The Grandfather Paradox: What Happens If You Travel Back In Time And Kill Your Grandpa?

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The grandfather paradox was first described in 1931, and it is the age-old argument of preventing your birth by killing your grandparents. Here are what this paradox is all about, and the theories that have been attempted to solve it.

The Grandfather Paradox is one of many mind-bending scenarios that physicists have imagined as they’ve considered the implications of time travel.

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Let’s say you have a time machine. You traveled back in time and killed your grandfather before he conceived your father. You wouldn’t be born, of course. If you would, how do you think you could even manage to travel back in time in the first place?

Let's make it a bit more complicated...

Killing your grandpa is the simplest version of this paradox, by the way. There are many other versions that can make your head spin. For example, Robert Heinlein’s classic short story All You Zombies is as follows:

A baby girl is mysteriously dropped off at an orphanage in Cleveland in 1945. "Jane" grows up lonely and dejected, not knowing who her parents are, until one day in 1963 she is strangely attracted to a drifter. She falls in love with him. But just when things are finally looking up for Jane, a series of disasters strike. First, she becomes pregnant by the drifter, who then disappears. Second, during the complicated delivery, doctors find that Jane has both sets of sex organs, and to save her life, they are forced to surgically convert "her" to a "him." Finally, a mysterious stranger kidnaps her baby from the delivery room.
Reeling from these disasters, rejected by society, scorned by fate, "he" becomes a drunkard and drifter. Not only has Jane lost her parents and her lover, but he has lost his only child as well. Years later, in 1970, he stumbles into a lonely bar, called Pop's Place, and spills out his pathetic story to an elderly bartender. The sympathetic bartender offers the drifter the chance to avenge the stranger who left her pregnant and abandoned, on the condition that he join the "time travelers corps." Both of them enter a time machine, and the bartender drops off the drifter in 1963. The drifter is strangely attracted to a young orphan woman, who subsequently becomes pregnant.

The bartender then goes forward 9 months, kidnaps the baby girl from the hospital, and drops off the baby in an orphanage back in 1945. Then the bartender drops off the thoroughly confused drifter in 1985, to enlist in the time travelers corps. The drifter eventually gets his life together, becomes a respected and elderly member of the time travelers corps, and then disguises himself as a bartender and has his most difficult mission: a date with destiny, meeting a certain drifter at Pop's Place in 1970.

Who is Jane's mother, father, grandfather, grandmother, son, daughter, granddaughter, and grandson? The girl, the drifter, and the bartender, of course, are all the same person.

Another version of the story is about wormholes.

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Imagine that you created a wormhole in your room, a time tunnel that stretches just one minute into the past. Through the wormhole, you can see yourself as you were one minute ago. But what if you use the wormhole to shoot your earlier self? You’re now dead. So who fired the shot?

Stephen Hawking’s way of explaining the grandfather paradox is by far the best: He held a party for time-travellers!

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As you may have guessed, no one showed up. 

“I have experimental evidence that time travel is not possible,” he said. “I gave a party for time-travellers, but I didn’t send out the invitations until after the party. I sat there a long time, but no one came.” Hawking also said that Einstein’s theories offer the possibility of traveling backward in time - but “it is likely that warping would trigger a bolt of radiation that would destroy the spaceship and maybe the space-time itself”.

Is there any way out of this paradox?

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You traveled to the past and shot your grandfather; will you suddenly evaporate when he dies? How could this even be possible? One of the interpretations brought to the paradox is that when you killed your grandfather, you killed a previous version of yourself. Because you are actually traveling to a copy of the past, not your own past, and everything you will do at that time will affect the future of the universe in which the events are happening, not the universe we now live in.

According to quantum mechanics, time travel and teleportation are basically suicide.

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Here’s how physicist Jennifer Oullette explains it:

"Think about it. All your atoms are scanned and destroyed in the process, because there’s no cloning allowed in quantum mechanics. Then all that information about you is teleported to the new location, where a bunch of new atoms are used to build a whole new you, complete with memories, personality quirks, and that oddly shaped birthmark on your left butt cheek."

But that's not good enough, because that solution kind of bypasses the paradox, rather than solving it.

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As MinutePhysics explains, subatomic particles regularly do multiple different things in parallel - a process known as quantum superposition. It's happening in the core of the Sun right now.

If you apply this kind of thinking to the grandfather paradox, you get something called a closed time loop, where your grandfather is simultaneously dead and alive, and so are you as a result.

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But could a closed time loop actually exist, according to the laws of physics? I guess we have to wait a bit more for a certain answer to that...

Sources: 1, 2, 3

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