# wanda-seldon's recent activity

1. ## Comment on Fermi problem game thread in ~misc

A water molecule consists of 3 = 10^0 atoms and weighs 18 = 10^1 atomic masses. A hydrogen atom weighs 10^-27 kg. So a water molecule weighs 10^1 times this; 10^-26 kg. A human consists mainly of...
• A water molecule consists of 3 = 10^0 atoms and weighs 18 = 10^1 atomic masses.
• A hydrogen atom weighs 10^-27 kg. So a water molecule weighs 10^1 times this; 10^-26 kg.
• A human consists mainly of water and weighs 10^2 kg.
• There are then 10^2/10^-26 = 10^(2+26) = 10^28 water molecules in a human body.
• There are 10^0 * 10^28 = 10^28 atoms in an average human body.
2. ## Comment on Fermi problem game thread in ~misc

I came up with the following: There are 10^6 people in Amsterdam, half of which are women. The average number of children per woman is 2. Women are able to have children over a period of ~30 years...

I came up with the following:

• There are 10^6 people in Amsterdam, half of which are women.
• The average number of children per woman is 2.
• Women are able to have children over a period of ~30 years (15 to 45).
• This means the probability of any of those women giving birth in a specific year is 2/30=1/15 = ~ 0.1 = 10^-1 (very close to your guess @gpl!).
• Assuming half of the women in Amsterdam are between 15 and 45, we get 10^6 / 4 =10^5.
• 10^(5-1) = 10^4 births in Amsterdam per year, so 10^2 births per day.
• A midwife will supervise in 10^0 births per working day.

So you would need around 10^(2-0) = 100 midwives in Amsterdam.

1 vote
3. ## Comment on Fermi problem game thread in ~misc

Ah yeah, that should definitely be 10^9. Gonna fix that. I rounded down the number of keystrokes per hour because the values quoted in keystrokes per minute or words per minute are usually not...

Ah yeah, that should definitely be 10^9. Gonna fix that. I rounded down the number of keystrokes per hour because the values quoted in keystrokes per minute or words per minute are usually not sustainable over long periods of time.

Edit: Actually, I'll change it to 10^10 as 7 billion is closer to that then to 10^9.

4. ## Comment on Fermi problem game thread in ~misc

How many midwives are there in Amsterdam?

How many midwives are there in Amsterdam?

1 vote
5. ## Comment on Fermi problem game thread in ~misc

This method is very common in physics, where we call these 'back of the envelope' calculations. Rounding to the nearest power of 10 for all values (coincidentally this is why I switched from...

This method is very common in physics, where we call these 'back of the envelope' calculations. Rounding to the nearest power of 10 for all values (coincidentally this is why I switched from astrophysics to material physics) is common in this scenario, because you do not know all data with a greater accuracy than that anyway. So let's try this.

I define a keystroke as taking place on a computer keyboard (so no smartphones etc).

``````An average person does 10^2 keystrokes per minute, or 10^3 keystrokes per hour.
The average person spends 10^0 hours per day typing. (Much more in the West probably, and much less in Africa, I feel it's definitely closer to 1 than to 10 hours).
There are 10^10 people in the world.
``````

This leads to 10^(3+0+10) = 10^13 or 1000000000000 keystrokes per day.

The Wikipedia page for Lev Landau (one of the most influential physicists of the previous century) has this gem:

The Wikipedia page for Lev Landau (one of the most influential physicists of the previous century) has this gem:

In 1937, Landau married Kora T. Drobanzeva from Kharkiv.[26] Their son Igor was born in 1946. Landau believed in "free love" rather than monogamy and encouraged his wife and his students to practise "free love". However, his wife was not enthusiastic.[17]

7. ## Comment on Intel Tried To Bribe Reseachers to Downplay The Severity of MDS Vulnerability in ~tech

Hoi, I am Dutch. This article is indeed in Dutch, you can tell from the lack of German characters like ü and ö and the inclusion of the Dutch digraph ij. The source you linked (which I'd say is...

Hoi, I am Dutch.

The source you linked (which I'd say is quite trusted) doesn't mention much about bribes.

De Amsterdamse universiteit krijgt ook als enige partij een beloning: 100.000 dollar (89.000 euro), Intels maximale beloning voor ontdekkers van kritische lekken.
Er zit wel een bijsmaakje aan de premie. Volgens de VU probeerde Intel de ernst van het lek te bagatelliseren door 40.000 dollar beloning officieel uit te keren en daarnaast nog eens 80.000 dollar ‘los’.

This says that intel wanted to give the VU 40k dollars officially as a reward for finding the big and another 80k seperately. This offer was refused and they ended up getting 100k on the books, the max amount available in the program.

8. ## Comment on What's a topic/comment on Tildes that you had previously bookmarked and meant to return to, but forgot about until you read the title of this post? And, what about it made you bookmark it? in ~talk

I'm still around. I might post some more in the future and see if my threads are more popular then. Currently real life is very busy though so it might be a while.

I'm still around. I might post some more in the future and see if my threads are more popular then. Currently real life is very busy though so it might be a while.

9. ## Comment on The Day the Dinosaurs Died in ~science

Done! And I have to agree it's a very interesting article.

Done! And I have to agree it's a very interesting article.

10. ## Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

Hi, sorry for the very late reply, I came down with the flu. So pilot wave theory is an example of a hidden-variable theory. Hidden-variable theories share the core idea that the randomness in...

Hi, sorry for the very late reply, I came down with the flu.

So pilot wave theory is an example of a hidden-variable theory. Hidden-variable theories share the core idea that the randomness in quantum mechanics is not really randomness but instead a deterministic hidden variable that we cannot measure.

There are two classes of hidden-variable theory; local hidden-variable and non-local hidden-variable. Local hidden-variable theories have the added requirement of local realism. Simply put this means that these theories require distant events (i.e. separated by some distance) cannot communicate instantaneously. As this class of theory is very sensible, they used to be very popular amongst physicists. So what happened? Bell wrote Bell's theorem which simply states "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.". This theorem has been proven.

So that leaves us with non-local hidden variable theories, which includes the modern version of Pilot wave theory, called De Broglie-Bohm theory. This is a valid interpretation of modern quantum mechanics (along with the many-worlds interpretation, Copenhagen interpretation, modal interpretation and objective-collapse interpretation) that has plenty of weirdness of its own. The wavefunctions given by this theorem have hidden variables that can depend on the state of the entire universe. Furthermore, the pilot waves of this theory are by themselves sufficient to explain the behaviour of particles. So once again you end up describing particles as waves, just pilot waves. However, there are definitely physicists that argue for this view (and for any of the other views that I mentioned) but in the end they all produce the same results.

11. ## Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

We do know, we just can't create an analogy to more everyday things. Next time I will elaborate on the model we currently have.

We do know, we just can't create an analogy to more everyday things. Next time I will elaborate on the model we currently have.

12. ## Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

I will elaborate on this in my next post. I'll @ you when it's up :)

I will elaborate on this in my next post. I'll @ you when it's up :)

13. ## Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

It can go from its base state (state 1) to a higher energy state (state 2) by absorbing a photon that would put in a third even higher energy state (state 3) and then reemit a photon that has an...

It can go from its base state (state 1) to a higher energy state (state 2) by absorbing a photon that would put in a third even higher energy state (state 3) and then reemit a photon that has an energy equal to the difference between the second and third state. This way it ends up at state 2 even though the photon didn't have the energy to fit the transition from 1 to 2.

14. ## Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

If the photon does not fit the electron's needs it will not interact with the electron. So it will not be absorbed.

If the photon does not fit the electron's needs it will not interact with the electron. So it will not be absorbed.

15. ## Comment on A layperson's introduction to the nature of light and matter, part 1 in ~science

In essence you are correct. Light that is too low frequency to allow the electron to escape will instead only bring the electron into a higher energy level within the metal. If it would then...

In essence you are correct. Light that is too low frequency to allow the electron to escape will instead only bring the electron into a higher energy level within the metal. If it would then absorb a second photon it could escape (or maybe it needs a third, fourth, etc). However, there are several rules that make this very tricky to achieve.

The energy of the photon must be exactly the energy difference between the energy level the electron is in and another allowed energy state. Remember, this is quantum mechanics so the electron can only have certain discrete energies. If the photon's energy doesn't match this energy difference nothing happens.

Say we manage to do this, then we have to get lucky and absorb a second photon before the electron falls back into its original state (and re-emits a photon). The average amount of time an electron spends in the higher energy state is called the lifetime, and it's usually very short in these cases.

If it needs more than two photons worth in energy to escape it becomes even trickier. Say it absorbed one photon. Then it gets lucky and comes into contact with another photon that has the same energy. Just because the photons have the right frequency to help the electron go from its original state to the state it's currently in, doesn't mean the same energy can bring it into an even higher state. Energy levels are rarely evenly spaced.

So I did lie a little bit, you could - if you have the right set of frequencies in your light that can help the photon raise its energy in steps - see a few photons being emitted but it would be very rare compared to what happens when you have the frequency that can punch out electrons in a single step. It's the difference between winning the lottery once and winning it twice within a short time span (the lifetime of the electron in the higher energy state).

Does that make sense? :)

1 vote
16. # A layperson's introduction to the nature of light and matter, part 1

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...

# 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!

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.

17. ## Comment on A layperson's introduction to Thermodynamics, part 2: Equilibrium, phase changes and steam engines in ~science

Thank you for posting this. An interesting aside on phase changes is that the theory is used much more widely than to just explain the form matter has when you heat it. The same theory can be used...

Thank you for posting this.

An interesting aside on phase changes is that the theory is used much more widely than to just explain the form matter has when you heat it. The same theory can be used to, for example, describe how materials magnetise (go from non-magnetic to magnetic) and how they go from normal conductivity to superconductivity.

The theory of phase changes and thermodynamics has its power in the fact that it can describe systems without knowing exactly what those systems are and therefore has been able to stay relevant throughout the quantum revolution of physics.

18. ## Comment on A layperson's introduction to LEDs in ~science

Quoting myself from elsewhere in the thread: I think that should answer the question. If not, please let me know!

Quoting myself from elsewhere in the thread:

So to create the photons we need the electrons to drop in energy. In order to explain this properly I'll first have to introduce a few more concepts.
A semiconductor has 2 energy bands, the conductance band and the valence band. If an electron has a high enough energy it will end up in the conductance band and be able to, well, conduct. If it doesn't have the required energy it ends up in the valence band where it's stuck being unable to move. Crucially, in semiconductor there's a gap between these two bands; the bandgap. What this means is that electrons cannot have an energy that falls within this gap.
So, now back to the junction. The two materials making up the junction do not have their valence and conductance bands at the same energies. The p-type's conductance band will start at a higher energy than that of the n-type. This means that an electron that flows into the barrier will end up in one of two scenarios. If it has enough energy to end up in the n-type's conductance band it will continue flowing. Otherwise, if it has enough energy to be in the p-type's conductance band but not in the n-type's, it is forced into valence band of the n-type. This means it will have to drop in energy - else its energy would end up in the band gap - and while doing this it will emit a photon. This is how the junction forces the electron to emit a photon.

I think that should answer the question. If not, please let me know!

19. ## Comment on A layperson's introduction to LEDs in ~science

So to create the photons we need the electrons to drop in energy. In order to explain this properly I'll first have to introduce a few more concepts. A semiconductor has 2 energy bands, the...

So to create the photons we need the electrons to drop in energy. In order to explain this properly I'll first have to introduce a few more concepts.

A semiconductor has 2 energy bands, the conductance band and the valence band. If an electron has a high enough energy it will end up in the conductance band and be able to, well, conduct. If it doesn't have the required energy it ends up in the valence band where it's stuck being unable to move. Crucially, in semiconductor there's a gap between these two bands; the bandgap. What this means is that electrons cannot have an energy that falls within this gap.

So, now back to the junction. The two materials making up the junction do not have their valence and conductance bands at the same energies. The p-type's conductance band will start at a higher energy than that of the n-type. This means that an electron that flows into the barrier will end up in one of two scenarios. If it has enough energy to end up in the n-type's conductance band it will continue flowing. Otherwise, if it has enough energy to be in the p-type's conductance band but not in the n-type's, it is forced into valence band of the n-type. This means it will have to drop in energy - else its energy would end up in the band gap - and while doing this it will emit a photon. This is how the junction forces the electron to emit a photon.

Also what do you mean by electrons interacting with phonons but not creating phonons, and how does the junction play a part in this?

What your previous comment seemed to imply is that you think electrons go bouncing around creating phonons. However, they usually just scatter off of phonons, making them change directions elastically (without a change in energy). Phonon creation can also happen at a junction, as I discussed previously, when the material has an indirect bandgap.

And what's the difference between crystal vibrations in the form of phonons compared to normal vibrations (like sound wave)?

A vibration from sound wave can be modelled as a certain kind of phonon, an acoustic phonon (with infinite wavelength).

20. ## Comment on A layperson's introduction to LEDs in ~science

That's a really cool idea. I hadn't considered it before. With current techniques we can create blue, green, red etc. LEDs but these are all made out of different semiconductors. So fusing them...

That's a really cool idea. I hadn't considered it before.

With current techniques we can create blue, green, red etc. LEDs but these are all made out of different semiconductors. So fusing them together into one diode is just impractical and would probably introduce secondary effects on the boundary between the materials.

However, in theory I don't see a problem with this approach - as long as your material can have a wide enough bandgap tunability (i.e. we can change the energy gap to range all the way from blue light to red light being emitted). In fact, I found a paper from 2011 where exactly this is done on a nanowire. The material they used has such a wide bandgap tunability that it can produce light in the entire visible spectrum. To quote:

The emerging nanotechnology has brought a variety of new opportunities for solid-state white lighting research. In particular, semiconductor nanostructures, such as semiconductor alloy nanowires, have shown the potential in constructing white lighting sources due to their high quantum efficiency and wide band gap tunability. Using the vapor liquid solid (VLS) nanowire growth mechanism, the element composition in the grown nanowires can directly be controlled by the corresponding element concentration in the source materials or the precursor vapor, and semiconductor alloys with different band gaps can be gradually grown into single wires along their length, through applying an in situ concentration changing of the source reagents during the growth. These band gap graded semiconductor nanostructures offer the opportunity to design novel white light-emitting materials or structures at microscale from the ground up.

Emphasis mine. It seems that the limiting factor here is how smoothly they can change the bandgap over the width of the material. Here's a photo they made of the nanowire they produced.

So thank you very much for asking this question, it led me to discover this very interesting paper :)