[MUSIC PLAYING] 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. [THEME MUSIC] 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." Thanks to The Great Courses Plus
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lus.com/spacetime. 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. [THEME MUSIC]
