- #1

SleepDeprived

- 31

- 8

So like the people in the flat land, we are trapped in a three dimension space in a supposedly four dimensional space time universe, and if the theory is correct and if we travel in a "straight line" we will eventually come back to where we started (assuming the universe is close and finite). But a "straight line" in a four dimensional space time is not the same "straight line" like on the surface of the earth. I mean when our spaceship takes off from earth, it will take off in a curved trajectory but even after that our spaceship will be affected by gravity from other celestial bodies the path will not be straight in a three dimensional sense. But you get my point. So the only way to know if our spaceship travels in a four dimensional straight line is the need of having a four dimensional compass to keep track of our path, but to visualize such a thing is a little bit foggy for my mind.

The closest thing I can visualize is shining a light beam into space and wait for it to come back again assuming the universe is close and finite and not expanding faster than the speed of light. We will inevitably use the flat Earth as an analogy, so when I look at the WMAP picture, I assume that any point in the universe, any observe will get the exactly the same picture of the cosmic radiation, just as any person on the Earth flat land will see the same picture.

Someone had said that if you can look far enough, you will eventually see the back of your head. I am still having a hard time able to visualize it though.