I've been wanting to make this post ever since I first saw this video on YouTube. Starting at 3:55, Brian Greene begins to explain how in special relativity, there is no absolute simultaneity. Motion affects what is "now" from your point of view.
The part that jumped out at me the most begins at 5:30 where he starts talking about an alien who lives in a galaxy 10 billion light years from earth. If the alien and some guy sitting on a bench are not moving relative to each other, they may share the same "now." But if the alien starts bicylcling away from earth at a slow pace, they will no longer share the same "now." The alien's "now" will actually be many years in the bench guy's past. And if the alien bikes in the opposite direction, his "now" will be in the man's future.
I put off making this post beause I wanted to learn the math behind it first so I could better understand it and explain it. But alas, I haven't learned the math, and I don't want to put this off anymore because it's so interesting to me.
Anywho, if you think about that, it means that it's not only possible to move forward and backward in time, but it's actual that we move forward and backward in time just by changing directions. If the alien bikes away from the man, then his "now" includes Beethoven and everything else going on 200 years ago. And if he bikes toward the man, then his "now" includes what will be going on 200 years from now.
If the alien moved faster or if he were farther away, the difference would be even bigger. So as the alien bikes away from us, toward us, and away from us again, his "now" could include the earth year 1800, then 2200, then 1800 again. Since events in our past and future would be his "now," then he is actually moving backward, forward, and backward again in time.
Likewise, as we move in one direction, then in another direction, then back again, there may be a planet 10 billion years away that has something resembling dinosaurs on it "right now," then people "right now," then dinosaurs again "right now." As we move back and forth relative to that distant planet, we can live at various moments in that planets history that can span hundreds, thousands, and maybe millions of years, depending on how fast we are moving.
I wish I knew the math so I can do the calculations and figure out how fast we'd have to be moving relative to each other to span different lengths of time in the past and future. I could do calculations on planets at different distances, including distances beyond the event horizon. Since the universe is expanding, I guess we're already moving away from each other, but we can still move back and forth in time since we can change the relative motion between us.
Isn't that crazy? If anybody knows the math and can explain it to me, please leave a comment. I suspect it has something to do with Lorenz transformations. I've seen various videos on that, but not applied to the specific scenario Brian Greene explains in that video.
