Why physicists never say: ‘potential mass/energy E=Mc^2’ ?
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In the early decades after Einstein introduced his SRT, the standard
practice was to make a distinction between rest mass and relativistic
mass, and many teacher who learned from old texts and older
professors have not yet broken that habit.
Among scientists who deal with relativity in practical situations
daily, no such distinction is made. Mass is just mass; the mass
of a particle increases as it is accelerated, and there is no need
to distinguish between the rest mass and the total mass.

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SRT is Special Relativity Theory, but I agree with billsuss'
premise It seems that some people really don't like to make
themselves clear when asking a question. This question
is a perfect example of that.
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Because they've both potential, both kinetic energy on their mind at the same moment? I don't see any problem here...

Why physicists never say: ‘potential mass/energy E=Mc^2’?
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Comment:
Because they've both potential, both kinetic energy
on their mind at the same moment?
I don't see any problem here...
/ Zephir./
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My opinion.
The problem is :
In Classic physics there are ‘ potential’ and ‘ kinetic’ energy.
And „The Law of Conservation and Transformation of Energy/ Mass"
connects them together.
In Quantum physics the ’rest mass/enegy’ looks as a static particle.
As you say: “ Because they've both potential, both kinetic energy
on their mind at the same moment? “
And you say: ‘I don't see any problem here... ‘.
I think that there is problem here.
Because in QT there are ’rest mass/enegy E=Mc^2’ and
active / kinetic energy E=hf. But how „The Law of
Conservation and Transformation of Energy/ Mass"
can unite them together nobody explains.
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Quantum physics doesn't "recognize" the concept of "static particle". In quantum physics the (behavior of) particle is described by its wave function, which is time dependent, i.e. it's never at rest.



The above animation illustrates the particle, trapped inside of potential hole. It's not quite statical, does it?

If you think that your picture shows the ’rest particle’
when you are mistecan.

Every atom have hidding potential energy E=Mc^2.
Your picture shows how this potential energy E=Mc^2
become free.
This is one-act, and not endless, as in your picture.

This is a rigorous Schrodinger wave equation simulation - a pure quantum mechanics without any interpretations, could be said. You can play with it on Java applet herein.