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Mass and Energy

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"Paul.B.Andersen" <relativity@paulba.no> wrote or quoted:
:that mass could be converted to energy as
:predicted by Einstein's E = mc².

  I can spot three mistakes here right off the bat.

       First, the phrase "mass could be converted to energy" is based on
  a misconception, since mass already /is/ a form of energy - there's no
  "conversion" happening. It's like saying, "water can be turned into a
  liquid" - no, water /is/ a liquid.

       Second, "E=mc^2" only applies to systems at rest. In general,
  it's "E^2=(mc^2)^2+(pc)^2", where "p" is the system's momentum. 
  (That basically says mass is the magnitude of the four-momentum.)

       Third, mass is conserved in nuclear fission. This point is often
  explained incorrectly, even in otherwise solid textbooks like Grif-
  fiths.

       I've laid out the full argument elsewhere, but here's the (suffi-
  cient) short version:

       Take a nucleus that's at rest, so its total momentum p=0.

       After fission, because momentum is conserved, the total momentum
  of all the fragments is still zero. That means the relation "E^2=(mc^2
  )^2+(pc)^2" simplifies to "E=mc^2" both before and after the split.

       Since energy is also conserved, the total energy E of the nucleus
  and its fragments - including their kinetic energy - stays the same,
  call it E. From "E=mc^2", it immediately follows: if E stays constant,
  m does too.

E_"before" = E_"after" - due to conservation of energy        (0)

E_"before" = m_"before" c^2 - as you said yourself            (1)

E_"after" = m_"after" c^2 - the same as (1), just later       (2)

m_"after" = m_"before" - substituting (1) and (2) into (0)    (3)
                         and dividing by c^2

  Only when you change what you call the "system", and treat the split 
  products separately, do you get systems with nonzero momentum and 
  find a mass defect and non-zero kinetic energies.

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Thread

Mass and Energy ram@zedat.fu-berlin.de (Stefan Ram) - 2026-01-01 16:19 +0000
  Re: Mass and Energy nospam@de-ster.demon.nl (J. J. Lodder) - 2026-01-01 22:38 +0100
  Re: Mass and Energy x <x@x.net> - 2026-01-02 10:03 -0800
  Re: Mass and Energy x <x@x.net> - 2026-01-02 10:06 -0800
    Re: Mass and Energy Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2026-01-02 21:07 +0100
      Re: Mass and Energy x <x@x.net> - 2026-01-02 15:40 -0800
      Re: Mass and Energy x <x@x.net> - 2026-01-02 16:05 -0800
        Re: Mass and Energy Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2026-01-03 14:30 +0100
      Re: Mass and Energy Thomas Heger <ttt_heg@web.de> - 2026-01-04 09:06 +0100
        Re: Mass and Energy nospam@de-ster.demon.nl (J. J. Lodder) - 2026-01-04 12:28 +0100
          Re: Mass and Energy Thomas Heger <ttt_heg@web.de> - 2026-01-06 09:03 +0100
            Re: Mass and Energy nospam@de-ster.demon.nl (J. J. Lodder) - 2026-01-06 10:56 +0100
              Re: Mass and Energy Ross Finlayson <ross.a.finlayson@gmail.com> - 2026-01-06 06:52 -0800
                Re: Mass and Energy nospam@de-ster.demon.nl (J. J. Lodder) - 2026-01-06 16:12 +0100
  Re: Mass and Energy "Paul.B.Andersen" <relativity@paulba.no> - 2026-01-04 20:51 +0100
    Re: Mass and Energy Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2026-01-06 03:04 +0100
      Re: Mass and Energy "Paul.B.Andersen" <relativity@paulba.no> - 2026-01-06 22:12 +0100
        Re: Mass and Energy "Paul.B.Andersen" <relativity@paulba.no> - 2026-01-06 22:27 +0100
        Re: Mass and Energy Ross Finlayson <ross.a.finlayson@gmail.com> - 2026-01-06 13:43 -0800
          Re: Mass and Energy Python <python@cccp.invalid> - 2026-01-06 21:46 +0000
            Re: Mass and Energy Ross Finlayson <ross.a.finlayson@gmail.com> - 2026-01-07 04:32 -0800
        Re: Mass and Energy Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2026-01-06 23:27 +0100
          Re: Mass and Energy Ross Finlayson <ross.a.finlayson@gmail.com> - 2026-01-07 04:14 -0800
          Re: Mass and Energy "Paul B. Andersen" <relativity@paulba.no> - 2026-01-07 22:18 +0100
    Re: Mass and Energy Thomas Heger <ttt_heg@web.de> - 2026-01-06 09:15 +0100
      Re: Mass and Energy "Paul.B.Andersen" <relativity@paulba.no> - 2026-01-06 22:57 +0100
        Re: Mass and Energy Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2026-01-06 23:51 +0100
          Re: Mass and Energy Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2026-01-07 05:12 +0100
            Re: Mass and Energy Maciej Woźniak <mlwozniak@wp.pl> - 2026-01-07 08:16 +0100
        Re: Mass and Energy "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-01-07 13:20 -0800
          Re: Mass and Energy "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-01-07 13:29 -0800
          Re: Mass and Energy Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2026-01-07 22:53 +0100

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