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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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