7 ms·

Neutrons on Classically Inexplicable Paths

31 points by anigbrowl 2y ago

ThinkBeat 2y ago
I loved this theory when I first heard about it. It makes intuitive sense to me, even though it is hardly possible from some perspectives with what we know today. The more I read the more interesting it gets.
- api 2y ago
  AFAIK the concept of a “particle” is kind of a construct, or at least every bit as much as a “wave.” What’s really happening down there is whatever the math says is happening and it does not have direct physical analogies to much of anything we experience microscopically on a day to day basis.
  - russdill 2y ago
    It's a little more concrete for things that are not elementary particles such as neurons
  - effie 2y ago
    > What’s really happening down there is whatever the math says is happening But math isn't saying what is happening in the real world. It's a model, a fake simplified world. The map is not the territory.
    - kadoban 2y ago
      We're really looking at two (or more) different maps here, not map vs territory. One of the maps is hey there's these little balls called particles, they bounce around and interact in ways X, Y, Z, etc. Another is a big page of math that involves excitations of fields that interact. The point is that _this_ one is the best we currently have, but it doesn't let you simply, intuitively and correctly talk about particles interacting (or other things we intuitively want to understand the world in terms of), because those aren't real in the model. They're just an abstraction that usually works at the scale we experience day-to-day. So yeah, the territory is the territory and neither of these are it, but even getting past that, our best map is just a bunch of math that you just have to take at face value if you want to be as correct as our maps currently allow. This is meant to address questions like "is light a particle or a wave?" The answer is: No. It's (according to our best model/map) this bunch of math that doesn't correspond to either one of those, have fun calculating.
munchler 2y ago
Does this rule out pilot wave theory, in which a particle always has a definite position at all times?
- deleted 2y ago
  [deleted]
- Filligree 2y ago
  No, but pilot wave theory is a bit weird since the wave contains all the same decoherent timelines as in many-worlds. They can both be true. Then there’s a single timeline which happens to have particles following it around, but everything else is still there and there’s no way from the inside to determine where the particles are, other than the assumption that only the particles create conscious scientists. But that’s a really weird assumption.
gus_massa 2y ago
Nice experiment, but they are still measuring neutrons, that are quite small. I don't understand why they try to sell this as a macroscopic experiment. Bell inecuality can be also measured at long distances, but it doesn't make it a macroscopic experiment.
- pottspotts 2y ago
  I would argue that the neutrino is relatively large compared to the quanta typical of these kinds of tests, but in the abstract they suggested a basketball!
  - gus_massa 2y ago
    I think most test of bell inequality are made using photons. I'm not sure about this inequality. Neutrons are much bigger than photons [1], but somewhat similar experiments of interference have been made with helium atoms and even small molecules (with 10 atoms or so). So if they say it's big in the quantum word, I expect at least a molecule. And the article saying macroscopic everywhere really sugest basketballs. From the initial description, I was optimistic and I was expecting small dots of "dust", or a tiny thin membrane. There are some experimental results with extremly small object, but I guess they have a lot of thechnical problems and are difficult to use them in complex measurements. [1] Ignoring a lot of technical details.
  - drdeca 2y ago
    Neutrino ≠ neutron. The “ino” in neutrino is like a diminutive. Neutrons are much heavier than a neutrino. I don’t think they have a positive lower bound on the size of the smallest mass eigenstate of neutrinos (though they do on the other two).
    - gus_massa 2y ago
      I agree. (I missed the "ino".) It's also important that (as far as we know) the neutrino is an elementary particle, but the neutron is composed by three real quarks and a lot of virtual quarks and gluons, it's a huge mess of smaller particles together.
ziofill 2y ago
Physicist working on QM here. At the macroscopic scale we are led to believe that the properties of a system have a value regardless of whether that value is “known” externally to the system. Then a measurement process would simply reveal the pre-existing value. But we can concoct a QM system such that it is impossible to assign a pre-existing value of a property in a consistent way (look for GHZ game to know more). This leads to true randomness in the most literal sense possible: there can exist no cause for a given value. That value materialized out of thin air.
- hilbert42 2y ago
  "…we are led to believe…" "That value materialized out of thin air." Then, you are questioning the whole premise?
  - Filligree 2y ago
    This article describes one in a long, long series of experiments demonstrating that particles don’t exist and our intuition is simply wrong. Imagine a gigantic, epic mountain of evidence tall enough to crush the resistance of even the most recalcitrant physicists. Particles are not real. Quantum fields are, which contain excitations, which because of conservation laws cannot simply disappear on their own and therefore in many common circumstances appear to persist and maintain an identity. There’s our “particles”. But it isn’t hard at all to devise an experiment that breaks our intuition like a dried up stick.
    - hilbert42 2y ago
      I'm not in QM but electronics, still I accept QFT on evidence. Had to give up intuition/bad training when coming to grips with Aharonov–Bohm and related. Still, on bad days when (4kRTΔf)^0.5 or such gets in my way, I default and accuse my noisy little 'charges' of being too big and that they rattle around like large ball-bearings in an empty oil drum.
fallingfrog 2y ago
There are many kinds of quantum weirdness. One of them is this: we think classically of the past as definite and the future in terms of possibilities- but in reality both the future and the past are indeterminate in exactly the same way (it’s just that typically there are a lot more possible futures for the current state than pasts, because of entropy). In fact you could define the past as being the direction in configuration space with the smallest uncertainty, and the future as being the direction with the most uncertainty. In this view, at the Big Bang since there is no direction with less uncertainty, there is no past at that point. That’s not the only quantum weirdness, but it’s a big one.
- fallingfrog 2y ago
  And, another big quantum weirdness is that matter has not only a probability but also a phase angle, so different possibilities can add but they can also cancel out, which is not how classical probability works where probabilities always add together.
- munksbeer 2y ago
  > it’s just that typically there are a lot more possible futures for the current state than pasts, because of entropy You've either just read "The Order of Time" or you already know a bit about this field. I'm reading it at the moment, as a non-physicist. Your statement above isn't clear to me. Could you simplify why "the future" has a lot more states than the past, because of entropy? Isn't this circular?
- effie 2y ago
  > we think classically of the past as definite and the future in terms of possibilities- but in reality both the future and the past are indeterminate in exactly the same way This has nothing to do with quantum theory. It's from classical physics already, from chaotic systems. For conservative systems, both predicting and retrodicting them is equally hard, due to high sensitivity of both to initial condition which we do not know exactly. > it’s just that typically there are a lot more possible futures for the current state than pasts, because of entropy This holds only in special cases, like when entropy of an isolated system increases (e.g. evolution towards equilibrium). When the system gets into equilibrium, entropy then remains constant, and both past macrostates and future macrostates have the same number of microstates, so for any state after that, "the number of futures" is the same as "the number of pasts". > In fact you could define the past as being the direction in configuration space with the smallest uncertainty In other words, you suggest to define future as that direction in which entropy increases. This is consistent with the usual meaning of future for an isolated thermodynamic system in a non-equilibrium process, evolving towards equilibrium; 2nd law implies entropy cannot decrease when comparing to the past macrostate, and entropy often increases when the new macrostate is reached. But this is not usable definition in general. Earth, Solar System, Galaxy interact with their neighborhood; they are not such isolated systems in a macrostate, moving towards equilibrium. We do not know how to measure their entropy, to find out whether they move towards past or future. And if we somehow found entropy of Earth decreases, nobody would seriously suggest Earth is moving towards the past. Instead, everything moves towards the future, just by definition, because that is how we think for ages. If spilled milk somewhere is observed to jump back into a cup, nobody will think the milk went back in time; instead, we will say that a very improbable thing just happened, violating prior experience and 2nd law, but that process would happen in time just as everything else, from the past towards the future. Redefining "future" as that direction of a process in which "entropy is increasing" is really an unnecessary and flawed idea of future. We knew what future was before people invented kinetic theory and entropy; it's those events which we approach, anticipate, and when they come, we label them with a number for date and time on the clock, which (if it is running correctly) never decreases, only stays the same or increases. What entropy did in the meantime does not matter at all; a good clock does not care.
scotty79 2y ago
Centimeter sized neutron beams are not really macroscopic because neutrons in them are not interacting with each other. Macroscopic objects consists of huge number of elementary particles that connect through interactions.
scotty79 2y ago
It is my belief that there is no reason to believe that elementary particles are ever point-like objects with momentum and location. They are fuzzy objects that always evolve in time according to wave equation. It's just that in case of very narrow, sharp waves they equation simplifies to Newtonian (or special relativity) equation of motion. Measurement is just exchange of momentum and energy between that fuzzy objects that happens very quickly and in quantized amount and the equations governing it simplify for narrow, sharp waves to equations of two balls bouncing of each other. Our intuitions developed for macroscopic objects which are a very peculiar form of matter, tightly bound by interactions, which make the constituent elementary particles very sharp and narrow all the time. They are just wrong for anything else. Newtonian mechanics is attractive because it's simple but it's equivalent of gas laws, that while very simple, only statistically capture the complexities of what actually happens in ideal gas and don't reflect nothing physically real.

31 points by anigbrowl 2y ago

I loved this theory when I first heard about it. It makes intuitive sense to me, even though it is hardly possible from some perspectives with what we know today. The more I read the more interesting it gets.

api 2y ago
AFAIK the concept of a “particle” is kind of a construct, or at least every bit as much as a “wave.” What’s really happening down there is whatever the math says is happening and it does not have direct physical analogies to much of anything we experience microscopically on a day to day basis.
- russdill 2y ago
  It's a little more concrete for things that are not elementary particles such as neurons
- effie 2y ago
  > What’s really happening down there is whatever the math says is happening But math isn't saying what is happening in the real world. It's a model, a fake simplified world. The map is not the territory.
  - kadoban 2y ago
    We're really looking at two (or more) different maps here, not map vs territory. One of the maps is hey there's these little balls called particles, they bounce around and interact in ways X, Y, Z, etc. Another is a big page of math that involves excitations of fields that interact. The point is that _this_ one is the best we currently have, but it doesn't let you simply, intuitively and correctly talk about particles interacting (or other things we intuitively want to understand the world in terms of), because those aren't real in the model. They're just an abstraction that usually works at the scale we experience day-to-day. So yeah, the territory is the territory and neither of these are it, but even getting past that, our best map is just a bunch of math that you just have to take at face value if you want to be as correct as our maps currently allow. This is meant to address questions like "is light a particle or a wave?" The answer is: No. It's (according to our best model/map) this bunch of math that doesn't correspond to either one of those, have fun calculating.

munchler 2y ago

Does this rule out pilot wave theory, in which a particle always has a definite position at all times?

deleted 2y ago
[deleted]
Filligree 2y ago
No, but pilot wave theory is a bit weird since the wave contains all the same decoherent timelines as in many-worlds. They can both be true. Then there’s a single timeline which happens to have particles following it around, but everything else is still there and there’s no way from the inside to determine where the particles are, other than the assumption that only the particles create conscious scientists. But that’s a really weird assumption.

gus_massa 2y ago

Nice experiment, but they are still measuring neutrons, that are quite small. I don't understand why they try to sell this as a macroscopic experiment. Bell inecuality can be also measured at long distances, but it doesn't make it a macroscopic experiment.

pottspotts 2y ago
I would argue that the neutrino is relatively large compared to the quanta typical of these kinds of tests, but in the abstract they suggested a basketball!
- gus_massa 2y ago
  I think most test of bell inequality are made using photons. I'm not sure about this inequality. Neutrons are much bigger than photons [1], but somewhat similar experiments of interference have been made with helium atoms and even small molecules (with 10 atoms or so). So if they say it's big in the quantum word, I expect at least a molecule. And the article saying macroscopic everywhere really sugest basketballs. From the initial description, I was optimistic and I was expecting small dots of "dust", or a tiny thin membrane. There are some experimental results with extremly small object, but I guess they have a lot of thechnical problems and are difficult to use them in complex measurements. [1] Ignoring a lot of technical details.
- drdeca 2y ago
  Neutrino ≠ neutron. The “ino” in neutrino is like a diminutive. Neutrons are much heavier than a neutrino. I don’t think they have a positive lower bound on the size of the smallest mass eigenstate of neutrinos (though they do on the other two).
  - gus_massa 2y ago
    I agree. (I missed the "ino".) It's also important that (as far as we know) the neutrino is an elementary particle, but the neutron is composed by three real quarks and a lot of virtual quarks and gluons, it's a huge mess of smaller particles together.

ziofill 2y ago

Physicist working on QM here. At the macroscopic scale we are led to believe that the properties of a system have a value regardless of whether that value is “known” externally to the system. Then a measurement process would simply reveal the pre-existing value. But we can concoct a QM system such that it is impossible to assign a pre-existing value of a property in a consistent way (look for GHZ game to know more). This leads to true randomness in the most literal sense possible: there can exist no cause for a given value. That value materialized out of thin air.

hilbert42 2y ago
"…we are led to believe…" "That value materialized out of thin air." Then, you are questioning the whole premise?
- Filligree 2y ago
  This article describes one in a long, long series of experiments demonstrating that particles don’t exist and our intuition is simply wrong. Imagine a gigantic, epic mountain of evidence tall enough to crush the resistance of even the most recalcitrant physicists. Particles are not real. Quantum fields are, which contain excitations, which because of conservation laws cannot simply disappear on their own and therefore in many common circumstances appear to persist and maintain an identity. There’s our “particles”. But it isn’t hard at all to devise an experiment that breaks our intuition like a dried up stick.
  - hilbert42 2y ago
    I'm not in QM but electronics, still I accept QFT on evidence. Had to give up intuition/bad training when coming to grips with Aharonov–Bohm and related. Still, on bad days when (4kRTΔf)^0.5 or such gets in my way, I default and accuse my noisy little 'charges' of being too big and that they rattle around like large ball-bearings in an empty oil drum.

fallingfrog 2y ago

There are many kinds of quantum weirdness. One of them is this: we think classically of the past as definite and the future in terms of possibilities- but in reality both the future and the past are indeterminate in exactly the same way (it’s just that typically there are a lot more possible futures for the current state than pasts, because of entropy). In fact you could define the past as being the direction in configuration space with the smallest uncertainty, and the future as being the direction with the most uncertainty. In this view, at the Big Bang since there is no direction with less uncertainty, there is no past at that point. That’s not the only quantum weirdness, but it’s a big one.

fallingfrog 2y ago
And, another big quantum weirdness is that matter has not only a probability but also a phase angle, so different possibilities can add but they can also cancel out, which is not how classical probability works where probabilities always add together.
munksbeer 2y ago
> it’s just that typically there are a lot more possible futures for the current state than pasts, because of entropy You've either just read "The Order of Time" or you already know a bit about this field. I'm reading it at the moment, as a non-physicist. Your statement above isn't clear to me. Could you simplify why "the future" has a lot more states than the past, because of entropy? Isn't this circular?
effie 2y ago
> we think classically of the past as definite and the future in terms of possibilities- but in reality both the future and the past are indeterminate in exactly the same way This has nothing to do with quantum theory. It's from classical physics already, from chaotic systems. For conservative systems, both predicting and retrodicting them is equally hard, due to high sensitivity of both to initial condition which we do not know exactly. > it’s just that typically there are a lot more possible futures for the current state than pasts, because of entropy This holds only in special cases, like when entropy of an isolated system increases (e.g. evolution towards equilibrium). When the system gets into equilibrium, entropy then remains constant, and both past macrostates and future macrostates have the same number of microstates, so for any state after that, "the number of futures" is the same as "the number of pasts". > In fact you could define the past as being the direction in configuration space with the smallest uncertainty In other words, you suggest to define future as that direction in which entropy increases. This is consistent with the usual meaning of future for an isolated thermodynamic system in a non-equilibrium process, evolving towards equilibrium; 2nd law implies entropy cannot decrease when comparing to the past macrostate, and entropy often increases when the new macrostate is reached. But this is not usable definition in general. Earth, Solar System, Galaxy interact with their neighborhood; they are not such isolated systems in a macrostate, moving towards equilibrium. We do not know how to measure their entropy, to find out whether they move towards past or future. And if we somehow found entropy of Earth decreases, nobody would seriously suggest Earth is moving towards the past. Instead, everything moves towards the future, just by definition, because that is how we think for ages. If spilled milk somewhere is observed to jump back into a cup, nobody will think the milk went back in time; instead, we will say that a very improbable thing just happened, violating prior experience and 2nd law, but that process would happen in time just as everything else, from the past towards the future. Redefining "future" as that direction of a process in which "entropy is increasing" is really an unnecessary and flawed idea of future. We knew what future was before people invented kinetic theory and entropy; it's those events which we approach, anticipate, and when they come, we label them with a number for date and time on the clock, which (if it is running correctly) never decreases, only stays the same or increases. What entropy did in the meantime does not matter at all; a good clock does not care.

scotty79 2y ago

Centimeter sized neutron beams are not really macroscopic because neutrons in them are not interacting with each other. Macroscopic objects consists of huge number of elementary particles that connect through interactions.

scotty79 2y ago

It is my belief that there is no reason to believe that elementary particles are ever point-like objects with momentum and location. They are fuzzy objects that always evolve in time according to wave equation. It's just that in case of very narrow, sharp waves they equation simplifies to Newtonian (or special relativity) equation of motion. Measurement is just exchange of momentum and energy between that fuzzy objects that happens very quickly and in quantized amount and the equations governing it simplify for narrow, sharp waves to equations of two balls bouncing of each other. Our intuitions developed for macroscopic objects which are a very peculiar form of matter, tightly bound by interactions, which make the constituent elementary particles very sharp and narrow all the time. They are just wrong for anything else. Newtonian mechanics is attractive because it's simple but it's equivalent of gas laws, that while very simple, only statistically capture the complexities of what actually happens in ideal gas and don't reflect nothing physically real.