# What's the name of this paradox? **URL:** \ **Category:** Factual Questions\ **Created:** [April 8, 2004, 3:47am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891 "2004-04-08T03:47:09Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![HPL](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@HPL](https://boards.straightdope.com/u/HPL)\ **Post date:** [April 8, 2004, 3:47am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/1 "2004-04-08T03:47:09Z")
There’s a paradox out there where a time travler is given the plans to the time machine by himself, which he then uses to build the time machine, then uses said machine to go back in time and give himself the plans. What is the name of this paradox? ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [April 8, 2004, 3:52am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/2 "2004-04-08T03:52:41Z")
It’s a rather generic time traveler paradox. I don’t believe there’s any name for most of them, with the exception of the grandfather paradox. ---
**Author:** ![SSgtBaloo](https://avatars.discourse-cdn.com/v4/letter/s/ac91a4/32.png) [@SSgtBaloo](https://boards.straightdope.com/u/SSgtBaloo)\ **Post date:** [April 8, 2004, 3:54am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/3 "2004-04-08T03:54:17Z")
I think it’s called a causality paradox. There’s no initial “cause” that begins this loop. So how did it start? Therefore “causality paradox”. Just my 2¢. –SSgtBaloo ---
**Author:** ![Jarrod87](https://avatars.discourse-cdn.com/v4/letter/j/8797f3/32.png) [@Jarrod87](https://boards.straightdope.com/u/Jarrod87)\ **Post date:** [April 8, 2004, 4:01am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/4 "2004-04-08T04:01:50Z")
But think about this paradox… In 2250CE, Jim goes back in time to 2050CE with plans for the time machine. In 2050CE, Jim goes back in time to 1950CE with plans for the time machine. In 1950CE, Jim goes back in time to 1850CE with plans for the time machine. Thats why i dont believe time travel will ever exist in the way people imagine it, because if it did we would already have it now, because no doubt someone would wanna take it back to make a dollar. Jarrod ---
**Author:** ![HPL](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@HPL](https://boards.straightdope.com/u/HPL)\ **Post date:** [April 8, 2004, 4:13am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/5 "2004-04-08T04:13:41Z")
> [@Jarrod87](#): > > But think about this paradox… > > In 2250CE, Jim goes back in time to 2050CE with plans for the time machine. > In 2050CE, Jim goes back in time to 1950CE with plans for the time machine. > In 1950CE, Jim goes back in time to 1850CE with plans for the time machine. > Thats why i dont believe time travel will ever exist in the way people imagine it, because if it did we would already have it now, because no doubt someone would wanna take it back to make a dollar. > Jarrod True, but the time machine would probably be buildable in one or two of the time periods mentioned above, so even if plans existed now, they wouldn’t be useful until certain key technologies are invented. And hypotehically, Time travel could be one of those technologies that would have the stricest of controls. Think the Atom bomb in 1945 but more security. Or there are time travelers, but nobody believes them because they appear on the art bell show. ---
**Author:** ![SSgtBaloo](https://avatars.discourse-cdn.com/v4/letter/s/ac91a4/32.png) [@SSgtBaloo](https://boards.straightdope.com/u/SSgtBaloo)\ **Post date:** [April 8, 2004, 4:23am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/6 "2004-04-08T04:23:10Z")
The most elegant “proof” of the impossibility of time travel (and I forget who actually came up with it): If time travel is possible, then _where are all the tourists_? –SSgtBaloo ---
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [April 8, 2004, 4:26am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/7 "2004-04-08T04:26:39Z")
Well, you know, this isn’t actually a _paradox._ A paradox is where two logically contradictory things both seem to be true. If you kill your grandfather before you were born, for instance, you were both born and not born. What you’re describing is self-causality, but there’s no logical contradiction. The time machine wasn’t both invented and not invented; it just exists without being invented. The real problem arises when an object is trapped in this kind of time loop instead of just information. After all, objects decay with the passage of time, but an object trapped in a time loop somehow remains perfectly unchanged even though it is _infinitely_ old. Such an object is sometimes called a “djinn particle.” ---
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [April 8, 2004, 4:31am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/8 "2004-04-08T04:31:53Z")
It was Stephen Hawking who speculated that time travel couldn’t exist because there were no tourists from the future. However, all time travel scenarios currently imagined by physicists do not allow travel to a time before the “machine” was created. Therefore, there are no tourists because time travel hasn’t been invented yet. Also, if you accept a “many-worlds” cosmology, time travellers cannot visit their own pasts, only alternate pasts. ---
**Author:** ![HPL](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@HPL](https://boards.straightdope.com/u/HPL)\ **Post date:** [April 8, 2004, 4:38am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/9 "2004-04-08T04:38:58Z")
> [@ricksummon](#): > > It was Stephen Hawking who speculated that time travel couldn’t exist because there were no tourists from the future. However, all time travel scenarios currently imagined by physicists do not allow travel to a time before the “machine” was created. Therefore, there are no tourists because time travel hasn’t been invented yet. Also, if you accept a “many-worlds” cosmology, time travellers cannot visit their own pasts, only alternate pasts. I can think of two other possibilities: 1. Not just anyone can use a time machine, either from laws or secrecy or certain conditions that have to be met to make it work(maybe you have to be less then 4’5’’ to be a time traveler or something). 2. There are tourists and nobody notices, either because most people regard those who claim they are time travelers like they do those who claim they’ve been abducted by aliens, or the tourists are smart enough not to make themselves known. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [April 8, 2004, 4:56am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/10 "2004-04-08T04:56:59Z")
Googling [“Closed Timelike Loop”](http://www.google.com/search?as_q=&num=10&hl=en&ie=ISO-8859-1&btnG=Google+Search&as_epq=closed+timelike+loop&as_oq=&as_eq=&lr=&as_ft=i&as_filetype=&as_qdr=all&as_nlo=&as_nhi=&as_occt=any&as_dt=i&as_sitesearch=&safe=off) returns some interesting stuff. If you wait a few days, this thread will probably show up in the results, and you’ll end up here again. ---
**Author:** ![SSgtBaloo](https://avatars.discourse-cdn.com/v4/letter/s/ac91a4/32.png) [@SSgtBaloo](https://boards.straightdope.com/u/SSgtBaloo)\ **Post date:** [April 8, 2004, 5:01am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/11 "2004-04-08T05:01:33Z")
> [@ricksummon](#): > > It was Stephen Hawking who speculated that time travel couldn’t exist because there were no tourists from the future. Thanks! > [@ricksummon](#): > > …if you accept a “many-worlds” cosmology, time travellers cannot visit their own pasts, only alternate pasts. Sign: “Warning! This time machine only works on alternate Thursdays.”😃 –SSgtBaloo ---
**Author:** ![Jarrod87](https://avatars.discourse-cdn.com/v4/letter/j/8797f3/32.png) [@Jarrod87](https://boards.straightdope.com/u/Jarrod87)\ **Post date:** [April 8, 2004, 5:25am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/12 "2004-04-08T05:25:48Z")
> [@HPL](#): > > True, but the time machine would probably be buildable in one or two of the time periods mentioned above, so even if plans existed now, they wouldn’t be useful until certain key technologies are invented. > > And hypotehically, Time travel could be one of those technologies that would have the stricest of controls. Think the Atom bomb in 1945 but more security. > > Or there are time travelers, but nobody believes them because they appear on the art bell show. Wouldnt it also be wise to bring these technologies back with them? I thought that was implied… ---
**Author:** ![HPL](https://avatars.discourse-cdn.com/v4/letter/h/3da27b/32.png) [@HPL](https://boards.straightdope.com/u/HPL)\ **Post date:** [April 8, 2004, 5:39am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/13 "2004-04-08T05:39:22Z")
> [@Jarrod87](#): > > Wouldnt it also be wise to bring these technologies back with them? > I thought that was implied… I wasn’t sure. Responsible time travelers probably wouldn’t so, because then they risk altering the timeline in significant ways. ---
**Author:** ![AHarris](https://avatars.discourse-cdn.com/v4/letter/a/bcef8e/32.png) [@AHarris](https://boards.straightdope.com/u/AHarris)\ **Post date:** [April 8, 2004, 5:53am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/14 "2004-04-08T05:53:46Z")
> [@Jarrod87](#): > > Wouldnt it also be wise to bring these technologies back with them? > I thought that was implied… You’d pretty much have to bring a kit with all the pre-made components as well as any tools needed to assemble the time machine in it, otherwise you’d run into the problem of not having the tools to make the tools. ---
**Author:** ![NoCoolUserName](https://avatars.discourse-cdn.com/v4/letter/n/5fc32e/32.png) [@NoCoolUserName](https://boards.straightdope.com/u/NoCoolUserName)\ **Post date:** [April 8, 2004, 6:00am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/15 "2004-04-08T06:00:52Z")
For the best “causality loop” story ever written, read _By His Bootstraps_ by **RAH**. For the definitive discussion of time travel, see _The Theory and Practice of Time Travel_ by **Larry Niven**. > [@](#): > > Niven’s Law: If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe. Collected in “All the Myriad Ways.” ---
**Author:** ![LuckySevens](https://avatars.discourse-cdn.com/v4/letter/l/ad7895/32.png) [@LuckySevens](https://boards.straightdope.com/u/LuckySevens)\ **Post date:** [April 8, 2004, 6:10am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/16 "2004-04-08T06:10:41Z")
What about producing more matter at any one time? Would a time traveller going back in time create a situation where there is more matter then there was in the universe before hand, and therefore implode it (or something 😛 ) Or what about someone bringing resources back; couldnt we effectivly ‘mine the future dry’? ---
**Author:** ![fezpp](https://avatars.discourse-cdn.com/v4/letter/f/da6949/32.png) [@fezpp](https://boards.straightdope.com/u/fezpp)\ **Post date:** [April 8, 2004, 9:49am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/17 "2004-04-08T09:49:56Z")
> [@LuckySevens](#): > > couldnt we effectivly ‘mine the future dry’? Is that not what we’re doing without time travel! ---
**Author:** ![alterego](https://avatars.discourse-cdn.com/v4/letter/a/6bbea6/32.png) [@alterego](https://boards.straightdope.com/u/alterego)\ **Post date:** [April 8, 2004, 11:12am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/18 "2004-04-08T11:12:59Z")
> [@Squink](#): > > Googling [“Closed Timelike Loop”](http://www.google.com/search?as_q=&num=10&hl=en&ie=ISO-8859-1&btnG=Google+Search&as_epq=closed+timelike+loop&as_oq=&as_eq=&lr=&as_ft=i&as_filetype=&as_qdr=all&as_nlo=&as_nhi=&as_occt=any&as_dt=i&as_sitesearch=&safe=off) returns some interesting stuff. > If you wait a few days, this thread will probably show up in the results, and you’ll end up here again. [http://boards.straightdope.com/robots.txt](http://boards.straightdope.com/robots.txt) > [@](#): > > User-agent: \* > Disallow: / ---
**Author:** ![Dan\_Norder](https://avatars.discourse-cdn.com/v4/letter/d/4af34b/32.png) [@Dan\_Norder](https://boards.straightdope.com/u/Dan_Norder)\ **Post date:** [April 8, 2004, 11:19am UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/19 "2004-04-08T11:19:55Z")
> [@ricksummon](#): > > However, all time travel scenarios currently imagined by physicists do not allow travel to a time before the “machine” was created. Uh, where did you get that idea? That sounds more like the sort of thing a fictional TV show would come up with to explain away weaknesses of its premise (Quantum Leap, for one, which never made sense). I’ve heard of plenty of scenarios that would get people to travel back in time well beyond the time this planet was even formed, which is more than enough the purposes of this discussion. Of course if people have traveled through time, odds are very good that they have enough advanced technology and training to make themselves fit in so that we don’t notice them too much. ---
**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:** [April 8, 2004, 12:00pm UTC](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891/20 "2004-04-08T12:00:35Z")
> [@](#): > > Niven’s Law: If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe. Niven’s Law works like this; everytime someone traveled into the past, they will change history in some fashion, small or large, random or controlled. So if time travel exists, the past is always being changed. Eventually, by blind random chance if nothing else, someone will change history in such a manner that time travel never existed. And at that point, there will be no more changes and history will get locked into one pattern. [Next page](https://boards.straightdope.com/t/whats-the-name-of-this-paradox/238891.md?page=2)