Relativity and the Block Universe
What Physics Avoids Facing
When the very way of posing a question has remained fixed for more than a century, it is not surprising that the answers remain confined within the original framework.
This repetition can sometimes stem from a form of ipsedixitism — the acceptance of a statement solely because it comes from a recognized authority, without critical examination.
Yet it is entirely legitimate — and even necessary — to approach a subject from a different angle in order to test its limits.
The Argument in Three Logical Steps
I therefore frame the problem differently, in three logical steps:
1 First Question
Does the invariance of the speed of light in one-way propagation imply the relativity of simultaneity at the physical level , and not merely as a convention for dating events?
Answer: Yes.
This should be recognized as a direct consequence of the postulate. However, ambiguity persists on this point: simultaneity is indeed said to be relative, but discussions often remain at the level of appearances or conventions for dating events, without ever clearly stating that this would imply a relativity of the very existence of events.
Yet this is a known implication, since we routinely speak of the block-universe model, in which past, present and future coexist within a four-dimensional structure.
In other words, the idea is accepted, but its full consequences are avoided. For my part, I take this ontological aspect seriously and carry the reasoning through to its conclusion. Physicists, curiously, tend to stop at the operational level. That is a mistake.
2 Second Question
Does this relativity of simultaneity, when all its implications are taken into account — as in the shuttle-and-missile objection — not lead to a logical contradiction with itself, particularly regarding the existence or non-existence of an event depending on a change of reference frame?
Answer: Yes.
If one and the same fact — a launched missile — can be said to be real in one frame and not yet realized in another, then we are dealing with a contradiction in being, not merely with a difference in measurement.
3 Third Question
Does the affirmative answer to the first two questions lead us to challenge the invariance of the one-way speed of light, at least as a universal axiom applicable in all situations?
Answer: Yes.
A logical contradiction in the consequences of a postulate calls the postulate itself into question.
Two Incompatible Logics at the Heart of Relativity
To assert that light has travelled from point A to point B at speed c, one must be able to assume that, at a given instant, it was located at a specific position between A and B.
In other words, this requires the ability to assign a common time to spatially separated points — that is, to points separated by a spacelike interval, which, according to relativity, are not connected by a causal relation.
This reveals an internal tension within relativity.
It relies on two distinct logical frameworks:
- a logic specific to timelike and lightlike intervals, where causality is preserved;
- another applying to spacelike intervals, where no causal connection can be established.
The difficulty arises when a change of inertial frame allows a temporal order, defined within a causal framework, to be transposed into a region of spacetime where such a framework is no longer valid.
An Example Involving a Train and a Distant Light Ray
Far behind the train, hundreds of billions of kilometres away, a light ray is emitted: this is event 1.
For both observers standing on the station platform, this event occurs while they are at rest in the same inertial frame.
After this event, one of the two station observers accelerates and then joins the observer on the train — before, according to the latter, the light ray has been emitted.
The First Dating
The station observer who remained at rest says to the observer who accelerated:
“The light ray was indeed emitted when you were still in the same inertial frame as I was, before your acceleration. In fact, I received it at such-and-such a date.”
This is a lightlike interval. Since both observers were in the same inertial frame at that time, this dating must also be valid for the observer who later changed frame.
The Second Dating
The train observer replies:
“When you joined me, the light ray had not yet been emitted. It was only emitted later. In fact, we both received it at such-and-such a date.”
This, again, is a lightlike interval. Since the observer who changed frame is now in the same frame as the train observer, this dating must also be valid for him.
The Resulting Contradiction
Two different datings are attributed to the same event for the same observer, in two distinct configurations.
In both cases, the dating is based on a lightlike interval — that is, an interval in which causality is supposed to be preserved.
If we accept that the event — the emission of the light ray — actually occurred, then the same observer cannot assign it two different dates, both based on causal relations. This contradiction highlights an internal inconsistency if one grants physical reality to events while maintaining the relativity of simultaneity as a real property of the physical world.
The causal approach in relativity is a convention that depends on the invariance postulate. If this postulate leads to a logical contradiction, as revealed by the shuttle-and-missile objection, then the entire framework becomes open to question.
Conclusion
Within the dominant paradigm, questions are rarely reframed from a different perspective. The coherence of the system is preserved by interpreting apparent contradictions as technical or conventional issues.
Paradoxes linked to the relativity of simultaneity are reduced to problems of clock synchronization. More importantly, it is often unclear whether the invariance of the speed of light refers to the one-way or the two-way case. This ambiguity prevents a proper examination of the logical consequences that genuine one-way invariance would entail.
Such resistance can be understood. But the fact that a system has endured over time does not mean that it is free from inconsistency. When a flaw emerges, one must still have the courage to point it out and explore all of its consequences.
Convention or Physical Reality? A Choice Must Be Made
If the discussion keeps going in circles, it is often because a fundamental point has not been clearly stated from the outset. It is therefore necessary to pose the question in a simple and decisive way:
Should the relativity of simultaneity be regarded as a mere convention of synchronization, or as a real physical property of the world?
First Possibility: It Is Conventional
If the relativity of simultaneity is a convention, then the invariance of the one-way speed of light is also a convention. It cannot, in that case, be regarded as a physical reality.
Second Possibility: It Is Physical
If, on the contrary, the relativity of simultaneity is held to be a physical property — in other words, if it reflects a reality of spacetime — then all its logical consequences must be acknowledged, including those revealed by the shuttle-and-missile objection.
Among these consequences is a striking contradiction: the same observer, after changing inertial frames, can assign two different dates to the same event, in configurations where causality is supposed to be preserved.
This is not a matter of subjective interpretation, but of a clear-cut logical alternative:
- either the relativity of simultaneity is conventional;
- or it is physical — but in that case, everything it entails must be accepted, even what is unsettling.
It is therefore essential that this question be asked clearly — and that the answer be equally clear .
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