Light & Coherence part 2: Spatial Coherence (and the Double Slit Experiment)

Summary

This video delves into the concept of spatial coherence, following up on the previous episode about temporal coherence and light's spectral properties. Using a pond analogy and numerical simulations, the presenter illustrates how waves become organized and spatially coherent over distance. The video explores the relationship between spatial coherence and geometry, explaining the formula for coherence area and applying it to the Sun and Antares, a star in the Scorpio constellation. It also touches on the historical misunderstandings in science communication, the double-slit experiment with white light, and the Fourier transform effect with multiple slits. The script concludes by pondering the classical versus quantum perspectives on light, setting the stage for further discussion in a future sequel.

Takeaways

• 📚 The video is a sequel focusing on spatial coherence, continuing from a previous episode about temporal coherence and its relation to the spectral properties of light.
• 🌌 The presenter uses a pond in Hilversum, Netherlands, named after Hendrik Antoon Lorentz, to illustrate spatial coherence by observing the behavior of waves in water.
• 🌊 Waves in the pond become more organized and linear as they move away from the chaotic splash zone, demonstrating how spatial coherence develops with distance from the source.
• 🔍 A numerical simulation by Nils Berglund is introduced to show in detail how waves develop over time and how spatial coherence grows with distance from multiple wave sources.
• 📉 The simulation highlights areas of strong wave amplitude separated by boundaries of low amplitude due to destructive interference, illustrating spatial coherence and decoherence.
• 📐 The spatial coherence of waves can be quantitatively understood through geometry, with the coherence area being related to the distance from the source, wavelength, and source diameter.
• 🌞 The formula for coherence area is applied to the Sun, revealing that its spatial coherence on Earth is quite small, comparable to the cross-section of a human hair.
• 🔬 Thomas Young's famous experiment on light and colors used sunlight and a pinhole to create a spatially coherent beam, despite common misconceptions about a double slit configuration.
• 🌈 A true double slit experiment with white light is demonstrated, showing how spatial coherence affects the interference pattern and the visibility of colors.
• 🌠 The video discusses the coherence area for stars like Antares, which is much larger than that of the Sun, emphasizing the uniformity of the field despite the star's immense size and distance.
• 🤔 The presenter reflects on the different perspectives on light between classical and quantum mechanics, questioning whether light itself is quantized or if only its interaction with matter is probabilistic.

Q & A

What is the main topic of the second video on coherence?

The main topic of the second video is spatial coherence, explaining how it arises and its implications for the properties of light.

What is the significance of the pond named after Hendrik Antoon Lorentz in demonstrating spatial coherence?

The pond, with its fountain and water jets, serves as a real-life example to illustrate how waves become spatially coherent as they move away from the source of disturbance.

How does the video script use the analogy of waves in a pond to explain spatial coherence?

The script describes how waves in the pond near the fountain are chaotic, but as one moves away, the waves become more organized and linear, demonstrating the concept of spatial coherence.

What role does the numerical simulation play in the explanation of spatial coherence?

The numerical simulation, created by Nils Berglund, provides a controlled environment to observe the development of waves over time and how spatial coherence grows with distance from the source.

How does the script relate temporal coherence to the frequency changes of waves?

The script mentions that the frequency of waves changes with time in the direction of wave propagation, linking this phenomenon to temporal coherence, which was covered in a previous video.

What is the formula given in the script for calculating the area of coherence for waves in 3D space?

The area of coherence is proportional to the distance from the source squared, the wavelength squared, and inversely proportional to the diameter of the source squared, applicable when the distance R is much larger than the size of the emitter.

How does the size of the emitter affect the area of coherence according to the formula?

The area of coherence is inversely proportional to the diameter of the source squared, meaning that a smaller emitter results in a larger area of coherence.

What historical figure's work is discussed in the script, and how does it relate to the understanding of light?

The script discusses Thomas Young and his experiments with light and colors. It highlights the historical misunderstanding around the double-slit experiment and emphasizes the wave nature of light.

How does the script use the double-slit experiment to demonstrate the effects of spatial coherence in light?

The script describes an experiment with two slits and white light, showing how a light source with a larger area of coherence produces an interference pattern with colored lines due to different wavelengths going in and out of phase.

What is the significance of the star Antares in illustrating the concept of spatial coherence?

Antares, being a massive star with a large area of coherence, demonstrates that even though it emits vast amounts of random emissions, the resulting field on Earth is almost uniform within a large area, showcasing the concept of spatial coherence.

How does the script differentiate between the classical and quantum mechanical perspectives on light?

The script suggests that a classical mindset views light primarily as an electromagnetic phenomenon, while a quantum mechanical perspective focuses on the emission and absorption processes involving discrete energy transfers.

What is the script's stance on the quantization of light?

The script questions whether experiments like the photoelectric effect and Compton scattering truly show that light is quantized into discrete energy packages or if they only demonstrate the probabilistic nature of electromagnetic radiation's interaction with matter.

Outlines

00:00

🌌 Introduction to Spatial Coherence

The script introduces the concept of spatial coherence, building upon a previous video on temporal coherence. It sets the stage for a discussion on spatial coherence's emergence and its implications for light properties. The presenter uses a real-life example of a pond named after Hendrik Antoon Lorentz to illustrate spatial coherence in waves. The pond's water, disturbed by a fountain, moves chaotically near the source but becomes more organized as one moves away, demonstrating spatial coherence. The video promises a sequel to further explore the topic, including numerical simulations to better understand wave development over time.

05:06

📚 Mathematical Insight into Spatial Coherence

The script delves into the mathematical characterization of spatial coherence, particularly for light in 3D space. It presents a formula that relates the coherence area to the distance from the source, wavelength, and source diameter. The explanation simplifies the formula's derivation, using an analogy of a flat round disk as a wave source. The relationship between phase shift, wavelength, distance from the source, and emitter size is discussed, leading to the conclusion that coherence area is influenced by these factors. The sun is used as an example to calculate coherence area, revealing it to be comparable to the cross-section of a human hair, with variations depending on wavelength and atmospheric conditions.

10:08

🔬 Historical Perspective on Light Coherence

This paragraph explores the historical context of light coherence experiments, focusing on Thomas Young's work. It corrects a common misconception about Young's double-slit experiment, clarifying that he initially used a single slit and a card that split the light, not a double slit. The script discusses how scientific communication often relies on metaphors rather than precise details, which can lead to confusion. It contrasts the traditional double-slit depiction with the original experiment's setup and emphasizes the importance of understanding the actual experimental conditions for a clearer grasp of scientific phenomena.

15:09

🧪 Practical Demonstrations of Spatial Coherence

The script describes practical demonstrations to illustrate the effects of spatial coherence in light, using a double-slit experiment with white light. It explains how changing the light source's proximity to the slits affects the resulting interference pattern. When using a light source with a small area of coherence, no pattern emerges due to the lack of phase relationships. However, with a source producing spatially coherent light, a clear pattern of colored lines appears, showcasing temporal decoherence. The experiment highlights the necessity of spatial coherence for accurate double-slit experiments and introduces the concept of a 'bunch-a-slits' experiment, which performs a Fourier transform on the light's temporal signal.

20:15

🌟 Coherence and the Wave-Particle Duality of Light

The final paragraph contemplates the vastness of stars like Antares and how their light, despite originating from a chaotic process, becomes spatially coherent over astronomical distances. It contrasts this with the small coherence area of the sun's light on Earth. The script then transitions to a discussion on the nature of light, highlighting the difference in perspectives between classical and quantum mechanics. It questions whether experiments like the photoelectric effect and Compton scattering truly demonstrate that light is quantized or merely show that interactions with matter are probabilistic. The paragraph concludes by anticipating further discussions on these topics in a future video.

Keywords

💡Coherence

Coherence refers to the property of waves where different points on the wavefront are in phase with each other, leading to a consistent pattern. In the video, coherence is a central theme, with a focus on both temporal and spatial coherence. Temporal coherence relates to the consistency of the frequency of light waves over time, while spatial coherence pertains to the phase relationship between points on a wavefront. The video discusses how spatial coherence emerges as waves move away from their source and how it can be observed in the organized movement of waves in a pond and through numerical simulations.

💡Spatial Coherence

Spatial coherence is a concept that describes the correlation between different points on a wavefront. It is a measure of how orderly and synchronized the wave motion is across space. In the video, spatial coherence is illustrated through the example of waves on a pond, which become more organized and linear as they move away from a chaotic source, such as a fountain. The video also explains how spatial coherence can be characterized by an area of coherence in 3D space, which is influenced by factors like distance from the source, wavelength, and the size of the source.

💡Temporal Coherence

Temporal coherence is the property of waves that have a constant frequency over time. It is related to the spectral properties of light, as discussed in the previous video mentioned in the script. The script implies that temporal coherence is a phenomenon where the frequency of waves does not change with time, which contrasts with spatial coherence that involves the organization of waves in space. The video builds upon the concept introduced in the previous episode, indicating the interconnectedness of temporal and spatial coherence in understanding wave behavior.

💡Wavefront

A wavefront is a surface of constant phase of a wave that moves through space. In the context of the video, when discussing spatial coherence, the script refers to considering a small enough part of the wavefront to observe how waves become organized and linear, indicating spatial coherence. The wavefront is used to illustrate how waves from different sources can interfere constructively or destructively, leading to areas of high or low amplitude, respectively.

💡Numerical Simulation

Numerical simulation is a method used to simulate physical phenomena using mathematical models and computer algorithms. In the video, a numerical simulation created by Nils Berglund is used to demonstrate how waves develop in time and how spatial coherence emerges as waves move away from multiple sources. This simulation provides a controlled environment to observe wave behavior, which complements the real-life example of the pond discussed earlier in the script.

💡Destructive Interference

Destructive interference occurs when two or more waves meet such that their amplitudes cancel each other out, resulting in a wave with a reduced amplitude or no wave at all. In the script, this concept is used to describe the boundaries between areas where waves are strong; these boundaries have low amplitude due to destructive interference. This phenomenon is key to understanding how spatial coherence can lead to a non-uniform distribution of wave energy in space.

💡Area of Coherence

The area of coherence is a region over which the waves are considered to be in phase and thus spatially coherent. The script provides a formula to calculate this area, showing that it is related to the distance from the source, the wavelength, and the size of the source. The concept is crucial for understanding how spatial coherence develops and can be quantified, as illustrated with the example of the sun and the star Antares.

💡Wavelength

Wavelength is the distance between two consecutive points on a wave that are in the same phase. It is a fundamental property of waves and plays a significant role in determining spatial coherence. The script explains that the area of coherence is proportional to the square of the wavelength, indicating that shorter wavelengths result in smaller areas of coherence. This is exemplified when discussing the coherence area for different colors of light from the sun.

💡Double Slit Experiment

The double slit experiment is a classic physics demonstration that shows the wave-like behavior of light (or other particles) as it passes through two closely spaced slits, creating an interference pattern. In the video, the script discusses a variation of this experiment using a single electron and an electron biprism, which challenges the traditional metaphor of the double slit and highlights the wave-particle duality of light and matter.

💡Fourier Transform

A Fourier transform is a mathematical technique used to decompose a signal into its constituent frequencies. In the context of the video, the script describes a 'bunch-of-slits' experiment that performs a Fourier transform on the temporal signal of light passing through the slits, resulting in a pattern that reveals the spectrum of frequencies present in the light source. This demonstrates how spatial arrangements of slits can be used to analyze and visualize the frequency components of light.

💡Quantum Mechanics

Quantum mechanics is a fundamental theory in physics that describes the behavior of matter and energy at very small scales. While not the main focus of the video, the script touches upon the differences in perspective between classical wave theory and quantum mechanics when it comes to understanding light. The video creator expresses a personal struggle with quantum mechanics and questions whether the quantized interactions observed in experiments like the photoelectric effect and Compton scattering necessarily imply that light itself is quantized.

Highlights

Introduction to the concept of spatial coherence and its relation to the properties of light.

Explanation of how spatial coherence emerges from a chaotic wave source, illustrated with a pond analogy.

Introduction of Hendrik Antoon Lorentz and the use of a pond named after him to demonstrate spatial coherence.

Numerical simulation by Nils Berglund to visualize the development of waves over time and spatial coherence.

Discussion on how wave patterns become more organized and coherent as they move away from the source.

Quantitative understanding of spatial coherence through a formula relating coherence area to source geometry.

Derivation of the coherence area formula for light and its dependence on distance, wavelength, and source size.

Calculation of the sun's coherence area using the derived formula and its implications for light observation.

Historical account of Thomas Young's experiments with light and colors, and the misunderstandings surrounding his methods.

Clarification of the actual setup used by Young in his famous 'double slit' experiment, involving a single slit and a card.

Critique of the use of metaphors in science communication and their potential to cause confusion.

Demonstration of a true double slit experiment with white light and the importance of spatial coherence for the experiment.

Observation of temporal decoherence in the double slit experiment and its effect on the interference pattern.

Experiment with multiple slits performing a Fourier transform on the temporal signal of the light wave.

Calculation of the coherence area for light from Antares, a star in the Scorpio constellation, and its surprising uniformity.

Reflection on the different perspectives on light between classical and quantum mechanics, and the implications for understanding light.

Introduction of the concept of probabilistic interaction between electromagnetic radiation and matter, contrasting with the idea of quantized light.

Transcripts

Hey Everyone, This is my second video on  

coherence. The previous episode was mostly about  temporal coherence and how it is directly related  

to the spectral properties of light. If you haven’  t seen it, it might be a good idea to watch that  

one first, because the current video it really  a sequel of part 1. Today, I will mainly talk  

about spatial coherence, how it comes about and  what it means for the properties of light. And,  

I will also take a few side tracks. I found that I  could not fit all the side tracks into this video,  

so good news for those of you who  like sequels: there will be a part 3.  

I live in Hilversum, the Netherlands and close  to my home there is this a long rectangular pond  

that is named after Hendrik Antoon Lorentz,  in my opinion one of the greatest Dutch  

scientists ever. And this pond is just ideal  to demonstrate real life spatial coherence.  

The pond has a fountain in it, which consists of  3 water jets that generate a lot of splashes. And  

so, in the vicinity of the fountain, every point  on the surface moves rather chaotically. However,  

if we move away from the actual splash zone, we  observe that the waves gradually all start moving  

in this same direction, away from the source and  become ever more organized. Shh, go away.If we  

consider a small enough part of the wavefront, we  observe that the waves are almost linear. Which  

means that all points in the direction transverse  to wave propagation move in unison or in sync.  

And so, if we consider a small enough area of  the wavefront, we can say that the waves have  

gradually become spatially coherent. If you look  at the spacing between the waves you will still  

observe that the frequency of the waves changes  with time in the direction of wave propagation.  

And this is related to temporal coherence, a  phenomenon that was covered in the previous video.  

What you observe in a general sense is that  the region of spatial coherence gradually  

expands when the waves move further away from  the sources. Now, although, quite illustrative,  

this pond is not ideal as a model, especially  with birds and the wind contributing to the  

total wave pattern. So let me take you the more  controlled environment of numerical simulation.  

The simulation that I’m about to show you here is  made by Nils Berglund who runs a YouTube channel  

in his name. On his channel, he regularly posts  visualizations of all kinds of physical phenomena,  

from chemical reactions to lasers solving mazes.  And he very kindly accepted the challenge to do  

this wave simulation, which allows us to look  in more detail how waves develop in time.  

Let’s go to the start of the simulation. It  involves 15 randomly positioned wave sources that  

each emit a longitudinal wave in one long burst.  The sources have a slightly different emission  

frequency and start at a random phase. As you can  see, they emit waves in all directions and what we  

observe is how, relatively close to the sources,  the wave patterns are quite chaotic, a bit like  

in the pond. But as the waves move away from the  disorder, we see that things settle down pretty  

quickly. And again, if we consider only a part  of the wavefront, we can actually see how spatial  

coherence grows with distance. But there is more:  we can identify these areas where the waves are  

relatively strong. These are separated by what  appear to be boundaries where the amplitude is  

low due to destructive interference. Now for the  casual observer it may seem that the waves and the  

wave energy is non-uniformly distributed in space.  But that is actually not the case, because the  

simulation basically only highlights potential  energy and not so much the kinetic energy in  

the waves. But what we do observe is that the  waves from apparently randomly emitting sources  

result in a wave pattern that gradually becomes  spatially coherent some distance further away.  

It also illustrates nicely the most important  aspects of temporal and spatial decoherence.  

But why do waves seem to add up to a  fairly regular pattern when they move  

away from the sources? Well, one important  aspect is that as they move further away,  

the wave propagation directions of different  sources are lining up. And what you should realize  

is that when waves of the same frequency  but with a different phase are lined up,  

they will result in one single regular wave  of the same frequency. And so, all the initial  

chaos related to just phase difference from  the various wave sources gradually disappears  

when we move further away. Now in this case,  the linear addition of individual waves will  

never show a 100% regular pattern, because the  sources all emit at a different frequency and so  

phase relationships are not constant in time and  space. And that is why we observe these areas of  

decoherence in both the direction of propagation  as well as in the transverse direction.Let’s  

see if we can understand quantitatively how  spatial coherence in waves is related to geometry.  

Spatial coherence of waves in 3D space, like  for example light, can be characterized by  

an area of coherence and it is interesting to  see what defines this area. I’ll just start by  

giving you a formula. You see that the coherence  area is proportional to the distance from the  

source squared, and the wavelength squared  and inversely proportional to the diameter  

of the source squared. I should add that this  is a simplification which only applies if the  

distance R is much larger than the size of the  emitter. Now there is actually quite a lot to it  

if you want to derive this formula property in the  case of light, which I don’t want to do. However,  

I think it would be good to give you some  very general insight into this formula.  

Imagine we have a source in the shape of a  flat round disk emitting waves. And let’s  

for now consider only those waves arriving from  the extreme outside of this source which arrive  

on a surface located somewhat further away. This  surface is perpendicular to the axis pointing in  

the direction of the source and we observe  that these waves are in phase on this axis.  

If we move some distance in the y-direction  away from the central axis, the waves will  

gradually get out of phase more and more  until they are completely out of phase.  

Now if we define Y as the distance over which  the phase shift between these 2 waves is lower  

than an acceptable value, say for example  60 degrees phase shift, we find that this  

distance Y is proportional to the wavelength:  so, the shorter the wavelength, the smaller  

this distance Y. And this relationship between  Y and wavelength is actually a linear one.The  

same kind of relationship holds for the distance  from the source: the longer the distance between  

source and area, the larger the value of Y  within which waves will still be considered  

to be in phase. However, for the size of the  emitter, it is exactly the opposite: the smaller  

the distance between the outer boundaries  of the emitter, the larger the value of Y.  

And so, we can summarize that the distance Y  is linearly proportional to the wavelength and  

the distance from the source, and is inversely  proportional to the size of the emitter. And that  

means that the Area of coherence which is defined  by Y as the radius of a circle, is proportional to  

Y squared. Now if you compare this result with  the formula I showed you, you may notice there  

is one detail which is still a bit out of place  and that is the one over pi. This is actually a  

proportionality factor that arises if we derive  this formula in a mathematically more rigorous  

way, based on the exact geometric configuration  of a round source and our coherence requirement.  

Now as stated, this is a somewhat simplified  representation, but nevertheless the resulting  

formula is actually quite useful. So, let’s  calculate the area of coherence for the sun using  

this formula. I must remark that, technically  speaking, on earth the constraint between  

distance and size is not truly satisfied.  You should also keep in mind that the sun  

is spectrally very broad, so we have to choose  a wavelength for which to do the calculation.  

But when we fill in the values for the diameter  of the sun, the distance between the sun and  

earth and choose a value for Lamba in the center  of the visible spectrum, say 500nm, we arrive at  

an area of coherence of almost 900 square microns  which is equivalent to an area with a diameter of  

34 um. This area is comparable to the cross  section of a human hair. Of course, this only  

works if there are no clouds in the sky because in  that case, the area of coherence will definitely  

be smaller. And since wavelength is a parameter,  for blue light with a shorter wavelength the area  

of coherence it will be somewhat smaller  and for red light it somewhat larger.It  

is interesting to note that when Thomas Young  wrote his paper about the theory of light and  

colors in 1801, he did of course not have a laser  or other really bright light sources available,  

other than the sun. And so Young was more or less  forced to use sunlight for his experiment. From  

what I’ve read on the internet, he used a mirror  to direct the sunlight through a tiny pinhole into  

a darkened room, to create a bright beam of light.  And with the knowledge you have just aquired about  

the area of coherence of the sun, I think it  should be fairly obvious to you what the function  

of the pinhole is: it’s intended to make all the  light entering the room spatially coherent.  

When I tried to confirm that this is how he  actually did it in the original paper, I could not  

find any reference to a mirror or an aperture in  the window. Or a double slit for that matter. Yes,  

that is correct: Thomas Youngs’ famous double  slit experiment actually did not involve a  

double slit configuration. Instead, he placed  a thin card in the beam that split it in half,  

which caused the diffraction pattern. And this  is how he could observe the separation of light  

into different colors. Even a few years later  in his 1803-1804 paper on the same subject,  

there is no reference to a double slit  configuration anywhere. However, six years  

after the original experiment, Young published  2 books that contained a large collection  

of lectures. And in volume 1, he did actually  mention a configuration resembling a double slit.  

The reason why I think this is interesting  is because it illustrates how we communicate  

about science. We don’t necessarily communicate  facts. Most of the time we communicate metaphors  

that represent the essence of an experiment  and not necessarily the details of the actual  

configuration. So somewhere in history, someone  decided to name it the Thomas Young’s double slit  

experiment and this then became synonymous with  the phenomenon observed. Now, don’t get me wrong:  

There is nothing like a good metaphor to explain  difficult concepts to an audience of non-experts.  

However, extending the use of metaphors beyond  what they were intended for can become pretty  

confusing. Take the double slit experiment for  single electrons instead of photons. Here are a  

few examples of how that experiment is depicted:  see if you can spot the confusing aspect.  

It’s not this cougar here, which might very well  be a metaphor for something else too, who knows.  

No, the confusing aspect is that all these visuals  show narrow slits with a very substantial distance  

between then. Which makes the experiment  intrinsically mysterious: because how can a  

particle small enough to pass through a narrow  slit, pass through two widely spaced slits at  

the same time?However, if we refer to the original  experiment performed at Hitachi, the interference  

was actually created by using an electron  biprism. This device basically consists of  

an extremely thin charged wire, with a thickness  much smaller than a micron. When you look at this  

configuration, the interference effect suddenly  becomes much less mysterious because it is  

basically about how the electron interacts with  an object small enough as to demonstrate that the  

electron has wave properties. And so, the metaphor  of the double slit and the way it is presented  

doesn’t necessarily help people understand.  If anything it just adds to the confusion.Ah  

well, all of this does not change the fact that  actual double slits do a good job in demonstrating  

the effects that Thomas Young observed. So, let’s  do a true double slit experiment that illustrates  

the effect of spatial coherence in light. And for  this demonstration we will use white light. So  

here are 2 slits etched in a layer of chromium on  a glass slide and we are looking at them under a  

microscope. They are quite small about 300 microns  long and 5 microns wide. Why so small? Well,  

because we really want to bring out the colors  and the interference here in a lot of detail. I’ll  

place a link to another video in the description  about how you can record this type of images.  

We will start out by looking at how the light  through the slits develops when the area of  

coherence is much smaller than the dimensions of  the current double slit configuration. Basically,  

this can be achieved by placing a white  light LED very near to the slits. If we  

now move away from the slits, we’ll observe  how the light passing the slits is diffracted  

and how this results in 2 blurry blobs that merge  without any noticeable interference pattern. And  

this is because the light does not contain any  phase relationships on points in the slit area.  

Not particularly fascinating right? Now, let’s  replace the close-by positioned LED with a light  

source that produces spatially coherent light.  And this means that the area of coherence is much  

larger than the total area of the 2 slits. So in  this case there is a fixed phase relationship for  

every wavelength at every point on the slits.  And look what happens now if we do the same  

experiment. Once light from the 2 beams starts to  overlap, we observe this fascinating pattern of  

colored lines developing, because of different  wavelengths going in and out of phase.What  

we observe in this pattern is actually temporal  decoherence. In the center of the pattern,  

all wavelengths are in phase with each other,  because the distance to the two slits is equal  

for every point on that line. But as we move  away from the center, every wavelength has its  

own interference periodicity and so the phase  differences between the light originating from  

the 2 different slits will be different for  every wavelength. So this is the reason that  

you need spatial coherence to do a proper  double slit experiment. Pretty cool huh?  

Want to see something even cooler? Here I’ve got  a bunch of slits on which we can do the famous  

bunch-a-slits experiment. Together these can  actually perform a little math operation for us.  

They can do a Fourier transforms on the temporal  signal of the light wave passing the area of the  

slits. Here you see how the transformation  develops in space as we move away from the  

grating. It results in a spectrum showing all  frequencies present in the LED used for the  

experiment. In other words, it’s a microscopically  small spectrometer in one pattern. Previously we  

calculated the area of coherence for light from  the sun and found that the area is pretty small in  

the visible range. But of course, for stars a bit  further away, we can do the same thing. That is,  

if we know their size and distance and that is for  example the case for this particular star here in  

the Scorpio constellation: The star is called  Antares after the Greek god of war. And indeed,  

Antares is not a friendly little neighborhood  star. Depending on which source you consult,  

Antares is about 600- 800 times larger than our  sun. And I mean larger in diameter. If we were  

to place this big boy in our own solar system,  it’s physical boundaries would extend beyond  

the orbit of the planet mars. It’s a monster, but  fortunately for us, it’s also 550 light years away  

from earth. Now, if we do the same calculation for  the area of coherence of light Antares on earth,  

we find a value of about 2.3 square meters. Now  I want you to let this sink in for a second.  

Here we have an object so massive it’s beyond  human comprehension. Which emits unimaginable  

amounts of random emissions every second. Yet  by the time the radiation arrives on earth,  

the sum of these individual contributions yields  a field that is as good as uniform within an area  

of several square meters. And you can measure  this with methods similar to the double slit  

experiment. Of course, as for the temporal field  properties: it still contains a lot of frequencies  

since the light emitted by Antares is spectrally  also very broad. But just think about how you  

could explain for example the phase relationship  from a corpuscular perspective on light.  

Also, keep in mind Antares is a huge  star, relatively close by. Smaller,  

further away stars can easily have an area  of coherence the size of a square kilometer.  

As you may have noticed, I’ve focused only on the  wave behavior of light so far, in this video but  

also in the previous one. And if you want, you  can consider this a “classical” perspective. In  

the previous video I made friends for life with  the quantum mechanics community by stating that  

the properties of light arise purely from wave  behavior. And as much as I would elaborate on  

this a bit further now, I don’t think it would be  wise to do that near the end of a video that is  

already too long. By the way, apart from the use  of the word photon in video part 1, I did not make  

a single reference to quantum mechanics. Because I  think it is safe to say I don’t understand Quantum  

Mechanics. I did take courses in the subject at  university and I did pass the exams without a  

problem. But somehow it never made sense to  me at. I guess I just wasn’t smart enough.  

There was one thing that struck me  during the discussion. And that is  

that at some fundamental level, we do not seem  to agree on what exactly we mean with the word  

“light”.If you’d ask someone like me with  a classical mindset to describe light,  

that person would probably mainly refer to the  electromagnetic phenomenon. So, I would say  

something like: we have a source that emits light.  This light propagates in space, is scattered,  

diffracted, reflected whatever and is then  detected or absorbed, for example in your eye.  

From this perspective, I’m referring to  the electromagnetic radiation as light.  

However, if you’d ask the same question to  someone deeply involved in quantum mechanics,  

the emphasis would not be on the radiation  aspect. It would likely be more on the emission  

and absorption processes and how these involve the  transfer of discrete energy packages. So I guess,  

from the viewpoint of someone in quantum  mechanics, double slit experiments are only  

marginally interesting at best. They would argue  that the quantized interaction between radiation  

and matter is proof that the radiation or the  field itself must also quantized. And that this  

proof is in many experiments, including the  photoelectric effect and Compton scattering.  

Now just to be clear, I’m not questioning the  quantized nature of the interaction between  

radiation and matter. But do these experiments  truly show that the radiation which I refer to  

as light, is quantized into discrete packages of  energy? Or do they merely demonstrate that the  

interaction of electromagnetic radiation with  all matter is fundamentally probabilistic? 

I have a feeling that we’ll have plenty  to talk about in part 3. And maybe we  

can even discover a suitable common metaphor.