光中的结什么时候会打结？
When do knots in light stay knotted?
摘要：
一个初始打结的光场将保持打结，如果它满足一组非线性几何约束，即零条件，对所有时空。然而，最初为零的光场何时保持为零的问题仍然难以回答。通过建立麦克斯韦方程组之间的映射和运输沿流动的无压欧拉流体，我们表明，一个最初的分析零光场保持为零，当且仅当初始坡印亭场的流动是无剪切，给出了一个设计规则的建设持续打结光场。此外，我们概述了最初打结零光场，最初零，无剪切光场的构建方法，并给出了充分条件的磁场（或电）场线的零光场的谎言切表面。我们的结果为持续打结光场的设计及其场线结构的研究奠定了基础。
英文摘要：
An initially knotted light field will stay knotted if it satisfies a set of nonlinear, geometric constraints, i.e. the null conditions, for all space-time. However, the question of when an initially null light field stays null has remained challenging to answer. By establishing a mapping between Maxwell's equations and transport along the flow of a pressureless Euler fluid, we show that an initially analytic null light field stays null if and only if the flow of the initial Poynting field is shear-free, giving a design rule for the construction of persistently knotted light fields. Furthermore we outline methods for constructing initially knotted null light fields, and initially null, shear-free light fields, and give sufficient conditions for the magnetic (or electric) field lines of a null light field to lie tangent to surfaces. Our results pave the way for the design of persistently knotted light fields and the study of their field line structure.
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