It is well-known that time travel into the past, and sometimes also into the future, if it were possible, could give rise to destructive paradoxes. In a previous post I offered a list of five different types of these paradoxes. Here I will explain in more detail the fourth type I mentioned there: the fact that time travel into the past and human freedom are incompatible. I will do so through a short science fiction story, divided into two scenarios.
First scenario
At 3:55 PM, my friend Max said to me, “I just
invented a time machine. Do you want to see it?” Of course, I agreed.
At 3:58 PM, Max and I entered the room where the machine was located. It looked like a simple metal chair. The machinery seemed to be under the seat.
At 3:59 PM, as Max and I watched the machine from
the doorway of the room, a second machine appeared beside it, an exact copy of
the first. Sitting on the second machine was a man, an exact copy of Max. Max
and I stared at him in amazement.
At 4:00 PM, my friend Max, who had entered with me,
sat on the first machine, pressed a control, and disappeared into the future.
The copy of Max, sitting in the copy of the machine, watched what he did.
At 4:01 PM, the copy of Max pressed a control on
the copy of the machine and disappeared into the future. Neither Max nor the
two machines ever returned.
After thinking about it, I understood what had
happened. At 4:00 PM, Max traveled in his time machine into the future, but
upon arriving, he reversed his journey and went back, arriving at 3:59 PM. Two
minutes later, after watching the original Max depart into the future, he
departed again. He never returned.
In the figure, the blue line is Max. The red line
is the time machine.
Second scenario
At 3:55 PM, my friend Max said to me, “I just
invented a time machine. Do you want to see it?” Of course, I agreed.
At 3:58 PM, Max and I entered the room where the
machine was located. It looked like a simple metal chair. The machinery seemed
to be under the seat.
At 3:59 PM, as Max and I watched the machine from
the doorway of the room, a second machine appeared next to it, an exact copy of
the first. Sitting on the second machine was a man, an exact copy of Max. Max
and I stared at him in amazement.
At 4:00 PM, the copy of Max got up from the copy of
the machine, sat on the first machine, pressed a control, and disappeared into
the future. My friend Max, standing next to me, watched what he did.
At 4:01 PM, my friend Max sat on the copy of the
machine, pressed a control, and disappeared into the future. Neither Max nor
the two machines ever returned.
After thinking about it, I understood what had
happened. At 4:01 PM, Max traveled in his time machine to the future. When he
arrived, he found the second Max and the other time machine waiting for him. He
switched machines and returned to the past, arriving at 3:59 PM. After
switching machines again, he set off again into the future. He never returned.
===============================================
Only these two scenarios are possible. Any attempt
to design any other would be inconsistent. Try it and you'll see.
The timeline for the machine is the same in both
scenarios. What varies is the timeline of the traveler. In the first scenario,
the traveler does not change machines, either in the present or in the future.
In the second scenario, the traveler changes machines in the present and in the
future. Therefore, if time travel were possible, travelers would not be free.
What they do in the present necessarily determines what they must do in the
future, and vice versa. If they change machines now, they must change them in
the future; if they don't change them now, they cannot change them in the
future. If they don’t do that, if they try to change machines only at the
beginning or only at the end of the travel, the universe would be inconsistent.
No one doubts that a human being is free to make a
decision as basic as whether or not to change chairs. Then we must deduce that
time travel to the past is impossible.
Thematic Thread about Time: Previous Next
Manuel Alfonseca
This paradox was devised several years ago by
Francisco José Soler Gil and myself
Have a nice Christmas. See you on January 8th
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