Chapter 20 The special theory of relativity Albert Einstein(1879-1955)
Chapter 20 The special theory of relativity Albert Einstein ( 1879 ~ 1955 )
20-1 Troubles with classical physics The kinematics developed by galileo and the mechanics developed by Newton, which form the basis of what we call"classical physics, had many triumphs. However, a number of experimental phenomena can not be understood with these otherwise successful classical theories 1. Troubles with our ideas about time The pions( or z created at rest are observed
20-1 Troubles with classical physics The kinematics developed by Galileo and the mechanics developed by Newton, which form the basis of what we call “classical physics”, had many triumphs. However, a number of experimental phenomena can not be understood with these otherwise successful classical theories. 1. Troubles with our ideas about time The pions ( or ) created at rest are observed + −
to decay( to other particles)with an average lifetime of only 26.0ns In one particular experiment, pions were created in motion at a speed ofv=0.913c. In this case they were observed to travel in the laboratory an average distance of D=17.4m before decaying, from which we conclude that they decay in a time given by D/=637ns, much larger than the lifetime measured for pions at rest This effect. called time dilation", which cannot be explained by Newtonian physics. In Newtonian physics time is a universal coordinate having identical values for all observers
to decay ( to other particles ) with an average lifetime of only . In one particular experiment, pions were created in motion at a speed of . In this case they were observed to travel in the laboratory an average distance of before decaying, from which we conclude that they decay in a time given by , much larger than the lifetime measured for pions at rest. This effect, called “time dilation”, which cannot be explained by Newtonian physics. In Newtonian physics time is a universal coordinate having identical values for all observers. 26.0ns v = 0.913c ns v D = 63.7 D =17.4m
2. Trouble with our ideas about length Suppose an observer in the above laboratory placed one marker at the location of the pion's formation and another at the location of its decay The distance between the markers is measured to be 17.4m. Now consider the observer who is traveling along with the pion at a speed of u=0.913c This observer, to whom the pion appear to be at rest, measures its lifetime to be 260ns, and the distance between the markers is (0913c)(260×103)=71m Thus two observers measure different value for the same length interval 3. Troubles with our ideas about light
2. Trouble with our ideas about length Suppose an observer in the above laboratory placed one marker at the location of the pion’s formation and another at the location of its decay. The distance between the markers is measured to be 17.4m. Now consider the observer who is traveling along with the pion at a speed of u=0.913c. This observer, to whom the pion appear to be at rest, measures its lifetime to be 26.0ns, and the distance between the markers is Thus two observers measure different value for the same length interval. (0.913c)(26.0 10 ) 7.1m 9 = − 3. Troubles with our ideas about light
20-2 The postulates of special relativity 1. Einstein offered two postulates that form the basis of his special theory of relativity o The principle of relativity: The laws of phys are the same in all inertial reference frames. sics (D The principle of the constancy of the speed of light The speed of light in free space has the same value c in all inertial reference frames 2. The first postulate declares that the laws of physics are absolute, universal, and same for all inertial observers
20-2 The postulates of special relativity 1. Einstein offered two postulates that form the basis of his special theory of relativity. (I) The principle of relativity: “The laws of physics are the same in all inertial reference frames.” (II) The principle of the constancy of the speed of light : “ The speed of light in free space has the same value c in all inertial reference frames.” 2. The first postulate declares that the laws of physics are absolute, universal, and same for all inertial observers