A paradox is a question or statement that completely goes against logic. From time travel, to the Twin Paradox, we're giving you our picks for the top 10 mind blowing paradoxes.
Paradox of Twin Earths
The paradox of Twin Earths is a thought experiment first presented by the philosohper Hilary Putnam, in his papers, "Meaning and Reference" and "The Meaning of 'Meaning,” used as an argument for what has become now known as semantic externalism. So we begin by supposing there is another universe with a planet just like our own and we'll call this Earth-2. On Earth-2, there is a match for every person, animal or thing on our Earth, except, Earth-2 does not have what we call water. Instead of H2O, they have a different liquid that we call "XYZ". Those from Earth-2, call their liquid "water" as well. Herein lies the paradox, if both Earthlings refer to water, are they referring to the same thing?
The Friendship Paradox
The friendship paradox was first observed by Scott L. Feld in 1991. He states that most people have fewer friends than that of the friends they have, on an average. Using a sampling bias as a base, Scott claims that people with a greater number of friends will face an increased chance of being observed with one's own friends. To really simplify it for you, he's pretty much saying that those with more friends, are more likely to be your friend to begin with, because they have a higher inclination to actually go out and make friends.
Imagine if you will a young mother walking along a riverbank with her small child - then suddenly the kid is snatched up by a crocodile. The mother panics and pleads with the crocodile to relinquish her child. The crocodile replies that if the woman can guess whether or not he will release the child, then he will release it. Of course, if she guesses right, the crocodile will release the child, then there is no paradox. However, if she replies saying the crocodile will not return the child, that’s where we are left with a paradox. If she is correct in stating that he won't, then the crocodile never planned on letting the child go in the first place, breaking his word to release the child if she guessed right. If the mother was wrong, and the crocodile did indeed plan on returning the child to her, the crocodile must now keep the child - though his intentions were not to do so, again making it break its word.
Causal Loop Paradox
The Causal Loop Paradox is another paradox having to do with time travel. This paradox happens when you use an event of the present, usually in the form of an object, action or bit of passed along information, to make the initial event happen in the first place; this is also called the Bootstrap Paradox. Much like a bootstrap is used to pull yourself up, this paradox can be exemplified using George Lucas. Say for instance, after the success of the original Star-Wars films, George would use a time machine to go back in time and give his younger self the scripts that spiraled his success. Ultimately, the causal loop paradox forces an existence without a creation point.
Hempel's Paradox (The Raven Paradox)
This paradox is brought into being by the questioning of what exactly evidence is. It was first proposed by Carl Gustav Hempel to show the contradiction between intuition and inductive logic. He starts the paradox with one simple statement, "All ravens are black." Now this is where the logical train of thought arrives: everything that is not black is certainly not a raven then. So in any instance of the second statement being true, the first would have to be true as well. The reason that this conclusion is considered to be paradoxical? Because it implies that information is produced about the raven by looking at any object that isn't black.
Zeno's Arrow Paradox
This paradox was proposed by Greek philosopher Zeno of Elea. He did it to support Parmenides's doctrine, which claims that, contrary to the evidence garnered by one's senses, the belief that plurality as a concept is true, specifically that motion is nothing more than an illusion, Zeno states that for motion to occur, an object must change the position which it occupies. Using an arrow as an example, he states that at any one instant of time, an arrow is neither moving to where it is, or where it is not. It can't move to where it isn't, because no time elapses for it to move; however, it can't move to where it is, because it is already there. Simply put, if everything is truly motionless at every instant of time, and time is composed of instants, then motion is an impossibility.
Now this one gets a little tricky, as it is based upon the hypothetical concept of time travel, and creating inconsistency by altering the past. This paradox is also sometimes referred to as Hitler's Murder Paradox. A time traveler goes back in time to kill his grandfather before he would meet his grandmother. As a direct result, the time traveler would never have existed in the first place to make the trip. So if he never existed, he could never go back in time and murder his grandparent, resulting in his undesired birth in the first place.
The Twin Paradox
The twin paradox is a thought experiment dealing with the laws of physics, in particular. The paradox involves identical twins - one who stays on Earth and the other who takes a high-speed rocket into space, only to return years later. When the space-traveling twin returns home, he finds that his twin on Earth had aged much more rapidly. The result is puzzling because each twin saw the other twin as moving.
Grandfather's Axe Paradox (Ship)
The grandfather's axe paradoxically, is also known as the ship of Theseus paradox. Imagine you have an object, a ship, or in this example, your grandfather's wood-axe. Now throughout regular use, the axe becomes damaged; first the handle splits, so you go out and buy a replacement. Later, the head of the axe chips and grinding it down is no longer an option, so you set out to replace it with a new axe-head. It's at this point, the ancient Greek historian Plutarch, raised the question; if the entirety of an object is replaced piece by piece, does it truly remain the same object in question?
The Liar's Paradox
The liars paradox is best illustrated with a piece of paper, written upon it the line "this sentence is false". (now follow along) So if the line, "this sentence is false", is true, then it is indeed false; however, if the sentence claims that it is false, then it must truly be false. Though if it's false... then it must be true. And there within lies the paradox; it's an ongoing contradiction of conclusions formed by illustrating the common belief about what true and false actually means, and even though the phrase is completely in accordance with grammatical rules, there is no value to determine absolute truth.