Evidence Against Quantum Gravity

[ranyart]

Originally posted by marcus
well
who is Joe Magueijo and what kind of reputation does he have as a theoretical physicist?

this could matter because what I called the Smolin paper is actually by
Maguieijo and Smolin.
It is called "Generalized Lorentz invariance with an invariant energy scale"

Magueijo is at the Blackett Lab of the Imperial College in London.

I don't know how it happened that I didnt see this paper earlier. an oversight.

Marcus,:who is Joe Magueijo and what kind of reputation does he have as a theoretical physicist?

Surely you must be joking!

The paper here quoted:"Generalized Lorentz invariance with an invariant energy scale"
Came out some years ago, my copy is well worn, and there are some significant insights contained within.

I say this with some conviction that I was a porter at Swansea University at the Halls of Residence. In the mid-nineties Ed Witten was giving a 're-scheduled' lecture to the worlds formost Post Grads,

It is certain that Joao, benefeted from a discussion I gave to a number of Post-Grads in the 'snooker-hall' one evening. I gave a precise talk on how the speed of light had to be different at the early-universe, I am in no way saying that I had been the first to state this 'Varying Speed of Light', but I certainly left a good impression, and being that Ed Witten himself was presenting his talks to the Post-grads in such a way, most(PGs) were contemplating falling out and leaving the Superstring 'M' Theory arena.

back to the said paper, there is relevence to a diverging speed of light and energy, I will be locating the papers I have on Joao, and will make some speculations, it will relate to the breaking of the Second Law of Thermo dynamics (the result of which has played an important part in the evolution of our observed Universe), this as I recall was touched upon in a paper by Joao, but until I retrieve the paperwork I will say no more.

 

[wolram]

MARCUS,
i had quick google for J MAGUEIJO, most of the sites
are advertising his book, it seems the media like him
for his contravertial views.
number 2 in my list may interest you.
-------------------------------------------------------------------


an interview with JOAO MAGUEIJO.

http://www.abc.net.au/rn/science/ss/stories/s212674.htm
-----------------------------------------------------------
can a changing "a"explain the supernova results.
john d. barrow and joao magueijo

http://xxx.lanl.gov/PS_cache/astro-ph/pdf/9907/9907354.pdf
-----------------------------------------------------------
joao magueijo, lee smolin.
generalized lorentz invariance---------

http://www.lns.cornell.edu/spr/2002-07/msg0043162.html

 

[marcus]

Thanks ranyart, thanks wolram!

the link to the discussion on SPR, by Jeremie Vinet,
was especially helpful in my case. I had somehow just
filtered out a lot of the talk about variable speed of light
as too wild to consider.

this whole business (whether one thinks of light's speed varying in the course of time or being constant in time but varying with the energy of elementary particles) is a can of worms and would be safer
left on the back shelf, as was suggested

It seems to me at this point that Rovelli must have been at pains to distance his "vanilla" (plain unvarying speed of light) version of LQG from this more exotic approach: one that obviously intrigues Smolin.
This would have been one of the motivations for the May 2002 paper
by Rovelli and Speziale "Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald constraction".

As a spectator I don't need to favor one over the other and have no business doing so, but my sympathies are more on the vanilla side.

As I understand it, the main reason (perhaps fundamentally the only reason) this comes up is that at least from a naive perspective ANY theory that distinguishes the Planck scale as a place where new things start to happen is effectively setting up a quantity which (like the speed of light) should be the same to all observers. Rovelli attempts to deflect this, and indeed may succeed in showing what is wrong with it. I am going to take another look at what he says (and also at the naive argument that all observers should agree on what lengths, energies etc are Planck-scale because new behavior starts to happen around that scale.)

there is more discussion of these issues at the start of this earlier thread:

https://www.physicsforums.com/showthread.php?s=&threadid=8514
"how area in LQG transforms under Lorentz boosts"

 

[PRodQuanta]

Im kinda jumping into the discussion late here, but I got a couple questions I need to clear up.

1) I understand the GLAST experiment, except for it's implications to LQG. How does this experiment relate to LQG's well known spin networks? Does it imply that because the gamma particle has more frequency (less wavelength) that it takes a shorter path in the network than the particle with less frequency? I'm a little lost here.
Paden Roder

 

[marcus]

hello Paden, anyone who can help out with explanation is invited to jump in
In what I've read the connection (which Smolin sees but Rovelli seems to be reserving judgement about) between Loop Gravity and some "DSR"-like theory with its energy-dependent speed is not a firm one.

It isn't based on any mechanism that one can visualize. In loop gravity space is not "made" of spin networks---the spin networks serve to catalog the quantum exitations of space. they provide a orthogonal basis of a hilbertspace of the quantum states space can be in.

so it doesn't seem to help to visualize a gammaray photon traveling thru a spin network! I haven't read anybody thinking of it that way

The intuitive argument for DSR (doubly special relativity) is of a different kind. Main proponent seems to be G Amelino-Camelia. He says that since quantum effects are expected to become important at Planck scale the main milestone or boundary-marker of that scale (namely Planck length or Planck energy) should be the same for all observers.

If we are going to see remarkable weird new behavior at Planck scale then moving observers ought to be able to agree what that scale is! what wavelengths and particle energies are at that scale and what are not.

This is a dreadful realization because it means that instead of ONLY ONE thing, the speed of light, that has to be the same for all observers and has to be invariant under lorentz group, you now have TWO quantities (speed of light AND Planck length or energy for elementary particles) that have to be invariant.

then there is a kind of mathematical thing that comes in, if you want TWO quantities to be invariant (and yet retain the principle of relativity that all observers are equal, there being no one distinguished frame) then there has to be some "give" in the system.
there can be a "normal" speed of light that is the same for all observers and which photons have in the low energy limit but we are forced to consider the possibility that very high energy photons
get away from this

Fortunately Rovelli has found a way around this, and has avoided getting on the DSR track. Things seem a lot simpler if you just have one invariant quantity (c) that looks the same to all observers. All light can travel that speed including very high energy photons. Whew! What a relief!

He describes his viewpoint in the paper called "Reconcile..." I gave a link earlier.

But at least for now Smolin is going a different way and exploring the consequences of Giovanni AmelinoCamelia's idea---what if there were not just one but instead TWO invariant quantities----the speed limit c and the boundary marker of the Planckscale domain (plancklength or Planckenergy). This is what the GLAST can test because it can see if there is some variation of speed with elementary particle energy.

Part of your question is what physical MECHANISM for varying speed has been imagined, and I don't know of any----it is more a mathematical revenge that nature takes on you if you try to pin her down at two points instead of just one---with two invariant quantities instead of only a single one.

 

[marcus]

the revenge is a non-linear distortion of the way the lorentz group transforms energy and momentum
with a side effect that the speed of high energy photons gets away from the norm to a noticeable extent---or one GLAST might notice anyway.

 

[PRodQuanta]

Thank you marcus. Although I was not implying that space is made up of spin networks,(although looking back, I did not take time to word my thoughts properly, for that I am sorry) just like outer space is not made up of little gridlines (like that is expressed when trying to visually show the warping of space that you most commonly see)I was just wondering how this these photons can move faster or slower than one another, and how this can be explained in a spin network.

But from what I got from your answer (please correct me if I am wrong) is that spin networks (or foams) are not involved or used to try and describe this phenomenon? Sorry for all of the questions, I am just not a expert in this category.
Paden Roder

 

[marcus]

Originally posted by PRodQuanta
...I was just wondering how this these photons can move faster or slower than one another, and how this can be explained in a spin network.

But from what I got from your answer (please correct me if I am wrong) is that spin networks (or foams) are not involved or used to try and describe this phenomenon? Sorry for all of the questions, I am just not a expert in this category.
Paden Roder

Yeah me neither. It just interests me. I think this is an historical moment in science where some new understanding (of nature of space and time) is likely to emerge in order to put together
the two main 20th C physical theories: GR and QM.

It is historical because people have been trying to do it since around 1930 without much success and there seem to be some signs that now the roadblocks are breaking up.

selfAdjoint just posted something that wasnt either loop or string---it was conditional renormalizability in vintage 1960-1970 quantum gravity. cracks in the ice

Sorry for all the questions! I have to laugh. The best we can hope for is someone will ask an intelligent question. You just asked a really intelligent one---in my estimation: is there some mechanism for the dispersion of velocity with particle energy?

AFAIK there is NOT. Maybe some visitor passing thru PF will correct me and say Yes there is a mechanism that so and so has imagined in some paper (and hopefully give a link to it!) That would be great. But so far I have never heard any mechanism described

Also the more I think about it the crazier DSR (doubly special rel) seems. I am more and more attracted and persuaded by Rovelli's argument that you do not need DSR. You can have relativity of frames and you can have agreement among observers about certain Planck scale things----like a minimal nonzero area eigenvalue----and still not have to make Planck-energy into an invariant or take some radical DSR step like that.

But Smolin is seeming now to take the DSR track. Seems so to me anyway. Is there something wrong with Rovelli's argument that you do not need to? If you do not need to, if you can do LQG without that, then why do it? DSR makes things a lot more complicated.

the hidden factor here is the cosmological constant. no one has a sure idea what it is or where it will eventually fit into physical theory----smolin has been trying to fit it into loop----other people try to fit it into other sorts of theories. Maybe the cc doesn't even exist---but most cosmologists accept that it is 70 percent of the average energy density of the world so it seems like it must exist.

If it exists then when it gets fitted into basic physical theory it could well force something besides the speed of light to be held constant or to be an invariant that all observers agree about. So then DSR or something like it could creep in---could infiltrate by way of the cosmological constant, the dark energy so-called.

You can tell I am confused and unsettled by this. Let us make a pact: you do not apologize for asking questions. I do not apologize for being confused and not being able to respond.

but basically no, there is no proposed mechanism for velocity dispersion that I know of, and I would love to be told about one
by anybody at PF

 

[PRodQuanta]

Lol... Thanks marcus. Let's see if I can find another question pertaining quantum gravity.

(If this is correct, it will make sense, if not, ignore it, lol)Since LCG gives rise to the fact that the smallest element of anything is the Planck length, then there shall be nothing smaller, correct? Now in SST, the smallest element is a string. So, in these theories, there is a discprency. Or does SST actually describe LCG's fact of Planck length units?
Paden Roder

 

[PRodQuanta]

And if this is correct, shouldn't there be a recipricol effect? If there is a minimum unit, shouldn't there be a maximum? In other words, In a world where LQG exists, there should be no such thing as infinities and asymtotes?

So, in general, there is (??) discrepency between SST and LQG, but yet there is a sense of similarities between them??

Steps toward unification?? Disappointment?? Time will tell I guess, but until then, questions will do!
Paden Roder

 

[MathematicalPhysicist]

Originally posted by marcus
These are two alternative links to the Ragazzoni, Turatto, Gaessler article (RTG).

I mentioned already the Jacobson, Liberati, Mattingly (JLM) article
http://arxiv.org/astro-ph/0212190

In case anyone is interested, just today a new Planck-scale phenomenology article was posted by Amelino-Camelia

http://arxiv.org./abs/astro-ph/0312014

It is 9 pages, dated 8 December 2003, and called
"Planck-scale structure of spacetime and some implications for astrophysics and cosmology"

It is the text of a talk he gave at some conference this September.
Whatever else you can say, the guy is prolific. This November's issue of "Physics World" magazine was devoted to Quantum Gravity and had three invited articles:
Amelino-Camelia on QG Phenomenology

does this qg phenomolgy has any resemblence to husserl phenomolgy which I've just now got acquinted to?

 

[marcus]

Originally posted by loop quantum gravity
does this qg phenomolgy has any resemblence to husserl phenomolgy which I've just now got acquinted to?

as far as I know there is no resemblance and no logical connection whatsoever

"Phenomenology" has two separate meanings:

A.
among physicists it refers to the empirical process of testing theories---see for example the "HEP-PH" category in the preprint archives (which means "High Energy Physics-Phenomenology")----papers in the area of research ask questions like "what experiment can we do to test this theory? what does this theory predict? what observations can we make and what can we measure to see if the theory is right?"

B.
but in the Religious Studies department of Stanford University
you can find signs of interest in the history of
Philosophy/Psychology/Religion
and old philosophical movements, including the
movement of Edmund Husserl (1859-1938) called "Phenomenology"

Here is some writing by Husserl at the Religious Studies website.
http://www.stanford.edu/dept/relstud/faculty/sheehan.bak/EHtrans/5-eb.pdf

Also in the Catholic University of Leuven, in the Netherlands, there is a history project to study the history of Husserl's philosophical movement.

Husserl was born a Jew but when he was 28 years old he changed over and became a Lutheran. He seems to have been very concerned with spiritual questions and in asking if life and consciousness has a purpose. He had an important influence on Martin Heidegger and J-P Sartre. (Being an important influence on Martin Heidegger is not necessarily a plus, or so I think.)

Personally I cannot imagine any connection between science and the Philosophy/Psychology/Religion teachings of Husserl.
The phenomenology on which all empirical science must be based (i.e. the business of testing models by experiment) is completely different from the Phenomenology Movement of Husserl back in the early 20th century.

 

[selfAdjoint]

Husserl's phenomonology is not only religious. He was a major source, along with Nietsche and Heidegger, for Sartre, who were certainly not religious. His method is an extreme form of introspection, characterized by "bracketing" or "discriminating" certain aspects of our perceiving, intending, etc. in order to free other, hopefully deeper aspects for study.

Here are two concerns or puzzles that motivated Husserl, and are important to Sartre too.

I am a small living being on a small world in the great universe, hardly noticible in that scheme of things. On the other hand everything I observe and sense is part of my own story, that belongs to me, and in this sense, from quarks to quasars, I own the universe. Contradiction. (Husserl would say the external object thus internalized is "for me", Sartre calls the internalizing ego the pour-soi).

Think of any familiar object - say a chair in your home. You have seen it thousands of times, under all different lighting conditions, from all different angles. From all these thousands of very different perceptions, how do you make sure they all concern or constitute some one definite thing in the world? How do they make THIS CHAIR in your perceptual world?

Of course a scientific or from-the-outside view of things makes nothing of these questions. But if we think of the persistent work of human beings as the making and care of perceptual worlds, which is what we do every moment of our waking lives, whether we do anything else or not, then they are critical.