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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

What would a universe be like with two time dimensions?

Pretty weird. I do not know the details, but because there are two time dimensions, it is very difficult to avoid creating 'loops' in which one traveler leaves location A and travels to location B and arrives at a specific time in the future, and another traveler making the same journey but arriving at 'B' before he left 'A'. Presumably, we would have the same freedom to move in this second time dimension as we do in a second space dimension, and because closed curves can be created in 2-dimensional spaces, the two time dimensions would allow us to 'move' along paths that violate causality. Every pair of points in space would be threaded by a multitude of such possible curves, and the concept of cause-and-effect would become meaningless.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.