May 25, 2004

A Small Universe

So Jem lives in this universe which is one light year across. This means that, if we shoot a beam of light in the right direction it will hit us one year later. In this universe, space wraps around on itself along certain directions. It has no edges but simply comes back in on itself. The easiest way to think about it is walking on the surface of the Earth. If we travel for long enough along a great circle we will end up where we started. Except in the case of the universe it is not curved like the surface of the Earth.
Now suppose Jem looks in the direction in which the universe is wrapped around itself. He will see light coming towards him from very far sources. From stuff which is close by but also from things that are quite far off. In particular he may, with the use of a telescope, see stuff which is a light year away. Including himself, as he was a year earlier.
He should also be able to see himself as he was two years earlier. Because some light may make the journey twice around the wrapped directions if there is nothing to stop it. Or even, three, four, five and so on, years earlier. The light ray just keeps on going until something (like our eyes, or a telescope) stops it in its path. Except there is one small problem. The moment he sees the light, he stops it in its path. Which means that he will see the light that he has emitted a year earlier, but he won’t see any images emitted in the previous years because he, as he was a year earlier, is blocking their path.
How can he see multiple images instead of just seeing the latest one? Imagine Jem holding out a torch to his side, with his arm slightly bent upwards. He will see the image of himself doing this at the distance of a light year. He will see the point of light slightly displaced from his head. From trigonometry one can work out that the angle is very roughly the distance at which he holds the torch away from himself divided by the distance to the copy of himself, one light year away. But he will also be able to see the torch which is two light years away. In this case the displacement angle will be half as big because one has to divide the distance he holds the torch away from his body by two light years. He will also be able to see the torch as it was three years before. Its displacement angle will be a third of the angle of the closest image. And so on. In fact he will see a series of torches getting closer and closer to his head. The closer the images are to the image of his head, the further away they are from him. By holding out the torch he has been able to avoid block off the light rays,
Is it like a hall of mirrors? The effect is very similar except in a hall of mirrors, all the odd images are reflections while the even image aren’t. Jem will see an alternating series of torches, with alternate sides of the torch as he looks deeper and deeper.

Posted by Pedro Ferreira at May 25, 2004 10:54 AM