A questioner on ABC's Q&A asks whether it is possible for a person who invents a time machine in the future to travel back in time and show us how to make one. Panelists Nalini Joshi and Professor Brian Cox give their views.
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Brian Cox says you can't travel into the past, but Nalini Joshi disagrees.
"Well, I don't know about whether there's an arrow that prevents you from going to the past or only into the future but, mathematically, I think that there are many, many possible answers to these questions. If I may? You know, if you take just a three dimensional cube and you look at one corner of this three-dimensional cube and then you compress one corner down to the diagonally opposite corner, then what you do is you flatten the cube and it becomes a hexagon. If you take that structure, which is hexagonally symmetric, it turns out that these types of symmetries are in crystals and there are a very knew few number of these regular symmetries that are exhibited in crystals in two dimensions. Now, if you think about increasing the number of dimensions, now go to three, four, five, six, presto, around 7 and 8 there are exceptional new structures that come up, that only exist when you get to 7 and 8 and maybe 9 dimensions."