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LIMITATIONS OF GAUGE INVARIANCE

LIMITATIONS OF GAUGE INVARIANCE

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By hypothesis, the laser pulse is finite, so<br />

H<br />

t<br />

H 0<br />

lim<br />

0 <br />

t<br />

We can organize the two complete sets of states and states so that they<br />

correspond at t - :<br />

lim<br />

t<br />

<br />

t<br />

t<br />

0<br />

n<br />

After the laser interaction has occurred, the only way for the laboratory instruments to<br />

discover what has happened is to form overlaps of all possible final f states with the<br />

state that began as a particular i state. This is the S matrix:<br />

S<br />

fi<br />

<br />

lim<br />

t<br />

<br />

<br />

n<br />

f<br />

, <br />

Subtract the amplitude that no transition has occurred:<br />

i<br />

<br />

M<br />

fi<br />

<br />

S<br />

1<br />

lim <br />

, <br />

lim <br />

, <br />

fi<br />

t<br />

f<br />

i<br />

t<br />

f<br />

i<br />

45

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