Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.
38
votes
2answers
591 views
Analog Hawking radiation
I am confused by most discussions of analog
Hawking radiation in fluids (see, for example,
the recent experimental result of Weinfurtner et
al. Phys. Rev. Lett. 106, 021302 (2011), ...
23
votes
5answers
2k views
A list of inconveniences between quantum mechanics and relativity?
It is well known that quantum mechanics and (special and/or general) relativity do not fit well. I am wondering whether it is possible to make a list of contradictions or problems between them?
E.g. ...
21
votes
3answers
679 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 ...
17
votes
3answers
107 views
Twistors in Curved Spacetime
I am looking for good and recent references to constructing twistor space for curved spacetime. This could be a general spacetime, or specific ones (say maximally symmetric spaces different from ...
16
votes
3answers
540 views
Is there a thought experiment which brings to light the contradiction between General Relativity and Quantum Mechanics?
I've been told that GR and QM are not compatible, is there an intuitive reason/thought experiment which demonstrates the issue? (Or one of the issues?)
16
votes
3answers
959 views
Is decoherence even possible in anti de Sitter space?
Is decoherence even possible in anti de Sitter space? The spatial conformal boundary acts as a repulsive wall, thus turning anti de Sitter space into an eternally closed quantum system. Superpositions ...
15
votes
6answers
322 views
Classic Literature in Quantum Gravity?
I've seen it said in various places that a major reason people like string theory as a theory of quantum gravity is that it does a good job of matching our prejudices about how a quantum gravity ...
15
votes
3answers
548 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 ...
14
votes
8answers
2k views
Reversing gravitational decoherence
[Update: Thanks, everyone, for the wonderful replies! I learned something extremely interesting and relevant (namely, the basic way decoherence works in QFT), even though it wasn't what I thought I ...
14
votes
4answers
2k views
Give a description of Loop Quantum Gravity your grandmother could understand
Of course, assuming your grandmother is not a theoretical physicist.
I'd like to hear the basics concepts that make LQG tick and the way it relates to the GR. I heard about spin-networks where one ...
14
votes
7answers
612 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 ...
14
votes
1answer
63 views
Instantons and Non Perturbative Amplitudes in Gravity
In perturbative QFT in flat spacetime the perturbation expansion typically does not converge, and estimates of the large order behaviour of perturbative amplitudes reveals ambiguity of the ...
12
votes
3answers
793 views
Has the black hole information loss paradox been settled?
This question was triggered by a comment of Peter Shor's (he is a skeptic, it seems.) I thought that the holographic principle and AdS/CFT dealt with that, and was enough for Hawking to give John ...
12
votes
6answers
1k views
Is the Planck length Lorentz invariant?
The planck length is defined as $l_P = \sqrt{\frac{\hbar G}{c^3}}$. So it is a combination of the constants $c, h, G$ which I believe are all Lorentz invariants. So I think the Planck length should ...
12
votes
1answer
103 views
6d Massive Gravity
Massive gravity (with a Fierz-Pauli mass) in 4 dimensions is very well-studied, involving exotic phenomena like the vDVZ discontinuity and the Vainshtein effect that all have an elegant and physically ...
11
votes
4answers
1k views
Is there a maximum possible acceleration?
I'm thinking equivalence principle, possibilities of unbounded space-time curvature, quantum gravity ...
11
votes
2answers
412 views
Extremal black hole with no angular momentum and no electric charge
A black hole will have a temperature that is a function of the mass, the angular momentum and the electric charge. For a fixed mass, Angular momentum and electric charge are bounded by the extremality ...
11
votes
1answer
287 views
Quantum uncertainty of particle falling in black hole
A stationary observer at infinity sees a particle of mass m falling in a supermassive Schwarzschild black hole. He observes an increasing redshift and sees the particle ceasing to progress when it ...
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 ...
10
votes
3answers
405 views
How do we know that nonperturbative canonical quantum gravity is wrong?
In these forums and elsewhere it is routinely agreed that "we do not have a theory of quantum gravity." My question is, how do we know that canonical quantum gravity is "wrong"? I understand that the ...
10
votes
3answers
438 views
Hawking radiation and quark confinement
The simple picture of Hawking radiation is that a pair-antiparticle pair is produced near the event horizon, then one falls into the black hole while the other escapes. Suppose the particles are ...
10
votes
5answers
668 views
Why is Mendel Sachs's work not taken seriously? Or is it? [closed]
Back in college I remember coming across a few books in the physics library by Mendel Sachs. Examples are:
General Relativity and Matter
Quantum Mechanics and Gravity
Quantum Mechanics from General ...
10
votes
1answer
187 views
What does the equivalence principle mean in quantum cases?
We know that electron trapped by nuclear, like the hydrogen system, is described by quantum state,and never fall to the nuclear.So is there any similar situation in the case of electron near the ...
9
votes
5answers
1k views
How to get Planck length
I know that what Planck length equals to.
The first question is, how do you get the formula
$$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length?
The second question is, will any ...
9
votes
4answers
1k views
Can Loop Quantum Gravity connect in any way with string theory?
The one difficulty I see with LQG is that it requires an enormous number of degrees of freedom, e.g. these spin variables in the net. This is in contrast to stringy holographic theory where the ...
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 ...
9
votes
4answers
874 views
Discussion of the Rovelli's paper on the black hole entropy in Loop Quantum Gravity
In a recent discussion about black holes, space_cadet provided me with the following paper of Rovelli: Black Hole Entropy from Loop Quantum Gravity which claims to derive the Bekenstein-Hawking ...
9
votes
3answers
442 views
Is there any quantum-gravity theory that has flat space-time and gravitons?
Many quantum-gravity theories are strongly interacting. It is not clear
if they produce the gravity as we know it at low energies. So I wonder, is there
any quantum-gravity theory that
a) is a well ...
9
votes
1answer
756 views
Diff(M) as a gauge group and local observables in theories with gravity
In a gauge theory like QED a gauge transformation transforms one mathematical representation of a physical system to another mathematical representation of the same system, where the two mathematical ...
9
votes
5answers
638 views
What is the role spin-networks play in Loop Quantum Gravity?
This is a second question (in what will probably become a series) in my attempt to understand LQG a little. In the previous question I asked about the general concepts behind LQG to which space_cadet ...
9
votes
3answers
324 views
Can the entropy density of a spacelike singularity arbitrarily exceed the inverse Planck volume?
For the purpose of this question, let's restrict ourselves to BKL singularities. BKL cosmologies are homogeneous Bianchi type XIII and IV cosmologies which exhibit oscillatory chaotic behavior, ...
9
votes
2answers
495 views
The Reeh-Schlieder theorem and quantum geometry
There have been some very nice discussions recently centered around the question of whether gravity and the geometry and topology of the classical world we see about us, could be phenomena which ...
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 ...
9
votes
2answers
216 views
How is causal patch complementarity compatible with behavior during inflation?
Causal patch complementarity is the conjecture that in de Sitter space with a positive cosmological constant, the states within the causal patch are sufficient to fully describe the universe with the ...
8
votes
5answers
825 views
Graduate School for Theoretical Physics
First off, let me just say that I am unsure if this question is appropriate for this site, and if the community deems it necessary, the question should be closed.
So right now I am a fourth year ...
8
votes
8answers
2k views
What are cosmological “firewalls”?
Reading the funny title of this talk, Black Holes and Firewalls, just made me LOL because I have no idea what it is about but a lively imagination :-P (Sorry Raphael Bousso but the title is just too ...
8
votes
1answer
524 views
Is String Theory formulated in flat or curved spacetime?
String Theory is formulated in 10 or 11 (or 26?) dimensions where it is assumed that all of the space dimensions except for 3 (large) space dimensions and 1 time dimension are a compact manifold with ...
8
votes
6answers
1k views
The final death of a black hole
What are the different death scenarios for a black hole? I know they can evaporate through Hawking radiation - but is there any other way? What if you just kept shoveling more and more mass and ...
8
votes
1answer
81 views
Quantum gravity at D = 3
Quantization of gravity (general relativity) seems to be impossible for spacetime dimension D >= 4. Instead, quantum gravity is described by string theory which is something more than quantization ...
8
votes
1answer
299 views
Summing over topologies
Sometimes in quantum cosmology, when we are thinking about 'wave functions of the universe' we have in mind some sort of formal path integral, where we include not just the variations in the dynamical ...
8
votes
3answers
343 views
Does the dimensionality of phase space go up as the universe expands?
Ever since Hubble, it is well known that the universe is expanding from a Big Bang. The size of the universe had gone up by many many orders of magnitude as space expanded. If the dimensionality of ...
8
votes
1answer
179 views
Derivation of the basic equation for Witten diagrams
I could understand the derivation of the "bulk-to-boundary" propagators ($K$) for scalar fields in $AdS$ but the iterative definition of the "bulk-to-bulk" propagators is not clear to me.
On is ...
8
votes
2answers
810 views
What happens when a black hole and an “anti-black-hole” collide?
Let's say we have one black hole that formed through the collapse of hydrogen gas and another that formed through the collapse of anti-hydrogen gas. What happens when they collide? Do they (1) ...
8
votes
5answers
462 views
How important is the cosmic censorship conjecture?
I would like to know how important the cosmic censorship conjecture is? Should a quantum theory of gravity must obey this? It was never rigorously proved in classical GR too. What would be the ...
8
votes
1answer
132 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, ...
8
votes
3answers
334 views
Thermodynamically reversed black holes, firewalls, Casimir effect, null energy condition violations
Scott Aaronson asked a very deep question at Hawking radiation and reversibility about what happens if black hole evolution is reversed thermodynamically. Most of the commenters missed his point ...
7
votes
2answers
1k views
Is reality discrete at the quantum level? (…and what does it imply not only mathematically?)
On a quantum scale the smallest unit is the Planck scale, which is a discrete measure.
There several question that come to mind:
Does that mean that particles can only live in a discrete grid-like ...
7
votes
3answers
239 views
Does a UV completion of gravity necessarily need to be so drastic as String Theory or LQG?
First of all I, it is my understanding that the problems one encounters with the non-renormalizability of gravity are very similar (if not the same) as one encounters in any non-renormalizable theory. ...
7
votes
1answer
90 views
Simple question on the foundations of spin foam formalism
To make it simple, take the spin foam formalism of ($SU(2)$) 3D gravity. My question is about the choice of the data that will replace the (smoothly defined) fields $e$ (the triad) and $\omega$ (the ...
7
votes
3answers
307 views
An electron falling into a black hole
If an electron falls into a black hole. How can the Heisenberg uncertainty principle hold? The electron has fallen into the singularity now so it has a well defined position which means that it ...
