Lecture 1 - Why Quantum?

disc’r# Class Logistics

• Quantum physics 2 Applications of quantum physics is M/R 4-6
• Understanding the properties of quantum physics is important for all science and technology fields (except biology)
• Talk to Persans about office hours

Classical Physics vs Quantum Physics

Classical Physics

• Classical mechanics concerns the movement of point masses
  • Electrons, Leptons, etc.
• Completely predictable. God does not play dice. All motion is regulated and can be predicted by differential equations.
• These are very good approximations for larger then atom objects.

Quantum Physics

• Simple experiments have proven that light has a wave-particle duality.
  • The photoelectric effect
    • Voltage across two plates to stop an electron ejected from a surface by incident light of a known wavelength.
  • Double slit interference
    • Intensity maxima given by
    $$d\sin \theta =n\lambda$$
  • X-ray diffraction aka Bragg diffraction
    • Bragg diffraction maxima
  • The Compton effect
    • The Compton effect allows us to calculate the wavelength shift caused by collisions between photons and electrons.
    • Utilizes Conservation of momentum and energy
    • Compton won the Nobel prize for this
  • Photon Coincidence Experiments
    • A more direct proof of the particle nature of light
    • Uses two-step decays of the calcium atom electrons from the 4p orbital to the 4s ground state.
    • The two decays occur within nanoseconds of each other, so a detector can be set up to detect only events co-incident with the first 551 nanometer decay.
    • A beam splitter is used to detect the photos emitted from the two step decay, and the photons would take either one or the other route. This proves further the indivisible particle nature of light.
• Overview
  • light can act both as a particle and a wave

$$E_{photon}=h\nu =\frac{hc}{\lambda }$$ $$p=\frac{h}{\lambda }$$ $$E=pc$$

• Babinet’s Principle
  • If you have waves incident on an object, you get the same diffraction patterns as if you had a hole of the same size.
• De Broglie wavelength

$$\lambda =\frac{h}{p}=\frac{h}{\sqrt{2mK}}$$

• Dr Eibenberger shoots things through holes. Seems fun.
• Black body radiation