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I'm not a physicist either. From what I understand, the parts of quantum mechanics that aren't still fully understood are where it overlaps with relativity. Rec

I'm not a physicist either. From what I understand, the parts of quantum mechanics that aren't still fully understood are where it overlaps with relativity. Reconciling the two is still one of the big open problems, and high energy particle physics does help in testing theories.

sanxiyn 8y ago
When people talk of understanding quantum mechanics, they usually mean measurement problem not quantum gravity. One prevailing attitude toward measurement problem is that it is a non-problem, but I and many others disagree.
codethief 8y ago
Physicist here. > From what I understand, the parts of quantum mechanics that aren't still fully understood are where it overlaps with relativity. Yes and no. You're right, one big question are high-energy completions of the quantum field theories that make up the standard model and which we expect to break down at very high energies. On the one hand, this expectation is due to the fact that general relativity is likely going to play a role at these scales, so we'll probably have to solve the puzzle of quantum gravity along the way. On the other hand, though, there are many more reasons why the quantum field theories we have right now are expected not to be the full story: 1) For reasons of, well, beauty. Right now, the standard model has many free parameters but even if you ignore the parameters for a second, it's also a somewhat random collection of specific gauge theories (the latter is basically another word for quantum field theory), i.e. specific gauge groups. Therefore, people hope that there is an underlying "grand unifying theory" (GUT) that should become visible at high energies. Note that the beauty that comes with a GUT is only in parts what Hossenfelder means when she discusses "beauty". 2) More importantly than 1), the usual interpretation of the renormalization procedure in QFT these days is that it is a way to exclude physics at the very high energy scales from our calculations/predictions. Put differently, without introducing a renormalization scale (basically a "maximum energy") the equations we have blow up and people take this as a sign that our theories are incorrect at extremely high energies. So in this sense, current quantum field theories are nowadays seen as effective theories (which are only valid at low-energy scales) of an underlying "complete" theory which will hopefully cover all energy (and thus length) scales. But apart from the lacking high-energy completion, there are more reasons to believe that we don't fully understand nature yet and that we might need to come up with a new theory: 3) Even after decades of research we're still lacking a rigorous mathematical underpinning for practically all relevant QFTs. Considering that nature so far has always been governed by laws that we could express in precise mathematical terms, this might suggest that we're doing something wrong at a very fundamental level. A lot of people (or at least those I have spoken to) hope that a theory of quantum gravity will solve this (and thereby reestablish mathematical sanity in high-energy physics). 4) Dark matter: As you might now, dark matter is a general name for the apparent stuff that causes our observations to differ slightly from the laws of gravity that we know (i.e. from general relativity). The existence of dark matter doesn't necessarily mean that we'll need an entirely new theory but if we're right in our assumption that dark matter consists of particle excitations of another quantum field (usually called WIMPs), we'll need to at least extend the current standard model. 5) The foundations of quantum mechanics: More specifically, the measurement problem together with the issue of having different interpretations of QM and, possibly, the issue of time. These are basically the issues danbruc meantioned above. For some background info see for instance Steven Weinberg's lecture on "What's the matter with quantum mechanics?" (discussed by Sabine Hossenfelder here: http://backreaction.blogspot.com/2016/11/steven-weinberg-doesnt-like-quantum.html?m=1 http://backreaction.blogspot.com/2016/11/steven-weinberg-doe...) and Carlo Rovelli's recent book (https://news.ycombinator.com/item?id=17376437 https://news.ycombinator.com/item?id=17376437). Again, some people hope that these issues, too, will eventually be solved by a theory of quantum gravity but this is anything but clear. (By the way, a friend of mine working on Bohmian Mechanics recently told me that there might be experiments to distinguish Bohm's interpretation from the Copenhagen one, so we might not even need to wait for quantum gravity.) Anyway, let's assume for a second that we really need a full quantum theory of gravity to solve all these issues. Then, what's our best shot at coming up with a theory of quantum gravity? Well, studying the high-energy behavior of particles and hoping to discover "new physics". So in this sense, you were absolutely right of course. I just wanted to point out that it's not reconciling relativity and quantum mechanics alone that drives us.
- gfodor 8y ago
  (Non-physicist, please bear with me) Of these 5 criticisms of contemporary theory, all of them except dark matter seem to stem from a suspicion about the eventual result through the lens of beauty, unification, etc. However, the dark matter problem stands out as one where measurements do not line up with theory, and seems more of a classic example of how we've typically "turned the crank" on making progress in the past. How big is the concerted effort to understand dark matter vs trying to attack these other concerns in theoretical physics? It certainly seems like an aggressive, field-wide effort is warranted given it's one of the few universally acknowledged and reproducible blatant predictive errors in current models. The discovery of these errors seems like a huge gift to physicists.
  - danbruc 8y ago
    Of these 5 criticisms of contemporary theory, all of them except dark matter seem to stem from a suspicion about the eventual result through the lens of beauty, unification, etc. I don't think it is really justified to label it this way. We know that quantum field theory is not the correct final theory because it breaks down at short lengths and high energies. It is in some sense most likely even the completely wrong way for looking at the problem. Quantum field theories are mathematical tools to deal with many particle systems and they have some nice properties like making locality manifest, they however also force things onto us that are non-physical or hard to work with, for example gauge symmetries and virtual particles. So even if there were no four different forces that could maybe be unified into one, there would still be a lot of issues with quantum field theory. However, the dark matter problem stands out as one where measurements do not line up with theory, and seems more of a classic example of how we've typically "turned the crank" on making progress in the past. It is of course true that dark matter is in a certain sense a more tangible problem, but on the other hand there is the problem that there are so many different suggested resolutions. Astronomical and cosmological observations may be the way to go for modified gravity or small black holes, particle detector experiments may be the way for weakly interacting massive particles, and each other resolution requires probably its own set of experiments. So that problem seems not so much that nobody is investing the effort but that we are not really sure where to look and for what. And in case of things like weakly interacting massive particles we somewhat come back to extending the standard model because such extensions may better inform us where to look.
danbruc 8y ago
I would disagree, I think we do not understand quantum mechanics at all. We have the equations and get the correct results, but we still don't know what's really behind measurements, how entanglement can be non-local, we are not even agreeing on whether time evolution is unitary or not.
- BeetleB 8y ago
  >We have the equations and get the correct results, but we still don't know what's really behind measurements That's pretty much true for classical physics as well.
  - danbruc 8y ago
    In classical physics there is no limit to how gentle a measurement can be, you can essentially measure everything without perturbation and to arbitrary precision, so measurements are not really a concern. Or am I misunderstanding what you wanted to hint at?
    - fjsolwmv 8y ago
      Measurements aren't a concern, but what's "behind" them is unknown.
      - danbruc 8y ago
        What do you think is missing in classical physics? There are physical objects with real properties an we are determining their values by carefully interacting with the physical object using a measurement device. If we, for example, want to know the position of an electron - and in classical physics the electron is just a classical particle with a definite position - we carefully throw something, for example photons, into the rough direction of the electron and look at the scattering result to find out where the electron is. I don't see what this picture is missing.
    - BeetleB 8y ago
      I'm referring to the notion that we understand classical mechanics beyond merely "having the equations and getting correct results" - we really do not. Because classical mechanics prevails on the scale that our brains operate on, we do not find it unintuitive. However, at the end of the day, it is merely that we have a bunch of equations that give us correct results - just as quantum mechanics does. This is true of all of physics - reducing observations to the level of predictability. Beyond that we are wading into philosophy. (BTW, classical physics has warts as well - what is the force between a charge of 1 Coulomb and -1 Coulomb when they are touching each other?)

sanxiyn 8y ago

When people talk of understanding quantum mechanics, they usually mean measurement problem not quantum gravity. One prevailing attitude toward measurement problem is that it is a non-problem, but I and many others disagree.

codethief 8y ago

Physicist here. > From what I understand, the parts of quantum mechanics that aren't still fully understood are where it overlaps with relativity. Yes and no. You're right, one big question are high-energy completions of the quantum field theories that make up the standard model and which we expect to break down at very high energies. On the one hand, this expectation is due to the fact that general relativity is likely going to play a role at these scales, so we'll probably have to solve the puzzle of quantum gravity along the way. On the other hand, though, there are many more reasons why the quantum field theories we have right now are expected not to be the full story: 1) For reasons of, well, beauty. Right now, the standard model has many free parameters but even if you ignore the parameters for a second, it's also a somewhat random collection of specific gauge theories (the latter is basically another word for quantum field theory), i.e. specific gauge groups. Therefore, people hope that there is an underlying "grand unifying theory" (GUT) that should become visible at high energies. Note that the beauty that comes with a GUT is only in parts what Hossenfelder means when she discusses "beauty". 2) More importantly than 1), the usual interpretation of the renormalization procedure in QFT these days is that it is a way to exclude physics at the very high energy scales from our calculations/predictions. Put differently, without introducing a renormalization scale (basically a "maximum energy") the equations we have blow up and people take this as a sign that our theories are incorrect at extremely high energies. So in this sense, current quantum field theories are nowadays seen as effective theories (which are only valid at low-energy scales) of an underlying "complete" theory which will hopefully cover all energy (and thus length) scales. But apart from the lacking high-energy completion, there are more reasons to believe that we don't fully understand nature yet and that we might need to come up with a new theory: 3) Even after decades of research we're still lacking a rigorous mathematical underpinning for practically all relevant QFTs. Considering that nature so far has always been governed by laws that we could express in precise mathematical terms, this might suggest that we're doing something wrong at a very fundamental level. A lot of people (or at least those I have spoken to) hope that a theory of quantum gravity will solve this (and thereby reestablish mathematical sanity in high-energy physics). 4) Dark matter: As you might now, dark matter is a general name for the apparent stuff that causes our observations to differ slightly from the laws of gravity that we know (i.e. from general relativity). The existence of dark matter doesn't necessarily mean that we'll need an entirely new theory but if we're right in our assumption that dark matter consists of particle excitations of another quantum field (usually called WIMPs), we'll need to at least extend the current standard model. 5) The foundations of quantum mechanics: More specifically, the measurement problem together with the issue of having different interpretations of QM and, possibly, the issue of time. These are basically the issues danbruc meantioned above. For some background info see for instance Steven Weinberg's lecture on "What's the matter with quantum mechanics?" (discussed by Sabine Hossenfelder here: http://backreaction.blogspot.com/2016/11/steven-weinberg-doesnt-like-quantum.html?m=1 http://backreaction.blogspot.com/2016/11/steven-weinberg-doe...) and Carlo Rovelli's recent book (https://news.ycombinator.com/item?id=17376437 https://news.ycombinator.com/item?id=17376437). Again, some people hope that these issues, too, will eventually be solved by a theory of quantum gravity but this is anything but clear. (By the way, a friend of mine working on Bohmian Mechanics recently told me that there might be experiments to distinguish Bohm's interpretation from the Copenhagen one, so we might not even need to wait for quantum gravity.) Anyway, let's assume for a second that we really need a full quantum theory of gravity to solve all these issues. Then, what's our best shot at coming up with a theory of quantum gravity? Well, studying the high-energy behavior of particles and hoping to discover "new physics". So in this sense, you were absolutely right of course. I just wanted to point out that it's not reconciling relativity and quantum mechanics alone that drives us.

gfodor 8y ago
(Non-physicist, please bear with me) Of these 5 criticisms of contemporary theory, all of them except dark matter seem to stem from a suspicion about the eventual result through the lens of beauty, unification, etc. However, the dark matter problem stands out as one where measurements do not line up with theory, and seems more of a classic example of how we've typically "turned the crank" on making progress in the past. How big is the concerted effort to understand dark matter vs trying to attack these other concerns in theoretical physics? It certainly seems like an aggressive, field-wide effort is warranted given it's one of the few universally acknowledged and reproducible blatant predictive errors in current models. The discovery of these errors seems like a huge gift to physicists.
- danbruc 8y ago
  Of these 5 criticisms of contemporary theory, all of them except dark matter seem to stem from a suspicion about the eventual result through the lens of beauty, unification, etc. I don't think it is really justified to label it this way. We know that quantum field theory is not the correct final theory because it breaks down at short lengths and high energies. It is in some sense most likely even the completely wrong way for looking at the problem. Quantum field theories are mathematical tools to deal with many particle systems and they have some nice properties like making locality manifest, they however also force things onto us that are non-physical or hard to work with, for example gauge symmetries and virtual particles. So even if there were no four different forces that could maybe be unified into one, there would still be a lot of issues with quantum field theory. However, the dark matter problem stands out as one where measurements do not line up with theory, and seems more of a classic example of how we've typically "turned the crank" on making progress in the past. It is of course true that dark matter is in a certain sense a more tangible problem, but on the other hand there is the problem that there are so many different suggested resolutions. Astronomical and cosmological observations may be the way to go for modified gravity or small black holes, particle detector experiments may be the way for weakly interacting massive particles, and each other resolution requires probably its own set of experiments. So that problem seems not so much that nobody is investing the effort but that we are not really sure where to look and for what. And in case of things like weakly interacting massive particles we somewhat come back to extending the standard model because such extensions may better inform us where to look.

danbruc 8y ago

I would disagree, I think we do not understand quantum mechanics at all. We have the equations and get the correct results, but we still don't know what's really behind measurements, how entanglement can be non-local, we are not even agreeing on whether time evolution is unitary or not.

BeetleB 8y ago
>We have the equations and get the correct results, but we still don't know what's really behind measurements That's pretty much true for classical physics as well.
- danbruc 8y ago
  In classical physics there is no limit to how gentle a measurement can be, you can essentially measure everything without perturbation and to arbitrary precision, so measurements are not really a concern. Or am I misunderstanding what you wanted to hint at?
  - fjsolwmv 8y ago
    Measurements aren't a concern, but what's "behind" them is unknown.
    - danbruc 8y ago
      What do you think is missing in classical physics? There are physical objects with real properties an we are determining their values by carefully interacting with the physical object using a measurement device. If we, for example, want to know the position of an electron - and in classical physics the electron is just a classical particle with a definite position - we carefully throw something, for example photons, into the rough direction of the electron and look at the scattering result to find out where the electron is. I don't see what this picture is missing.
  - BeetleB 8y ago
    I'm referring to the notion that we understand classical mechanics beyond merely "having the equations and getting correct results" - we really do not. Because classical mechanics prevails on the scale that our brains operate on, we do not find it unintuitive. However, at the end of the day, it is merely that we have a bunch of equations that give us correct results - just as quantum mechanics does. This is true of all of physics - reducing observations to the level of predictability. Beyond that we are wading into philosophy. (BTW, classical physics has warts as well - what is the force between a charge of 1 Coulomb and -1 Coulomb when they are touching each other?)