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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Does 'now' have a size instead of being a point in spacetime?

The concept of 'now' is rather well defined in classical physics, but as we have seen in a previous question, Heisenberg's Uncertainty Principle prevents us from assigning BOTH a specific place and time to any event in spacetime. All events are in actuality, small fuzzy regions rather than mathematical points with zero size. This means that 'now' has a size, and this size is determined by the condition that DE x DT < h, or DM x Dx < h. If you tried to specify exactly when something occurred, you would have to give up a precise knowledge of where the particle was when it was in a state defined by a particular energy and momentum, at the time you observed it, so that the concept of 'now' at a specific spatial point in spacetime becomes less well defined in size.

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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.