If the flex capacitor has been invented, where's Marty Mcfly?

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Because if they made their presence known they would change history and so change the time line.

When Marty McFly went back in time he changed the time line and he changed history. He turned his dad from a nerd into someone cool and he made his neighbor into a lackey instead of a threat to his father. When Marty went back in time he didn't have his own car, when he came back to the future he got a cool new monster truck. He is no longer the same Marty McFly that went back in time.

There was one time line, when Marty McFly went back in time he changed the time line and started a new one, for him a better one. It could be that if time travelers came back in time then they would change the time line and laws would be passed to make it illegal to research time travel so the time traveler could not travel back in time. Therefore by going back in time and showing himself as a time traveler he assures that he no longer exists. Since he can't exist he can't go back in time to tell people about time travel so the secret is kept.

It is more likely that no one has gone back in time yet, and that those who have don’t want to change their past so they make sure no one knows that they are traveling back in time.

I don’t claim to understand the physics, but according to Steven Hawking time travel, back in time is possible. Provided you have a super heavy object like a black hole. The problem is that you have to go so close to the black hole to use the time travel method that it may be impossible to go that close and survive. If that is true then time travel may be theoretically possible, but actually physically impossible. We don’t know, we just know that with the science we have you can’t travel back in time. We can imagine a situation where you could, but we lack the technology to do it, and we may never have the technology to be able to do it. Just like we may never develop a method of traveling faster than the speed of light.

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it's a load of crap to me.
i don't even think it's possible to do that.

it'd be super cool if they could though.
but.. if it was possible, don't you think the whole world would know?

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