# Blow to quantum gravity

by wolram
Tags: blow, gravity, quantum
 PF Gold P: 3,682 http://www.spacedaily.com/news/cosmology-03i.html For the second time in as many months, images gathered by the Hubble Space Telescope (HST) are raising questions about the structures of time and gravity, and the fabric of space. -------------------------------------------------------------------- no comment.
Astronomy
PF Gold
P: 23,083
 Originally posted by wolram http://www.spacedaily.com/news/cosmology-03i.html For the second time in as many months, images gathered by the Hubble Space Telescope (HST) are raising questions about the structures of time and gravity, and the fabric of space. -------------------------------------------------------------------- no comment.
The news item is from March 2003 and refers to a paper by Roberto Ragazzoni and others
http://arxiv.org./abs/astro-ph/0303043
"Lack of evidence for quantum structure of space-time at Planck scales"

The previous paper (Ragazzoni's was the second) was by Rieu and Hillman
http://arxiv.org./abs/astro-ph/0211402
"Stringent limits on the existence of Planck time from stellar interferometry"

The implications of this, and another Rieu and Hillman paper, for LQG were discussed by Smolin in March 2003 on page 19 of "How far are we from the quantum theory of gravity?"
http://arxiv.org./abs/hep-th/0303185

Smolin discusses the implications of a half a dozen or so such papers.
So far they have not succeeded in restricting the parameter space by very much. But it is a hopeful development that LQG is already getting the attention of observational workers (like Ragazzoni, Rieu, Hillman and a dozen or more others) and they are already trying to narrow down the range of parameters for the theory. Smolin describes some future programs which hopefully will do this still further.

Since Smolin cites it too, I will add another Reiu Hillman paper
http://arxiv.org./abs/astro-ph/0301184
My opinion is that one needs patience---the real tests will not be for some years, with better instruments, and the papers one is
getting now are just warming up for the real show-down. Their headline titles somewhat overplay their significance. But that is
just my personal take on it.
 PF Gold P: 3,682 how can a purly optical observation prove anything about the ultimate make up of space time? i understand the theory about distant objects exhibiting fuzziness, but how can one predict how out of phase, step, alignment,these planckian packets of quanta are.
Astronomy
PF Gold
P: 23,083
Blow to quantum gravity

 Originally posted by wolram how can a purly optical observation prove anything about the ultimate make up of space time? i understand the theory about distant objects exhibiting fuzziness, but how can one predict how out of phase, step, alignment,these planckian packets of quanta are.
I do not fully understand the testing of LQG that (in a small gradual way) has already begun. The most concise description is pages 17-20 of Smolin's March 2003 paper
"How far are we from the quantum theory of gravity?"
http://arxiv.org./abs/hep-th/0303185

Someone else may of course step in and make it all clear, but supposing they dont, then here is what I suggest you do.

Read pages 17-20 whether you can understand them or not.
This will acquaint you with two possibilities "Scenario A and B"
Smolin says that Scenario A is already ruled out by the observations.
It will also acquaint you with two numbers alpha and beta
which occur in formula (2) at the top of page 18.
These numbers are the numbers that the observations are struggling to squeeze down into a narrowing range of possibility---to "constrain" as stringently as possible.

Formula (2) is extremely interesting because it is a variation of the famous "Eee equals emcee square" formula. You cant immediately see this because for neatness he has set c = 1, so that c does not appear when the formula is written. But one could put the c back in.
Also both sides have been squared so the famous formula becomes

E2 = M2c4+....(other terms)

the other terms include the momentum p2c2 which was always there but was often set equal to zero or ignored in popular discussions, so it is not controversial. And then there are the really controversial other terms that have alpha and beta in them. These are special uses of alpha and beta (which can mean various things in different branches of physics.) Here alpha and beta just mean the unknown coefficients of those other terms you can see in equation (2).

The observation worker's job is to force those numbers alpha and beta as close to zero as possible and leave the theorists as little leeway or slack as possible in their theory.

Smolin describes the progress made by the observation people. Good idea to look at pages 17-20 even if not all is comprehensible.
Emeritus
PF Gold
P: 4,014
marcus, thanks for bringing this to the attention of PF readers.
 So far they have not succeeded in restricting the parameter space by very much.
Yep. What's heartening is that there are observations within reach which may constrain ST or LQG.
 My opinion is that one needs patience---the real tests will not be for some years, with better instruments, and the papers one is getting now are just warming up for the real show-down. Their headline titles somewhat overplay their significance.
Well said!

We should have more discussion of how observations may constrain Planck-length theories.
 PF Gold P: 3,682 there is a lot to take in when reading these papers but i now have a glimer of understanding. it seems one of the tasks is to define or rule out the planck scale, part of this task is already underway useing optical methods to define how grainy space time is,if images looked at from great distances are sharp then it is unlikly that space time is grainy. as i say it is only a glimer, but i still have a lot to read.
Astronomy
PF Gold
P: 23,083
 Originally posted by wolram there is a lot to take in when reading these papers but i now have a glimer of understanding. it seems one of the tasks is to define or rule out the planck scale, part of this task is already underway useing optical methods to define how grainy space time is,if images looked at from great distances are sharp then it is unlikly that space time is grainy. as i say it is only a glimer, but i still have a lot to read.
I would say something now like "relax we have a long road ahead".
You have looked at those 3 or 4 pages which describe two numbers that they are trying to narrow down the estimates on.
As long as you have looked at pages 17-20 of Smolin you know what I mean when I say alpha and beta.

"Constrain" is not the same thing as "rule out". It would help the LQG theorists if some experiments or astronomical observations could say "well, we cant say those two numbers are exactly zero but we are pretty sure they are less than such-and-such" Less than
0.1 or less than 0.01 or whatever upper bound the astronomers can get on them.

this in turn would give the theorists something to go on "OK we have to construct the theory so that alpha and beta turn out to be at least this small"

or in an extreme case where they have no leeway they have to throw out a whole chunk of theory and come up with a replacement. but so far the observations are nowhere near that point.

whatever impression the journalists give, right now observations that have a bearing on LQG and constrain the parameters are welcomed. At quantum gravity conferences, the observational people are invited to give talks. "Phenomenology" (looking for measurable phenomena or effects that could be used to test the theory, working out predictions that might in future be checkable) is something to encourage.

but the process of constraining---narrowing down the possibilities---zeroing in on the range of possible alpha and beta parameters (or if not those precise ones then other ones that grow up to take their place)----is very slow

so far it has not proceeded very far

but I still find it interesting that in Smolin's paper there is this modified "ee equals emcee-square" formula that has a bit of flexibility or slack in it, as exemplified by those two unknown numbers. A very slight room for bending, to possibly get just a slightly better fit to nature.

for now there is not much more to say about that aspect of quantum gravity. Smolin's paper looks ahead to IIRC 2006 and 2010 when some new observational tools come into play. I dont remember exactly what the projected timetable is, and I dont understand the detail. Time enough to find out about that when there is more of it happening.
 PF Gold P: 3,682 MARCUS OR SOMEONE, can you explain a little more about the greek alpha and beta, i think alpha is a elecric coupling constant and beta is some finite area, but i could be up a gum tree.
Astronomy
PF Gold
P: 23,083
 Originally posted by wolram MARCUS OR SOMEONE, can you explain a little more about the greek alpha and beta, i think alpha is a elecric coupling constant and beta is some finite area, but i could be up a gum tree.
You are right that in the theory of atoms (just to be a little more general let's say quantum electrodynamics) and throughout a lot of other physics the letter alpha is commonly used for the "fine structure constant". It is a 'coupling constant' that tells how strong the (square of the) electron charge is in natural units.

But that doesn't stop Smolin and other from using alpha to mean other things. There are so few letters in the latin and greek alphabets and so many numbers to keep track of. So they "re-use" symbols a lot.

Please stay out of the gum tree and climb trees that are more tidy, like maple.

You have now gone and gotten me interested in this modification of the "ee equals ..." relation and I have been reading papers about it
today. It is fairly recent stuff. I have not been able to get a clear focussed view of it.

I have been reading http://arxiv.org/hep-th/0306134. And several other previous papers. I will not recommend any of them. But as soon as I think I have some perspective on this I will post.

one of the players in this free-for-all of ideas is called "doubly special relativity" I used to not pay attention to DSR but now I have gotten curious, might be something to it. It is like Special Relativity (where the speed of light looks the same to all observers) except that there are now TWO things that look the same---the speed of light and some very small length (I think) or the cosmological constant or something. Have to go but will be back on this tomorrow
 Emeritus Sci Advisor PF Gold P: 4,014 marcus, wolfram, I've been thinking about the observational paper, and have some wild thoughts on how we could use publicly available data to do some more, at least somewhat similar, tests. Interested in working on this with me, on PF?
Astronomy
PF Gold
P: 23,083
 Originally posted by Nereid marcus, wolfram, I've been thinking about the observational paper, and have some wild thoughts on how we could use publicly available data to do some more, at least somewhat similar, tests. Interested in working on this with me, on PF?
of course Nereid (speaking for myself) you know a lot and would be fun to try to work with. I would like to hear what public-access data you have found and what your ideas are.

if I find I either dont understand enough or am not handy enough with data-analysis, I will drop out. but there is a possibility that I can help and that would make it extra fun.

 Emeritus Sci Advisor PF Gold P: 4,014 Working from the Ragozonni and Turatto paper (marcus: "The news item is from March 2003 and refers to a paper by Roberto Ragazzoni and others http://arxiv.org./abs/astro-ph/0303043 "Lack of evidence for quantum structure of space-time at Planck scales""). (5) is: $$\eta = \displaystyle{\frac{\Delta\theta}{\lambda/D}} = \alpha_0\displaystyle{\frac{L}{\lambda}} \displaystyle{(\frac{l_P}{\lambda})^\alpha} }$$ $$\Delta\theta$$ is the blurring introduced by Planck-scale phenomena D is the diameter of the telescope (or separation distance of the components in an interferometer) L is the distance to the (cosmologically distant) source $$\lambda$$ is the wavelength at which the observation is being made $$l_P$$ is the Planck length $$\alpha_0$$ and $$\alpha$$ are the parameters we are trying to constrain "The meaning of $$\eta$$ is that it directly influences fringe visibility in the case of an interferometer, or the Strehl ratio S of deterioration in point spread function in the case of a telescope." Leave aside Chandra and XMM-Newton (and a few other interesting 'observatories'), what sort of $$\Delta\theta$$, D, L, and $$\lambda$$ space can we search in (from public domain data)? IMHO, the wavelength domains are light (inc near IR) and radio; D is 2.4m (Hubble) and ~8-10m (Keck, Subaru, Geminis, VLTs); L is what it is; $$\Delta\theta$$ is the best that the respective telescopes (and interferometers?) can do. We should also be looking for observations whose 'non-blurred' angular size is way below $$\Delta\theta$$. What sorts of objects/observations might we consider?