# Time travel does not create paradox **URL:** \ **Category:** Factual Questions\ **Created:** [May 10, 2011, 1:42am UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356 "2011-05-10T01:42:20Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![yoyodyne](https://avatars.discourse-cdn.com/v4/letter/y/a9a28c/32.png) [@yoyodyne](https://boards.straightdope.com/u/yoyodyne)\ **Post date:** [May 10, 2011, 3:24pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/21 "2011-05-10T15:24:13Z")
> [@RealityChuck](#): > > [Alfred Bester beat you to it a half century ago.](http://arthursbookshelf.com/.../alfred%20bester%20-%20the%20men%20who%20murdered%20mohammed.pd) [Fixed link](http://arthursbookshelf.com/sci-fi/bester/alfred%20bester%20-%20the%20men%20who%20murdered%20mohammed.pdf) ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [May 10, 2011, 6:55pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/22 "2011-05-10T18:55:49Z")
Okay, this may be worth a try. I’ll back-step about six hours and see if a paradox waveform collapses or propagates. ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [May 10, 2011, 7:45pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/23 "2011-05-10T19:45:28Z")
> [@Hambil](#): > > Yeah, there is the problem of the time traveler ‘instantly’ traveling through time. But isn’t that a problem with all time travel stories/theories? All I’m saying is that IF you could jump around in time you would never have any effect on yourself. You will always be ahead of the changes you make. But the time traveler is anchored to the timeline from before he started traveling. Except maybe for the person in [this story.](http://en.wikipedia.org/wiki/%E2%80%94All_You_Zombies%E2%80%94) ---
**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:** [May 10, 2011, 7:47pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/24 "2011-05-10T19:47:45Z")
I’m not really sure this is material for a factual discussion, lest such point at which time travel is achieved. ---
**Author:** ![Farmer\_Jane](https://avatars.discourse-cdn.com/v4/letter/f/bbe5ce/32.png) [@Farmer\_Jane](https://boards.straightdope.com/u/Farmer_Jane)\ **Post date:** [May 10, 2011, 7:58pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/25 "2011-05-10T19:58:16Z")
> [@Kobal2](#): > > And since Hitler wasn’t shot by a time travelling Vatican Assassin Warlock A+ ---
**Author:** ![Farmer\_Jane](https://avatars.discourse-cdn.com/v4/letter/f/bbe5ce/32.png) [@Farmer\_Jane](https://boards.straightdope.com/u/Farmer_Jane)\ **Post date:** [May 10, 2011, 8:01pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/26 "2011-05-10T20:01:29Z")
> [@Mijin](#): > > “Motion is relative” does not mean “really everything is moving even if it feels like it’s still”. > It literally means that motion is relative. It’s no more correct to say the earth is orbiting the sun as orbiting the centre of the galaxy as being stationary. > > Consider this: temperature is broadly molecular motion. If everything on earth is really moving at thousands of miles per hour, how have they managed to cool substances to near absolute zero? To the point where they become a Bose-Einstein condensate? > > Another example, for the OP, is light, which from its own frame of reference moves through space but not time. Um. Doesn’t light move through time if time were at ‘0’? Or what about light bending? Or do you just mean that light does not age? 😕 //never took physics and didn’t pay attention in chemistry or math edit: I guess when light emits, it just _is_ all at once. ---
**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:** [May 10, 2011, 8:05pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/27 "2011-05-10T20:05:48Z")
What I think would be an interesting and somewhat amusing problem with time travel is if it were invented several billions of people from all of future-history (?) would blink into existence around Hitler and try to kill him at pretty much every point in his life. Or this: [http://www.abyssandapex.com/200710-wikihistory.html](http://www.abyssandapex.com/200710-wikihistory.html) (I imagine Chronos as SilverFox316 in that story) 😃 ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [May 10, 2011, 9:55pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/28 "2011-05-10T21:55:42Z")
> [@CitizenPained](#): > > Or do you just mean that light does not age? 😕 Think so. IANAPhysicist but I believe that according to the theory of relativity, time gets contracted as you approach the speed of light such that at the speed of light itself, any journey takes zero time, from your own frame of reference (which is as valid as any other). Of course, it’s arguable whether this means light does not travel through the time dimension. From our POV light travels (with us) into our future. But from a photon’s POV its emission, journey and absorption are simultaneous, even for a cross-galaxy journey. ---
**Author:** ![Hambil](https://avatars.discourse-cdn.com/v4/letter/h/3be4f8/32.png) [@Hambil](https://boards.straightdope.com/u/Hambil)\ **Post date:** [May 10, 2011, 10:33pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/29 "2011-05-10T22:33:12Z")
Well, if you’re going to bring light into the discussion you’re gonna screw everything up. Pretty much any discussion of light I’ve read or heard so far tends to end with “well, we just don’t really know yet” 😛 ---
**Author:** ![Farmer\_Jane](https://avatars.discourse-cdn.com/v4/letter/f/bbe5ce/32.png) [@Farmer\_Jane](https://boards.straightdope.com/u/Farmer_Jane)\ **Post date:** [May 10, 2011, 10:36pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/30 "2011-05-10T22:36:56Z")
> [@Mijin](#): > > Think so. IANAPhysicist but I believe that according to the theory of relativity, time gets contracted as you approach the speed of light such that at the speed of light itself, any journey takes zero time, from your own frame of reference (which is as valid as any other). > > Of course, it’s arguable whether this means light does not travel through the time dimension. From our POV light travels (with us) into our future. > But from a photon’s POV its emission, journey and absorption are simultaneous, even for a cross-galaxy journey. Meh! So it’s a perspective thing? Hmm…if people in the future figured this stuff out, then how do we know they want to kill Hitler? Maybe they’ll figure life would be too dramatically changed and what’s done is done. Maybe they’re going to kill someone else…like…Kennedy. ---
**Author:** ![Strassia](https://avatars.discourse-cdn.com/v4/letter/s/e8c25b/32.png) [@Strassia](https://boards.straightdope.com/u/Strassia)\ **Post date:** [May 10, 2011, 10:53pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/31 "2011-05-10T22:53:01Z")
> [@DrFidelius](#): > > See? No paradox at all. > [@DrFidelius](#): > > Okay, this may be worth a try. I’ll back-step about six hours and see if a paradox waveform collapses or propagates. Well, that settles it then. ---
**Author:** ![brocks](https://avatars.discourse-cdn.com/v4/letter/b/7feea3/32.png) [@brocks](https://boards.straightdope.com/u/brocks)\ **Post date:** [May 11, 2011, 1:50am UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/32 "2011-05-11T01:50:18Z")
> [@RealityChuck](#): > > [Alfred Bester beat you to it a half century ago.](http://arthursbookshelf.com/.../alfred%20bester%20-%20the%20men%20who%20murdered%20mohammed.pd) Very strange. I read that story with no recognition of it, until I came to the sentence about such was his fury, and such was his genius. I _remembered_ that sentence, so I must have read the story before. But I didn’t remember a thing about it — not the title, author, plot, or anything else, except that one line. IMO it’s a forgettable story, but it’s kind of scary that if it hadn’t been for that one line, I would swear in a court of law that I had never seen that story before. ---
**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:** [May 11, 2011, 4:16am UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/33 "2011-05-11T04:16:17Z")
> [@](#): > > Well, if you’re going to bring light into the discussion you’re gonna screw everything up. Pretty much any discussion of light I’ve read or heard so far tends to end with “well, we just don’t really know yet” Nah, light we understand perfectly well. It’s just matter that we’re puzzled about. ---
**Author:** ![Hambil](https://avatars.discourse-cdn.com/v4/letter/h/3be4f8/32.png) [@Hambil](https://boards.straightdope.com/u/Hambil)\ **Post date:** [May 11, 2011, 6:01am UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/34 "2011-05-11T06:01:31Z")
> [@Chronos](#): > > Nah, light we understand perfectly well. It’s just matter that we’re puzzled about. Yeah? So, lights a wave - waves move through things, so what is it moving through? Space-time? Are we bringing back the concept of [ether](http://en.wikipedia.org/wiki/Luminiferous_aether)? ---
**Author:** ![digs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/digs/32/14889_2.png) [@digs](https://boards.straightdope.com/u/digs)\ **Post date:** [May 11, 2011, 2:06pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/35 "2011-05-11T14:06:44Z")
> [@Exapno\_Mapcase](#): > > Upward isn’t a dimension, it’s a direction. > > You as a person always inhabit the three dimensions of length, width and height. You can’t possibly move only one dimension of you. Since I hit adulthood, I’ve been expanding horizontally instead of vertically… ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [May 11, 2011, 2:17pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/36 "2011-05-11T14:17:13Z")
> [@Hambil](#): > > Yeah? So, lights a wave - waves move through things, so what is it moving through? Space-time? Are we bringing back the concept of [ether](http://en.wikipedia.org/wiki/Luminiferous_aether)? Not exactly. There isn’t a good simple English word to describe what a photon is (wave? particle? wavical?), but the math is very clear. ---
**Author:** ![BlinkingDuck](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@BlinkingDuck](https://boards.straightdope.com/u/BlinkingDuck)\ **Post date:** [May 11, 2011, 2:17pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/37 "2011-05-11T14:17:19Z")
> [@Hambil](#): > > Had this thought while driving. I was thinking of the stupid ways in which alternate dimensions are treated in television and movies. You travel to an ‘alternate’ dimension where everything is just like here except some small difference. Or, you travel through an ‘alternate’ dimension to achieve faster than light travel… > > You cannot travel through one dimension without traveling though them all. When you travel through space you also travel through time. This (I believe) holds true no matter how many dimensions they discover. You cannot move through one without moving through them all. > > That being the case, events in time are no different than events in space. So, imagine you are traveling through space at 50 miles and hour and someone behind you throws a baseball at your head at 50 miles per hour. That baseball will never catch you, never have any effect on you. So, if you travel into the past and kill your own father so you are never born you will create a change that is moving forward at the exact same rate that you are. Thus that change will never reach you, never affect you. > > It’s just a thought in it’s infant stages, but I find it interesting. I had the exact same thought…that if you go back in time and make a change then that goes forward in time at 1 sec per sec so not ever reaching ‘you’. If I make a huge change (like blowing up the world killing everyone then when I come back to the present everyone would be here. This is no different, I think, then any change you make ‘branching’ the universe into 2 branches…except that if I built a time machine and went back 1000 years that past may be entirely different from the history books from ‘my’ past beacuse it may have already been changed by someone else with a time machine. I prefer the branching idea better because your history is not destroyed. It always exists in a previous branch. The OP’s theory destroys your history. ---
**Author:** ![Chessic\_Sense](https://avatars.discourse-cdn.com/v4/letter/c/7c8e57/32.png) [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)\ **Post date:** [May 11, 2011, 3:32pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/38 "2011-05-11T15:32:44Z")
Here’s my preferred “escape the paradox” scenario: Suppose time were like a stack Lego bricks, laid on their side. Each brick is a moment in time and there’s some unknown force called “A Child” adding bricks to the right end of the sideways stack. A red one, then a blue one, then another red one, then a yellow one, and so on at random. This is our timeline. The added blocks represent the passage of time. On top of the stack is a Lego man. He’s always walking forward and is always standing on the last brick to be added. As he steps forward, another brick is added to catch his footfall. He can turn around and look at where he’s been- yellow, red before that, and blue before that…but he can’t walk backwards. This man is the time traveller. He remembers the past but can’t return to it. So add time travel. Now most people imagine that this is like our Lego man jumping up and passing backward over the lego stack so that he lands on, say, the first red brick. Then he’ll walk forward over the blue, then red, then yellow, just like he did before. Some say when he gets to the end brick again, he’ll jump back again in a loop. Some say that he can change the brick so that maybe green comes instead of yellow and then he can’t go backward. But I toss all this out the window. I say that when the time traveler jumps, what he actually does is to ‘control’ The Child so that the next bricks to be added are exact clones of the bricks that came before. So when he looks back, he still sees yellow preceded by red preceded by blue preceded by red. But he knows that that combination will also be coming up. So Lego man can still change bricks, kill his grandfather, stop bellbottoms from coming into fashion, or make sure his parents hook up at the Under the Sea dance, and it won’t affect the past one bit. He can still exist because he already exists. His grandfather won’t sire Lego man’s father, but instead, a clone of Lego man’s father…or, at least, he would have if Lego man hadn’t interfered. See? If the mountain comes to Mohammed, there’s no paradox at all. ---
**Author:** ![colonial](https://avatars.discourse-cdn.com/v4/letter/c/34f0e0/32.png) [@colonial](https://boards.straightdope.com/u/colonial)\ **Post date:** [May 11, 2011, 3:44pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/39 "2011-05-11T15:44:08Z")
We really need some expert commentary here. > [@Mijin](#): > > Think so. IANAPhysicist but I believe that according to the theory of relativity, time gets contracted as you approach the speed of light such that at the speed of light itself, any journey takes zero time, from your own frame of reference (which is as valid as any other). IANA Physicist either, but I believe this is certainly untrue. See link: [Time Dilation](http://www.fourmilab.ch/cship/timedial.html) Note that lapsed time increases with velocity. What happens is that it increases at a progressively slower rate than for an observer at rest. However, it in meaningless to say what happens, or that anything happens for an object when it attains the speed of light: Note the equation:t’ = t/square root(v^2/c^2) If v=c then the numerator of the right side of the equation = 0, and that is not allowed! The internal logic of Special Relativity forbids any massive object from attaining the speed of light, and renders the concept meaningless. > [@Mijin](#): > > Of course, it’s arguable whether this means light does not travel through the time dimension. From our POV light travels (with us) into our future. But from a photon’s POV its emission, journey and absorption are simultaneous, even for a cross-galaxy journey. Although the existence of non-local effect seems be becoming increasingly apparent, I do not believe this has been shwon to apply to the passage of time for a proton in its own frame of reference. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [May 11, 2011, 4:41pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/40 "2011-05-11T16:41:32Z")
> [@colonial](#): > > IANA Physicist either, but I believe this is certainly untrue. See link: > > [Time Dilation](http://www.fourmilab.ch/cship/timedial.html) > > Note that lapsed time increases with velocity. What happens is that it > increases at a progressively slower rate than for an observer at rest. I think you’ve misunderstood me and maybe the physics. Let’s forget about c itself and just describe someone going very close to the speed of light. Say I travel so fast I get to a star 5 light years away from earth in 1 week, my time, then I turn around and come back at the same speed. Now, what you’re calling time elapsed is the time as measured by an atomic clock _on earth_, say, which would say that my journey took a little more than 10 years. This is the frame of reference that’s important to most earthlings as it’s basically the same as their frame of reference. Virtually all humans would agree that this amount of time had elapsed. However, from my frame of reference only two weeks have elapsed, and my frame _is as valid as any other_. Essentially what is happening from my POV is as I approach c two dimensions are being contracted: time and the spatial dimension in which I’m travelling. > [@](#): > > However, it in meaningless to say what happens, or that anything happens > for an object when it attains the speed of light: > > Note the equation:t’ = t/square root(v^2/c^2) > > If v=c then the numerator of the right side of the equation = 0, and that > is not allowed! The internal logic of Special Relativity forbids any massive > object from attaining the speed of light, and renders the concept meaningless. I’m aware that massive objects cannot reach c. I wasn’t talking about actually going at that speed, just anthropomorphizing a description of photons. The theory of relativity does imply that time will not pass in a reference frame at c, firstly because as velocity tends to c, time tends to zero. But also because it would imply absolute time (and contradict the theory), if you could measure time passing in such a reference frame. [Previous page](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356.md?page=1) [Next page](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356.md?page=3)