Re: Mass and Energy

Posted by Uncle Al on Mar 03, 2004 at 09:42

Re: Mass and Energy (Sam)

"but my question is can you convert energy to mass... can you give me an example, where energy is converted to mass..."

A gamma ray photon with energy exceeding 1.022 MeV grazes a heavy atomic nucleus. One photon goes in, an electron-positron pair emerges. Energy is converted to mass, all conserved physical properties are conserved in the transformation.

There is NO conversion of mass to energy in any of the examples given in textbooks, including chemistry, nuclear fission, and nuclear fusion. The numbers of leptons and baryons before and immediately after the "conversion" are rigorously identical. All you are seeing is the mass equivalent associated with fields being liberated. It starts as energy and it comes out as energy.

Postronium annihalating to two 511 keV photons is the conversion of mass to energy.

It gets even nicer. Imagine accreting the Earth from a dispersed mist of stuff. As the pieces fall down the growing gravitational well they lose potential energy. Energy has mass equivalent. The whole Earth accreted weighs less than its parts fully separated by about 0.46 parts-er-billion. For a homogeneous isotropic spherical ball of mass M and radius R, the gravitational self-energy is

U = -(3/5)(GM2/R)

where "G" is Newton's constant. The sun's gravitational self-energy (missing mass) is about 2.3x1041 joules. Convert to kilograms and compare with the mass of the moon.

Uncle Al
(Do something naughty to physics)

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