期刊文献+
共找到101篇文章
< 1 2 6 >
每页显示 20 50 100
普通情况下的多普勒效应表达式 被引量:13
1
作者 魏国柱 李林 杜安 《东北工学院学报》 CSCD 北大核心 2004年第6期602-605,共4页
考虑到波源和接收器相对于介质同时沿不同方向运动,推导出普遍情况下的机械波多普勒效应表达式·基于爱因斯坦的相对性原理和光速不变原理,利用狭义相对论的时间膨胀公式,推导出光在真空中的多普勒效应表达式·这一推导方法简... 考虑到波源和接收器相对于介质同时沿不同方向运动,推导出普遍情况下的机械波多普勒效应表达式·基于爱因斯坦的相对性原理和光速不变原理,利用狭义相对论的时间膨胀公式,推导出光在真空中的多普勒效应表达式·这一推导方法简单明了,物理概念清晰·最后给出了应用多普勒效应表达式的两个实例· 展开更多
关键词 多普勒效应表达式 光速不变原理 狭义相对论 时间膨胀公式 介质
下载PDF
爱因斯坦建立狭义相对论的关键一步——同时性定义 被引量:10
2
作者 张元仲 《物理与工程》 2015年第4期3-8,共6页
本文阐明了双程(回路)光速的数值可以由实验直接给出,但是单向光速的数值不能由实验获得而只能通过单向光速各向同性的假设使之等于双程光速的数值.有了这个(假定的)单向光速的数值就可以同步惯性系中的所有时钟,因而定义了惯性系的时... 本文阐明了双程(回路)光速的数值可以由实验直接给出,但是单向光速的数值不能由实验获得而只能通过单向光速各向同性的假设使之等于双程光速的数值.有了这个(假定的)单向光速的数值就可以同步惯性系中的所有时钟,因而定义了惯性系的时间坐标,正是由于这种同时性定义,爱因斯坦才进而发现了狭义相对论.迄今为止所有测量光速的实验给出的都是双程而非单程光速的数值.为了从理论上阐述单程光速无法测量,我们介绍了爱德瓦兹同时性,它是使用双程光速不变而单程光速任意的假设来定义的;同时给出了与之相应的爱德瓦兹变换,进而讨论了这种同时性与爱因斯坦同时性的关系.通过时间膨胀和长度收缩效应阐明了二者在物理上是等价的,也就是说单向光速的效应在实验中不会显现. 展开更多
关键词 狭义相对论 洛伦兹变换 单向光速 双程光速 时间膨胀 长度收缩
下载PDF
电子束时间展宽皮秒分幅相机 被引量:10
3
作者 蔡厚智 龙井华 +4 位作者 刘进元 谢维信 白雁力 雷云飞 廖昱博 《红外与激光工程》 EI CSCD 北大核心 2016年第12期54-59,共6页
研制了基于电子束时间展宽技术和微通道板(microchannel plate,MCP)选通技术的时间展宽分幅相机。相机有三条厚度80 nm、宽度8 mm的微带阴极,阴极上加载斜率为2.1 V/ps的高压斜坡脉冲,使得先发射的电子较后面的电子速度快,经过50 cm的... 研制了基于电子束时间展宽技术和微通道板(microchannel plate,MCP)选通技术的时间展宽分幅相机。相机有三条厚度80 nm、宽度8 mm的微带阴极,阴极上加载斜率为2.1 V/ps的高压斜坡脉冲,使得先发射的电子较后面的电子速度快,经过50 cm的漂移区后,电子束产生时间展宽,从而提高相机时间分辨率。阴极和MCP均加载了脉冲电压,因此,需要精确同步光脉冲、阴极脉冲和MCP选通脉冲,分析了完整的同步过程。当阴极仅加直流电压,无电子束时间展宽时,获得相机的时间分辨率为78 ps。当阴极加载高压斜坡脉冲时,电子束时间展宽技术将系统的时间分辨率提高至12 ps。改变延时,将光脉冲分别同步在斜坡脉冲不同位置,获得了时间分辨率与同步位置的关系。 展开更多
关键词 分幅相机 时间展宽 时间分辨率 惯性约束聚变 Z箍缩
下载PDF
狭义相对论教学中不该出现的错误 被引量:7
4
作者 张元仲 《物理与工程》 2017年第4期3-9,共7页
本文为了阐明个别大学物理教材中对光速不变假设的错误评论而详细说明了光速不变原理的两个用途:首要用途是用来定义惯性系中的时间坐标(即定义同时性);第二个用途是用来推导洛伦兹变换。随后,在分析了钟慢效应、尺缩效应、同时性的相... 本文为了阐明个别大学物理教材中对光速不变假设的错误评论而详细说明了光速不变原理的两个用途:首要用途是用来定义惯性系中的时间坐标(即定义同时性);第二个用途是用来推导洛伦兹变换。随后,在分析了钟慢效应、尺缩效应、同时性的相对性之后解释了尺缩佯谬,并特别指出了某大学物理教材中对一道习题提供的答案是错误的并用两种解法给出了正确答案。 展开更多
关键词 光速不变原理 相对性原理 洛伦兹变换 时钟变慢 尺子缩短 同时性的相对性
下载PDF
Time Dilation Cosmology 2
5
作者 Joseph H. (Cass) Forrington 《Journal of Modern Physics》 2024年第4期448-461,共14页
This paper is a further elaboration of the author’s Time Dilation Cosmology (TDC) holographic model that ties gravitation and celestial mechanics and kinematics directly to time dilation, resolving all the major conu... This paper is a further elaboration of the author’s Time Dilation Cosmology (TDC) holographic model that ties gravitation and celestial mechanics and kinematics directly to time dilation, resolving all the major conundrums in astrophysics, and ties astrophysics directly to quantum physics. It begins with a brief summary of the TDC model and contains the new derivation for the time dilation version of the formula for summing relativistic velocities, Einstein’s gravitational constant and the time dilation versions for the Lorentz factor and the Euclidean norm of the 3d velocity vector, the two of which can then be used in the Four-velocity formula. It is demonstrated how orbital curvature is manifested as the resultant of two time dilation-manifested velocities. It also explains why an interferometer cannot distinguish free fall from zero gravity and further elaborates on the author’s previous explanations of how spiral galaxies are formed, and contains mathematical proof that Black Holes are actually Magnetospheric Eternally Collapsing Objects (MECOs) that are massless spacetime vortices. 展开更多
关键词 GRAVITATION time time dilation Celestial Mechanics ISM: Kinematics and Dynamics Cosmology: Theory Galaxies: Evolution
下载PDF
The Unified Field
6
作者 Joseph H. (Cass) Forrington 《Journal of Modern Physics》 2024年第7期1010-1035,共26页
This is a Unified Field description based on the holographic Time Dilation Cosmology, TDC, model, which is an eternal continuum evolving forward in the forward direction of time, at the speed of light, c, at an invari... This is a Unified Field description based on the holographic Time Dilation Cosmology, TDC, model, which is an eternal continuum evolving forward in the forward direction of time, at the speed of light, c, at an invariant 1 s/s rate of time. This is the Fundamental Direction of Evolution, FDE. There is also an evolution down time dilation gradients, the Gravitational Direction of Evolution, GDE. These evolutions are gravity, which is the evolutionary force in time. Gravitational velocities are compensation for the difference in the rate of time, dRt, in a dilation field, and the dRtis equal to the compensatory velocity’s percentage of c, and is a measure of the force in time inducing the velocity. In applied force induced velocities, the dRt is a measure of the resistance in time to the induced velocity, which might be called “anti-gravity” or “negative gravity”. The two effects keep the continuum uniformly evolving forward at c. It is demonstrated that gravity is already a part of the electromagnetic field equations in way of the dRt element contained in the TDC velocity formula. Einstein’s energy formula is defined as a velocity formula and a modified version is used for charged elementary particle solutions. A time dilation-based derivation of the Lorentz force ties gravity directly to the electromagnetic field proving the unified field of gravity and the EMF. It is noted how we could possibly create gravity drives. This is followed by a discussion of black holes, proving supermassive objects, like massive black hole singularities, are impossible, and that black holes are massless Magnetospheric Eternally Collapsing Objects (MECOs) that are vortices in spacetime. . 展开更多
关键词 Unified Field GRAVITY Anti-Gravity Astrophysics Einstein General Relativity Special Relativity Galactic Rotation Velocities time dilation SPACEtime Space time Spacetime Continuum Quantum Continuum MECO Black Hole Event Horizon timelike Spacelike Lightlike
下载PDF
Physics of Clocks in Absolute Space-Time 被引量:3
7
作者 Edwin Eugene Klingman 《Journal of Modern Physics》 2020年第12期1950-1968,共19页
20th century physics experimentally established beyond doubt the fact that moving clocks read differently from “static” clocks. This fact is typically interpreted as support for special relativity. On the other hand... 20th century physics experimentally established beyond doubt the fact that moving clocks read differently from “static” clocks. This fact is typically interpreted as support for special relativity. On the other hand, the same century produced proof that clocks at various locations in the gravitational field also read differently, and this fact is explained by general relativity, which is, in general, not Lorentz transformable. This paper establishes a common framework for the physics of clocks in these different situations. 展开更多
关键词 RELATIVITY time dilation Space-time Ontology Clock Slowdown Inertial Clocks Absolute Space and time Inertial Mass Covariance Principle Ideal Clocks
下载PDF
狭义相对论的逻辑不自洽问题和新伽利略时空观 被引量:6
8
作者 马青平 《北京石油化工学院学报》 2006年第4期4-16,共13页
科学理论需要在逻辑上自洽。分析了狭义相对论对高速运动的不稳定微观粒子寿命延长的解释,发现狭义相对论的解释否定了光速不变原理或相对论性时空观。进一步分析发现,在狭义相对论“时间膨胀”和“长度收缩”的推导过程中,观察系和被... 科学理论需要在逻辑上自洽。分析了狭义相对论对高速运动的不稳定微观粒子寿命延长的解释,发现狭义相对论的解释否定了光速不变原理或相对论性时空观。进一步分析发现,在狭义相对论“时间膨胀”和“长度收缩”的推导过程中,观察系和被观察系的指定完全相反。这一现象说明“时间膨胀”和“长度收缩”不是狭义相对论的逻辑结果,狭义相对论的逻辑结论应该是“时间收缩”和“长度收缩”(或“时间膨胀”和“长度膨胀”)。在狭义相对论的理论框架内不能正确地推导出运动物体的横质量,在狭义相对论的框架内得出正确横质量公式的推导都存在逻辑或物理错误。因此,狭义相对论在逻辑上是不自洽的,相对论性时空观没有实验证据。现代物理学应该建立在以局部优势引力场为相对优越参照系的新伽利略时空观的基础上。从光传播媒介对应于局部优势引力场这一原则出发,解释了迈克尔逊-莫雷实验、菲索流水光速实验和光行差现象。“运动质量”可由运动物体在受电磁场或引力场作用时受力的大小与该物体的运动速度有关得到解释。不稳定粒子寿命和物理时钟受各自的运动状态影响,也可以分别由粒子衰变和物理时钟依赖的具体物理过程受其相对于局部优势引力场的运动状态的影响来解释。 展开更多
关键词 时间膨胀 长度收缩 运动质量 洛伦兹变换 光行差
下载PDF
Physical Space Is Finite
9
作者 Youqi Wang 《Journal of Modern Physics》 2024年第5期550-595,共46页
Metrological analysis shows that any clock in inertial motion in infinite space shall not have time dilation, due to relativity of such motion in such space. On the other hand, atomic clock in inertial motion in finit... Metrological analysis shows that any clock in inertial motion in infinite space shall not have time dilation, due to relativity of such motion in such space. On the other hand, atomic clock in inertial motion in finite space shall exhibit time dilation, due to alteration of momentum of clock-defining particle caused by nonzero curvature of trajectory of such motion in such space. Therefore, time dilation experiment of atomic clock in inertial motion in physical space provides a direct and decisive way of determining geometry of physical space in real-time. Phenomenon of time dilation of atomic clock in inertial motion in physical space has long been observed and confirmed experimentally. Therefore, extent of physical space has to be finite, consistent with result of high precision experiment of free particle in high-speed motion conducted a decade ago.Keywords Geometry of Physical Space, Time Dilation, Atomic Clock, Special Relativity Theory. 展开更多
关键词 Geometry of Physical Space time dilation Atomic Clock Special Relativity Theory
下载PDF
The Space Dilation: Analyzing Progressive Speed Reduction and Its Relativistic Parallels
10
作者 Kreshnik Feka 《Journal of Applied Mathematics and Physics》 2024年第8期2990-3005,共16页
This paper presents a conceptual exploration that draws an intriguing parallelbetween a hypothetical travel scenario and the principles of special relativity. The scenario involves a traveler who reduces their speed b... This paper presents a conceptual exploration that draws an intriguing parallelbetween a hypothetical travel scenario and the principles of special relativity. The scenario involves a traveler who reduces their speed by an amount proportional to the distance traveled. Despite initially traveling at a high speed towards a given destination, the continual reduction in speed results in an asymptotic approach to the goal, analogous to the unattainable speed of light in relativity. Mathematically, the scenario is expressed through the Harmonic Series, demonstrating that the total travel time increases without bound, making the destination theoretically unreachable within a finite timeframe. This exploration mirrors the relativistic velocity addition and time dilation effects, providing a compelling analogy for understanding asymptotic limits. By highlighting the profound implications of diminishing returns and unattainable goals, this paper aims to stimulate further discussion and exploration of these fascinating parallels. 展开更多
关键词 Asymptotic Limits Special Relativity Harmonic Series time dilation Diminishing Returns
下载PDF
Encoding Energy-Density as Geometry 被引量:3
11
作者 Edwin Eugene Klingman 《Journal of Modern Physics》 2021年第9期1190-1209,共20页
Physicists possess an intuitive awareness of Euclidian space and time and Galilean transformation, and are then challenged with Minkowski space-time and Einstein’s curved space-time. Relativistic experiments support ... Physicists possess an intuitive awareness of Euclidian space and time and Galilean transformation, and are then challenged with Minkowski space-time and Einstein’s curved space-time. Relativistic experiments support the “time-dilation” interpretation and others support “curved space-time” interpretation. In this, and related work, we investigate the key issues in terms of the intuitive space-time frame. In particular, we provide alternative approaches to explain “time dilation” and to explain the energy density for gravity systems. We approach the latter problem from an information perspective. 展开更多
关键词 Curved Space-time Gravitational Energy Stress-Energy Tensor Encoding Information in Geometry time dilation Equivalence Principle Minkowski Relation Schwarzschild Metric Linearized General Relativity COSMOLOGY
下载PDF
磁聚焦成像电子束时间展宽分幅相机的时空特性 被引量:5
12
作者 蔡厚智 付文勇 +3 位作者 雷云飞 廖昱博 刘进元 龙井华 《激光与光电子学进展》 CSCD 北大核心 2018年第7期56-64,共9页
研制了一款新型磁聚焦成像电子束时间展宽分幅相机,研究了其空间分辨率与磁聚焦透镜数目及电流间的关系,实验结果表明:采用三个磁聚焦透镜且电流适合时,相机的空间分辨率最高;离轴越远,空间分辨率越低。研究了相机的空间分辨率与电压间... 研制了一款新型磁聚焦成像电子束时间展宽分幅相机,研究了其空间分辨率与磁聚焦透镜数目及电流间的关系,实验结果表明:采用三个磁聚焦透镜且电流适合时,相机的空间分辨率最高;离轴越远,空间分辨率越低。研究了相机的空间分辨率与电压间的关系,发现空间分辨率随着阴栅极间电压的增大而提高。该新型分幅相机成像面为一个曲面,整个阴极面不能同时清晰成像。采用电子束时间展宽技术后,该新型分幅相机的时间分辨率提高至11ps;当成像倍率为2…1时,相机的静态空间分辨率为5lp·mm^(-1)。 展开更多
关键词 探测器 分幅相机 时间展宽 空间分辨率 惯性约束聚变(ICF)
原文传递
时间展宽X射线分幅相机空间分辨特性 被引量:5
13
作者 张珂 蔡厚智 +1 位作者 刘进元 付文勇 《光子学报》 EI CAS CSCD 北大核心 2018年第2期35-41,共7页
利用Monte Carlo方法、有限元法和有限差分法建立了时间展宽X射线分幅相机的理论模型,对相机静态空间分辨特性进行了理论研究。当光电阴极的电压为-3kV,采用三个磁透镜,成像倍率为2:1时,相机的静态空间分辨率优于110μm.研究了空间分辨... 利用Monte Carlo方法、有限元法和有限差分法建立了时间展宽X射线分幅相机的理论模型,对相机静态空间分辨特性进行了理论研究。当光电阴极的电压为-3kV,采用三个磁透镜,成像倍率为2:1时,相机的静态空间分辨率优于110μm.研究了空间分辨率与发射位置、阴极电压、磁聚焦透镜数量的关系.模拟结果表明,发射位置离中心越近,阴极电压越高,磁聚焦透镜个数越多,空间分辨率越好.此外,平面的光电阴极经磁聚焦透镜成像后,像面不是一个平面而是一个曲面. 展开更多
关键词 分幅相机 时间展宽 空间分辨率 惯性约束聚变 Z箍缩
下载PDF
A Bridge Connecting Classical Physics and Modern Physics 被引量:2
14
作者 Tower Chen Zeon Chen 《Journal of Modern Physics》 2016年第11期1378-1387,共10页
In classical physics, time and space are absolute and independent, so time and space can be treated separately. However, in modern physics, time and space are relative and dependent: time and space must be treated tog... In classical physics, time and space are absolute and independent, so time and space can be treated separately. However, in modern physics, time and space are relative and dependent: time and space must be treated together. In 4-d s-t frames, we treat time and space independently, then add a constraint to link them together. In teaching, there is a big gap between classical and modern physics. We hope that we are able to find a frame connecting them to make learning simpler. 3-d s-t frame is the best candidate to serve this purpose: time and space are able to be treated dependently by defining the unit of time as T and the unit of space as λ in this frame. Furthermore, the ratio, λ/T, is the velocity, c, of the medium. This paper shows the equivalence between a 4-d s-t frame and a 3-d s-t frame by properly converting coordinates of two frames. 展开更多
关键词 4-d s-t Frames 3-d s-t Frame Proper time Proper Length time dilation Length Contraction
下载PDF
Einstein’s Theory of Relativity in the Scenario of the Higgs Quantum Space Dynamics
15
作者 Jacob Schaf 《Journal of Modern Physics》 2018年第5期1111-1143,共33页
This work is concerned about the true physical mechanism of gravity. The Higgs theory introduces important changes in Einstein’s view about the nature of empty space, about the meaning of motions and about the nature... This work is concerned about the true physical mechanism of gravity. The Higgs theory introduces important changes in Einstein’s view about the nature of empty space, about the meaning of motions and about the nature of the gravitational physics. The Higgs Quantum Space (HQS) is a real quantum fluid spatial medium, giving inertial mass to the elementary particles by the Higgs mechanism. Therefore, the HQS rules the inertial motion of matter-energy and is the local ultimate reference for rest and for motions. In this new scenario, velocity with respect to the local HQS and not relative velocity is the origin of all the effects of motion. The HQS also necessarily is responsible for the gravitational fields because it is mass that creates the gravitational fields. The observed absence of the gravitational slowing of the GPS clocks by the solar field and the absence of light anisotropy with respect to earth are both the obvious signature of the true physical mechanism of gravity in action. These observations show that the HQS is moving itself round the sun according to a Keplerian velocity field, consistent with the planetary motions, which will be shown to accurately create, besides the observed gravitational dynamics, all the observed effects of the gravitational fields on light and on clocks. Current theories explain the absence of the solar gravitational slowing of the GPS clocks with base in the principle of equivalence. In the language of the present work, the HQS materializes the local Lorentz frames (LFs), turning them into local proper LFs, intrinsically stationary with respect to the local moving HQS. In its motions, the HQS carries these local proper LFs with it round the sun, so that the planets of the solar system are stationary with respect to them, which directly predicts the absence of the gravitational slowing of the GPS clocks by the solar field, the absence of light anisotropy with respect to earth and all the other effects of the gravitational fields observed on earth. 展开更多
关键词 Fundamental Physics THEORY of RELATIVITY RELATIVISTIC Effects time dilation Gravitational time dilation HIGGS THEORY RELATIVISTIC Experiments
下载PDF
GPS Satellite Clock Corrections without Relativity Theory
16
作者 Stephan J. G. Gift 《Journal of Applied Mathematics and Physics》 2021年第10期2476-2482,共7页
The GPS satellite clock corrections (along with gravitational redshift) which are necessary for the proper operation of the GPS are fully described without invoking relativity theory as is the practice today.
关键词 GPS Selleri Transformations time dilation Gravitational time dilation Special Relativity General Relativity
下载PDF
对爱因斯坦“尺缩”、“钟慢”效应的再认识 被引量:4
17
作者 文兴吾 《大自然探索》 1991年第2期109-117,共9页
关键词 爱因斯坦 尺缩效应 钟慢效应
下载PDF
洛伦兹变换的引入及其时空图像讨论 被引量:3
18
作者 缪劲松 胡海云 《物理与工程》 2016年第S1期10-16,21,共8页
狭义相对论是理工科大学物理课程教学中的一个难点,这是因为狭义相对论所涉及的时空效应在实际中都很难被观测到,而反映相对论时空观的洛伦兹变换又较为抽象,学生在学习过程中通常是机械地记忆公式,而并没有真正理解相对论的时空观及其... 狭义相对论是理工科大学物理课程教学中的一个难点,这是因为狭义相对论所涉及的时空效应在实际中都很难被观测到,而反映相对论时空观的洛伦兹变换又较为抽象,学生在学习过程中通常是机械地记忆公式,而并没有真正理解相对论的时空观及其与洛伦兹变换之间的联系.针对这一问题,本文介绍了一种引入洛伦兹变换的新方式:首先通过直观的思想实验定量地导出时间延缓、长度收缩和同时性的相对性等时空效应的数学表达式,然后在此基础上推导得到洛伦兹变换的x坐标变换式和时间变换式,并结合推导过程对其时空图像进行讨论.通过洛伦兹变换的这种引入方式可将相对论时空效应与洛伦兹变换紧密地联系起来,突出了洛伦兹变换的物理图像,从而加深学生对洛伦兹变换及相对论时空观的理解. 展开更多
关键词 狭义相对论 时间延缓 长度收缩 同时性的相对性 洛伦兹变换 时空观
下载PDF
Cosmic Time Transformations in Cosmological Relativity 被引量:1
19
作者 Firmin J. Oliveira 《Journal of High Energy Physics, Gravitation and Cosmology》 2016年第2期253-279,共27页
The relativity of cosmic time is developed within the framework of Cosmological Relativity in five dimensions of space, time and velocity. A general linearized metric element is defined to have the form , where the co... The relativity of cosmic time is developed within the framework of Cosmological Relativity in five dimensions of space, time and velocity. A general linearized metric element is defined to have the form , where the coordinates are time , radial distance for spatials x, y and z, and velocity v, with c the speed of light in vacuum and t the Hubble-Carmeli time constant. The metric is accurate to first order in and v/c . The fields and are general functions of the coordinates. By showing that =, a metric of the form is obtained from the general metric, implying that the universe is flat. For cosmological redshift z, the luminosity distance relation is used to fit combined distance moduli from Type 1a supernovae up to z is obtained for the matter density parameter at the present epoch. Assuming a baryon density of , a rest mass energy of (9.79+ 0.47) GeV is predicted for the anti-baryonic and the particles which decay from a hypothetical particle. The cosmic aging function makes good fits to light curve data from two reports of Type 1a supernovae and in fitting to simulated quasar like light curve power spectra separated by redshift . We determine the multipole of the first acoustic peak of the Cosmic Microwave Background radiation anisotropy to be and a sound horizon of on today’s sky. 展开更多
关键词 Flat Space Cosmic time time dilation Dark Matter
下载PDF
爱因斯坦相对论的时空特性研究 被引量:3
20
作者 张宁 《北京联合大学学报》 CAS 2005年第1期15-18,共4页
 时间和空间是相对论研究的重要物理问题,在物质的空间中,存在引力场,而由于引力场的存在,则会导致时间的变化,空间的弯曲,光线轨迹的改变等。对相对论中的时间和空间特性进行了探讨,分析了广义相对论中的等效原理和广义协变原理,讨论...  时间和空间是相对论研究的重要物理问题,在物质的空间中,存在引力场,而由于引力场的存在,则会导致时间的变化,空间的弯曲,光线轨迹的改变等。对相对论中的时间和空间特性进行了探讨,分析了广义相对论中的等效原理和广义协变原理,讨论了爱因斯坦引力场方程,研究了相对性原理、引力场中的时间膨胀、长度收缩和引力波。 展开更多
关键词 广义相对论 等效原理 引力场 时间膨胀 长度收缩 引力波
下载PDF
上一页 1 2 6 下一页 到第
使用帮助 返回顶部