I was thinking about time travel, we are traveling in time now at a rate of one second per second but I think it is impossible to travel backwards in time. I honestly think that the past doesn't exist, it isn't a place that you could visit.
If time travel to the past was possible, it would be very difficult and dangerous, at least I think it would be.
Firstly, the position we were in 5 years ago was different to where we are now. The solar system is orbiting a giant Black hole at the center of the Galaxy so the position we are in now was just empty space recently.
Secondly, if you managed to appear in the past in the position of you are now then I think you would be smashed into the surroundings because the Earth's speed is constantly changing because its orbit is slightly elliptical. Whether you are moving faster or slower than the earth the hazard is the same, you would be smashed into the nearest thing that is in your way. Sometimes the earth is closer to the sun and moves faster and other times it is further from the sun and slows down.
The other problem I thought of is that when the Earth is in between the sun and the center of the galaxy it is moving faster then the sun and when it is on the other side six months later it is moving slower because the sun is orbiting the the center of the galaxy and drags the Earth along with it. It is hard to know what speed the Earth is moving at in the past so a doorway to the past would be a death trap for anyone that walks through it.
Ok
Thirdly, you make a space ship that materialises in empty space in the past, that is fine but... When the ship appears it brings energy with it and that energy makes the net energy of the universe increase, now because the time travel becomes a historical event it repeats so you travel back in time and appear in the same place you appeared before. The energy keeps coming and increasing until it causes an explosion.
Finally, If you manage to travel back in time and not destroy yourself and the things around you then you would have to worry about the grandfather paradox and you would alter the events in the time you came from.
Anyway, I don't think it is possible. There would be a lot of evidence of time travel all around us otherwise.
