Three Roads To Quantum Gravity

HOW TO COUNT SPACE
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®lings. However, there is a situation in which the ®eld lines
really are discrete. If you pass a magnetic ®eld through a
superconductor, the magnetic ®eld breaks up into discrete
®eld lines, each of which carries a fundamental unit of
magnetic ¯ux (Figure 20). Experiments show that the amount
of magnetic ¯ux passing through a superconductor is always
an integer multiple of this fundamental unit.
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FIGURE 19
Field lines between two poles of an ordinary magnet, in air.
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Ordinary magnetic field lines
Quantized magnetic field lines
in a superconductor
FIGURE 20
The magnetic ®eld of a superconductor breaks up into discrete ¯ux lines,
each carrying a certain minimum amount of the ®eld.