I've read it somewhere saying that even "do nothing just watch" would change the past. It doesn't obey the physical law because the person who travels backwards in time absorbs light and thus is interacting with the past universe and therefore changing it.

I don't quite understand this. Could someone explain it? Thanks!

(There is only one single timeline of course. I'm not going into a parallel universe)

This is how time travel in my question works:

You go back to a certain point in time, physically, regardless of means. Be it a spaceship or a time machine.

So you've reached to the past. You know certain big things are going to happen because you are from the future, you've had your history class.

You've decided not to do anything about it, just being an observer. You just eat and sleep everyday.

Will this kind of behavior change the past?

If so, is it because daily activities still involve interaction with people in the past and thus triggers butterfly effect?

Also, I've read it somewhere saying that even "do nothing just watch" would change the past. It doesn't obey the physical law because the person who travels backwards in time absorbs light and thus is interacting with the past universe and therefore changing it.

I don't quite understand this whole "the person who travels backwards in time absorbs light and thus is interacting with the past universe" thing. What light is this person absorbing? Why is this person absorbing light? Could someone explain it? Thanks!

(There is only one single timeline of course. I'm not going into a parallel universe)