A common misunderstanding about wave-particle duality
chemistryworld.com
chemistryworld.com
In the real world, a particle can be a lot of different things, but usually we use this word to describe small parts of larger objects. There is no much physical difference between a small grain of sand and a planet, but we use different words for them, while we use the same word for small things, which can be different by many orders of magnitude.
In the real world, a wave, is just a form of group behavior of particles, i.e. many particles are doing the same thing at once because of an interaction between them and some energy in a system. For example, merging of black holes makes waves in a galaxy, so we can watch that in super slow motion.
I thought the idea that a particle is an excitation of a quantum field was broadly accepted.
Is QFT analogous to binary? Everything builds up from QFT?
Two recent popular books about QFT are:
Matt Strassler’s Waves in an Impossible Sea (no math, just hand-wavy analogies): https://www.amazon.com/dp/B0CB3LZXR9/
Sean Carroll’s Quanta and Fields (tries to give you a basic understanding of the actual math): https://www.amazon.com/dp/0593186605/
Unfortunately you're probably not going to be satisfied with that, at some point you need to connect these simulations to real measurable parameters and this is exceedingly non-trivial :/
Starting from the other end, pure old-school undergrad quantum mechanics is really the easiest way to connect to measurable quantities and this is why it's used in all of engineering, quantum optics etc. It just provides the right amount of detail.
I suspect that solving those collapses is key to connecting quantum mechanics to relativity, seems like a much more reasonable place to look than to invent even more fields like string theory does.
But for all normal practical purposes you don't need to solve that and can ignore it (which is both a blessing and a curse of course)
I don't think you really have to go beyond that - except that is not how things behave in the macro world. The problem is trying to map Wave/Particles to the macro scale. Just don't do it.
Not this specific topic, but that approach. Science is a model, and as soon as you're generalizing past experiments and observations, you're outside of where you should be.
I thought measurement along the path collapses the wave, removing the wave pattern over the full source to destination that would be displayed if there was no observation? Eg in dual slit if you observe at one of the slits, the pattern disappears. Wave-like behaviour only exists between observations.
For example, we can make sand to behave like water, by bubbling air through it, but it doesn't break classical mechanics.
In the information theory of Physics there are thought experiments about deep space, where one particle may be essentially all alone, in such cases the particle's decoherence becomes quite dramatic.
As you once again just said, you eventually, after emitting multiple electrons see the wave like properties emerge.
Macroscopic waves also change in behavior when you poke them, this is not mysterious. But the relative energy of the poking to the wave on quantum mechanics is huge for out macroscopic standards.
One unfortunate effect of the presenting it as "wave-particle duality" to the laymen audience is that it's as if physicists don't have a good understanding of what's going on and are puzzled by the behavior, whereas QFT is well-established.
> This doesn’t mean that the atoms themselves are smeared out like waves; rather, what spreads is the probability distribution of them being found subsequently in a given location
No, it really does mean they're smeared out like waves. Prior to the measurement they are in superposition, relative to you. When your experiment images the atoms, that's a measurement that entangles you with the atoms.
If the observable value takes values in (A,B) then when you get entangled you end up in a state (A, measured A) + (B, measured B), each of which perceives a definite value of the measurement. The whole system would still be (to an outside, non-entangled observer, if you could pull such a thing off) in a superposition which could continue to interfere with itself, but the observer who's inside the superposition will never be able to tell.
Afaik this is the standard interpretation nowadays. Particles are waves (well, in the sense that what we call a particle is usually a momentum eigenstate that evolves in space like a wave), but the measurement process that entangles us to the causes them to come in quantized packets which we call particles.
Maybe I'm missing the point of the article somehow though...
You've described the Everett interpretation. It should be the standard. There's still a surprising amount of resistance for some reason.
At least in what I read it seems like the remaining debate is over how literally to interpret the words "many worlds", rather than whether the basic idea is correct. Ideas like entangling with the experiment and the decoherence of quantum states are very well-established.
Yes, but neither of those explains why we observe measurements to have single results. The MWI "explains" that by saying that actually, measurements have all possible results (which is what a straightforward application of the Schrodinger equation says), but we don't experience that, we experience a single result in each branch of the overall entangled wave function.
This "explanation" is treated with skepticism by many because it is completely different from how we treat individual branches of an entangled superposition in any other context besides "measurement". Normally, when two quantum systems are in an entangled superposition, we say that neither of them individually has any well-defined state at all; only the total joint system containing both of them does. We don't say that each branch of the entangled superposition is a different "world" in which each individual system has a well-defined state. The MWI has to claim that somehow when a "measurement" involved we can say that.
IMO Everett is in fact the only interpretation that explains this quite clearly.
"Why do we observe measurements to have single results?"
"Because when you entangle with (an observable in a superposition), you enter a superposition. I.e., a mapping from states of you to states of (observable)"
People who don't believe it's possible for themselves to enter a superposition should ask themselves whether they believe atoms may, and whether they (themselves) are made of atoms.
"Because when you entangle with (an observable in a superposition), you enter a superposition. I.e., a mapping from states of you to states of (observable)"
This doesn't explain why we observe single results. A single result is not an entangled state. Mathematically, it's just one term of the entangled state: so observing a single result would mean the entangled state would be replaced by just one of its terms. This is what "objective collapse" interpretations say happens. But it's not what the MWI says happens.
> People who don't believe it's possible for themselves to enter a superposition
"Superposition" is not the correct term here; "entanglement" is. "Superposition" is basis dependent. "Entanglement" is not.
The relevant question is not whether people can become entangled. A state in which a measurement has a single result is not entangled. At least, not if you interpret the math the way standard QM normally interprets the math.
Edit: I should add here that this whole discussion is assuming a "realist" interpretation, where the quantum state describes the actual physical state of an individual quantum system (which might consist of subsystems, such as a measured object and the measuring device and the brain of the person reading the result off the measuring device). Not all interpretations are like that; for example, ensemble interpretations or statistical interpretations. In those interpretations the issue we are discussing doesn't even arise and "many worlds" seems like a straightforward confusion of concepts, like thinking that (to use an example from Beyond the Fringe) Venezuela must be all blue because it's colored blue on a map.
It's not surprising at all; it's a perfectly rational unwillingness to treat our current quantum mechanics (and not even relativistic quantum field theory, but non-relativistic QM, which is already known to be just an approximation) as though it were an exactly complete fundamental theory.
Would be cool if there were of course. I'm not saying it isn't interesting, I'm just saying I completely understand why most physicists don't bother as it doesn't bring anything new to their table - there are plenty of other practical quantum effects that require research as well.
The MWI doesn't offer any other way to calculate anything (as it's an interpretation, not a theory). It's a good mental image of what happens though.
I guess at some point there will be some experiment that can be performed that requires a more advanced theoretical method than what current QM provides and then maybe the MWI (or other interpretations) get a chance.
Not really. In non-relativistic QM, it's a wave packet. True momentum eigenstates are not physically realizable.
In quantum field theory, "particle" is just a shorthand name for certain quantum field states. Quantum fields aren't waves or particles, they're quantum fields.
But others in this thread have mentioned that you can interfere a single electron with itself (which to my mind rules out classical-phenonema-which-appear-quantum.). And 'superposition' seems like a better word for that.
This is what makes quantum mechanics hard, if it was just a probability distribution nobody would find that vexing its just a normal probability.
https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...
Do quantum effects make solar sails work ? My High School physics said F=MA and photons have zero mass.. so where does the force come from ?
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Objects in motion have kinetic energy, and Einstein says mass and energy are equivalent. This means in a very real sense objects in motion have additional "relativistic mass". When you annihilate that photon it's energy is transferred to whatever absorbed it.
Confused? You're not alone! Physicists are trying to move away from the terms "rest mass" and "relativistic mass" for reasons including one you've already identified: what does it even mean for a photon to be "at rest"?
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On a sunny day, I'd guess that sunlight exerts about the same force on an acre on the Earth's surface as a postage stamp lying on the ground.
For particles with mass the momentum of such a particle is p = mv, and so you can use that to yield F = ma. However for a massless particle like a photon, its momentum is p = E / c. If you use that momentum to describe a beam of photons being absorbed by a material, then you get F = n * E / c, where E is the average energy of the photons, n is the rate of photons per second, and c is the speed of light.
edit: btw - thats a REALLY nice explanation :-)
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Imagine implementing a QED simulator: some EM source emitting billions of photons, each with a vector clock rotating to indicate the wavelength. You could code this up as an array tracking each photon.
Alternatively, at very large numbers of photons, you'd notice that each pixel on your screen would have so many (maybe millions!) that you could just simulate the aggregate behaviour of each little square patch of space instead of individual particles.
Ta-da... it's a continuum. No particles.
You can simulate waves in space either using a Monte Carlo particle simulation or by subdividing the space into finite elements and tracking exchanges over their boundaries.
Superficially the maths looks different, but the result is the same, and the finite element method has locality and makes the speed of light limit manifest.
Why do we keep insisting on covering only the particle model in text books?
Saying that an electron "isn't a wave" when it is in motion, because the wave is probability, not the electron, is equivalent to saying the electron doesn't exist between emissions and absorption. This is a valid interpretation, but even more conunterintuitive to novice, and raises more questions. Ultimately, arguing over vocabulary as interpretation is a distraction. What the thing does is what the thing is. Interpretations are intuitive guides.
One metaphor usually comes closer than the other depending on what system you're looking at, but it's never changing back and forth between some "particle state" and "wave state".
I don’t recall from where I’m stealing it, but I thought it captures the "paradox" quite well.
https://www.science.org/doi/10.1126/science.1201959
We can't say, in any satisfying way. The mathematics is uncontroversial, but all of the simple natural-language explanations fail under scrutiny.
Where is the electron in the double slit experiment? Is it a particle or a wave?
Similarly, we can't say. We don't have a good way of talking about this by analogy, or using natural language. As with the bicycle, the mathematics is bulletproof and boring.
This is not to say that quantum mechanics is unmysterious - I think it is very mysterious. However, the bicycle example shows how this characteristic, frustrating elusiveness of good natural-language descriptions is not limited to exotic quantum systems.
We don’t know that there is no “natural language” story and no analogies for either bikes or quanta, what we know is we have some analogies that don’t work. History is full of instances of things we had incorrect stories for, and then found better ones. We have no reason to believe that won’t happen for photons and electrons too.
If you fire a single particle between two slits just once and it still forms a wave-like interference pattern, then surely what is being observed is more than just a probability distribution?
Can you ever be sure you only fired a single particle?
“The atoms themselves are only ever observed in a given experimental run as particles – just as quantum mechanics says they should be. The wavelike behaviour – which is to say, the smeared-out probability distribution – is reconstructed from many particle-like observations.”
That's not a thing. There's no pattern to observe from a single particle.
The situation you're describing doesn't exist.
Wave behavior and interference patterns can only be observed in statistically significant collections of particle interactions. Not single interactions.
My point was that 1 sample in isolation won’t give you information about the underlying distribution, except that it’s non zero at that point, maybe?
I'm on the completely opposite end of the spectrum. I see no reason to say that quantum entities are ever really pointlike particles.
I'd rather see them as smeared wave-like entities that occasionally rapidly reshape while exchanging energy and momentum through fields as if the were two small billiard balls bouncing.
There's really nothing particle-like about those quantum objects apart from this momentum and energy exchanges (and even that is weird because it's quantized) and that evolution of their center of mass in time seems like a thing flying in space according to Newtonian dynamics.
We draw our simplest intuitions from macroscopic objects that are built of huge number of actual elemental material objects so tightly bound with one another that they are barely smeared.
It's not an accident that macroscopic object obeys the same equations that a tight quantum objects obey.
But it's a huge mistake to think that those equations that we wrote for this very bizarre state of matter that are macroscopic objects is anything primary just because the math describing them is as simple as it goes.
Take a look at ideal gas equations about pressure volume and temperature. They are childishly simple when compared to the math you'd need to accurately describe what actually happens in a gas.
Framing quantum mechanics in terms of "observation" instead of "instance of momentum and energy exchange" might be a very computationally convenient interpretation of what happens but I don't think it's real in any sense of the word.
In broader context we have very many interpretations in physics l that are the simplest possible interpretations of the mathematical abstractions of our models. With complete disregard for how sensible they seem. Even though there are completely reasonable alternative interpretations of the same math available.
Physics educators seem to delight in the quirkiness of the interpretations that theoretical physicists love because they are just their equations narrated, nothing more, nothing less, instead of exploring more reasonable interpretations or even mention that they exist.
In absence of new math, bringing new insights to our fundamental knowledge, one of the goals of physics should be to get real. New, or old but rekindled, more plausible interpretations might inspire new generations of young physicists to visit avenues less explored. Because abstract narratives we globally adopted failed to do that for many decades already.
For example, atoms are fluctuating electrons, neutrons and protons, all of which are fluctuating subatomic particles and so on. And what we describe as particles are essentially the maxima of these fluctuations.