Some reasons for including the Relativity DS in Unit 3:

1. Some reasons for including the Relativity DS in Unit 3: Students fascinated by the idea of Einstein and Relativity. Expands their thinking and opens their minds – and not only in physics. It’s what they want from Physics! We need to attract ‘non-physics’ students – who will end up as journalists, teachers, business people, and even politicians. (Especially politicians!) Relativity is an excellent example of the way science works: Apparent inconsistencies  creative thinking  experiments  new theories Science as creative and imaginative. Fundamental physics: Newton’s laws, frames of reference, mass and gravity, relative motion, waves and light, nature of space and time, electromagnetism. ‘Classic’ physics experiments: Michelson-Morley, speed of light, Eddington’s eclipse, travelling clocks, muon lifetimes, etc. Much modern technology can not be understood without relativity: magnetism, nuclear energy, GPS, the synchrotron.

2. Einstein’s Special Relativityin VCE Physics PHYSCON February 2011 Keith Burrows AIP Education Committee

3. Why Relativity? • What is physics really all about?

4. What physics really is all about:

6. Newton realised that we assume that time and space are ‘straight’... • and that time and space are unrelated.

7. Einstein said that we should not assume this: • He said that travel through time and space were intimately related. • Special Relativity is about that relationship. These illustrations from Hawking: Universe in a Nutshell These illustrations from Hawking: Universe in a Nutshell

8. Why study relativity? • Relativity represents a ‘giant step’ in the story of physics. Why leave out the climax of the story? • It is an excellent illustration of the process and nature of physics. Through it we can get a feel for real physics. “Imagination is more important than knowledge” Albert Einstein

9. In many ways Einstein represents a new way of thinking, not just about space and time, but about everything. Will this new way of thinking have an impact on the way we see our world as profound as that of the physicists of the Enlightenment? (Not yet apparently ) Einstein’s Legacy

10. “When the ideas involved in relativity have become familiar, as they will do when they are taught in schools, certain changes in our habits of thought are likely to result, and to have great importance in the long run.” Bertrand Russell ‘ABC of Relativity’ Relativity could change the way we think!

11. Albert Michelson (1898) “While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established …”

12. The view in the early 1900’s • In 1900 the mechanical world view seemed capable of explaining just about everything. • Did this lead to the materialism and economic rationalism of the twentieth century?

13. Could relativity really change the way we think? Science is not about collecting facts, but finding new ways of thinking about them. (Bragg) Relativity could change the way we think! • Imagine what could happen if we applied new ways of thinking to, say, energy production and consumption!

14. So why study relativity? • It’s great physics! • Relativity is about questioning common assumptions and finding new ways of looking at a situation. It’s great thinking! • What the world needs now… • Our students – future leaders!

15. Relativity for all • This is not (just) for the specialists, it is for future... • Journalists • Teachers • Politicians • Lawyers • Hairdressers • Mothers and Fathers • Citizens

16. ok – so how do we do it?

17. Developing the story: Overview • 1. Two principles Einstein did NOT want to give up • 2. Einstein's crazy idea • 3. Time is not as it seems: Time Dilation • 4. If time is strange, what about space? • 5. Faster than light? Momentum, Energy and E = mc² • This sequence includes all of the points in the SD, but orders them in a more historically logical way.

18. Summary of 1:Two principles Einstein did NOT want to give up • 1. The principle of relativity (no absolute zero of velocity) seems universal. • 2. Maxwell’s (very ‘elegant’) equations suggested: • light is an electromagnetic wave • and has a fixed speed whatever frame of reference! • this speed was assumed to be the speed through the aether (to make it consistent with the principle of relativity) • But Michelson and Morley could not detect the aether – and Maxwell’s equations didn’t want it. • The principle of relativity seemed inconsistent with the predictions of Maxwell’s equations!

19. Galilean/Newtonian “principle of relativity”: Nothing special about a velocity of zero Velocity can only be measured relative to some other frame of reference No absolute velocity Force changes velocity The laws of physics are the same in any inertial frame. 1. Two principles Einstein did NOT want to give up (1)

20. 1. Two principles Einstein did NOT want to give up (1) The principle of relativity

21. 1. Two principles Einstein did NOT want to give up (2) Maxwell and the speed of light • In the 1830’s Michael Faraday suggested that light may be some sort of electromagnetic wave phenomenon. • In the 1860’s James Clerk Maxwell developed Faraday’s idea into his famous electromagnetic equations. PS: Ideal examples of the excellent experimentalist and the excellent theoretician

22. 1. Two principles Einstein did NOT want to give up (2) Maxwell and the speed of light • The equations suggested the possibility of electromagnetic waves travelling through space from an accelerated charge.

23. 1. Two principles Einstein did NOT want to give up (2)Maxwell and the speed of light • Maxwell’s equations predicted that electromagnetic waves would travel at a speed given by a simple expression involving electric and magnetic constants.

24. 1. Two principles Einstein did NOT want to give up (2) Maxwell and the speed of light • But this expression suggested that electromagnetic waves would travel at this fixed speed in any frame of reference. • Most physicists, including Maxwell, thought this must be wrong and that perhaps this speed was relative to the aether... • …the aether being a hypothetical medium which filled all space.

25. Was the speed of electromagnetic waves really relative to the ‘aether’ – an absolute frame of reference? Michelson and Morley decided to look for evidence of the Earth’s motion through the aether. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether

26. 1. Two principles Einstein did NOT want to give up. Michelson and Morley look for the aether • The principle of their experiment – an analogy:

27. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether • The principle of their experiment – an analogy:

28. 1. Two principles Einstein did NOT want to give up. Michelson and Morley look for the aether • The principle of their experiment:

29. They couldn’t measure the actual speed accurately enough, but they could compare speeds in two perpendicular directions very accurately. They knew that these speeds should be a little different if the Earth was speeding through the aether at 30 km/sec. 1. Two principles Einstein did NOT want to give up. Michelson and Morley look for the aether

30. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether • A water analogy: The boat travels at 5 m/s in a river flowing at 3 m/s

31. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether • It travels at 4 m/s across the river and so takes 2000/4 = 500 sec to complete a two way trip

32. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether • But it travels at 2 m/s upstream (5 – 3) and 8 m/s downstream (5 + 3) and so takes:

33. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether • 1000/2 + 1000/8 = 500 + 125 = 625 sec for the two way trip parallel to the water.

34. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether • So the average speed seems faster going across than going up/down the stream 500 sec 625 sec

35. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether • They arranged to compare the speed of light in perpendicular directions to the Earth’s motion through the aether.

36. 1. Two principles Einstein did NOT want to give up Michelson and Morley look for the aether • When they rotated their apparatus they found NO DIFFERENCE in the speed of light in the two perpendicular directions!

37. 1. Two principles Einstein did NOT want to give up • Einstein apparently knew about the Michelson-Morley experiment, but he does not seem to have been particularly interested in it… • his main interest was in Maxwell’s equations and their predictions about the speed of light and it’s relativity.

38. 1. Two principles Einstein did NOT want to give up • Einstein thought the principle of relativity was so fundamental it should apply in all areas of physics – including electromagnetism. • He also thought Maxwell’s equations were so elegant they, and their prediction about the speed of electromagnetic waves, had to be true. • But how could these two great ideas be reconciled? Surely the speed of light (like every other speed) should depend on one’s frame of reference!

39. Summary of 2: Einstein’s crazy idea • Einstein thought about light and decided that it should be impossible to catch up to it. • He also thought the aether did not make sense and scrapped the whole idea. • This led to his two famous postulates: I No law of physics can identify a state of absolute rest. II The speed of light is the same for all observers. • The implication of taking these postulates at face value is that time seems to be relative!

40. 2. Einstein's crazy idea (1) • Einstein had spent a lot of time wondering about the nature of light…

41. 2. Einstein's crazy idea (2) • ... and concluded that you could not catch up to light – or the electromagnetic waves would be ‘frozen’ in space. This was something that had never been seen – and seemed impossible.

42. 2. Einstein's crazy idea (2) I wonder what would happen if… • Einstein did all his experiments in his head – Gedanken experiments. (Much neater than messy apparatus!) • This might sound odd – but many of the greatest discoveries in physics were made through ‘Gedanken’ experiments… • Including Newton’s three laws!

43. 2. Einstein's crazy idea (2) • Einstein realised that the principle of relativity was an extremely ‘elegant’ principle. • The real world would be very messy without it!

44. 2. Einstein's crazy idea (2) • If there was a way to find an absolute velocity (a ‘veelo’) – whose system is absolute? • It seemed more likely that the universe was truly democratic!

45. 2. Einstein's crazy idea (2) • All motion is relative – there is no absolute frame of reference. • (However changes of velocity are absolute – acceleration is absolute.)

46. 2. Einstein's crazy idea (2) • But then, what is the point of the aether? • Einstein decided that it should not be possible to use an aether or light to determine an absolute velocity.

47. 2. Einstein's crazy idea (2) • He therefore scrapped the idea of the aether… • and concluded that any measurement of the speed of light must give the same result:

48. I No law of physics can identify a state of absolute rest. II The speed of light is the same for all observers. He decided to keep these two great principles and so put forward two postulates which embodied them: 2. Einstein's crazy idea (3)

49. 2. Einstein's crazy idea (4) • At first sight these seem quite simple and straightforward – except that in classical physics they are inconsistent! • Einstein said the aether was unnecessary, but this still left the problem of the relative velocity of light in different frames.

50. There was another difficulty: Einstein realised that there was a problem with ‘simultaneity’ if all observers saw light with the same speed... Ana and Ben see the light hit the ends at the same time... 2. Einstein's crazy idea (5)