Crusade in Jeans, anybody watched this (Time travel question)

[Pawelk198604]

It's about boy, who lost in soccer match, he's time lost match because him. His mother a quantum physics invented time machine, he secretly used it, to win a match which he lose, like Sam Becket from Quantum Leap to Set Right What Once Went Wrong:)

But something he massed up, and landed in medieval time.

SPOILER

The boy at the end of the film goes back to rescue the girl in whom he felt in love that she could be his 1000 X

great-grandmother, but that's not what I mean

Then he goes back to the ill-fated match that his team lost, the event took place

before he took his first trip back in time, if he won the match due to fall back on

time, the application had no reason to even travel in time :-)

Thus, these events did not happen right, it's time paradox

[drakesdoom]

This whole time paradox thing is looking mighty sketchy these days. Even nasa is funding research into controlled gravity fields assisting acceleration to allow ftl travel, which allows a light form of time travel. Saying you can't do something because logic only shows you do not yet fully understand how to do something.

[Thaniel]

It's a movie. If they can bend relativity/realism to make a story seem the least bit reasonable, they will. The paradox here seems to be of a psychological sort, where the need to do something is negated. In principle, it is a paradox.

If someone went back and killed some horrible massmurderer before he got to do it, would the posterity then still think it would be necesary to do it? The paradox does however not follow the person who went back in time and did it, wich is the person that conceviably will go back to his/her/its rightfull place in time, and the resulting history will have been changed. What reality that person goes back to however is anybody's guess.

[Pawelk198604]

I forget to put link

http://en.wikipedia.org/wiki/Crusade_in_Jeans_(film)

[Kerbface]

Realistically if you went back in time any significant time, like a year or more, tiny actions would have massive changes in the future due to the butterfly effect. Not saying everyone would be seahorses or a nuclear bomb would blow up your city, but even just people being in the tiniest different place at different times would cause different coincidences to occur, when someone conceived a different sperm would get to the egg, that's a whole different person born, some people would never meet, other people would meet, a person would not arrive at work at the same time, which means that they will not get in the way of a man walking perpendicular to him, which means that man will not walk a bit to the left, which means he won't notice the cafe, which means he won't order a coffee, which means that the person the server would have screwed up the order for will go to another server, which means he won't get yelled at, which means he won't go and complain about it to his friend later, which means he won't get into a conversation with that friend about music, which means that the friend won't get a song stuck in his head, which means that he won't be humming it in the middle of a lecture, which means he won't meet a girl who he would date for a while, which means she won't influence him and cause him to change his career path to fine art, which means he won't fail to achieve his goals and become homeless and that the girl won't be broken up about their relationship and decide to go for a drive in the country to calm her nerves, which means her car won't break down, which means she won't hear a story from someone at a diner while waiting for her car to be repaired, which means she won't turn that story into a bestselling novel.

So yeah, if time travel worked, things wouldn't fit together. They would surely diverge into different timelines. And it wouldn't be like in movies when it only affects things relevant to the plot and the rest of the world stays more or less the same with the same people in the same relationships as long as it's not the central character's relationships.

[K^2]

The time travel paradoxes only appear in classical mechanics. Any field theory completely resolves grandfather paradox. I can go into details, but there is a lot of math. In practice, the easiest way to think of it is the alternate time lines explanation. E.g., if somebody travels back in time to prevent a catastrophe that led to him being sent into the past in the first place, you can think of it as there being two worlds, one where things are bad and one where things are good. Person from the "bad" world comes back and ensures that the "good" world can exist. It's kind of up to interpretations whether that's what is actually going on, but if you use this sort of logic to sort through what would actually happen, you should get the right outcome.

In terms of where we are with understanding of time travel, the particular field theory we use to resolve paradoxes is Quantum Field Theory. And because we understand how time travel works in confines of General Relativity, we can actually solve a certain class of time travel problems on paper. Specifically, ones where the time machine exists regardless of any choices made, but whether somebody uses it to go back in time can depend on the circumstance. Unfortunately, if the very creation of time machine is subject to changes you make by time-traveling, the exact solutions are not known. Using the aforementioned tools, we would need theory of Quantum Gravity to do that. But in general, it should behave the same way.

Again, I'm oversimplifying a lot of this. Just explanation of how time travel in GR works is rather involved, because there are all sorts of rules and limitations. But this is the gist of it.