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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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)

8. (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

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

10. 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

11. 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)

12. Everything is made from atoms

13. 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

14. 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:

15. 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

16. 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”

17. 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!

18. 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

19. 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!

20. 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

21. 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:

22. 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

23. 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

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

25. Quantum Communication
Possible Future Quantum Technologies
Quantum Communication
Quantum Computer

26. 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, …)