Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.
8
votes
1answer
131 views
Gauge symmetries and elementary particles
The Weinberg-Witten theorem (disclaimer: I don't know this wikipedia entry) is usually mentioned as the reason why gravitons may not be composite particles. I do understand the proof of the theorem, ...
1
vote
1answer
142 views
Is time quantized? Is there a fundamental time unit that cannot be divided? [duplicate]
Is the present just a sharp line between the past and the future with no time at all, or is the present a short frozen unit of time?
Could time be quantized into a fundamental units? Like Planck ...
-4
votes
0answers
31 views
Basic force in gravity responsible for attracting everything including nonmetal [closed]
Why do things get attracted? It is said that there is a magnetic field around the earth, but how can the attraction of non-metals be explained in terms of magnetic fields?
How can one generate this ...
2
votes
1answer
86 views
Planck length is straight, curved, volumetric or what?
Is shape of Planck length straight line or wavelength or what? And how to free your mind from Euclidean thinking or Least action principle? Tried some QM videos but cannot ignore straight line ...
1
vote
0answers
26 views
Why are polymer representations called “polymer representations”?
Why are polymer representations called "polymer representations"?
Polymer representations deal with non-continuous unitary representations of groups acting on nonseparable Hilbert spaces (see e.g. ...
2
votes
0answers
74 views
How can any QM interpretations which use a linear Schrödinger Equation be used to quantitize gravity?
Since general relativity is nonlinear, how could we quantitize gravity with QM interpretations which use the linear Schrödinger Equation? Or is this fundamentally unworkable?
2
votes
2answers
139 views
Was TP Singh right to say that a theory of quantum gravity necessitates the Copenhagen Interpretation?
http://iopscience.iop.org/1742-6596/174/1/012024
In the above link we see TP Singh arguing that only Copenhagen will work for a theory of quantum gravity.
Some of his key points are "quantum theory ...
6
votes
0answers
176 views
Is the 125 GeV Higgs boson some kind of a “almost-commutative graviton” at the electroweak scale?
The clumsy "almost-commutative graviton" is provocative. I use it on purpose, to ask two questions in one :
The observation of only one Higgs and no supersymmetric particle below 7 TeV (up to now) : ...
-6
votes
0answers
114 views
Criticism of String Theory by other string theorists [closed]
*EDIT: Due to policy I will edit this into a question:
Do you know any string theorist that have criticized string theory and if so please could you give me references? thanks.
End of EDIT. ...
0
votes
1answer
100 views
can be exist the negative mass? [duplicate]
I'm not sure about this but I guess there must be negative masses in the universe because of the symmetry. If the gravity is one of the main forces in nature it must has negatives mass to be able to ...
5
votes
1answer
88 views
T-Duality between Type HE String theory and Type HO string theory
My question is regarding T-Duality between the 2 Type H string theories.
I know that the Type II String theories are T-dual to each other because T-Duality changes the sign of the Gamma Matrix so
...
0
votes
0answers
88 views
How can it be seen that ST unifies GR and QM as the quantum gravity scale is not directly accessible [closed]
I am a newbie to superstring theories, but I came into this question:
so superstring theories purport to unify general relativity and quantum theory.
However, there is yet no definitive way to test ...
1
vote
1answer
87 views
What are the implications of the Nesvizhevsky experiment and followup experiments with ultracold neutrons?
I remember reading about the groundbreaking experiment by Nesvizhevsky (et al. 2001) some 12 years ago using ultra-cold neutrons which showed the first experimental evidence of quantum gravity. It is ...
0
votes
1answer
75 views
Why Planck scale is so important?
I know that Planck scale is the scale where both, gravity and quantum effects are relevant simultaneously. Are there more reasons?
0
votes
1answer
74 views
Explanation for the notion that physical laws break down at the Big Bang
I've often heard the phrase "physical laws break down at the big bang".
Why is this? Divide by zero?
Please provide the mathematics.
0
votes
0answers
63 views
Why there can't be infinitely small existing?
Why there can't be infinitely small existing?
I am not suggesting it can or cannot. I am asking can there be an absolute or reasonable answer to that question.
1
vote
1answer
86 views
What is the smallest existing thing in theory and law?
What is the smallest existing thing in theory and law?
4
votes
2answers
103 views
Are all points in the universe connected?
Is it true that every point in the universe is connected or could be so theoretically? If so how is this mediated?
Is it through the quantum nature of the fabric of space or is it through the ...
1
vote
1answer
31 views
measure higher frequencies then half of Planck-frequency?
The maximum frequency is defined by the Planck frequency $\omega_P$. Also there is the Shannon theorem which tells us that to lossless capture a signal, you have to sample it with minimum of the ...
2
votes
1answer
108 views
Phys.org Spectral geometry to unite relativity and quantum mechanics, restate in laymens terms?
Lingua Franca links relativity and quantum theories with spectral geometry
Could someone give me a short synopsis of this article in laymens terms? What implications does this have in the physics ...
9
votes
2answers
352 views
What is the current state of research in quantum gravity?
I was browsing through this and was wondering what progress in quantum gravity research has taken place since the (preprint) publication.
If anyone can provide some helpful feedback I would be ...
7
votes
0answers
104 views
Why is the Planck length the shortest measurable length? [duplicate]
I quote from the Wikipedia article on Planck length:
According to the generalized uncertainty principle, the Planck length
is in principle, within a factor of order unity, the shortest
...
5
votes
0answers
249 views
Can the laws of quantum mechanics be derived from a more fundamental theory? [closed]
String theory takes quantum mechanics for granted and tries to make it compatible with gravity but if it turns out to be a theory of everything then shouldn't it in principle explain why our world is ...
3
votes
1answer
219 views
How do I quantize a classical field theory
I have not been able to find any information about this on the Internet. I am a middle-schooler, 14, who self-studies physics, and I know up to and including ODEs, and some of the calculus of ...
6
votes
0answers
105 views
Is it believed that all UV completions have “Maldacena duals”?
I have heard occasional rumors that effective field theories have gravity duals. For example, I've been told that UV momentum cutoffs in N=4 SYM become finite radii in AdS. I've heard speculations ...
2
votes
0answers
57 views
Status of Teleparallel Quantum Gravity
There are several approches to incorporate torsion into a theory of gravity.
According to this article, requiring a consistent coupling to Dirac fields leaves us with the Einstein-Cartan approach or ...
-1
votes
3answers
254 views
Why isn't the Bekenstein-Hawking Entropy considered the quantum gravitational unification?
Based on the Bekenstein-Hawking Equation for Entropy, hasn't the relationship between quantum mechanics and gravity already been established.
19
votes
3answers
626 views
Why is gravity such a unique force?
My knowledge on this particular field of physics is very sketchy, but I frequently hear of a theoretical "graviton", the quantum of the gravitational field. So I guess most physicists' assumption is ...
5
votes
2answers
336 views
The Uncertainty Principle and Black Holes
What are the consequences of applying the uncertainty principle to black holes?
Does the uncertainty principle need to be modified in the context of a black hole and if so what are the implications ...
0
votes
4answers
341 views
Could all strings be one single string which weaves the fabric of the universe?
This question popped out of another discussion, about if the photon needs a receiver to exist. Can a photon get emitted without a receiver? A universe containing only one electron was hypothetically ...
10
votes
1answer
417 views
Why one-dimensional strings, but not higher-dimensional shells/membranes?
One way that I've seen to sort-of motivate string theory is to 'generalize' the relativistic point particle action, resulting in the Nambu-Goto action. However, once you see how to make this ...
9
votes
0answers
177 views
How is Matter Modelled in Loop Quantum Gravity?
I have a basic understanding of how gravitational degrees of freedom are modelled in loop quantum gravity, but as far as I know, the main machinery - spin network states and observables, does not ...
4
votes
1answer
177 views
Is the quantization of gravity necessary for a quantum theory of gravity? Part II
(At the suggestion of the user markovchain, I have decided to take a very large edit/addition to the original question, and ask it as a separate question altogether.)
Here it is:
I have since ...
15
votes
3answers
546 views
Why is gravity so hard to unify with the other 3 fundamental forces?
Electricity and magnetism was unified in the 19th century, and unification of electromagnetism with the weak force followed suit, bringing into play the electroweak force.
I've been told that ...
3
votes
1answer
230 views
Deriving the reduced Green's functions in Polchinski's volume 1
In equation 6.2.7, Polchinski defines his reduced Green's functions $G'$ on the 2-manifold to satisfy the equation,
$$ \frac{-1}{2\pi \alpha '}\nabla ^2 G'(\sigma_1, \sigma_2) = ...
1
vote
1answer
172 views
Is Space-Time Quantisation necessary or even meaningful?
It is believed among people working on Quantum Gravity, that space-time must be quantised at the Planck scale. Although it is very hard to verify such proposition, it is interesting from a ...
2
votes
1answer
151 views
What happens to atoms inside the black hole?
Black holes have very high gravitational force intending to crush everything. So as we know atoms in a molecule have inter atomic spacing between then and further electron,s also revolve at a certain ...
7
votes
0answers
152 views
Does local physics depend on global topology?
Motivating Example
In standard treatments of AdS/CFT (MAGOO for example), one defines $\mathrm{AdS}_{p+2}$ as a particular embedded submanifold of $\mathbb R^{2,p+1}$ which gives it topology ...
4
votes
3answers
319 views
Higgs-Boson/Graviton
The Higgs boson gives particles mass. And the graviton is the theoretical force-carrier of gravity. Gravity depends on mass. So if the Higgs Boson gives things mass, it therefore gives them gravity. ...
4
votes
2answers
334 views
Is spacetime an illusion?
In consistent histories, for gauge theories, can the projection operators used in the chains be not gauge invariant?
In quantum gravity, for a projection operator to be gauge invariant means it has ...
14
votes
7answers
611 views
Is the quantization of gravity necessary for a quantum theory of gravity?
The other day in my string theory class, I asked the professor why we wanted to quantize gravity, in the sense that we want to treat the metric on space-time as a quantum field, as opposed to, for ...
1
vote
1answer
110 views
Divergence in Supergravity
I'm not familiar with supergravity so here's my question: I've heard in talks that if one finds divergence for five-loop 4-graviton scattering amplitudes in five dimensions this translates to a ...
1
vote
1answer
209 views
Dirac Equation in General Relativity
Dirac equation for the massless fermions in curved spase time is $γ^ae^μ_aD_μΨ=0$, where $e^μ_a$ are the tetrads. I have to show that Dirac spinors obey the following equation:
...
6
votes
4answers
309 views
What is the “foamy space” hypothesis that has been debunked recently?
In "Space-Time Is Smooth, Not Foamy", a Space.com article, it is stated:
In his general theory of relativity, Einstein described space-time as fundamentally smooth, warping only under the strain ...
1
vote
0answers
114 views
Approaches to Quantum gravity [closed]
I'm going to start my graduate studies in theoretical physics. My supervisor wants me to work on quantum gravity. He gave me the liberty to chose a particular approach to Quantum gravity (Excluding ...
3
votes
3answers
227 views
Question on inflation
I have two particular questions regarding the inflationary scenario. They are:
1.) What is the physical origin of the inflaton field?
2.) Why has the potential of the inflation field its particular ...
4
votes
1answer
90 views
Where and how is the entropy of a black hole stored?
Where and how is the entropy of a black hole stored?
Is it around the horizon? Most of the entanglement entropy across the event horizon lies within Planck distances of it and are short lived.
Is ...
1
vote
0answers
75 views
Black hole entropy from collapsed entangled pure light
Consider the following scenario, very similar to the one proposed in this question, but this time, the pure quantum radiation used for the black hole collapse, is now being split with down-converter ...
3
votes
1answer
130 views
Question on inflation as a phase transition
I have just finished watching the following video http://www.youtube.com/watch?v=beQ9fZ0jVdE where Laughlin, Gross and some students discuss e.g. about inflation. The following question is risen:
Is ...
1
vote
0answers
186 views
Curiosity episode with Stephen Hawking. The Big-Bang
In an episode of Discovery's Curiosity with host Stephen Hawking, he claims the Big Bang event can be explained from physics alone, and does not require the intervention of a creator.
1) His ...
