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MATTHEW O'DOWD: This episode is supported by The Great Courses
Plus.
One of the strangest experimental results
ever observed has got to be that of the single particle
double-slit experiment.
It's one of the most stunning illustrations
of how the quantum world is very, very
different from the large-scale world
of our physical intuition.
In fact, it hints that the fundamental nature of reality
may not be physical at all, at least in any sense
that we're familiar with.
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Let's start with the familiar.
In fact, let's start with a rubber duckie.
It bobs up and down in a pool, causing periodic ripples
to spread out.
Some distance away, those waves encounter a barrier
with two gaps cut in it.
Most of the wave is blocked but ripples pass through the gaps.
When the new ripples start to overlap each other,
they produce this really cool pattern.
It's called an "interference pattern."
It's due to the fact that in some places,
the peak of the ripple from one gap stacks on top of the peak
from the other gap, making a more extreme peak.
You also get more extreme dips when two troughs overlap.
We call this "constructive interference."
But when the peak from one wave encounters the trough
from another, they cancel out, leaving nothing,
"destructive interference."
So we have these alternating tracks of wavy and flat water.
Any type of wave should make an interference pattern like this,
for example, water waves and sound
waves but also light waves.
This double-slit interference of light
was first observed by Thomas Young back in 1801.
A source of light passing through two very thin slits
produces bands of light and dark stripes, alternating
regions of constructive and destructive interference,
on a screen.
Of course, we now know that light
is a wave in the electromagnetic field
thanks to the work of James Clerk Maxwell a century later.
So it makes perfect sense that it should produce
an interference pattern, right?
But wait, we also know that light
comes in indivisible little bundles
of electromagnetic energy called "photons."
Einstein demonstrated this through the photoelectric
effect but his clue came from the quantized energy
levels of Max Planck's black-body radiation law.
Check out our episode on this for the details.
OK.
So each photon is a little bundle
of waves, waves of electromagnetic field,
and each bundle can't be broken into smaller parts.
That means that each photon should
have to decide whether it's going to go
through one slit or the other.
It can't split in half and then recombine on the other side.
That shouldn't be a problem as long as you
have at least two photons.
One photon passes through each slit
and then the two photons interact
with each other on the other side
and produce our interference pattern.
But here, we get to one of the craziest experimental results
in all of physics.
The interference pattern is seen even if you fire
those photons one at a time.
Well, let me back up a bit.
The first photon is detected as having
arrived at a very particular location on the screen.
The second, third, and fourth photons, also-- they
deliver their energy at a single spot
and so they appear to be acting like particles
of well-determined position.
But check it out.
If you keep firing those single photons,
you start to see our interference pattern emerge
once again.
By the way, Veritasium actually conducts this experiment
in his excellent series on the double-slit experiment--
really worth a look.
This is so bizarre.
This pattern has nothing to do with how
each photon's energy gets spread out,
as was the case with the water wave.
Each photon dumps all of its energy at a single point.
No, the pattern emerges in the distribution
of final positions of many completely unrelated photons.
How can that be?
Each photon has no idea where previous photons landed
or where future photons will land yet
each photon reaches the screen knowing which regions
are the most likely landing spots
and which are the least likely.
It knows the interference pattern
of a pure wave that passed through both slits equally
and it chooses its landing point based on that.
It turns out that the photon isn't
the only thing that does this.
Shoot a single electron through a pair of slits
and it'll also appear to land at a single spot on the screen
but fire many electrons and they slowly
build up the same sort of interference pattern.
This crazy effect has even been observed with whole atoms, even
whole molecules.
Buckminsterfullerene, buckyballs,
are gigantic spherical molecules of 60 carbon atoms
and have been observed to produce
double-slit interference under special conditions.
We have to conclude that each individual photon, electron,
or buckyball travels through both slits
as some sort of wave.
That wave then interacts with itself
to produce an interference pattern,
except that here, the peaks of that pattern
are regions where there's more chance that the particle will
find itself.
It looks like a wave of possible undefined positions
that at some point, for some reason,
resolves itself into a single certain position.
We also saw this waviness in position
when we talked about quantum tunneling.
In fact, several quantum properties, like momentum,
energy, and spin, all display similar waviness
in different situations.
We call the mathematical description
of this wave-like distribution of properties a "wave
function."
Describing the behavior of the wave function
is the heart of quantum mechanics.
But what does the wave function represent?
What are these waves of or waves in?
Let's start with what we do know about the double-slit result.
We know where the particle is at both ends.
It starts its journey wherever we put the laser or electron
gun or buckyball trebuchet and it releases its energy
at a well-defined spot on the screen.
So the particle seems to be more particle-like at either end
but wave-like in between.
That wave holds the information about all
the possible final positions of the particle
but also about its possible positions
at every stage in the journey.
In fact, the wave must map out all possible paths
that the particle could take.
We have this family of could-be trajectories
from start to finish and for some reason, when
the wave reaches the screen, it chooses a final location
and that implies choosing from these possible paths.
So what causes this transition between a wave
of many possibilities and a well-defined thing
at a particular spot?
Within that mysterious span between the creation
and the detection, is the particle anything
more than a space of possibility?
OK.
We're adding more questions than we're answering.
We still couldn't figure out what the wave is made of.
In fact, the answers aren't known
but the various interpretations of quantum mechanics do try.
Let's talk about the view favored
by Werner Heisenberg and Niels Bohr,
who pioneered quantum mechanics at the University of Copenhagen
in the 1920s.
The Copenhagen interpretation says that the wave function
doesn't have a physical nature.
Instead, it's comprised of pure possibility.
It suggests that a particle traversing
the double-slit experiment exists only
as a wave of possible locations that ultimately encompasses
all possible paths.
It's only when the particle is detected
that a location and the path it took to get there are decided.
The Copenhagen interpretation calls this transition
from a possibility space to a defined set
of properties "the collapse of the wave function."
It tells us that prior to the collapse,
it's meaningless to try to define a particle's properties.
It's almost like the universe is allowing all possibilities
to exist simultaneously but holds off
choosing which actually happened until the last instant.
Weirder, those different possible paths,
those different possible realities,
interact with each other.
That interaction increases the chance
that some paths become real and decreases the chance of others.
There's an interaction between possible realities
that is seen in the distribution of final positions
in the interference pattern.
That pattern is real, even though the vast majority
of paths involved in producing the interference never
attain reality.
In the Copenhagen interpretation,
that final choice of the experiment of the universe
is fundamentally random within the constraints
of the final wave function.
The theory of quantum mechanics produces
stunningly accurate predictions of reality
and it's completely consistent with the Copenhagen
interpretation but this is not the only interpretation
that works.
There are interpretations that give the wave function
a physical reality.
Remember, we know that light is a wave
in the electromagnetic field and quantum field theory tells us
that all fundamental particles are waves in their own fields.
This may give us a more physical medium
that drives these waves of possibility.
And if you thought the Copenhagen interpretation was
freaky, wait until we get to the many worlds
interpretation, which we will right here on "Space Time."
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OK.
Let's look at some of the comments
from our episode on the role of Jupiter
in the formation of our solar system.
Jason Blank asks, "Wasn't Jupiter almost a star?"
Well, the lowest mass stars are around 7.5% the mass
of the Sun, while Jupiter is 1/10,000 of a solar mass.
So it's not really all that close.
You'd need around 75 Jupiters piled on top of each other
to ignite sustained fusion in its core.
A few of you wonder why we think Jupiter even
needs to have a rocky core.
Well, the Sun and other stars don't need rocky cores
because they are massive enough for all of that gas
to collapse by itself.
There's a minimum mass that's capable of doing that.
It's called "the Jeans mass."
It depends on cloud size, temperature, rotation rate,
and composition.
For typical interstellar clouds, the Jeans mass is quite a bit
smaller than the Sun's mass but still much,
much larger than Jupiter's.
For Jupiter to form its giant ball of gas,
it needed a rocky core to start the process.
That core may have dissolved since Jupiter first formed.
Juno will figure that out by carefully
mapping Jupiter's gravitational and magnetic fields.
Bike Jake would like me to talk more
about resonant frequencies.
My pleasure-- a resonant frequency
is when two orbiting bodies have orbital periods that form
a neat ratio of small integers.
For example, for every one orbit of Jupiter's moon Io,
its moon Europa orbits twice and Ganymede four times.
For every eight Earth orbits, Venus does 13.
These integer ratios maximize the amount of time
that the planets spend in closest proximity.
When these bodies are closest together,
they have the strongest gravitational pull
on each other and that pull stops them
from straying out of that resonant frequency.
An Imposter complains that the Jupiter episode was way
too understandable.
Don't worry.
We've got some incomprehensible content
coming your way real soon.
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