[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]