The Quantum Experiment that Broke Reality | Space Time | PBS Digital Studios

Summary

The video explores the mind-boggling results of the double-slit experiment, which reveals the strange nature of quantum particles. Photons, electrons, and even large molecules like buckyballs exhibit wave-like interference patterns, even when fired one at a time. This suggests that particles travel as waves of possibilities, only choosing a definite position when observed. The Copenhagen interpretation is discussed, which posits that particles exist in a state of pure possibility until measured, raising profound questions about the nature of reality. The video also touches on alternative interpretations and promises a follow-up on the many-worlds theory.

Takeaways

• 💡 The double-slit experiment reveals that quantum particles, like photons, behave both like particles and waves.
• 🌊 When waves pass through two slits, they create an interference pattern of constructive and destructive interference, which also happens with photons.
• 🔬 Even when photons are fired one at a time, an interference pattern eventually emerges, hinting that each photon somehow interacts with the possibility of other photons' paths.
• 🔄 This wave-like behavior is observed not just with photons, but also electrons, atoms, and even large molecules like buckyballs.
• 🌐 The Copenhagen interpretation of quantum mechanics suggests that particles exist as waves of possibilities until they are observed or measured, causing the 'collapse of the wave function.'
• 🌀 The wave function describes the distribution of possible outcomes, mapping all potential paths a particle could take before it chooses a single outcome.
• 🤯 The nature of reality in the quantum world is different from our classical intuition; particles seem to traverse all possible paths simultaneously before 'deciding' their final position.
• 🔮 Other interpretations, like quantum field theory, propose that particles are waves in their own fields, possibly giving physical reality to the wave function.
• 📚 Quantum mechanics provides highly accurate predictions, but its interpretations, such as the Copenhagen interpretation and the many worlds interpretation, remain debated.
• 🎓 The Great Courses Plus provides educational content, including a thorough review of quantum mechanics, offering more insights into these complex topics.

Q & A

What is the double-slit experiment, and why is it significant in quantum mechanics?

The double-slit experiment demonstrates the wave-particle duality of quantum particles. When particles like photons, electrons, or even molecules pass through two slits, they create an interference pattern, suggesting wave-like behavior. However, when detected, they appear as particles. This experiment is significant because it reveals the strange behavior of quantum particles, which don't conform to classical physics expectations.

How does the interference pattern form when particles are fired one at a time?

Even when particles are fired one at a time, they still create an interference pattern after many particles have passed through the slits. Each particle seems to interfere with itself as though it travels through both slits simultaneously as a wave, but it ultimately lands at a single point like a particle. This reveals that particles behave as waves of probability until measured.

What does the Copenhagen interpretation say about the wave function?

The Copenhagen interpretation, favored by Niels Bohr and Werner Heisenberg, posits that the wave function represents a range of possibilities, not physical reality. When a measurement is made, the wave function 'collapses,' selecting one outcome. Before this collapse, it's meaningless to assign definite properties to the particle, such as position or momentum.

What is constructive and destructive interference in the context of the double-slit experiment?

Constructive interference occurs when the peaks of waves from both slits align, resulting in a higher intensity, while destructive interference occurs when a peak from one wave cancels out a trough from another, resulting in no intensity. This is why the interference pattern on the screen shows alternating bands of light and dark.

How does quantum mechanics challenge our classical understanding of particles?

Quantum mechanics shows that particles, such as photons or electrons, don't behave like classical objects. They exhibit both wave-like and particle-like behavior. Before being measured, their position, momentum, and other properties are not definite but exist as probabilities described by a wave function. This challenges the deterministic view of classical mechanics.

What does it mean for a particle to be in a 'wave of possibilities'?

In quantum mechanics, before a particle is measured, it is described by a wave function that represents all possible positions and paths the particle could take. This wave of possibilities means the particle doesn't have a single, defined location until it's observed, at which point the wave function collapses, and the particle takes on definite properties.

How does the behavior of larger particles, such as buckyballs, differ in the double-slit experiment?

Surprisingly, even large molecules like buckyballs, consisting of 60 carbon atoms, have been observed to produce interference patterns under special conditions. This suggests that wave-like behavior isn't limited to subatomic particles, and even relatively large objects can exhibit quantum effects like wave-particle duality.

Why is the double-slit experiment considered one of the strangest experimental results in physics?

The experiment is strange because it shows that individual particles, like photons or electrons, seem to interfere with themselves as though they pass through both slits simultaneously as waves, even though they are detected as particles. The emergence of the interference pattern over time, when firing particles one by one, defies classical intuition.

What role does randomness play in the Copenhagen interpretation?

In the Copenhagen interpretation, the outcome of a particle’s position or momentum after the wave function collapses is fundamentally random. While the wave function gives the probability distribution of where the particle is likely to be found, the specific outcome is not determined until measured, and this outcome is random within those probabilities.

What are some alternative interpretations to the Copenhagen interpretation?

One alternative is the Many-Worlds interpretation, which suggests that all possible outcomes of the wave function occur in separate, branching realities, avoiding the need for wave function collapse. Another approach is to treat the wave function as a real physical entity, rather than just a mathematical tool, as is done in quantum field theory.

Outlines

00:00

🌍 Exploring the Quantum World through Double-Slit Experiment

The script introduces the double-slit experiment, one of the most intriguing phenomena in quantum physics. Matthew O'Dowd explains how the experiment demonstrates the profound difference between the quantum world and the large-scale physical world. He draws comparisons to familiar waves like water ripples, showing how light behaves similarly by producing interference patterns. The experiment ultimately challenges our understanding of reality, suggesting that photons, even when fired one at a time, create the same interference pattern as waves, hinting that particles possess wave-like properties.

05:01

⚛️ Quantum Behavior in Larger Particles

The script shifts focus to buckyballs (large molecules made of 60 carbon atoms) and how they exhibit similar wave-like behavior in the double-slit experiment. O'Dowd explores how not only photons but also larger particles like electrons and even molecules follow this wave-particle duality. He introduces the concept of a 'wave function,' which describes the probability distribution of a particle's various properties, such as position. These wave-like behaviors imply that particles travel as waves of possibilities, only choosing a definite position upon detection.

10:02

🌊 The Copenhagen Interpretation and Wave Function Collapse

The Copenhagen interpretation of quantum mechanics is explored, as O'Dowd explains how particles exist in a state of possibility until observed. The wave function, according to this theory, represents pure potential, and only collapses into a definite state upon detection. The interaction of potential realities manifests in interference patterns, where only some possible paths become real. This interpretation highlights how quantum theory embraces randomness and possibility, though alternative interpretations, like the many-worlds theory, also exist.

Keywords

💡Double-slit experiment

The double-slit experiment demonstrates that particles, like photons or electrons, can display both wave-like and particle-like behavior. When particles pass through two slits, they create an interference pattern, suggesting that they travel through both slits simultaneously as waves. This experiment is central to understanding quantum mechanics and challenges our classical notions of reality.

💡Interference pattern

An interference pattern occurs when waves overlap and either reinforce each other (constructive interference) or cancel each other out (destructive interference). In the context of the video, this pattern appears even when particles like photons or electrons are fired one at a time, implying they still behave like waves interacting with themselves.

💡Photon

A photon is a fundamental particle of light, representing a quantum of electromagnetic energy. In the double-slit experiment, photons act as both particles (landing at specific points on the screen) and waves (creating an interference pattern when many photons are observed). This duality highlights a key quantum property of light.

💡Wave function

The wave function is a mathematical description in quantum mechanics that represents the probabilities of all possible outcomes for a particle's properties, such as position or momentum. In the double-slit experiment, it describes how a particle behaves like a wave, spreading out over multiple paths before collapsing to a specific location upon measurement.

💡Quantum mechanics

Quantum mechanics is the branch of physics that studies the behavior of particles on the atomic and subatomic scale. It introduces concepts such as wave-particle duality and the uncertainty inherent in measuring properties like position and momentum. The video's focus on the double-slit experiment is a key illustration of quantum mechanics in action.

💡Copenhagen interpretation

The Copenhagen interpretation, favored by physicists like Niels Bohr and Werner Heisenberg, suggests that the wave function represents probabilities and that a quantum system exists in all possible states until it is observed. Upon observation, the wave function 'collapses,' and the system takes on a definite state. This interpretation is discussed in the video as a way to understand the behavior of particles in the double-slit experiment.

💡Wave-particle duality

Wave-particle duality refers to the idea that particles such as photons and electrons exhibit both wave-like and particle-like properties. In the double-slit experiment, particles create an interference pattern (wave behavior) but are detected at discrete points (particle behavior). This duality is a core concept in quantum mechanics.

💡Collapse of the wave function

The collapse of the wave function refers to the moment when a particle's wave-like probability distribution resolves into a definite, particle-like state upon measurement. Before this collapse, the particle exists in a superposition of possibilities. The video uses this concept to explain how a photon or electron's final landing spot in the double-slit experiment is determined.

💡Constructive interference

Constructive interference occurs when two waves overlap and their peaks combine, resulting in a higher amplitude. In the double-slit experiment, constructive interference creates bright bands on the screen where many photons or electrons land. This is a direct result of their wave-like behavior.

💡Destructive interference

Destructive interference happens when the peak of one wave meets the trough of another, canceling each other out. In the double-slit experiment, this leads to dark bands on the screen where few or no photons or electrons are detected, again highlighting their wave-like nature.

Highlights

The double-slit experiment shows the strange behavior of particles like photons and electrons, demonstrating wave-particle duality.

The interference pattern seen in the double-slit experiment suggests that particles can act as waves, even when fired one at a time.

Photons, when shot one at a time, still create an interference pattern over time, suggesting that each photon somehow 'knows' where to land based on wave interference.

The experiment has been repeated with electrons, atoms, and even large molecules like buckyballs, confirming the wave-like behavior of matter at the quantum level.

Quantum mechanics describes the behavior of particles with a wave function, which encodes the probabilities of different outcomes.

The Copenhagen interpretation suggests that the wave function represents pure possibility and collapses into a definite state when observed.

According to the Copenhagen interpretation, particles don't have definite properties like position until they are measured.

Quantum mechanics allows for multiple possible realities to interact, influencing which outcomes are more or less likely.

In the double-slit experiment, the wave function encompasses all possible paths a particle could take, collapsing to one upon detection.

Quantum tunneling, where particles pass through barriers they seemingly shouldn't be able to, also reflects the wave-like nature of quantum particles.

There are different interpretations of quantum mechanics, including some that suggest the wave function has a physical reality, unlike the Copenhagen interpretation.

Quantum field theory offers another interpretation, where particles are waves in fields, providing a more tangible basis for the wave function.

The many worlds interpretation, an alternative to Copenhagen, suggests that all possible outcomes of a quantum event actually happen in separate universes.

Resonant frequencies occur when two orbiting bodies have periods in simple integer ratios, leading to stronger gravitational interactions.

Jupiter's role in solar system formation highlights how its large mass influences the orbits of other celestial bodies, preventing planetary migration.

Transcripts

[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 for sponsoring this episode.

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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.

[THEME MUSIC]