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===== Massless Particles =====

(Lecture: 5 minutes)

See (the end of) (ss)[[gsr>book:gsr:energy|9.4]] of [[gsr>|the text]].

==== Reflections ====

This mini-lecture is really a continuation of the
[[swbq:srsw:rfswenergy|preceding SWBQ]], and represents the
limiting case of the hyperbolic triangle relating energy, momentum,
and rest mass when the hypotenuse becomes lightlike.

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