The Strange World of Quantum Physics Johannes Kofler Anton Bruckner International School Linz, Austria 30 Oct. 2017

What is Physics? Physics means “knowledge of nature“ Physics is the study of nature It is the most fundamental of the natural sciences (what are other natural sciences?) Goal of physics: to understand how the universe works

Classical Mechanics (16th–19th century) Movement of bodies due to forces Which forces and examples do you know that have to do with mechanics? Collision of billiard balls Aerodynamics of an airplane Movement of planets around the sun

Reflection of light on water surface Optics (17th–19th century) Behavior and properties of light Which examples do you know that have to do with light? Reflection of light on water surface Rainbow Eyes, glasses

Lightnings (electric discharges) Electricity and Magnetism (17th–19th century) Electric and magnetic fields Which examples do you know that have to do with electricity or magnetism? Wind turbines Lightnings (electric discharges) Compass

Freezing of water to ice Thermodynamics (19th century) Temperature, heat and energy Which examples do you know that have to do with temperature? Steam trains Weather Freezing of water to ice

Classical Physics Mechanics, Optics, Electricity and Magnetism, Thermodynamics Describes the macroscopic world of large objects (balls, trains, planets,…) Macroscopic: what you can see with your eyes Objects have definitive properties (e.g. position, velocity) Two different objects cannot be in the same place at the same time The whole universe is like a precise clock; everything can in principle be calculated and predicted (determinism)

(the microscopic world of atoms and photons) The Pillars of Physics Physics Classical Mechanics Optics Electricity & Magnetism Thermo- dynamics Quantum Physics (the microscopic world of atoms and photons) Relativity Theory

Physics of the Atom (20th century) What are atoms? All matter consists of tiny particles called atoms How many types of atoms?

Atoms are very small How big are atoms? The number of atoms that fit into an apple is approximately the number of apples that fit into Earth

What is Chemistry? Chemistry is the natural science of compounds composed of atoms It studies their properties and changes during reactions Atoms can stick together (“bond”) and form molecules and crystals Oxygen Water Sugar Salt (crystal)

Everything is made from atoms

Atoms are somewhat like tiny solar systems Planets move around the sun Atom: Electrons move around the atomic nucleus Described by classical physics Described by quantum physics

But how do atoms really work? Electrons are not like macroscopic classical objects (balls or planets) Electrons belong to the microscopic world; they are quantum particles Their behavior in the atom is really strange An electron is not in one place, but it is “smeared out” over some space – like a wave or cloud Just a nice picture: More realistic:

Bodies and Waves Which types of bodies do you know? Which types of waves do you know? salt grains water waves soccer balls sound waves planets seismic waves

Bodies and Waves What happens when two bodies meet? What happens when two waves meet? Bodies collide: Two bodies cannot be in the same place Waves can show interference: They “add” (form high peaks and deep valleys) or “cancel out”

In the (microscopic) quantum world, things can be both! Classical and Quantum Classical world Quantum world In the (macroscopic) classical world, things are either bodies or waves In the (microscopic) quantum world, things can be both!

What is Light? Optics Electromagnetism Quantum physics Christiaan Huygens (1629–1695) Isaac Newton (1643–1727) James Clerk Maxwell (1831–1879) Albert Einstein (1879–1955) Photons (quantum particles) Wave Particles Electromagnetic wave

Photons on a beam splitter detector 1 detector 2 For each photon, only one detector clicks → the photon behaves like a particle Which detector will click, cannot be predicted!

Interferometer (two beam splitters) 50/50 A single photons B detector 1 detector 2 All photons end up in detector 1 Each photon travels like a wave and like a particle at once Waves towards detector 1 add up, waves towards detector 2 cancel out

The Double-Slit Experiment Classical Physics Quantum Physics Bodies (e.g. marbles) Waves (e.g. water, sound) Quantum particles (photons, electrons, atoms, molecules, …) individual detections and interference quantum particles are both bodies and waves individual detections, no interference no individual detections, interference Bilder: http://www.blacklightpower.com/theory/DoubleSlit.shtml

What is Randomness? Classical Randomness Quantum Randomness (e.g. dice, weather) Quantum Randomness (e.g. radioactive decay, photon on a beam splitter) events seem to happen without reason but every event has a cause in principle everything can be predicted (deterministic chaos) some events really happen without a cause individual events cannot be predicted

No, cats don’t show interference Are there quantum effects in our every-day life? No, cats don’t show interference But maybe… large objects usually behave classically Schrödinger’s thought experiment: cat is dead and alive at the same time

Revolutions in Technology Technologies relying on classical physics Technologies relying on quantum physics

Quantum Communication Possible Future Quantum Technologies Quantum Communication Quantum Computer

Summary The macroscopic world is described by classical physics All objects (rocks, water, humans) consist of atoms which are bond together Light consists of photons The microscopic world of quantum particles (electrons, atoms, molecules, photons) is described by quantum physics Quantum particles have strange properties; they behave like particles and waves at the same time There is true randomness in quantum physics; events do not have a cause Quantum physics is important for many technologies (computers, lasers, …)