1. Possible reasons:

1) They haven't contacted anyone that would be a creditable source

2) The have come only to observe and have not made their presence known

3) Secret government cover-up

I don't know if time travel is possible or not. But to make the statement that its impossible because we have no proof showing that it is possible is wrong. Now if you said "at this moment in time, we are not able to do time traveling" that would be accurate.

2. As Adrian has pointed out this is the grandfather paradox, interestingly Hawking has propose the "Chronology Protection Conjecture" as a possible solution.

The basic idea is that the universe acts to prevent paradoxes, either by making the act of time travel impossible, or by preventing you (or anyone else) doing paradoxical things like killling your grandfather.

The gun would jam, you would get interupted, grandfather would happen to duck at exactly the right moment etc. etc.

This avoids the direct paradox but still leaves the problem of information coming from nowhere, technically this doesn't seem to violate any laws, but it still seems very odd.

Consider that you go back in time and you hand your grandfather a note, on it is your name, your grandfather then tells your mother/father to name her/his child the name you gave him. Effectively you have named yourself. The information appears to come from nowhere. Odd no?

3. 3) Secret government cover-up
LOL, riiiiight.

I don't know if time travel is possible or not. But to make the statement that its impossible because we have no proof showing that it is possible is wrong. Now if you said "at this moment in time, we are not able to do time traveling" that would be accurate.
Some things really are impossible, and at the moment time-travel looks to be one of them.

4. Now if you said "at this moment in time, we are not able to do time traveling" that would be accurate.
This isn't right exactly. You see, if time travel EVER becomes possible we would have seen evidence of time travelers already. those that have come back would have shown up. So I think you may be able to say "We will never discover how to time-travel" but to say that it is possible does not require that we find out how some day.

5. This isn't right exactly. You see, if time travel EVER becomes possible we would have seen evidence of time travelers already
Not if humanity were wiped out shortly after its invention and only handfull fo people managed to actually travel in time.

6. As I said I think time travel will only be able to go forwards. There is logic that you can follow in there but how would you be able to go BACK? There is an answer to the time travel theory in a book I own and it explains how going into the future would work, but going back would be entirely different.

7. ## Re: Killing someones grandparents

Originally posted by stovellp
During a rather boring english lesson, a friend of mine decided he wanted to kill me . His plan was to go back in time, and kill my grandparents so that I would never exist. He's an idiot so I didn't pay much attention to what he said anyway.

But I got to thinking, would that be possible? If he killed my gradparents, I would never be born. So if I was never born, he would never have met me. If he never met me, he would have no reason to go back in time to kill my grandparents. If he never killed them, they they would exist again and so would I.

How does that sound? I guess its a lot like the grandfather theory, where you can't kill your own grandparents because you would not exist, and so would never come back to kill them. The only difference is its someone else doing the killing. With me not existing, he has no reason to go back in time, and so I would exist again.

Does what I said make sense? Would it then be impossible to go back in time (if we assume time travel IS possible) and kill someone? Then wouldn't it be impossible to kill hitler and not have world war II, because if he didn't exist you would have no reason to go back, and so then he would?
Yes, you wouldn't be here, if someone went back and killed your grandparents(before they gave birth to one of your parents).

It would be like if someone(anyone) really had killed your grandparents before they gave birth to one of your parents.

And then the second time around, you never would've existed, and the person who killed your grandparents won't have a reason to kill your grandparents, because you don't exist and your grandparents were already killed by him.

Make sense?
Anyways, just watch back to the future II again!

8. And then the second time around, you never would've existed, and the person who killed your grandparents won't have a reason to kill your grandparents, because you don't exist and your grandparents were already killed by him.
No it doesnt make sense.

9. And then the second time around, you never would've existed, and the person who killed your grandparents won't have a reason to kill your grandparents, because you don't exist and your grandparents were already killed by him.
Ok, so his past self doesn't go back to kill them, wouldn't they exist again?

Why should going back to the past change the future, if not going back can't? By not going back the second time, he will also change the future by allowing my grandparents to live. See what I'm getting at?

But why haven't we encountered visitors from the future?
1) This thread isn't concerned with if time travel is possible, I'm just saying if it was, then wouldn't that be the case?
2) Maybe, time travel is possible but humans never get to the stage to developing it? Perhaps something bad happens to the human race in the near future before the technology can be created? Or, maybe as soon as it is invented, the government make it absolutely illegal to go back in time even a few seconds, beacause of the whole "changing the past thing".

10. Originally posted by Fountain
No it doesnt make sense.

Like I said, just watch back to the future 2. If you go into the past, and do something that will dramatically alter the future, it changes it permanently.

You(the person who started this thread) would have to go into the past, before the other kid killed your grandparents, and kill him/his parents/his grandparents/etc and stop him from ever killing your grandparents. Then he would never have the ability to kill your grandparents in the first place.

Make sense?

11. Ok how abt the concept of time line.. every time a change is made in the past a new time line is spawned... so the new time line has no relation to the old time line from the point the changes were made.. so this will solve the problem of looping... So a person travelling to the future.. say 2500 from the old time line and a erson travelling to the same year from the new time line will not be the same... the year 2500 on the 2 time lines will be completely different...so every time any minute change like even a stick is moved a new time line can be spawned... we can even say that the old time line will be destroyed once the new one is created.. hopw i am not confusing

Code:
```                                                                                      2500
------------------------------
2100           /                    2500
/----------------------/ -----------------------------
original time line       /          2100                                 2500
----------------------------------------------------------------------------------
\
-----------------------------
2500```

so there can be multiple time lines or only the last one can exist.. so any changes made in time line 1 will create a new time line..

i think this can explain everything.. so even if anyone from the future visited us they created a new time line and we are not affected by it.. but i dont think time travel will ever be possible.. but who said we cannot dream...

12. Well, you said it yourself: If the guy killed your grandparents, he would not kill them. In other words, travelling to the past is not possible if you see time as a stream of events. If would only be possible if you allow multiple lines that can be splittet.
In that case, your grandparents, their decendents and the guy who attempted to kill them would get caught in a real infinite loop and become unknown to their previous timeline. I guess thats what hell must be.

omg, I sounded like a Trekkie!

13. >>You see, if time travel EVER becomes possible we would have seen evidence of time travelers already<<

Can you imagine the conversation.

Hi, this is a future you. Yes, you from the future.

I've come from your future to warn you about that trip to Bali and don't have much time, Ha Ha.

Time Travelers joke.

Don't go bec..........Hello?

14. Annual General Meeting of the Danish Time Travellers club will be held next Wednesday, or if raining, last Tuesday.

15. The Solution to the Grandfather Paradox:

An age old paradox has been the debate of time travel for quite some time. I'f one were to travel back in time and kill their grandfater (or mother) this would quickly make them non existant. And if indeed the person became non existant how could he go back in time and kill himself?

The trouble of this question comes from the simple assumption that time is a single entity and that the past is the related past and the future is the related future.

In actuallity time is dimensional -- WHA?? Thats right boys and girls the time line branches...
It branches at every significant and insignificant event of time - infinatly. If a coin flips - time branches - each branch continueing from heads result and tales result respectively. Why? This is the only way to compensate for the grandfather paradox.

An example.
time line A is moving along nicely - unfortunatly Billy A hates himself and wishes never to be born. He decides to jump into time machine and jump back to kill his grandfather. Bam, he jumps back 60 years and kills his grandfather before he has a son BLAM SPLIT POW timeline B starts traveling along having never known Billy A's father or billy A at all. But as billy A reinters the "presant" he sees hes still in existant - father still was born and grandfather never killed? WTF. Hes returned to time line A - time line B is where he doesnt exist. OOps. All well.

This appropriately allows for time travel both to the past and future WITH OUT paradoxs. You can never leap to the future of your own time line as it hasnt been lived yet but you can leap to another time line that HAS been lived - unfortunatly its only a "possibility" thus you can change the future.

The astute of you might be thinking if thats true and you cant leap to YOUR future as it hasnt been made yet - then how can any of the OTHER branches be in the future if they are parallel in time just with different actions/choices associated with them?

Thats a little more complicated - think of when billy A traveled back to his grandfather and killed him. a new branch was created - but this is the PAST!! You've already lived in the future 60 years ahead of the time line - this branch was created with a time line offset... doh. Billy A cant travel to the future - but timmy B (billy b's would be friend if he existed) can travel to Billy A's present which is timmy b's possible future (by 60 years).
Thank you offset. But how does Billy A get to go to the future?
Well Billy A' who traveled to billy A's past and saved his grandfather from an accident that injured his knee and prevented him from playing foot ball. (this billy has traveled for a different reason but the branch time offset has been created) billy A can now travel to Billy A' 's present or Billy A 's future..

Confused? I bet. But it makes sense.

Now Realize that the time travel thing with timmy B and Billy A' (prime) is just a metaphor at how a time delay can be created by a branch being trigured by an event in the future. How this really happens is beyon anything I or YOU could know. A possible explination is a look at the big bang (scientific theory) if we travel back to the first moment that the super dense super pact thing of matter started expanding into the known universe - it COULD have started at that exact moment or it COULD have started a moment later, or a moment later or a moment sooner - time branched for every one of the infinant momentary possibilities creating nano-nano-infinatly-nano seconds that the universe could have been created. So here in time line 330289c-28930x02 its 2003 - but in time line 3302843c-29389430x022.3 its 2230.

I love dimensions.