With regard to adding "energy", it is important to realize that there
is really no such thing as "pure energy". Energy is an accounting
convention, representing the ability to do work.
There are many forms of energy: heat energy (random molecular motion),
kinetic energy (organized motion), potential energy (due to gravitational,
electric, or other fields), radiation energy (carried by photons),
and so on.
Most people are assuming that photons are "pure energy". It is true that
photons have no rest mass, but I think it is better to think of them
as particles that carry energy. An ordinary particle carries kinetic
energy as well, unless its velocity is absolutely zero (which will never
happen exactly). A photon also can carry different amounts of energy,
arbitrarily close to zero.
The energy carried by a photon is not invariant, but depends on the
motion of the observer. Move towards the photon and its measured
energy increases (blue shift), move away and it decreases (red shift).
The same effect is seen with kinetic energy carried by a massive particle.
When people were talking earlier about "becoming beings of pure energy"
that is really much more ambiguous and less meaningful than might be
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