Modern Physics:
a Revolution of Concepts
Modern Physics
• One of the greatest intellectual achievements
of mankind.
• Role in society affects everyone:
whether or not people understand it, or even
know about it !
Classical physics by the end of the
19th century
• Mechanics:
– Galileo 1564-1642 ; Newton 1642-1727…
• Electromagnetism:
– Coulomb 1736-1806 ; Ampere 1775-1836
– Faraday 1791-1867 ; Maxwell 1831-1879…
• Thermodynamics:
– Carnot 1796-1832 ; Kelvin 1824-1907…
Kinetic Theory of Gases
• Thermodynamics and “atomic” theory of matter
– Gay-Lussac 1778-1850: ideal gas: PV=nRT
– Avogadro 1776-1856: the Avogadro number
– 1895’s: Maxwell-Boltzmann velocity distribution
• It works… but, in 1895, nobody knows what an
atom really is !
Outstanding issues at the end of
the 19th century (I)
• Electromagnetic medium:
Michelson
– All waves propagate through a medium
(example: sound waves in air, water…)
– What about light ?
Hypothesis: the luminiferous ether
• Electrodynamics:
Einstein
– Maxwell equations do not keep the same form
under a Galilean transformation
Theory of Relativity
Outstanding issues at the end of
the 19th century (II)
• Blackbody radiation:
Planck
– The emission spectrum of a blackbody cannot be
reproduced by classical theories
• The nature of light:
Einstein,…
– The photoelectric effect
Quantum Theory
A revolution of concepts (I)
• VERY FAST:
– Special relativity replaces Newton mechanics
• VERY SMALL:
– Quantum physics replaces Newton mechanics
• VERY LARGE:
– General relativity replaces Newton gravitation
A revolution of concepts (II)
(… not a refinement of old ideas !)
New Experiments
New Theories
New Concepts !
Measurement of c
Relativity
Space & Time
Spectrum of Light
Quantum Mechanics
Determinism and
Measurements
(a) Hot, glowing objects [Blackbody]
(b) Electrical breakdown in gases [Atomic structure]
Challenges: Measuring accurately the very fast and the very small…
(One reason why these new measurements occur towards the end of the 19th century)
Modern Physics and Philosophy ?
• Before 1895, people believe that classical
mechanics could explain the motion of *any*
macroscopic object: DETERMINISM
• Given all the information about a system, one
should be able to predict his future.
Example of a macroscopic system: the human brain.
If one could predict the motion of everyone of its
constituents… no more free will !
(Note: Not everybody believed in such an interpretation.)
The World in 1895
Why 1895 ?
• Major discoveries to happen within 2-3 years
Politics
Three countries research active:
England, France and Germany
• England:
– at the top of their colonial empire
• France:
– Defeated by the German in 1870
– Pasteur: the recovery can come through advances in
Science
• Germany:
– Rapidly ascending, Strong military power
Technology
• No airplane
• Virtually no telephone.
– People communicate by mail. Paris has a
pneumatic mail system for example
• No car… coming soon.
• Streets lit by gas
• Steamships cross the Atlantic. Still use
sails now and then.
• New developments of apparatus
Doing physics then…
•
•
•
•
One professor
A few assistants
Slow communication between scientists
Facilities ranked according to their
potential of:
– Producing strong current/bias
– Cooling capacity
– Vacuum capability
A hot topic
• The kinetic theory of gases
– Investigates the “atomic” nature of gases
– The atom hypothesis is not widely accepted.
The study of the microscopic
world is the key.
This and much more when we
returns…
… for now, back to PHGN 200 or so
(e.g. before 1895)