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  "canonicalUrl": "https://serpentsquiggles.neocities.org//posts/essays/time-travel",
  "description": "|",
  "path": "/posts/essays/time-travel",
  "publishedAt": "2020-04-28T00:00:00.000Z",
  "site": "at://did:plc:ivoe7cntxuy6at7uzmxzs2ft/site.standard.publication/3mfk6cpprzt2t",
  "textContent": "[timpost]: https://old.reddit.com/r/HPMOR/comments/2xie39/time_travel_and_why_everyone_gets_it_wrong/\n\n; foreword\n: First posted many years ago elsewhere, now hosted here alongside my\nother essays. While my style has developed subtantially since this was\nfirst written, I think the contents remain worthwhile.\n\n: related\n: - [Time Travel and why everyone gets it \"wrong\"][timpost]\n  - What's Expected of Us\n  - [](/posts/time-splice.html)\n\n; aside\n\n: Abstract: I discuss several models for assigning probability to\ntimelines under the assumption that time travel is possible, but\nparadoxes are absolutely impossible, as is the case in many fictional\nworlds. The models are mathematically precise, and illuminate issues\nthat have previously confused many people about what sort of timelines\nare \"most likely\". I discuss an example due to /u/TimTravel in a old\npost on /r/HPMOR, then analyze whether time travel can be used to\nsolve the halting problem. I outline how timeline probability may\ninteract with physical probabilities, often used to justify physics\n\"conspiring\" or contriving a certain outcome to prevent paradox.\n\n  Edit: commenters have pointed out similarities between this and\n  the Ted Chiang story,\"What's Expected of Us.\" The similarity wasn't\n  intentional, but it's pretty interesting.\n\nIntroduction\n\nLet's say you're walking down the street one day when a wizard appears\nin a clap of thunder and places a strange gray device of buttons and\nswitches into your hands.\n\nYou're looking down at it, struggling to make heads or tails of it, and\nthen you look up and the wizard is gone.\n\nAt the top of the device, there is a slider, already set to the\nleftmost extreme. Below it, two switches: a power switch already set\nto ON, and a stiff, unlabeled switch, the exact gray of the surface,\nrising so inconspicuously low off the surface you almost miss it.\nBelow that, two LED buttons, both inactive.\n\nSuddenly, the left LED glows blue. Confused, you press the button (it\ngoes in with a satisfying click) and the light flashes off instantly.\n\nFurrowing your brow, you decide to press the button again. The blue\nlight quickly comes on while your finger's still moving, and it again\nwinks out immediately as the button is depressed. You try pressing the\nbutton again and again, and each time the blue light turn on, seeming to\npredict or anticipate the button press.\n\nThen, the other LED button glows red. You press it, and it turns off;\nseveral tries later, you conclude it behaves exactly the same.\n\nYou decide now to deliberately not press either button, even if the\nlights were to shine encouragingly. But nothing happens; neither light\ncomes back on. You move your finger closer to a button, determined to\narrest its motion at the last possible second. But the light doesn't\ncome on, even when your skin is brushing the cool metal. You forget it\nand press the button. The light blinks bright blue milliseconds before\nyou've even decided.\n\nNow, you (you, dear reader, not the above character) have already read\nthe title of this post. This is strange device sends information\nbackward in time. Specifically, it sends a single bit back in time one\nsecond.\n\nOr well, you fiddle with the slider, and notice it controls the\ninterval; you can set it to one minute, an hour, or even a day.\n\nAll that established, it's time to test something. \"Red is heads, and\nblue tails,\" you say. A coin from your pockets is flipping in the air\nuntil you catch it and slap it down on your wrist.\n\nThe device shines blue. You lift your hand. It's heads.\n\nYou push the blue button anyway, out of habit, the light flashing off.\nAnd then it hits you: you have to commit intently to pressing the right\nbutton even when (especially when) the device is wrong.\n\nAnother test: if the device shines red again, you'll press blue. But if\nit shines blue, you'll press still blue.\n\nThere's a noticeable delay before the device tentatively shines a\nlight.\n\nIt's blue.\n\nCall this act forcing. You can force the device to be red or blue.\n\nYou try the coin flip test just a few more times. Now, the device is\nalways right, even if it seems to pause a random interval before shining\na light.\n\nThe opposite of forcing would be splinting (for 'splinterpoint').\nThis is, pressing the button for whichever light comes on next, with no\ntricks and no conditionals.\n\nFinally, the last thing you can do --- for a broad notion of 'can' ---\nis what we'll call crashing. This is: pressing the button of\nwhichever light doesn't blink on. It's less that you can do this,\nand more that you can intend this, and reality responds to that.\n\nYou give it a try right now: you commit to crashing if your next coin\ntoss doesn't come up heads.\n\nYou flip the coin, anxiously watching it's path through the air, catch\nit, slap it down on your wrist, spend a few seconds working up the nerve\nand then lifting your hand. It's tails.\n\nYou take a deep breath, and look expectantly at the device.\n\nNo light comes on. You're waiting for a few minutes.\n\nAnd then it hits you; the device isn't binary, it's trinary. Sure,\nit can shine red or blue --- but so too can it refuse to shine at all!\nAnd if it either light leads to paradox, why would any light come on?\nThe only winning move is not to play.\n\nIs that it, then? Are your dreams of munchkinry doomed to fail? Was it\njust a coincidence that 'forcing' seemed to work earlier?\n\nAnd then the red light comes on. You grin triumphantly, with not a\nlittle dread. You're about to destroy the universe! Before the\nimplications catch up to, you're flinging your hand forward, jabbing it\nat the device. You don't want to lose your nerve.\n\nYou look down and see that you missed, pressing the red button rather\nthan the blue like you planned.\n\nIs this fate? Is the world itself conspiring to prevent paradox, just\nlike in the stories? You want to give crashing another try, but the last\nthing you want is to wait those long minutes for the light to come on\nagain. You glare down at the device, and then you notice the second\nswitch. You'd almost forgotten about it.\n\nYou idly flick it, and immediately the blue light comes on.\n\nIt forces a prediction? Maybe your plans aren't doomed. You consider\ngiving crashes another try, but maybe destroying the whole timeline is\nnot worth the risk. You decide to spare the universe, and press the blue\nbutton.\n\nYou need to understand how this device works before you can really\nexploit it. And you have just the idea for another experiment. What if\nyou splint, and if the splint comes out blue, you force blue again, but\notherwise you just splint again. After two button presses, you turn off\nthe device.\n\nIt's clear there are three possibilities: blue-blue, red-blue and\nred-red. But which are most likely?\n\nYou run this experiment a hundred times, and keep track of the results.\n\nCall it the double blue experiment.\n\nThere are a few ways it could turn out:\n\nModel A: Path Realism\n\nIt seems that consistent timelines are the only thing that matters.\nIt's as if the universe has already set aside exactly the number of\ntimelines there needs to be, and you're already in a certain\ntimeline, you just don't know which one yet.\n\nIn the double blue experiment, there are three possibilities, and every\none is equally likely.\n\n> p(red,red) = p(red,blue) = p(blue,blue) = ⅓\n\nYou find it strange, as a follow-up experiment aptly demonstrates:\n\nSplint once. If it comes out blue, force blue twenty-nine times.\nOtherwise, do nothing. Turn off the device. This results in one red or\nthirty blues in a row.\n\nOn the face of it, it's crazy that you can even experience the second\npossibility. It's like winning the lottery half the time. Then again,\nmaybe it's not so crazy? If you were to just force blue twenty-nine\ntimes, it's equally unlikely on the face of it; like flipping dozens of\ncoins that all come up heads.\n\nThere's a weirder consequence, though. If you splint ten times, you can\nsee any combination of reds and blues;\nred-blue-blue-red-red-red-blue-red-red-red and all the others, with\nuniform probability.\n\nBut if you splint ten times, and if and only if every splint came up\nblue, you splint ten more times, you'll find that the first set of\nsplints come up all blue almost half the time!\n\nThis is easy to reconcile with path realism. There are 2^10^= 1024\nthrough the ten splints. Each is as likely as the other.\n\nBut if you commit to doing ten more splints if and only if the first\nset comes up all blues, then there are 2^11^-1 = 2047 paths down the\ntime-tree.  If each is as likely as the other, then over half of them\nare located under one branch!\n\nModel B: Local Branch Realism\n\nIt seems that splints are basically coin tosses; it either comes up blue\nor it comes up red. The exception is if one of those options always\nleads to paradox. If you commit to causing paradox when the light shines\nblue, then it will always shine red. If you commit to splinting then\ncrashing when the first splint comes out blue, then the splint will\nsimilarly always shine red.\n\nThe intermediate is more interesting: in the double blue experiment (you\nsplint twice and crash if both splints are blue), then half the time the\nfirst splint will come out red, but if the first splint comes out blue,\nthe next one always comes out red. In numbers, the possibilities are\np(red,red) = p(red,blue) = ¼, and p(blue,red) = ½.\n\nIt's like the universe is savescumming, just as a gamer might: it\neffectively \"saves\" to a new \"slot\" to every time a time travel event\nis about to happen. If a paradox happens, it reloads from recent\n\"saves\" one after another, finding the newest one that lets it avoid\nthe paradox.\n\nModel C: Reroll Realism (or, Bayesian Branch Realism)\n\nYou're not sure if paradoxes really don't happen. You've looked at the\nnumbers. What it suggests is that, rather than avoiding paradoxes,\nparadoxes might simply cause the universe to effectively restart.\n\nThe stats from the double blue experiment don't lie: p(red) =\n⅔; p(blue,blue) = ⅓.\n\nImag",
  "title": "A Closer Look At Time Travel and Probability"
}