Is light fundamentally a wave or a particle?
forbes.com
forbes.com
https://www.lesswrong.com/posts/hc9Eg6erp6hk9bWhn/the-quantu...
> An electron is not a billiard ball, and it’s not a crest and trough moving through a pool of water. An electron is a mathematically different sort of entity, all the time and under all circumstances, and it has to be accepted on its own terms.
> The universe is not wavering between using particles and waves, unable to make up its mind. It’s only human intuitions about quantum mechanics that swap back and forth. The intuitions we have for billiard balls, and the intuitions we have for crests and troughs in a pool of water, both look sort of like they’re applicable to electrons, at different times and under different circumstances. But the truth is that both intuitions simply aren’t applicable.
> If you try to think of an electron as being like a billiard ball on some days, and like an ocean wave on other days, you will confuse the living daylights out of yourself.
> Yet it’s your eyes that are wobbling and unstable, not the world.
This also applies to light.
IMHO, light is light. When we view it as a particle, we are wrong. When we view it as a wave we are also wrong, but in a different way.
The problem is, people still want to map the mathematical description of how light and particles behave of QED to common sense analogies and that just does not work.
There is no commonly known and understood thing that lends itself as a working analogy.
I’m going to simultaneously upvote and downvote you.
Wave or particle are interpretations we attach to the results of applying the Schrodinger equation.
They might end up saying, "so EM waves are invisible like Air, but they move around like Water waves, so really EM waves display Air-Water duality?"
That's how stupid the idea of wave-particle duality is. When you upgrade to a better model of the world, you have to drop the old concepts, and start anew with new concepts.
But I don't see your quote in the link. Did it come from another source, or did the linked sequence change in the interim, or something else?
"Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.....
There is one lucky break, however—electrons behave just like light. The quantum behavior of atomic objects (electrons, protons, neutrons, photons, and so on) is the same for all, they are all “particle waves,” or whatever you want to call them. So what we learn about the properties of electrons (which we shall use for our examples) will apply also to all “particles,” including photons of light."
If everything is a field, then light is simply a three dimensional wave in that field. This makes the double slit experiment seem much easier to grasp:
A single photon (which is a wave with a discreet amount of energy) travels through both slits at the same time. This causes the wave to interfere with itself and when it hits the wall it registers as a single point, as the photon must always be a discreet quantum of energy.
To me, this makes intuitive sense and removes a lot of the mystery and confusion that gets invoked whenever pop-sci writers explain the experiment. They tend to say something along the lines of: photons are like billiard balls, so they can only go through one slit or the other, but if you don't detect which slit it goes through it seems to go through both.
Am I close to what's actually going on, or am I way off?
There are experiments that show that light is not a particle (in the classical sense) and other experiments that show that light is not a wave (in the classical sense). Quantum mechanics is the result of trying to find a theory consistent with both sets of experiments.
Originally, the double slit experiment was seen as proof that the wave theory is correct. So if you want to challenge your wave theory, look to different experiments. Wave theory of light was predominant from the mid-19th century to the emergence of quantum mechanics in the early 20th century, so I would focus on science from the early 20th century, such as the ultraviolet catastrophe and the photoelectric effect.
Consider the photoelectric effect. If light were a wave that carried energy, this wouldn’t explain why an equal amount of energy has a different effect depending on its wavelength. The quantum explanation is that one packet of light contains a different amount of energy, which depends on wavelength. “Light is a wave” kind of falls apart as a theory, because it is unable to explain this.
Not by itself but the accompanying theory - classical EM theory - does explain why different frequencies interact differently. Absorption/dispersion spectra are well described by wave theory. What the classical wave theory can't explain easily are coincidence counts in extremely weak light (antibunching), or things such as the Compton scattering.
Im fairly certain you could make an experiment where one photon went through some apparatus and two specks of light appeared at the photo detector (with new wavelengths of course). What mechanism in your wave-only theory describes this split? Why do these waves sometime split or fuse and other times stay as single particles? The 'it stays as one particle cause it started as one particle' explanation would lead to a very monochromatic universe!
If you literally try one photon only once, well, you can't really talk about probabilities; whatever happened happened (possibly you witnessed an unlikely event). But if you repeat the experiment N times, even if you do it spaced by many miles and many days, and collect the data you'll notice that the measurements cluster in ways that are compatible with interference bands.
If an actual physicist can tell me whether that I wrote above is fundamentally wrong, I'd be happy and have more questions
Moreover, if we assume conservation of energy, changing a photon w/ wavelength A into B leaves over some energy C, that again, assuming some mechanism of changing energy into light, a new photon could be emitted.
So which assumption is wrong? (1) Light comes in many different wavelengths (2) Conservation of energy (3) Non-light energy can be changed into light energy (4) Light color follows the photon model for the relation between energy and wavelength.
(or just do a simple google search: https://arxiv.org/abs/2009.03039 where it's sounds like it's been observed directly)
Unfortunately, I would say it doesn't help (and could actually block you) when you want to build intuition around QFT and high-energy processes.
In your mind, you would still then think of a photon as a wave running around interacting with electronwaves that then oscillate and produce new photon waves.. and you'd think for example that "hey, I can just solve the wave equation and I solve the experiment".
While it's really so much more messy, as you'd note when you start considering the number of degrees of freedom involved required in your simulation to approximate reality. It's not classically interacting waves timestepped forward in time, it just looks like that in the most trivial situations.
Photons do interact! There is no two-photon vertex, that’s true, since there is no fundamental interaction between two photons. But put four photon/charged particle vertices together and you’ll get the 4-photon box diagram [1], whose contribution is nonzero.
[1]: Since I’m not good at painting diagrams in ASCII on HN, here’s a link to the Wikipedia article with the box rendered nicely: https://en.wikipedia.org/wiki/Two-photon_physics
Maybe you meant there’s no 2–>1 vertex.
electron + positron = 2x photon
If you squint hard enough, it looks like leptons behave like sufficiently energetic photons self-interacting to the point that they form a localised circulation. This requires a charge separation to be stable, hence the requirement for a pair of oppositely charged leptons to be formed from a photon.
Get it?
Feynman explained nothing by saying that particles are "not like" two other familiar concepts. That's flipping two bits in an infinite set of to "false". You can't gain understanding by flipping all but one remaining bit in that infinite set to false! Particles aren't like tiny springs either, or tiny metronomes, or tiny bolts of lightning, or... an infinite list of things they aren't!
Similarly, Feynman's Path Integral method is trotted out as a mental model of how particles "work" at the Quantum Level, but this is literally just a mathematical trick for solving a class of problems efficiently. This is not my opinion, Feynman said so. It's not a "model of the world", it is literally just a specific case of Monte Carlo Integration!
Quantum field theory describes reality with wave functions. In this theory there is a field for each type of fundamental particle spread across all of space and time. Regions of a field can evolve into a localized excited state. Particles are a useful mathematical approximation of these excited states.
For example, an electron is a region where the electron field has more amplitude, or bigger ripples. The electron field exists everywhere, but electrons are more likely to be measured where the field amplitude is larger.
Fields interact with other fields, and with themselves. These interactions can be approximated with a Feynman diagram, which treats everything as particles.
Using Feynman diagrams electron field interactions generally model well as particles. This is because the most complex electron interactions are weak enough to be mathematically canceled out. Quarks, the particles inside protons and neutrons, interact through the "strong force". This force has frequent complex interactions that don't cancel out, making the particle model less useful for calculations.
"So what is a particle?"
"Well, it acts like a wave but is not a wave."
"Aha..."
"It also acts like a particle but is not a particle."
"Okay, but then what is it?"
"I just told you! It's neither a particle, nor a wave, it's something else entirely!"
"I accept that it isn't either of those two things, but then describe the thing that it is!"
"I can't do that! You have to study a pile of textbooks this high to understand what it is!"
"Why? I understand relativity without having to have read a pile of textbooks! That's also an unintuitive physical concept with no classical analogy with which we are familiar with in every day life. Despite this it has intuitive visualisations to aid understanding! [1] Why can't you do something similar for quantum mechanics?"
"Wave-particle duality just has to be understood through the mathematics, there is no simpler way"
"Okay, okay, never mind. Now what about Spin?"
"Go away."
QM is one of those transitions and the jump bothers many.
I'll talk about electrons first. An electron is a point particle. The position of the electron is governed by a wave (wave function). The wave is not made out of "electron". It is made out of something else, kind of like "probability". The wave tells you where you find the electron. But when you do find the electron, it is always a point particle. You will never detect the electron smeared out over space. (As an aside, for those who think about field theory, I still see the electron as point in field theory, though some might see it differently.)
Light is a field/wave made of the electric field. And it has a wave function too. The wave function again is made of probability. But it is probability for different electric field configurations, rather than photon locations, so it might be harder to picture.
Why does a light behave like a particle at all? Though the light has spatial extent it does come in discrete chunks, specific quanta of energy. When light is absorbed it can only be absorbed in these discrete quanta. In most detectors (like photographic paper) the light is absorbed by an electron. Since the electron is a point particle, the photon absorption happens at a single point. This might make it look as if a "particle" of light hit the detector.
This is in much the same way that there is always a possible observer that sees only electrical interactions when magnetic fields are involved for other observers.
Can't help but think that whatever supposedly "happened" is not that relevant, what matters is the measurement.
Clearly "the photon" is only the effect of the interaction between light and the act of measuring it. You just can't remove the measurement from the phenomenon.
In our reality ("to us"), light doesn't make sense if it's not measured, and the result of that measurement is not light itself, but the product or outcome of the interaction of the measuring device/method and whatever "light is" (when not being measured), if light can even exist without being measured (which is unknowable).
Yes
> At that point you probably have a more complex model than admitting the photon was not a particle.
Yes again. But then why a more complex model? To be able to deal with interference of complex systems of entangled particles.
Let me summon the annoying physicist take on this by Sabine:
I'm still a mere trainee so I'm basically neutral on her opinion of HEP and Accelerators, but I know enough that it looks a little weird for an academic.
I linked this part in the video because it points out that it's quite common to try to categorize things in one box or another while it should really be given a new box even if that box contains one item only.
https://ed.ted.com/lessons/making-sense-of-a-visible-quantum...
Okay, I get that the theory is cool. But what's the point of showing audience an object, where they can't see anything interesting anyway? And perhaps, if you are quantum physicist, stick to quantum physics and don't meddle in psychology. You should know best how annoying it is when psychologists start meddling in quantum physics, so don't do the same thing in reverse.
No
Would love to see how alien species with fundamentally different perceptions approach these concepts.
Splitting the universe into galaxies, those into star systems, things in them into cells and molecules and then atoms and eventually elementary particles and all the success we had with this might give the impression that reductionism, that decomposing and reassembling things, is the only way that the world can be and that this will always work, but this is not true. And there is no a priori reason that the universe should be easily decomposable all the way down.
Even I might behave like a wave, it's just extremely unlikely :)
Light is neither a wave nor a particle. It is a quantum thing whose formula can be simplified to either wavelike or particlelike under certain assumptions, but not in the general case. Experiments that force those assumptions will reliably turn up wavelike and particlelike behavior, while more general experiments reveal the harder case where the assumptions don't hold.
This really isn't a hard thing. This has been known for pretty much a century. People keep insisting that quantum mechanics is really just classical mechanics with an extra bag on the side, and that approach will always fail in some limit case.
When you accept that it's the reverse -- that classical mechanics is a special case of quantum mechanics -- you can work with QM in a very straightforward way. It's just not the straightforward way you're used to.
Because they share behavior with two very distinct objects we have macroscopic experience with, we try to categorize them as such.