2. T. K. Ng (HKUST) The classical Universe

3. Plan: This lecture: Review Pre-relativity/Quantum Mechanics picture of our physical universe You will learn some basic concepts in relativity & QM in the coming days last lecture: Modern view of our universe

4. Hidden assumptions in Newton’s Law (F = ma) 4 elements in describing events in our universe space + time  trajectory matter (particle) (moving in space-time) force (acting between matter)

5. Question: What properties are assumed in these 4 elements?

6. Space & Time Classical (absolute) reality (objectivity) - everyone has same observation on what has happened! Existence independent of other 2 factors (matter & force).

7.  (classical) objectiveness in science We measure what we want with certainty?

8.  (classical) objectiveness in science Can you imagine an universe otherwise?

9. Matter & Force Matter: Force: Mass, Energy, momentum,…..etc. How is force defined? Can they exist without matter?

10. In Newton’s World Space & Time exists independent of matter. In fact, space is independent of time. They provide a rigid background for matter & force Force is physically distinct from matter! but its existence is detected only through matter.

11. Are you satisfied with the above relations? If not, how should they be modified? Physicists: Is there an underlying principle governing these relations?

12. “Same” questions asked differently What is the origin of our universe (space & time)? What is the origin of matters? What is the origin of different forces? No answer (God given) in Newton‘s World

13. Electricity & Magnetism At late 19th century physicists realized that Newton’s picture of space, time, matter & force has to be modified if we want to understand E&M! Main problem: existence of Light = Electromagnetic wave!

14. First problem: the “matter” nature of electromagnetic force Light = oscillating electromagnetic field electromagnetic waves

15. Notice: Electric field = Electric force per unit charge Light = oscillating electromagnetic field electromagnetic waves

16. Notice: Electric field = Electric force per unit charge Light = oscillating electromagnetic force electromagnetic waves

17. Light carries energy and can exist independent of electric charge Light = oscillating electromagnetic force electromagnetic waves

18.  Electromagnetic force carries energy. Does it represents also a form of matter !? How about gravitational force (and other forces)?

19.  Electromagnetic force represents also a form of matter !? Nowadays understanding, Force is associated with particular class of matters called vector bosons

20. Second problem: Speed of light = c Can you imagine anything in classical mechanics that travels always with constant velocity? Notice: velocity addition  c’ = c+v

21. c’ = c + v (from ground) Throwing a ball in a train,

22. What does a constant c means?

23. Notice: c is defined with reference to “observer” of light! Hint: Think of sound wave!

24. c = velocity in a (fixed) medium?  Vacuum (space) is a medium! (a kind of matter?) c Universe is filled with “ether”

25. i.e. E&M  suggests that even vacuum has “matter” property c Universe is filled with “ether”

26. In a way, relativity started with people trying to “detect” this medium c Universe is filled with “ether”

27. What Happens next? Exercise: Tell me the picture of our universe after you have learned relativity & Quantum physics.

28. examples: What is reality in relatively and QM? (Twin paradox and Schrodinger Cat) How about concept of vacuum as a “medium”? How are space, time, matter and force related to each other in General Relativity or Quantum Physics?

29. Last lecture Modern picture of space-time, matter & forces (Standard Model) Towards the still unknowns (Grand Unification, Quantum gravity, String theory) Microscopic world & Cosmology