Time travel. If time travel was possible, then presumably someone from the future would have already gone back in time to change the past. Therefore, when someone says they, for example, would have stopped Hitler, they actually wouldn’t because someone already would have made that correction in time. Instead, that must have been, unfortunately, the best possible outcome out of all possible outcomes. Either that or time travel just isn’t possible which seems significantly more likely.
Zeno’s Paradox. If you want to reach a wall and you’re 10 meters away, then travel half the distance. Then, travel half of THAT distance, and do it again, and again, and again. Mathematically, you will never reach 0, thus you will never reach the wall, but physically, you will.
You take a ship and replace every single part in it with a new one. Is it still the same ship? If not, at what point does it stop being the ship you knew? Also, if you take all the parts you replaced and build another ship with them, is it the original ship?
Camus regards The Absurd as a paradox. Humans are constantly driven to seek meaning in a world that doesn’t appear to have any.
The Bootstrap Paradox
You receive a strange machine and a note that says “This is a time machine, recreate the time machine and send the machine back in time to the day you received it (today).” Bewildered, you set to work to recreate the time machine. When you are done and have tested it, you send the original machine and the note back in time to the day you received it.
You didn’t create the machine you received from yourself, where did it come from?
The Ship of Theseus, also known as Theseus’s paradox. Sometimes also known as the Axe paradox
The axe version is that if you are given an Axe and replace the wooden handle is it the same axe? What if you then replace the blade? How many times can you repeat this process before the Axe is no longer the same Axe?
The cool thing about this is that your entire body is made up of a different set of cells every 7 years or so (as cells die). So can you really say you’re the same person you were 7 years ago?
The Fermi Paradox
It’s basically about the existence of aliens and can be summed up as:
1- The Milky Way contains hundreds of billions of stars, and billions of them are similar to the sun.
2- It is highly likely that some of these stars will have planets that are similar to Earth.
3- If we assume – via the Copernican principle – that Earth is not particularly special, then intelligent life should also exist on some fraction of these Earth-like planets.
4- Some of these intelligent life-forms might develop advanced technology, and even interstellar travel.
5- Interstellar travel would take a long time, but as there are many sun-like stars that are billions of years older, there has been plenty of time for such travel to have occurred.
6- Given all this, why haven’t we met or seen any trace of aliens? Where is everybody?
No one goes there because it’s crowded.
If you ask Rick Astley for his copy of the movie Up, he cannot give it to you as he will never give you up. However, in doing so he lets you down. Thus creating the Astley Paradox
The human brain paradox.
You see, our brains are so complex that we can’t fully understand how they work. If they were simpler, we totally could. Except that if our brains were simpler, we’d be more stupid, and still unable to fully understand our own brains.
The heap of sand one.
If you have what you agree to be a “heap” of sand, and remove one grain, then it’s still a heap, right? So if a heap -1 is still a heap, you should logically be able to keep going until you have a “heap” of 1 grain of sand.
The grandfather paradox – if you travel back in time and kill your grandfather then you wouldn’t have been born, but if you weren’t born then you couldn’t kill your grandfather which means you did get born, etc..