It is widely acknowledged that the Galilean Relativity Principle, according to which the laws of classical systems are the same in all inertial frames in relative motion, has played an important role in the development of modern physics. It is also commonly believed that this principle holds the key to answering why, for example, we do not notice the orbital velocity of the Earth as we go about our day. And yet, I argue in this paper that the precise content of this principle is ambiguous: standard presentations, in both physics and philosophy, fail to distinguish between two principles that are ultimately inequivalent, the “External Galilean Relativity Principle” (EGRP) and the “Internal Galilean Relativity Principle” (IGRP). I demonstrate that EGRP and IGRP play distinct roles in physics practice (e.g., EGRP is connected to the concept of Galilean invariance, but IGRP is not) and that many classical systems that satisfy IGRP fail to satisfy EGRP. I further show that the Relativity Principle famously discussed by Einstein in 1905—which is not restricted to classical systems—also leads to two inequivalent principles, an external one analogous to EGRP, and an internal one analogous to IGRP. I conclude by highlighting that the phenomenon originally captured by Galileo's famous ship passage is much more general than contemporary discussions in the philosophy of symmetries suggest.

中文翻译：

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伽利略的船和相对论原理


人们普遍认为，伽利略相对论在现代物理学的发展中发挥了重要作用，根据该原理，经典系统的定律在所有相对运动的惯性系中都是相同的。人们还普遍认为，这一原理是回答为什么我们在日常生活中没有注意到地球轨道速度的关键。然而，我在本文中认为，这一原理的确切内容是含糊不清的：物理学和哲学中的标准表述都未能区分两个最终不等价的原理，即“外伽利略相对论原理”（EGRP）和“内伽利略相对论原理”（IGRP）。我证明了 EGRP 和 IGRP 在物理实践中发挥着不同的作用（例如，EGRP 与伽利略不变性的概念相关，但 IGRP 则不然），并且许多满足 IGRP 的经典系统无法满足 EGRP。我进一步表明，爱因斯坦在 1905 年著名讨论的相对论原理（不仅限于经典系统）也导致了两个不等价的原理，一个类似于 EGRP 的外部原理，一个类似于 IGRP 的内部原理。最后我要强调的是，伽利略著名的航船航道最初捕捉到的现象比当代对称哲学的讨论所暗示的更为普遍。