I found myself in need of a rough form for the 
-decay spectrum, so I went and fetched Fermi’s Nuclear Physics from the library. I thought I would share a passage which suddenly made a lot of things go click for me:
According to the relativistic theory of the electron, an electron has energy 
. This equation permits negative energy values.
In Dirac’s theory, practically all negative states are filled at all points in space. A vacuum is then a sea of electrons in negative energy states. The presence of this charge is not observed because it is uniformly distributed.
A photon of sufficiently high energy may lift an electron from a negative energy state. The energy threshold for the photon is 
, since for a free electron there are no states between 
and 
. Physically, this means that the photon must supply enough energy to create two particles of mass 
. Momentum must be conserved and this requires either that the negative energy electron be near a nuclear or an electron, i.e., not free, or that two photons coming from different directions coalesce and lift an electron from a negative energy state. If the electron is near a nucleus it may occupy discrete states just below . These are within a few eV of 510,000eV. Strictly, then, the threshold for pair formation near a nucleus is 
. This is of no importance because binding energy 
and because transitions from negative energy states to the discrete part of the spectrum are improbable and not yet observed.
madhadron :: Apr.24.2008 ::
physics ::
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