And neither observer is privileged. Special relativity says there is no physically preferred inertial frame that tells us which simultaneity judgment is the “real” one.
2. Putnam’s philosophical move
Now add a seemingly innocent principle:
If an event is real for one observer, its reality cannot depend merely on another observer’s state of motion.
Alice exists now.
A distant event X_1, simultaneous with Alice according to Alice’s frame, therefore exists.
Bob, standing right beside Alice, also exists.
But Bob’s present includes X_2, an event that Alice calls future.
Therefore X_2 must also exist.
Yet if X_2 exists, something that Alice regards as her future is already part of reality.
Repeat the reasoning across observers and spacetime, and the distinction between
- real present,
- unreal future
becomes extremely hard to sustain.
Hence Rietdijk–Putnam:
\boxed{\text{Relativity of simultaneity}}
+
\boxed{\text{observer-independent reality}}
\Rightarrow
\boxed{\text{future events are real}}
And therefore:
\boxed{\text{Eternalism / Block Universe}}
Its deepest implication for time
It attacks presentism.
Presentism says:
Only what exists now is real.
Caesar no longer exists; your death does not yet exist.
The Rietdijk–Putnam argument asks:
Whose now?
Relativity contains no unique cosmic present. If ontology tracks “the present,” nature gives us no invariant surface on which to put it.
The alternative is Minkowski spacetime:
\text{Reality} =
\{\text{all spacetime events}\}
Your birth, this conversation, and events occurring decades from now occupy different coordinates in one four-dimensional spacetime structure.
In that sense:
Time may not be something that reality moves through.
Rather, temporal locations may be something that things occupy.
This is the conceptual revolution.
We normally picture reality as:
t_1\rightarrow t_2\rightarrow\boxed{t_3}\rightarrow t_4\rightarrow t_5
with only t_3 existing.
The block-universe picture is instead:
\boxed{
t_1\quad t_2\quad t_3\quad t_4\quad t_5
}
All are parts of spacetime.
There is no objective metaphysical spotlight moving from t_1 to t_2 to t_3.
But this does
not
quite prove that “the future is predetermined”
This distinction is important.
The argument is primarily about existence, not causation or determinism.
Three claims are different:
\text{Future exists}
\neq
\text{Future is predictable}
\neq
\text{Future is causally determined by the past}.
A block universe could, at least conceptually, contain genuinely probabilistic quantum events. The entire four-dimensional history would simply include whichever outcomes occur.
So eternalism does not automatically imply Laplacean determinism.
And something stranger follows: perhaps time does not genuinely “flow”
This, I think, is the most interesting implication.
Physics gives us an invariant distinction between:
- timelike separation,
- lightlike separation,
- spacelike separation,
and gives us causal order inside light cones.
But it gives us nothing corresponding straightforwardly to the experiential statement:
“The present is moving.”
There is no v_{\text{time}} describing the velocity of Now through spacetime.
So the Rietdijk–Putnam argument moves us toward a distinction between two things:
\textbf{temporal order}
\quad\neq\quad
\textbf{temporal passage}.
Physics unquestionably contains temporal order:
A \prec B \prec C.
Whether there is additionally an objective becoming—
A \rightarrow B \rightarrow C
—is another question.
Einstein’s relativity makes that second notion surprisingly difficult to put into fundamental physics.
And here we arrive at the famous tension between Einstein and Bergson.
Bergson would say: lived duration—durée—cannot simply be identified with coordinates in Minkowski spacetime.
The physicist can tell you:
(x,y,z,t)
but this does not explain why there is the extraordinary phenomenological difference between:
not yet → now → no longer.
So I would put the philosophical consequence of Rietdijk–Putnam this way:
\boxed{\text{Relativity kills the universal Now}}
and perhaps therefore
\boxed{\text{physics gives us time without becoming}.}
But consciousness seems to give us precisely the opposite:
\boxed{\text{becoming without access to the whole of time}.}
That gap—between Minkowski’s block and our experience that something is happening—remains one of the deepest unresolved problems in the philosophy of time.
