19 ms·

So do we finally have one unified theory or are we still none the wiser?

swiftcoder 27d ago
From the article: "The result does not unite quantum mechanics and gravity, nor does it show that gravity itself is quantum". I must say, it's actually quite refreshing to read an article about a science topic that conveys the caveats and limitations of the study. Far too many of these studies get filtered through the news outlet hype-machine
- Mizza 27d ago
  If you'd learn about a proposed experiment to test the quantum nature of gravity, this is a cool video: https://www.youtube.com/watch?v=Uey_mUy1vN0 https://www.youtube.com/watch?v=Uey_mUy1vN0 Hopefully we'll see a result in the next decade
rhdunn 27d ago
The article says that this proves that Einstein's equivalence principle (resulting in relativity) holds in this test of a falling quantum particle (where gravity results in a phase shift in the quantum state). It doesn't show/prove how general relativity and quantum mechanics interact. NOTE: The Dirac equation and Quantum Electro Dynamics (QED) unify quantum mechanics and special relativity (non-accelerating frames of reference). So the remaining piece is either to extend QED/QCD to accelerating frames of reference or to quantize general relativity. That would likely predict the phase shift observed in this experiment.
- pdonis 27d ago
  > The Dirac equation and Quantum Electro Dynamics (QED) unify quantum mechanics and special relativity And more generally the Standard Model, which includes the weak and strong interactions. The SM is a quantum field theory, which, as you say, unifies QM and SR. > (non-accelerating frames of reference). No, SR and QFT are not limited to non-accelerating frames. They are limited to small enough regions of spacetime that spacetime curvature is negligible. This experiment is an illustration of that: it compares an accelerated atom with a free-falling atom to show the phase shift between them, and the lab frame in which it is done is accelerated--but the SM and SR work just fine. But the experiment does not show any effects of spacetime curvature. > the remaining piece is either to extend QED/QCD to accelerating frames of reference No, that's already done. See above. > or to quantize general relativity. That's the big missing piece, yes. We know how to write the QFT of a massless spin-2 field (which is our naive expectation of what a QFT for gravity would look like), and we know that the classical limit of that QFT is the classical GR we have now. But we know that QFT has to be just an effective theory, just like the Standard Model; it can't be the final answer. > That would likely predict the phase shift observed in this experiment. The theories we already have (Standard Model + the equivalence principle are all we actually need) are sufficient to predict that. Of course any more comprehensive theory will have to reproduce that prediction, yes.
  - uecker 27d ago
    Can one really derive classical GR completely from a QFT of a spin-2 field? Or only a linear approximation?
    - pdonis 26d ago
      All of it. The theoretical work that showed this was done in the 1960s and early 1970s by Feynman, Deser, and others. One of the key insights was figuring out how to reformulate the theory so that it did not require an infinite series of terms, whose sum nobody knew how to calculate, in order to derive the exact field equation correct to all orders. IIRC Deser was the one who figured that out, and he published a paper in the early 1970s that summarized the research.
      - uecker 26d ago
        Thanks! This is cool. In need to do some reading...
  - rhdunn 27d ago
    My understanding is that 1) SR doesn't consider acceleration (it's an extension of Galilean/uniform motion), and that 2) when Einstein considered acceleration as well as gravity via the equivalence principle which lead to GR [1]. The key insight of the equivalence principle was that the force from gravity (e.g. standing on the Earth) is no different to the observer in their frame of reference to them being in a room in a rocket accelerating at the same rate as gravity [1], [2]. Thus, if you extend QED/QCD/SM in a similar way (thinking of QED/QCD/SM extensions in terms of acceleration and curved space with the equivalence principle in mind) that may lead to a quantized theory of gravity. -- Sir Roger Penrose has a similar idea/thinking [3]. One of the key challenges with quantizing gravity is in how the terms in the expressions resulting from analyzing the Feynman diagram interactions behave [4] which prevent them being renormalized. For electromagnetism you can formulate the terms using the fine structure constant (via the coulomb potential, ħ, and c) which results in successive terms decreasing in value and thus stabilizing to a single value. For gravity using Newton's relationship between two masses in a similar way to deriving the fine structure constant you get Gm^2/ħc. Applying E=mc^2 gives GE^2/ħc^5. Using the Planck energy constant gives (E/E_p)^2 for the energy coupling strength. This means that unlike electromagnetism, the successive terms in the Feynman diagram analysis grows exponentially instead of decreasing to 0. Thus, this approach to quantization doesn't work for gravity. Note: you can still use this to analyze quantum gravitational effects at small energies by evaluating to a given number of terms. [1] https://www.britannica.com/story/how-albert-einstein-developed-the-theory-of-general-relativity https://www.britannica.com/story/how-albert-einstein-develop... [2] https://www.ebsco.com/research-starters/physics/equivalence-principle https://www.ebsco.com/research-starters/physics/equivalence-... [3] https://www.youtube.com/watch?v=VQM0OtxvZ-Y https://www.youtube.com/watch?v=VQM0OtxvZ-Y "We need to 'gravitise' quantum mechanics, not quantise gravity | Roger Penrose | Full interview" [4] https://www.youtube.com/watch?v=yTEPm5d6mrI https://www.youtube.com/watch?v=yTEPm5d6mrI "Why Quantum Gravity Doesn't Work"
- T-A 27d ago
  > the remaining piece is either to extend QED/QCD to accelerating frames of reference or to quantize general relativity Quantum field theory in accelerating frames of reference is old hat; poster children like the Unruh effect [1] and Hawking radiation [2] are from the 1970s. [1] https://en.wikipedia.org/wiki/Unruh_effect https://en.wikipedia.org/wiki/Unruh_effect [2] https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation
elevaet 27d ago
I'm way in over my depth here, but does this maybe that the unification of gravity and quantum physics is further out of reach than we might have hoped? Because it would be easier if gravity disappeared at quantum scales - then it could be understood as an emergent property that emerges out of quantum when you move to bigger scales. But now we have to find something that underlies both.
qsera 27d ago
I have, but no one takes it seriously. But who cares, one needs to answer ones own questions, and not of the entire world.

From the article: "The result does not unite quantum mechanics and gravity, nor does it show that gravity itself is quantum". I must say, it's actually quite refreshing to read an article about a science topic that conveys the caveats and limitations of the study. Far too many of these studies get filtered through the news outlet hype-machine

Mizza 27d ago
If you'd learn about a proposed experiment to test the quantum nature of gravity, this is a cool video: https://www.youtube.com/watch?v=Uey_mUy1vN0 https://www.youtube.com/watch?v=Uey_mUy1vN0 Hopefully we'll see a result in the next decade

rhdunn 27d ago

The article says that this proves that Einstein's equivalence principle (resulting in relativity) holds in this test of a falling quantum particle (where gravity results in a phase shift in the quantum state). It doesn't show/prove how general relativity and quantum mechanics interact. NOTE: The Dirac equation and Quantum Electro Dynamics (QED) unify quantum mechanics and special relativity (non-accelerating frames of reference). So the remaining piece is either to extend QED/QCD to accelerating frames of reference or to quantize general relativity. That would likely predict the phase shift observed in this experiment.

pdonis 27d ago
> The Dirac equation and Quantum Electro Dynamics (QED) unify quantum mechanics and special relativity And more generally the Standard Model, which includes the weak and strong interactions. The SM is a quantum field theory, which, as you say, unifies QM and SR. > (non-accelerating frames of reference). No, SR and QFT are not limited to non-accelerating frames. They are limited to small enough regions of spacetime that spacetime curvature is negligible. This experiment is an illustration of that: it compares an accelerated atom with a free-falling atom to show the phase shift between them, and the lab frame in which it is done is accelerated--but the SM and SR work just fine. But the experiment does not show any effects of spacetime curvature. > the remaining piece is either to extend QED/QCD to accelerating frames of reference No, that's already done. See above. > or to quantize general relativity. That's the big missing piece, yes. We know how to write the QFT of a massless spin-2 field (which is our naive expectation of what a QFT for gravity would look like), and we know that the classical limit of that QFT is the classical GR we have now. But we know that QFT has to be just an effective theory, just like the Standard Model; it can't be the final answer. > That would likely predict the phase shift observed in this experiment. The theories we already have (Standard Model + the equivalence principle are all we actually need) are sufficient to predict that. Of course any more comprehensive theory will have to reproduce that prediction, yes.
- uecker 27d ago
  Can one really derive classical GR completely from a QFT of a spin-2 field? Or only a linear approximation?
  - pdonis 26d ago
    All of it. The theoretical work that showed this was done in the 1960s and early 1970s by Feynman, Deser, and others. One of the key insights was figuring out how to reformulate the theory so that it did not require an infinite series of terms, whose sum nobody knew how to calculate, in order to derive the exact field equation correct to all orders. IIRC Deser was the one who figured that out, and he published a paper in the early 1970s that summarized the research.
    - uecker 26d ago
      Thanks! This is cool. In need to do some reading...
- rhdunn 27d ago
  My understanding is that 1) SR doesn't consider acceleration (it's an extension of Galilean/uniform motion), and that 2) when Einstein considered acceleration as well as gravity via the equivalence principle which lead to GR [1]. The key insight of the equivalence principle was that the force from gravity (e.g. standing on the Earth) is no different to the observer in their frame of reference to them being in a room in a rocket accelerating at the same rate as gravity [1], [2]. Thus, if you extend QED/QCD/SM in a similar way (thinking of QED/QCD/SM extensions in terms of acceleration and curved space with the equivalence principle in mind) that may lead to a quantized theory of gravity. -- Sir Roger Penrose has a similar idea/thinking [3]. One of the key challenges with quantizing gravity is in how the terms in the expressions resulting from analyzing the Feynman diagram interactions behave [4] which prevent them being renormalized. For electromagnetism you can formulate the terms using the fine structure constant (via the coulomb potential, ħ, and c) which results in successive terms decreasing in value and thus stabilizing to a single value. For gravity using Newton's relationship between two masses in a similar way to deriving the fine structure constant you get Gm^2/ħc. Applying E=mc^2 gives GE^2/ħc^5. Using the Planck energy constant gives (E/E_p)^2 for the energy coupling strength. This means that unlike electromagnetism, the successive terms in the Feynman diagram analysis grows exponentially instead of decreasing to 0. Thus, this approach to quantization doesn't work for gravity. Note: you can still use this to analyze quantum gravitational effects at small energies by evaluating to a given number of terms. [1] https://www.britannica.com/story/how-albert-einstein-developed-the-theory-of-general-relativity https://www.britannica.com/story/how-albert-einstein-develop... [2] https://www.ebsco.com/research-starters/physics/equivalence-principle https://www.ebsco.com/research-starters/physics/equivalence-... [3] https://www.youtube.com/watch?v=VQM0OtxvZ-Y https://www.youtube.com/watch?v=VQM0OtxvZ-Y "We need to 'gravitise' quantum mechanics, not quantise gravity | Roger Penrose | Full interview" [4] https://www.youtube.com/watch?v=yTEPm5d6mrI https://www.youtube.com/watch?v=yTEPm5d6mrI "Why Quantum Gravity Doesn't Work"
T-A 27d ago
> the remaining piece is either to extend QED/QCD to accelerating frames of reference or to quantize general relativity Quantum field theory in accelerating frames of reference is old hat; poster children like the Unruh effect [1] and Hawking radiation [2] are from the 1970s. [1] https://en.wikipedia.org/wiki/Unruh_effect https://en.wikipedia.org/wiki/Unruh_effect [2] https://en.wikipedia.org/wiki/Hawking_radiation https://en.wikipedia.org/wiki/Hawking_radiation

elevaet 27d ago

I'm way in over my depth here, but does this maybe that the unification of gravity and quantum physics is further out of reach than we might have hoped? Because it would be easier if gravity disappeared at quantum scales - then it could be understood as an emergent property that emerges out of quantum when you move to bigger scales. But now we have to find something that underlies both.

qsera 27d ago

I have, but no one takes it seriously. But who cares, one needs to answer ones own questions, and not of the entire world.