物理与天文2024nature.com
旋转物体对电磁场的放大
Amplification of electromagnetic fields by a rotating body
专业 · 约 90 分钟读完前置知识：电动力学、角动量守恒、散射理论基础
导读
这篇论文从理论上研究旋转物体如何放大电磁场：当物体旋转速度足够快时，旋转会通过角动量转移放大入射的电磁波（一种“旋转多普勒”效应）。这是理论性结果，尚未涉及具体实验实现。
HN 关注点：是因为“旋转放大电磁场”听起来接近免费能量/超光速类话题，讨论焦点集中在能量守恒、能否做引擎以及为什么难以高速旋转等物理直觉问题。
HN 评论精选
这几条评论里：质疑 2 补充 2
质疑猜测放大的能量来自旋转体的角动量，旋转体会减速
（我没看懂论文里的数学）这是否意味着太空中的旋转体会因此逐渐减速？换句话说，放大电磁辐射是在消耗角动量的能量？
bloopernova查看英文原文
(I didn't understand the math in the paper) Would this mean that a rotating body in space would eventually slow down? In other words, amplifying EM radiation draws energy from angular momentum?
质疑联想到 Steorn 永磁骗局和超导材料，猜测该效应可用于能量增益
所以，如果用涡流把激励场的相位延迟足够久，让物体在响应前转过 90 度以上，响应的涡流场就会从抵抗原场变成增强原场？这听起来很像多年前 Steorn 做的事。如果“超导体”（ultraconductors，专利声称电导率 10^11 S/cm，比铜高 15 万倍）真的可行，也许能造出损耗低到在临界转速以上真正产生能量的机械装置。
mikewarot查看英文原文
So, if you use eddy currents to delay the phase of an exciting field long enough that the object those eddy currents are inside of can spin more than 90 degrees, the response eddy current fields now AID instead of opposing the original field? This sounds quite a bit like what Steorm[1] was doing years ago. If ultraconductors[2] worked, you could actually build a mechanical device that had losses low enough to actually gain energy once a critical speed were obtained. [1] https://en.wikipedia.org/wiki/Steorn [2] https://patents.google.com/patent/US5777292A/en (Claim 7 is for material with a conductivity of 10^11 S/cm, which is 150,000 times better than copper)
补充用角动量守恒的比喻提问：高速旋转的物理瓶颈到底在哪
笨问题：为什么把东西转得非常快很难？举例：让中间的“无摩擦球形牛”拉着两根绳，绳另一端各接一头 10 米外的全尺寸牛，让它们以 1Hz 旋转，然后中间的牛收绳到 10cm——由角动量守恒，转速变成 1kHz；收到 10mm 就是 1MHz。在真空微重力下甚至不需要中间的牛。当然牛尺度下绳的张力大得离谱——那小尺度下为什么不可行？
sfink查看英文原文
Dumb question: why is it hard to make something spin really fast? Simple example: put your frictionless spherical cow on a spinny plate. Make it a very small cow; it's only there to have a point of rotation. Why frictionless? You don't want its butt to catch fire. Why spherical? It'll need to maximize volume dedicated to arm muscles; see below. Have the cow hold two ropes, each leading to a full-sized cow 10m away. Apply force to those cows (blow on them, or magnetize them and do a solenoid thing, or just make them very gassy cows and orient their spherical butts in opposite directions). Get them spinning at 1Hz. (This is very fast; remember the diameter is 20m.) Now have the middle cow pull the ropes, shortening them to 10cm. It's now spinning at 1Khz. 10mm gives 1Mhz. Conservation of angular momentum, baby. Do this in a vacuum in microgravity, and you don't need the center cow. Sure, if you're doing this at a bovine scale, the tension is ridiculously large. What makes it infeasible at a small scale?
补充好奇该效应能否用于推进/引擎
这能不能做成某种引擎？旋转体发出电磁波，再用类似太阳帆的东西捕获这些波？
cdiamand查看英文原文
Could this be used as an engine of some kind? The spinny thing giving off EM waves and those waves are caught by something like a solar sail?
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