Against the Block Universe Part II
Back to the Source
2025 Aug 13
In Part 1, I pulled apart the block universe argument in the form you’re most likely to meet it today: a neat, confident little story that claims relativity has already proved the future is fixed. We saw that, once the physics is described accurately, the case falls apart — and that much of its apparent force came from a kind of stage trick, the introduction of a “second observer” who didn’t actually contribute anything essential.
Now I’m going to rewind to where this argument first took root in the philosophical literature, in papers by Hilary Putnam and by Willem Rietdijk. These are the ur-texts — the sources that have been cited for decades as if they contained the real, rigorous version of the case. The trouble is, they’re not an easy read. They were written more than sixty years ago, and they carry the stylistic baggage of their time: dense, formal, and sprinkled with technical asides that can make it hard to see the wood for the trees.
My aim here isn’t to translate their language word-for-word, but to unpack what they were actually saying — in plain English — so that it’s clear both what their argument is and where it goes wrong. And, just as in Part 1, we’ll find that the extra complications they introduce are not new ideas, but distractions. Putnam builds his case by dragging in one set of side issues, Rietdijk by another. Both make the argument seem richer and more formidable than it really is. Strip those away, and you’ll see the same fragile core we met in Part 1 — still unconvincing, just dressed in older clothes.
Where we’re headed We’ll start with Rietdijk’s original paper, see how it frames the argument, and untangle the physics from the philosophical gloss. Then we’ll turn to Putnam’s well-known paper, see how his version differs, and why his additions don’t rescue the case. Along the way, I’ll strip out the side issues, restate the reasoning in plain language, and show that — as in Part 1 — the argument simply doesn’t do what it claims to do.
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Rietdijk’s Determinism Argument — At a Glance
Core Claim: Every event you think of as in your future is, for some distant observer, already in the past. That means no event is genuinely indeterminate.
Key Mechanism: This relies on relativity of simultaneity: different observers, moving differently, have different notions of what counts as “now,” “past,” or “future.”
Thought Experiment Structure:
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Imagine a process (like a random event) that triggers for you at what is your “now.”
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Move to another observer travelling relative to you.
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For that observer, the event’s time—and hence its status as past—can shift.
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If, for them, the event is already past, it must be determined—even if for you it’s only just happening.
Consequences Drawn:
If any event is already past (and thus fixed) relative to any possible observer, then all events are, in a sense, predetermined.
Rietdijk argues that only the radical idea—“what cannot be observed does not yet exist”—could resist this conclusion. Otherwise, determinism seems forced.
Rietdijk’s “Rigorous Proof” — Without the Formalities
Imagine you believe the future is open — it doesn’t yet exist, and what happens tomorrow depends on choices and chance.
Rietdijk’s idea is to use relativity to argue that this can’t be true. His starting point is a key fact from Einstein’s theory: in special relativity, what you call “now” isn’t the same for everyone. Two observers moving differently will disagree about which events are happening right now, which are in the past, and which are still to come.
Here’s how he builds on that:
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Pick an event in your future — say, you tossing a coin tomorrow.
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In relativity, there’s some other observer moving at the right speed and direction for whom that coin toss is already in the past.
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If it’s in their past, it’s already fixed — it’s something that’s happened.
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But if it’s fixed for them, then, says Rietdijk, it must also be fixed for you, because reality can’t depend on which observer you happen to be.
By linking different observers together in this way — each one’s “future” overlapping with someone else’s “past” — Rietdijk concludes that every event is already settled. The future, he says, is as fixed as the past, and determinism follows directly from relativity.
Why Rietdijk’s Argument Fails
His argument is essentially the same as that we encountered in the ‘Andromeda Paradox’ in part one of this essay, stated differently. He is ignoring the adjustments that SR demands we make in our notions about past, future, and causality. A hidden assumption (point 3, above) is that if something is in the ‘past’ it must be settled. But he applies this reasoning to events that are space-like separated, and this is not true of those events.
Another unspoken assumption – events fall into two categories – fixed, or changeable. If something is not changeable it must therefore be fixed. But events ‘elsewhere’ are neither changeable or fixed.
Putnam’s Argument—Put Simply
The Everyday View Challenged: Putnam starts from the common-sense idea—presentism—that only what exists “now” is real.
Relativity Undermines “Now”: Special Relativity teaches that what counts as “now” depends on the observer—each moving observer slices space-time differently into past, present, and future . Putnam argues we can’t privilege one observer’s “now” over another’s.
Either Strange Plurality or No Flow of Time: If we accept that every observer’s “now” truly exists, then we must allow many different “presents” (each with its own set of events). Putnam finds that absurd. His alternative is that the world should be viewed tenselessly: past, present, and future events all equally “real” .
Reality Is Frame-Independent: From this, he concludes the only coherent way to think of reality is as a four-dimensional structure—including events we normally think of as “future”—and that time doesn’t “flow” in the usual sense .
In short:
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Presentism (only the now is real) seems natural.
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But Special Relativity shows “now” depends on your motion—there’s no single universal now.
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If we accept one observer’s “now” is privileged, we get multiple incompatible “presents”—also absurd.
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So we must adopt a tenseless, block-universe view, where all events—past, present, future—are equally real, and the idea that “time flows” is just an illusion.
Now, just as with Rietdijk’s “rigorous proof,” the heart of Putnam’s argument is actually quite small and simple: he takes relativity’s relativity of simultaneity, combines it with the belief that reality must be the same for all observers, and then makes the leap to “all of space-time exists equally.” But instead of keeping this bare-bones, he builds in a layer of philosophical scaffolding about ontology — the nature of existence — that makes the whole thing seem more weighty and inevitable than it really is.
This is the extra complication here. Where Rietdijk brought in a chain of observers to link one person’s future to another’s past, Putnam brings in a sweeping philosophical principle: that if we can’t pick a single “now” as privileged, then we must abandon the very idea that only the present is real. This turns a modest observation from physics into a radical metaphysical claim.
In the next part, I’ll strip away that philosophical scaffolding and show that, once you set aside the jargon and the framing, Putnam’s argument doesn’t give you any more reason to believe in a block universe than the popular version we looked at in Part 1 — and it rests on the same hidden assumption that the relativity of simultaneity somehow forces determinism.
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Putnam’s Argument — Without the Jargon
Let’s imagine you believe only the present is real. The past is gone, the future doesn’t yet exist — only the “now” is truly out there. That’s the common-sense view most of us start with.
Special Relativity throws a spanner in the works. In relativity, what counts as “now” depends on how you’re moving. Two people moving differently will slice space-time into “past, present, and future” in different ways. If they’re far enough apart, one person’s “now” can include events the other says haven’t happened yet — or have already happened.
Putnam says: if we think reality should be the same for everyone, we can’t have it that one person’s “now” is real but another’s isn’t. That would mean reality changes depending on who you are and how you’re moving. The only way out, he argues, is to say that all of these different “nows” are equally real. And if they’re all equally real, then everything — past, present, and future — must exist together in a four-dimensional “block.”
So, in its bare form, his reasoning goes like this:
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The present is different for different observers.
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Reality can’t depend on which observer you pick.
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Therefore, all events in all observers’ presents are equally real.
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If that’s true, then past, present, and future all exist equally — so time doesn’t “flow,” it just is.
Why Putnam’s Leap Doesn’t Follow
At first glance, Putnam’s chain of reasoning feels neat and airtight. If different observers have different “presents,” and we want reality to be the same for all of them, then surely we have to accept all those presents as equally real — right?
But reality isn’t the same for everyone. I am sitting in my hut in the forest. you are not. OK, he says, our experience isn’t identical, but what counts as real is the same for everyone.
No it isn’t. For me on Monday, Sunday has past. For Jane, who lives next door, on Saturday, Sunday is yet to come. Jane, on Saturday, has a different reality from me on Sunday. Ah. he says. But that is because they are at different times. Two observers at the same time should have the same reality.
But there is no “at the same time”. It’s relative. Putnam himself points out that an event that is ‘at the same time’ for one observer is the past for another. So why should any two observers ‘have the same reality’? No one is experiencing ’the same time’ as anyone else. We are all experiencing our own time.
If you then insist that what is real for all observers at all times must be equally real you are assuming the very point you are trying to prove.
Relativity tells us how different observers label events as past, present, or future, depending on their motion. It doesn’t tell us that all those labelled “presents” must be ontologically equal — that they must all “exist” in the same sense. That’s not physics speaking; that’s a philosophical choice.
Putnam is quietly sliding from a statement about measurement and coordinate systems — “different observers slice space-time differently” — to a sweeping claim about the nature of reality — “everything in all those slices must be real.” These are two very different things. The first is a well-tested feature of the theory; the second is an extra assumption you could accept or reject without contradicting relativity.
That’s not a logical necessity — it’s a reinterpretation, and one you don’t have to buy. Relativity lets you keep presentism if you want, as long as you’re careful about how you define “present” in a relativistic world. You can also adopt other metaphysical views about time without forcing yourself into a block universe.
So the step from “relativity of simultaneity” to “block universe” isn’t a deduction. It’s a philosophical jump, and once you notice it, the apparent inevitability of Putnam’s conclusion evaporates.
In the end, Putnam’s version of the block universe case turns out to be doing the same job as Rietdijk’s — only with a different kind of camouflage. Rietdijk used a chain of observers to connect one person’s future to another’s past; Putnam uses a grand philosophical principle about “observer equality” to connect the relativity of simultaneity to the claim that all of time exists at once. But in both cases, the extra machinery hides the same basic move: taking a feature of how relativity describes the timing of events and smuggling in an assumption that this must reflect the deepest nature of reality. Strip away those additions, and the argument is no stronger than the popular version we examined in Part 1.