# What are your favorite thought experiments? **URL:** \ **Category:** In My Humble Opinion\ **Created:** [November 19, 2023, 10:57pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394 "2023-11-19T22:57:35Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Cmyers1980](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyers1980/32/11814_2.png) [@Cmyers1980](https://boards.straightdope.com/u/Cmyers1980)\ **Post date:** [November 19, 2023, 10:57pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/1 "2023-11-19T22:57:35Z")
What are your favorite thought experiments and why? My example is the experience machine by Robert Nozick. It serves to show whether the person being asked values hedonism over anything else, whether they value what’s real over what’s not real and to what degree are they satisfied with their current life. I personally would choose to enter the machine though my answer would change depending on what my life is currently like. ---
**Author:** ![Spice\_Weasel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spice_weasel/32/5435_2.png) [@Spice\_Weasel](https://boards.straightdope.com/u/Spice_Weasel)\ **Post date:** [November 19, 2023, 11:56pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/2 "2023-11-19T23:56:19Z")
I don’t know if this counts, but someone here introduced me to the Veil of Ignorance and I haven’t been able to stop thinking about it. If I had no control over the state I was born into, what kind of society would I want to be born into? ---
**Author:** ![griffin1977](https://avatars.discourse-cdn.com/v4/letter/g/977dab/32.png) [@griffin1977](https://boards.straightdope.com/u/griffin1977)\ **Post date:** [November 20, 2023, 12:11am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/3 "2023-11-20T00:11:07Z")
So the first thing that comes to mind for me are the various “thought experiments” involved in game theory. The most famous being [The Prisoners Dilemma](https://en.m.wikipedia.org/wiki/Prisoner%27s_dilemma), which accurately describes a bunch of situations in human society and non-human biology (though in most cases you encounter it IRL the result is not the “rational” outcome predicted by game theory). Though I’m not 100% sure it counts as a thought experiment, I mean you can carry it out IRL, and people do (the former president of the US [is finding himself of on wrong end of the prisoners dilemma](https://www.google.com/amp/s/amp.cnn.com/cnn/2023/10/21/politics/trump-georgia-guilty-pleas-what-matters/index.html)) Similarly (and less well known) is the [dollar auction](https://en.m.wikipedia.org/wiki/Dollar_auction) which IMO more accurate in predicting behavior in situations like WW1 where the belligerents continue to bare costs far higher than any benefit they could receive by winning the conflict. ---
**Author:** ![griffin1977](https://avatars.discourse-cdn.com/v4/letter/g/977dab/32.png) [@griffin1977](https://boards.straightdope.com/u/griffin1977)\ **Post date:** [November 20, 2023, 1:46am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/4 "2023-11-20T01:46:22Z")
Also a couple of more classical (or rather non-classical 😉 ) examples: Schrodinger’s Kittens: similar to Schrodinger’s cat, but instead of just one cat you have two kittens, same non-PETA approved setup, but you use two entangled particles and ensure one kitten is poisoned if the spin is in one direction, and saved in the opposite direction. You send the (presumably very long lived) kittens to opposite ends of the galaxy, then open one box. As the original Schrodinger’s cat you have now collapsed the wave function for that kitten, but you also instantly collapse the wave function of the other kitten, even though it’s now many light years away. [The Train/Tunnel Paradox](https://hep.physics.illinois.edu/home/g-gollin/relativity/p112_relativity_11.html): You drive a 1000m long train into a 800m long tunnel. As the train in travelling at 0.8c, to a stationary observer it appears that the train shrinks due to relativity so the entire train is inside the tunnel. Which is fine except that stationary now presses a button to close the gates at either end of the tunnel once the train is entirely inside the tunnel, and press it again to open them before the train leaves. What about the trains frame of reference where the train is never entirely inside the tunnel? It turns out in that frame of reference the doors at the start and end still open and close _but not at the same time_. As the train sees it the door at the end opens and closes before the front of the train reaches the exit, and the door at the beginning then closes after the back of the train enters the tunnel. I find this super trippy. ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [November 20, 2023, 2:31am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/5 "2023-11-20T02:31:45Z")
While I was a student at Defense Language Institute, I was introduced to this one: > A man murders a wino on Skid Row and then cuts off the wino’s arms. The man mails each arm to a different address. The man then goes the train station. Another man sees him, becomes enraged, and kills the first man. Why did the last man become enraged and kill the first man? IIRC, the person posing the question can only answer Yes/No questions. One variant is the poser (heh) can answer “Yes”, “No”, “Irrelevant”, or a couple of other choices that really don’t dissuade from the experiment. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [November 20, 2023, 2:38am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/6 "2023-11-20T02:38:22Z")
This might not quite be what you mean by thought experiment, but I often think of the concept of the Library of Babel, which contains books with every possible sequence of letters. So within it are sets books that with 100% accuracy describe, for instance, every day of your life from your birth to your death. And also sets of books that are 100% accurate except for getting what you had for breakfast on one day wrong (with versions spanning every breakfast of your life). > **[The Library of Babel](https://en.wikipedia.org/wiki/The_Library_of_Babel)** > > "The Library of Babel" (Spanish: La biblioteca de Babel) is a short story by Argentine author and librarian Jorge Luis Borges (1899–1986), conceiving of a universe in the form of a vast library containing all possible 410-page books of a certain format and character set. > The story was originally published in Spanish in Borges' 1941 collection of stories El jardín de senderos que se bifurcan (The Garden of Forking Paths). That entire book was, in turn, included within his much-reprinted Ficciones... I think of it more larely in terms of generative AI and of a “photo album” of Babel. A library of every possible combination of 1024x1024 pixels x 16 million colors is beyond vast, and contains accurate images of every moment in the history of the universe, plus images that are accurate except that everyone is wearing funny hats. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 20, 2023, 3:03am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/7 "2023-11-20T03:03:28Z")
> [@Monty](#): > > IIRC, the person posing the question can only answer Yes/No questions. One variant is the poser (heh) can answer “Yes”, “No”, “Irrelevant”, or a couple of other choices that really don’t dissuade from the experiment. Those are what we’re calling “lateral thinking puzzles”, over in Thread Games. I don’t think we’ve done that one, so if you remember the solution, go ahead and pose it over there. ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [November 20, 2023, 3:35am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/8 "2023-11-20T03:35:31Z")
Happy to so do. Can you give me a link so I head to the right place? ---
**Author:** ![Little\_Nemo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/little_nemo/32/3120_2.png) [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)\ **Post date:** [November 20, 2023, 3:36am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/9 "2023-11-20T03:36:22Z")
The two envelope paradox. You have two sealed envelopes. Each envelope contains an amount of money. One envelope contains exactly twice as much money as the other. You cannot tell by examining the unopened envelopes how much money is in them. You are allowed to pick one envelope, open it, and keep the money in it. Your goal is to obtain as much money as possible. So you pick one of the envelopes. But before you open it, you’re offered an opportunity to swap envelopes. Should you swap? Math says you should swap. Let’s say the amount of money in the envelope you’re holding is X. That means the other envelope contains either .5X or 2X. Which means the expected value of the other envelope is 1.25X. Plug in a specific amount. Let’s say the first envelope you chose hold one thousand dollars. That means other envelope holds either five hundred dollars or two thousand dollars. If you swap you have a fifty percent chance of gaining one thousand dollars and a fifty percent chance of losing five hundred dollars. Your potential gain is greater than your potential loss. So you should swap. But when you swap and pick up the other envelope, you’re offered another chance to swap back. And now the same math which told you to swap the first time says you should swap a second time. No matter which envelope you’re holding in your hands, the other envelope has an expected value that is twenty-five percent greater. ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [November 20, 2023, 5:50am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/10 "2023-11-20T05:50:36Z")
> [@Monty](#): > > Happy to so do. Can you give me a link so I head to the right place? Found the thread and posted it. > [@Little\_Nemo](#): > > The two envelope paradox. Also known as [Two Envelopes Problem](https://en.wikipedia.org/wiki/Two_envelopes_problem). That wiki entry on it is fascinating. ---
**Author:** ![Babale](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/babale/32/15666_2.png) [@Babale](https://boards.straightdope.com/u/Babale)\ **Post date:** [November 20, 2023, 5:53am UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/11 "2023-11-20T05:53:15Z")
> [@Spice\_Weasel](#): > > I don’t know if this counts, but someone here introduced me to the Veil of Ignorance and I haven’t been able to stop thinking about it I came here to post about the Veil of Ignorance. I learned about it years and years ago and like you I still think about it very often. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 20, 2023, 1:26pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/12 "2023-11-20T13:26:18Z")
> [@Darren\_Garrison](#): > > This might not quite be what you mean by thought experiment, but I often think of the concept of the Library of Babel, which contains books with every possible sequence of letters. So within it are sets books that with 100% accuracy describe, for instance, every day of your life from your birth to your death. And also sets of books that are 100% accurate except for getting what you had for breakfast on one day wrong (with versions spanning every breakfast of your life). > > [The Library of Babel - Wikipedia](https://en.wikipedia.org/wiki/The_Library_of_Babel) Years ago on a comedy website I saw an article that talked about the old idea that a monkey at a typewriter, pounding randomly on the keys, would eventually manage to type out Hamlet. The author posited that there were probably many other things the monkey would produce before managing to achieve that feat, and then listed numerous examples: - Several perfect cryptograms of Hamlet. - A long Usenet argument over whether Boba Fett is alive, complete with spam-blocked e-mail addresses. - The phrase “Jesus Christ my ass is chafed” repeated for the length of two letter-sized pages. - The text of Hamlet, except everyone dies of food poisoning in Act II. - A brief but accurate write-up of the most embarrassing thing you ever did, with full names, dates, and places. - Hop on Pop - The Denny’s Kids Menu. - This article, including HTML mark-up. - A short story entitled “Babysitter’s Passion.” - “Iii#jd89 pp98&(\*(^9 879j; FF” - The text of Hamlet, except that Horatio is named “Elvis.” ---
**Author:** ![Ponderoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponderoid/32/19221_2.png) [@Ponderoid](https://boards.straightdope.com/u/Ponderoid)\ **Post date:** [November 20, 2023, 1:29pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/13 "2023-11-20T13:29:23Z")
> [@Babale](#): > > I came here to post about the Veil of Ignorance. I learned about it years and years ago and like you I still think about it very often. This sounds like a potential information hazard. Am I better off not looking this thing up? ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [November 20, 2023, 1:43pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/14 "2023-11-20T13:43:46Z")
> [@Ponderoid](#): > > This sounds like a potential information hazard. Am I better off not looking this thing up? Consider a world with a veil of ignorance in it, but you don’t know if you will be in the group that looks it up or the group that doesn’t look it up… ---
**Author:** ![Babale](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/babale/32/15666_2.png) [@Babale](https://boards.straightdope.com/u/Babale)\ **Post date:** [November 20, 2023, 2:23pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/15 "2023-11-20T14:23:05Z")
> [@Ponderoid](#): > > This sounds like a potential information hazard. Am I better off not looking this thing up? > [@Darren\_Garrison](#): > > Consider a world with a veil of ignorance in it, but you don’t know if you will be in the group that looks it up or the group that doesn’t look it up…. The world in which everyone is familiar with and applied the Veil of Ignorance is clearly better regardless of who you end up being 😛 Here’s the short version. If you want to figure out what the more moral outcome to a situation is, imagine that the task of deciding which world is better is up to ethereal beings who exist outside the universe. They know that once they pick the best world, they will be born into that world as a random member of it. So a system like Feudalism would be rejected by this framework because even though kings get to live like… kings, most people are poor peasants with incredibly shitty lives, and these ethereal beings wouldn’t want to risk being a peasant just for a tiny chance at being a king. > **[Original position](https://en.m.wikipedia.org/wiki/Original_position)** > > The original position is a hypothetical position from which members of society would consider which principles they would select for the basic structure of their society if they had no knowledge ahead of time regarding the position which they would end up occupying in that society. The idea of having no such knowledge, because everyone is behind a veil of ignorance, represents a thought experiment often associated with the work of the American philosopher John Rawls. > In this "original positio... ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [November 20, 2023, 2:28pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/16 "2023-11-20T14:28:36Z")
> [@Monty](#): > > A man murders a wino on Skid Row and then cuts off the wino’s arms. The man mails each arm to a different address. The man then goes the train station. Another man sees him, becomes enraged, and kills the first man. Why did the last man become enraged and kill the first man? Are you going to submit that to the [lateral puzzles thread](https://boards.straightdope.com/t/lateral-thinking-puzzles-third-time-is-best/987214)? ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 20, 2023, 2:31pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/17 "2023-11-20T14:31:23Z")
> [@Darren\_Garrison](#): > > This might not quite be what you mean by thought experiment, but I often think of the concept of the Library of Babel, which contains books with every possible sequence of letters. So within it are sets books that with 100% accuracy describe, for instance, every day of your life from your birth to your death. And also sets of books that are 100% accurate except for getting what you had for breakfast on one day wrong (with versions spanning every breakfast of your life). > > [The Library of Babel - Wikipedia](https://en.wikipedia.org/wiki/The_Library_of_Babel) I’ve had a similar idea about computer monitors. A 4K monitor is 3840 x 2160 pixels (8,294,400), and each pixel is capable of 2563 unique colors. So a 4K display could show: (2563)8,294,400 unique images. **[Wolfram Alpha says that in standard base-10 notation, this is a number about 60 million digits long](https://www.wolframalpha.com/input?i=%28256%5E3%29%5E8294400)**. One year of 4K video footage at 30 frames per second is about a billion frames - a one followed by nine zeros - so the above gigantic number represents X years of footage, where X is a number that’s about 51 million digits long. To be clear, that’s not 51 million years; 51 million is a number only eight digits long. This is much, much bigger. I’m pretty sure my monitor could show every page of every 410-page book in the Library of Babel, and still have enough unused images to show every moment of every human being’s life from every angle. And lots of other stuff. ---
**Author:** ![griffin1977](https://avatars.discourse-cdn.com/v4/letter/g/977dab/32.png) [@griffin1977](https://boards.straightdope.com/u/griffin1977)\ **Post date:** [November 20, 2023, 3:02pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/18 "2023-11-20T15:02:32Z")
I guess where does logical paradox become a thought experiment? E.g.: Alan Turing’s proof of the halting problem. Namely: is it possible to tell if a turing machine (a generic hypothetical computer, capable of computing anything any real computer can compute) will finish in finite time, or “halt”? If is this is possible you could encode that logic (whatever it is) as a turing machine (because anything computable can be encoded as a turing machine), so you’d have a turing machine that could tell if another turing machine will halt. It would be then trivial to create a turing machine that will loop forever (i.e. never halt) if the input machine halts, otherwise return true. You then point this machine at itself. What will happen? If it halts, it will loop forever, but that means it won’t halt, but then it won’t loop forever, but now it will halt so it will loop forever, and so on and so on. This is a logical paradox, and hence a proof that it is not possible to know if a turing machine will halt. ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [November 20, 2023, 5:53pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/19 "2023-11-20T17:53:43Z")
Check a couple of posts up. I already did. 🙂 ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [November 20, 2023, 6:58pm UTC](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394/20 "2023-11-20T18:58:58Z")
A cousin and best friend discussed this last night. Some think that the Universe ends, and it’s just ‘nothing’ beyond it. Well if it’s defined as nothing, and has a definition, that means it’s something. A complete and total vacuum if it could exist with our mortal powers, is actually something. Ergo, it is something. Have a nice day. [Next page](https://boards.straightdope.com/t/what-are-your-favorite-thought-experiments/993394.md?page=2)