A QM's interpretations vs GR

MathematicalPhysicist

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As far as I can tell QM has several interpretations, but GR doesn't have such a diversity, am I correct?

Why is that?
Will a theory of QGR suffer also from the disease of QM and will yield several interpretations?
 

MathematicalPhysicist

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Because GR is an ontological theory. Space-time curvature is supposed to be there even if nobody measures it.
And in QG, is it still there when no one measures it?
 

MathematicalPhysicist

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It depends on the interpretation of QG. In other words QG, by itself, does not resolve the interpretation problems of QM.
What do QG theories tell us about the measurement of curvature of spacetime?

I mean on the one macroscopic scale (which its limits need to be defined strictly contrary to microscopic and mesoscopic scales) limit the curvature should be there whether someone is measuring or not, but on the quantum regime which is microscopic we have the measurement problem, the curvature may not be there if no one is measuring it.

Another question which seems rather vague but interesting nonetheless.
Can individual particles measure macroscopic entities?

I mean, an observer performs a measurement on particles, assuming he is composed of several particles, then it seems like an ensemble of particles can perform a measurement on single particles, can the vise versa process occur?

How do we even define the notion of "measurement" coherently?
I mean even in non-laboratory conditions we make measurements all the time in this dynamical world.
 
How do we even define the notion of "measurement" coherently?
I like the view that "measurement" is a process where measuring apparatus becomes quantum-entangled with the measured object. Depending on the design of the apparatus, entanglement with different parameters occurs.

It's the Schrodinger's cat all over again: cat measures the state of the radioactive atom. Similarly, a CCD camera measures arrival of a photon. When photon from a distant star hits a CCD detector, now you have your CCD in a superposition of states, in each state one of its CCD cells has a trapped electron. As soon as you observe (nee "measure") your CCD, now _you_ and the CCD are in a superposition of states of seeing a CCD with a particular cell having that electron.
 

MathematicalPhysicist

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I like the view that "measurement" is a process where measuring apparatus becomes quantum-entangled with the measured object. Depending on the design of the apparatus, entanglement with different parameters occurs.

It's the Schrodinger's cat all over again: cat measures the state of the radioactive atom. Similarly, a CCD camera measures arrival of a photon. When photon from a distant star hits a CCD detector, now you have your CCD in a superposition of states, in each state one of its CCD cells has a trapped electron. As soon as you observe (nee "measure") your CCD, now _you_ and the CCD are in a superposition of states of seeing a CCD with a particular cell having that electron.
The question is more like: who is the one making the measurement, the observation; I mean with your definition all the universe can be seen as entangled, I mean one parameter you measure here will have some correlation with a parameter observed by some other scientist in alpha centauri for example.
 

Fra

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Another question which seems rather vague but interesting nonetheless.
Can individual particles measure macroscopic entities?

I mean, an observer performs a measurement on particles, assuming he is composed of several particles, then it seems like an ensemble of particles can perform a measurement on single particles, can the vise versa process occur?
I agree this is a key question. The question you raise here is, how to describe a "cosmological measurement", which is effectively what you have when a small inside observer wants to "measure" something in a dominant environment.

It is clear that the abstraction used for scientific measurement, and statistics in particle physics which is clean, mathematical precise and allows us to extract timeless mathematical laws, breaks fown for inside observers. (Smolins point in reality of time, evolution of law).

IMO: So does inside observations take place? Of course! but they can only be described as an evolutionary process. Evolutionary models requires a different type of mathematics. The focus is more on algorithms and information processing, than on a set of rules the defines a flow in state space.

/Fredrik
 
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As far as I can tell QM has several interpretations, but GR doesn't have such a diversity, am I correct?
Why is that?
Will a theory of QGR suffer also from the disease of QM and will yield several interpretations?
In QM there was also a long period when interpretations other than Copenhagen/shut up and calculate were anathema, in relativity this is yet so, to discuss the Lorentz ether is essentially forbidden.

To generalize the Lorentz ether to gravity is quite simple, use the Einstein equations in harmonic coordinates, and interpret the harmonic conditions as continuity and Euler equations for the Lorentz ether. But anything with the e-word is a no go.
 
The question is more like: who is the one making the measurement, the observation;
No one. In MWI interpretation, there is no special interaction called "observation". The "observation" is the entanglement.

I mean with your definition all the universe can be seen as entangled, I mean one parameter you measure here will have some correlation with a parameter observed by some other scientist in alpha centauri for example.
This part is too vague to understand.
 

MathematicalPhysicist

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In QM there was also a long period when interpretations other than Copenhagen/shut up and calculate were anathema, in relativity this is yet so, to discuss the Lorentz ether is essentially forbidden.

To generalize the Lorentz ether to gravity is quite simple, use the Einstein equations in harmonic coordinates, and interpret the harmonic conditions as continuity and Euler equations for the Lorentz ether. But anything with the e-word is a no go.
So I see that you're Ether proponent.
Did the ether really disappear or now it just goes by the "dark energy" theme?
 
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So I see that you're Ether proponent.
Did the ether really disappear or now it just goes by the "dark energy" theme?
Dark energy is a strange word for the cosmological term in GR and has no relation to ether interpretations of the GR equations. To seriously discuss something related with the e-word is anyway not allowed here, with the simple information that there is also more than one interpretation of the GR equations I have probably already reached the boundaries of what is allowed now and here.
 

Fra

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About "classical GR", i think Demystifier answered in well in post 6. Then one need not make it more complicated.

But if we look at GR from the point of view of QG and unification, ie. try to rephrase the principles of GR, in a framework that is required for unification it IMO boils down to "interpreting" what is gauge equivalence? is it an ontology or the result of a communication?

More food for thought relating to this is here.

https://www.physicsforums.com/threads/is-gravity-a-gauge-theory.938304/
https://www.physicsforums.com/threads/ed-witten-on-symmetry-and-emergence.927897/

The problem is that the ontological interpretation of gauge equivalence, that works PERFECT for high energy physics in principle, fundamentally fails for cosmological perspective. At least its my firm understanding.

But not everyone would even agree on this characterisation, which is why efforts top make progress are so divergent.

/Fredrik
 

Demystifier

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As I argue in the paper linked below in my signature, the Bohmian interpretation of QM is best viewed as a theory in which relativistic field theory is not fundamental. As GR is also a relativistic field theory, it follows that GR is not fundamental.
 

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