Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Illusion of Time: Is the Future Already Written?

  1. Nov 14, 2011 #1
    In Brian Greene's "The Fabric of the Cosmos: Illusion of Time" on PBS, http://www.pbs.org/wgbh/nova/physics/fabric-of-cosmos.html#fabric-time", he explains that if an alien were 10 billion light years away but not moving, the alien's current time (his "now slice") would be in sync with someone here on earth. However, if the alien then started moving away from Earth (even at a slow speed) , his "now slice" would be angled, resulting in his future becoming "in sync" with Earth's distant past. Even more strange, if he were to move towards Earth instead of away from it, his "now slice" would be angled towards the future so that his future would be in sync with Earth's distant future. Very Confusing! Is this implying that Earth's future is already written? And how does the alien moving towards us make his "now slice" become in sync with our distant future?
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Nov 14, 2011 #2


    User Avatar
    Science Advisor

    Terry, It's true the "now slice" changes depending on your velocity, but this has no practical significance. The information you receive does not come from "now", it comes to you from the past light cone. What a distant galaxy is doing "now" cannot be observed, because signals do not propagate infinitely fast. Likewise, a person on the distant galaxy can only observe what we were doing 10 billion years ago.
  4. Nov 14, 2011 #3
    Hi TerryWalsh, Brian Greene is correctly describing the situation you have with the 4-dimensional continuum represented in the mathematics of special relativity. And in his post, Bill_K has put an appropriate perspective on this.

    This situation is very similar to the situation described with Roger Penroe's "Andromeda Paradox." I've sketched it below. Some aliens in the distant Andromeda galaxy are making a decision about whether to attack earth. Bill and Ruth walk past each other on the street somewhere on earth. Since they are moving relative to each other they do not occupy the exact same simultaneous 3-D space. Note the difference in the time axes they follow as they move along their respective 4th dimensions. And their X axes (3-D space) point in different directions in the 4-dimensional continuum. In Bill's simultaneous 3-D space, the aliens are meeting to decide whether to attack. But, in Ruth's 3-D simultaneous space the aliens have already launched their attack.

    But, to answer your question, yes, in this model of the universe (embraced by some physicists, but not all) the future is already written. Einstein and Kurt Godel apparently had conversations about this, and both seem to have adapted this interpretation of special relativity--although I'm not completely satsified with the references usually presented when making this claim about Einstein's convictions on this.

    Last edited by a moderator: Apr 26, 2017
  5. Nov 15, 2011 #4
    Thanks for the diagram and intro to the Andromeda Paradox, bobc2! So, it seems like, as Bill_K has pointed out, it is more of a theoretical curiosity rather than a reality since there is no "practical" way of an alien ever being actually connected or knowing anything about our distant future. So if there's no information about it, is it safe to say that is equivalent to "it hasn't been written yet"? Or is it a "if tree falls in forest" problem? Again, because of this show, I am reading a lot of blogs by people stating that Brian Greene is explaining the future has already happened! For example, PeterNew writes in his blog http://fortyinfourbyforty.wordpress.com/2011/11/14/my-slice-of-now/" [Broken]
    And LogyExpress writes in her blog http://logyexpress.com/2011/11/14/the-bread-loaf-of-time/"

    According to her, the Aliens can see our future! They just can't tell us about it though, but it has been written!

    So again, has the future been written? Or is Brian Greene simply stating that our clocks are just out of sync relative to an alien's, but in no way implying that our future has happened already.
    Last edited by a moderator: May 5, 2017
  6. Nov 15, 2011 #5


    Staff: Mentor

    In the model you are talking about, the "future is already written" only in the sense that the universe is completely deterministic; given a set of initial conditions and the laws of physics, the entire 4-dimensional spacetime is completely determined. This model does *not* say that the aliens on Andromeda at a given time have information about portions of our worldline that are outside their past light cone at that time, and vice versa. In the diagram you drew, the aliens' launching the attack is outside the past light cone of both Bill and Ruth when their paths cross, and vice versa; the events are spacelike separated, so neither can have information about the other. The different "simultaneous spaces" have no actual physical meaning; they're just different conventions about how to slice up the spacetime.

    I don't think Brian Greene claims that the universe is completely deterministic, because he talks a lot about quantum uncertainty. So I don't think what he's saying actually implies that our future has happened already.
  7. Nov 15, 2011 #6
    Peter and Terry, I really think that Brian Greene is presenting a universe that is all there at once as a 4-dimensional continuum--actually existing as such in a real physical sense. With this kind of concept you don't view the universe as evolving in accordance with laws of physics. The laws of physics do not control anything. Rather, the laws of physics arise after-the-fact, i.e., as a consequence of the way the fabric of the universe is woven. If you have a long coiled spring, then run a long wire along the inside axis along the length of the spring, you would not sit back looking at it and say, "The spring is in orbit around the wire because there is a force of attraction trying to pull the two together like earth and moon." There is no force there--it's just the way the spring and wire are layed out--there is no causality between the wire and spring. But, you can make up the illusion that the wire is pulling on the spring.

    So, it's not the laws of physics that make for a deterministic universe, it's just that the 4-dimensional continuum is already all there. Einstein used words like "stubborn, persistent illusion", and "we physicists make no distinction between past present and future..."
    Last edited: Nov 15, 2011
  8. Nov 15, 2011 #7
    This was a vivid illustration of the fact that simultaneity has no intrinsic meaning in special relativity. I must admit, when I saw this NOVA documentary, it was the first time I realized there was this implication that time might be an illusion, though I learned special relativity a long time ago and even took a graduate level general relativity class.

    But there is still a distinction between future, past, and present that can be made.

    Any point in space-time has a past and a future. You can picture space time as being filled with a field of cones, called light cones, one at each point in space-time. Actually, it's a two-sided cone, so two cones that meet at a point. The inside of one of the cones is in the future of that point in space-time, and the inside of the other cone is the past. These cones are relativistically invariant. They don't depend on any reference frames or how fast anyone is moving--they are an intrinsic feature to space-time. So, given any two points in space-time, you can ask if one is in the future of the other. But, the concept of past and future doesn't really make sense if you are not inside one of these two cones--and it depends on picking out a point in space-time. So, there are infinitely many pasts and futures, one for each point of space-time, and, for each such point, not every event can be considered to be in the past or future. This is very counter-intuitive. We grow up thinking that something is either in the past of the future of some now slice.

    However, what we see here is that what we call our future has not "already" happened. The very definition of "already" ought to be that it is inside our past light cone. But that doesn't share any points with our future, namely the inside of the future light cone. There may be a danger of equivocation, here. You might want to use the word "already" in a different sense.

    Now slices are dependent on your reference frame or space-time coordinates, so they don't have an intrinsic meaning in space-time. That's quite a shock. However, the now slices, though they are not intrinsic to space-time are well-defined, in terms of stuff moving around within space-time, in some sense. Given a choice of clocks and rulers and something moving along in space-time, you get a local coordinate system, and a now slice. The now slice doesn't depend on the clocks and rulers. So, I would say it does have a physical meaning--it's just not intrinsic to the background space-time itself.

    It's not that time is an illusion. It's just that it isn't what we thought it was.

    It's not completely clear how to make sense of the concept of determinism in relativistic terms, since determinism is a concept that depends on time, which is relative in relativity. My definition of determinism would be this. A universe is deterministic if, given what happens in any given past light cone, everything in the future cone is determined. Now, quantum field theory, for example, wouldn't satisfy this condition.
  9. Nov 15, 2011 #8


    Staff: Mentor

    I understand the viewpoint, but I also understand that it requires determinism, at least in the following sense: if the entire 4-dimensional continuum is "already all there", then there is no way to "change" it. (BTW, strictly speaking the words "already" and "change" are incorrect, since they imply temporal relationships and the 4-dimensional continuum as a whole is not in a temporal relationship to anything; temporal relationships exist only *within* the continuum. It's hard to talk about this stuff without slipping into temporal language, though; hopefully my meaning above is clear.)

    At any rate, whether or not the above merits the term "determinism" (which also has temporal connotations that, strictly speaking, do not apply to the above viewpoint), my point (which I didn't really make clear in my last post) is that in so far as this viewpoint means that "our future is already written", that fact has *nothing* to do with the relativity of simultaneity for spacelike separated events. The reason is simple: the relativity of simultaneity is perfectly consistent, not just with the above view (that the 4-dimensional continuum is "already all there"), but also with the alternate view I proposed, that the universe "evolves" according to physical laws from some set of initial conditions, and that our "future", meaning the set of events in the future light cone of us now, is only "determined" to the extent that the universe's evolution as a whole is deterministic. So observing that, to us here on Earth, which event is "now" in the Andromeda galaxy can vary by several years if we change our speed (and vice versa), does *not* in itself tell us which of the above viewpoints is "true".

    I don't know whether Greene actually tries to argue that the relativity of simultaneity is evidence for his viewpoint (that the 4-dimensional continuum is "already all there") or not. I suspect not--I suspect he was only using it as an illustration of how there are different ways to "slice up" the 4-dimensional continuum.
  10. Nov 15, 2011 #9


    Staff: Mentor

    This is more or less how it's defined in GR; this Wikipedia page talks about the "initial value formulation" of GR, which is a way of making your intuitive definition above mathematically precise:

  11. Nov 15, 2011 #10
    In her defense, I don't know if she meant to claim that aliens have information about us. The point may have just been that this now-slice stuff doesn't have real-world significance, but she expressed herself poorly.
  12. Nov 15, 2011 #11
    I've always thought of time, in general, as the evolution of our universe. It doesn't seem that anything that's been said here contradicts that view.

    How about thinking of the '4D spacetime continuum' as an index, an artiface that we can 'move about' in? Then we don't have to entertain the notion that time is an illusion, and there's no difficulty in talking about the deterministic evolution of the universe.
  13. Nov 15, 2011 #12
    This seems a little bit vague. This is probably part of the reason that many scientific and mathematical discussions get too bogged down in heaps of impenetrable formalism. It's hard to talk about these things precisely.

    But, yes, we can do something like this. There's something called wrist-watch time. This is the time measured by your wristwatch. This has an invariant meaning, independent of reference frames. This assigns a quantity (after a choice of clock--i.e. unit of time) called the time elapsed to any curve in space-time (only time-like curves are physically realizable--that is, those that stay within light cones).
  14. Nov 15, 2011 #13
    I think the now slice has no significance at all. It's a number that can be calculated: so, what? As far as we know the aliens can have no influence on our now.
    Last edited by a moderator: Apr 26, 2017
  15. Nov 15, 2011 #14

    BRIAN GREENE: And if that's not strange enough, the direction you move makes a difference, too. Watch what happens when the alien turns around and bikes toward Earth. The alien's new "now slice" is angled to…toward the future, and so it includes events that won't happen on Earth for 200 years: perhaps our friend's great-great-great granddaughter teleporting from Paris to New York.

    Once we know that your now can be what I consider the past, or your now can be what I consider the future, and your now is every bit as valid as my now, then we learn that the past must be real, the future must be real. They could be your now. That means past, present, future…all equally real; they all exist.
  16. Nov 15, 2011 #15


    Staff: Mentor

    Hmm...that makes me want to give Brian Greene a kick in the pants. :grumpy:

    What he conspicuously *fails* to mention in the above is that the alien *cannot exchange information* with *any* of the events on the Earth's worldline that could be in his *now slice* by him changing his state of motion. He is spacelike separated from *all* of those events, so there is no causal contact between them. So the part in bold above is simply a non sequitur; the only sense of "past" and "future" that would justify saying that "the past must be real, the future must be real" is causal past and future, and the alien and Earth are *not* in each other's causal past or future, no matter how they move.

    Put another way, take any event on Earth's worldline that the alien could label as part of his "now slice", and call it event E. If event E is in the past for you, on Earth, then you, at your "now" instant, know what happened at that event, but the alien *can't* possibly know. And if event E is in the future for you, on Earth, then you can have a causal influence on that event but the alien can't. So either way, it doesn't matter which event the alien labels "now", or whether it's in your future or your past; whether or not that event is "real" has *nothing* to do with the alien at all.

    I'm disappointed that Greene muddies these waters, but unfortunately I'm not surprised. I remember having a similar feeling a number of times while reading one of his books (Elegant Universe or Fabric of the Cosmos, can't remember which). I've seen much better discussions of how spacetime works in other popular science books; IIRC Penrose has a good one in one of his (again, can't remember which), along the lines of: your "past" is your past light cone, and has already happened; your "future" is your future light cone, and has not happened yet; the rest of spacetime is "elsewhere", and you can't exchange information with it either way so there's not much point in worrying about it, or trying to pin down where exactly "now" is within "elsewhere". It's just not a big deal.
  17. Nov 15, 2011 #16
    I think you've summed it up pretty well, PatrickPowers. You can take the view that if something is going on outside of your light cones it doesn't matter. And that's a fair position to take. But, I don't think it is in the spirit of fundamental theoretical physics, which strives to understand what is out there, existing independent of observers.

    Einstein was the first to put physics in a position to pose serious questions about the structure of the universe, a subject encompasing much more than our limited light cones. If one does not wonder with curiosity about these things, that's fine. But, I don't think the scope of physics is based on whether or not we could personally be affected by the events of "elsewhere" (outside of light cones).

    I would not presume to have the knowledge and expertese to judge anyone's views on this. However, I think Brian Greene's presentation brings a very relevant and useful perspective to the special relativity discussion. I'm not sure I would know how you arrive at a logical definitive conclusion about this view of reality.

    There are certainly some bizaare outcomes if you think through some of the implications. And those bother me personally. Objectively, I can't find any aspect of Greene's concept that is not consistent with the 4-dimensional continuum described by relativity, but my problems with it are entirely subjective at this point. As Einstein said, my illusions are too stubborn and persistent to give up. At this point I regard the problem as a deep unresolved mystery.
  18. Nov 15, 2011 #17


    Staff: Mentor

    I don't disagree. But my point is that what Greene is describing tells us *nothing* either way. It is equally consistent with the view that the entire 4-dimensional spacetime continuum is "all there", so our future is already "written", *and* with the view that the universe is evolving in time, and the 4-dimensional viewpoint is a useful way of thinking about particular possible time evolutions but does not imply that our actual future is "already there".

    I don't react the same way to Einstein's pronouncements on the subject because he was careful to say, as you note, that his beliefs were "illusions...too stubborn and persistent to give up", *not* that they were somehow necessary implications of the relativity of simultaneity. Greene, in the quote PatrickPowers gave, seems to be strongly implying the latter, that the relativity of simultaneity somehow *requires* us to believe that "the future is already written", when in fact it is consistent with that view but in no way *requires* it.
  19. Nov 16, 2011 #18
    I too am irritated with Greene. How come he gets to be on NPR and I don't? I could do better than that. I know, I know, he wrote books that sell and I haven't...

    So let's see whether I can back that up. I tend to agree with the view that the past, present, and future all have equal status. But that doesn't mean that the future can't be changed. That means that the past can be changed, to precisely the same degree that the future can be changed.

    At first this seems outrageous, but think about how many events there are in the past of which mankind is completely unaware. The percentage is so high that it is hard to put a number on it. 99.999999999%? If one of those events changed, how would you know? You couldn't, because you were completely unaware of it. So how can you say that the past can't change? It could be that a great deal of it has. You would never know.

    The easy way is to exclude all those events of which we feel certain, but I don't feel like doing it the easy way. Suppose a few of those events of which we have a record decided not to happen and the record remains anyway. How would we know? All that we know for certain is that the records don't change. How do we know that the same particle didn't make two records? We don't. Many of the laws that we have are statistical and violations that cancel out in the long run would be undetectable.

    Looking at the future, there are a great many events that are virtually certain. See what I mean? The status is quite similar.

    The present doesn't really exist at all. All we can sense is the past.

    Great swatches of the past remain undefined or forgotten. If they were to change, where is the problem?

    That's enough for now.
  20. Nov 16, 2011 #19
    Maybe part of the point of relativity is that there is no "now", except that one cone point at each point in spacetime where you are.

    But I think the thing we are interested in as human beings, rather than as physicists, IS dependent on the observer. That is, we care about OUR now. We don't care about any other now. That's exactly the shocking element here. Physics is telling us our now doesn't really exist. That is exactly what those bloggers are worked up about. I think that's all Greene is trying to get across. It's probably better to argue in terms of space-like separation than in terms of now slices.

    The ironic thing is, I think this thread is just confirming what Greene is trying to say. "Future already written." It's not very clear what that means. I wouldn't jump all over it as though it were a very precise scientific statement. It's not. I think it's just a way to emphasize Einstein's discovery that simultaneity doesn't exist, except, maybe approximately at short distances and low speeds.
  21. Nov 16, 2011 #20
    Not a bad way to size it up. Thanks, Peter.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Similar Threads for Illusion Future Already
I Using Black Holes to Time Travel Into the Future