You’ve probably heard this one before. You build a time machine, decide to go back in time, and kill your grandfather. What happens then? Do you cease to exist, meaning you never built your time machine and went back in time—creating an impossible loop?
This is the grandfather paradox, one of the most famous of all time travel paradoxes. It is one of many that posit what happens in some illogical scenarios if time travel is possible. For the most part, these are fun thought experiments—but some can actually be tested too, either mathematically or even experimentally. The results are compelling because they hint at potential breakthroughs in our knowledge of how time works, even though they don’t necessarily mean time travel itself is possible.
“We should not be thinking time travel is impossible because it would lead to paradoxes,” says Lorenzo Gavassino, PhD, a mathematical physicist at the University of Cambridge in the U.K. “The problem is that the universe tends to break down before,” meaning the universe might find ways to stop it—such as forming a singularity before it could happen.
Time travel would work in multiple hypothetical ways, scientists think. One way is the many-worlds theory, in which our universe splits into parallel timelines. Going back in time would create a new timeline, so killing your grandfather would begin a new branch of reality, and no paradox is invoked.
The "autocidal machine paradox" critique of the possiblity of faster than light communication is particularly interesting. Look it up.
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