.1 Basic concepts of special relativity world would be unable to perform any mechanics experiment that would reveal the uniform velocity of the box,even though any acceleration could be easily detected.(We are all familiar with the feeling of being pressed back in our seats when a train or car accelerates forward.)These notions provided the basis for the first theory of relativity,which is now known as Galilean relativity in honour of Galileo's original insight.This theory of relativity assumes that all inertial observers will agree about the laws of Newtonian mechanics. Einstein believed that inertial observers would agree about the laws of physics quite generally,not just in mechanics.But he was not convinced that Galilean relativity was correct,which brought Newtonian mechanics into question.The only statement that he wanted to presume as a law of physics was that all inertial observers agreed about the speed of light in a vacuum.Starting from this minimal assumption,Einstein was led to a new theory of relativity that was markedly different from Galilean relativity.The new theory,the special theory of relativity, supported Maxwell's laws of electromagnetism but caused the laws of mechanics to be substantially rewritten.It also provided extraordinary new insights into space and time that will occupy us for the rest of this chapter. Einstein based the special theory of relativity on two postulates,that is,two statements that he believed to be true on the basis of the physics that he knew.The first postulate is often referred to as the principle of relativity. The first postulate of special relativity The laws of physics can be written in the same form in all inertial frames. This is a bold extension of the earlier belief that observers would agree about the laws of mechanics,but it is not at first sight exceptionally outrageous.It will, however,have profound consequences. The second postulate is the one that gives primacy to the behaviour of light, a subject that was already known as a source of difficulty.This postulate is sometimes referred to as the principle of the constancy of the speed of light. The second postulate of special relativity The speed of light in a vacuum has the same constant value, c=3×l0的ms-1,in all inertial frames. This postulate certainly accounts for Michelson and Morley's failure to detect any variations in the speed of light,but at first sight it still seems crazy.Our experience with everyday objects moving at speeds that are small compared with the speed of light tells us that if someone in a car that is travelling forward at speed v throws something forward at speed w relative to the car,then,according to an observer standing on the roadside,the thrown object will move forward with speed v+w.But the second postulate tells us that if the traveller in the car turns on a torch,effectively throwing forward some light moving at speed c relative to the car,then the roadside observer will find that the light travels at speed c,not the v+c that might have been expected.Einstein realized that for this to be true, space and time must behave in previously unexpected ways. 151.1 Basic concepts of special relativity world would be unable to perform any mechanics experiment that would reveal the uniform velocity of the box, even though any acceleration could be easily detected. (We are all familiar with the feeling of being pressed back in our seats when a train or car accelerates forward.) These notions provided the basis for the first theory of relativity, which is now known as Galilean relativity in honour of Galileo’s original insight. This theory of relativity assumes that all inertial observers will agree about the laws of Newtonian mechanics. Einstein believed that inertial observers would agree about the laws of physics quite generally, not just in mechanics. But he was not convinced that Galilean relativity was correct, which brought Newtonian mechanics into question. The only statement that he wanted to presume as a law of physics was that all inertial observers agreed about the speed of light in a vacuum. Starting from this minimal assumption, Einstein was led to a new theory of relativity that was markedly different from Galilean relativity. The new theory, the special theory of relativity, supported Maxwell’s laws of electromagnetism but caused the laws of mechanics to be substantially rewritten. It also provided extraordinary new insights into space and time that will occupy us for the rest of this chapter. Einstein based the special theory of relativity on two postulates, that is, two statements that he believed to be true on the basis of the physics that he knew. The first postulate is often referred to as the principle of relativity. The first postulate of special relativity The laws of physics can be written in the same form in all inertial frames. This is a bold extension of the earlier belief that observers would agree about the laws of mechanics, but it is not at first sight exceptionally outrageous. It will, however, have profound consequences. The second postulate is the one that gives primacy to the behaviour of light, a subject that was already known as a source of difficulty. This postulate is sometimes referred to as the principle of the constancy of the speed of light. The second postulate of special relativity The speed of light in a vacuum has the same constant value, c = 3 × 108 m s−1 , in all inertial frames. This postulate certainly accounts for Michelson and Morley’s failure to detect any variations in the speed of light, but at first sight it still seems crazy. Our experience with everyday objects moving at speeds that are small compared with the speed of light tells us that if someone in a car that is travelling forward at speed v throws something forward at speed w relative to the car, then, according to an observer standing on the roadside, the thrown object will move forward with speed v + w. But the second postulate tells us that if the traveller in the car turns on a torch, effectively throwing forward some light moving at speed c relative to the car, then the roadside observer will find that the light travels at speed c, not the v + c that might have been expected. Einstein realized that for this to be true, space and time must behave in previously unexpected ways. 15