Why have we never seen anyone from the future? Because you cannot go back in time.
Going forward in time is not even time traveling, it's simply just "freezing" yourself or slowing down time so that less time has passed for yourself , so you basically end up in the future
You can not go forward or back in time because it does not exist. It's simply a manmade construct.
You as a human are constrained by the universe you are in and the observable laws it contains.
You yourself could not go back to a different point in the lifespan of the universe but perhaps if there was someway to move the universe itself or step outside it then maybe you're getting somewhere but still a long way from what you want.
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Sorry, I had to.
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You can make time pass slower for yourself with time dilation. All within the laws of the universe, and there's evidence to show that it's possible.
Time is just the name we use as a tl;dr for the law of thermodynamics.
"In order to maintain a tolerant society, the society must be intolerant of intolerance." Paradox of tolerance
I alter time constantly, and you can't prove that I dont!
It is by caffeine alone I set my mind in motion. It is by the beans of Java that thoughts acquire speed, the hands acquire shakes, the shakes become a warning.
-Kujako-
You should probably take a peek at Sean Carroll's Rules for Time Travelers. (And if you have an hour to kill, there's an in-depth lecture which explains it all in detail.)
While we're on the subject of time travel, john titor http://www.johntitor.strategicbrains.com/ is probably the most convincing one to date that I know of.
We don't yet know nearly enough about the complexity of time to assume it wouldn't be possible traveling in time. I don't see a problem traveling forward in time, however traveling backward in time seems disputable.
Timelines is such a cheap explanation to a real paradox, doubt the universe will create a whole new universe simply because you spit that gum on a different place.
Either way, even by those standards yes, it is impossible to change anything as you alter a different one and maybe return to a different one, the original you were trying to change is unchanged.
I just simply don't believe time travel will ever exist. Call me ignorant but the way I see it, time isn't something that can be altered in any way, rewound or sped up.
'Twas a cutlass swipe or an ounce of lead
Or a yawing hole in a battered head
And the scuppers clogged with rotting red
And there they lay I damn me eyes
All lookouts clapped on Paradise
All souls bound just contrarywise, yo ho ho and a bottle of rum!
Can be slown down by in my view it has to do with atomic interaction that slow down entropy, not an actual fabric of time...
That is not just an idea, it does happen and you have to account for that with gps satelites, there is a time difference.
But i too share the idea that one cannot travel in time, specially backwards
If them dying was the reason you made the time machine to go in time and stop them from dying, then yes, there is a paradox, since if they never died you would never have made the time machine.
If time travel already existed through some other means, you could go back and stop them from dying, which created a new time line in which they survive, and the old time line where they died never happens.
My name is Cernunnos, I will love you like no other, I have died a thousand deaths, each time I died I thought of you.