> 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.
Imagine talking to an ancient philosopher who strongly believed that the everything in the world was made of Earth, Air, Water and Fire. Explain electromagnetic waves to them.
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.
So if I understand your point correctly, approximations are a priori simplifications of problems, while we are talking about after-the-fact interpretations here, and therefore they are not the same? That sounds logical, thank you.
We already have, its called QED and is the most preise physical theory ever created and tested. Theoretical predictions agree with experiment inside the statistical uncertainties which are now at the 15th or so decimal digit.
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.
Wave particle duality used to stress me out before I read Feynman talk about it.
"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."
Single beam of electrons diffracts like single beam of light, but otherwise electrons behave very differently than light: two electron beams interfere with each other, while two light beams do not.
Am I right that thinking of everything as waves isn't a bad mental model?
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?
If you want to think about it, I suggest that you keep a “library” of classic experiments in your head.
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.
> 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.
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.
"When it hits the walls it registers as a single point" is pretty vague and seems to me to be where you're kinda glossing over a lot of the not-just-a-wave complexity.
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!
What you can do with one photon is to observe that it almost never appears in the screen in positions where geometry dictates there would be destructive interference of the photon was a wave.
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
So, 'something' must be able to turn a photon of one wavelength into a photon of another wavelength, otherwise we'd only have 1 wavelength of light in our universe.
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.
Thinking of them as waves helps your intuition for entry-level QM phenomena like the dual-slit experiment and other lab-scale interferometry experiments.
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.
Thinking of photons as waves in 3d space breaks down as soon as there's entanglement around (eg delayed choice eraser). You need to think of the system as a whole as a wave travelling through configuration space.
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.
I think that was meant to mean that the two beams interact. Two crossing light beams will pass right through each other without much happening while two electron beams will interact. But this is because electrons are charged and photons are not. On careful inspection and with high energy photons one would also notice an interaction between two light beams due to photon photon scattering [1]. But one could reasonably call this a kind of distraction due to different charges of different particles. Ignoring those makes photons and electrons behave identical in respect to the phenomena discussed in the article.
Electron beams interfere in ways that light beams don't, though. If they cross in vacuum, they deflect each other due to the electric charge carried by electrons. Light beams can only deflect each other in media with non-linear response to electromagnetic fields, i.e. the response to one beam modifies the response to the other beam.
Don't try to understand transistors in concepts you're familiar with, such as cars and bananas. A transistor is neither a car, nor a banana. You have to have a new kind of understanding, one that transcends such mundane, familiar concepts.
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!
Just to add a nitpick here, apart from them sharing some wave-like descriptions at some levels in the theory, there is not that much in common between photons and electrons. Fundamentally they play different parts in the theory (one being a matter particle the other a force carrier) and mathematically they are represented differently and behaves very differently (one being a fermion and the other a boson). In particular the boson/fermion distinction is usually glossed over in popular treatments of QED (because it's so unintuitive and messy).
Meanwhile, in a very real sense the following equation holds:
electron + positron = 2x photon
So it's somewhat (very?) false to say that they're fundamentally distinct, when both leptons and photons are fundamental components of electromagnetic phenomena, and are even interconvertible!
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.
Is nature made of waves or particles? The annoying answer is neither. The wave model and the particle model are approximations of nature that are each accurate for different conditions.
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.
These discussions always frustrate me, because for the ten thousandth time the same conversation plays out like a needle stuck on a record:
"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"
Your undergraduate physics education is a series of approximations. Every so often, you will go to a new class and the professor will say, "Okay, that was nice, but that was an approximation. You were wading in the shallow end of the pool. Let's go a little deeper. Throw away what you thought you knew and how you previously modeled things, it isn't as accurate as this ..." and then you're on to the next thing.
QM is one of those transitions and the jump bothers many.
Most people with exposure to any physics learn some basics about electrons, light and quantum mechanics, but I think it is tough to put these different ideas together. The hard part about quantum mechanics for physicists is not the rules for quantum mechanics but how that manifests itself in the real world. This is how I have understand it.
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.
By the way, light has no speed. Light is not a "thing" that "moves" through space, even if it is a particle. Source and target of light incidence simply exchange their excitations through entanglement, symmetrically. There is no direction of movement. And there is no movement.
I believe you're right. While I'm not a physicist by profession, I took enough high level physics to be dangerous. The way I interpret lorentz contraction and time dilation, there will always be a possible observer for which the photon does not need to exist because the interacting particles collided.
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.
I haven't read this somewhere. I was just consolidating my thoughts through reasoning. The first abnormality I observed was light propagation having a direction that is asymmetric between source and target. There is no room for that asymmetry. Next is, assumption that time, space and causality exist independent of light, and movement of light being described in terms of those concepts. This can't be true, because all these (light, time, space, change, causality, direction etc) only have mutual existence but not independent or absolute existence. So, light can't be described in terms of others as if others already exist. For more discussion, you can ping me at vrpbkp_at_gmail.
> we can swap one type of detector for another at the very last instant, implying that the photon was “always a wave” or “always a particle” in order to produce the result that we see.
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).
Does that explain the two slit experiment, with a single particle inferring with its own wave function? If you consider it to be a particle, then you probably need to make the interact of the many-worlds be wavelike. At that point you probably have a more complex model than admitting the photon was not a particle.
I assume it's ultimately in good humour but some of the things she does seem quite "out" for a physicist e.g. the annoying physicist things looks like position yourself towards outsiders.
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 dig her style, but don't subscribe to all her views either.
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.
Watch this TED talk by Aaron O'Conell's experiment if you haven't seen it. Even objects large enough to be seen can exhibit quantum effects. It's not the size of the object that matters but the condition of its environment. Of course the smaller the object, the easier it gets to create those conditions
I watched, and was disappointed. First I had to listen to the debunked psychological theory of two brains, which anyway was unrelated to the topic. The they brought the thing... and it was a tiny dot that did nothing. Then he excitedly talked about how cool it is.
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.
I'm not so sure that other intelligent life could have evolved without having the same way of reducing complexity as we do.
It is the essence of modelling: when you are interested only in certain aspects of an environment you try to categorise and classify your subject accordingly.
Which was helpful till now but there is no a priori reason that this must always be the case. And we might actually have reached to bottom in this regard with quantum physics where splitting things into pieces, studying them separately, and putting them together again does not really work anymore. A system of two or more quantum particles is not just the sum of those particles, the combined system can be in states that are not decomposable, i.e. the particles can be entangled.
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.
What do you mean with it? The quantum state of a two particle system? In that case I am pretty sure - even though I am not a physicist - that you are wrong. The quantum state of a two particle system can not be decomposed into two states, one state for each particle, in the general case. You can find the density matrix for each particle which will tell you how each corresponding particle will behave in isolation but you will lose information about the system by doing so in the general case.
One of my physics professors liked to say that a photon is an event. The particle we know as a photon is a point in spacetime where there is some sort of interaction, but the electromagnetic field is closer to a wave.
Betteridge's Law applies nicely here. The answer is "no".
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.
It is both, or probably even better neither. Elementary particles are not classical particles and they are not classical waves. They are things that behave similar to classical particles in some circumstances and similar to classical waves in others, but they are neither. But as humans have no experience with such things in the macroscopic world, as we lack intuition for them, we desperately want them to be either or the other in order to comprehend them. Ironically this desire is probably what makes them actually more incomprehensible.
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.