Twin Paradox — Two Competing Interpretations

Two readings, two worldviews

Grégoire says in his video about Interstellar:

« L'effet temporel est dû à la vitesse dans le premier cas et à la gravité dans le second cas. On va voir qu'en fait c'est exactement le même effet. Ce lien entre gravité et vitesse est très rarement évoqué alors que c'est capital. Cela va nous permettre de comprendre la raison physique qui fait que Cooper vieillit moins que sa fille. »

“The time effect is due to speed in the first case and to gravity in the second case. We’ll see that in fact it’s exactly the same effect. This link between gravity and speed is very rarely discussed, yet it’s crucial. It will allow us to understand the physical reason why Cooper ages less than his daughter.”

Here, Grégoire raises a real issue: the current explanation of the twin paradox in special relativity is not fully physical, because it relies on a change in geometry (the line of simultaneity) without an actual cause. This leaves a gap in the interpretation.

The Relativistic (Standard) Interpretation

In the standard interpretation:

- The differential aging comes from the change of inertial frame by the traveling twin;

- This change causes a jump in the line of simultaneity, according to the relativity of simultaneity;

- This is not a physical cause, but a geometrical consequence of Einstein’s postulate that the speed of light is invariant in all inertial frames (even in a one-way trip).

But it is precisely this relativity of simultaneity that is problematic: it implies that a mere change of reference frame can make the same event appear or disappear — something that is philosophically and physically questionable.

The Relational Interpretation (Real Spatial Configuration)

In contrast, a second reading proposes to base the paradox on a real relationship to space, rather than on a mere play of reference frames.

In this view:

- The speed of a body is defined with respect to a real spatial configuration (for example, a distribution of light particles or a gravitational reference space);

- Escape velocity has here a real meaning: it reflects a form of effective gravity, linked to motion relative to a physical background;

It is the one who has the greatest motion relative to this configuration who undergoes a real slowing down of proper time — as if being more strongly subject to an extended form of gravity.

And above all:

- This interpretation does not rely on the relativity of simultaneity;

- It assumes that one can define an objective present moment, so that the invariance of the one-way speed of light is no longer universal: it becomes an operational idealization, not a fundamental property of reality.

The Role of Real Doppler: Number of Cycles Received

The problem can also be formulated differently:

Instead of reasoning in terms of coordinates, one can count the actual number of clock ticks received via light signals.

In the twin paradox:

- The traveling twin sees the other’s clock ticking slowly on the outbound leg (red Doppler shift);

- Then much faster on the return leg (blue Doppler shift);

- But they receive more cycles on the return than they missed on the outbound;

- Because the return “catches up” with the signals emitted during the separation (the photons’ memory persists);

This measurable imbalance — and not a mere convention of simultaneity — accounts for the differential aging.

Two Competing Interpretations:

- One (special relativity) relies on the relativity of simultaneity, which implies the invariance of the one-way speed of light, but at the cost of lacking a genuine physical cause;

- The other (relational) relies on a physical relation to a spatial configuration, allowing for a unification of motion and gravity, and restoring a real status to proper time without going through a complete geometrization.

Grégoire is right to say that the paradox is not truly resolved. But to resolve it physically, one must clarify the nature of speed, of time, and of the link between gravity and the structure of space. This requires stepping outside the strict framework of special relativity.