Transcript Physics before the 20th century

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
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)
Question:
What properties are assumed in these 4
elements?
Space & Time
• Classical (absolute) reality (objectivity) everyone has same observation on what has
happened!
• Existence independent of other 2 factors
(matter & force).
 (classical) objectiveness in science
We measure what we want with
certainty?
 (classical) objectiveness in science
Can you imagine an universe
otherwise?
Matter & Force
• Matter:
Mass, Energy, momentum,…..etc.
• Force:
How is force defined?
Can they exist without matter?
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.
Are you satisfied with the above
relations?
If not, how should they be modified?
Physicists: Is there an underlying principle
governing these relations?
“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
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!
First problem:
the “matter” nature of electromagnetic
force
• Light = oscillating electromagnetic field
electromagnetic waves
Notice:
Electric field = Electric force per unit
charge
• Light = oscillating electromagnetic field
electromagnetic waves
Notice:
Electric field = Electric force per unit
charge
• Light = oscillating electromagnetic force
electromagnetic waves
Light carries energy and can exist
independent of electric charge
• Light = oscillating electromagnetic force
electromagnetic waves
 Electromagnetic force carries
energy. Does it represents also a
form of matter !?
How about gravitational force
(and other forces)?
 Electromagnetic force represents
also a form of matter !?
Nowadays understanding,
Force is associated with particular class
of matters called vector bosons
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
Throwing a ball in a train,
c’ = c + v (from ground)
What does a constant c
means?
Notice: c is defined with reference to
“observer” of light!
Hint:
Think of sound wave!
c = velocity in a (fixed) medium?
 Vacuum (space) is a medium!
(a kind of matter?)
Universe
is filled
with
“ether”
c
i.e. E&M  suggests that even
vacuum has “matter” property
Universe
is filled
with
“ether”
c
In a way, relativity started with
people trying to “detect” this
medium
Universe
is filled
with
“ether”
c
What Happens next?
• Exercise:
• Tell me the picture of our universe after you
have learned relativity & Quantum physics.
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?
Last lecture
• Modern picture of space-time, matter &
forces (Standard Model)
• Towards the still unknowns (Grand
Unification, Quantum gravity, String
theory)
• Microscopic world & Cosmology