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

The Grandfather Paradox seemed complex until I broke it down into actual steps:
Grandfather lives
Grandfather has children
Grandfather has grandchildren
Grandchild goes to past
Grandchild kills grandfather
Grandfather is dead
Grandfather has no children
Life for everyone else proceeds forward from that point.

The grandfather, being dead and having had no granchildren, never has children and the grandchild never exists.
But only from the point at which the grandfather has already died, does the future change.
The death is not negated by the lack of the grandchild's birth, for at the time of the death, the grandchild had been born and was there.

The piece, btw, doesn't cover the subject of conservation of matter and energy. Traveling backward in time would be tantamount to adding matter to the previous universe. It would require the exchange of an identical chunk of matter to keep the laws satisfied.
And, if one built a time machine that went back six months, you would probably not change your position in space. You would therefore appear at least 190 million miles from the Earth.

Actually, Einstein said that space and time are part of one element called "space-time" to travel time is to travel space and vice versa.

As someone travels back in time, adding matter to an old universe, the matter is already displaced. What i mean is,there energy and matter, does not have to be instantly displaced. This is because there matter is already definatly displaced, always, with out question, just at a later date. So the laws of thermodynamics are disturbed, but not brocken, because it does balance itself out.

About this having to add matter to a universe if you travel back through time. It seems to me that this would not be neccesary. Say you travelled back in time a week, then you would have always travelled back in time to that week. This comes from the fact that if you can travel back in time, then the future must already have happened. This means that you would not have to add matter to a previous universe because, as you would always have travelled back in time the amount of matter in the universe would not be affected.

I doubt that I've explained myself properly, but hopefully you can see what I mean.

Simon, cheers for adding to my little theory, somewhere it didnt add up, and it seemed a little thin, however, you have backed it up, cheers

The article, probably due to its age, doesn't say much about the splitting universes theories. Here, every event triggers the birth of another universe, so the grandfather killing merely starts off a new universe where the grandchild is never born.

They can also use this to explain quantum effects, for example an apparently single particle being able to pass through two holes at the same time. In fact the particle goes through one hole in each of two universes, but you don't know which one until you observe it at which time the situation resolves itself to a single hole, in a single universe (yours). Or something.

To reply to the initial post in this thread: The problem with resolving the Grandfather Paradox in this fashion is that in the final state, there is no reason for the grandfather to die. The grandchild never existed, so (s)he never went back in time to kill the grandfather, so the grandfather was never killed. Saying, "He dies before he doesn't have children, so not having children can't affect his dying," doesn't work, because in a universe with time travel, causality can go backwards in time.

As far as the location one would travel to: This is why I imagine that any real time machine would need to be more like a doorway or wormhole than like a spaceship. A time machine would be a connection between two points in spacetime which are not otherwise connected. So there would be no risk of going back six months and finding yourself on the wrong side of the sun; you'd actually be going to a point in spacetime defined as "Six months ago, at the same location on Earth."

For parallel universes: I actually made an updated guide entry with a section discussing the effects of quantum splitting; it's entry A662447 (http://www.bbc.co.uk/dna/h2g2/alabaster/A662447). However, the editor of the Edited Guide version of the entry never replied to my requests to integrate the changes into the edited version, and I'm not sure how to initiate that process.

And lastly, about conservation of matter. If one imagines the time-machine-as-doorway, this isn't as uncomfortable; it's like the door were to open up and new matter were to appear through it. With a time-machine-as-car (that is, the time machine will just pop into existence whenever it goes), then you do have the problem of replacing the matter in the space that the time machine appears. Perhaps it could swap out the air in its new location with the hole it leaves behind it its old location. Of course, since the time machine would presumably need to consume energy in order to travel, the waste heat would need to be dissapated somewhere, and perhaps this could be used to balance massenergy.

Physics man, i almost totally agree with you on every piont. Howevr you then added that space would not be constant. As the grandchild goes back in time, he enters his own universe at an earlier date, and murders his grandfather. Then returning to his own universe to find that he no longer exists.
Surely in a world with time travel, causality does not go back in time, as time still goes forwrds, and as shown in HGTTG, time figures it self in the end, causality cannot go backwards, as effects do not go backwards.
eg: when you kill your grandfather in the past, you die, but you do not survivre because it was at n earlier date, this is where all the other universes crap comes in...and i cant be arsed to go into that.
To conclude, who cares, time travel is almost impossible, no one will kill thier grandfather in the past anyway, and i think we all have better things to do, but as time travel is only possible by moving super dense stars, and then you can only go back to when time travel started, is it really worth discussing?

Have read the updated entry; you should definately try to get that in the guide. Two things I don't quite understand though:

1. There is much discussion about returning to "your own" universe, and the possibility of the paradox returning if another you were to kill your grandfather at the same time you kill his, etc. Surely the idea is that each resolution of any quantum duality spawns a new universe? That way there can never be a paradox. Presumably the one you would return to would also be new

2. In the "parallel universes" section, there is an assumption that all universes are exactly the same as they all started at the same time. I can't see this; surely random fluctuations would change things over time? If we can't accurately predict the motion of nine planets thousands of years into the future, I don't see how the same human could invent a time machine in several universes simultaneously 14 billion years down the line.

Anyway, there's a damn good argument for not encouraging your grandchildren to go into physics, just in case they invent a time machine and try to prove a point.

Sorry its me again, and although you could fit all my knoledge of quantum mecanics into a rather small and arogant photon who wont give me a ride, i dont see why people must bring up the other universes crap to describe the grandfather paradox? The whole idea of the grandfather paradox id that you travel in your own universe, aome are taking this way too seriously. When thye grandfather paradox was disscussed who gave a damn about other universes and killing other versions of you grandfather, because as it has been stated for over 40 years, as you enter a parralel universe you cease to exist.
To conclude, there is nothing left here to be said on the grandfather paradox
1. Because it is impossible
2. Because why would you do it if you knew the effects, and how many of us are murderers
and 3. Because doing it could cause the universe to be destroye, even having repercusions on the multiverse
and lastly 4. I dont care anymore, im done

The reason the grandfather paradox might not be a problem is simply this, one theory is that because you are going back in time, what you have done has already happened, so you have no free will to change it, if you do try to kill your grandfather, something will happen to prevent it, you might get lost or kill the wrong person, or maybe you're grandfather did get killed by someone in the past, and it turned out to be you.

First off i don't agree with the killing your grandfather will end the universe. to me thats just arrogant, just because it doesn't make sense to us, doesn't mean the universe can't cope with it. Besides doesn't that mean that even by going back in time, by disturbing a couple of air molecules, you could change the future so you don't go back in time, and thustly ending the universe, to me that just doesn't make sense.

Its not arrogance, its logic. As you travel bac in time you are adding energy and matter to a previous universe, thus disrupting the laws of thermodynamics, nay breaking them. However, I have suggested that people who saay this look at time far too linearly. You are not adding matter to the universe, you are displacing it from one time stream and transfering it to another, thuis only bending the laws not breaking. And since in say...20 years you will travel back anyway a circle of time is created, thus the universe survives. However, if the laws are cleanly broken, why should we assume that the universe ends. Perhaps it may warp space or time, but im sure the universe will survive, with 11 dimensions, im sure a little law can be broken.

Feeble, you need to work on spelling and grammar. It's damn hard to take you seriously in a discussion about thermodynamics when you talk like a child.

Anyway the conservation of matter isn't a problem in a multiverse, because it would then apply to the entire multiverse. Meaning, if you leave universa A and go back in time, you are now in universe B. So UA is -1 you, while UB is now +1 you. Matter is still conserved. Granted a multiverse. Time travel in a single timeline is not possible because of the conservation, and the paradox. But in a multiverse it seems that anything is possible

To feeble, about wether or not the grandfather paradox is stupid to talk about:

It isn't a questions of killing your grandfather. It's anything that can be done that will change the events into the future into a way that would keep you from going back in time. Literally using the grandfather is just a simple way to look at it. But there problem really is this: if you can go back in time and kill your grandfather, that means that your grandfather is dead and you where never born. You need to think about this from the begining. For example if you go back in time, and say hi to yourself 5 years before. Then 5 years before you went back in time you would have seen yourself saying hi to yourself. You get what I am saying? Anything that hapens from the future will always happen, therefore if you go back to kill your grandfather you will never have been born. So you were never born. You grandfather died. Before you even made a time machine, and got the idea to kill him, you never existed. And therefore, no one ever killed him, there is the paradox.

Despite you're amazing semantics, u have not solved anything, merely stated what the paradox was. Also you seem to have assumed the multiverse, which is ok...but what has that got to do with you're little rant about the paradox. Also, you have used my theory on matter conservation in your hypothesis, and for this to work the multiverse is again irrelevant.

I don't see how I either used "your" thoery of consevation, or how that would make the multiverse irrelevent. I used the multiverse, to say the law of conservation would be satisfied.

Let me try to say the grandfather things again. You can never go back to kill your grandfather. Because if your could, then your wouldn't be around to even think about it. If time travel is possible, there can't be a begining to it. There is not "First time" the machine is used. Your grandfather would be dead, and you would never be born, you will never be alive to ever get into a machine. Yet you killed him anyway, thats the paradox.

However in a multiverse, both the Grandfather paradox, and the law of conservation is not a problem.

But the whole paradox is that depite never being there to kill him, you did. Thus it is a loop, where u are alive and kill him or are not. Theortically tow universes develop where you were born and wernt, thus the pardox in nullified.

Oh yeah i am a smoker, and i oppose the phrase 'f**k smokers rights'. Whats you're beef against fags?

Firstly I am not sure what you mean, when did I ever say I have anything against fags, unless you are trying to use the proper form of fags, in which case, don't try to bring that back.

Either way if you want to discuss this you should either do it on my personal websites forum, or in my personal space here on h2g2, not in a conversation about the grandfather paradox. I would be glad to tell you all about my beef with "fags" in those other forums.