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Quantum Physics in a Nutshell Wave-Particle Duality Classical Physics • This is most of what you studied in grade 11 physics • Physics of motion, energies, and interactions (collisions) of objects in the everyday world around us Classical Physics – Historical Timeline • Greek Scientists: studied how the Earth moved in relation to the Sun – Not just mythology and religion – Aristotle: Attempt to explain motion and even gravity – Ptolemy: Astronomy (geocentric views) • Indian and Chinese physicists also had similar views but these views were based more on logic rather than experimentation (even some semiheliocentric views of our Solar system) Classical Physics • Muslim physicists – Ibn al-Haytham (“Alhazen”) believed that light traveled in rays to our eyes (rather than our eyes emitting the light as European physicists believed). – Nasir al-Din al-Tusi believed that the planets orbited in ellipses rather than circles Classical Physics • Galileo – Went against the Church – Heliocentric view of the Solar system – Centre of gravity • Newton – Laws of Motion (explaining motion, inertia, dynamics) Maxwell • Proposed that electromagnetic radiation acted as a wave (but not a mechanical wave) • This theory states that electromagnetism behaves in a continuous way • Example: Your hand will get continuously warmer as it is near a hot object. • He also proposed that all electromagnetic waves traveled at c (in a vacuum). • Page 380: Electromagnetic spectrum Visible Light Spectrum • At a lower range of these hot temperatures, red light begins to be emitted. Higher frequency colours are seen as it gets hotter: we will see blue and eventually white (all colours being emitted). Questions • 1) Why don’t we see our hand getting hotter – we can only feel it? • 2) The picture below is taken of stars in the globular star cluster Omega Centauri by Hubble. What can you say about the different colours in the picture? Which colour stars have the most energy? • 3) We can “see” thermal energy from a fireplace. If energy is radiated by all objects, why can’t we see this energy for all objects? Answers • 1) The energy is not in the visible range of the spectrum. • All objects radiate energy we just can’t see it! • 2) The colours mean that there are different wave lengths being emitted. Red stars have longer wavelengths (smaller frequencies) so have less energy than blue and white stars. • 3) Energy is radiated from all objects but not always visible to us (must be in the visible range). Quantum Physics • Physics after 1900/Modern Physics • Quantum Theory took almost 3 decades to come about. It cannot be credited to any one scientist. It is now the basis for explaining the structure of matter and very small subatomic objects (electrons, protons, neutrons). • Note: Quantum physics is an attempt to explain and predict what we observe in a way that we can understand. Models and theories support each other and are supported experimentally, but they may not actually represent what is really happening. We cannot see what electrons and photons actually are! Key Terms and Ideas • Quantum: a discrete amount of energy (not continuous) • Photon: a quantum of light or electromagnetic wave (“particle of light”) • Electromagnetic Force: the non-contact force that occurs between electrically charged particles. Example: electricity, magnetism, and light. Key Terms and Ideas • Radiation: Electromagnetic Waves (EMR): wave consisting of oscillating electric and magnetic fields that move at the speed of light through space. • Incandescence: light emitted by an object heated beyond a specific temperature. Examples: light in lightbulbs • Black Body: physical body that absorbs all incident (incoming) radiation. No reflection. Quantum Physics • 1900: Max Planck (German) • Planck was trying to explain incandescent light spectrums • A light spectrum is the unique frequencies of visible light that blend together to produce the colour emitted by any incandescent object. Every incandescent object has its own unique spectrum. • Since the spectrum of light emitted in incandescence is made up of specific frequencies, and not a range of frequencies, it is called a DISCRETE or LINE spectrum (rather than a continuous spectrum). Why is this useful? • Elements give off characteristic colours (chemistry lab?) and that is how astronomers can determine what elements are present on a distant star. • Look at the emission spectra of these 2 elements. Note how they are discrete and unique. • Top: Emission spectra of hydrogen. • Bottom: Absorption spectra of hydrogen: • Note: It is continuous but has some gaps – these are frequencies that are not absorbed. Quantum Physics • 1900: Max Planck (German) • Planck was trying to explain incandescence and black body radiation • Ultraviolet Catastrophe: Maxwell’s electromagnetic wave theory did not accurately predict the observed spectrum of light that Planck saw. Planck’s Conclusions • Planck studied the emission spectrums of incandescent objects in greater detail and made the following conclusions: • The energy of electromagnetic radiation was directly related to frequency. Higher frequency waves have more energy. • Any spectral line (frequency) can vary in intensity (energy). BUT any variations in energy were always multiples of some integer “n”. Planck’s Conclusions • Formula for energy of radiation: E = nhf • • • • E = Energy (Joules) h = Planck’s constant (6.626 x 10-34 J/Hz or Js) f = frequency (Hz) n = integer (so the values of energy are DISCRETE) Poor Planck • The idea that energy exists only in discrete amounts was a revolutionary idea. The smallest amount of energy possible (hf) is called a quantum of energy. It is extremely small and is not significant in everyday situations. • Planck was not happy though. He believed he just found the math to support the answer but not a discovery. He had no basis for suggesting this concept of a quantum of energy other than his observations. Einstein • 1905: “n” is understood. Einstein argues the existence of photons are particles of waves. “n” is the number of photons that make up a wave of light. • Ephoton = hf • h = Planck’s constant (6.626 x 10-34 J/Hz or Js) • f = frequency (Hz) Example • A molecule vibrates with a frequency of 8.32 x 1015 Hz. What is the energy of a photon of this molecule? • 5.51 x 10 -18 J Try This • A light source of frequency 5.0 x 10 14 Hz produces a spectral line with an energy of 1.657 x 10 -16 J. How many photons were ejected to produce that? • 500 photons The Birth of Quantum Physics • Planck’s discovery lead to Quantum Mechanics. Excited substances can only emit light of certain frequencies. • Quantum physics addresses things like energy that only exists in discrete quantities. Light Light… and its problem of identity • Light may or may not behave as a wave. • Wave-Particle Duality: – There are two theories for how light behaves • Light behaves as a wave • Light behaves as a particle (massless particles called photons) Remember back to the Special Theory of Relativity… • Remember how the Michelson-Morely experiment was trying to prove that “ether” existed? This was because they thought light behaved as a mechanical wave (needed a medium for the energy to travel through). • Then, once this was disproven, it was thought that light behaved as an electromagnetic wave (see how as new data/observations come in the theory changes?) Two Methods for Transferring Energy • 1) Particle method • - Example: A baseball traveling through the air has kinetic energy which can be transferred to another object in a collision. • – proposed by Newton • 2) Wave method • - Example: Water waves transfer energy to the shore and cause erosion. • – proposed by Huygens (Hoi-gens) Light as a Particle • Key Ideas: • Particles transfer energy from one particle to another after collisions • Refraction occurs because of forces acting on boundaries between different materials • Problems: Newton’s theory could not explain how refraction AND reflection occurred simultaneously Your Turn • Write down what you just learned about light being a particle… everything! Light as a Wave • Huygens model gained more acceptance by 1923 because: – double-slit experiment showed that light passing through two slits demonstrated the same interference pattern as two sources of water waves – speed of light was shown to be lower in water than in air (relativity…), which supported Huygen’s theory of refraction that light traveled slower in denser materials • Problems: – Where is the ether?! Your Turn • What do you remember about Light as a wave? Discussion… • Which theory do you like more? Why? So how about this double slit experiment??? • We can do it! (sort of…) Double Slit Experiment • http://hrsbstaff.ednet.ns.ca/pboudrea/Phys ics_12/Notes/quantum/8%20%20the%20double%20slit%20experiment. pdf • https://www.youtube.com/watch?v=zKdoE 1vX7k4 Double-Slit Experiment – Classical Particles • Tennis balls (classical particles like sugar) move as localized particles through slits • Once they hit the screen they produce the following distribution: Double-slit … Again • If we use water waves, sound, or any other classical waves they spread out behind the double slit (diffraction – see the review notes from Physics 11) • http://www.launc.tased.edu.au/online/scien ces/physics/diffrac.html Double-slit with electrons • Using small subatomic particles (electrons), it was found that – Each single electron hits the detection screen as a particle – After many electrons hit, an interference pattern forms, demonstrating wave behaviour – The same interference pattern occurs if we fire each electron singly or more than one at a time – SAY WHAT? That makes no sense and tons of sense all at the same time… kind of like time dilation and length contraction. The Photoelectric Effect • - the ejection of electrons from an atom (which flow as electric current) of a photoemissive surface when struck by light or other forms of EMR (electromagnetic radiation). • A photoemissive material is one that can exhibit this effect. Example: material used in solar panels. • The photoelectric effect can be seen in: – photo sensors in digital cameras – infrared remote controls – camera film The Photoelectric Effect • The Photoelectric Effect: when light shines on a metal surface, electrons can be emitted from the surface (these electrons are called photoelectrons). This only happens when you have a high enough frequency (or more energy). Videos to Help • https://www.youtube.com/watch?v=0qKrO F-gJZ4 • (first 4 min) • http://aplusphysics.com/courses/honors/vi deos/PhotoelectricEffect/PhotoelectricEffe ct.html Photoelectric Effect • The maximum kinetic energy of a photoelectron is the difference of the energy of the photon and the work function of the metal emitter. E k (max) hf W • Where Ek(max) is the maximum kinetic energy of a photoelectron (ejected electron) in Joules • f is the frequency of the EMR (in Hz) • W is the work function of a metal (in Joules) – This is the work needed to eject an electron • h is Planck’s constant Photoelectric Effect E k (max) hf W • Think about this equation for a second: • hf is what? • W is what? Work Functions of Common Metals • Table page 853 • These are all in eV (electron volts). We need them in Joules. • Conversion ratio: 1 eV = 1.60 x 10-19 J (on formula sheet) Example Problem • Light with a wavelength of 581nm (581 x 10-9) strikes a Cesium metal surface inside a vacuum tube. • What is the maximum kinetic energy of the electrons emitted from the surface? • HINT: We need the wave speed equation from grade 11. • v = λf Try This • 1. Photons with a frequency of 4.86 x 1015 HZ are directed toward a nickel surface in a vacuum tube. • What will be the maximum kinetic energy of the electrons emitted from the nickel surface? Photon Momentum • Einstein theorized that energy and matter were related. • He thought that photons (energy) may have some of the same characteristics as matter. • Photons have momentum – just like matter! The Compton Effect • In 1923, Arthur Compton observed that when a high-energy photon strikes a free electron at rest, the electron gains some of the energy and momentum of the photon and is scattered off. The other photon is also scattered. This confirms the conservation of momentum in the collision. • So even tiny particles (photons) have momentum – just like matter. • So waves act like matter… The Particle (Quantum) Nature of Waves – Summary • By 1923, the particle nature of waves was largely accepted by the scientific community due mainly to 3 pieces of evidence: • Planck’s discovery that incandescent bodies emit unique EM spectra that are discrete. Meaning energy is quantized not continuous as previously thought. • Einstein’s explanation of the Photoelectric Effect – light exists as particles called photons. • Photon Momentum The Particle (Quantum) Nature of Waves – Summary • These 3 observations demonstrate the EM: – Exists as particles called photons – These particles possess kinetic energy and momentum, which are fundamental properties of matter • New Definition of Electromagnetic Wave – consists of discrete particles of energy called photons that travel through space in a wave-like fashion, transferring energy The Wave Nature of Matter • 1924 – Louis deBrogli proposes the opposite is true – if waves have properties of matter, then matter must have properties of waves. • People thought he was crazy until Einstein endorsed this (and then he received the Nobel prize). De Broglie • (“Broy”) • https://www.youtube.com/watch?v=JIGIeXK0tg So What are Matter Waves? • There are 3 fundamental wave types in the universe: – Mechanical waves – they need a medium. Examples: Sound, water – Electromagnetic waves – they consist of photons – Matter – the wave nature of matter in motion. Also called deBroglie waves. Some good videos for summary • http://www.youtube.com/watch?v=DfPepr Q7oGc • http://www.perimeterinstitute.ca/news/schr -dinger-s-cat-wanted-dead-and-alive Video part 3 So electrons… • Behave as particles and waves. • Wave-particle Duality So what happens with Light? • Just like electrons, light behaves like waves and particles • Light hits the screen as an individual particle but then produces an interference pattern (like waves) over time • https://www.youtube.com/watch?v=DfPepr Q7oGc So what happens with all quantum objects? • All exhibit wave-particle duality – Electrons, protons, neutrons, atoms, molecules What if we want to see it in more detail??? • Video Part 4 • Who wouldn’t be interested in the electron in this experiment – What is it doing at each key phase? – How does it decide which slit to go through? – Does it split into two parts? – Does it go through both parts at once? Curiosity Killed the Cat… • When we look at the electron to see what it is doing while passing through the double-slit barrier, we are making a measurement which perturbs the electron and destroys the interference pattern. • This is called measurement disturbance. Why? Interpretations… • We can predict the overall behaviour of the electrons and light in the double-slit experiment, but nobody really knows what the electrons are doing between the source and the detector. • There are 4 interpretations to complete this picture… Collapse Interpretation • Thinking of electrons as spread-out waves that collapse to point-like particles once they are measured • Pilot Wave • Thinking of electrons as particles that are guided by an invisible wave • Many Worlds • Parallel universes that come into being when we make measurements at the quantum level • Copenhagen Interpretation • Just think about the results from the measurements • Who cares about what happens in the middle! Pilot Wave Interpretation • Thinking of electrons as particles that are guided by an invisible wave Many Worlds Interpretation • Parallel universes that come into being when we make measurements at the quantum level Copenhagen Interpretation • Just think about the results from the measurements • Who cares about what happens in the middle! • Come up with a skit/song/way to explain one of these interpretations and present to the class • Video Part 5 What if we want to see it in more detail??? • https://www.youtube.com/watch?v=IOYyC HGWJq4