1. Waves and Particles Motivation Topics Light as a wave;
Light as a particle;
Particle-wave duality;
The wave-like nature of matter;
The Copenhagen Interpretation;
The great debate begins.
Motivation
Learn about the particle-wave duality of matter and energy.
Learn how physics can blow your brain.

2. Light as a Wave Characteristics of typical waves
Waves transmit energy;
Waves move;
Waves are diffuse, without well defined edges;
Waves travel through a medium;
Waves may be longitudinal or transverse;
Objects can move slower, faster, or the same speed as waves.
Examples of light being wavelike
Interference;
Antireflective coatings.

3. Maxwell’s Equations Gauss’s Law Gauss’s Law of Magnetism
Faraday’s Law of Induction
Ampère’s Law
Maxwell’s correction
Maxwell added the final term in Ampere’s Law, showing that electrical fields and magnetic fields are related. A consequence of that term is that pulses in these fields travel at a speed “s”:
Electricity and magnetism are united, and to optics as well!!!!

4. The Aether
Wave Speed
Different types of waves travel at different speeds, Seismic waves: P (5000 m/s), S (3000 m/s)
-- Light and sound waves (hence thunder delays)
The medium affects the wave speed:
-- vsound = 343 m/s (air), 1484 m/s(water), m/s (steel)
The nature of the luminiferous aether
A medium for light (aether) was proposed in 1678.
Aether must be very stiff to account for light’s high speed.
Michelson interferometer could not detect it in 1887.
We will wait for Einstein (1905) to understand why.

5. Light as a Particle Photoelectric effect
In 1887, Hertz noted that high energy light (ultraviolet) on polished metal can knock off electrons.
In 1905, Einstein explained this “photo-electric effect” as discrete light particles (photons) knocking the electrons out of the metal surface. This earned Einstein the Nobel Prize in 1921.

6. Particle-Wave Duality
Wave or Particle?
Low energy light (radio waves, microwaves) act very wave-like.
High-energy light (X-ray, gamma rays) act very particle-like.
In general, photons can act either like waves OR particles. Light is BOTH wave-like and particle-like.
This is called particle-wave duality.
Example: Double-slit diffraction patterns.
Weirder still, we will see that even matter is both wave-like and particle-like—electron diffraction patterns can be generated.

7. Optics and Matter: Quantum physics
The three postulates of the Bohr atom
Electrons occur in stable orbits around a nucleus; each orbit has an energy.
An electron can move from one energy level to another; the change in energy is associated with a photon of equivalent energy being emitted or absorbed.
The orbits can have only certain specific energies (these energy levels are said to be quantized).
But….how do particles move from one orbit to another? How do they jump through the forbidden zone?

8. Electrons as Waves?
Electrons are wave-like! The de Broglie wavelength;
λ=h/p
(~ 0.4 nm, the size of atoms.)
Bohr’s electron orbits are set by the sizes needed to have perfectly arranged de Broglie wavelengths.

9. Everything as Waves!
Schrödinger found that any particle could be described as a wave. And since any material object consists of particles, the object in turn is really just a wave.
Furthermore, any physical object does not really exist in a single state; it exists as a “wave function,” with characteristics of many states at the same time.
Unlike the idea that particles that sharp, clearly defined edges, we see that particles—close up—are better thought of as just probability distributions. Their edges are fuzzy.

10. A New Way to Think of Matter
Old perspective
You can say where a particle in a box is. This perspective of physics is called “deterministic.”
New perspective
A particle in a box is potentially anywhere in the box. It exists as a “wave function.” Its location is not determined until you measure it.
The wave function exists throughout the entire Universe, and it is possible that the particle is anywhere in the Universe.
It is the observing of the particle that determines reality, by “collapsing the wave function” into a value.

11. Duality, Multiplicity, Infinitisity
At any moment, you could move to another point in the Universe, even if you couldn’t normally get there! (Ex. Quantum tunneling)
Consider the double electron gun. Which hole does the electron travel through? Top or bottom?
Why choose? Schrödinger says the electron exists in TWO states, one state passes through the top gap, the second passes through the bottom gap.
This works, even if you fire just one electron at a time!

12. A Famous Oddity of Reality
An unmeasured system exists, simultaneously, as a combination of many (even contradictory) states. Hence, Schrödinger’s cat.
As weird as these concepts of Quantum Mechanics are, they made accurate predictions in the laboratory, and this pleased the “instrumentalists”.

13. What Does it Mean to be a Wave?
Just as photons reflect and transmit at a glass wall, matter reflects and transmits at potential energy boundaries!

14. The 1927 Solvay Conference
At this historic conference in Brussels, Einstein, Schrödinger, Bohr, Heisenberg, Curie, and 24 other great physicists—17 of whom would be Nobel Prize winners--arrived to fight!
Einstein and Schrödinger led the way for the realists.
Bohr and Heisenberg led the way for the instrumentalists.
Realists: The belief that objects have attributes, even if you don’t happen to be observing them.
Instrumentalists: The belief that objects do not have attributes until they are observed or measured by instruments.

15. Copenhagen interpretation
In time, Bohr and Heisenberg formulated the “Copenhagen interpretation of quantum mechanics” as, essentially, particles do not have inherent attributes like position or energy, until the moment of measurement collapses the wave function.
Einstein, who believed in determinism in physics, despised this notion, saying instead that Quantum Mechanics was simply incomplete, and missed “hidden variables.”
Ex: Temperature and pressure are useful to talk about, but are really emergent properties based upon the hidden variables of energy and density.
Bohr and Einstein argued for years about this, but never came to a resolution.
Modern physics education and thought follows Bohr, because it works. Don’t ask for explanations, just use the equations.

16. Locality to the forefront
Key topic:
How do forces and particles propagate through space?
Newton’s “Action at a Distance” was a mystery. We shall see that Einstein fixes Newton, by explaining—in General Relativity—how gravity works locally, using spacetime to convey the force at the speed of light.
How do quantum particles jump across space? Dyson, Born, Feynman, and others developed Quantum Field Theory. This says that space is filled with fields, and these fields can support waves (or excitations). Such waves are the particles themselves, in a state of uncertain, and unknowable probability states.
Spoiler alert!
These are heady topics and deep waters, leading ultimately to quantum entanglement, spooky action at a distance, and great philosophical problems!

17. Particle-Wave Duality Summary
Particle-wave duality applies to light;
Particle-wave duality applies to matter;
Particle-wave duality applies to you
Do unmeasured objects have actual attributes?