Special & General Relativity Questions and Answers
Can loops in time exist in nature?
We do not think so. Such loops would violate 'causality' and allow effects to come before their causes. All that we know about the mechanics of the universe at scales larger than the quantum scale, show that no present phenomena require closed loops to be required. The universe appears to obey causality very accurately.
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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.