|MadSci Network: Physics|
If you really manage to overcome the problem of getting a beam into a closed loop (closed means closed to getting out as well as closed to getting in!), it would go around a few times, weakening at every reflection (real mirrors are imperfect and impose losses) and through interaction with athmospheric particles, until you cannot see it any more. This takes a few milliseconds, at most. With (theoretical) perfect mirrors and in perfect vacuum, the beam would circle forever. Greetings from Indonesia, Frank Berauer
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