U1L3k.txt

#
# File:   content-mit-8370x-subtitles/U1L3k.txt
#
# Captions for course module
#
# This file has 73 caption lines.
#
# Do not add or delete any lines.  If there is text missing at the end, please add it to the last line.
#
#----------------------------------------

How many of you have heard of interpretations
of quantum mechanics?
Yeah.
So everyone's heard of the Copenhagen interpretation?
So the Copenhagen interpretation--

Copenhagen interpretation, is due to--
or supposedly due to Niels Bohr.

I guess when-- in the early days of quantum mechanics,
Copenhagen--
Niels Bohr with Copenhagen that was a big center of research
in quantum mechanics.
And a lot of physicists came through and well
were maybe taught quantum mechanics by Niels Bohr.
Anyway, much later, David Mermen started
teaching a course in quantum computation at Cornell.
And you know he writes--
I mean he's a physicist, but he also
occasionally writes philosophical columns
for I think Physics Today.
And one of his columns he said well,
you know starting to teach quantum mechanics to all
these computer scientists, I found myself
drifting into the Copenhagen interpretation.
And Niels Bohr of course was teaching quantum mechanics
to all sorts of physicists who didn't know it.
And he came up with the Copenhagen interpretation
to explain it.
And so the Copenhagen interpretation
is really good at well explaining how to calculate
with quantum mechanics etc.
And the other thing Niels Bohr did with the Copenhagan
interpretation is he added a layer of mysticism to it.
So we'll try not to do that.
But you know good for teaching quantum mechanics.

And what it says is it says that quantum mechanical processes
can be divided into two--

I don't want to say steps, but two kinds of evolution.

Isolated processes undergo unitary evolution.

And every so often, an experiment or maybe the
universe itself--
somebody looks at them.

Looks at them.

And this is called measurement.

And it's awfully convenient to treat measurement as something
that happens you know--
something goes under-- undergoes unitary evolution for a while.
Then it gets measured, then maybe undergoes
more evolution-- unitary evolution.
And then it gets measured etc.
Of course, in real life these are happening both at once.
And what you get is a master equation.
And there are all sorts of oh I don't
know what the right word is--
and you can look at master equations
and figure out their dynamics and work with them etc.
But that's much more complicated.
And this is the easy way to explain things.
So this is what we're doing.

So unitary evolution is what happens to quantum systems
when they're isolated from the environment
and nobody is looking at them.