The b-theory of time is often described as a film reel where the whole reel exists, but we can only view one frame at a time.

The problem I have with this analogy is that it starts to feel too theological. Where did the film reel come from? If it all began to exist "simultaneously" then biological evolution cannot explain diversity in nature because the current creatures already existed somewhere in the file, even from the beginning of the playback. I'd have to imagine the film reel being created by a director that builds it frame by frame, then sets it in motion.

I'm hoping that I'm not understanding the idea correctly and am taking the analogy too far. Is there a better way to think about it?

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    The answer might depend on whether the person endorsing the B-theory of time also endorses the view that states of the world at later times are generated by some type of mathematical function (deterministic or stochastic) operating on states at earlier times. If so, it may be that the development of complex organisms at a later time is implicit in the dynamical laws plus the initial conditions (and would be for a wide variety of initial conditions), in much the same way that nearly every complex mathematical theorem we've discovered is implicit in some set of axioms we knew about much earlier.
    – Hypnosifl
    Mar 18, 2020 at 14:56
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    I would endorse the view that states of the world at later times are generated by some type of mathematical function. In this case I could maybe reconciling it by imagining all of 4D spacetime including our worldlines as coming into existence based on some initial conditions. Then somehow our consciousness is moving along that worldline, but that seems to imply dualism though since our consciousness would have to be outside 4D spacetime in some sort of "supertime" in order to move along the worldline, right?
    – Physeo
    Mar 18, 2020 at 18:06
  • For B-theorists who are not reductive materialists about consciousness (they believe there are truths about subjective experience/qualia distinct from 'objective' truths about arrangements of matter/energy), there may be some difficulty with how to reconcile the B-theory "objective" view of spacetime with the subjective experience of flowing time, I imagine different B-theorists would have different answers to this problem. But this seems like a diff. issue from the one in your question above about whether evolution still explains complexity in the B-theory, might be worth a separate question.
    – Hypnosifl
    Mar 18, 2020 at 18:22
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    "Began to exist" and "already existed" are incompatible with B-theory, there are no tenses on existence there. It sounds like imagining some sort of meta-time of A-theory with B-theory spacetimes as slices? Playing a tape shot by shot no more explains what's on it than looking at it all at once. Conversely, what evolution does explain, gradual increase in complexity, does not depend on whether it is shown on a direction labeled still graph or "played out" live.
    – Conifold
    Mar 19, 2020 at 6:53
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    In the block universe, time doesn't pass So from our perspective, it appears that time flows or passes. But in the block universe model, time doesn't flow. In other words, in a block universe, there is no specific present moment, and "past" and "future" moments are relative.Dean Buonomano in his Book entitled "Brain as Time Machine" argued that: The human brain, is a complex system that not only tells time but creates it; it constructs our sense of chronological flow and enables "mental time travel" - simulations of future and past events.
    – user47436
    Aug 17, 2020 at 8:57

3 Answers 3


It is a fallacy to consider physical processes such as evolution existing for an observer "looking down" on block time. For that observer there is no such thing as space or time, they are by definition outside of both. The observer thus has no concept of simultaneity outside of a slice through the block map. The idea of the whole map coming into being "simultaneously" has no meaning to them. Whatever way they might appreciate the flow of causality in their higher universe, it will not be the same way the laws of physics work in ours.


Picture a pile of sand, being added to grain by grain. The exact way it piles and slips is unpredictable, and stochastic. Yet, the general behaviour averages to very predictable, forming a pyramid pile in a way related to the specific grains.

When we look at what dimensions are defined as, they are symmetries under transformation, which simplify descriptions, and are directly equivalent to conservation laws (Noether's theorem). We can picture this where say you move a spinning top one metre to the left, and the momentum is the same. But rotate around the axis or around the middle, and the results are different (ie it has rotational but not translational momentum). Same if something has momentum in a spatial direction: there is an assymetry in transformation. This kind of pattern is the core of what we call a dimension. Space and time are sets of transformations and localised properties within them.

Now consider fractional dimensions, fractals. These represent not simple linear or reversing changes as you move across a system (perform a transformation), but fractional changes. The simplest example is the Koch Snowflake, which can be interpreted as niether a line, nor a plane - it has a dimension of 1.26. Fractals occur everywhere in nature, like the branching patterns of trees or alveoli in lungs (you can see a good explanation of how simple rules generate patterns like this here). (my favourite example of behaviour 'simplifying' across a dimension in a fractional way, is blackhole turbulence)

So. In a block universe evolution is a stochastic fractal, pattern. It is a shape, that's all. Like the shapes sandpiles make. Sandpiles have a shape in 3D,and they have an evolving shape in 4D too.

Personally, I'm inclined to see it not as a single 4D block, but a stack of blocks, in 5D. The Holographic Principle, which explains where the information goes when blackholes evaporate, points to this. We could picture quantum event splitting as branching the 4D block, into an additional dimension.

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    This is an interesting way to look at it. The whole tree of evolution is a just a 4D shape. I'm picturing it as one of those Lichtenberg figures, but in a block of spacetime. Like the whole thing popped into existence based on the initial laws of physics.
    – Physeo
    Mar 18, 2020 at 20:25

Many philosophers of physics assume The Past Hypothesis, that the universe was extremely low entropy compared to today at the Big Bang, "over and above" (Harvey S. Brown) the laws of physics.

With unitary and symmetric laws of physics, the Big Bang is a necessary prior/initial condition to pick out a direction of time, e.g. the second law.

I say all this because this prior/initial condition is what stands in for the director in your analogy. Instead of director creating a reel of film to set in motion, it is initial conditions + physical laws of evolution that predicts rather than creates one frame to the next. And with symmetric and unitary laws of physics, any frame predicts its adjoining frames in either direction.

For the "setting in motion" part, the B-theorist has to say the feeling of time flowing is a subjective illusion. It's not that this flow of time, this subjective illusion in the brain, is unphysical or dualistic necessarily. Rather it has the same status as any other subjective experience, like colors, warmth, and pain. And B-theorists often say subjective experiences like these will be explained in time by neuroscientists and psychologists.

Why set it up this way instead of just having an objective flow of time? Because the laws of physics are completely symmetric. Thus we have to posit this extra Past Hypothesis to explain why world looks like it does today. So either the laws of physics are wrong/incomplete, or the laws are pretty accurate and it's the initial condition we have to explain.

Finally I can now say, from this block perspective of the laws, there is no privileged present. There is not even a privileged past or future. The Big Bang is just a prior condition, not necessarily an initial one. This equity of the laws, that any "frame" predicts and other frame is very attractive.

So as far as evolution, all we are committed to admitting is that prior conditions were such that together with the laws of physics predicted/created a branching structure of complexity of life. No extra extra commitments to a director, creator, theology, etc. We can happily predict backwards and forwards (within the laws' limits) to our hearts contents. Maybe that's all we can ask of science.

A final analogy from Harvey Brown might help. The laws of physics determine/predict one patch of spacetime to the next in any "direction". This situation is like a patterned rug, where one patch of the rug predicts the patches around it. No objective flow of time, no "creation", just predictions about patches. The psychological flow of time has to be be explained to complete the story, but many are optimistic.

Hopefully all this shows how a B-theorist might hang things together. Creatures evolved later (like us) really just "coexist" with all the other past creatures. There is no privileged present or slice of spacetime. (Again, physicalist minded B-theorists admit the further task of explaining conscious experiences including flow of time, but which philosophy doesn't have to ultimately explain conscious experience at some point).

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