This is my attempt at contributing to "A Layperson's Introduction" series, here on Tildes. It's why it's here on ~science, rather than ~enviro

Many people have heard about how global warming works. “We are emitting greenhouse gases, and these trap heat, leading to further warming.” So how does this process occur in more detail? What is its physical basis? In this post, I will try to explain the physical basis of these questions in a simple way that is a bit more detailed than what is usually seen.

Electromagnetic Spectrum and Thermal Radiation

The electromagnetic spectrum is a broad spectrum that includes visible light. There are long wavelengths, such as radio waves and infrared light, and short wavelengths, such as ultraviolet light, X-rays, and gamma rays.

Visualization of the electromagnetic spectrum

Thermal radiation is the radiation emitted by the molecules of an object due to thermal movement. It can be in the visible light wavelength, shorter wavelength, or longer wavelength. The length of these wavelengths varies depending on the temperature of the object that is the source of thermal radiation. For example, the thermal radiation emitted by Earth falls into the infrared spectrum, which is at lower energy, because Earth is not as hot as a star. The shift of thermal radiation emitted by colder objects to longer wavelengths is also known as Wien's law.

Energy Budget and Stefan-Boltzmann Law

Our planet Earth has a certain energy budget. In other words, the energy coming to the planet and the energy going out from the planet are specific. The source of the energy coming to the Earth is the Sun, and on average, approximately 340 Watt/m² energy reaches the surface of the planet. In order for this energy to be balanced, the energy radiated from Earth into space must be equal to this amount. This happens in two ways. First, some of the incoming energy is reflected into space by the Earth itself. Both the atmosphere (especially clouds) and the surface make this reflection. The second part can be explained by a physical law called Stefan Boltzmann law. According to this law, each object emits a certain amount of energy as thermal radiation, and the amount of this energy increases with temperature. This increase does not occur linearly, but as the fourth power of temperature. The mathematical expression of the law is given below.

E = σT⁴

In this equation, "E" is the energy, "σ" (sigma) is the Stefan-Boltzmann constant, and "T" is the temperature in Kelvin. However, the law cannot be applied to any object in its current form. The above equation is valid for ideal bodies called "black bodies". In physics, a black body is the name given to an ideal body that absorbs and emits all incoming radiation. However, Earth differs from a black body due to reflection. Therefore, the following equation is more appropriate.

E = εσT⁴

Here, ε (epsilon) means emissivity. Emissivity is the effectiveness of the surface of a material in emitting energy as thermal radiation. For a black body, ε = 1. The Earth's mean ε is less than 1, because it is not a black body. At the same time, emissivity changes depending on which part of the Earth is examined. For example, the emissivity of a vegetated surface and a desert or glacier are different. However, it is more important for us at this point to remember that the mean ε is less than 1.

When we look at the formulae above, we see that, in accordance with the Stefan-Boltzmann law, the Earth emits thermal radiation depending on the temperature, even though it is not a black body. This constitutes the second part of the Earth's energy budget, namely thermal radiation. In summary, Earth receives energy from the Sun and radiates this energy through reflection and thermal radiation.

Radiative Forcing and Greenhouse Effect

The energy budget is very important for our planet. Any change in the budget causes Earth to warm or cool. Natural or human-induced changes that change the balance between incoming and outgoing energy are called radiative forcing. This is the mechanism by which greenhouse gases warm the planet. Some gases in the atmosphere, such as carbon dioxide (CO²) or methane (CH⁴), have physical properties that absorb the thermal radiation emitted by Earth. If you remember, Earth's thermal radiation was in the infrared spectrum. That is, these gases absorb at certain points in the infrared spectrum. As a result of this absorption, the gases emit it again in the form of thermal radiation in all directions. While some of the emitted radiation escapes into space, some of it remains on Earth, causing warming. Since the energy emitted by Earth will increase as it warms up, at a certain point, the incoming and outgoing energy becomes equal again.

CO² emissions, concentration, and radiative forcing

In the image above, in different climate change scenarios, emissions of the greenhouse gas CO²) (left), the corresponding increase in CO² concentration in the atmosphere (middle), and the increasing radiative forcing due to this increase are shown (right). Note that the radiative forcing is shown in Watts/m². It is shown this way because it is calculated based on the change in Earth's energy budget, and Earth's energy budget is shown as Watt/m².

In other words, although the incoming energy is the same, there is a certain decrease in the energy going into space due to the greenhouse effect. This leads to what we call radiative forcing. As a result of radiative forcing, the temperature of Earth increases, and as the temperature increases, the thermal radiation energy emitted by the planet increases. This causes the incoming and outgoing energy to become equal again. As a result, in the long run, radiative forcing (and the greenhouse effect) does not lead to a change in the energy budget. However, it causes solar energy to remain in the atmosphere for a longer period of time, causing a certain amount of warming. This is what we call global warming due to the greenhouse effect.

This process is, of course, more complex than described here. Since the atmosphere has a layered and fluid structure, there are factors that make the job more complicated. For example, while the increase in CO² warms the troposphere (what we call global warming), the lowest layer of the atmosphere, it causes the stratosphere, its upper layer, to cool. Despite these and similar complexities, the physical basis of global warming is still based on the mechanisms described in this post.

Sources

• Schmittner, A. (2018). Introduction to Climate Science. Oregan State University
• van Vuuren, D. P., Edmonds, J., Kainuma, M., Riahi, K., Thomson, A., Hibbard, K., Hurtt, G. C., Kram, T., Krey, V., Lamarque, J.-F., Masui, T., Meinshausen, M., Nakicenovic, N., Smith, S. J., & Rose, S. K. (2011). The Representative Concentration Pathways: An overview. Climatic Change, 109(1-2), 5–31. https://doi.org/10.1007/s10584-011-0148-z
• Wild, M., Folini, D., Schär, C., Loeb, N., Dutton, E.G., König-Langlo, G. (2013). The global energy balance from a surface perspective. Clim Dyn 40, 3107–3134. https://doi.org/10.1007/s00382-012-1569-8
• Zohuri, B., McDaniel, P. (2021). Basic of heat transfer. Introduction to Energy Essentials, 569–578. https://doi.org/10.1016/b978-0-323-90152-9.00017-7

Image Sources

• Image 1: https://science.nasa.gov/ems/01_intro/
• Image 2: van Vuuren et al., 2011

Introduction

I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Previous topics

Bookmarkable meta post with links to all previous topics

Today's topic

Today's topic is the dual nature of light and matter, the wave-particle duality. It is a central concept in quantum mechanics that - as is tradition - violates common sense. I will first discuss the duality for light and then, in the next post, for matter.

The dual nature of light

In what terms can we think of light so that its behaviour becomes understandable to us? As waves? Or as particles? There are arguments to be made for both. Let's look at what phenomena we can explain if we treat light as a wave.

The wave nature of light

Let's start with an analogy. Drop two stones in a pond, imagine what happens to the ripples in the pond when they meet each other. They will interact, when two troughs meet they amplify each other, forming a deeper trough. When two crests meet they do the same. When a crest and a trough meet they cancel out.

Now if we shine light through two small openings and observe the resulting pattern, we see it's just like ripples in a pond, forming an interference pattern. When looking at the pattern formed on a screen placed at some distance from the openings, we see a striped pattern Light can be described as an electromagnetic wave, with crests and troughs. It sure seems like light is wavey! The wave nature of light allows us to describe phenomena like refraction and diffraction.

The particle nature of light

When we shine light on some metals, they will start tossing out electrons. This is called the photoelectric effect. How can we understand this process? Well we know light is a wave, so we imagine that the wave crashes into the electron that is chilling out near the surface of the metal. Once the electron has absorbed enough of the light's energy it will be able to overcome the attractive forces between itself and the positively charged atom core (remember, an electron has negative charge and so is attracted to the atom cores). So a higher intensity of light should make the electron absorb the required amount of energy more quickly. Easy, done!

However, there's something very peculiar going on with the photoelectric effect. If we shine low frequency light on said metal, no matter how intense the light, not a single electron will emerge. Meanwhile if we shine very little high frequency light on the metal, no matter how low the intensity, the electron will emerge. But how can this be? A higher intensity of light should mean the electron is receiving more energy. Why does frequency enter into this?

It seems that the electron needs a single solid punch in order to escape the metal. In other words, it seems it needs to be hit by something like a microscopic billiard ball that will punch it out of the metal in one go. The way physicists understand this is by saying light is made up out of particles called photons, and that the energy a photon carries is linked to its frequency. So, now we can understand the photoelectric effect! When the frequency is high enough, the photons in the light beam all individually carry enough energy to convince an electron to leave the metal. When the frequency is too low, none of the photons individually can knock an electron out of the metal. So even if we fire a single photon, with high enough frequency, at the metal we will see one electron emerging. If we shine low frequency light with a super high intensity at the metal, not a single photon will emerge.

So there you have it! Light is made out of particles. Wait, what? You just told us it's made out of electromagnetic waves!

The wave-particle duality of light

So, maybe light is just particles and the wave are some sort of emerging behaviour? This was a popular idea, one that Einstein held for some time. Remember the experiment where we shone light through two small openings and saw interference (commonly known as the double slit experiment)? Let's just take a single photon and shoot it at the openings! Because light is particles we'll see the photon just goes through either opening - like a particle would. Then all the non-believers will have to admit light is made out of particles! However, when we do the experiment we see the photon interfere with itself, like it was a wave. Remember this picture which we said was due to wave interference of light? When a single photon goes through the openings, it will land somewhere on the screen, but it can only ever land in an area where the light waves wouldn't cancel out. If we shoot a bunch of photons through the openings one at a time, we will see that the photons create the same pattern as the one we said is due to wave interference!

Implications

So it would seem light acts like a particle in some cases, but it acts like a wave in some others. Let's take a step back and question these results. Why are we trying to fit light into either description? Just because it's convenient for us to think about things like waves and particles - we understand them intuitively. But really, there is no reason nature needs to behave in ways we find easy to understand. Why can't a photon be a bit wavey and a bit particley at the same time? Is it really that weird, or is it just our intuition being confused by this world we have no intuitive experience with? I would love to hear your opinions in the comments!

Observing photons

To add one final helping of crazy to this story; if we measure the photon's location right after it emerges from the slit we find that it doesn't interfere with itself and that it just went through a single slit. This links back to my previous post where I described superpositions in quantum mechanics. By observing the photon at the slits, we collapsed its superposition and it will behave as if it's really located at one spot, instead of being somehow spread out like a wave and interacting with itself. The self interaction is a result of its wavefunction interacting with itself, a concept that I will explain in the next post.

Conclusion

We learned that light cannot be described fully by treating it simply as a wave or simply as a bunch of particles. It seems to be a bit of both - but neither - at the same time. This forces us to abandon our intuition and accept that the quantum world is just fundamentally different from our every day life.

Next time

Next time we will talk about the dual nature of matter and try to unify the wave and particle descriptions through a concept known as the wavefunction.

Feedback

As usual, please let me know where I missed the mark. Also let me know if things are not clear to you, I will try to explain further in the comments!

Addendum

The photoelectric effect is actually what gave Einstein his Nobel prize! Although he is famous for his work on relativity theory he was very influential in the development of quantum mechanics too.

EDIT: With the help of @ducks the post now has illustrations to clear up the experimental set-up.

Introduction

I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Previous topics

Spintronics
Quantum Oscillations
Quantisation and spin, part 1

Today's topic

Today's topic will be a continuation of the topics discussed in my last post. So if you haven't, please read part 1 first (see link above). We will be sending particles through two Stern-Gerlach apparatuses and then we'll put the particles through three of them. We will discuss our observations and draw some very interesting conclusions from it on the quantum nature of our universe. Not bad for a single experiment that can be performed easily!

Rotating the Stern-Gerlach apparatus

We will start simple and rotate the set-up of the last post 90 degrees so that the magnets face left and right instead of up and down. Now let's think for a moment what we expect would happen if we sent silver atoms through this setup. Logically, there should not be in any difference in outcome if we rotate our experiment 90 degrees (neglecting gravity, whose strength is very low compared to the strength of the magnets). This is a core concept of physics, there are no "privileged" frames of reference in which the results would be more correct. So it is reasonable to assume that the atoms would split left and right in the same way they split up and down last time. This is indeed what happens when we perform the experiment. Great!

Two Stern-Gerlach apparatuses

Let's continue our discussion by chaining two Stern-Gerlach apparatuses together. The first apparatus will be oriented up-down, the second one left-right. We will be sending silver atoms with unknown spin through the first apparatus. As we learned in the previous post, this will cause them to separate into spin-up and spin-down states. Now we take only the spin-up silver atoms and send them into the second apparatus, which is rotated 90 degrees compared to the first one. Let's think for a moment what we expect would happen. It would be reasonable to assume that spin-left and spin-right would both appear 50% of the time, even if the silver atoms all have spin-up too. We don't really have a reason to assume a particle cannot both have spin up and spin right, or spin up and spin left. And indeed, once again we find a 50% split between spin-left and spin-right at the end of our second apparatus. Illustration here.

Three Stern-Gerlach apparatuses and a massive violation of common sense

So it would seem silver atoms have spin up or down as a property, and spin left or spin right as another property. Makes sense to me. To be sure, we take all the silver atoms that went up at the end of the first apparatus and right at the end of the second apparatus and send them through a third apparatus which is oriented up-down (so the same way as the first). Surely, all these atoms are spin-up so they will all come out up top again. We test this and find... a 50-50 split between up and down. Wait, what?

Remember that in the previous post I briefly mentioned that if you take two apparatuses who are both up-down oriented and send only the spin-up atoms through the second one they all come out up top again. So why now suddenly do they decide to split 50-50 again? We have to conclude that being forced to choose spin-left or spin-right causes the atoms to forget if they were spin-up or spin-down.

This result forces us to fundamentally reconsider how we describe the universe. We have to introduce the concepts of superposition and wave function collapse to be able to explain these results.

Superpositions, collapse and the meaning of observing in quantum physics

The way physicists make sense of the kind of behaviour described above is by saying the particles start out in a superposition; before the first experiment they are 50% in the up-state and 50% in the down-state at the same time. We can write this as 50%[spin up]+50%[spin down], and we call this a wave function. Once we send the particles through the first Stern-Gerlach apparatus each one will be forced to choose to exhibit spin-up or spin-down behaviour. At this point they are said to undergo (wave function) collapse; they are now in either the 100%[spin up] or 100%[spin down] state. This is the meaning of observing in quantum mechanics, once we interact with a property of an atom (or any particle, or even a cat) that is in a superposition this superposition is forced to collapse into a single definite state, in this case the property spin is in a superposition and upon observing is forced to collapse to spin up or spin down.

However, once we send our particles through the second apparatus, they are forced to collapse into 100%[spin left] or 100%[spin right]. As we saw above, this somehow also makes them go back into the 50%[spin up]+50%[spin down] state. The particles cannot collapse into both a definite [spin up] or [spin down] state and a definite [spin left] or [spin right] state. Knowing one precludes knowing the other. An illustration can be seen here.

This has far reaching consequences for how knowable our universe it. Even if we can perfectly describe the universe and everything in it, we still cannot know such simple things as whether a silver atom will go left or right in a magnetic field - if we know it would go up or down. It's not just that we aren't good enough at measuring, it's fundamentally unknowable. Our universe is inherently random.

Conclusion

In these two posts we have broken the laws of classical physics and were forced to create a whole new theory to describe how our universe works. We found out our universe is unknowable and inherently random. Even if we could know all the information of the state our universe is in right now, we still would not be able to track perfectly how our universe would evolve, due to the inherent chance that is baked into it.

Next time

Well that was quite mind-blowing. Next time I might discuss fermions vs bosons, two types of particles that classify all (normal) matter in the universe and that have wildly different properties. But first @ducks will take over this series for a few posts and talk about classical physics and engineering.

Feedback

As always, please feel free to ask for clarification and give me feedback on which parts of the post could me made clearer. Feel free to discuss the implications for humanity to exist in a universe that is inherently random and unknowable.

Addendum

Observant readers might argue that in this particular case we could just as well have described spin as a simple property that will align itself to the magnets. However, we find the same type of behaviour happens with angles other than 90 degrees. Say the second apparatus is at an angle phi to the first apparatus, then the chance of the particles deflecting one way is cos^2(phi/2)[up] and sin^2(phi/2)[down]. So even if there's only a 1 degree difference between the two apparatuses, there's still a chance that the spin will come out 89 degrees rotated rather than 1 degree rotated.

Introduction

I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Previous topics

Spintronics
Quantum Oscillations

Today's topic

Today's topic will be quantisation, explained through the results of the Stern-Gerlach experiment which was first performed in 1922. This topic treats a much more fundamental concept of quantum physics than my previous topics.

What is the Stern-Gerlach experiment?

In 1922 physicists Stern and Gerlach set up an experiment where they shot silver atoms through a magnetic field, the results of this experiment gave conclusive support for the concept of quantisation. I will now first explain the experiment and then, using the results, explain what quantisation is. If you would rather watch a video on the experiment, wikipedia provided one here, it can be watched without sound. Note that I will dive a bit deeper into the results than this video does.

The experiment consists of two magnets, put on top of each other with a gap in the middle. The top magnet has its north pole facing the gap, the bottom magnet has its south pole facing the gap. See this illustration. Now we can shoot things through the gap. What do we expect would happen? Let's first shoot through simple bar magnets. Depending on how its poles are oriented, it will either bend downwards, upwards or not at all. If the bar magnet's north pole is facing the top magnet, it will be pushed downwards (because then north is facing north). If the bar magnet's south pole is facing the top magnet, it will instead be pushed upwards. If the bar magnet's poles are at a 90 degree angle to the two magnets it will fly straight through, without bending. Lastly, if the bar magnet's poles are at any other angle, say 45 degrees, it will still bend but less so. If we send through a lot of magnets, all with a random orientation, and measure how much they got deflected at the other side of the set-up we expect to see a line, see 4 in the illustration.

Now we'll send through atoms, Stern and Gerlach chose silver atoms because they were easy to generate back in 1922 and because they have so-called spin, which we will get back to shortly. We send these silver atoms through in the same way we sent through the bar magnets; lots of them and all of them with a random orientation. Now what will happen? As it turns out all the atoms will either end up being deflected all the way up or all the way down, with nothing in between. 50% will be bent upwards, 50% downwards. So silver atoms seem to respond as if they were bar magnets that either bend maximally up or maximally down. In the illustration this is labeled 5.

If we were to take only the silver atoms that bent upwards and sent them through the experiment again, all of them would bend upwards again. They seem to remember if they previously went up or down rather than just deciding on the spot each time if they go up or down. What model can we think of that would explain this behaviour? The silver atoms must have some property that will make them decide to bend up or down. Let's call this property spin, and say that if the silver atoms chose to bend up they have spin up, if they chose to bend down they have spin down. It seems that these are the only two values spin can have, because we see them bend either maximally up or maximally down. So we can say the spin is quantised; it has two discrete values, up or down, and nothing in between.

Conclusion

We have found a property of atoms (and indeed other particles like electrons have spin too) that is quantised. This goes against classical physics where properties are continuous. This shows one of the ways in which physics at the smallest scales is fundamentally different from the physics of everyday life.

Next time

Next time we will investigate what happens when we rotate the angle of the magnets used in the experiment. This will lead us to discover other fundamental aspects of physics and nature, quantum superpositions and the inherent randomness of nature.

EDIT: part 2 is now up here.

Feedback

As discussed in the last post, I am trying something different for this post. Talking about more fundamental quantum physics that was discovered 100 years ago rather than modern physics. Did you like it? Let me know in the comments!

Introduction and motivation

In an effort to get more content on Tildes, I want to try and give an introduction on several 'hot topics' in semiconductor physics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Today's topic

I will start this series with an introduction to spintronics and spin transistors.

What is spintronics?

Spintronics is named in analogy to electronics. In electronics, the flow of current (consisting of electrons) is studied. Each electron has an electric charge, and by pulling at this charge we can move electrons through wires, transistors, creating modern electronics. Spintronics also studies the flow of electrons, but it uses another property of the electrons, spin, to create new kinds of transistors.

What are transistors?

Transistors are small electronic devices that act as an on-off switch for current. We can flip this on-off switch by sending a signal to the transistor, so that the current will flow. Transistors are the basis for all computers and as such are used very widely in modern life.

What is spin?

Spin arises from quantum mechanics. However, for the purpose of explaining spin transistors we can think of an electron's spin as a bar magnet. Each electron can be thought of as a bar magnet that will align itself to a nearby magnetic field. Think of it as a compass (the electron) aligning itself to a fridge magnet when it's held near the compass.

What are spin transistors and how do they work?

Spin transistors are a type of transistor whose on-off switch is created by magnets. We take two bar magnets, whose north poles are pointed in the same way, and put them next to each other, leaving a small gap between them. This gap is filled with a material through which the electrons can move. Now we connect wires to the big bar magnets and let current (electrons!) flow through both magnets, via the gap. When the electrons go through the first magnet, their internal magnets will align themselves to the big bar magnet. However, once they are in the gap the electrons' internal magnets will start rotating and no longer point in the same direction as the big bar magnets. So that when the electrons arrive at the second magnet, they will be repelled just like when you try to push the north poles of two magnets together. This means the current will not flow, and the device is off! So, how do we get it to turn on?

By exposing the gap to an electric field, we can control the amount of rotation the electrons experience (this is called the Rashba effect). If we change the strength of this electric field so that the electrons will make exactly one full rotation while crossing the gap, then by the time they reach the second big bar magnet they will once again be pointing in the right direction. Now the electrons are able to move through the second big bar magnet, and out its other end. So by turning this electric field on, the spin transistor will let current flow, and if we turn the electric field off, no current will flow. We have created an on-off switch using magnets and spin!

That's cool, but why go through the effort of doing this when we have perfectly fine electronics already?

The process of switching between the on and off states of these spin transistors is a lot more energy efficient than with regular transistors. These types of transistors leak a lot less too. Normal transistors will leak, meaning that a small amount of current will go through even when the transistor is off. With spin transistors, this leak is a lot smaller. This once again improves the energy efficiency of these devices. So in short, spin transistors will make your computer more energy efficient. This type of transistor can also be made smaller than normal transistors, which leads to more powerful computers.

Feedback and interest

As I mentioned, I wrote this post as a challenge to myself to explain modern physics to laypeople. Please let me know where I succeeded and where I failed. Also let me know if you like this type of content and if I should continue posting other similar topics in the same format.