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

The counter-argument that explains why we don't see time travelers visiting from the future isn't actually vague. It refers specifically to the wormhole method, and it's worth a bit more text in the entry. I'll try to explain it in concrete terms.

You create a wormhole, with ends X and Y. Then you move Y at near-light speeds until you've built up a relative discrepancy of, say, 50 years between the two ends. When you've finished, if you enter X, when you pop out of Y the universe will have aged 50 years without you. If you enter Y, when you exit X the universe will be 50 years younger than it was when you entered Y. This Guide entry explains all this quite well.

But this doesn't help you if you want to go back beyond the creation of the wormhole. You still need both ends, and a temporal discrepancy between them, for your wormhole to work at all. Before you created the wormhole, you didn't have an end of it through which any travellers from the future could arrive.

So, then, what would happen if you entered Y only ten years after creating such a time machine? What time will it be when you arrive at X? The answer is that you wouldn't get the full 50 years out of it yet. You would emerge at some moment after you created the wormhole, not before.

Why is this? Well, the 50-year discrepancy didn't happen instantaneously; it took you some time to build up. As you moved end Y around, the discrepancy grew from zero to 50 years. But as you pass backward through the wormhole and approach X, the time discrepancy decreases.

That's probably not all that clear anyway. But my point is that this is an area of the entry that could use a little expansion.

Bryce

another reason why might be to do with the grandfather paradox, say a time machine (whatever method u may like) is created in the year 2050, then someone decides to come to our year and tell us how to create it, we would create it, then when we get to the year 2050 we wouldnt go back in time because the first people went to our time to tell us how to build it but if it was already built, then why would we need to explain it to our past?

If you have seen the movie the time machine, (if not dont read this or it will spoil the end for you) it has a similar example. This man's soon-to-be-wife is shot and killed, so he is determined to bring her back, after four years he builts the time machine, travels back in time and saves her from getting shot, a few minutes after saving her she gets run over by a wagon and dies, the man is confused and travels into the futur to find out why, he meets a man who tells him why, he says "you built the time machine to save her, if you saved her, then you wouldnt need to build the time machine, so how could you save her?)

There are many theories concerning time travel. Some say no, citing various paradoxes. Others say yes, bypassing the paradoxes by introdusing parralell universes (many worlds) etc.
Both the pro and con theoies neglect the cause and effect.
The "killing your gandfather" paradox state that if you go back in time and kill your grandfather you would never be born and thus could never go back and kill your grandfather.
But if you think about cause and effect this is not nesecarily so. Lets put the events in order.
1.grandfather lives
2.you are born
3.you travel "backward" in time
4.you kill your grandfather
All would agree to this, I presume. Any paradox only comes into play when you rearange the secvense.
So that you get
1.grandfather lives
4.you kill your grandfather
2.you are born
3.you travel "backward" in time
If you think linear only the first secvense is valid. The point is that the "kill your grandfather" will always come after you are born. so changing your past is imposible, but everyone else can have a future diffrent from your past if you change your past(which really is your future ). So in a linear secvense any time travel would only change the future.
This sekvense would unfold:
1.history happens
2.time machine is invented
3.time machine is used to change the "past"
4.the past happens (with changes)
as you see it is all linear and follows the cause and effect without introdusing "many worlds"

Traveling forward in time would only require that you where unaware of time moving. A stasis field would do nice. Going backwards is the tricky part. (get a stasis field and make time flow backwards for everthing else.)

Just thought you guys may like to hear about a "real life" time traveller. It's true apparently, well. Make of it what you want: www.johntitor.com

I have a friend who studies philosophy MA and quite rates the site; he isn't a strong believer though.

I am busy reading that site, i love the way that he gives such good pictures of the time machine and physics equations and everything BUT mucks this up:


he says "When I was 13, I was a soldier" but then later he says "In my 2012, I was 14 years old spending most of my time living, running and hiding in the woods and rivers of central Florida" so that makes him sound like somebody running from soldiers not fighting.

he said he is from 2036, but he also says "or check out the information I've given you about the UNIX failure in 2038." so has he come from 2036 or 38? or just visited 38?

hey ya i remember that

its kind of like final destination, when your time is up, your time is up and thats it, nothing you can do

Bryce, you're absolutely right. However, the little bit at the end was meant more as a way to close the article then as a complete argument. I'll consider adding it, though.

I've read John Titor's site, and I find his discussions to be unusually accurate (that is to say, he's not completely wrong). Of course, that doen't mean he's telling the truth; I could write a site that sounded equally valid, and I'm not actually a time traveler. But it does mean that at the very least, he did his research.