At last we have reached the Solvay conference of 1927,
where the two most powerful voices in physics
Einstein and Bohr
began an intellectual sparring match to determine the very way we understand reality
while aspects of these debates continue even to today,
Einstein's main goal
was to take down the idea of complementarity
and the uncertainty principle,
concepts upon which
many of the most unsettling aspects of quantum mechanics hang.
It was like an elaborate game.
Einstein would offer brilliant thought experiments that
violated complementarity and the uncertainty principle
and Bohr would have to dismantle them
to keep his understanding of quantum mechanics alive.
So get your brains ready.
This is gonna get science-y.
Brussels, October 27th,
the fifth Solvay conference begins.
This may be the greatest meeting of minds humanity has ever assembled.
Heisenberg,
Dirac,
Curie,
De Broglie,
Planck,
Pauli,
Lawrence,
Born,
and of course,
Bohr and Einstein
are all in attendance.
One by one, lectures are given
and long stretches of discussion while away the hours
Then,
on the third day of the conference,
its Heisenberg and Born's turn to speak.
As they near the close of their remarks
they include a short statement saying that they considered
the fundamental tenets of quantum mechanics to be established to the point
where no modification to them would occur.
Everyone knows Einstein famously disagrees.
But he remained silent.
Before the conference began,
Einstein had said that he would probably just be an observer,
citing that quantum mechanics, the theme of the conference,
was not his area of expertise.
And so, he kept his objections to himself.
But then later,
Bohr restates the idea that the fundamentals of quantum mechanics are settled.
At this, Einstein gets up
and walks over to Lorentz,
who was running the conference,
and says:
"Despite being conscious of the fact
that I have not entered deeply enough into the essence of quantum mechanics,
nevertheless,
I'd like to present
some general remarks."
by which he means:
"I would very much like to lay the smack-down, please."
He walks up to a chalkboard and proposes a mechanism that utilizes the idea of conservation of momentum
to violate
the uncertainty principle.
His argument went like this:
imagine our double slit experiment from the first episode.
Now put another barrier with a single slit in it
between our beam of light and our double slit wall.
When we fire our light
through that slit,
we have to treat it as a wave form
which means
just as in our original experiment,
when that wave form passes through the two slits on the second wall
the two wave forms it produces
will interfere with each other
and give us the interference pattern
we are so used to.
But since light's also a particle,
the only thing that could have possibly
deflected the particle-like nature of the light
would be bouncing off the first wall.
There are no other variables or components in the experiment.
So that's the only possibility.
But now,
imagine that first wall is on ball bearings and can move.
By the laws of conservation of momentum,
if we measure which way and by how much
the first wall was moved by the impact of the photon bouncing off it,
we should be able to trace its path
and tell which slit the photon will eventually go through on the second wall.
If the wall was pushed a little bit south,
then we know the photon bounced off it and got reflected north,
heading towards the northernly slit
on the second wall,
right ?
But that can't be the case
because
if you remember from our discussion of wave- particle duality
while basically everything operates like a wave
and a particle,
things don't operate as both at once.
Things like light are a possibility wave
until that wave is collapsed
by someone trying to measure it;
at which point,
stuff like the location of the photon within the space of possibility
has to be resolved.
So, by that logic,
we shouldn't be able to tell which slit
on the second wall
a photon will go through,
which is particle-like behavior,
and
get an interference pattern
for where those photons land
on the screen behind it.
Because,
in order for the wave pattern to
interfere with each other on the other side,
the light would have to be acting
like it was a wave as it passed through both slits.
If we could observe both the wave
aspects and of the particle aspects of light simultaneously,
that would violate
indeterminacy and thus unseat the uncertainty principle.
But Bohr
had an answer.
He basically said that Einstein had set up a strawman here.
He hadn't removed uncertainty;
he had just moved the uncertainty from one place to another without realizing it.
You see,
in order to measure how much rolling that first wall did as a result of being hit with one photon,
you'd have to know the exact location and the movement of the wall itself
before the photon hit it.
And
because we're dealing with one photon here,
you'd have to be
incredibly precise:
precise to a level that quantum rules kick in.
And dealing at the quantum level means we run into
indeterminancy,
You can't know
both the momentum and the location of the wall
with perfect accuracy.
So, yes,
Einstein's example would violate
indeterminacy,
but the only way his example works is if you ignore
indeterminacy first.
Q. E. D.
Not bad for a first round.
Over the rest of the conference,
with a spirit of camaraderie,
Einstein would assail Bohr with other, less intense thought experiments
and Bohr would respond
parrying and thrusting back.
Soon,
the real conference was going on in the dining halls in the hotel hallways,
with everyone getting in on the discussion.
And so,
with the spirit of inquiry and joy,
the first real test of quantum mechanics was overcome.
But Einstein wasn't totally satisfied.
The next Solvay Conference would come in three year's time and
this time,
Einstein would come bearing gifts.
More specifically,
he would bring Bohr a box of light.
Einstein shows up to the next conference and walks up to Bohr and he says,
"Imagine a box
whose walls are perfectly
reflective mirrors.
Inside the box,
there's light bouncing around.
There's also a clock
built into the box and
at a specific time,
this clock will open a hole in the side of the box
for however long it takes for one photon to get out."
Einstein then waits to see if Bohr will see what's coming.
Then, he says,
"Weigh the box."
Bohr goes pale.
He starts wandering around the conference like a man in the aftermath of a natural disaster,
going from person to person, saying,
"It can't be true. It can't be true."
You see,
he realized what Einstein is saying.
If you weigh the box, before the photon leaves it,
and then weigh it again after,
you'll know the mass of the photon.
And thanks to Einstein's own insight that E=mc²,
if you know the mass of a thing,
you know
its energy.
When the uncertainty principle and
complementarity are batted around in science fiction novels or Popular Mechanics articles,
usually all anyone ever talks about
is the fact that you can't find the position
and momentum of a particle with absolute certainty.
But there are a number of other complementary pieces of information
which these principles also say
you can't know both aspects of absolutely.
One of them
is energy and time.
According to the uncertainty principle.
you can't know the exact energy of a particle at an exact time.
But that's what Einstein's box appears to successfully do.
The clock gives an absolute time
and the weight of the photon allows us to determine its energy at that exact moment.
So Bohr retreats to his room,
stunned,
muttering that, "It's the end of physics if Einstein's right."
Sleepless, he works the problem through the night.
He draws a diagram of the box;
he imagines the experiment as if he was actually going to do it.
Then,
as dawn nears, it hits him.
With a smile,
he lays down to catch a few hours of sleep.
He'll be ready to unveil his total triumph in the morning.
And so, it was with mounting excitement in the crowd
he addressed the gathering the next morning.
He explained that,
if a photon escapes the box,
then the box itself must recoil by the laws of conservation of momentum.
By how much?
Well,
due to the uncertainty principle,
we can't know precisely.
But this fact itself proved nothing.
This was an issue of momentum and
Einstein's argument was dealing specifically with energy and time.
But then,
Bohr administered the coup de grâce:
in order to weigh the box,
that box would have to be within a
gravitational field.
After all,
weight means nothing without gravity.
But, by Einstein's very own theory of relativity,
we know that gravity distorts spacetime
and, for an object moving through a gravitational field,
how quickly time moves is affected
by that distortion.
It's why you'll hear people talk about how time moves faster on a mountaintop
or why time might stop near the center of a black hole.
Thanks to gravity's distortion of spacetime,
we can't tell exactly how much the clock within it
was affected by that movement.
HENCE, UNCERTAINTY.
Now that was a gross
generalization of Bohr's answer;
and there are plenty of physicists,
who over the years, have suggested that Bohr's solution here was flawed
or that they have a better solution.
But the principle still stood:
Bohr had bested Einstein using his own theory.
And so,
Bohr's understanding of the quantum world
came to be accepted by the majority of the scientific community.
Basically, through today.
So why walk through the Solvay conferences?
Well,
because, without seeing the pieces being put into place,
it's hard to fully accept
what comes next.
The real thing that Einstein found so upsetting about quantum theory
was something he called,
"spooky action at a distance",
which we know today is the violation of local realism.
Because once we have that
and the uncertainty principle,
the idea that energy comes in quanta
and the fact you can superimpose two quantum states and
have them interfere with each other, like we saw with the waves in the double slit experiment;
once we've got all that stuff,
which you've hopefully come to at least grudgingly accept here,
then,
we can come to understand the power
and the challenges
of the quantum computer.
I Know Zoe,
my head's spinning, too.