HHTIMRTTER AN 1 1 ANTI-UNIVERSES 193
if parity-reversed and charge-reversed universes are not possible, an
anti-universe is still possible, but it would be a strange one. If we reversed the charges, the parity, and the march of time, then the resulting universe would obey all the laws of physics. The CPT-reversed
universe is allowed.
Time reversal is a bizarre symmetry. In a T-reversed universe,
fried eggs jump off the dinner plate, reform on the frying pan, and then
jump back into the egg, sealing the cracks. Corpses rise from the dead,
get younger, turn into babies, and then jump into their mother's womb.
Common sense tells us that the T-reversed universe is not possible.
But the mathematical equations of subatomic particles tell us otherwise. Newton's laws run perfectly well backward or forward. Imagine
videotaping a billiard game. Each collision of the balls obeys Newton's
laws of motion; running such a videotape would make for a bizarre
game, but it is allowed by the laws of Newton.
In the quantum theory things are more complicated. T-reversal by
itself violates the laws of quantum mechanics, but the full CPT-reversed universe is allowed. This means that a universe in which left
and right are reversed, matter turns into antimatter, and time runs
backward is a fully acceptable universe obeying the laws of physics!
(Ironically, we cannot communicate with such a CPT-reversed
world. If time runs backward on their planet, it means that everything
we tell them by radio will be part of their future, so they would forget
everything we told them as soon as we spoke to them. So even though
the CPT-reversed universe is allowed under the laws of physics, we
cannot talk to any CPT-reversed alien by radio.)
In summary, antimatter engines may give us a realistic possibility for
fueling a starship in the distant future, if enough antimatter could be
made on Earth, or found in outer space. There is a slight imbalance between matter and antimatter because of CP violation, and this in turn
may mean that pockets of antimatter still exist and can be harvested.
But because of the technical difficulties involved in antimatter en-
if parity-reversed and charge-reversed universes are not possible, an
anti-universe is still possible, but it would be a strange one. If we reversed the charges, the parity, and the march of time, then the resulting universe would obey all the laws of physics. The CPT-reversed
universe is allowed.
Time reversal is a bizarre symmetry. In a T-reversed universe,
fried eggs jump off the dinner plate, reform on the frying pan, and then
jump back into the egg, sealing the cracks. Corpses rise from the dead,
get younger, turn into babies, and then jump into their mother's womb.
Common sense tells us that the T-reversed universe is not possible.
But the mathematical equations of subatomic particles tell us otherwise. Newton's laws run perfectly well backward or forward. Imagine
videotaping a billiard game. Each collision of the balls obeys Newton's
laws of motion; running such a videotape would make for a bizarre
game, but it is allowed by the laws of Newton.
In the quantum theory things are more complicated. T-reversal by
itself violates the laws of quantum mechanics, but the full CPT-reversed universe is allowed. This means that a universe in which left
and right are reversed, matter turns into antimatter, and time runs
backward is a fully acceptable universe obeying the laws of physics!
(Ironically, we cannot communicate with such a CPT-reversed
world. If time runs backward on their planet, it means that everything
we tell them by radio will be part of their future, so they would forget
everything we told them as soon as we spoke to them. So even though
the CPT-reversed universe is allowed under the laws of physics, we
cannot talk to any CPT-reversed alien by radio.)
In summary, antimatter engines may give us a realistic possibility for
fueling a starship in the distant future, if enough antimatter could be
made on Earth, or found in outer space. There is a slight imbalance between matter and antimatter because of CP violation, and this in turn
may mean that pockets of antimatter still exist and can be harvested.
But because of the technical difficulties involved in antimatter en-