A talk given to the IB class from Schenectady high School, 5/24/10.

1. What Every Dog Should Know
About Quantum Physics

xp 
2 I like
cheese
Chad Orzel
http://scienceblogs.com/principles/

2. Quantum Physics
~20 Nobel Prizes
For quantum pioneers

3. Why Should Dogs Care About Quantum
Physics?
Three reasons:
1) Computers
Semiconductor chips
2) Telecommunications
Lasers, fiber optics
3) Sunlight
Quantum physics allows
the Sun to shine

4. What‟s the Deal with Quantum Physics?
Four essential elements:
I) Wavefunctions
Everything in the universe is described by a wavefunction
II) Allowed States
Quantum objects are only found in specific states
III) Probability
You can only predict the probability of a given state
IV) Measurement
An object‟s state is indeterminate until you measure it

5. Wavefunctions
I) Wavefunctions
Everything in the universe is
described by a wavefunction
Mathematical function,
describes properties of object
Put in position and time
 Get out what‟s going on
Has both particle and wave properties
Detect at specific position  like particle
Function extends over space  like wave

6. Electrical Conductors
Electrical properties of materials depend on electron motion:

7. Electron Waves
Electrons behave like waves

8. Semiconductor Chips
Control electron properties by
modifying material
Key to making silicon computer chips
 Computers are Quantum!

9. Allowed States
II) Allowed States
Quantum objects are only
found in specific states
“State”  collection of properties
position, velocity, energy,
momentum, etc.
Only certain special states are allowed
 NEVER find object with other properties
Property that gives quantum physics its name
“quantum” = “how much” in Latin

10. Atomic Spectra

11. Bohr Model
Niels Bohr, 1913
Explains light emitted by atoms
Strange idea, but effective
Gets hydrogen almost perfectly
(not as successful with other elements)
 Not the full story, but gets started in right direction

12. Lasers
Lasers work using allowed states
Start with excited atoms
Emit light, drop down in energy
Light causes other atoms to emit
 Amplification
Build up between mirrors

13. Laser Communications
First Laser: May 16, 1960
“A solution looking for a
problem…”
Gordon Gould, Union „41 One problem it solves:
T. Maiman
Fiber optic communications
Telephone, Internet sent using laser
light
 Telephones are Quantum!

14. Probability
III) Probability
You can only predict the
probability of a given state
Wavefunction gives probability
of each of the allowed states
Can‟t say definitely where a
quantum object will be located
 Outcome of a single experiment is completely random
 Some chance of things happening even when they shouldn‟t

15. Sunlight
Temperature of Sun: 10,000,000 degrees
Shines by nuclear fusion:
Stick 4 protons together, make helium

16. Sunlight
Temperature of Sun: 10,000,000 degrees
Shines by nuclear fusion:
Stick 4 protons together, make helium
Problem: Protons repel each other
(unless they‟re really, really close)
Temperature needed to get fusion directly:
15,000,000,000 degrees
1,500 times the Sun‟s temperature!

17. Sunlight
Temperature of Sun: 10,000,000 degrees
Shines by nuclear fusion:
Stick 4 protons together, make helium
Problem: need 15,000,000,000 degrees
Solution: Quantum Tunneling
Small probability of particles passing
through impassable barriers
Allows enough fusion to power Sun
 Sunlight is Quantum!

18. Probability is a Problem
“The theory delivers a lot,
but hardly brings us closer
to the secret of the Old
One. I for one am
convinced that He does
not throw dice.”
-- Albert Einstein,
letter to Max Born,1926

19. “Physicists are silly…”
Can never know exactly where thrown
object will land, only probability

20. Measurement
IV) Measurement
An object‟s state is
indeterminate until you
measure it
Wavefunction tells you probability
Until you measure it, the object is
in ALL possible states,
AT THE SAME TIME

21. Schrödinger‟s Cat
(from http://commons.wikimedia.org/wiki/File:Schrodinger_cat_everret.png )

22. Schrödinger‟s Cat
Cat is both alive and dead
AT THE SAME TIME
Invented by Erwin
Schrödinger to show
philosophical problem
with quantum physics
Situation seems completely ridiculous
 BUT IT‟S TRUE

23. Tennis Experiment

24. Tennis Experiment
 Find probability distribution with two stripes, one each slit

25. Electron Experiment
Send electrons at two slits in a barrier:
Image and video from Hitachi:
http://www.hitachi.com/rd/research/em/doubleslit.html

26. What‟s Going On?
Phenomenon called “Interference”
Waves from two different sources
add to give bright and dark spots
Peak + Peak = Bigger Peak Peak + Valley = Nothing
= =

27. Double Slit
Pattern depends on distance from slits
Get alternating pattern of “bright” and “dark” stripes

28. Double Slit with Electrons
Electrons behave like waves
Probability shows the
effects of interference
BUT: electrons detected
as particles
One at a time, at single
spots on screen
 Each electron must pass through BOTH slits at SAME TIME

29. Double Slit with Electrons
“[A] phenomenon which is
impossible, absolutely
impossible, to explain in any
classical way, and which has
in it the heart of quantum
mechanics. In reality, it
contains the only mystery.”
-- Richard Feynman

30. More Interference
Double-slit only scratches the
surface of quantum weirdness
Interference of molecules
(C60, at right)
“Quantum Eraser”
Quantum Zeno Effect
Entanglement and Teleportation
“Spooky action at a distance”
etc, etc,…
Huge variety of weird stuff involving quantum physics

31. What Every Dog Should Know
About Quantum Physics
Four essential elements, shown in double-slit experiment:
I) Wavefunctions
Electrons (particles) show interference (waves)
II) Allowed States
Each slit defines a state
III) Probability
Where each electron lands is random
IV) Measurement
Each electron goes through BOTH slits at once

32. Schrödinger‟s Dog

33. The End