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A reminder that the energy of a
system is a very abstract idea! By definition, the value of an
energy is always arbitrary up to an additive constant, and the
same object observed from different inertial frames obviously
has a totally different kinetic energy in each frame. This is
vitally important in particle physics, where the largest
possible energy needs to be brought into the system of two
colliding particles, in order to find new things. As a result of
the way energy transforms relativistically, high energy
experiments MUST be done in the center-of-momentum frame, which
requires two colliding beams instead of a beam incident on a
fixed target. Otherwise, almost all the beam energy
provided by a particle accelerator is lost. [See pages 124
- 5 in our text.]
