In time travel...?

Question

...why do you have to go extremely fast to experience time travel? If ppl from the future time traveled to the past, wouldn't they change the time line?

Answer 1

You mistake time dilation with time travel.

time dilation means, that the time inside your spacecraft runs slower when measured by a "stationary" observer.

It is not time travel at all.

Answer 2

There is a problem with time travel, not to mention the many paradoxes which would happen, that is called the Spacial Displacement Problem. Let's say you want to travel into the future six months. Six months from now the earth will be on the other side of the sun in its orbit. It's possible to compute exactly where the earth will be in its orbit around the sun six months from now and, by the way, your time machine must also be a spacecraft. Now, here's the rub: not only will the earth be in a different position in orbit, but also the sun has moved in its orbit around the Milky Way galaxy. Not only that, but the Milky Way galaxy has moved through the universe. Since there is no point of reference for a galaxy in the universe except another galaxy, oh, yeah, and that galgaxy moves, too, there is no way to compute where you should be in the universe at the time you want to travel to.

Answer 3

Time dilation is time travel - into the future.  How else would you experience the year 2500.  But going fast won't get you to the past. Unless you're Superman, i guess.

There are hints that time travel to the past may be possible.  It will require manipulating masses on the order of the size of the sun.  It may be so unstable that you can only go back once.  And you can only go back to the time that the machine was built.  From there, though, could the time line be changed?  No idea.  If you do build a time machine of this type, you might expect that the first thing that happens is that something comes out of it, from the future.