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P H V SICS OF THE IMPOSSIBLE
whom the machines have been unable to kill. Frustrated, the machines
send a series of killer robots back to the past, before the great leader
was born, to kill off his mother. But after epic battles, human civilization is eventually destroyed at the end of the movie, as it was meant
to be.
Back to the Future explored the third possibility. Dr. Brown invents
a plutonium-fired DeLorean car, actually a time machine for traveling
to the past. Michael J. Fox (Marty McFly) enters the machine and goes
back and meets his teenage mother, who then falls in love with him.
This poses a sticky problem. If Marty McFly's teenage mother spurns
his future father, then they never would have married, and Michael J.
Fox's character would never have been born.
The problem is clarified a bit by Doc Brown. He goes to the blackboard and draws a horizontal line, representing the time line of our
universe. Then he draws a second line, which branches off the first
line, representing a parallel universe that opens up when you change
the past. Thus, whenever we go back into the river of time, the river
forks into two rivers, and one time line becomes two time lines, or
what is called the "many worlds" approach, which we will discuss in
the next chapter.
This means that all time travel paradoxes can be solved. If you
have killed your parents before you were born, it simply means you
have killed some people who are genetically identical to your parents,
with the same memories and personalities, but they are not your true
parents.
The "many worlds" idea solves at least one main problem with
time travel. To a physicist, the number one criticism of time travel (besides finding negative energy) is that radiation effects will build up until either you are killed the instant you enter the machine or the
wormhole collapses on you. Radiation effects build up because any radiation entering the time portal will be sent back into the past, where
it will eventually wander around the universe until it reaches the present day, and then it will fall into the wormhole again. Since radiation
can enter the mouth of the wormhole an infinite number of times, the
P H V SICS OF THE IMPOSSIBLE
whom the machines have been unable to kill. Frustrated, the machines
send a series of killer robots back to the past, before the great leader
was born, to kill off his mother. But after epic battles, human civilization is eventually destroyed at the end of the movie, as it was meant
to be.
Back to the Future explored the third possibility. Dr. Brown invents
a plutonium-fired DeLorean car, actually a time machine for traveling
to the past. Michael J. Fox (Marty McFly) enters the machine and goes
back and meets his teenage mother, who then falls in love with him.
This poses a sticky problem. If Marty McFly's teenage mother spurns
his future father, then they never would have married, and Michael J.
Fox's character would never have been born.
The problem is clarified a bit by Doc Brown. He goes to the blackboard and draws a horizontal line, representing the time line of our
universe. Then he draws a second line, which branches off the first
line, representing a parallel universe that opens up when you change
the past. Thus, whenever we go back into the river of time, the river
forks into two rivers, and one time line becomes two time lines, or
what is called the "many worlds" approach, which we will discuss in
the next chapter.
This means that all time travel paradoxes can be solved. If you
have killed your parents before you were born, it simply means you
have killed some people who are genetically identical to your parents,
with the same memories and personalities, but they are not your true
parents.
The "many worlds" idea solves at least one main problem with
time travel. To a physicist, the number one criticism of time travel (besides finding negative energy) is that radiation effects will build up until either you are killed the instant you enter the machine or the
wormhole collapses on you. Radiation effects build up because any radiation entering the time portal will be sent back into the past, where
it will eventually wander around the universe until it reaches the present day, and then it will fall into the wormhole again. Since radiation
can enter the mouth of the wormhole an infinite number of times, the