Virtual Pair Production
Recall Heisenberg's Uncertainty Principle. It basically puts a limit on how
much we can reduce the disturbance we introduce in a system by doing a
measurement on it. There are a number of forms of the principle, and here we
shall use only one of them:
The uncertainty in any measurement of the energy of an object times the
uncertainty in when the object had that energy will always be at least equal to
a universal constant.
Technical note: The universal constant is Planck's constant h divided
by 2 pi.
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A moment's reflection on the implications of this form of the
Uncertainty Principle may convince you that this means that the energy
does not even have a definite value but only a lower and upper
bound.
Thus the principle of conservation of energy can be violated so long as
the violation occurs for only a brief period of time.
Now consider Dirac's infinite sea of negative energy electrons. One of
those electrons can violate conservation of energy by spontaneously
jumping into a positive energy state provided it falls back into the hole
quickly enough. You will recall that we interpret the hole in the sea as a
positron. Thus, we believe that this virtual pair production is
occurring everywhere in the universe. The pair can only exist for a time
of about 10-35 seconds, i.e. 34 zeroes followed by a 1 to the
right of the decimal point; this is called the Planck time. |
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Similarly we believe virtual pairs of proton-antiprotons,
neutron-antineutrons etc. are continually being formed and disappearing
everywhere in the universe. Wheeler, then, characterises the vacuum at a scale
of very small distances as being quantum foam.
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