TTC Quantum Mechanics: The Physics of the Microscopic World

welcome to our course on quantum

mechanics the physics of the microscopic

world I'm Ben Schumacher I'm a professor

of physics at Kenyon College in Ohio and

i'll be your instructor for this course

in this course we are embarking on a

journey to a distant world a world that

is governed by strange and unfamiliar

laws by distant I mean a world far from

our everyday experience I mean a world

that is distant not in space but in size

it's the world of the microscopic the

microscopic world so how small do I mean

by microscopic in fact i don't mean

something you can see in a microscope I

mean something far smaller than you can

see in an ordinary microscope the

characters of our drama are dark

entities like atoms and subatomic

particles such as electrons and protons

and neutrons other characters of our

drama include photons which are the

particles of light and these are all

exceedingly small how small is small how

small is an atom well I have here a 1

inch cube of aluminum it's one inch long

one inch high one inch wide and in one

cubic inch of aluminum there are 10 to

the 24th power aluminium atoms 10 to the

24th power a one followed by 24 zeros

that's one trillion trillion aluminum

atoms there are a hundred million

aluminum atoms on this edge a hundred

million aluminum atoms on this edge a

hundred million aluminum atoms on that

edge about a hundred million aluminum

atoms to the inch that's a lot well what

about something you actually could see

in a microscope how about

single-celled organism well a typical

single-celled organism contains about

one trillion atoms a one followed by 12

zeros 10 to the 12th power okay I've

been throwing around the number one

trillion how big is a trillion well

imagine how big a grain of fine sand is

now imagine an entire large truck load

of sand that turns out to be about a

trillion grains of sand or here's

another way of thinking about a trillion

imagine how many heartbeats you have in

a day distinct heartbeats that's not

actually a very big number it's only a

hundred thousand well maybe that is a

pretty big number one hundred thousand

heartbeats in a day now imagine the

entire population of New York City about

10 million people that means that there

are about one trillion human heart beats

in New York City in the course of a day

so what trillion is a really big number

and there are a trillion times a

trillion atoms of aluminum in this cube

of aluminum okay so atoms are pretty

small what about photons how big your

photons well to do that let's consider

an ordinary light bulb like this one um

an ordinary 20 watt light bulb emits

about 100 million trillion photons in

one second a hundred million trillion

that's that's ten to the twentieth power

a one followed by twenty zeros that's

the number of photons that come out of a

20 watt light bulb in a second these are

huge numbers atoms electrons and photons

are extremely small compared with our

everyday experience and quantum

mechanics is the basic physics of this

realm the realm of atoms electrons and

photons now the microscopic realm is not

just a miniature copy of the world of

our experience if that were true this

wouldn't be that interesting a subject

but in fact in the microscopic realm

there are lots of surprises and we'll

talk about things that will at first be

hard to believe but are nevertheless

true about the microscopic realm but i

want to emphasize one important fact the

quantum world is nevertheless our own

world the quantum world is the world we

live in and quantum mechanics will

explain not just funny things about

atoms but will explain basic facts about

the world we see every day okay what is

quantum mechanics let's take that apart

first of all what is mechanics mechanics

is the branch of physics that studies

force and motion how things move why

they move the laws governing motion in

the world the way things in the universe

evolved over time that's all mechanics

now there's a kind of mechanics called

classical mechanics classical mechanics

is the theory of mechanics based on

Newton's laws of motion developed by

Isaac Newton in the late 17th century

and classical mechanics is a branch of a

somewhat larger realm called classical

physics which also includes other

subjects such as thermodynamics which is

about heat and energy transformations on

electromagnetism which is about the

physics of electric and magnetic fields

optics which is about how light

propagates all these are branches of

classical physics and classical

mechanics is sort of at the center of

classical physics for more than 200

years

classical mechanics was the best theory

of physics the gold standard of theories

far-reaching and extremely precise if

you wanted to know how to predict where

a projectile would land or how to build

a strong bridge or or how the planets

were going to move a thousand years from

now classical mechanics was the tool you

would use to figure that out and until

about the year 1900 the entire story of

physics was largely the story of the

success of Newtonian mechanics however

around 1900 it became clear that

classical physics could not explain the

behavior of the microscopic world

classical mechanics wasn't enough and so

quantum mechanics was the new theory

developed to replace Newton's laws and

especially to account for microscopic

pieces of matter and how they behaved

but also to account for puzzles about

the world that we could see and touch

with our ordinary senses it was called

quantum mechanics because in this theory

energy was found in many circumstances

to come in distinct lumps distinct

packages or quanta of energy that's the

quantum in quantum mechanics now the

first quantum ideas were proposed around

1900 but the theory itself was not fully

developed until the 1920s so quantum

mechanics was not constructed in a day

the process of its discovery was was a

remarkable process with many steps and

we'll talk about that process there are

two other terms I want to explain I'll

use the phrase quantum theory sometimes

I'll say quantum physics now technically

speaking quantum mechanics quantum

theory quantum physics are not quite the

same thing each term is a little more

encompassing than the last but for our

purposes we can regard them as as near

sims for quantum mechanics I'll use one

or the other just for variety sake so so

don't worry about the distinction

quantum mechanics is now the most

successful theory of physics ever

devised some phenomenon explained by

quantum mechanics are and we'll discuss

all of these the basic internal

structure of nuclei atoms and molecules

we understand that structure because of

quantum mechanics quantum mechanics

explains how light is absorbed and

emitted by matter a very important a

very important subject quantum mechanics

explains how solids and liquids behave

especially at very low temperatures

where their behavior can be quite

surprising quantum mechanics is also the

theory that accounts for how elementary

particles interact with each other so

quantum mechanics has a tremendous range

of applicability even though the basic

quantum ideas are now more than a

hundred years old even though quantum

theory or quantum mechanics itself is

more than 80 years old quantum theory

remains the basic framework within which

modern theories of physics are framed

it's it's the it's the basic language of

physics for example superstring theory

which is an exciting but but speculative

theory of elementary particles and

forces superstring theory is a quantum

theory now our course is not going to to

cover superstring theory but I just want

to point out that that some that all of

these modern theories like super string