T. K. Ng (HKUST)

1
Physics Summer School 2002
The classical Universe

• T. K. Ng (HKUST)

2
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

3
Hidden assumptions in Newtons Law(F ma)

• 4 elements in describing events in our universe
• space time ? trajectory
• matter (particle) (moving in space-time)
• force (acting between matter)

4
QuestionWhat properties are assumed in these
4 elements?
5
Space Time

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

6
? (classical) objectiveness in science We
measure what we want with certainty?
7
? (classical) objectiveness in science Can you
imagine an universe otherwise?
8
Matter Force

• Matter
• Force

• Mass, Energy, momentum,..etc.
• How is force defined?
• Can they exist without matter?

9
In Newtons 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.

10
Are you satisfied with the above relations?
If not, how should they be modified? Physicists
Is there an underlying principle governing these
relations?
11
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 Newtons World

12
Electricity Magnetism

• At late 19th century physicists realized that
  Newtons picture of space, time, matter force
  has to be modified if we want to understand EM!
• Main problem existence of Light
  Electromagnetic wave!

13
First problem the matter nature of
electromagnetic force

• Light oscillating electromagnetic field

14
Notice Electric field Electric force per unit
charge

• Light oscillating electromagnetic field

15
Notice Electric field Electric force per unit
charge

• Light oscillating electromagnetic force

16
Light carries energy and can exist independent of
electric charge

• Light oscillating electromagnetic force

17
? Electromagnetic force carries
energy. Does it represents also a form of matter
!?
How about gravitational force (and other forces)?
18
? Electromagnetic force represents also a form
of matter !?
Nowadays understanding, Force is associated with
particular class of matters called vector bosons
19
Second problem Speed of light c

• Can you imagine anything in classical mechanics
  that travels always with constant velocity?
• Notice velocity addition ? c cv

20
Throwing a ball in a train,
c c v (from ground)
21
What does a constant c means?
22
Notice c is defined with reference to observer
of light!

• Hint
• Think of sound wave!

23
c velocity in a (fixed) medium? ? Vacuum
(space) is a medium! (a kind of matter?)
24
i.e. EM ? suggests that even vacuum has matter
property
25
In a way, relativity started with people trying
to detect this medium
26
What Happens next?

• Exercise
• Tell me the picture of our universe after you
  have learned relativity Quantum physics.

27
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?

28
Last lecture

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