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- Thread starter aditya ver.2.0
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In summary, there have been many developments in the field of General Relativity since Einstein's original idea. These include the development of global techniques by Penrose and others in the 1960s and 1970s, as well as the modification of GR to allow for nonzero torsion and accommodate quantum mechanics. However, the underlying structure of GR has remained remarkably resilient over the years, unlike quantum mechanics which has been modified multiple times. There have also been attempts to find alternatives to quantum mechanics, but none have been successful so far. Overall, GR has stood the test of time and continues to be a major area of study in physics.

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One of the famous things that happened in the 60's and 70's was the development of the global techniques by Penrose and the others.

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martinbn said:

One of the famous things that happened in the 60's and 70's was the development of the global techniques by Penrose and the others.

The original post title said "Modifications..." Development of new techniques is not a modification of the theory.

As far as modifying the equations, it's actually pretty remarkable how well they have stood up over the years. In contrast, quantum theory has been modified a bunch of times, to take into account relativity and intrinsic spin and particle creation, etc.

One modification of GR that I think most people believe is necessary for consistency, although it's not going to make much difference to most observational consequences of GR is to allow nonzero torsion. This is necessary (as I understand it) to take into account particles with intrinsic spin, such as an electron. It's also clear that GR will have to be modified in some ways to accommodate quantum mechanics. For the latter, there has been a tremendous amount of work done, although there is no definitive theory yet.

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stevendaryl said:In contrast, quantum theory has been modified a bunch of times, to take into account relativity and intrinsic spin and particle creation, etc.

I don't agree with this. Many new

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LastOneStanding said:I don't agree with this. Many newapplicationsof quantum theory have been developed, such as those you mention. None of them represent a modification of the basic axioms of quantum mechanics, but rather the application of quantum mechanics to increasingly more interesting choices of Hamiltonians. The underlying structure of quantum mechanics has not changed since the previous mish-mash of heuristics usually called "old quantum mechanics" was formalized by Dirac, von Neumann and others. Indeed, we've found that mathematical structure of quantum mechanics is even more resilient than that of general relativity. Therearetheories similar to GR, like Brans-Dicke theory, which are still compatible with experimental data though GR is generally the simpler choice. On the other hand, as Weinberg noted, it's so far not been possible to find something "close to" quantum mechanics in some sense that doesn't have some extremely undesirable properties and isn't quantum mechanics itself.

I sort of agree. "Quantum mechanics" is broad enough that all the various theories, Schrodinger's equation, Dirac's equation, the Klein-Gordon equation, QED, QCD, Weinberg-Salam model, etc., are all instances of the general axioms of quantum mechanics. However, it's only the instances of quantum mechanics that make testable predictions. I don't think that the general framework makes any predictions at all--only the specific theories. Or the specific choice of Hamiltonian, state space, etc.

In contrast, the Einstein Field equations are a very specific theory--they are more comparable (in my opinion) to a specific theory such as QED than to the broad category of quantum mechanics.

QM is a framework for theories, rather than a theory in itself. It's sort of like Special Relativity in that sense (there are many theories that are capable of being put into Special Relativistic form), but not like GR.

That's just a matter of opinion.

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stevendaryl said:I sort of agree. "Quantum mechanics" is broad enough that all the various theories, Schrodinger's equation, Dirac's equation, the Klein-Gordon equation, QED, QCD, Weinberg-Salam model, etc., are all instances of the general axioms of quantum mechanics. However, it's only the instances of quantum mechanics that make testable predictions. I don't think that the general framework makes any predictions at all--only the specific theories. Or the specific choice of Hamiltonian, state space, etc.

The general framework makes falsifiable predictions about things like how strong non-local correlations in nature can be (via the CHSH inequality and related Bell inequalities), what kinds of physical processes should not be possible under any particular realization (via the various no-go theorems, e.g., no cloning of states), etc. I do see what you're saying and agree that "quantum theory" is best understood as a constraint on theories (though I'd argue its a much, much tighter constraint than, say, "classical theory") especially compared to general relativity. But the general structure of it does make concrete predictions on its own without appealing to any particular realization.

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aditya ver.2.0 said:

I can't provide a comprehensive update, (and I don't quite see how it can be an update unless you already know GR as it arose in the mind of EInstein. Which I rather doubt.)

A few points which are not a comprehensive update do come to mind though. We now have a better theoretical understanding of black holes than Einstein did. We have a rather larger number of alternate theories of gravity besides GR as well. Most (and perhaps even all) of these alternate theories of gravity are either equivalent experimentally to GR to the best of our ability to measure with currrent technology, or have been proven false by observation.

I'm not quite sure what you're fishing for here. I'm sure there has been a lot more done than I mentioned, these are a few of the main developments that come to my mind.

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Good Onepervect said:I can't provide a comprehensive update, (and I don't quite see how it can be an update unless you already know GR as it arose in the mind of EInstein. Which I rather doubt.)

A few points which are not a comprehensive update do come to mind though. We now have a better theoretical understanding of black holes than Einstein did. We have a rather larger number of alternate theories of gravity besides GR as well. Most (and perhaps even all) of these alternate theories of gravity are either equivalent experimentally to GR to the best of our ability to measure with currrent technology, or have been proven false by observation.

I'm not quite sure what you're fishing for here. I'm sure there has been a lot more done than I mentioned, these are a few of the main developments that come to my mind.

General Relativity is a theory of gravity developed by Albert Einstein in the early 20th century. It describes how massive objects, such as planets and stars, curve the fabric of space and time, causing objects to move in certain ways.

Modifications in General Relativity refer to changes or additions made to the original theory proposed by Einstein. These modifications aim to address some of the limitations and unanswered questions of General Relativity, such as the inability to explain the behavior of dark matter and dark energy.

Some examples of Modifications in General Relativity include theories such as Modified Newtonian Dynamics (MOND), Scalar-Tensor-Vector Gravity (STVG), and Brans-Dicke theory. These theories introduce new concepts and equations to improve upon the predictions of General Relativity.

Modifications in General Relativity have the potential to significantly change our understanding of the universe. They may provide explanations for phenomena that cannot be explained by General Relativity, such as the accelerated expansion of the universe. However, these modifications are still being studied and tested, and their impact on our understanding is still being debated.

Yes, there have been several experiments and observations that support Modifications in General Relativity. For example, the observation of the accelerated expansion of the universe is better explained by some modifications, such as the addition of a cosmological constant. Additionally, the bending of starlight around massive objects, known as gravitational lensing, has been observed and supports the predictions of modified theories of gravity.

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