Exploring the World of Atoms and Light in Quantum Physics

1. Atomic Particles nucleus • Atoms are made of protons, neutrons and electrons • 99.999999999999% of the atom is empty space • Electrons have locations described by probability functions • Nuclei have protons and neutrons mp = 1836 me

2. Atomic sizes • Atoms are about 10-10 m • Nuclei are about 10-14 m • Protons are about 10-15 m • The size of electrons and quarks has not been measured, but they are at least 1000 times smaller than a proton

3. What is Light? • Properties of light • Reflection, Refraction • A property of both particles and waves • Interference and Diffraction • Young’s double slits • A Property of Waves Only • Polarisation • A Property of Waves Only

4. Classical Physics • Light is a wave • Young’s Double Slit Experiment • Faraday’s experiments • Maxwell’s equations

5. Line-Emission Spectrum excited state ENERGY IN PHOTON OUT ground state

6. Bohr Model • e- exist only in orbits with specific amounts of energy called energy levels • Therefore… • e- can only gain or lose certain amounts of energy • only certain photons are produced

7. Energy of photon depends on the difference in energy levels Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom Bohr Model 6 5 4 3 2 1

8. Other Elements • Each element has a unique bright-line emission spectrum. • “Atomic Fingerprint” Helium • Bohr’s calculations only worked for hydrogen! 

10. The Birth of the Quantum • Max Planck • The energy contained in radiation is related to the frequency of the radiation by the relationship • n is a positive integer called the quantum number • f is the frequency of the oscillation • A discreet packet of energy, later to become known as “a photon”

11. Implications of Planck’s Law • The energy levels of the molecules must be discreet • Only transitions by an amount E=hf are allowed • The implication is that light is discreet or quantised energy n energy 4hf 3hf 2hf 1hf 0 4 3 2 1 0 These quantum levels are now known as number states

12. Photoelectric effect • When light strikes the cathode, electrons are emitted • Electrons moving between the two plates constitute a current

13. Photoelectric Effect • Explanation • Einstein: the quanta of energy are in fact localised “particle like” energy packets • Each having an energy given by hf • Emitted electrons will have an energy given by • Where f is known as the “work function” of the material hft where ft is the threshold frequency for the metal.

14. Louis de Broglie1892 - 1987

15. Wave Properties of Matter • In 1923 Louis de Broglie postulated that perhaps matter exhibits the same “duality” that light exhibits • Perhaps all matter has both characteristics as well • For photons, • Which says that the wavelength of light is related to its momentum • Making the same comparison for matter we find…

16. Quantum Theory • Particles act like waves?! • The best we can do is predict theprobability that something will happen. Heisenberg Dirac Schrodinger

17. Erwin Schrödinger (1887 – 1961) “The task is not so much to see what no-one has yet seen, but to think what nobody has yet thought, about that which everybody sees.”

18. Schrodinger’s cat • After consultation with Einstein, Schrodinger proposed a thought experiment in which he highlighted the apparent inconsistencies between the so-called Copenhagen interpretation of Quantum Mechanics and the reality of macroscopic measurements. • He proposed that a cat be placed in a sealed box. The release of a poison is then subject to the probabilistic decay of a radioactive isotope. If the isotope decays, the poison is released. If no decay occurs, the poison is not released. • The result is that the cat is in a superposition of states between being dead, and being alive. This is very unintuitive.

19. Quantum mechanics • Wave-particle duality • Waves and particles have interchangeable properties • This is an example of a system with complementary properties • The mechanics for dealing with systems when these properties become important is called “Quantum Mechanics”

20. The Uncertainty Principle Measurement disturbes the system

21. The Uncertainty Principle • Classical physics • Measurement uncertainty is due to limitations of the measurement apparatus • There is no limit in principle to how accurate a measurement can be made • Quantum Mechanics • There is a fundamental limit to the accuracy of a measurement determined by the Heisenberg uncertainty principle • If a measurement of position is made with precision Dx and a simultaneous measurement of linear momentum is made with precision Dp, then the product of the two uncertainties can never be less than h/2p

22. The Uncertainty Principle Virtual particles: created due to the UP

23. In Search of the Higgs Boson • Higgs boson is “cosmic molasses” – the Holy Grail of particle physics • Interactions with the Higgs Field are theorized to give all the particles their masses • LHC detectors should be able to confirm or disprove initial hints for Higgs at E=115 GeV