Converting mass entirely into energy

[g.lemaitre]

In The First Three Minutes Weinberg wrote:

When I speak of a given energy
as being equivalent to a given mass, I mean of course that this is the energy that would be released according to the Einstein formula E=mc^2, if the mass were converted entirely to energy.

When he is says mass converted entirely into energy does he mean the massive particles get converted into new particles? or does mass literally disappear and get converted into some magical etherial, ineffable substance, known as pure energy. I'm still confused as to what pure energy is. I think of it as something tantamount to nothing but it can be made in massive particles which move and can perform work and all that with some bit of magic.

 

[Dale]

Usually they are referring to a matter antimatter annihilation reaction, where a particle and it's anti particle combine to form a couple of photons. It is kind of a sloppy way of talking which is usually reserved for pop-science type books.

The equation E=mc^2 isn't about conversions of mass to energy and vice versa. It simply states that all energy has a little mass and all mass has a lot of energy. Even in an annihilation reaction, the resulting system of photons has all of the original mass (the mass of the system is greater than the sum of the masses of the constituents) and all of the original energy (and all of the original momentum, although that is being set to 0 for this equation).

 

[g.lemaitre]

so when he says all of the mass being converted to energy that would be a complete conversion of protons neutrons into photons, neutrinos, electrons and positrons?

 

[harrylin]

so when he says all of the mass being converted to energy that would be a complete conversion of protons neutrons into photons, neutrinos, electrons and positrons?

Certainly not: electrons have rest mass.
He must have meant if all of the contained energy is transformed into radiation energy (photons).
Thus the transformation of matter into radiation, as Dalespam rather well elaborated.

Note also the precise formulation by Einstein in his first paper on that topic:

"If a body gives off the energy L in the form of radiation, its mass diminishes by L/c²."
-http://www.fourmilab.ch/etexts/einstein/E_mc2/www/

 

[jtbell]

some magical etherial, ineffable substance, known as pure energy. I'm still confused as to what pure energy is.

It doesn't exist. Energy is always a property of something. In the case of electron/positron annihilation, for example, the electron and positron start out with energy due to their motion (kinetic energy) and energy due to their masses (rest-energy). All of that energy gets converted into the energies of the outgoing photons.