What do you think about time travelling.

https://en.wikipedia.org/wiki/Twin_paradox
https://en.wikipedia.org/wiki/Gödel_metric
https://en.wikipedia.org/wiki/Closed_timelike_curve

 

Why is this in General Discussion forum? And have you done a search on this very topic for the numerous threads already posted in this forum?

Zz.

 

Since it is impossible, I don't think of it at all.

 

It is possible! The twin paradox is a perfectly valid way to travel time. Just because we understand it, doesn't mean that it's not time travel. And travelling backwards in time is not contradicting any physics either.

 

In the twin paradox, the twin leaves in the present and returns to the present. Only the passage of time was different for the twins.

I forget where I read it, but it was explicitly stated that time travel is impossible, perhaps due to entropy. In Feynman diagrams, antiparticles travel backward in time, but I expect this mere convention.

 

It is possible in principle to leave, to travel for 1 second, come back and be 5000 years in the future. That's time travel no?

 

I'm curious, how can this be done?

 

Time dilation. Just travel for one second for a speed sufficiently close to light. On earth, 5000 years will pass, while for you it looks like 1 second.

 

How would you accelerate an object to a fraction of the speed of light, slow it down, turn it around and accelerate it back to earth, and slow it down once again, all in one second?

 

That's an engineering problem. Physics says it's possible.

 

What about the forces on the body? That's a physics problem.

 

I consider it an engineering problem. Of course we can't time travel right now. But saying that the physics laws forbid it, that's false. It's very likely we'll never be able to do it though.

 

And sure, you're probably right that the forces on the human body forbid this kind of thing. But where do you draw the line: travelling 5000 years in 1 second is not possible. But traveling 2 seconds in 1 second might be more feasible. Travelling 1.001 seconds in 1 second maybe even more. Where do you draw the line as to what is time travel and what is not? I consider them all to be time travel.

 

"Solar" (i. e. stellar) sails. Although it'd take considerably longer than a second. The current engineering problem isn't achieving a significant fraction of light speed, it is escaping or resisting micro collisions with cosmic dust or anything larger.

 

We are all time traveling with respect to another (e. g. the aliens) all the time.

 

Time travel is an economics problem. It's a problem of affordable energy.

http://spaceplace.nasa.gov/review/dr-marc-space/time-travel.html

Theoretically, tachyons are all traveling back in time, all the time.

 

As a question of terminology, what is the definition of "state" when we refer to "travelling backward in time" ? - which presumably means travelling to some previous "state" of a system in time.

The various popular depictions of travelling backward in time have the time traveller arrive at some state in the past where he is a novelty, or can see his younger self or a deceased relative etc. But if a "state" is a complete description of conditions at a given time, it should include the information that the time traveller is present. So if the traveller travels backward to some state in time where "state" refers to a complete description of things, it is a state where he was present at that time. Of course, we can imagine that in the state of the world at the present moment there are time travellers from the future among us, but this seems outside the scope of the twin paradox.

So, paying attention to the definition of "state", isn't there a distinction between the twin paradox and a scenario where a person would travel backward in time ? Or is the notion of "twin" supposed to invite some limiting argument where person T only differs from his twin by epsilon and we can let epsilon go to zero and argue that T can meet up with himself ?

 

Twin Paradox, photographic proof:
http://www.countryliving.com/life/kids-pets/a39262/this-photo-proves-that-time-travel-is-possible/