# Why does special relativity predict the future already exists? **URL:** \ **Category:** Factual Questions\ **Created:** [January 31, 2017, 6:19pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677 "2017-01-31T18:19:04Z")\ **Posts on this page:** 10\ **Page:** 5
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [February 6, 2017, 12:28pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/81 "2017-02-06T12:28:50Z")
> [@Francis\_Vaughan](#): > > Couple of random thoughts whilst I wait for lunch. > > Causality is transitive and (we currently assume) local in all dimensions. That covers a lot of the basis of the questions. > > However. A question of the form “does the future exist now?” Has a really difficult problem without going much further. What does “now” mean in reference to the future? Has the future always existed? What does that even mean? What does always mean? Time has become self referential. The “now” in reference to the future would really be a 4th dimensional being’s “now,” which would encompass our universe’s past, present, and future. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [February 6, 2017, 1:10pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/82 "2017-02-06T13:10:07Z")
> [@KidCharlemagne](#): > > The “now” in reference to the future would really be a 4th dimensional being’s “now,” which would encompass our universe’s past, present, and future. So it is meaningless to us then. This 4th dimensional being isn’t being suggested as a real entity. Nobody is positing that such a beast exists, and I don’t think the current laws of physics can support the existence of one, no matter what. Rather the creature is a device for thinking about the problem. So, given this beast doesn’t exist, it seems that indeed, it makes no sense to talk about the future existing “now”. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 6, 2017, 2:01pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/83 "2017-02-06T14:01:48Z")
> [@Francis\_Vaughan](#): > > So it is meaningless to us then. > > This 4th dimensional being isn’t being suggested as a real entity. Nobody is positing that such a beast exists, and I don’t think the current laws of physics can support the existence of one, no matter what. Rather the creature is a device for thinking about the problem. So, given this beast doesn’t exist, it seems that indeed, it makes no sense to talk about the future existing “now”. That’s my (admittedly very amateur) take. > [@Half\_Man\_Half\_Wit](#): > > … > A way to make sense of this is to imagine the universe as 2d, and glue all the present moments of Newtonian time together, into one long world sausage. You can then slice this sausage at different angles—these slicings correspond to different inertial observers, each slice being a given present moment. Now suppose two slices intersevt—say, one cuts the sausage orthogonally, one at a 30° angle to the first. These are the present moments of two observers, and the intersection corresponds to their meeting. However, each one observer’s present contains events to the future and the past of events in the other observer’s present. > > In the analogy, this is only possible because the full sausage was on the table; but translating that back to spacetime just is eternalism. Nice analogy. But it assumes the existence of a POV able to observe the entire sausage from any angle and hence any slice. And do so at any “tyme” as “tyme” has meaning to that POV. Which is a very different idea from the concept of time that exists within the sausage where sausage inhabitants see these slices as instances of observer-relative simultaneity. Having to posit the existence of this POV and “tyme” to force the model to exhibit eternalism strikes me as pre-Copernican epicycles. We’re thinking wrongly and force-fitting a non-physical explanation onto some real phenomenon. The (apparent to me) fact that we’re force-fitting is close to proof that we’re thinking wrongly. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [February 6, 2017, 2:36pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/84 "2017-02-06T14:36:54Z")
> [@Asympotically\_fat](#): > > I’d disagree, the slicing is not _that_ important that is should suggest eternalism; what us important is the causal structure. > > In Minkowski space, for any event p, the rest of the spacetime into 3 disjoint sets: the causal past of p, the causal future of p and those events which are in neither the causal future or past of p. For the purposes of eternalism I would say this is no different to the way Newtonian physics, for a given event, divides all other events into past, future and present which are also disjoint. Well, there certainly are ways to resist the Rietdijk-Putnam argument, but to the best of my knowledge, what I’ve written above is a reasonable account of it. One possible rejoinder, which might be what you have in mind, is to restrict the ‘present moment’ as really just being the one single point at the tip of the past lightcone—i.e. having each event’s present moment be that event alone. But this makes a couple of ways we usually talk rather awkward. For instance, what exactly is meant by things like ‘five minutes ago on the sun’? If there’s no notion of present moment that extends to the sun, then there’s nothing five minutes earlier to that moment; yet, we can perfectly well, and truthfully, claim that ‘if there was a massive solar flare five minutes ago on the sun, we’ll see its effect in roughly three minutes and a half’. But if things like ‘five minutes ago on the sun’ pick out some spacetime event, then so does ‘yesterday on Andromeda’, and the Rietdijk-Putnam argument works out. Also, one has to be careful not to get tangled up in the seductive image of the present moment being a point ‘traveling along’ the worldline of some observer—for in what time would this ‘traveling along’ take place? > [@LSLGuy](#): > > Nice analogy. > > But it assumes the existence of a POV able to observe the entire sausage from any angle and hence any slice. No, we can conceive of it completely abstractly—just as a two-dimensional spherical surface doesn’t imply any three-dimensional space to be embedded within. No need for any ‘tyme’, or ‘thyme’, anymore than there are additional directions of space (or should that by ‘spice’?) needed to intelligibly talk about a two-dimensional curved surface, or indeed a one-dimensional line. ---
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [February 6, 2017, 3:56pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/85 "2017-02-06T15:56:35Z")
> [@Francis\_Vaughan](#): > > So it is meaningless to us then. > > This 4th dimensional being isn’t being suggested as a real entity. Nobody is positing that such a beast exists, and I don’t think the current laws of physics can support the existence of one, no matter what. Rather the creature is a device for thinking about the problem. So, given this beast doesn’t exist, it seems that indeed, it makes no sense to talk about the future existing “now”. Of course it’s a device but that doesn’t make it impossible that in the 5th dimension, the first four are spatial. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [February 6, 2017, 4:12pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/86 "2017-02-06T16:12:29Z")
> [@KidCharlemagne](#): > > Of course it’s a device but that doesn’t make it impossible that in the 5th dimension, the first four are spatial. What 5th? The whole point of the idea is that our universe must have more dimensions than we perceive, and that an entity that somehow does perceive another dimension will see us. We both inhabit the same universe. Otherwise it can’t see us. One probably needs to belabour one point. Time dimensions behave differently to spatial ones. You can’t just deem a time dimension to suddenly become a spatial one. So far, although we have theories that include higher dimensions, there are no workable theories that have more than one time dimension. Thus our higher dimensional being will need to have **the same** time dimension as us. That does indeed mean that the first four are spatial. But what it does mean is that three of them are shared with us, and the fourth spatial dimension is the new dimension that the 5D being uses that we don’t. This makes it a bit useless, since we reduce to precisely the Flatland argument. (The idea that the 4th dimension must be time, and that the 5th dimension is the next spatial one is a bit useless. The numbering is arbitrary. Similarly there is no ordering. I would make time the zeroth dimension personally.) The alternative is that the original question becomes a tautology. It ends up being, “in this alternative made-up universe where there is a 5 dimensional being that has a different time dimension to us and sees our time as a spatial dimension, thus seeing our entire time-line in a lump, does the future already exist?” The answer of course is yes, because you have defined the universe in such a way that it does. But it isn’t our universe. So it isn’t clear the exercise has been helpful. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [February 7, 2017, 1:45am UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/87 "2017-02-07T01:45:33Z")
Robert Heinlein’s first SF story, “Lifeline,” dealt with this idea. Every person is a “tube” in 4-space, from their birth to their death. A character works out a way to send a signal-pulse along that tube, and the time-delay to the echo tells how far away the ends are. Since we know the date of one end – your birth – the other end has to be your death, and so now we can learn how long people will live. The person’s lifeline is described as being “fixed” in 4-space, and each person’s death is predestined. As SF, it works! (Isaac Asimov, in The End of Eternity, also deals with time travel that has two separate time dimensions. Fun book!) ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 7, 2017, 2:57pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/88 "2017-02-07T14:57:11Z")
> [@Half\_Man\_Half\_Wit](#): > > Well, there certainly are ways to resist the Rietdijk-Putnam argument, but to the best of my knowledge, what I’ve written above is a reasonable account of it. One possible rejoinder, which might be what you have in mind, is to restrict the ‘present moment’ as really just being the one single point at the tip of the past lightcone—i.e. having each event’s present moment be that event alone. I’m not putting forward an alternative, I just don’t find the argument very convincing, it seems to mix and match ideas about past, present and future, whilst not even referencing the most important description of causality in relativity: causal structure. > [@](#): > > But this makes a couple of ways we usually talk rather awkward. For instance, what exactly is meant by things like ‘five minutes ago on the sun’? If there’s no notion of present moment that extends to the sun, then there’s nothing five minutes earlier to that moment; yet, we can perfectly well, and truthfully, claim that ‘if there was a massive solar flare five minutes ago on the sun, we’ll see its effect in roughly three minutes and a half’. But if things like ‘five minutes ago on the sun’ pick out some spacetime event, then so does ‘yesterday on Andromeda’, and the Rietdijk-Putnam argument works out. Clearly reference frames do provide a notion of the present, but as always you need to be careful about drawing conclusions from them. For example would it make sense to talk about 5 minutes ao at the edge of the observable Universe? What about accelerated observers for whom we don’t have a clear procedure to construct global frames of reference in special relativity? [quote] Also, one has to be careful not to get tangled up in the seductive image of the present moment being a point ‘traveling along’ the worldline of some observer—for in what time would this ‘traveling along’ take place?]/quote] I’m not trying to answer questions as to the metaphysical setting of special relativity, but the actual physics an observer experiences is about what happens along there wordline, regardless of any reference frame they construct E.g. when they see a faraway object, what they see is determined by the light arriving from that object at that moment. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [February 7, 2017, 4:41pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/89 "2017-02-07T16:41:18Z")
> [@KidCharlemagne](#): > > They try and make it seem as if the universe is consciously conspiring to stymie our attempts to figure out if light is a particle or a wave when, at least according to the physicists I’ve spoken to, the real revelation is if we attempt to detect a particle, we detect a particle and if we attempt to detect a wave, we detect a wave. Maybe a highjack, but this is exactly the wrong way to think about light’s nature. The right way is to understand that a ‘particle’ is a simple mathematical model of a thing, and a perfect wave is a different simple mathematical model. But there’s no reason any particular thing has to match up to one of these two simple models, and it turns out that light is a complicated thing that can’t be represented perfectly by either simple model. There are situations and particular questions (say, making eyeglasses) where the simple wave model is a good enough fit to give good answers; and there are situations and particular questions (say, calculating spectral lines) where the simple particle model gives good enough answers. But that doesn’t mean light ‘is’ a wave or a particle; light is just what it is: something that’s too complicated for either simple model. ---
**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:** [February 7, 2017, 6:39pm UTC](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677/90 "2017-02-07T18:39:57Z")
> [@Quercus](#): > > Maybe a highjack, but this is exactly the wrong way to think about light’s nature. The right way is to understand that a ‘particle’ is a simple mathematical model of a thing, and a perfect wave is a different simple mathematical model. But there’s no reason any particular thing has to match up to one of these two simple models, and it turns out that light is a complicated thing that can’t be represented perfectly by either simple model. I thought the Double-Slit experiment shows us light behaves as a particle AND a wave at the same time. The screen at the back of the experiment shows particles of light hitting it but arranged in a pattern that only makes sense if they behaved like waves going through the slits. [Previous page](https://boards.straightdope.com/t/why-does-special-relativity-predict-the-future-already-exists/778677.md?page=4)