Modern Physics

1. Modern Physics

4. Modern physics relativity Quantum Classic physics

5. relativity Einstein:The founder of modern space-time

6. Atomic Bomb

8. Relativity Relative nature of simultaneity Principle of relativity Lorentz transformation Time-space view Length contraction Time dilation dynamics

9. Einstain’s theory took us into a world far beyond that of ordinary experience,it led us to a deeper and more satisfying view of the nature of space and time introduction

10. A story In modern long range navigation,the precise location and speed of moving craft are continuously monitored and updated.A system of navigation satellites called NAVSTAR permits locations and speeds anywhere on earth to be determined to within about 16m and 2cm/s.However if relativity effects were not taken into account,speeds could not be determined any closer than about 20cm/s,which is unacceptable for modern navigation systems.How can something as abstract as Einstain’s relativity be involved in something as practical as navigation?

11. twin paradox A pair of twins ,A remains on earth,and B make the milk run to a nearby solar system in high speed,when B come back,who’s younger?

12. §1 Galileo relativity principle 1.Galileo transformation Take an account on a event in two different frame

14. is constant Velocity and acceleration transformation

15. In two inertial frame conclusion: 1.time interval is absolute t=t = 2.space separation is absolute for： so： 3.the invariability of Newton’s law

16. For example：conservation of momentum 2.Galileo relativity P957

17. 3.the trouble in electromagnetic equation 1) C :to which reference frame ?

19. Even though electromagnetism shares with mechanics concept such as energy and momentum.there appear to be a major difference between these two fundamental discipline.the laws of mechanics look the same in all inertial frames,but electromagnetism appears to violate the general law.According to Maxwell’s equation,electromagnetic waves propagate at speed C,with no restrictions on the state of the source of detector,this suggests the existence of an absolute frame for electromagnetism.

21. discussion physicsrule §2 the postulates 1.the relativity postulate:the laws of physics are the same for observers in all inertial reference frames.no frames is preferred 2.the speed postulate:the speed of light in vacuum has the same value c in all direction and in all inertial reference frames Einstein relativity develop Newton’s theory Mechanics rule

22.  The light speed invariability is opposed to Galileo velocity transformation  Difference in view Time scale Length scale Mass measure Has no relation with frame Newton Time,space,mass has relation with reference frame relativity C is constant

23. 2.lorentz transformation 1.transformation formula transformation Inverse transformation

24. << discussion  Has relation with  Galileo transformation

25. >  No meaning. Maximum Speed is C 4.procedure to solve the problem 1)establish coordinate 2)determine the moving frame and rest frame 3)u is the positive speed of S’ to S 4)use formula to get relation among x,  t,  x’  t’ and solve problem

26. Example:two persons a,b observe lighting pulses, from point of a, x1=6104m， t1=2 10-4 s ； x2=12 104m，t2=1 10-4 s，from the point of b, two events happen at the same time （1）find the relative velocity of b to a （2）find the space separation of lighting pulses measured by b Solution:

27. We get From lorentz transformation

28. Example:a race track with length 100m,a sports man Run from the origin point to end point with time interval 10s,a craft with velocity 0.8c fly along the direction of run way.from the point of craft man find the space separation and time interval Solution:x=100, t=10s,u=0.8c Negative sign means the sportsman run in the opposite direction

30. § 2 time and space of relativity 1、relative nature of simultaneity In Newton’s theory But in Einstain theory: so， if Relative nature of simultaneity。 We have

31. Conclusion:in general,two events that appear simultaneous in one frame of reference don’t appear simultaneous in a second,unless the two events happen in the same place.

32. Give a signal Einstain’s train experiment Einstein train rest frame In train There’s a signal source In the middle

33. Give a signal Receive flash Receive flash Receive the flash at same time Move with Give a signal Event 1 Event 2 S Receive light early than Two events happen in different time

34. discussion 1）simultaneous is absolutely only when two events happen in same place

35. A君 B君 2) Relative nature of simultaneous is the result of constant c 3) When speed is far more less than C,the result in two inertial frame is same

36. Example:two trains leave from two station A,B with space separation 1000km at the same time,a craft with u=9km/s along the direction ab,find The interval from the point of spaceman Solution:x=106 t=0 Negative means b train go first

37. 2. Time dilation 1)proper time a time interval between two events at the same space point in a frame is called a proper time in that frame

38. 2、proper time is shortest time in all frames Take an account on a clock in In s,from lorentz transformation Time dilation,proper time is minimum time

39. y′ u M′ d l d A′ x′ u t C′ C′ 3)physics reason of time dilation In S ′， there’s a light source in A ′ M′is a reflected mirror

40. y′ u M′ d l d A′ x′ u t C′ C′ In S： Time dilation

41. discussion （1）time dilation is the space-time effect of relativity,it has no relation with the structure of clock (2)the proper time is the shortest one in all frames （3）there’re a lot of experiment to proof the time dilation effect

42. example a rocket v=0.95c，the time interval is 10min measured in rocket，how long is it in earth frame？ Solution:

43. Example:the lifelong time for  is 2.5×10-8s while in rest,if its u=0.99c,the passing distance is 52m,is this ok？ Solution:if with t′=2.5 ×10-8s times u，we get 7.4m。Take account of time dilation So s=uΔt=53m,it’s ok

44. 3.length contraction 1、proper length A length measured in the rest frame of the body is called proper length 2、proper length is the longest in all frames (length contraction)

45. Notes:in S we measure the rod length,we must measure end points in same time From lorenze transformation

46. discussion Proper length is longest • Relativity effect • 2) In low speed  Galileo transformation 3)proper length is the longest , 4)length contraction is relativity effect,it’s different from what we say that the body become smaller.

47. Example:a length of rocket measured in rocket frame is 15m, suppose v=0.95c,find the length in earth frame？ Solution:

48. Example:a 1m rod rest in O’x’y’。the angle is 450 with x’ axis measured from s’。find the length of the rod and angle with x in s。The related velocity is y y' S S' solution: v ly' l' q' lx' x' O O x z' z

49. from lorentz transformation:

50. Conclusion:not only the rod have contraction,but also rotate in some angle.