How Quantum Gravity Can Alter Dynamics at Astrophysical Scales by Samir D. Mathur

Samir Mathur

Essay Abstract

It is generally assumed that the effects of quantum gravity will be manifest only at lengths scales comparable to or smaller than the planck length $l_p$. We argue that this belief is false; quantum gravity effects make a dramatic change to our understanding of physics on arbitrarily large length scales. The reason this can happen is that when a large number $N$ of particles are involved, the length scale of quantum gravitational effects need not be $l_p$ but $N^\alpha l_p$ for some $\alpha>0$. We explain the mechanism of `fractionation' which leads to the emergence of such a new length scale in string theory. For black holes, this new length scale turns out to be order the horizon radius, and resolves puzzles like the black hole information paradox. In the early Universe we have another situation where a large number $N$ of quanta are crushed together, and we argue that the effect of the emergent length scale is to provide an inflationary `push' to larger volumes resulting from the quantum mechanical measure term in the path integral.

Author Bio

I obtained my Ph.D from the University of Bombay. I held postdoc positions at TIFR and Harvard, was a junior faculty at MIT and am currently a professor at Ohio State. My research interests are gravity and string theory, with a particular focus on resolving the black hole information paradox.

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James Hoover

Samir,

This is a needed and vital effort -- and intriguing in concept:

"In this essay I will argue that this belief is wrong; quantum gravitational physics does not stay confined to planck scales, but in fact leaks out to affect all scales, including the largest scales of astrophysics and Cosmology."

How do you do it in words?

Empty space is defined by Lawrence Krauss as "a boiling brew of virtual particles that pop in and out of existence in a time so short we cannot see them directly. Quantum gravity theories speak of quantum foam. On a large scale we have the smoothness described by the cosmological principle.

Jim

[deleted]

Hi Jim,

Indeed, empty space boils at the planck scale. The question of interest here was: if we put a large number N of particles in this space, does their energy create long distance effects in this `boiling brew' which is N times planck length? This is what happens in string theory, (due to the effect I termed fractionation). And in that situation, we get quantum gravity effects across distances as big as a black hole horizon, or perhaps even on Cosmological scales ...

So what happens is that taking a large number N of densely clustered particles causes the smoothness of spactime to break down .. and in string theory we were able to write down the wavefunction for what it becomes instead -- (this is the fuzzball wavefunction)

Hope this helps,

Samir

Harlan Swyers

Samir,

I like your paper, particularly for this line:

"it prevents the formation of a traditional black hole with horizon."

Does that necessitate that in order for the theory to be consistent that the graviton itself can not collapse to a point? Or is the point ground state of the graviton a zero energy state for the string?

[deleted]

hi Harian,

the graviton stays collapsed to a point, but what happens is that the other states of the string become very easy to excite, once they are part of a large bound state. So the energy of the hole is not distributed among gravitons, but among the `fractionated' string theory modes...

samir

Harlan Swyers

Thank Samir,

I read through fractionation and like the concept. Here are my thoughts. I see 1/L and understand L as fundamental length. Whenever I see that I automatically think fundamental frequency (which of course could be understood as energy). I see the string wrapped around an additional dimension, and see it connect to itself. Since these dimensions are curled up, the implication is that the dimension is connected at a point in some ordinary space (a la some variation of Hopf fibration). Ok, we extend the idea to string wrapped around 5 dimension branes. So, is it really then the 5-brane wrapped around the fifth dimension that connects to the same 4 dimension spactime, and are we not talk about some analgous set of 5 fundamental lengths (or 5 fundamental frequencies)?

Benjamin Dribus

Samir,

Very interesting essay. There are not many essays about string theory here. A couple of questions come to mind.

1. I am not quite clear on what role the compactified dimensions are playing in this besides providing the fractionation mechanism. In particular, when you are integrating over 3-geometries, are you taking the compactification to be constant? Is it not dynamically coupled to the matter-energy?

2. It seems as though this idea predicts a nonconstant dark energy effect, since the "push" in a given region depends on the energy density. Would the resulting fluctuations be large enough to observe?

3. Presumably dark matter cannot be described along similar lines, since the effect seems to be the opposite one?

What's motivating these questions is my interest in scale-dependence of phenomena and the theory of path integrals over geometries or analogous structures. You can see my general point of view here:

[link:fqxi.org/community/forum/topic/1386]On the Foundational Assumptions of Modern Physics[link]

I'm a mathematician, not committed to any particular theory, and I know less about string theory than I would like. Your essay supplemented that deficiency a bit!

Take care,

Ben Dribus

Take care,

Ben Dribus

Benjamin Dribus

(sorry for the link format; I left out the slash!)

[deleted]

hi Benjamin,

(1) As you say, the compactified directions help in getting the fractionation, and that is of course the crucial point. But more abstractly, the compact directions provide extra degrees of freedom, and one can imagine other ways of taking degrees of freedom that will also give similar effects. What is needed are enough degrees of freedom to account for the Bekenstein entropy, and these can come in many ways.

(2) The time dependence (and position dependence) is a good question, which we have not studied yet. I would say that the story is quite well understood for black holes, but very few aspects have been worked out for Cosmology,

regards

Samir

Peter Morgan

An interesting essay, once I had decided not to apply my usual no-string/M-theory-filter for this case. I found it interesting because it seems that your argument may not be confined to a string mathematics. Or do you think it only applies to strings? Thanks.

[deleted]

Hi Peter,

I think the lessons here are general, not just for a given theory like strings. Whenever we have enough degrees of freedom to account for the bekenstein entropy, then we have to worry about what so many states will do in the path integral. Of course we knew about the large entropy since the 1972 work of bekenstein, but since the states for this entropy were not manifest, I think we never took its consequence seriously,

Samir

[deleted]

Seems like a very good idea!

Steve Dufourny

Hello Mr.Mathur,

I am also very intrigued by these BH. The works of Hawking and Bekenstein are relevant considering the steps of entropy and informations. They have a lot of properties. The heat and themo. are essential. The principle of pure themrodynamics are universal after all. My equations can help for a better understanding of these BH. Of course they are spheres with a mass, a volume, a rotation spinal, a rotation orbital,a temperature and its steps of heat. The proportions are numerous in fact even in their quantum correlation. I beleive strongly that the codes of singularities are under my eqations. The Entropy is purelly correlated with the spheres. It is very important for the correct correlations with the principles of thermodynamics. Furthermore the serie of uniqueness for the two scales in 3D near these walls is universal. This serie is specific and precise with the correlated volumes. The BH are central spheres.And more we go towards our main central BH(hypothetical BB) of our Universal evolutive sphere(variable volumes), more the spheres increase in volume.So the central universal sphere is the most important BH.The main singularities. With correlated informations and entropy.

It is fascinating these BH and correlated informative radiations of sortings and synchronizations. All has a pure rule of complementarity at all scales and the spheres answer to all. The states of entropy can be classed when we understand the volumes differenciating the quantum 3D scale and the cosmological 3D scale. Now when the fermions and the bosons are differenciated with two senses of rotation, so it becomes very simple and relevant if and only if the groups are finite considering the universal finite fractal.

The fact that this entropy inside the physicality is in a kind of pure cooling seems relevant when we insert this entropy, physical and its steps.and also this ionfinite light above our walls. It is paradoxal. In all case this entropy creates the physicality with specific codes inside the singularities.

Best Regards

Jayakar Joseph

Dear Samir D. Mathur,

As quantization is indispensable to describe infinite universe in that quantization of universe in entirety is not empirical, we may think of quantizing the entire universe in its current segmental-fluctuation cycle.

Quantization of the entire 3-D universe is incomplete in the current scenario of dimensionality without string dynamics, as the plank volume by plank length that evolves from speed of light, is assigned in a framework of dynamics of 0-D discrete photons that predicts the universe with black hole singularity.

Thus we need to rethink the dynamics of universe that endorses another scenario of dimensionality, in that the quantization of gravity that emerges as a tensor product of 3-D tetrahedral-branes of 1-D eigen-rotational strings, without any black hole predictions. This implies the definition of unit volume of tetrahedral-brane instead of unit length of plank length that is variable in this scenario, whereas the unit volume of brane is invariant for the segment of holarchial universe it belongs.

With best wishes,

Jayakar

Jayakar Joseph

Regret for the repeated spelling mistake by me on this post. Please read, 'Planck' instead of 'Plank'

Jayakar

[deleted]

Samir

Look my table

Big Bang; Present; Big Crunch

c=10^30; c=10^10; c=10^-10

G=10^22; G=10^-8; G=10^-28

h=10^-28; h=10^-28; h=10^-28

alfa =10^-3; 1/ 137; 1

e=0,1 ; e=e ; e=12

String in moment Big Bang c x h = 10^2 erg x sm

String in Present moment c x h = 10^-18 erg x sm

String in moment Big Crunch c x h = 10^-38 erg x sm

[deleted]

10^2 erg x sm /10^-38 erg x sm =10^40 Dirac number

Sergey Fedosin

Dear Samir,

In Covariant theory of gravitation it is found the meaning of cosmological constant and supposed the absence of black holes. Also for quantum gravity may be used strong gravitation. Can you comment about it?

Sergey Fedosin

Avtar Singh

Dear Samir:

I enjoyed reading your well-written and intuitive essay.

Your statement - "....quantum gravitational physics does not stay confined to Planck scales, but in fact leaks out to affect all scales, including the largest scales of astrophysics and Cosmology" is strongly vindicated in my paper - -" From Absurd to Elegant Universe".

My paper shows that once the proper missing physics of spontaneous decay is considered it successfully predicts the observed data at all scales from below Planck scale to beyond cosmological scales. The proposed model not only resolves black hole singularities but also the unresolved paradoxes of physics and cosmology. It also explains the inner workings of QM and eliminates its inconsistencies with relativity.

I would greatly appreciate your comments on my paper. You can contact me at avsingh@alum.mit.edu.

Best of Luck and Regards

Avtar Singh

[deleted]

For better clarification my approach

I sending to you Frank Wilczek's 3 keen articles

http://ctpweb.lns.mit.edu/physics_today/phystoday/Abs_limits393.pdf

http://ctpweb.lns.mit.edu/physics_today/phystoday/Abs_limits400.pdf

All the best

Yuri danoyan

[deleted]

For better clarification my approach

I sending to you Frank Wilczek's 3 keen articles

http://ctpweb.lns.mit.edu/physics_today/phystoday/Abs_limits388.pdf

http://ctpweb.lns.mit.edu/physics_today/phystoday/Abs_limits393.pdf

http://ctpweb.lns.mit.edu/physics_today/phystoday/Abs_limits400.pdf

All the best

i missed part 1