192 P H V SIC S OF TIE IMPOSSIBLE
But Feynman then raised an unsettling thought. What happens if
the aliens stick out their left hand instead? This means that we have
made a fatal mistake, that we have failed to communicate the concept
of "left" and "right." Worse, it means that the alien is actually made of
antimatter, and that he performed all the experiments backward, and
hence got "left" and "right" mixed up. It means when we shake hands,
we will explode!
That was our understanding until the 1960s. It was impossible to
tell the difference between our universe and a universe in which
everything was made of antimatter and was parity-reversed. If you
flipped both the parity and the charge, the resulting universe would
obey the laws of physics. Parity by itself was overthrown, but charge
and parity was still a good symmetry of the universe. So a CP-reversed
universe was still possible.
This meant that if we were talking to aliens on the phone, we could
not tell the difference between an ordinary universe and one that was
both parity- and charge-reversed (i.e., left and right are interchanged,
and all matter is turned into antimatter).
Then in 1964 physicists received a second shock: the CP-reversed
universe cannot exist. By analyzing the properties of subatomic particles,
it is still possible to tell the difference between left-right, clockwisecounterclockwise if you are talking by radio to another CP-reversed universe. For this result, James Cronin and Val Fitch won the Nobel Prize in
1980.
(Although many physicists were upset when the CP-reversed universe was shown to be inconsistent with the laws of physics, in hindsight the discovery was a good thing, as we discussed earlier. If the
CP-reversed universe were possible, then the original big bang would
have involved precisely the same amount of matter and antimatter,
and hence 100 percent annihilation would have taken place, and our
atoms would not have been possible! The fact that we exist as a leftover
from the annihilation of unequal amounts of matter and antimatter is
proof of CP violation.)
Are any reversed anti-universes possible? The answer is yes. Even
But Feynman then raised an unsettling thought. What happens if
the aliens stick out their left hand instead? This means that we have
made a fatal mistake, that we have failed to communicate the concept
of "left" and "right." Worse, it means that the alien is actually made of
antimatter, and that he performed all the experiments backward, and
hence got "left" and "right" mixed up. It means when we shake hands,
we will explode!
That was our understanding until the 1960s. It was impossible to
tell the difference between our universe and a universe in which
everything was made of antimatter and was parity-reversed. If you
flipped both the parity and the charge, the resulting universe would
obey the laws of physics. Parity by itself was overthrown, but charge
and parity was still a good symmetry of the universe. So a CP-reversed
universe was still possible.
This meant that if we were talking to aliens on the phone, we could
not tell the difference between an ordinary universe and one that was
both parity- and charge-reversed (i.e., left and right are interchanged,
and all matter is turned into antimatter).
Then in 1964 physicists received a second shock: the CP-reversed
universe cannot exist. By analyzing the properties of subatomic particles,
it is still possible to tell the difference between left-right, clockwisecounterclockwise if you are talking by radio to another CP-reversed universe. For this result, James Cronin and Val Fitch won the Nobel Prize in
1980.
(Although many physicists were upset when the CP-reversed universe was shown to be inconsistent with the laws of physics, in hindsight the discovery was a good thing, as we discussed earlier. If the
CP-reversed universe were possible, then the original big bang would
have involved precisely the same amount of matter and antimatter,
and hence 100 percent annihilation would have taken place, and our
atoms would not have been possible! The fact that we exist as a leftover
from the annihilation of unequal amounts of matter and antimatter is
proof of CP violation.)
Are any reversed anti-universes possible? The answer is yes. Even