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 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.
Awesome topic once again. And I actually have a question this time! :)
This seems really counter-intuitive. I would have assumed that a singular powerful "knock" would essentially be no different than several slightly less powerful ones in rapid succession, but clearly that is not the case. So do we know why that is? Does the energy put in to the electrons by the low frequency light just dissipate too quickly for them to achieve "escape velocity", so to speak, or is something else going on there that prevents low frequency light from knocking off electrons?
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? :)
What happens to the energy? Is it theoretically possible for a single atom to absorb an infinite number of wrong-energy photons without emitting any?
If the photon does not fit the electron's needs it will not interact with the electron. So it will not be absorbed.
What if it's just a little bit too much? Does the electron take what it needs and spit out the leftovers as like a lower frequency/energy/??? photon?
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.
It does, thank you. :)
I briefly touched on this during my own education. I never got an answer when I asked if light was a wave, particle or something else. So we still don't really know?
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.
Guess I've misunderstood some things. Thanks for clarifying.
Light is light - sometimes it behaves like a wave, sometimes it behaves like a particle. You could just as well say "why do billiard balls not interfere with themselves like light does? Do we not understand what a billiard ball is?" (at least, that's my understanding of the topic).
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.
So light as a particle is nowhere in particular until something comes along to force it to be somewhere? Why doesn't it go back to being nowhere after that?
I will elaborate on this in my next post. I'll @ you when it's up :)