4 ms·

Can you be more precise about what you mean by "objective reality"? I would say that QM shows the world is not classical, but it doesn't say there's no objecti

Can you be more precise about what you mean by "objective reality"? I would say that QM shows the world is not classical, but it doesn't say there's no objective reality: the predictions it makes about what we observe (reality) are extremely reliable and accurate (i.e. objective). Yes, those predictions are just probabilistic for any single system, but when you have a lot of systems the probability that you will observe a specific outcome (to within observational error) can approach 1. A lot of our technology, such as lasers, transistors, etc., relies on this. I don't see how you make sense of that while denying there's objective reality.

d4rkn0d3z 1y ago
I mean there is no perspective from which one can obtain a view of all properties of all systems that will not be invalid to another observer.
- platz 1y ago
  if your definition requires universal observer agreement you already have that issue with special relativity / light cones / the spacetime metric. many worlds posits a single universal quantum state it's just only partially accessible to observers, which is different from saying that it simply doesn't objectively exist. maybe it depends on your definition of objective
- griffzhowl 1y ago
  In my view there is such a perspective: quantum mechanics. So far as we know its predictions are valid for all observers. But what that means is that we have to readjust our classical conceptions about what a "property of a system" is. The word "property" in general is just a logical concept, and doesn't carry any intrinsic ontological implications. There can be mathematical properties, physical properties, properties of thoughts and dreams etc., and this way of talking about things doesn't by itself imply any specific ontological interpretation. It's just a feature of the structure of language. About physical properties specifically, if we derive our concept of physical property from quantum mechanics, instead of trying to retain the inadequate classical meaning, then physical properties are exactly those represented by the state vector: e.g. its projections on to each of the basis vectors corresponding to some observable. True, as is well-known from the Bell and Kochen-Specker theorems, we can't consistently say that a quantum state has some specific value of its observables independently of interactions with other systems, but this is just the classical conception of a physical property (formalized, e.g., by a real-valued function on phase space). But quantum mechanics doesn't thereby force us to say that a physical system has no definite properties. Instead, we can reconfigure our conception of physical property to make it compatible with quantum mechanics. Then in general the properties of quantum states are probabilistic (at least some of them - the dimension of its state space, for example, is not), but the theory unambiguously assigns to a state the probabilities that the various possible measurement outcomes will be observed. These probabilities are among that state's properties, and all indications are that these probabilities are objective features of the state, independently of our ways of representing the state. In fact the dependence goes in the other direction: this (objectively) probabilistic character of quantum states (among other things, like the quantization of energy exchanges) is what forced us to change the way we think of physical states.
- fellowniusmonk 1y ago
  No objective reality or no single universal frame? Reproducable reality from x frame seems non-arbitrary if not objective.
- GoblinSlayer 1y ago
  That phenomenon was known in classical physics since 500 BCE: https://en.wikipedia.org/wiki/Blind_men_and_an_elephant https://en.wikipedia.org/wiki/Blind_men_and_an_elephant
ridgeguy 1y ago
Amateur here, certainly. But I recall that one of the two consequences of Bell's inequality (shown to be valid AFAIK) is that there isn't an "objective reality". Kind of like nature makes it up depending on what the observer is up to. Yes? No? Maybe?
- griffzhowl 1y ago
  Not really, no, but I can see why you might think that, because Bell's theorem is often described as saying that quantum mechanics contradicts "local realism", but "realism" in this sense has a precise technical meaning, which is that a physical system at some time has a complete set of well-defined values for all the possible measurements that we might do on it (and "local" just means that physical effects can't propagate faster than the speed of light). It was known since the beginning that quantum mechanics, as a theory, doesn't have this feature because of complementary observables, aka the uncertainty principle, which says that the more precisely some quantities are known, the less precisely the theory determines other quantities, e.g. in the case of a spin-1/2 particle, knowing the value of its spin along one axis means that it's value along any axis at right-angles is completely uncertain - it can be up or down with equal probability. Nevertheless, it was thought (e.g. in the Einstein Podolsky Rosen paper in 1935) that it might be possible to formulate a theory that could reproduce all the correct predictions of quantum mechanics, while also ascribing simultaneous well-defined values to all the physical quantities possessed by a quantum system, i.e a locally realistic theory. These are also known as local "hidden variable" theories, where the idea was that some of the values of the variables might be unobserved simply because of measurement practicalities - we can't measure the spin of a particle along two orthogonal axes simultaneously because the measurement needs a magnetic field gradient along the direction we're measuring in. Bell derived an inequality that any locally realistic theory must satisfy, and showed that quantum mechanics in fact violates this inequality, so no locally realistic theory can reproduce the predictions of quantum mechanics. Alain Aspect and others later implemented Bell's thought experiment in the lab and showed that the physical world obeys quantum mechanics, and so is not describable by a locally realistic theory. In my view, none of that shows that there is "no objective reality". Rather, it shows that objective reality is as far as we can tell quantum mechanical, and not locally realistic in the sense described above. It's certainly the case that quantum mechanics requires a modification of the classical concepts of reality, i.e. of classical ideas about what a physical system is, but you would only accept that conclusion if you agree that quantum mechanics is telling you something objective about reality... At least according to how I understand those words. So I think what people really mean when they say quantum mechanics shows there's no objective reality is just that it contradicts classical conceptions of physical systems, which is clearly true but sounds less sexy and mysterious.

d4rkn0d3z 1y ago

I mean there is no perspective from which one can obtain a view of all properties of all systems that will not be invalid to another observer.

platz 1y ago
if your definition requires universal observer agreement you already have that issue with special relativity / light cones / the spacetime metric. many worlds posits a single universal quantum state it's just only partially accessible to observers, which is different from saying that it simply doesn't objectively exist. maybe it depends on your definition of objective
griffzhowl 1y ago
In my view there is such a perspective: quantum mechanics. So far as we know its predictions are valid for all observers. But what that means is that we have to readjust our classical conceptions about what a "property of a system" is. The word "property" in general is just a logical concept, and doesn't carry any intrinsic ontological implications. There can be mathematical properties, physical properties, properties of thoughts and dreams etc., and this way of talking about things doesn't by itself imply any specific ontological interpretation. It's just a feature of the structure of language. About physical properties specifically, if we derive our concept of physical property from quantum mechanics, instead of trying to retain the inadequate classical meaning, then physical properties are exactly those represented by the state vector: e.g. its projections on to each of the basis vectors corresponding to some observable. True, as is well-known from the Bell and Kochen-Specker theorems, we can't consistently say that a quantum state has some specific value of its observables independently of interactions with other systems, but this is just the classical conception of a physical property (formalized, e.g., by a real-valued function on phase space). But quantum mechanics doesn't thereby force us to say that a physical system has no definite properties. Instead, we can reconfigure our conception of physical property to make it compatible with quantum mechanics. Then in general the properties of quantum states are probabilistic (at least some of them - the dimension of its state space, for example, is not), but the theory unambiguously assigns to a state the probabilities that the various possible measurement outcomes will be observed. These probabilities are among that state's properties, and all indications are that these probabilities are objective features of the state, independently of our ways of representing the state. In fact the dependence goes in the other direction: this (objectively) probabilistic character of quantum states (among other things, like the quantization of energy exchanges) is what forced us to change the way we think of physical states.
fellowniusmonk 1y ago
No objective reality or no single universal frame? Reproducable reality from x frame seems non-arbitrary if not objective.
GoblinSlayer 1y ago
That phenomenon was known in classical physics since 500 BCE: https://en.wikipedia.org/wiki/Blind_men_and_an_elephant https://en.wikipedia.org/wiki/Blind_men_and_an_elephant

ridgeguy 1y ago

Amateur here, certainly. But I recall that one of the two consequences of Bell's inequality (shown to be valid AFAIK) is that there isn't an "objective reality". Kind of like nature makes it up depending on what the observer is up to. Yes? No? Maybe?

griffzhowl 1y ago
Not really, no, but I can see why you might think that, because Bell's theorem is often described as saying that quantum mechanics contradicts "local realism", but "realism" in this sense has a precise technical meaning, which is that a physical system at some time has a complete set of well-defined values for all the possible measurements that we might do on it (and "local" just means that physical effects can't propagate faster than the speed of light). It was known since the beginning that quantum mechanics, as a theory, doesn't have this feature because of complementary observables, aka the uncertainty principle, which says that the more precisely some quantities are known, the less precisely the theory determines other quantities, e.g. in the case of a spin-1/2 particle, knowing the value of its spin along one axis means that it's value along any axis at right-angles is completely uncertain - it can be up or down with equal probability. Nevertheless, it was thought (e.g. in the Einstein Podolsky Rosen paper in 1935) that it might be possible to formulate a theory that could reproduce all the correct predictions of quantum mechanics, while also ascribing simultaneous well-defined values to all the physical quantities possessed by a quantum system, i.e a locally realistic theory. These are also known as local "hidden variable" theories, where the idea was that some of the values of the variables might be unobserved simply because of measurement practicalities - we can't measure the spin of a particle along two orthogonal axes simultaneously because the measurement needs a magnetic field gradient along the direction we're measuring in. Bell derived an inequality that any locally realistic theory must satisfy, and showed that quantum mechanics in fact violates this inequality, so no locally realistic theory can reproduce the predictions of quantum mechanics. Alain Aspect and others later implemented Bell's thought experiment in the lab and showed that the physical world obeys quantum mechanics, and so is not describable by a locally realistic theory. In my view, none of that shows that there is "no objective reality". Rather, it shows that objective reality is as far as we can tell quantum mechanical, and not locally realistic in the sense described above. It's certainly the case that quantum mechanics requires a modification of the classical concepts of reality, i.e. of classical ideas about what a physical system is, but you would only accept that conclusion if you agree that quantum mechanics is telling you something objective about reality... At least according to how I understand those words. So I think what people really mean when they say quantum mechanics shows there's no objective reality is just that it contradicts classical conceptions of physical systems, which is clearly true but sounds less sexy and mysterious.