# What if

1. Aug 4, 2005

### saretesotu

what if you cuold go back in time?

2. Aug 5, 2005

### Sempiternity

If you could go back into time, then what comes after would fall either under general relativity or quantum mechanics.

In general relativity, you would fulfill the past. That is, anything that is known to have happened will continue to have happened and you will not be able to change anything that is known to have happened. You would only be able to fulfill the past, not change it. For example, if you went back to visit your ten-year-old self, you yourself would have a memory of meeting a stranger claiming to be you of the future when you were ten. If you do not have such a memory, then you will know that when you go back to the past, you will not meet your ten-year-old self and not claim to be of the future. You couldn't kill your grandfather when he was born because you know your grandfather was alive long enough to bear children, and your timeline doesn't follow a line where he was killed before you were born. There would be something which would prevent you from doing so. This is due to conservation of world-lines in general relativity.

In quantum mechanics, when you go back in time, the timeline splits into two separate timelines. One timeline would contain everything you remember, and the other timeline would contain things you do not remember. For example, you know that you did not meet a stranger that claimed to be your future self when you were ten. Though, when you go back to the past, quantum mechanics says that you can freely go and visit your ten-year-old self and claim that you are of the future, even so you don't remember such an event. The reason is that the timelines split. You could kill your grandfather at his birth, but being that it's a split timeline, it wouldn't affect your own timeline.

These two views, however, are as of yet incomplete because both physical systems don't fit together. To get a better idea of what would happen, a theory of everything will have to be used.

3. Aug 5, 2005

### Pengwuino

Sounds like something out of the Outer Limits....