# Time travel implications **URL:** \ **Category:** Factual Questions\ **Created:** [April 24, 2000, 6:16am UTC](https://boards.straightdope.com/t/time-travel-implications/11204 "2000-04-24T06:16:00Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![JJ\_Richard](https://avatars.discourse-cdn.com/v4/letter/j/a587f6/32.png) [@JJ\_Richard](https://boards.straightdope.com/u/JJ_Richard)\ **Post date:** [April 24, 2000, 6:16am UTC](https://boards.straightdope.com/t/time-travel-implications/11204/1 "2000-04-24T06:16:00Z")
My position on time travel is this: Forward time travel is possible. Time travel into the past is impossible (hopefully). Reasoning against travel into the past: 1. If time traveling is only possible by surpassing the speed of light, then doesn’t it stand to reason that any time traveled via this method would have to be in the forward (future) direction? After all, to go backwards in time, what are you gonna do…put your ship in reverse and go backwards at faster than light speed??? 2. One day, someone pops into our year (2000), and says “Hi, I’m from the future…the year 2099”. Your immediate reaction would be “but that’s not possible…we haven’t gotten there yet!”. Which in itself seems valid, however, if you were to pop into any period in the past, would that not be their exact response to you as well?? 1. Here’s the kicker…suppose for a minute that fore and aft time travel were possible. Not only possible, but even commonplace amongst humans. Yeah, people could walk up to high tech “phone booths” and, with the swipe of an ATM card, be whisked away instantly to any year of their choosing. Consider that this technology is available to all persons, world-wide. Now, you’ve got millions of people zipping in and out of time at their choosing. Sooner or later, someone would raise the question: “Hey, what is today’s real date, anyway?” There could be no answer! Not to mention the cessation of all sports on Earth, as well as gambling, prison sentences, warranties on purchased goods, etc., ect., let your mind run loose on the possibilities… The day AFT time travel is possible would be the end of humanity. You’d better hope like hell it NEVER happens! Forward time travel, however seems more plausible! Reasoning: 1. Say you’re on Earth with a super telescope. I’m walking around on the moon. You can see me walking on the moon through your telescope. I wave at you! Then, I plant an American flag into the “soil” of the moon, and then I run over to my “space ship”, start it up, and blast on over to your position on Earth at faster than light speed. Then, I jump out of my ship, and run over to your telescope, and (since I beat the light traveling image) can actually see myself back on the moon, waving at you! To me, this is forward time travel, since I beat the image back to Earth. Question: Have I traveled in time? If so, in which direction?? And if your answer is forward in time, then how would I go about going BACK in time?? JJ Richard ---
**Author:** ![Mousseduck](https://avatars.discourse-cdn.com/v4/letter/m/ecc23a/32.png) [@Mousseduck](https://boards.straightdope.com/u/Mousseduck)\ **Post date:** [April 24, 2000, 2:10pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/2 "2000-04-24T14:10:00Z")
I would say you are going back in time, because you are going back to before you were waving, before you jumped in your spaceship etc. To go forward in time, you would have to be where there is only one of you (ie not you on the moon and you watching). ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [April 24, 2000, 2:15pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/3 "2000-04-24T14:15:00Z")
First, you explain how you travelled faster than the speed of light, and then we’ll chat about the rest… In other words, you pose a theoretical question which can only yield a theoretical debate…not really an answer. * * * “They’re coming to take me away ha-ha, ho-ho, hee-hee, to the funny farm where life is beautiful all the time… :)” - Napoleon IV ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [April 24, 2000, 2:28pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/4 "2000-04-24T14:28:00Z")
First off–Time travel is THEORETICALLY possible but PRACTICALLY impossible (i.e. yanking theoretical wormholes around and travelling through them). Second Thing–NOTHING can travel faster than the speed of light. At the speed of light your mass becomes infinite hence you’d need an infinite amount of energy to move youself. There isn’t that much energy in the Universe. Third Thing–If what you propose is true then the future has already happened. Hence destiny exists and every ‘free’ choice you make is already determined (and was determined at the beginning of the universe). Free choice becomes an illusion…you’re merely like a picture on a frame of film. Fourth Thing–If you COULD travel to the future but can’t travel into the past then your trip to the future would be one-way. You couldn’t return to the ‘present’ as backwards time travel is impossible (to follow your proposal). ---
**Author:** ![sdimbert](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sdimbert/32/20204_2.png) [@sdimbert](https://boards.straightdope.com/u/sdimbert)\ **Post date:** [April 24, 2000, 2:29pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/5 "2000-04-24T14:29:00Z")
> [@](#): > > Originally posted by JJ Richard: > **3. Here’s the kicker…suppose for a minute that fore and aft time travel were possible. Not only possible, but even commonplace amongst humans. Yeah, people could walk up to high tech “phone booths” and, with the swipe of an ATM card, be whisked away instantly to any year of their choosing** I would go back a couple of days and close my back account. Free Trip! ---
**Author:** ![Fenris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fenris/32/498_2.png) [@Fenris](https://boards.straightdope.com/u/Fenris)\ **Post date:** [April 24, 2000, 2:31pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/6 "2000-04-24T14:31:00Z")
> [@](#): > > Originally posted by JJ Richard: > \*\*My position on time travel is this: > > Forward time travel is possible. Time travel into the past is impossible (hopefully). > > Reasoning against travel into the past: > > JJ Richard\*\* SF author Larry Niven dealt with a number of your ideas and objections in the essay “The Theory and Practice of Time Travel” which can be found in (I believe) his book All the Myriad Ways. The book also has the great essay “Man of Steel, Woman of Kleenex” which explains why Superman is a virgin. Fenris ---
**Author:** ![SingleDad](https://avatars.discourse-cdn.com/v4/letter/s/9e8a1a/32.png) [@SingleDad](https://boards.straightdope.com/u/SingleDad)\ **Post date:** [April 25, 2000, 5:37am UTC](https://boards.straightdope.com/t/time-travel-implications/11204/7 "2000-04-25T05:37:00Z")
> [@](#): > > First off–Time travel is THEORETICALLY possible but PRACTICALLY impossible (i.e. yanking theoretical wormholes around and travelling through them). It is not yet know for certain whether or not time travel is theoretically possible. It is clear that to use the wormhole method, you have to have _exotic matter_, a substance with a negative energy density in some reference frames. It has been proven that such exotic matter does exist near the event horizon of a black hole. If you have a wormhole with time travel capability, it is not known yet whether random quantum fluctuations would become amplified and destroy the wormhole. In fact, it may be that the fluctuations would be _precisely enough_ to destroy the wormhole; such an exact correlation would perhaps tell us something deep about the nature of time as it relates to QM. * * * Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam. ---
**Author:** ![SingleDad](https://avatars.discourse-cdn.com/v4/letter/s/9e8a1a/32.png) [@SingleDad](https://boards.straightdope.com/u/SingleDad)\ **Post date:** [April 25, 2000, 5:40am UTC](https://boards.straightdope.com/t/time-travel-implications/11204/8 "2000-04-25T05:40:00Z")
The relevant academic authority for time travel is Dr. Kip S. Thorne, who writes about it in [Black Holes and Time Warps: Einstein’s Outrageous Legacy.](http://www.amazon.com/exec/obidos/ASIN/0393312763/o/qid=956597907/sr=8-1/ref=aps_sr_b_1_1/104-7498661-7022807) ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [April 24, 2000, 6:05pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/9 "2000-04-24T18:05:00Z")
You’ll note I said theoretically possible and practically impossible. As I understand it physicists have found several methods that should work for time travel: 1. The Wormhole: If you move one end of a wormhole at near the speed of light then (presumably) a spaceman could fly in one end and pop out the other before he entered. Problems: Wormholes probably aren’t big enough to fly through. The presence of the spaceman may be enough to destabilize the wormhole and make it collapse. The difficulty in moving the other end of the wormhole around (i.e. dragging a large mass object nearby the wormhole terminus so its gravity tugs the end along with it). 2. Please forgive this one because I’m not sure I remember the details correctly. However, I believe there is some effect between to charged metal plates that in some way ‘connects’ them. Place one plate on a spaceship and fly it around at near the speed of light for awhile. Now…if you could move through the connection between the plates you could time travel (that description sounds awful even to me…I’ll try and see if Ican find a reference to it). Problems…If I remember correctly the energy required to allow a person through this would be more than converting the entire mass of Jupiter into energy. With that much energy involved pretty much anything entering that ‘portal’ would be torn to pieces. There may be other ways but the upshot is that time travel can actually be conceived of as a real possibility in this universe even if it is highly unlikely or impractical. ---
**Author:** ![JoeyBlades](https://avatars.discourse-cdn.com/v4/letter/j/a87d85/32.png) [@JoeyBlades](https://boards.straightdope.com/u/JoeyBlades)\ **Post date:** [April 24, 2000, 6:12pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/10 "2000-04-24T18:12:00Z")
Let’s not forget about the Tipler Cylinder if we’re discussing theoretical mechanisms for time travel. The Tipler Cylinder is much more reasonable than dealing with wormholes, if you ask me… As to the OP: > [@](#): > > Forward time travel, however seems more plausible! I should hope so since we are all experiencing forward time travel everyday… ---
**Author:** ![InutilisVisEst](https://avatars.discourse-cdn.com/v4/letter/i/d2c977/32.png) [@InutilisVisEst](https://boards.straightdope.com/u/InutilisVisEst)\ **Post date:** [April 24, 2000, 7:25pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/11 "2000-04-24T19:25:00Z")
The mechanics of building a Tipler cylinder seem as daunting as those of stabilizing a wormhole, IMHO. Niven’s monograph, as mentioned by Fenris, is a must-read for quick and easy time travel info. There’s also one dealing with teleportation, since we’ve mentioned faster-than-light travel. Speaking of FTL travel: there’s no prohibition in physics against travelling FTL. You simply can’t **accelerate** past the speed of light due to the mass/energy problem mentioned by Jeff\_42. BTW, Niven concludes that if time travel is possible according to physics, then it **won’t** happen. * * * I lead a boring life of relative unimportance. Really. ---
**Author:** ![vandal](https://avatars.discourse-cdn.com/v4/letter/v/858c86/32.png) [@vandal](https://boards.straightdope.com/u/vandal)\ **Post date:** [April 25, 2000, 6:08am UTC](https://boards.straightdope.com/t/time-travel-implications/11204/12 "2000-04-25T06:08:00Z")
There was a thread about this a few weeks ago. I said it then, and I’ll say it here as well. Time travel is not possible because matter cannot be created or destroyed. By time traveling, both points of this law are broken. If you travel into the future, you leave your current point in time, thus destroying matter, and upon entering the future, you are creating matter. As for the wormhole theory expressed by SingleDad and Jeff, I agree with this. The fact that a piece of mass as large as a human, or most likely, the spacecraft you would be traveling in, would inevitably cause the wormhole to crumble. ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [April 25, 2000, 6:47am UTC](https://boards.straightdope.com/t/time-travel-implications/11204/13 "2000-04-25T06:47:00Z")
> [@](#): > > Originally posted by **InutilisVisEst** : > Speaking of FTL travel: there’s no prohibition in physics against travelling FTL. Umm…Actually, the principle of local causality says that no information can travel faster than light. This principle is very dear to the philosophy of physics, even more important than the principle that reality is objective. * * * Virtually yours, DrMatrix If I’ve told you once, I’ve told you 0.99999999… times. ---
**Author:** ![JoeyBlades](https://avatars.discourse-cdn.com/v4/letter/j/a87d85/32.png) [@JoeyBlades](https://boards.straightdope.com/u/JoeyBlades)\ **Post date:** [April 25, 2000, 1:09pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/14 "2000-04-25T13:09:00Z")
InutilisVisEst writes: > [@](#): > > The mechanics of building a Tipler cylinder seem as daunting as those of stabilizing a wormhole, IMHO. Ahh… but some theorist believe that the fundamental building blocks for a Tipler Cylinder already exist in the form of neutron stars. It’s just a matter of bringing several of them together. As for wormholes, there are more challenges than simply getting them stable. You have to find a way to open one up big enough to get a vehicle through, you may have to find a way to neutralize gravitational and inertial forces on this vehicle, you have to find a way to place and anchor both ends of the wormhole, and of course, there is the question of the energy source… Really, though, it’s kind of silly to talk about either one as being more probable. It’s a bit like the primordial ooze discussing the virtues of Macintosh OS versus Windows… I just wanted to mention another alternative to FTL travel and wormholes. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [April 25, 2000, 1:39pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/15 "2000-04-25T13:39:00Z")
Dr Matrix: I think you are wrong. There is an experiment where two particles are created with an opposite spin to each other (as it has to be). If you reverse the spin on one of the particles the spin on the other particle automatically reverses itself. If you move these two particles VERY far apart and try this the same thing happens instantaneously. Let’s say the particles are a light year apart the ‘information’ that one particle reversed its spin moves much faster than the speed of light to the second particle so it reverses its spin. I believe Einstein called this action at a distance and HATED it. Unfortunately it seems to hold true anyway. ---
**Author:** ![InutilisVisEst](https://avatars.discourse-cdn.com/v4/letter/i/d2c977/32.png) [@InutilisVisEst](https://boards.straightdope.com/u/InutilisVisEst)\ **Post date:** [April 25, 2000, 1:55pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/16 "2000-04-25T13:55:00Z")
DrMatrix: Superluminal particles still don’t allow information to travel faster than light, so there’s no causality violation there. I don’t recall the proof of this, but it also negates the usefullness of wave function collapse and spin reversal (as mentioned by Jeff\_42) to transmit information faster than light. I’m sure there’s a bit of Heinsenberg in it, but I can’t recall the details. Roger Penrose’s excellent book “The Emperor’s New Mind” has some great wave function and spin discussions, but I don’t have it handy with me right now. He has a couple of chapters on time travel, but physical and information only. Lots of light cones and Minkowskian space-time stuff. Dang, Joey: no matter how much I wash now, I still feel like ooze. Thanks a whole bunch. You have a good point – both techniques are (currently) infinitely impractical, so neither is more likely than the other. I promise not to hijack this into a discussion of transfinite numbers. Not sure how you’d assemble neutron stars into a cylinder, anyway: seems like you’d just get a larger sphere each time. Oh, well. ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [April 25, 2000, 2:34pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/17 "2000-04-25T14:34:00Z")
You are referring (I believe) to Bell’s theorem (AKA Bell’s inequality) You do _not_ reverse the spin. What happens is you detect the value of the spin. The angle difference that you measure determines the correlation between the measurements. Basically you cannot have local causality and objective reality. Objective reality says that there is a definite value to the spin without a measurement. The accepted interpretation is that local causality is to valuable to lose and if one of the two (local causality and objective reality) must go then it is objective reality. ## Einstein held both too close to let go of either. Virtually yours, DrMatrix If I’ve told you once, I’ve told you 0.99999999… times. ---
**Author:** ![SingleDad](https://avatars.discourse-cdn.com/v4/letter/s/9e8a1a/32.png) [@SingleDad](https://boards.straightdope.com/u/SingleDad)\ **Post date:** [April 25, 2000, 2:59pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/18 "2000-04-25T14:59:00Z")
DrMatrix is correct. (BTW: Are you a Martin Gardner fan?) The reason you can’t use the phenomenon described by Bell’s theorem, is that you can’t detect the correlation between the distant measurements until you transmit those measurements at the speed of light to a common location. Essentially Bell proved that by adjusting the instrument at one location, you can change the outcome of the measurement at the other location from **one random sequence to another random sequence**. But you can’t extract a message from the random sequence at one instrument until you’ve received the random sequence from the other instrument by ordinary, sub-luminal means. * * * Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam. ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [April 25, 2000, 3:25pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/19 "2000-04-25T15:25:00Z")
> [@](#): > > Originally posted by **InutilisVisEst** : > DrMatrix: Superluminal particles still don’t allow information to travel faster than light, so there’s no causality violation there. If Superluminal particals don’t allow information to travel faster than light, then I think that must imply they cannot be detected. If they cannot be detected then I would say that they don’t exist. **SingleDad** Yes, I _love_ Martin Gardner, I used to go to the library to read back issues of Scientific American for his columns. When I registered I wanted a name that would be associated with math and Dr. Matrix just came to mind (from Mathematical Games, of course). * * * Virtually yours, DrMatrix If I’ve told you once, I’ve told you 0.99999999… times. ---
**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:** [April 25, 2000, 8:50pm UTC](https://boards.straightdope.com/t/time-travel-implications/11204/20 "2000-04-25T20:50:00Z")
Just to be a Devil’s Advocate here, what’s so special about unidirectional causality, anyway? Once you get around that, there’s no real objection to time travel or FTL. Yes, an acausal world would be weird, but then, so is a _causal_ world. * * * “There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe” –A. Einstein [Next page](https://boards.straightdope.com/t/time-travel-implications/11204.md?page=2)