Schrodinger's Cat and The Universe

[A. Neumaier]

To develop successful technology it's sufficient to successfully describe what's observable, and that's what science is all about.

No. One needs to describe much more - namely what would be observable if observed - even when nothing observes it. We observe of a computer (using quantum knowledge in its chips) its input and output but in order to be sure the output relates to the input the way we expect we need to be sure that every gate works as required even while nobody observes it. The same holds for everything technologically used, from computer tomography to nuclear bombs to quantum cryptography.

 

[hutchphd]

No. One needs to describe much more - namely what would be observable if observed - even when nothing observes it.

Surely this statement is far too broad.
For instance the technology required to construct a Spanish Galleon in the 16th century did not require Rutherford's description of the atom. I must not understand your point.

 

[vanhees71]

No. One needs to describe much more - namely what would be observable if observed - even when nothing observes it. We observe of a computer (using quantum knowledge in its chips) its input and output but in order to be sure the output relates to the input the way we expect we need to be sure that every gate works as required even while nobody observes it. The same holds for everything technologically used, from computer tomography to nuclear bombs to quantum cryptography.

No, I'm not interested to know about the functioning of my laptop when I don't use it. I only need to be sure that it is switched off or at least doesn't explode when I'm not looking ;-)).

 

[A. Neumaier]

Surely this statement is far too broad.
For instance the technology required to construct a Spanish Galleon in the 16th century did not require Rutherford's description of the atom. I must not understand your point.

We are now in the 21st century.

 

[A. Neumaier]

No, I'm not interested to know about the functioning of my laptop when I don't use it. I only need to be sure that it is switched off or at least doesn't explode when I'm not looking ;-)).

But when you use it you observe only keyboard, mouse, and screen, while the physics must cover all the unobserved stuff that makes you get the results you want to see (e.g., correctly formatted PhysicsForums pages).

 

[AlexCaledin]

physics must cover all the unobserved stuff

- exactly, just like the picture on a game computer screen is "working" according to the game rules while "covering" all the processes inside the comp.

 

[EPR]

But when you use it you observe only keyboard, mouse, and screen, while the physics must cover all the unobserved stuff that makes you get the results you want to see (e.g., correctly formatted PhysicsForums pages).

Who is going to build a Universe like that? Of classical stuff ala Rutherford. What a waste compared to the current Zero total energy Universe.

 

[hutchphd]

physics must cover all the unobserved stuff

(my italics)
Unless you define "all", this statement is essentially devoid of meaning in this context.

 

[vanhees71]

Well, physics covers all the yet known stuff. I don't see, where this is not the case in contemporary physics. We interpret the observations concerning the motion of heavenly bodies in galaxies as well as the fluctuations in the cosmic microwave background as to be described as the result of the existence of hitherto not found kinds of matter (dark matter) and an even more enigmatic "cosmological constant" (or "dark energy"). That's where theoretical physics is incomplete, but QT in itself describes everything known and observable, whether it's observed or not.

The only thing one must get used to is the notion of state, which in quantum mechanics describes probabilities for the outcome of feasible experiments. There's no determinism in QT, i.e., observables do not need to take certain values independent of the state the system is in, but that doesn't mean that there's anything incomplete in our description, because the randomness of the outcome of measurements is an observed fact, and the prediction of QT concerning the probabilities are consistent with the observations to a high confidence level. In this sense QT covers all known observable stuff, no matter whether it's observed or not.

There's also no doubt that the moon is there when nobody looks at it, because it has been observed in the past, and there are conservation laws telling us that it is still there no matter whether one looks or not. It's pretty sure though, that it will be eaten up together with the planets in our solar system when the Sun gets a red giant within some billion years.

 

[A. Neumaier]

physics covers all the yet known stuff

...even when not observed. That's the point relevant for the applications.

QT in itself describes everything known and observable, whether it's observed or not.

Therefore physics is not only about the observable, but about what actually happens, even in the absence of observation. Thus phyiscs is not only about the observable. We cannot observe much inside the sun, but physics still describes the processes there.

 

[vanhees71]

Physics is only about the objectively, reproducible observable facts of nature. Concerning the Sun we have some ideas about what happens inside from applying the known laws though we cannot directly observe his interior directly (at least not with our today's technical abilities). As long as nothing observable about the Sun disproves our model about what's going on in its interior, we can at least conclude that the known laws work for the matter in the interior of the Sun as far as observable facts are concerned. Of course, we cannot deduce that our model is right in all non-observable details, but that doesn't invalidate these models nor is there anything special with the application of QT, which we inevitably have to use to describe what's going on in the Sun. It doesn't matter whether we really observe it.

 

[Elias1960]

It's typical that one can only say what "physical reality" is not, but there seems to be no clear definition of it. If it's not the sum of (at least in principle possible) objective observations of phenomena, what is it then?

I would suggest to compare realism with the Lagrange formalism.

Theories with a Lagrange formalism have a certain structure. It consists of some configuration space, a parameter named time, and the Lagrangian, a function which defines some action S for a given continuous trajectory. And there is a formula which defines the Euler-Lagrange equations as the evolution equations of the theory, a formula derived from some (quite metaphysical) minimum principle.

Is there some "clear definition of the Lagrangian"? No, not in general. Is the Lagrangian observable? Not at all. Is it useful to have a Lagrange formalism? Certainly.

The situation with realism is quite similar. It is the particular realist theory which defines what, according to this theory, really exists. This is its ontology. In classical theories with a Lagrange formalism, this ontology is simply defined by the configuration space of that theory.

So, a realistic theory is a theory with some additional structure, and such an additional structure is useful. This is quite typical for fundamental principles like realism, causality, minimum principle, Hamilton formalism and so on: They require some additional restrictive structure.

There's no determinism in QT, i.e., observables do not need to take certain values independent of the state the system is in, but that doesn't mean that there's anything incomplete in our description, because the randomness of the outcome of measurements is an observed fact, and the prediction of QT concerning the probabilities are consistent with the observations to a high confidence level.

There cannot be such an observable fact like randomness. There are well-known deterministic theories which, because of our inability to specify the initial values with sufficient accuracy, show random results for all observations. The question if this randomness is some genuine, fundamental one or simply deterministic chaos is nothing one can decide by looking only at the outcome of experiments.

In this sense QT covers all known observable stuff, no matter whether it's observed or not.

 

[Lord Jestocost]

There's also no doubt that the moon is there when nobody looks at it, because it has been observed in the past, and there are conservation laws telling us that it is still there no matter whether one looks or not.

- The moon was "factual" somewhere (time, coordinates) when it was observed in the past, and the moon is "factual" somewhere, when it is observed at present.

- Quantum theory: There are no facts without an observation. There is nothing "factual" to say about the situation between observations as this is the realm of the potentially possible.

- A statement like "There's also no doubt that the moon is there when nobody looks at it" represents nothing else but your ideological belief and belongs to the realm of metaphysics.

- The correct statement would be: "I believe hat the moon is there when nobody looks at it."

 

[Elias1960]

It's typical that one can only say what "physical reality" is not, but there seems to be no clear definition of it. If it's not the sum of (at least in principle possible) objective observations of phenomena, what is it then?

I would suggest to compare realism with the Lagrange formalism.

Theories with a Lagrange formalism have a certain structure. It consists of some configuration space, a parameter named time, and the Lagrangian, a function which defines some action S for a given continuous trajectory. And there is a formula which defines the Euler-Lagrange equations as the evolution equations of the theory, a formula derived from some (quite metaphysical) minimum principle.

Is there some "clear definition of the Lagrangian"? No, not in general. Is the Lagrangian observable? Not at all. Is it useful to have a Lagrange formalism? Certainly.

The situation with realism is quite similar. It is the particular realist theory which defines what, according to this theory, really exists. This is its ontology. In classical theories with a Lagrange formalism, this ontology is simply defined by the configuration space of that theory.

So, realism is a structural requirement. A realistic theory has to have some structure, namely an ontology. This structure has some properties. So, the evolution equations of the theory describe how that reality changes in time.

There's no determinism in QT, i.e., observables do not need to take certain values independent of the state the system is in, but that doesn't mean that there's anything incomplete in our description, because the randomness of the outcome of measurements is an observed fact,

Randomness cannot be an observed fact, because randomness may be the consequence of deterministic chaos. Randomness is a property of a theory, and the next more fundamental theory can easily switch to determinism.

 

[hutchphd]

Randomness cannot be an observed fact, because randomness may be the consequence of deterministic chaos.

Please do not misquote. Clearly @vanhees71 says only the randomness manifests in our observations and refrains from drawing unwarranted causal inferences. Would that you were as careful.

 

[Elias1960]

Please do not misquote.

Sorry, but my quote was correct.

 

[PeterDonis]

Thread closed for moderation.