第三个粒子的悖论是经典的
The Paradox of the Third Particle is classical
AI总结：
研究第三个粒子的悖论，发现其在经典情况下就已出现，建立不可行定理，表明当物理子系统作参考系且要求框架变换信息保持时，无法有意义地将世界划分为子系统。
AI中文摘要：
当粒子相对于其中一个作为参考系来描述时，就会出现第三个粒子的悖论。虽然它通常被归因于量子叠加，但我们表明，该悖论及其后果，如子系统的相对性，在经典情况下就已经出现。鉴于这一见解，我们建立了一个不可行定理，无论物理子系统是经典的还是量子的，只要用作参考系：如果要求框架变换是信息保持的，那么从每个框架的角度来看，就不可能有意义地将世界划分为子系统，如单个粒子。
英文摘要：
The Paradox of the Third Particle arises when particles are described relative to one of them serving as a reference frame. Although it is typically attributed to quantum superpositions, we show that the paradox and its ramifications$\unicode{x2014}$such as the relativity of subsystems$\unicode{x2014}$already occur classically. In light of this insight, we establish a no-go theorem that holds whenever physical subsystems, whether classical or quantum, are used as reference frames: if one demands that frame transformations be information-preserving, then it is impossible to meaningfully partition the world into subsystems, like individual particles, from the perspective of each frame.
