In the special theory of relativity, massive particles can travel at neither the speed of light c nor faster. Meanwhile, since the photon was quantized, many have thought of it as a point particle. How pointed? The id...In the special theory of relativity, massive particles can travel at neither the speed of light c nor faster. Meanwhile, since the photon was quantized, many have thought of it as a point particle. How pointed? The idea could be a mathematical device or physical simplification. By contrast, the preceding notion of wave-group duality has two velocities: a group velocity vg and a phase velocity vp. In light vp = vg = c;but it follows from special relativity that, in massive particles, vp > c. The phase velocity is the product of the two best measured variables, and so their product constitutes internal motion that travels, verifiably, faster than light. How does vp then appear in Minkowski space? For light, the spatio-temporal Lorentz invariant metric is s2=c2t2−x2−y2−z2, the same in whatever frame it is viewed. The space is divided into 3 parts: firstly a cone, symmetric about the vertical axis ct > 0 that represents the world line of a stationary particle while the conical surface at s = 0 represents the locus for light rays that travel at the speed of light c. Since no real thing travels faster than the speed of light c, the surface is also a horizon for what can be seen by an observer starting from the origin at time t = 0. Secondly, an inverted cone represents, equivalently, time past. Thirdly, outside the cones, inaccessible space. The phase velocity vp, group velocity vg and speed of light are all equal in free space, vp = vg = c, constant. By contrast, for particles, where causality is due to particle interactions having rest mass mo > 0, we have to employ the Klein-Gordon equation with s2=c2t2−x2−y2−z2+mo2c2. Now special relativity requires a complication: vp.vg = c2 where vg c and therefore vp > c. In the volume outside the cones, causality due to light interactions cannot extend beyond the cones. However, since vp > c and even vp >> c when wavelength λ is long, extreme phase velocities are then limited in their causal effects by the particle uncertainty σ, i.e. to vgt ± σ/ω, where ω is the particle angular frequency. This is the first time the phase range has been described for a massive particle.展开更多
随着传感、信号处理和通信技术的快速发展,关于网络控制系统(Networked control systems,NCSs)的研究引起了极大的关注.本文拟回顾关于网络控制系统的最新研究进展.为揭示反馈通信网络对控制系统的影响,主要讨论了为满足不同控制目的所...随着传感、信号处理和通信技术的快速发展,关于网络控制系统(Networked control systems,NCSs)的研究引起了极大的关注.本文拟回顾关于网络控制系统的最新研究进展.为揭示反馈通信网络对控制系统的影响,主要讨论了为满足不同控制目的所需的各种网络条件,例如:在时变信道的环境下,保证线性系统可镇定性所需的最低编码率;在间断观测的环境下,保证卡尔曼滤波器稳定性的临界丢包条件;在时不变连接图的环境下,达到线性多自主体系统趋同性所需的网络拓扑结构;在通信资源有限的情况下,基于事件驱动的采样与控制方法等.展开更多
精确的时间和频率信息对于通信系统各种应用非常重要。传统电路交换通过TDM链路帧同步实现精确频率同步,而分组网络的"存储-转发"特性为同步消息引入延时,要实现精确的能够满足传统通信需要的频率和时间同步更加困难。IEEE158...精确的时间和频率信息对于通信系统各种应用非常重要。传统电路交换通过TDM链路帧同步实现精确频率同步,而分组网络的"存储-转发"特性为同步消息引入延时,要实现精确的能够满足传统通信需要的频率和时间同步更加困难。IEEE1588精密时间同步协议(Precision Time Protocol,PTP)是一个原来应用于工业控制、测试测量等领域低成本高精度的同步协议,根据分组通信网络的特点,提出了IEEE1588在通信中的应用框架,采用虚拟和真实网络环境相结合的仿真方法,分析了所提框架的同步性能及各种性能补偿措施的补偿效果。虚拟网络仿真采用OMNeT++工具,给出网络业务特性对精确时间同步的定性和定量影响。真实网络仿真基于校园网环境,采用软件时间戳标记达到50μs的同步精度。展开更多
文摘In the special theory of relativity, massive particles can travel at neither the speed of light c nor faster. Meanwhile, since the photon was quantized, many have thought of it as a point particle. How pointed? The idea could be a mathematical device or physical simplification. By contrast, the preceding notion of wave-group duality has two velocities: a group velocity vg and a phase velocity vp. In light vp = vg = c;but it follows from special relativity that, in massive particles, vp > c. The phase velocity is the product of the two best measured variables, and so their product constitutes internal motion that travels, verifiably, faster than light. How does vp then appear in Minkowski space? For light, the spatio-temporal Lorentz invariant metric is s2=c2t2−x2−y2−z2, the same in whatever frame it is viewed. The space is divided into 3 parts: firstly a cone, symmetric about the vertical axis ct > 0 that represents the world line of a stationary particle while the conical surface at s = 0 represents the locus for light rays that travel at the speed of light c. Since no real thing travels faster than the speed of light c, the surface is also a horizon for what can be seen by an observer starting from the origin at time t = 0. Secondly, an inverted cone represents, equivalently, time past. Thirdly, outside the cones, inaccessible space. The phase velocity vp, group velocity vg and speed of light are all equal in free space, vp = vg = c, constant. By contrast, for particles, where causality is due to particle interactions having rest mass mo > 0, we have to employ the Klein-Gordon equation with s2=c2t2−x2−y2−z2+mo2c2. Now special relativity requires a complication: vp.vg = c2 where vg c and therefore vp > c. In the volume outside the cones, causality due to light interactions cannot extend beyond the cones. However, since vp > c and even vp >> c when wavelength λ is long, extreme phase velocities are then limited in their causal effects by the particle uncertainty σ, i.e. to vgt ± σ/ω, where ω is the particle angular frequency. This is the first time the phase range has been described for a massive particle.
文摘随着传感、信号处理和通信技术的快速发展,关于网络控制系统(Networked control systems,NCSs)的研究引起了极大的关注.本文拟回顾关于网络控制系统的最新研究进展.为揭示反馈通信网络对控制系统的影响,主要讨论了为满足不同控制目的所需的各种网络条件,例如:在时变信道的环境下,保证线性系统可镇定性所需的最低编码率;在间断观测的环境下,保证卡尔曼滤波器稳定性的临界丢包条件;在时不变连接图的环境下,达到线性多自主体系统趋同性所需的网络拓扑结构;在通信资源有限的情况下,基于事件驱动的采样与控制方法等.
文摘精确的时间和频率信息对于通信系统各种应用非常重要。传统电路交换通过TDM链路帧同步实现精确频率同步,而分组网络的"存储-转发"特性为同步消息引入延时,要实现精确的能够满足传统通信需要的频率和时间同步更加困难。IEEE1588精密时间同步协议(Precision Time Protocol,PTP)是一个原来应用于工业控制、测试测量等领域低成本高精度的同步协议,根据分组通信网络的特点,提出了IEEE1588在通信中的应用框架,采用虚拟和真实网络环境相结合的仿真方法,分析了所提框架的同步性能及各种性能补偿措施的补偿效果。虚拟网络仿真采用OMNeT++工具,给出网络业务特性对精确时间同步的定性和定量影响。真实网络仿真基于校园网环境,采用软件时间戳标记达到50μs的同步精度。
基金Supported by Key Project of Science and Technology Commission of Shanghai Municipality(14JC1402200,15JC1401900)National Key Scientific Instrument and Equipment Development Project(2012YQ15008703)+1 种基金National Science Foundation of China(61473182)Shanghai Rising-Star Program(13QA 1401600)
