# 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:** 8\ **Page:** 3
**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, 5:02pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/41 "2011-05-11T17:02:44Z")
> [@](#): > > 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. How would this mean that the change never catches up, though? If I go back to 1935 and kill Hitler, then the change has had 76 years to propagate forward, and it’s now 76 years after then, so the change has reached now. ---
**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, 5:23pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/42 "2011-05-11T17:23:09Z")
> [@Chronos](#): > > How would this mean that the change never catches up, though? If I go back to 1935 and kill Hitler, then the change has had 76 years to propagate forward, and it’s now 76 years after then, so the change has reached now. I believe the questioner would expect us to be another 76 years further on by the time the changefront reaches the present Nowpoint. When the Change reaches 2011, we will be in 2087. Our subjective Nowpoint will continue to be 76 years ahead of the changefront, given a propagation of one second per second. ---
**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, 5:34pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/43 "2011-05-11T17:34:42Z")
Think of it another way - when you travel to the past you are still aging. You are always moving forward in time regardless of what time ‘anchor point’ you are moving forward from. If this where not true you would still be unable to create paradox because as you ‘traveled’ back in time you’d ‘de-age’ and before you reached your father to kill him you’d simply vanish as you reach age zero. ---
**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, 8:43pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/44 "2011-05-11T20:43:41Z")
> [@Mijin](#): > > I think you’ve misunderstood me and maybe the physics. I am subject to correction on any point of physics, but so are you, and you seem to me to be confusingly interchanging a hypothetical entity which may accelerate to c, with a photon which can possess velocity c and none other. > [@Mijin](#): > > 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. One light year is the distance light travels in 52 weeks. Five LY would then be the distance light travels in 260 weeks. Subsequently the person in your example is not “going very close to the speed of light” rather he is going 260 times faster than light! Therefore the numbers provided cannot be used in a discussion which assumes that the Theory of Special Relativity is valid. > [@Mijin](#): > > 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. According to the table in my citation 10 years in my frame of reference would be equivalent to 2 weeks in the frame of reference of an traveler moving at a bit more than .99999c > [@Mijin](#): > > 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. I am not sure how there could be meaningful difference for anyone who is earthbound since his velocities would be insignificant compared to .99999c > [@Mijin](#): > > However, from my frame of reference only two weeks have elapsed, and my frame is as valid as any other. OK, if your velocity is .99999c > [@Mijin](#): > > Essentially what is happening from my POV is as I approach c two dimensions are being contracted: time According to my understanding your time does not contract from your point of view, your time contracts from the point of view of an observer in another, slower frame of reference. > [@Mijin](#): > > and the spatial dimension in which I’m travelling. OK, and this was a point I had not been aware of- thanks for pointing it out to me. I had heard of Lorentz-Fitzgerald and knew contraction applied to the moving object itself. I did not know it also applied to the space in the object’s direction of travel. The link I provided before has a page on the subject: [Length Contraction](http://www.fourmilab.ch/cship/lorentz.html) > [@Mijin](#): > > 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. This was not clear because you spoke of transition from “approach (of) the speed of light” to “the speed of light itself” which would be uncharacteristic of a photon. > [@Mijin](#): > > 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. SR should surely be explicit on such a fundamental point. > [@Mijin](#): > > But also because it would imply absolute time (and contradict the theory), if you could measure time passing in such a reference frame. I do not follow this. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [May 11, 2011, 9:17pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/45 "2011-05-11T21:17:24Z")
If events can only travel forward in time at a speed of one hour per hour, then people can only travel forward at that speed as well. If people and events can only travel forward at the normal speed, and never catch up to cause a paradox, then how is it a person can travel backwards in time instantaneously? If people can travel backwards instantly to the past, and instantly forward again to any point in time, then why can’t “events”? ---
**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, 11:12pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/46 "2011-05-11T23:12:19Z")
> [@Bear\_Nenno](#): > > If events can only travel forward in time at a speed of one hour per hour, then people can only travel forward at that speed as well. If people and events can only travel forward at the normal speed, and never catch up to cause a paradox, then how is it a person can travel backwards in time instantaneously? > If people can travel backwards instantly to the past, and instantly forward again to any point in time, then why can’t “events”? Well, for the sake of having any discussion at all we wave a hand a bit over the idea of someone jumping instantly around in time. However, I would say a couple things on the subject. 1. The man is an event. So, if he can jump instantly in time then events, indeed, can - by definition. But… 2. The man is an intelligent event, with a ‘time machine’ or whatever technology he/she is armed with. Whereas normal ‘events’ have to wait around for some 1 in a billion billion quantum event to move them. In an infinite universe that will eventually happen of course (has already happened an infinite number of times and will happen an infinite number of times again). Or - it’s never happened. In an infinite universe things tend to those extremes - they either never happen, or happen an infinite number of times. Of course, some infinites are more dense than others, making the events they represent more common, but it’s still infinite. This is also the argument I use to state that mankind is not alone in the universe. In an infinite universe things simply do not happen only once. ---
**Author:** ![BlinkingDuck](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@BlinkingDuck](https://boards.straightdope.com/u/BlinkingDuck)\ **Post date:** [May 12, 2011, 1:29pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/47 "2011-05-12T13:29:23Z")
> [@Hambil](#): > > This is also the argument I use to state that mankind is not alone in the universe. In an infinite universe things simply do not happen only once. Why not? You can Integrate a function over ‘infinity’ and get an answer of 1. ---
**Author:** ![Ann\_Hedonia](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ann_hedonia/32/319_2.png) [@Ann\_Hedonia](https://boards.straightdope.com/u/Ann_Hedonia)\ **Post date:** [May 12, 2011, 4:46pm UTC](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356/48 "2011-05-12T16:46:30Z")
There is only a time travel paradox if you believe in free will and a non-deterministic universe. Otherwise, all the time travel has been factored into the final equation and it’s kind of like rewinding a video, no paradoxes at all. [Previous page](https://boards.straightdope.com/t/time-travel-does-not-create-paradox/581356.md?page=2)