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

See, I told you I would!

I predict that I will create and then utilise a disposable time machine that will be capable of travelling a few seconds back in time but which will cease to function thereafter.

Hah, I've done that allready, threw it away though

I predict that timetravel will not exist at at all (on earth) in the next millenium, and that if it is discovered it will instantly be rejected because of all the falling-into-the-wrong-hands stuff.

Reverse time travel will not be achieved. Forward time travel on the other hand is easy to do.
The chronology protection hypothesis holds that it would be impossible to change history. Although no mechanism has been proposed by which the Universe would protect itself from time travellers with homicidal designs on their own ancestors, there would have to be some such thing.
If we do set up a time paradox, perhaps we might go back in time and shoot our grandfather. What happens then? Fifty years later, you do not exist. Therefore you do not go back and shoot your grandfather. So he survives. The Universe continues for the next fifty years, when you make your attempt at time travel. Now what?
The future is not set; so there are random differences - probably fairly slight - between this Universe and the first one, which have built up in the last fifty years (quantum uncertainty and things like that). These will probably not be enough to stop you, so off you trot into the past and blow your grandfather's head off.
And the Universe loops again, and we come to another time travel attempt, and another, and another... and sooner or later the random differences will be such that the time travel attempt is unsuccessful. Perhaps you never had the idea to build it, perhaps you never wanted to, perhaps you realized the deadly threat you posed to causality, perhaps it blew up killing you and everyone in a five-mile radius when you turned it on, perhaps the whole Earth collapsed into a black hole due to temporal stresses... could be anything. The Universe will loop until something happens to stop it (if the word 'until' means anything in this context).
The only way to travel back in time, as far as I know, is to dive into a black hole. Then the warped nature of spacetime in such regions can drag you back to the beginning of time, crush you out of existence at a singularity and spray your energy, momentum and charge out into the Big Bang. Which doesn't much help.

Now my head hurts.

Yes, hm-hmm.
That's interesting, but tell me:
do you often sing or whistle just for fun?

Hey phobos.

Interesting lines of thought you've put there, but you don't seem to allow of the possibility of parallel universes?

Hey phobos.

Interesting lines of thought you've put there, but you don't seem to allow of the possibility of parallel universes?

Yes, hm-hmm.
That's interesting, but tell me:
do you often sing or whistle just for fun?

LOL

How about going forward in time to murder your grandson?
Would that present any ruptures and/or loops in the system?

Huh?

And what if that grandson you murdered had initially planned on murdering you? I go back to kill my grandpa, who was out to kill me.
But I got him so he couldn't kill me, so I was never born in the first place. And I wouldn't have gone back to knock him, so he could kill me. But I wasn't born yet. Help!

I've posted this idea of mine up elsewhere but what the hey...
I thought that energy/matter had to be 'conserved' in the universe - i.e. can't be created nor destroyed. So therefore would it not follow that the amount of energy in the universe is a constant.
So, if anyone could travel backwards in time they would be 'adding' energy/matter to that previous state of the universe. e.g. if I travel back in time to yesterday, the universe has had its 'energy constant' increased to the tune of one wisest fool. Isn't this officially impossible?

By Presidential Decree
I Hereby Declare
That It Is
Possibly Impossible.

Happy?

Conservation of mass / energy: yes, the Universe in the past would have had several hundred Hiroshima's worth of energy added by your presence - but the Universe in the present would have lost a corresponding amount of energy.
However, if we let conservation-of-energy apply only when considered in four dimensions, we should expect the other laws of thermodynamics to do the same. In a Universe where time travel is possible, then it must be possible (it may be extremely unlikely, but in quantum mechanics it could happen) for a particle to travel in time naturally. In this case then we could expect the Universe to become much the same in all regions of time, as well as in all regions of space. As someone once said, 'The past is like a foreign country; they do things exactly the same there.' This is clearly a problem, as the Universe is very definitely changing with time.
As for alternate universes, in what sense can we say that they exist? If we can observe them, or be observed by them, or affect them, or travel to them in any way, then they are really regions of our own Universe. If not, then they may as well not exist. However, you might say that parallel universes do exist in the Universe-looping scenario I suggested; it is entirely possible that the state of the Universe be very different once the paradox is resolved. Biff Tannen might well rule the world, because if he comes to power, then Doc Brown gets put in the loony bin and the time machine is never invented - so resolving the paradox. He doesn't need that Sports Almanac after all!
And finally, about going forward in time to murder your grandson - no problem. Forwards time travel is easy - you just have to wait. People have murdered their grandsons before and the Universe is quite OK.
If you want to get to the future quickly, get in a spaceship and fly close to the speed of light. Einstein will do the rest - unfortunately, there's no way to return.