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Engineering Newsletter

Timothy van der Linden writes:
 > ReplyTo   : Steve
 > ReplyFrom : Timothy
 > Subject   : Lost letter?
 > Steve, I haven't heard it from you, yet: Do we agree that at
 > least between us the word relativistic mass is merely an other
 > word for relativistic energy?

What you call "relativistic energy" or "relativistic mass" I call
energy.  I reserve the term mass for invariant mass.  I believe
this avoids confusion.

 > Also in my letter of november 29th, I replied to you:
 > >Yes, this is also true.  Hot objects are heavier than cold ones
 > >(although not by an amount we have equipment to measure).  A
 > >mirrored box full of photons is heavier than the empty box.
 > And it doesn't matter if they all move parallel and in the same direction
 > all the time, right?

Even if the photons all move parallel in the same direction, the
combination of the photons and the box is more massive than the
box alone, even though the photons alone have no mass.

 > Here is another one: If two particles feel the gravity of each other, then
 > they are heavier together than if they are separate because of the extra
 > gravitational energy.

I do not have the knowledge to answer this.