Block Universe Implications

  1. Chestermiller

    Staff: Mentor

    I’m relatively new to Physics Forums, so please go easy on me guys. I’m trying to get an understanding of the so-called Block Universe concept and its implications, but I need help. I’m going to start out by articulating my current understanding, and then try to apply this understanding to develop some interpretations (which may or may not be correct). I welcome your comments and criticisms.
    My understanding is that, inherent in the Block Universe concept are the following three features:
    1. the acceptance of time as a bona-fide spatial dimension, within the constraints of the Minkowski metric.
    2. the requirement that all physical objects, both animate and inanimate, travel along their world lines at the speed of light.
    3. The notion that all events occurring along an object’s world line exist simultaneously (in a 4D sense) and are equally real.
    The only true axiom on this list is the first. The other two features seem to follow automatically from the predictions of the Lorentz Transformation.
    Now for some interpretations:
    In connection with feature 3, if all the events in a person’s life exist simultaneously in a 4D Block Universe and are all equally real, then it would seem to me that the only way this can happen is if there exists a continuous sequence of younger- and older versions of this same person strung out along his world line. For a person of a specific age, the younger versions must be situated behind him on his world line, and the older versions must be situated ahead of him. The actual distances between these versions can be calculated as the differences in age times the speed of light. But all the versions would exist simultaneously.
    It would also seem to me that this very same type of interpretation can be applied not only to animate objects, but also to inanimate objects. Thus, in the case of the usual Train Example, if I am an observer in the S frame of reference on the ground and the train represents the S’ frame of reference, the middle of the train might be directly opposite me, and exhibiting a clock time of t’; but there would be another observer within my ground frame of reference further up the track that sees a clock in the caboose that exhibits a time reading greater than t’, and still another observer further down the track that sees a clock in the locomotive reading a time less than t’. Under this interpretation of the Block Universe concept, the clock in the caboose would be “older” than the one I see opposite me, and the clock in the locomotive would be “younger” than the one I see opposite me (assuming that all three clocks were fabricated at the same time in the S’ reference frame).
    It would also seem that, in this Block Universe model, the rate at which each version of a person is aging is exactly one year for every light year of travel along his world line. As a result of this feature, from the perspective of a “stationary” observer viewing the Block Universe from outside, nothing within the Block Universe would appear to be changing. Younger versions would replace older versions at precisely the right rate so that the Block Universe would seem to be in a permanent condition of steady state equilibrium…frozen. The reality, however, would be much more dynamic than this. There would be a continuous flow of younger and older versions of each animate and inanimate object along each world line.
    The flow of all the various versions of objects along each world line at the speed of light would also seem to translate into a continuous steady flow of mass along that world line. The sum total of all these continuous mass flow rates along all the world lines throughout hyperspace would represent an enormous continuous rate of mass flow. Where could all this continuous mass flow be coming from? The only logical answer, if we trace the flow back to its source, would seem to be the big bang. But, the big bang has always been viewed an instantaneous isolated mass injection event that took place ~14 billion years ago. The Block Universe interpretation of the big bang would appear to markedly differ from this. It would imply that, rather than representing an a single isolated injection event, the big bang could be viewed as an ongoing event representing a steady continuous point source of mass flow into 4D hyperspace… a sort of a black hole in reverse (in a 4D sense).
    This completes what I wanted to discuss. Please give me feedback on whether my general understanding of the Block Universe concept is in any way correct, and if the interpretation I have presented makes any sense whatsoever. Thank you.
  2. jcsd
  3. bcrowell

    bcrowell 5,682
    Staff Emeritus
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    Most of this is not correct, most of it has nothing to do with the block universe, and most of it isn't physics.

    "the acceptance of time as a bona-fide spatial dimension"
    Time is a timelike dimension, not a spacelike one. Viewing time as a fourth dimension is not specific to the block universe picture. "Bona-fide" is meaningless here.

    "the requirement that all physical objects, both animate and inanimate, travel along their world lines at the speed of light"
    This sounds like Brian Greene's description, which is misleading and not a description widely used by physicists. In any case, it is something generic about relativity, not something specific to the block universe picture.

    "The notion that all events occurring along an object’s world line exist simultaneously"
    This is wrong. Relativity says that if event B lies inside event A's light cone, then they are not simultaneous in any frame.

    "and are equally real."
    This is meaningless, because "real" hasn't been defined.

    "The only true axiom on this list is the first."
    Huh? Why?

    "The other two features seem to follow automatically from the predictions of the Lorentz Transformation."
    In science, a "prediction" is something that can be tested against experiment. Neither 1 nor 3 can be tested against experiment, so neither is a prediction.

    The block universe picture is philosophy, not physics.

    Before we could meaningfully discuss your statements about what is "real" or "bona-fide," you would have to define what those terms meant to you.
  4. Chestermiller

    Staff: Mentor

    This is in response to the reply by bcrowell. Thank you for responding so quickly to my post, and thank you for identifying what you deemed to be wrong with my understanding of the Block Universe concept. I was hoping that in addition to pointing out what is wrong, responders would also articulate their own (correct) understanding of Block Universe to those of us who have puzzled long hours over this. I have seen many posts to PF on Block Universe asking about its basic features, but none of these seemed to make the concept very clear. I realize that there is substantial controversy over this subject, and, as an expert, you may not hold it in very high regard. As for myself, I haven't reached an opinion yet. But before I can, I need to have a better understanding of what it is saying. Do you think you would be capable of providing a balanced explanation in a few hundred words (or less) so that less experienced scientists like myself could get a clearer picture?
    As for some of your other comments: I must admit that I am proud to be lumped in the same basket with Brian Greene, even if his description may not be mainstream. Also when I observed "that all events occurring along an object’s world line exist simultaneously", I didn't mean to use the s-word in its usual sense; I should have qualified it by saying that I was referring to simultaneity in a 4D Block Universe sense, in which an observer is viewing space-time from the outside. Finally, what I meant by "real" was, in the case of animate objects "living."
    I'm really looking forward to reading your articulation of the Block Universe concept. Thanks so much.
  5. bcrowell

    bcrowell 5,682
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    It's not that I don't hold it in high regard, it's just that it's not physics, it's philosophy. If you want a clear explanation of it, you should ask a philosopher. PF does have a philosophy forum under general discussion.
  6. Here's a different view of the block universe: It is altogether physics. Don't look for help in the philosopher forum (I've tried it). They will get you way off into discussions of esoteric stuff that we physicists have no interest it. The block concept is of enormous significance to many physicists who are motivated to do theoretical physics because they feel it beckons one to pursue the understanding of external objective reality (see "The Road to Reality" by Roger Penrose). Fundamentally, that's the loftiest goal of physics.

    However, physicists do not fully comprehend the block concept because at this point we are limited in our fundamental understanding of time and of consciousness.

    Here is a space-time diagram sketch illustrating the motivation for the block universe concept. As you pointed out, it is based on the Lorentz transformations of special relativity.
  7. pervect

    pervect 7,878
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    "Block time" basically is a philosophical concept as Ben says. I'm not sure how much luck you'll have finding someone to talk about it, I regard philosophy as a sort of "necessary evil", so I'm not particularly good at it, or well read on it. You might or might not have better luck in the philosophy forums - you'll at least find people interested in philosophy there, but I'm not sure how many of them will be interested in this particular part of philosophy.

    I do occasionally find myself having to discuss philosophy in order to get to the more interesting parts, which is the actual physics, but because philosophical discussions can drag on forever, I don't generally follow up much.

    The current wiki article linked to "block time" seems to me to be relatively sane at the moment (this wasn't always true), see

    I think you did get some of the elements of the phiilosophy right (from reading the article, at least).

    From a physical point of view, though, all that's really necessary is to say that the set of events in space-time that observer "A" regards as being simultaneous is DIFFERENT from the set of points in space-time that observer "B" regards as being simultaneous, where A and B are observers at the same event in space-time who are moving at different veolcities.

    In my view, "Block time" aka Eteranalism doesn't have some of the probelms that what Wiki calls "Presentism" has - if "only the present is real", but the present is different for observer A and observer B, then you have the problem of "reality" being different for A and B.

    But realities are (in my view, at least) supposed to be the same for different observers. I"m pretty sure that not all philosphers will agree with this simple statement above, though (my opinion , for what it's worth, is that that's almost always a safe statement to say that philosphers will disagre about almost anytihng.). Thus it is better to say "observer dependent" and "observer independent" rather than "real" or "not real" - it's more precise - there is less extraneous "baggage".

    So, being as precise as possible, we can say that the concept of "now" is observer-dependent in SR, where it was not observer dependent in classical physics.

    The rest of "Block time" you can use if you like, or not use if you don't like, like most philosophies it doesn't matter to anything you can measure If it did matter to something you could measure, it wouldn't be a philosophical issue anymore, but one that could be decided by experiment - i.e. a scientific issue.
  8. atyy

    atyy 9,799
    Science Advisor


    Lewis, Coordinate Confusion in Conformal Cosmology: "The choice of coordinates is down to personal preference, as both must give the same predictions. From all of this, it should be clear that it is futile to ask the question “is space really expanding?”; the standard-FLRW metric and its conformal representation are the same spacetime. No experiment can be formulated to differentiate one personal choice of coordinates from another."

    Block universe just means the primary entity is spacetime not space + global time. There are many ways to split spacetime into space +global time, but none are "the best", so in the block time view spacetime is primary, and global time is not.
    Last edited: Mar 4, 2012
  9. bcrowell

    bcrowell 5,682
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    This is going to depend completely on what one means by "real."
  10. pervect

    pervect 7,878
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    Yes. That's why it's best to say "observer dependent" or "observer independent". Which I tried to mention later on.

    It's my own personal phillosophy (and not any generally agreed on statement) that says that observer independent quantities are 'real" and observer dependent quantities are not. I''d guess this is a common enough position to have a name in the philosophical literature, but I don't know what that name is, alas.

    There are some other issues here as to how one might define observer dependent or observer independent. The individual components of tensor quantities vary between observers, for instance, but I'd regard the tensor as a whole as falling in the "observer independent" category.
  11. pervect

    pervect 7,878
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    A bit off-topic, perhaps, but it seems my position has finally found a mirror in the literature that the whole expanding / not expanding thing is equivalent to arguing about coordinates.

    Now they only need to note that relative velocity, as they've defined it, is ALSO down to a similar coordinate choice (along with distance). Specifically, you need to specify a surface of constant time to measure either - you don't need to specify the rest of the coordinates though.

  12. But the point of the space-time diagram is that both events, R1 and R2, exist and are real, and the Red guy exists and is real at both R1 and R2 events. That is, when the brown guy and the blue guy meet, R1 exists in Blue's instaneous 3-D world while R2 exists in Brown's instantaneous simultaneous world.

    Don't overcomplicate the problem by asking what you mean by real. I think we all know what we are talking about. We simply use a protype example of real. When Blue and Brown meet, Brown observes that Blue is a real observer and Blue observes that Brown is a real observer. If you and I are standing in a room talking, I identify you as a real body and you identify me as a real body--that's our prototype example of real--unless you are a Solipsist--but Einstein was quite emphatic about prohibiting solipsism in relativity). In the purely physical sense each observes the other as an existing 3-dimensional physical body. In that same sense, each observes the Red observer to be a real 3-dimensional physical body.

    Now, it is not a question of whether Brown or Blue correctly identifies the Red observer as being real. Both Brown and Blue are correct in observing that a real Red body exists in their respective 3-D worlds at the event of Brown's and Blue's world lines intersecting (Brown and Blue meet).

    That's the whole point of the block concept: the Red body must be a 4-dimensional body for both Brown and Blue to each have Red existing as a 3-D body in their respective instantaneous 3-D worlds. Brown and Blue each observe a different 3-D cross-section of Red's 4-dimensional body.

    Putting a little more into the 4-dimensional reality picture, we have added in another observer to the previous space-time diagram: Another Red guy who is a companion to the first Red guy--he is at rest in the original Red's inertial frame of reference (moving at the same velocity as the original Red, with respect to the other observers). The event, R3, on the original Red world line is simultaneous with the new Red when the new Red meets with Brown and Blue at the intersection of all three world lines. Thus, the original Red is real and exists at event R3 as a 3-D body when the new Red guy meets up with Brown and Blue. The new red guy is viewing yet another 3-D cross-section view of the original 4-dimensional Red guy.

    Again, by extension, we could keep adding more and more observers with different Lorentz boosts until the original Red world line is seen to be a continuous 4-dimensional body, i.e., a real physical body extending continuously along Red's world line. We could then develop the concept for every other observer and every other object in the universe until we recognize the entire universe is 4-dimensional populated by 4-dimensional objects. That's the block universe concept.

    I think Chestermiller was looking for help on understanding the implications of the block universe. But, I yield the floor after such extended verbage. Hopefully someone will help Chestermiller with some of those implications.
    Last edited: Mar 4, 2012
  13. Chestermiller

    Staff: Mentor

  14. You can measure the path of a photon as either time, length or the two at the same time. I can't see how this could be done in a block universe "reality". It's a continuum. Space is isotropic. Mass is measurable. All sorts of things....

    I have almost 12 months of PF "experience" in the field of internet forum layman physics. (raising the point of your experience in the field is moot, unless you are a "leading expert" in the field there is no additional authority due to experience with regard to interpretation )

    Lastly just from a conceptual perspective, I cannot envision 4 spatial dimensions, it makes no sense to me. 3 isotropic spatial dimensions with an, invariant limited speed does make sense to me and is more sensical in comparison to eternalism.
    Last edited: Mar 6, 2012
  15. Here is an experiment to verify predictions of the block universe model. We have three observers, Brown, Blue and Light Brown. The experiment predicts observations associated with events indicated in the space-time diagram below. The events along with predictions of observations are planned in advance of the initiation of the events.

    The space-time diagram presents the three world lines associated with the three observers along with the events. The green lines represent the world lines of signals transmitted between observers during the experiment (traveling at the speed of light in all inertial reference frames).

    We wish to provide experimental verification of the existence of the light brown observer as a real physical 4-dimensional body (real body as defined in the earlier post #11). The experiment accomplishes this by providing experimental evidence that when the Blue observer and Brown observer meet, each establishes a separate 3-D existence for the Light Brown observer. That is the Light Brown observer exists at both event 11 and event 12 while Blue and Brown are together. The existence of the Light Brown observer is established at other points along Light Brown’s world line as well (events 8, 9, 10, and 13). Existence of separate 3-D bodies (observed simultaneously by two different observers--Blue and Brown in our experiment) require the existence of a 4-dimensional body, and different observers moving at different velocities relative to each other will observe different 3-D cross-sections of the 4-D body.

    All three observers establish their existence as real bodies as they meet at event 1 before proceeding with trips into space-time. Each observer keeps data on each of the events indicated in the space-time diagram. Data is exchanged throughout the experiment with signal transmissions (including photographs of the observers as they perform their measurements, and time data along with computed distances, etc.). The Blue observer and Light Brown observer initially travel at high speed to event 8, where they both come to rest with respect to the Brown inertial frame of reference. The Blue observer then leaves the Light Brown observer, moving in the opposite direction, headed back for the encounter with the Brown observer at event 5.

    The culmination of the experiment (not shown in the diagram) occurs with both Light Brown and Blue returning to join Brown. All three are together comparing their data to determine whether all observations and measurements are in agreement with the predictions made based on the Block Universe model. The predicted data include clock times and computed distances to be recorded by each observer at each of the events planned (as indicated in the space-time diagram)-- along with predictions of times and other content of transmitted information (transmissions indicated by green photon world lines). Particularly important is the prediction of content of information that is to be received at planned events.

    At the final meeting of Brown, Light Brown and Blue, the observations and measurements are found to be in agreement with the predictions for the experiment (otherwise, special relativity would be invalidated).
    Last edited: Mar 6, 2012
  16. As far as I know Special Relativity does not take a position on whether the universe is deterministic or not. The block universe seems to adopt Special Relativity and take the deterministic position. Since there is no experiment that at least in principle could distinguish between a deterministic or non deterministic universe, any discussion of the block universe would have to be purely philosophical as it does not and cannot predict anything that is not already predicted by SR.
  17. But, I just presented an experiment that would predict outcomes based on the Block Universe Model.

    Please indicate where in my example you fee that it fails to demonstrate a positive result.
  18. In reply to post #16

    Referring to Space and Time in Specal Relativity by Mermin p41

  19. Yes. One should be very careful. I was very careful to assure the constant velocity travel paths were very very long compared to the turn-around paths for traveling observers. On the scale of my space-time sketch you would have to zoom in at great magnification to detect the curvature of that short turn-around path. We can assure a negligible effect on the final data from the turn-around.

    I'm sure folks on this forum do not discount the results of the twin paradox thought experiment on the basis of the turn-around for the traveling twin.
  20. DaleSpam

    Staff: Mentor

    There is no possible way to make an experimental test that would distinguish between a block universe and a deterministic non-block universe.
  21. Why? Please explain. I provided an explanation via the thought experiment. Point out where the experiment produces a negative result.

    And please explain what you mean by a "deterministic non-block universe."
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