The Grandfather Paradox

This is a bootstrap paradox of time travel in which inconsistencies emerge through changing the past. The time traveler prevents its existence by traveling to the past and kills their own grandfather before the conception of the traveler’s father or mother. This is just one among the many time travel paradoxes that test out your ability to think.

I used to work in a really dull job and the day we discovered this paradox was the happiest day of my entire career there. We spent the next 2 years trying to solve this paradox in increasingly ridiculous ways. Simply put, this famous paradox involves someone traveling back in time and killing his grandfather in a date before his own birth. So far so good, if a little heartless. Ah, but if he killed his grandfather before he was born then he was never born. So who killed the old dude? I’ll be darned if I know. Jeez, that was 2 years of my life well spent.

The Achilles and the Tortoise Paradox

This is a paradox created by Zeno of Elea of the 5th Century BCE which baffled all of Zeno’s contemporaries along with his several “paradoxes of motion”. The Achilles and the Tortoise Paradox is one of the famous and easiest to solve without an infinite number.

This is one of those famous paradoxes which cause me to go and lie down after reading it. In this case, we need to picture a race between the famous Greek warrior Achilles and a humble tortoise. The tortoise gets a head start, which we are going to call 100 meters. Achilles runs the 100 meters really quickly but by the time he gets to the starting point of the tortoise the flipping animal has moved a smaller distance ahead, maybe a meter. Now our fighting buddy runs that extra meter really quickly but by the time he gets there the annoying reptile has moved maybe 10 centimeters ahead of him. By this reckoning, he will keep closing the gap but he’ll never overtake the tortoise. How could that even happen? Try this one at home if you don’t believe me.

The Paradox of Value

​This one is really simple, although a bit mind-bending at the same time. Basically, it says that if water is so much more valuable to humans than diamonds then why are the precious stones so much more expensive to buy? The answer is something to do with there being far more water than diamonds on the planet.

​The Liar Paradox

​This sentence is false. Eh? This seemingly simple sentence is an example of a paradox which has confused and entertained people for centuries. You see, if the sentence is false then it must be true, as that’s what it is saying that it is. However, if it’s true then it’s false because it says that it’s false, not true. Oh goodness me, I wish I’d never mentioned this confounded paradox.

The Ship of Theseus Paradox

​This famous paradox goes by a number of names but fans of cheesy British sitcoms may know it as Trigger’s Broom paradox. You take an object – in this case, a ship (or a broom) – and say that if you replace parts in it one by one it is still the same ship. That must mean that if you took all the old parts and just built a new ship from scratch it would be the same ship as the one which is sitting beside it. Eh?

The Surprise Hanging Paradox

​This is probably my favorite paradox. A prisoner gets sentenced to hanging by a judge and told that he will be hanged one day next week at noon but that it will be a surprise to him, as he won’t know the day of the hanging. The prisoner calculates that it can’t happen on Friday since it wouldn’t be a surprise if that happened on that day, as it is the last day of the week. Hang on though, if Friday is out of the question then it can’t be Thursday either, as by the time he gets to Wednesday afternoon and hasn’t been hanged then Thursday is the only possible day. After ticking off the days of the week like this one by one the prisoner is happy, as he is sure he can’t be hanged by surprise any day. The executioner comes and knocks on his door at noon on Wednesday. It was a surprise after all.