Quantum 101 clarifications please (from those who actually know)

Wrestling with quantum physics at very basic level. I would appreciate some clarification on my specific questions from people who genuinely understand the topic, in Plain English.

The challenge
Quantum physics behaves differently, so physicists often use images or metaphors to explain it. We now have all these metaphors and explanations everywhere, but many expressed so badly that they have created a kind of mysticism. Some journalists seem to love the shock value of counter-intuitive, and deliberately play on it with confusing articles (or perhaps some simply have no idea what they are talking about.) There really is a mysticism now picked up by ordinary people who refer to quantum mechanics and conclude that our mind influences reality. Sigh.

Please don’t post -
• Links to articles (I’m tired of reading articles about it. So many bad ones.)
• Mathematics/formulas. (I only speak English, and would proffer that an explanation in Plain English is evidence of your clarity of thought.)
• Your best guess. (If you don’t really understand this, please don’t post your confusion. Sufficient for the day is my confusion thereof.)
• Take the conversation into lots of side topics unless it directly helps these questions. Learning is about grasping basic concepts and standing on them to go further. I need to grasp these basics.

A critical point of confusion: I’ve observed that those who write about quantum theory are often not clear whether they are referring to:
A) ACTUAL - The inherent property or position of a particle;
B) OBSERVER - The observer’s knowledge or ability to know.
This is shoddy communication that muddies the waters, and it’s present in most of what I read. If they actually know which one they are referring to, why can’t they make that distinction and state it clearly? And if it’s just about the B) the observer - well, I really don’t care much if we can’t predict precise location. This is not some super shock or mystery. The inability to predict informs us of its properties and how to comprehend it.

Here is my kindergarten level of conclusion so far. Please affirm or correct:
1. OBSERVATION IS NOT PSYCHIC INTERFERENCE: ‘Observing changes the outcome’ - so people infer that it’s a form of psychic interference, as if it’s our conscious awareness rather than our act of measuring creates the change. Nonsense. It’s the act of measuring. It is also sometimes expressed to refer to our inability to precisely predict.
2. WAVE vs FIELD: I think a wave is a movement within a field? And a particle is a manifestation in either a field or a wave? And that wave and particle can both be influenced by multiple fields.
3. WAVE OR PARTICLE: The particle does not behave like a marble that rolls from here to arrive there in its precise position so we should stop trying to think of it that way. It’s a manifestation of a wave function that we observe as a particle, and so it behaves differently to a marble… Ok. That doesn’t seem very complex.
My own metaphor: Remember those old-fashioned biscuit tins? If the lid became slightly misshapen, a buckle would develop. If you pushed the buckle in it would randomly pop up somewhere else. That’s because the restricted space of the damaged lid forced the compression to manifest as a buckle.
A particle is a manifestation of the field, and it manifests within the field. The field is bigger than the specific space a particle is in. For example an electron. It isn’t like a marble that has to move in a line to be over there - it can pop like the tin lid in another place anywhere within the constraints of the forces acting on it. The field of an electron exists more like a cloud around the atom (arranged in a certain way) and the electron pops like lightning anywhere within that cloud. Nothing difficult to understand there. The whole (area covered by the wave function or field) is greater than the part (particle).
a) OBSERVER: We can’t predict where the particle will be in the slit experiment. It could be anywhere within the wavelength in the wave function (within certain limits we can model). But it will be somewhere specific at any given time, observed or not.
B) REALITY: The wave function as a whole passes through the slits. It breaks into two waves that interfere with each other. Since the particle is part of a wave function that passes through both slits and forms two waves. It doesn’t matter if it manifests in one slit /wave or the other as it passes through, it will still ‘pop’ somewhere within those waves at different moments.
5. THE CAT REALLY REFERS TO THE OBSERVER’S ABILITY TO PREDICT, NOT THE PARTICLE: At any snap frozen moment the particle is somewhere specific, observed or not. The cat in the box refers to our inability to predict for certain. If I’m correct Schroedinger’s cat is a dreadful metaphor evoking all kinds of mystery to explain a simple idea. We can’t predict precisely where the particle will be at a precise moment in time, but it will be somewhere. Ok… so what? You could put Schroedinger’s Football team game result in a box. He doesn’t know if they won or lost until he opens the box. That tells me about the Schroedinger’s knowledge, not the result. Wow, spooky, not. If so, it’s a patently stupid and misleading illustration, in my view, as evidenced by all the published nonsense surrounding it.

Ok, I’m expecting people to inform me I haven’t even begun to comprehend it.

Maybe your hope is futile.

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Precisely unhelpful contribution.
So then how would you know I don’t understand it?

Masters degree in physics, specializing in theoretical physics.

The scientist always talks about what is observed not some imagined actuality. The observed is the only actuality that science will acknowledge. But therein lies a fundamental problem in quantum physics because it leads us to the ultimate conclusion that the act of observation sometimes alters and creates the results which are observed. This isn’t inconsistent because you don’t just observe once and the point is that a second observation can alter what you previously observed.

Correct. It is the interaction with the measuring device which does it. Why does it do such a thing? Because the measuring device is by necessity something which has an enormous number of particles.

Incorrect. A wave is a pattern which emerges from the change of a field over time. That can include the pattern of a moving wave. You are probably used to the example of string where the wave happens because of transverse motion of the string. Sound waves also involves motion of air molecules. But there are other examples which do not involve motion in the field itself but only changes in the field values.

Take for example light, an electromagnetic wave. This is not a wave based on something moving but on the changing values of the electric and magnetic fields. The moving wave, or light, is like I said, just an pattern which you see in the changing values of these fields.

Both and neither. It is a mistake to think one is more correct than the other. If it was really a wave in the classical sense then there would no smallest wave – but there is, the waves are “quantized.” The point here is that you can poke holes in the wave characterization of these things just as well as you have poked holes in the particle characterization of them.

Incorrect. The particle is the wave. For it to be in a particular place at a particular time you have to measure it there. The measurement alters the wave to make it look more like particle at that instant.

I don’t really understand your division into observer and reality. The usual contrast is between whether you put a detector on the slits or not. Without the detector, it goes like a wave through both slits and you get an interference pattern. With the detector it goes like a particle through only one slit and there is no interference pattern.

Schrodinger’s cat is a thought experiment showing the absurdity of thinking this has anything whatsoever to do with the conscious observer. It is the measuring device controlling the release of the poison which causes the decoherence before anybody opens the box, and thus the cat is either dead or alive and not both.

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Very helpful. Let me reorganise my assumptions and rethink them and get back to you with clarifications. You can navigate when you have some bearings you are clear on.

A little joke. Lighten up.

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Aha! Partly similar to waves on the sea - in the sense that the water doesn’t move along the ocean, it’s primarily kinetic energy passing through. (Except there is some real motion of the water.) Which partly explains why a photon has no mass? It’s not a particle of stuff, it’s a changing value in a field.

Yes, ok but… for me this sounds like Pluto didn’t really exist, even if we saw its gravitational effects, until we observed it. Well, it absolutely was somewhere specific, observed or not; acknowledged by science or not. This is my point about observer.

Ok great. That’s us as observers. Important for observer discipline. Dark matter remains theoretical for this reason. Whatever IS causing the effects - dark matter or something else - exists already even if we never figure it out.

Light clearly exists unobserved. I think you’re saying, ‘for us to verify it is in a particular place at a particular time you have to measure it there.’ I’m saying, it is in a particular place at a particular time whether we can measure and acknowledge it or not. Your point is that the fundamental nature of this and our need to verify is fraught because we can’t predict with certainty, we can’t measure without interfering. Correct?

Surely it’s not a difficult thing to stop and conclude that at this stage we cannot predict precisely where a photon or electron will be because of its nature, even though we can predict probabilities. When we tip sand out of a truck, physicists don’t wring their hands because they can’t predict where every grain will land. Behind the truck in a pile is less accurate than quantum mechanics.

Clearest statement I’ve ever read on that topic - particularly combining it with your other statement about the nature of waves.

Let’s see if I’m close: a photon is a changing value in a field caused by a wave function. When observed at a given moment it appears to us like a ‘particle’ - but in reality is the wave itself and partly shares behaviours that we associate with both waves and particles - and we have some predictive mathematical models for behaviour and that’s as close as we can get.

Sorry if this is like talking to a high school student.

Sorry Dale. I did edit my comment earlier to kind of acknowledge it in my second statement and lighten up.

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That’s a good start.

I’m not sure thinking about a particle “manifesting” is the best way to approach the phenomenon. Rather instead I would say that these are useful frameworks for understanding reality - is QFT the best approach for understanding reality and should that be the basis? We aren’t exactly sure… but it works so it is helpful for understanding certain phenomena. But at the end of the day, maybe all that exists are fields and so that’s the best way to think about things.

I will try to comment more later.

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I don’t think it is that simple. Bosonic particles are in general considered field manifestations in the same way as photons but not all are mass-less.

“Particle of stuff” tends to imply composition of smaller particle, which photons certainly are not. You could say that in some sense they are composed of energy (if you want to call that “stuff”), because we can convert a quantity of energy from one form to another, even the energy of motion into a couple of photons.

Two problems with this

  1. I didn’t say that particle doesn’t exist until you observe it. It most certainly does exist – just not in a particular point in space. Saying an abstract quantity doesn’t exist until you measure it is very different than saying a particle or planet doesn’t exist until you measure it.
  2. Quantum sized things are fundamentally different from large bodies like Pluto or even things the size of cats. Quantum objects can be in a superposition (probability distribution) of states. In fact they pretty much have to be because of incompatible measurements. This is even connected to the wave and particle nature of light. The particle represents the measurement of position which forces the photon (or electron etc…) into a superposition of many (infinite actually) values for the momentum. The plane wave represents the measurement of momentum which forces the photon (or electron etc…) into a superpostion of many (infinite number actually) values for the position. This is in turn connected to the uncertainty principle.

“Dark matter” is just a place holder filling in for the fact that we don’t really know what is going on. And it could simply be something missing in our equations for gravity.

NOPE! I am certainly not saying that because it is demonstrably incorrect. The idea that this is simply something we don’t know is hidden variable theory which has been disproved. There are no hidden variables. It is not a position which is unknown. It really is a wave just as it really is particle, and different measurements make it more like one than the other. As a wave it is spread out over space and not in a particular position. As a particle it is the velocity which is spread out over a larger number of values so there is a kind of symmetry between the two – momentum space versus position space.

Look here is a picture of things which might help – one which I devised all myself. It is not without flaws but it may help to get you more in the ballpark of understanding, as it did me. It uses the concept of virtual particles which are supposedly appearing and disappearing everywhere. So the idea is that a real particle gets lost among all these virtual particles because in any instant they are identical so how do you tell the real particle from the virtual particles all around it? The only real difference is that the virtual particles don’t last. But you cannot really say which one will disappear and which will last. So a position measurement simply singles one of these out and makes it last while all the others disappear as virtual particles. (The italics is just to indicate this is a visualization of the science rather than the science itself… and you certainly will not see this in any textbooks)

A photon is a propagating wave pattern in the changing values of electric and magnetic fields with a unit of energy fixed by the wavelength (or frequency). The particulate nature is found in the fact that light comes in those minimum units of energy. The cause is the way electric and magnetic fields interact and create each other – a changing electric field creates a magnetic field and a changing magnetic field creates an electric field. In the more general physics one can represent these particle/waves mathematically in a number of different ways and Schrodinger’s wave functions acted on by his wave equation one of the ways of doing so.

Not at all… This is something which even physicists are wrestling with and that is part of the reason for the quote from Feynman above.


Because it is not understandable. This I know. But it is so. This I know too. Like all of reality when you get down to it. Which has nothing to do with the irrelevant supernatural.

PhD in experimental particle physics

I don’t have time to dig in here, but I want to focus on one issue…

As @mitchellmckain already noted, we don’t have access the the ‘actual’ state. All we have are observations and our models to explain those observations. Within quantum mechanical models there a a number of conceptual entities – the wave function, position and momentum operators, etc – but the ‘actual position of a particle’ isn’t one of them. There simply is no concept with that meaning in the model; it’s a concept imported from models based on our experience with observing macroscopic objects. Because we start building the latter models in infancy and they’re so good at handling everyday macroscopic observations, we tend to think of them as directly reflecting the nature of reality: a baseball is a hard object with a well-defined location, so we think all physical entities must be objects with well-defined location. But macroscopic baseballs, particles, and waves aren’t reality – they’re models of reality, and those models don’t work in all domains.


I thought I should add some other examples of incompatible measurements like this one between position and momentum… each with their own uncertainty principle. I will begin by stating the uncertainty principle.

The uncertainty principle: the product of the uncertainty (smallest error bars for measuring) of two incompatible measurements is about plank’s constant 6.62607004 × 10-34 m2 kg / s. It is pretty small number so this only comes into play when we are talking about very precise measurements of things. But the point is that you cannot even theoretically measure both things simultaneously with absolute precision.

Measurements of the spin of a fermion (like an electron) in orthogonal directions are incompatible with each other. Measuring the spin along the “x axis” (however you define that) is going to put that particle in superposition of states with regards to the two orthogonal directions y and z, so that subsequent measures of the spin will be up or down in those directions with equal probability.

Measurements of the energy of a particle is incompatible with pinpointing the location in time. The corresponding uncertainty principle plays a role in the logic behind virtual particles which basically says that pair of particles with total energy E can pop into existence out of nothing as long as they disappear again in a time given by Plank’s constant divided by this energy.

I can also add that the position momentum uncertainty principle is really three different ones for each of the three orthogonal directions in space. The measurement of position in the x direction is only incompatible with the measurement of momentum in that same direction.

PS. For simplicity sake, I am ignoring some mathematical details which puts a factors of 2 and/or pi in these equations for the uncertainty principle. Though… this can be considered a matter of matching up the units of measurement.

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