# Time travel: You can't change the past. **URL:** \ **Category:** Great Debates\ **Created:** [August 7, 2003, 4:10pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609 "2003-08-07T16:10:38Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![El\_Mariachi](https://avatars.discourse-cdn.com/v4/letter/e/71c47a/32.png) [@El\_Mariachi](https://boards.straightdope.com/u/El_Mariachi)\ **Post date:** [August 7, 2003, 4:10pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/1 "2003-08-07T16:10:38Z")
Let’s push aside the whole debate on whether time travel is possible and just focus on how going back in time will not do anything to change the past. Many movies that concern time travel often show someone going back in time to change something in the past, and some hero stopping them from doing it. I am of the opinion that it doesn’t matter. I’ll describe the best I can a scenario and why you can’t change the past. Let’s say some maniac (we’ll call him Bill) had acquired the ability to travel back in time. His goal was to kill your mother (we’ll call her Esther) so that you weren’t born. I can tell you with 100% certainty that he failed, and always will. How? Well by the simple fact you’re alive today. There’s more to it than that. I can tell you that there’s nothing anyone can do by traveling into the past to stop you from being born. Its impossible. Simply because you’re alive today. Tomorrow, someone can go back into time and try to kill Esther, but she’ll survive. Geesh, I’m not really making my point by creating a scenario so I’ll just describe what is happening: There is no history where Bill did not try to kill Esther. That never changed. Even if he hasn’t gone back in time to do it yet, he will try, and he already has tried. So in short, there’s nothing you can do to change the past. ---
**Author:** ![Joe\_Random](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Joe\_Random](https://boards.straightdope.com/u/Joe_Random)\ **Post date:** [August 7, 2003, 4:28pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/2 "2003-08-07T16:28:16Z")
Devil’s Advocate time: I’m not following. I there a reason _why_ you think the past can’t be changed? For all you know, travelling into the past places a person somehow outside of the normal flow of cause and effect, so that going into the past and killing my own grandfather would not cause a paradox. Couldn’t going into the past and changing something cause the “current” future to cease existing? After all, If you’re allowing time travel, then the “flow” of time is no longer an issue, and every instant in time can be considered to occur simultaneously. ---
**Author:** ![Svt4Him](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@Svt4Him](https://boards.straightdope.com/u/Svt4Him)\ **Post date:** [August 7, 2003, 4:28pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/3 "2003-08-07T16:28:39Z")
That’s why is called Science Fiction. The fact that you are alive today is only applicable if you were alive today. But let me change the scene. Some maniac (Arnie) goes back in time to kill the mother of (Johnny), who you don’t know. So maybe Johnny’s mother has been killed, and since you don’t know Johnny, then it proves that he did exist, but doesn’t now. That’s the problem with the whole scenario. ---
**Author:** ![hilltopper](https://avatars.discourse-cdn.com/v4/letter/h/c68b51/32.png) [@hilltopper](https://boards.straightdope.com/u/hilltopper)\ **Post date:** [August 7, 2003, 4:31pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/4 "2003-08-07T16:31:37Z")
[kobe] If I could just turn back the hands of time…[end kobe] ---
**Author:** ![TVAA](https://avatars.discourse-cdn.com/v4/letter/t/c57346/32.png) [@TVAA](https://boards.straightdope.com/u/TVAA)\ **Post date:** [August 7, 2003, 4:32pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/5 "2003-08-07T16:32:03Z")
You can’t change the past. You can’t change the present. You can’t change the future. Unfortunately, few people are willing to acknowledge this. There are occasional exceptions, though. See the movie “Twelve Monkeys” for an example. ---
**Author:** ![CharlesW](https://avatars.discourse-cdn.com/v4/letter/c/87869e/32.png) [@CharlesW](https://boards.straightdope.com/u/CharlesW)\ **Post date:** [August 7, 2003, 5:01pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/6 "2003-08-07T17:01:55Z")
I always liked Heinlien’s idea on time travel. Time travel does not change your time line it creats a new one with the change that you caused. So in your example that evil person did go back and kill my mother, this created a new time line (or demention if you prefer) where my mother was killed, and I was not born. This does not cause the current time line to stop. Now there are two different time lines one where my mother lived, and I was born, and another where she was killed, and I was never born. Expand this out, and you could have child time lines of either of those, and child time lines of those etc. Think in the books Heinlen described it as the multiverse theory. For a better discription read “the cat that walks through walls” and “too sail beyond the sunset”. Also talked about in other books, but I can’t remember their titles. ---
**Author:** ![Meatros](https://avatars.discourse-cdn.com/v4/letter/m/b4bc9f/32.png) [@Meatros](https://boards.straightdope.com/u/Meatros)\ **Post date:** [August 7, 2003, 5:11pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/7 "2003-08-07T17:11:10Z")
> [@](#): > > \*Originally posted by Svt4Him \* > \*\*That’s why is called Science Fiction. The fact that you are alive today is only applicable if you were alive today. But let me change the scene. Some maniac (Arnie) goes back in time to kill the mother of (Johnny), who you don’t know. So maybe Johnny’s mother has been killed, and since you don’t know Johnny, then it proves that he did exist, but doesn’t now. That’s the problem with the whole scenario. \*\* But Arnie failed the first time, so they had to send the T-1000 back, remember? Also, Johnny’s mother died of natural causes after the second film. 😃 ---
**Author:** ![robertliguori](https://avatars.discourse-cdn.com/v4/letter/r/46a35a/32.png) [@robertliguori](https://boards.straightdope.com/u/robertliguori)\ **Post date:** [August 7, 2003, 5:17pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/8 "2003-08-07T17:17:06Z")
TVAA: isn’t a purely deterministic future contingent on a lack of random events? Are vacuum fluctuations writ in stone? ---
**Author:** ![Debaser](https://avatars.discourse-cdn.com/v4/letter/d/3da27b/32.png) [@Debaser](https://boards.straightdope.com/u/Debaser)\ **Post date:** [August 7, 2003, 5:18pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/9 "2003-08-07T17:18:33Z")
Sure, time travel is possible. They spell it out in [this article](http://www.wired.com/wired/archive/11.08/pwr_timetravel.html). I think if the dopers pool up our piggy banks we might be able to build Throne Plates: > [@](#): > > Miracle Studios > When Carl Sagan was writing his 1985 novel Contact, he asked Caltech physicist Kip Thorne how to abbreviate the lengthy flight time required for a trip to a distant star. Thorne suggested a wormhole, a shortcut through space-time that almost certainly exists as a consequence of Einsteinian principles, although one has yet to be detected. A few years later, Thorne suggested that a wormhole’s entrances could be positioned in space and time as desired. Unlike some other time machines, this Thorne-inspired design allows round trips. However, it can’t take you back to a time before the machine was built. Here’s how it works: > > •Obtain four large conductive plates at least a few miles in diameter. Arrange them in parallel, very close together. The space between each plate will teem with negative energy - a proven phenomenon known as the Casimir effect - creating slices of identical space-time. > > •Separate the plates into two pairs. A wormhole will connect the pairs like an umbilical cord. > > •Place one pair in a rocket ship and accelerate to almost the speed of light, preferably in a circular path so the rocket doesn’t stray too far. Time will nearly freeze for that set while the other, still on the ground, ages at the usual rate. With each passing moment, the space-borne plates will go farther back in time relative to the others. > > •When a sufficient amount of time has passed - preferably decades - step between the earthbound plates. You’ll immediately be transported back in time and across space to the other pair. > > Fine print: To activate Thorne plates, the distance between each plate must be less than the width of an atom. The resulting wormhole will be equally small, so getting in and out might be difficult. To widen the portal, some scientists suggest using a laser to inject immense amounts of negative energy. In addition, Thorne believes that radiation effects created by gravitons, or particles of gravity, might fry you as you enter the wormhole. According to string theory, however, this probably won’t happen, so it’s scant reason to cancel your trip. That article is a great read. They make it sound so matter of fact, like baking instructions. ---
**Author:** ![Debaser](https://avatars.discourse-cdn.com/v4/letter/d/3da27b/32.png) [@Debaser](https://boards.straightdope.com/u/Debaser)\ **Post date:** [August 7, 2003, 5:22pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/10 "2003-08-07T17:22:56Z")
Doh! _Thorne_ Plates. Not Throne plates. How silly of me. Now, everybody go ahead and send me a $100 through paypal and I will start building the first plate. …Hmm, two plates few miles in diamater and spaced apart by the width of one atom. Where did I leave my tape measure?.. ---
**Author:** ![TVAA](https://avatars.discourse-cdn.com/v4/letter/t/c57346/32.png) [@TVAA](https://boards.straightdope.com/u/TVAA)\ **Post date:** [August 7, 2003, 5:23pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/11 "2003-08-07T17:23:07Z")
> [@](#): > > \*Originally posted by robertliguori \* > \*\*TVAA: isn’t a purely deterministic future contingent on a lack of random events? Are vacuum fluctuations writ in stone? \*\* Who says vacuum fluctuations are random? Superficially, roulette wheels appear to be random. Does that mean that they can’t be predicted? More to the point, does that mean that they don’t obey causality? The universe is the sum and superposition of all possibilities. Events are what they must be. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [August 7, 2003, 5:28pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/12 "2003-08-07T17:28:06Z")
**Debaser** that’s a very speculative article, I wouldn’t draw too many conclusions from it. That said relativity doesn’t forbid time travel there are few situations, that though they have deemed to be unphysical (i.e. they involve infinitely long cylinders or require a rotating universe)have time loops in them. **TVAA** , that fiew is subjective, we simply don’t know this. Also the idea of detirministic universe seems to contradict quantum mechanics. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [August 7, 2003, 5:31pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/13 "2003-08-07T17:31:29Z")
> [@](#): > > \*Originally posted by TVAA \* > \*\*Who says vacuum fluctuations are random? > > Superficially, roulette wheels appear to be random. Does that mean that they can’t be predicted? More to the point, does that mean that they don’t obey causality? > > The universe is the sum and superposition of all possibilities. Events are what they must be. \*\* Then are you suggesting some kind of hidden variable theory, local hidden variable theories are forbidden by Bell’s inequalities. General hidden variable theories also seem to counteract many of the fundamnetal aspects of physics. ---
**Author:** ![Lobelia\_Overhill](https://avatars.discourse-cdn.com/v4/letter/l/54ee81/32.png) [@Lobelia\_Overhill](https://boards.straightdope.com/u/Lobelia_Overhill)\ **Post date:** [August 7, 2003, 5:34pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/14 "2003-08-07T17:34:36Z")
Pilfered from the remake of The Time Machine … If Bill goes back in time and kills Esther, then he has no reason [today] to go back in time, so he won’t go back in time … However, if Bill went back in time and accidentally kills Esther, well that’s different … ---
**Author:** ![TVAA](https://avatars.discourse-cdn.com/v4/letter/t/c57346/32.png) [@TVAA](https://boards.straightdope.com/u/TVAA)\ **Post date:** [August 7, 2003, 5:40pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/15 "2003-08-07T17:40:28Z")
As my physics professors were once fond of pointing out, quantum mechanics itself is a deterministic theory. The uncertainties about what quantum mechanics actually means are due to the fact that the theory allows us to determinstically predict _probabilities_. Again – is a roulette wheel deterministic or non-deterministic? When we view them with sufficiently precise instruments and apply the proper calculations, we can predict their outcomes fairly accurately. But even if we assume a deterministic universe, no model could ever predict their behavior without complete accuracy. So, conceptually, what’s the difference between a determinstic system that can never be modeled perfectly and a non-deterministic system that can never be modeled perfectly? ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [August 7, 2003, 5:45pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/16 "2003-08-07T17:45:55Z")
No quantum physics is a non-detirministic theory, the wave function is deterministic and evolves via the TDSE, but the collapse of the wavefunction is probabiltistic/stochastic, which is not the same as detirministic. The roulette wheel is chaotic, which is not the same as non-detirministic. QM is non-detirministic on a much more fundamental level, in that you can know all that there is possible to know about a system yet you will not be able to predict the exact outcome of a measurement. ---
**Author:** ![TVAA](https://avatars.discourse-cdn.com/v4/letter/t/c57346/32.png) [@TVAA](https://boards.straightdope.com/u/TVAA)\ **Post date:** [August 7, 2003, 6:25pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/17 "2003-08-07T18:25:43Z")
There’s a wavefunction “collapse” in only some of the interpretations of quantum mechanics. And it’s never possible to know everything about a system. So what difference is there between a chaotic and a non-deterministic system? ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [August 7, 2003, 6:30pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/18 "2003-08-07T18:30:26Z")
As I suggested in [this thread](http://boards.straightdope.com/sdmb/showthread.php?s=&postid=3740952#post3740952), going back in time will simply create a new timeline; the time traveller will only exist in that timeline from that time forward; from the point of view of the time traveller’s contemporaries all that happens is that the time traveller disappears forever. * * * A new thought occurs- perhaps we **should** expect time travellers to visit us, but only from futures **which differ from our own** ; the mere fact that they arrive here means that they cannot come from the future which we experience, as their arrival will create a divergent timeline. 😕 * * * SF worldbuilding at [http://www.orionsarm.com/main.html](http://www.orionsarm.com/main.html) ---
**Author:** ![Bricker](https://avatars.discourse-cdn.com/v4/letter/b/977dab/32.png) [@Bricker](https://boards.straightdope.com/u/Bricker)\ **Post date:** [August 7, 2003, 6:56pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/19 "2003-08-07T18:56:04Z")
> [@](#): > > \*Originally posted by El Mariachi \* > [Geesh, I’m not really making my point by creating a scenario so I’ll just describe what is happening: There is no history where Bill did not try to kill Esther. That never changed. Even if he hasn’t gone back in time to do it yet, he will try, and he already has tried. > > So in short, there’s nothing you can do to change the past. No, you’re quite wrong. Some time ago, to settle a bar bet, I actually built a time machine and went back to alter history. I attended President William Henery Harrison’s inauguration and infected him with a fatal version of influenza. As you know, history used to record Harrison’s two terms in office as highly significant in American history. Now, of course, he’s remembered only as the shortest-serving president, and John Tyler did things VERY differently. Oh, wait - what am I thinking of? As a product of the altered timeline, you probably have ALWAYS thought that Harrison died after only a month in office; you’ve never heard of the Indian Accords of 1846 and you don’t know that Daniel Webster was elected President in 1848. But that’s how it was, before I changed the timeline. Does that answer your theory? - Rick ---
**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:** [August 7, 2003, 7:30pm UTC](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609/20 "2003-08-07T19:30:01Z")
> [@](#): > > \*Originally posted by El Mariachi \* > \*\*Let’s say some maniac (we’ll call him Bill) had acquired the ability to travel back in time. His goal was to kill your mother (we’ll call her Esther) so that you weren’t born. I can tell you with 100% certainty that he failed, and always will. > > How? Well by the simple fact you’re alive today. There’s more to it than that. I can tell you that there’s nothing anyone can do by traveling into the past to stop you from being born. Its impossible. Simply because you’re alive today. Tomorrow, someone can go back into time and try to kill Esther, but she’ll survive.\*\* This is not really an argument. If someone went back in time and killed the parent (before they conceived the person) of someone who is sitting next to you at this very moment you wouldn’t have a clue. Assuming this is all possible the person next to you would wink out of existence along with ALL memory you had of that person. For all you know this has happened to you dozens of times and you;d never be the wiser. Star Trek:TNG had a neat episode that covered this. I believe the episode was called ‘Yesterday’s Enterprise’. In the episode a rift opens in space in front of the ‘present day’ Enterprise ‘D’ and out flies the Enterprise ‘C’ (or ‘B’…I forget but it isn’t important for this). The bridge of the Enterprise-D shimmers and everything is subtly changed. The Enterprise-C has jumped to the future from the past and as a result the Enterprise-C essentially disappeared from its present and did not do things they otherwise would have done. This causes a ‘new’ future in which the Federation is at war with the Klingons and losing badly. Of course no one on the Enterprise-D think anything is amiss. The present is what it is. Guinan (Whoopi Goldberg) is the only one who posseses a sense that a new timeline has been created and that the Enterprise-C needs to be returned through the rift (even though they face certain death in a battle they has just escaped from). Captain Picard takes some convincing but ultimately agrees. Here’s the kicker though. Tasha Yar (Denise Richards) is alive again in the ‘new’ timeline as the new timeline never got her to the place where she died in an earlier episode. Guinan tells her that she thinks Tasha is dead in the ‘proper’ timeline so Tasha convinces Picard to let her go on the Enterprise-C and he agrees. Ultimately they succeed in sending the Enterprise-C back through the rift. The Enterprise-D was moments away from certain destruction by some Klingons who showed up and then _POOF_…Enterprise-C enters the rift, bridge shimmers again and all is back as it was at the bginning of the episode. Everyone aboard the Enterprise-D has no clue that they had spent several hours (a day or two) fighting Klingons and repairing the Enterpris-C. For them it essentially never happened. Now for the real, real kicker. Tasha Yar, having gone into the past, survives the battle that destroys the Enterprise-C and is captured by the Romulans. She has a daughter with a Romulan who looks just like her and eventually the ‘new’ Tasha Yar (her daughter actually) faces off against the Enterprise-D. Those aboard the Enterprise-D, having no knowledge of the events that led to Tasha #2 are thoroughly puzzled and unable to explain it. Cool huh? [Next page](https://boards.straightdope.com/t/time-travel-you-cant-change-the-past/193609.md?page=2)