1. Philosophical Interpretations of
Quantum Mechanics

2. Outline Classical “Newtonian” Mechanics Elementary Quantum Mechanics
Young’s Double-Slit Experiment
Uncertainty Principle due to Heisenberg
Schrödinger’s Cat Thought Experiment
Interpretations of Quantum Mechanics
The Copenhagen Interpretation
The Many-Worlds Interpretation

3. Classical Newtonian Mechanics
Determinism
universe has a starting point (Big Bang?)
correct formulations for laws of nature allow histories of all particles to be traced and predicted into the future
everything is predictable, universe functions like clockwork
Free will?
Sir Isaac Newton

4. Young’s Double-Slit Experiment
Thomas Young
light consists of waves, not particles
wave interference
electrons, protons
wave-particle duality
matter sometimes behaves like a wave, sometimes like a particle

5. Pauli’s Exclusion Principle
particles around an atom are assigned quantum numbers {n, l, ml, ms}, which define their quantum state
no two particles can occupy the same quantum state
Wolfgang Pauli (1945)

6. Heisenberg’s Uncertainty Principle
two properties of a particle are unknowable to arbitrary accuracy
momentum (p) and position (x) of a particle cannot be known exactly at the same time
Standard Deviation (Δ) of momentum (p) or position (x) measurement multiplied together are larger than or equal to half the reduced Planck constant (ħ)

7. Bohr‘s Complementarity Principle
connects to the uncertainty principle
characteristics which are uncertain are complementary
wave and particle behavior is complementary as well
Niels Bohr and Albert Einstein (1925) during the Bohr-Einstein Debates

8. The Copenhagen Interpretation
Wavefunction ψ (Psi) describes a quantum mechanical system.
The nature of a system can be described by probabilistic values; probability of an event is equal to the square of the amplitude of the wavefunction (|ψ|²).
Impossible to know all properties of a system at the same time, each must be given by probabilistic values (uncertainty principle).
Matter exhibits wave-particle duality; particles may exhibit both particle and wave properties, but not both at the same time (complementarity principle).
Measuring devices are classical devices, and as such do not measure probabilities, but only classical properties.
Quantum mechanical descriptions of the system will closely approximate the values of the classical descriptions of the system.

9. Schrödinger‘s Cat (1935) Erwin Schrödinger
cat in a box with a vial of poison and a Geiger counter
possible decay of atom or not
if atom decays, cat dies; if not, cat lives
cat is both alive AND dead before one checks
Superposition of Quantum States demonstrated by the Schrödinger Cat Thought Experiment

10. The EPR Paradox
paradox of the CI formulated by Einstein, Boris Podolsky, and Nathan Rosen in reaction to the CI
quantum entanglement – connection of two or more particles
anti-correlation of e- and e+ spin
if spin is measured in one, the wavefunction of the other collapses; superluminal information transfer
Copenhagen Interpretation: second observer cannot benefit until results were relayed, at luminal or subluminal speed

11. Wavefunction Collapse
quantum system interacts with an observer; wavefunction collapses into a single state
“opening the box with the cat”
quantum systems are holistic; each particle contains information about the whole system
only measuring a specific particle causes wavefunction collapse

12. The Many-Worlds Interpretation
universal wavefunction exists
all alternative histories and futures of the wavefunction progression are followed in different parallel “worlds” or “universes”
Schrödinger‘s Cat as a visualization of the Many-Worlds Interpretation of Quantum Mechanics

13. The Universal Wavefunction
describes the universe in its entirety as a single quantum state
does not collapse; worlds split if an event with different possible outcomes occurs
interpretation makes no real difference between itself and CI, since observable results are the same for MWI and CI
no evidence for it as of now

14. Conclusion Arthur Eddington’s View
Why describe the world with quantum theories?
Connections to Hawking?
Scientific determinism?
Many worlds?