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Questions tagged [quantum-gravity]

Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.

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Does gravity need to be explained by quantum mechanics if spacetime is discrete? [on hold]

I have read that Einstein‘s theory of general relativity, explaining gravity as a warping of space time, is inconsistent with quantum mechanical theories attempting to explain gravity. My question is ...
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Can we measure the amount by which LIGO black hole mergers warp space itself, relative to the mass of the merger?

In our day and age we can calculate the mass and maximum volume of a black hole. With LIGO we can also now calculate by how much is space warped, relative to the distance to a black hole merger, such ...
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Why can we detect single photons, but not single gravitons?

Everybody can buy a single photon counter. Why are there no single graviton counters? Obviously, graviton sources are rare. But why are graviton detectors so hard to make? Is it just because the ...
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Can Newtonian gravity be quantized?

Today, nobody knows how GR is truly supposed to be married with QFT. As a result, the standard model as it is typically presented does not include gravity. Could it be modified to include Newtonian ...
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Quantum waves as waves of space itself?

Are there any interpretations of quantum mechanics were quantum waves are waves of space itself? I'm not talking about gravitational waves, more like pilot-wave theory. There seems to me like it would ...
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1answer
64 views

Would a graviton if it exists have energy?

If it has energy and comes from all matter would then matter lose mass as it releases these gravitons that have energy given that energy and mass can be converted to each other?
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Constraints on models with spontaneous symmetry breaking

As I understand it, one of the loopholes of the Coleman-Mandula theorem is that models with spontaneous symmetry breaking (SSB) escape its scope. Is it therefore right to say that a quantum field ...
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Unified theory of gravity [duplicate]

I have heard that a lot of times, but I have never completely understood what is a force theory unified with quantum mechanics, and also, why do we need this kind of gravitational theory to fully ...
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What are all the features required for a Quantum theory of gravity? [closed]

I do understand why are we seeking Theory of Quantum Gravity. What I am specifically asking is if there was a Theory of Quantum Gravity what would be its key features. Such as when would it be ...
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Why is there no gauge-invariant local operator in GR?

I have a hard time understanding why the bulk locality is a question. I know some operator which depends on a particular coordinate $x$, $O(x)$, and its correlation function like $ \langle O(x)O(y) \...
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How can a non-expert think of Black Hole microstates?

For someone familiar with basic QFT and GR and semi-classical physics but doesn't know string theory nor AdS/CFT is there some intuitive way to think about what the microstate of a black hole means? ...
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Penrose experiment

I just read about Penrose interpretation theory about the wave function collapse: https://en.wikipedia.org/wiki/Penrose_interpretation, which could be confirmed/infirmed by the following experiment: ...
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Renormalization of minisuperspace models

It often appears in the literature that the full quantum gravity theories are plagued by the quantum field theory problems, specially non-renormalization. And they also state that in the context of ...
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What is the Equation? [closed]

I am refreshing my Physics skills and I cannot remember what this equation is. Will someone please tell me what this formula is for? It sort of looks like Stephen Hawkings equation for black holes. $$...
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Greatest relativistc mass possible for a single photon [closed]

I have been told that photons can not be blue shifted to the point where they become black holes, although a photon with a Planck scale wavelength would also have a relativistic mass equal to a black ...
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Existence of the Graviton [duplicate]

The PDG http://pdg.lbl.gov/2019/html/authors_2018.html lists the Graviton as an observable particle (measurements). Is this an accepted fact? All other information says it is just hypothetical i.e. ...
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Why don't strings have a Planck mass? (version 2)

The Energy $E$ of a fundamental string due to its length $L$ goes like $$E\sim TL$$ where string tension $T$ is given by $$T \sim \frac{1}{l_P^2}$$ (Using natural units $\hbar=c=1$ with planck ...
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Why don't strings have a Planck mass?

I understand that strings have a size of roughly the Planck length $l_P$ of $10^{-35}$ m. If that is the case then one would expect that their mass would be roughly the Planck mass which is an ...
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Why strings, and confusion about string coupling constant

Source: Brian Greene's Elegant Universe 1) I finally understand why string theory is a theory of quantum gravity after reading chapter 6 - the quantum fluctuations/foam that made a mess of the ...
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Gravitons from linearized gravity

Linearizing gravity as follows: $$ g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu} $$ Up to this point, everything is a 4x4 matrice. How does one eventually recover a spin-2 particle which, according to https:/...
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Is it possible for a particle to decay via gravity?

Is it possible for a particle to decay via gravity? I know gravity is immensely weaker than the other forces, but all the other forces interact with particles. Do we need an understanding of quantum ...
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Conformal dimension of fields over groups

Group Field Theory (GFT) deals with field theory over group space for instance $$S_2 = \int dg_1dg_2 \mathcal{K}(g_1, g_2)\phi(g_1)\phi(g_2) +\int dg_1dg_2dg_3\mathcal{V}(g_1, g_2, g_3)\phi(g_1)\phi(...
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Difference between QFT In curved spacetime, semiclassical, and quantum gravity?

Could someone describe the difference, qualitatively, between QFT in curved spacetime, semiclassical gravity, and quantum gravity? I know that each is an approximation to the next and the end goal is ...
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2answers
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What really is GW quanta?

I have read this question: What is the difference between gravitons and gravitational waves? where annav says: So photons are the building blocks of light, and gravitons are (hopefully) the ...
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Gravitons (real) passing through a black hole

This is not a duplicate. I do not ask about the gravitational field of the black hole, or why it extends farther out then the event horizon. I do understand that the gravitational field is described ...
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Why is quantum gravity non-renormalizable?

The book The Ideas Of Particle Physics contains a brief treatment of quantum gravity, in which the claim is asserted that if one attempts to construct a model of gravity along the same lines as QED, ...
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108 views

Wouldn't gravitons themselves theoretically be affected by gravity? [duplicate]

(I wrote this using the assumption that the graviton exists, which I know is not necessarily true; this is asked from a theoretical standpoint) It is my understanding that General Relativity states ...
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Wheeler-deWitt equation as a renormalization group flow

I recently heard a comment that Wheeler-deWitt equation can be interpreted as RG flow equations. However, I haven't been able to find appropriate references for the same. Could someone suggest any ...
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How does this study affect the holographic principle and holographic screens?

In a recent study (https://www.nature.com/articles/s41467-018-05433-9) an important flaw in Emergent Gravity/Entropic Gravity has been spotted because it has been discovered that holographic screens ...
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First quantization of a graviton in 4 dimensions and related quantum numbers

1. Could you explain relations between independent components of the metric tensor field, its helicity, polarization and spin? Let me specify the question. The metric tensor in 4D has 10 independent ...
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What happens in theory if we focus gravitational waves into a point?

https://www.youtube.com/watch?v=iWKFPTgkpXo Basically inspired by this video and its comparison with wormholes. While many other questions on the site have already covered the weak strength of ...
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Update: Does the Planck scale imply that spacetime is discrete? [duplicate]

This question has already been asked Does the Planck scale imply that spacetime is discrete? however I'm wondering if there has been any change in the community since it was asked roughly 8 years ...
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Which predictions can a theory of quantum gravity make - update since 2014? [duplicate]

I have read the answers and comments to What predictions can a quantum gravity theory make? and I wonder if anything new can be added since 2014. Today we have an image of the Einstein shadow of the ...
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Must off-shell graviton field be symmetric?

In weak field expansion, one can expand the metric tensor in general relativity as $$g_{\mu\nu}=\eta_{\mu\nu}+h_{\mu\nu}$$ where $\eta_{\mu\nu}$ is the Minkowski metric and $h_{\mu\nu}$ can be ...
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Quantum information = quantum gravity?

I have recently listened to a podcast by Sean Carroll and Leonard Susskind (strongly recommended https://www.preposterousuniverse.com/podcast/2019/05/06/episode-45-leonard-susskind-on-quantum-...
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What are details of this claim of Penrose about gravity and QFT being at odds with each other?

Question Can someone flesh out the details of the argument Roger Penrose makes in this video of a lecture he gave on twistors (starting around 1:25:15) or recommend me the appropriate literature (...
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What happens to the time evolution equations in canonical quantum gravity?

Many expositions on canonical quantum gravity start from a 3+1 type formalism, where spacetime is foliated along the time dimension. The Einstein equations then decompose into constraint equations on ...
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How do Graviton-based theories of gravity explain the expansion of the universe?

In General Relativity, the expansion of the universe is modeled using the Friedmann–Lemaître–Robertson–Walker metric, and the expansion itself is a metric expansion by which the scale of space itself ...
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Is this a good clock for quantum cosmology?

I was thinking about Quantum Cosmology and the wave function of the universe $\Psi[g,\phi]$ in the no boundary proposal. I was thinking about what would make a good clock. It would be some function ...
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How does the direction of time work with timeless wave functions?

In thermodynamical theory, if we have a set of states for example: A) gas all in top left corner of box. B) gas spread out near the left. C) gas spread out to fill entire box. We can give the ...
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4answers
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When GR is not valid passing the event horizon?

A test body crosses the event horizon of a black hole. When can we say that GR breaks down? Only at the singularity? At any point CLOSE enough to the singularity? How much close? In summary, do we ...
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Graviton and spacetime

General Relativity and the concept of curved spacetime replacing the "force" of gravitation is really beautiful, and I thought one could probably find similar descriptions of other forces like ...
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Are the factors in the de Broglie identities operators, scalar real numbers, or both? Or something else?

I understand that $\hbar$ is an honest element of $\mathbb{R}$. But I am trying to understand how the de Broglie wavelength is related to the wave function mathematically. In the identity $p = \hbar k$...
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If gravity turns out to be mediated by exchange particles, would that imply a problem with gravitational fields around a black hole?

In general relativity, gravity is a distortion of spacetime due to mass. Its effects travel (if that's the right word) at the speed of light. In the SM all 3 other known interactions are mediated by ...
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As anyone attempted to show that QM and GR cannot be unified? [duplicate]

Sometimes it possible to make very general statement about functional form of things and show, for example, that QM cannot be described by the use of hidden variable (I'm not a physicist in any sense, ...
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Can gravity be quantized in a way that doesn't require a particle? [duplicate]

I understand what gravitons are and why they are theorized to exist, but is there any way to quantize gravity without a boson? In other words, possibly geometrically with no particle at all? For ...
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Gauge fixing in canonical quantum gravity

In analogy with QFT, the partition function in canonical quantum gravity is defined as a functional integral over the metric tensor (which is now the quantum field), $$ \int \mathcal{D} g \mathcal{D}\...
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How to show no boundary wave function satisifies Wheeler-de-Witt equation?

I've seen various places it says the no boundary wave function (Hartle Hawking state) satisfies the Wheeler-de-Witt equation. But I don't think I've seen a proof. Is there a proof of this somewhere? ...
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What does it mean for gravity in $(2+1)$ dimensions to be topological?

I have been studying gravity in $(2+1)$-dimensions and I have come across the idea that gravity in this lower-dimensional spacetime is topological but I haven’t been able to find a simple explanation ...
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What is going on in “nonlinear gravity from entanglement in conformal field theories”?

EDIT: I am now convinced that the sign of the logarithmic terms in the equations after 3.29 and 3.30 are wrong (unless I have missed something else). These identites come from looking at ...