Back to Block

A crushed flower.

This post has nothing to do with Amy Winehouse, sadly on the list of great talents who, poorly served by those in their lives, lost their way and died tragically and long before their time. (It’s bad enough when the ravages of life — disease and accident — steal away those with gifts. Losing people to human foibles is a more painful loss.)

The topic here is the Block Universe Hypothesis, which I’m revisiting, so the title kinda grabbed me (and I am a Winehouse fan). I’ve written about the BUH before, but a second debate with the same opponent turned up a few points worth exploring.

So it’s back to basic block (everyone looks good in block?)…

Or do they? If the BUH is true, then poor Amy Winehouse never stood a chance of a different life; the arc of her life was fixed when the block universe was created.

Which seems to demand the question: What, then, is the point of the universe? What is the point of this unwinding sense of «now» we all experience? Why does it seem as though we have free will and make choices?

As I wrote in the first post, the implication is that the entire universe (not just what we see now, but all of it past and distant future) was created all at once.

The opposing view is that the universe is essentially a quantum computer running a simulation (called “Classical Reality”) in real-time; it “knits” reality as it goes. The past is a woven tapestry of histories, the future is yet to be woven, and the «now» we experience is the leading edge of the ongoing calculation of history.

In that reality, Amy Winehouse might have found a better path.

(Because only in an evolving universe is there any shred of hope for genuine free will, and a universe with free will is a lot more interesting.)

§ §

The metaphysics of the BUH depend, firstly, on accepting a determined universe. Everything in the BUH has, in some sense, already happened.

Secondly, it depends on how Special Relativity transforms simultaneity between frames in motion relative to each other. The rotation into the apparent future is interpreted as meaning it must already exist.

One problem is that simultaneity can only be established after the fact. It’s meaningless in the moment (because, once again, it’s virtual).


This was apparently a big issue in the debate, so I’d like to explore it in detail. Let’s start with a definition of the word:

virtual: adjective; 1. In effect or essence, if not in fact or reality; imitated, simulated.

The secondary definitions all include some notion of “not in fact or reality” one way or another. To be virtual is to lack reality. To say, under SR, that simultaneity is virtual is to assert that simultaneity isn’t real.

Which is exactly what SR does assert.

In my Special Relativity Series, I spent four posts (see: SR#10, SR#11, SR#12 & SR#13), writing about how we define simultaneity; about what it means to say two events happened simultaneously. I also wrote about it in the Blocking the Universe post. The reader is urged to be familiar with those posts.

Very briefly, then, we define simultaneity by considering how long the light from something takes to reach us and subtracting the time it took light to travel the distance. That tells us when the event happened, relative to us.

Figure 1. Our frame of reference. We define when distant events occurred by how long it takes their light to reach us. In particular, we can only say all the events shown (the black dots) happened (simultaneously) at t0 after their light reaches us. (Note the three events labeled X, Y, and Z. These are referred to below.)

A crucial point is that we cannot make this determination until after the light reaches  us. It’s only in retrospect we’re able to define events as having happened simultaneously. I can’t over-emphasize the importance of this.

To illustrate that simultaneity is virtual, first consider Alex (Frame A), who is moving at 0.5c from left to right in the diagram:

Figure 2. Alex (green line) sees event Z as happening first, then event Y, and finally event X. The blue lines are Alex’s lines of simultaneity.

From Alex’s perspective, the events occur in the order Z⇒Y⇒X. They are not simultaneous as they are in our frame (Figure 1). Note that Alex (and Blair below) are shown from our perspective.

Crucially here, the events are causally separated — they cannot affect each other. That no event causes the others is important because all frames of reference respect causality. If event P causes event Q, all reference frames agree P came before Q. It’s only with causally separated events that we can have differing accounts of their order.

Now consider Blair (Frame B) who is moving (also at 0.5c) from right-to-left:

Figure 3. Blair (green line) sees event X as happening first, then event Y, and finally event Z. The blue lines are Blair’s lines of simultaneity.

From Blair’s perspective, the events occur in the order X⇒Y⇒Z. When we move towards something, more distant events seem to shift backwards in time. The greater the distance, the greater the shift.

This is why Blair sees the most distant event, X, as first, while Alex, coming from the opposite direction, sees event Z as first. When we move away from events, this reverses — we’ll define the closer events as having happened first.

(BTW: There’s a nice animation showing this same example on the Wiki Spacetime page.)

§ §

One thing to understand about the “surface” of simultaneity is that it’s infinite. Another is that it has the same spatial dimension as the spacetime.

With regard to the latter, a spacetime has N spatial dimensions plus one of time. Our 3D reality is actually a 4D spacetime. Our surface of simultaneity therefore also had three spatial dimensions. (None of time because, by definition, a surface of simultaneity is a single instant of time.)

It’s called a “surface” to make it more generic. In our 2D spacetime diagrams (1 space; 1 time), it’s an infinite one-dimensional line. In more sophisticated (and therefore complicated) 3D spacetime diagrams (2 space; 1 time), it’s an infinite two-dimensional plane of simultaneity — a thin slice of time.

Figure 4. 3D spacetime diagram (2 space; 1 time). Notice the simultaneous events and plane of simultaneity.

As just mentioned, in our 4D spacetime, it’s a volume of simultaneity, and that volume is infinite. It’s all of space at a given instant.

But now keep in mind that a fundamental tenant of SR is that we can make no statements about distant points. «Now» is strictly local and personal. SR expressly prohibits statements about distant events until their light reaches us.

Motion shifts (transforms) how we will perceive the infinite volume of another frame. Our perception of causally separated events varies depending on our motion. (In other words, it’s virtual.)

§ §

So what to make of this spacetime transformation that seems to “rotate” what we define locally as «now» into the apparent future along our line of motion?

As we move, the farther ahead we imagine, the further into the future those locations are. (Note that we have to imagine those distant locations, because their light hasn’t had time to reach us. Our virtual 3D surface of simultaneity includes the entire universe. Further note that locations behind us shift into the past. As we move our simultaneity “rotates” to include both “past” and “future.”)

Figure 5. 3D spacetime diagram showing Alex moving through our frame. Note Alex’s rotated plane of simultaneity.

Simultaneity also shifts further into the future (and the past behind us) the faster we move. At the extreme, light speed, the shift is maximizes such that it matches the travel time. For example, moving at c towards an object one light-year away sees time at the object shift one year into the future. The photon is simultaneous with its arrival time (for all points along its path).

Think about that for a moment. To the photon, all times along its path are simultaneous. What that does that say about just how virtual simultaneity is?


Let’s put this in more concrete terms. Let’s talk about the Andromeda Galaxy (2.537 million LY away) and what happens to simultaneity with a brisk walking speed of 3 m/s (meters per second).

That’s 6.71 MPH. For reference, when I’m walking regularly, my target is 4 MPH (1.79 m/s), which is a reasonable working stride. Usain Bolt was clocked at 27.8 MPH (12.4 m/s). The point is, 3 m/s is easily doable on foot.

The math.

I picked 3 m/s because light speed is 300,000,000 m/s, so our velocity as a simple fraction is 1/100,000,000 c. It makes the math a little easier.

But good luck finding any regular calculator capable of giving a result for numbers that small. I had to use my arbitrary precision suite to get an answer I could use:

γ = 1.00000000000000005000000000000000375

Which is very, very close to 1.0, which is gamma (γ) standing still. (I have the value to over 100 digits of precision, and the pattern above continues; a long string of zeros alternated with clumps of other digits.)

I’ll spare you the calculation. What it boils down to is this:

If one trots towards the Andromeda Galaxy at 3 m/s, one shifts «now» there just over 800,000 seconds into the future. That’s about 9.25 days. (If one trots away, the shift is into the past.)

The shift is greater with increased speed. At light speed, it shifts as far as possible, 2.537 million years.

The shift is also greater with increased distance. The CMB photons we see, generated shortly after the universe began, saw Earth, 13.5 billion years into the future, as simultaneous with them when they were created (and ever since).


Many interpret this to mean spacetime must already exist.

But any analysis shows that, firstly, there’s no way to ever leverage such simultaneity (because it’s virtual), and, secondly, it can’t even be demonstrated. So it becomes a metaphysical interpretation.

Let’s think about the Andromeda and Milky Way galaxies. I think it’s a safe assumption that the particles involved in both — compared to far more distant particles — have roughly shared a broad fuzzy “world tube” since the Big Bang.

Figure 6. A 2D spacetime diagram of the Milky Way and Andromeda galaxies since the Big Bang. Note time scale change in modern era!

The diagram above shows the Milky Way and Andromeda galaxies since the Big Bang when their particles were created. We can access our own past up to «now», but we only have access to the Andromeda galaxy up to about 2.5 million years ago. (Light rays from point X have had time to reach us.)

Because the particles in both galaxies have evolved in roughly the same reference frame, we can assume there is a current «now» in Andromeda, but any knowledge of such is 2.5 million years in our future. (For all we know, Andromeda vanished 2.4 million years ago and isn’t there «now».)

In fact, the area marked as “unknown!” should extend from point X forward. (I just wanted to make the point we are forbidden — by Special Relativity — from making definite statements about «now» in Andromeda.

Photons emitted now from the Milky Way and headed towards Andromeda (destination point Z) consider Andromeda 2.5 million years from now as simultaneous. But as mentioned, Andromeda may not even be there anymore, so there’s nothing we can say about that future (until another 2.5 million years after it).


More importantly, although we can believe there is a «now» in Andromeda based on reality knitting there as it does here, there is no reason to believe the unknit future exists.

Simultaneity is an after-the-fact assessment of spacetime, and it’s entirely virtual depending on our frame of reference.

Bottom line, one is free to commit to the metaphysics of an evolving universe or a block one. Special Relativity does not require the future to already exist.

Stay unblocked, my friends! Go forth and spread beauty and light.

About Wyrd Smythe

The canonical fool on the hill watching the sunset and the rotation of the planet and thinking what he imagines are large thoughts. View all posts by Wyrd Smythe

28 responses to “Back to Block

  • Wyrd Smythe

    I finally decided to buy a scanner, so you may be seeing a lot more hand-drawn diagrams. 😀

    • Wyrd Smythe

      And old photos! The scanner does slides and film negatives, which I thought might allow me to scan in some very old photos on slides. But it turns out color slides from the 1970s and 1980s have not fared well over time. The blues and greens are almost gone, and the red is serious amplified — some slides are mostly a sea of red tones. 😦

      • Wyrd Smythe

        For instance:

        Which is from 1978 when I was doing some product photography for an ad brochure. At one point I took a couple shots of myself reflected in the reflective windows of the location.

        The scanner comes with a pretty nice photo editor, and it was able to do a lot with the image:

        But the color is still pretty awful, and the slide obviously needs cleaning. I may have the negatives, and it’ll be interesting to see how the scanner handles them, since they aren’t the usual sort of consumer film. (Long story, but it’s the same film most movies were shot on.)

  • Anonymole

    Well, as long as there’s beer. And brie. And smoked oysters.

  • SelfAwarePatterns

    I think the block universe remains an interesting possibility. But I totally agree that special relativity, by itself, doesn’t get us there. We have to have a deterministic interpretation of quantum mechanics, at least.

    But even then, I don’t find the block universe, in and of itself, all that meaningful. Even if it’s true, we’re still embedded within it, so to us, we’re still in a dynamic and ever changing universe. We can’t, even in principle, ever take up a position outside of time and space to observe it all in its static blockiness. And we’ll never be in a position to predict everything, so we have epistemic indeterminism, even if there isn’t ontological indeterminism.

    Interestingly enough, Alastair Reynolds, in one of his books: House of Suns, actually does have Andromeda disappear. Taking place millions of years in the future with slower than light travel, it’s one of the mysteries of the story, with an interesting answer.

    • Wyrd Smythe

      I can see the attraction for you. 🙂 The MWI ends up being effectively a BU in that the putative “universal wave-function” necessarily is a complete description of the entire universe, all world branches, from start to (whatever that finish is).

      Without observational data, though, it’s all just speculation. And I’m swearing off speculation this year. I wanna use my time to learn real things!

      FWIW, it strikes me that we do have epistemic indeterminism at the classical level, and it appears there is ontological indeterminism at the quantum level. (Even if MWI is true, in a given branch quantum events appear random.) At root, nothing — other than a hypothesis — suggests Newton’s clockwork universe turns out to be the truth.

      From an information point of view, an evolving universe automatically explains all the structure we see now. As we’ve discussed before, even just the proposition of a determined universe (not necessarily a block) implies an impossible amount of information compressed into an extraordinarily tiny space. (I wonder if anyone has done a holographic analysis of that.) Adding a block — the assumption it all exists — amplifies that issue.

      There does seem two ways of seeing a BU. The “lucite block” view sees it as definitely physically existing, all of spacetime. But there is also what I think of as the “Einstein” view — «now» may be the leading edge of the “loom” of reality, but the pattern is completely determined. The future doesn’t physically exist, but it definitely will according to current (and past) conditions. The MWI-BU does raise some interesting questions about where «now» comes from.

      • SelfAwarePatterns

        The MWI is compatible with the BU, but as you note, it makes our experience as systems embedded in the universe even further from the idea of an overall static reality. I’ve noticed people really taken with the BU tend to favor something like pilot-wave theory, or other hidden variable theories that might provide a clockwork universe at the classical level.

        If you’re swearing off speculation, I have to wonder why you did a post on the block universe. Personally, I find speculation too difficult to resist. Generally what’s left is too mundane.

        I’m not sure Einstein held the view you’re citing, at least not near the end of his life. The BU advocates like to quote the message to his deceased friend’s family as evidence he held a stronger eternalist stance. I was skeptical, since it could have just been Einstein finding polite comforting words, but this was actually for one of his closest lifelong friends, so it might be an accurate representation of his view at that point.

      • Wyrd Smythe

        “If you’re swearing off speculation, I have to wonder why you did a post on the block universe.”

        Ha! Arguing against fantastic speculation isn’t itself fantastic speculation. I’ve been clear what I mean.

        “Personally, I find speculation too difficult to resist.”

        Yes, I’ve noticed. 🙂

        “Generally what’s left is too mundane.”

        Everyone is different. I’ve found, after being forced to deal with far too many beyond the pale social fantasies, that — outside of fiction — I am really fed up with fantasy bullshit. Even the more benign forms have gotten caught in the fallout. I’m just completely sick to death of it.

        I suppose I really am an über-geek, because that “mundane” stuff has always fascinated me. (And I do mean always, a word I rarely use like this. Apparently even as an infant I was fascinated by the physical world, and that interest just grew as I did.)

        I’ve realized there isn’t enough time in my lifetime to study all the mundane stuff I’d like to know about, whether it be baseball or quantum mechanics. As you know, I love SF, and I have a rich fantasy life, but I do tend to place it all in the fictional world. When it comes to science, I’m kind of a hard-core realist.

        “I’m not sure Einstein held the view you’re citing, at least not near the end of his life.”

        I think it’s interesting people single out that one line, from a personal letter to a friend consoling them about death, and ignore all his scientific writing.

        And I’m not sure any of us should be judged by our thinking near the end of our lives (assuming we’re old at the time).

        One example I’ve always wanted to learn more about: I recall a story about Christopher Hitchens, as his death grew near, becoming religion-curious. (As they say, no atheists in the trenches.) What I recall is he was in conversations with a priest friend of his. Not converting, of course, but a bit more interested in what the other side thought about what might come next.

        Point is, I think both Hitchens and Einstein should be taken on the bulk of their work. Citing that letter, or Hitchens facing death, seems like cherry-picking to me.

        Einstein definitely believed in a deterministic universe throughout his life. That much is certain!

      • SelfAwarePatterns

        I’d argue that asserting that a speculative proposition is false, is itself speculation, unless you just say, “It’s all too speculative!” and stop there.

        And I think there are different kinds of speculation. Careful speculation, grounded in successful scientific theories, is a different thing than smoking weed in a dorm room and mulling sci-fi scenarios. (Not that there’s anything wrong with doing the dorm room thing occasionally, as long as we’re clear that’s what we’re doing.) And it’s all different from claiming to know things that are only speculative, which I find a much bigger problem.

        Hitchens actually warned his fans that some religious people would make up stories about him having a deathbed conversion after he was gone, and no longer able to deny it. So it’s not surprising that’s what happened.

        The problem with Einstein is he seemed to value his public image, and knew some of his statements tended to get into the press. A lot of his casual writing, once he was famous, have to be read with that in mind.

      • Wyrd Smythe

        Yes, in the broad sense, “speculation”=”pondering”, it means thinking about something, so thinking about and rejecting an idea is a form of speculation, absolutely.

        Here I don’t think we mean such a broad sense, but one aligned with “speculative fiction” — speculation as a creative act, bringing an idea into being. In that sense my post is not speculation. There’s nothing in the post but established physics. I only demonstrate that the requirement the BU proponents claim is false.

        Totally agree about different levels of speculation! And that claiming it as truth is an error. My point has always been that even the most logical mathematical scientific speculation… remains speculation. I’m not always certain, when it comes to topics like brain uploading or multiverse theories, that everyone involved understands that. (Sean Carroll springs to mind.)

        Or, for that matter, Hitchens. That he speculated people would make up stories doesn’t mean there isn’t truth to what (IIRC) a friend of his said actually happened. The memory I have of what I read is that Hitchens simply became curious, by no means converted.

        I can see it happening. I saw Hitchens as both certain and arrogant, but facing imminent painful death can have a humbling effect. Again, not saying he changed his mind, I’m sure he didn’t. One thing I’ve noticed about atheists is that many are rejecting the organization of religion. Very few I’ve encountered have a deep understanding of spirituality or theology. What I can see is Hitchens becoming more open to learning about what he’d so flatly rejected as nonsense.

        I’m not saying it’s true (unless I could locate that account and it stood up), but I can see it. It fits with my understanding of people. And just to repeat, the claim isn’t that he changed his mind in any way. If anything maybe he just realized the truth: he didn’t know for sure. None of us do.

        We can speculate about Einstein, but all I know for sure is that he believed in a determined universe.

    • paultorek

      “Even if it’s true … to us, we’re still in a dynamic and ever changing universe.” I agree, yet I would dispense with the “to us” part.

  • Wyrd Smythe

    Wow, this post about Greg Egan’s novel Quarantine is really blowing up! 123 views in the last three days. That’s a bit more than the 64 views it’s gotten since August 2019 when I published it (27 in 2019; 37 in 2020).

    I wonder what happened?

  • Wyrd Smythe

    In other news, tired of Firefox’s memory bloat (over a gig of memory for a browser? wtf?), I’ve decided to drink the Kool-Aid and start using Edge.

    Which has a few foibles I don’t care for, but is fast and lean.

    Pity, I’ve been a Firefox user and supporter since Mozilla (some of those folks go all the way back to Netscape Navigator). But I’m just underwhelmed by FF these days. Edge is noticeably faster.

  • paultorek

    Elon Musk turns out to have been right, and immortality is technologically achieved within your lifetime. 5 million years later in your proper time, you meet an Andromedan who has read this piece. It turns out that 5 million years before you met, in her proper time, she was writing a similar piece. She wonders: when you were writing yours, was hers real? Did it only become real when the light from her writings reached you? Is there some frontier across spacetime at which merely-potential things are becoming real, and if so how is it oriented? Or instead, is reality relative to each observer?

    • Wyrd Smythe

      You are describing a situation similar to Figure 6. In 2.5 MY the information about my post reaches her (and, likewise, information from hers reaches me). At that point we can both recognize we wrote our posts at roughly the same time. Another 2.5 MY years allows us to inform each other of that fact.

      Given the right observational data, it would be possible to establish a rough simultaneity to the writing of both posts. Problem is that simultaneity with 2.5 MLY separation is a hugely relative affair. That said, one could define a rough inertial frame where the Milky Way and Andromeda galaxies are approximately motionless with respect to each other. That provides a single time axis that would order blogging events, but the accuracy is likely in the 100,000 year range or something. It would be a crude approximation at best (and more likely utter fantasy).

      But, the evolving scenario is that the particles in the MW and Andromeda galaxies are roughly the same age, approximately 13.8 BY. As such, it is not unreasonable to assume something is going on “right now” in Andromeda galaxy. SR forbids any definite statements about that “right now” until 2.5 MY from now, but saying Andromeda is real “right now” is entirely reasonable.

      It’s how motion “rotates” the volume of simultaneity into the distant future, and the claim the distant future is real, that I question. With regard to Andromeda, the maximum rotation is 2.5 MY, and that requires going at light speed. But in more distant galaxies the shift is greater. The volume of simultaneity isn’t limited to the visible universe, so it includes huge shifts into the past and future. I’m saying that shift is virtual and doesn’t imply an already existing future.

      • paultorek

        I get all that, and have no dispute there. It’s just that I think something more is needed to avoid the idea that the future is just as real as the present. (Which I take to be what “block universe” is supposed to mean, although the term seems a little too metaphorical for my taste.)

        If the future isn’t real but merely potential, then either reality is relative to observers in some way (be it objective inertial reference frames, or purely subjective), or there is some frontier along which the merely-possible is becoming real. I can’t think of a third alternative, can you?

        The first alternative seems terribly costly. It seems like there has to be some better way to describe things than saying that there is no univocal reality. The second alternative is easier to swallow, and doesn’t *contradict* relativity per se. But it’s a very big posit, i.e. speculative, and likely impossible to verify.

        Now I agree with Mike (SelfAwarePatterns) that the denial of the speculative is also speculative. But there are degrees. Some speculations are awfully specific, and it seems to me we need very good reasons to take them seriously.

      • Wyrd Smythe

        I quite agree. ‘Specific speculation’ is what I mean by creative speculation.

        I also agree with your alternatives and can’t think of a third. As a realist, I reject observer-dependent views (although that gets complicated at the quantum level). For me, the only alternative is that the present is generated in real-time.

        It may be, as you say, a big posit, but all putative stories about existence require such. Whether it be God or the Big Bang or a static BU or Tegmark’s MUH or Roger Penrose’s CCC, the nature of reality is… well, at the very least, big.

        FWIW, I do see an evolving “single-verse” universe as the “smallest” big posit. 🙂

      • paultorek

        It all comes down, in my view, to what actually follows from the “eternalist” viewpoint that the future is ontologically real. Surprisingly little. It doesn’t follow that evolution isn’t real, or that our viewpoint doesn’t move through time – all these are within-reality processes that span many time indices. The “block universe” metaphor hides this fact, which is why I don’t like it. The label “eternalism” isn’t very clear either, but at least it’s open to the right interpretation along with some wrong ones.

        And what advantages do the “presentist” or “growing block” views have. In my view, almost none – they are kinder to intuitions about time and causality that we develop on medium-sized dry goods, and we (over)extend to every size scale. But they buy this at the cost of a complication that yields no predictive value.

      • Wyrd Smythe

        Are you then rejecting both the static and evolving blocks? What remains?

        (Don’t know if you read the first post, but in that one I touched on the issue of viewpoint in the BU, so I didn’t get into it here.)

      • paultorek

        I hadn’t read that post. By its terms I subscribe to an “evolving block” universe. That is, “computations” occur as the universe evolves over time; it’s not like God wrote the whole story in advance and what happens on page 233 doesn’t cause what happens on 234.

      • Wyrd Smythe

        Yeah,… “Let there be a Block!” kinda beggars the imagination.

      • Wyrd Smythe

        FWIW, it occurs to me that our metaphysical “theories” tend to be reactive rather than predictive (which is why some don’t see them as real theories but hypotheses). They’re more big picture explanations than small physics explorations, so they less likely to offer specific predictions.

        To some extent they’re just comfortable bedtime stories. 🙂

  • Wyrd Smythe

    I was looking at an old printout I have from Purdue, a long technical text about Special Relativity. It was talking about length contraction, and a small light bulb went on regarding the notion of virtual simultaneity.

    The thing about length contraction is that it’s relative (i.e. virtual). To the train the tunnel seems shorter; to the tunnel, the train seems shorter. The universe sees the spaceship as shorter, the spaceship sees the universe as shorter.

    That length seems to change (yet obviously doesn’t really) illustrates the virtual nature of what is seen in moving frames. It’s not real.

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