Pages 891, 892
The principle of relativity and the constancy of the velocity of light
The paper raises two presuppositions: that the same electrodynamic and optical laws hold in every coordinate system in which the equations of mechanics hold, which it calls the principle of relativity, and that light in empty space always travels with a definite velocity , independent of the state of motion of the emitting body; together, it says, they suffice for a simple and consistent electrodynamics of moving bodies, in which, it says, a light ether will prove superfluous, insofar as no absolutely resting space is introduced and no point of empty space is given a velocity.
As printed
Beispiele ähnlicher Art, sowie die mißlungenen Versuche, eine Bewegung der Erde relativ zum „Lichtmedium“ zu konstatieren, führen zu der Vermutung, daß dem Begriffe der absoluten Ruhe nicht nur in der Mechanik, sondern auch in der Elektrodynamik keine Eigenschaften der Erscheinungen entsprechen, sondern daß vielmehr für alle Koordinatensysteme, für welche die mechanischen Gleichungen gelten, auch die gleichen elektrodynamischen und optischen Gesetze gelten, wie dies für die Größen erster Ordnung bereits erwiesen ist. Wir wollen diese Vermutung (deren Inhalt im folgenden „Prinzip der Relativität“ genannt werden wird) zur Voraussetzung erheben und außerdem die mit ihm nur scheinbar unverträgliche Voraussetzung einführen, daß sich das Licht im leeren Raume stets mit einer bestimmten, vom Bewegungszustande des emittierenden Körpers unabhängigen Geschwindigkeit fortpflanze. Diese beiden Voraussetzungen genügen, um zu einer einfachen und widerspruchsfreien Elektrodynamik bewegter Körper zu gelangen unter Zugrundelegung der Maxwellschen Theorie für ruhende Körper. Die Einführung eines „Lichtäthers“ wird sich insofern als überflüssig erweisen, als nach der zu entwickelnden Auffassung weder ein mit besonderen Eigenschaften ausgestatteter „absolut ruhender Raum“ eingeführt, noch einem Punkte des leeren Raumes, in welchem elektromagnetische Prozesse stattfinden, ein Geschwindigkeitsvektor zugeordnet wird.
- Premise: Both are presuppositions, and the paper does not derive them. Together with Maxwell's theory for bodies at rest, it says, they suffice for its electrodynamics of moving bodies.
- Premise: The paper cites the magnet and the conductor, and the unsuccessful attempts to detect the earth's motion through the light medium, as what suggests the principle, not as a proof of it.
- A comparison, not a premise: Lorentz's theory predicts the same currents at the speeds of real apparatus, so the introduction objects to an asymmetry in the explanation, not to a failed prediction.
Decoder
- "Prinzip der Relativität"
- The principle of relativity: the conjecture, raised to a presupposition, that absolute rest corresponds to no property of the phenomena, in electrodynamics as in mechanics.
- The velocity of light in empty space, printed V; today it is written c.
- "nur scheinbar unverträglich"
- Only apparently incompatible: light keeping one velocity seems to contradict the principle of relativity, and § 3 shows that the two agree.
- "Lichtmedium", "Lichtäther"
- The light medium, or light ether: the unsuccessful attempts the paper mentions tried to detect the earth's motion relative to it. The paper introduces no absolutely resting space and assigns no velocity to a point of empty space.
Try it
SR-02: Move the magnet past the resting conductor, then the conductor past the resting magnet at the same relative speed: is the current the same, and how does each description account for it? Choose the preset “Nagging fact (resolution withheld)” there.
SR-02: Now show the resolution: how does one description, depending only on the relative motion, cover both cases? Choose the preset “Apparatus with resolution shown” there.
Where this stops: A simulation built on the postulates cannot prove them. The model of a magnet and a conductor loop does not cover every induction experiment. Read the argument
Where this is used later
Common wrong turns
- Argument
- Two presuppositions, from which the paper derives the rest
- Model
- The theory's starting points, not results of it
- Evidence
- A conjecture raised to a presupposition, suggested by examples such as the magnet and the conductor and by the failed attempts to detect the earth's motion through the light medium
- History
- The magnet-and-conductor case was a textbook example, discussed in Föppl's 1894 introduction to Maxwell's theory; the two presuppositions are introduced in this paper