't live long enough to give birth to man so he never got born and never went into the past. Universe 1 evil genious never exists so therefore...


What happens in Universe 1 (past - changed by the act of time travelling):

Evil genious from universe 2 arrives in his time machine and kills his mom (who is actually from universe 1). Then he travels to the present of universe 1.


What happens in Universe 2:

Man's mom lives to give birth to pyschotic genious. Pyschotic genious dissapears with his machine, never to be seen again (in that universe).

Universe 1 and 2 are now different. In order to preserve the "fabric of space-time", parallel universes 3 and 4 get created, which are duplicates of 1 and 2 respectively.


Of course, it could be even simpler. It could be that whenever someone travels back in time, a parallel universe gets created in order to perserve natural order to things. There are many other concepts and theories out there. Just remember that just about any of these are possible. Then again, our current perception of time could be as inacurate as "the world is flat." We perceive time's passing a certain way. That certain way is uncertainly. Apparently it goes faster or slower depending on how fast and/or cold you are. We have been able to prove this with atomic clocks that have circled the globe.

[email protected]

You have many interesting theories. I will try to respond to them individualy:
1. "Desteny will prevent you from killing your mother": This is possible, I guess. It is, however, very hard to test. One problem with this theory is it assumes that the universe knows how history is supposed to happen. This is just one step away from saying the universe itself is a living being (the 'Gaia' theory on a larger scale). This may be true, but I doubt it on principle.
2. "The universe will sort itself out": Again, this assumes that the universe is a living being and can alter itself to insure its own continuity. I can't disprove this, but I still don't think it is true. This theory also brings up an interesting point: You say that the universe must be able to create matter spontaniously. If this is true (ie, the Law of Conservation of Mass is false), then much of physics ceases to be true. I really think the Law of Conservation of Mass has more validity than your theory. Sorry.
3. "All parallel universe traverlers end up in the same universe, and only mess it up.": You say that, because each iteration of our matricidal friend uses the same calculations, all would end up in the same universe. But if prior to this moment, all universes were the same, then no calculations could result in a specific universe being pinpointed by all traverlers. It is more likely that the calculations would result in each traveler going to the next universe to the left (metaphoricly speaking), and, because all universes would be in a circle (or a similarly equal space), each universe would recieve one traveler. There is nothing unique about universe #423 to make all travelers end up there. Also: as far as the rest of this theory (all travelers merging into one, etc), I believe them to be a moot point.
4. "When travelars from 1 and 2 go to the past, 3 and 4 are created.": Same thing. Why would both travelars end up in 1?

All in all, you have some very interesting theories. In my opinion, they aren't very likely, but that doesn't make them not true.

PhysicsMan ([email protected])

Hi Physics Man, - Hope you are still monitoring this because I've come to it a bit late. First of all let me congratulate you on reducing the go-back-in-time-and-kill paradox to killing your own mum. I could never figure out why the text books kept going on about killing your own Grandad (bit hard on Gramps - I say).
You were getting close when you talked about your murderous friend writing a note to himself to use the machine to go back and kill this woman in the past. The issue is information. Where did this note - or the idea to write the note come from? I prefer the going back in time and winning the lottery analagies better. You go back in time and tell your former self this week's winning lottery numbers. (I think this was a theme in a *recent* TV series about a time machine in someone's flat). The issue is one of getting that information into the past. One of your correspondents write about the need for having a time machine at both ends - well I don't need to meet myself to 'know' next weeks winning lottery numbers - I imagine that the easiest thing to transmit back in time is not 'me' but a simple radio signal. So, every Sunday night at the same time I tune in in the hope that I (or anybody) will have developed a time machine capable of transmitting a signal back to me. I would be very surprised if there isn't a lab somewhere already doing this. OK the lottery number case is a trivial one so how about the instructions on how to build a time machine sent back from the future where they exist. - But they didn't exist until the information about how to build them arrived from the future. As I say - the issue is about where did that information originate from? - It is no surprise that this links in with the laser spot on the moon thought experiment - it is information that cannot travel faster than light.

I disagree with you that information must have a source. Let's say I took a complete copy of Shakespeare's works, went back in time, and found Shakespeare. I then handed him the copy, and said, "Rewrite all of these, publish them as plays in your theater, and you will be eternally famous." He then rewrites them, and, since Shakespeare did write them, I could take them, and go back in time... While this would probably suffer from the "infinite possibilites" loop, it also raises your question: Where did Shakespeare's plays come from? In my opinion, they just were, thry did not have a real source. There is no real contradiction here.