
#127
Feb1312, 07:31 PM

P: 381

Craig Hogan is building the Holometer as this month SciAm detailed. What's funny is that if it produces nonnull. It confirms the discreteness of spacetime and supporting digital universe. But what does it support, the discreteness of spacetime due to LQG or the digital feature due to the Ads/CFT? So String Theory can only be truly background independent if the universe supports the holographic principle? Yet I think the holographic principle is not widely supported and even on the speculative side. So it means there are some String Theorists who still think nature doesn't have to be background independent and GR is just some side effect of the theory? 



#128
Feb1312, 07:41 PM

Sci Advisor
P: 8,004





#129
Feb1312, 08:59 PM

P: 381





#130
Feb1412, 03:20 AM

P: 344





#131
Feb1412, 04:46 AM

P: 381

GR = mass/stress/energy causing spacetime curvature LQG = spin networks/foam make up spacetime Strings = Strings modes create spacetime irregardless of the backgrounds Therefore background independence means differently in each case. And maybe we must not prefer one over the other. About QFT. Maybe it just ignores the mass/stress/energy effect on spacetime because it's negligible anyways. 



#132
Feb1412, 07:15 AM

P: 381

Also you seem to be saying that perturbative string theory can do that. How does this differs to nonperturbative string theory (is this about AdS/CFT?)? 



#133
Feb1412, 08:23 AM

Sci Advisor
P: 8,004

In AdS/CFT, even strings are not fundamental, and instead emerge holographically from the boundary theory. 



#134
Feb1412, 08:36 AM

P: 381

Earlier when I mentioned about the idea of flat spacetime + gravitons = curve spacetime. Marcus emphasized it was not standard in string theory. Now you are saying it's standard. Or maybe if we add strings in the context. Then it's standard in string theory. When no strings and just the idea of flat spacetime + gravitons = curve spacetime , then not standard. Is this it? Please elaborate as this got me confused for 5 years already. Thanks. 



#135
Feb1412, 08:47 AM

Sci Advisor
P: 8,004





#136
Feb1412, 09:15 AM

P: 381

"I think some relevant comment is contained in the posts that follow #4. By starting with a flat background you rule out big bang and black hole stuff. Also rule out one of the more common spatially finite versions of standard cosmology. As I recall someone in the thread was pointing that out. Basically it is inconvenient, one could say crippling, to start out that way but you can recover a sector of the geometric theory, at least locally. I'd say no QG approach has to explicitly deal with this special flat model because it empirically indistinguishable where it applies. (and since it doesn't cover all the cases it would be a botherso people normally use the full theory.) but mathematically interesting certainly." Marcus seems to disagree. If it's standard, why didn't he agree? Now I'm confused. 



#137
Feb1412, 09:24 AM

Astronomy
Sci Advisor
PF Gold
P: 22,796

But it has recognized limitations as a way to think about reality. The "flat space+small curvature perturbations" picture is not taken as fundamental. In nonstring QG there was a bunch of papers about gravitons, doing calculations. In Loop the graviton papers started coming in around 2007, certain things had to be checked so people did that. 



#138
Feb1412, 09:32 AM

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P: 8,004





#139
Feb1412, 09:34 AM

Astronomy
Sci Advisor
PF Gold
P: 22,796

I don't take issue with that. It's not a good way to picture reality when you are thinking about big bang cosmology. I take issue with someone calling the flat picture the real "territory" and the GR picture a mere "map". 



#140
Feb1412, 09:42 AM

P: 381

"Everything the whole universe, would be happening in some fixed eternal Euclidean space, and everything includes BH collapse. Your theory would then have to explain how a "graviton" gets from the heart of a black hole out past the horizon to exert a "pull" on somebody orbiting the BH. And all the stuff about how the clock on the mountain top runs faster than the one in the valley. I guess because the "gravitons" slow clocks down." Marcus didn't agree with it. But you Atyy agreed that we could be living in a flat spacetime and gravitons giving us GR. Marcus. I think Atyy is saying the flat picture is the real "territory" and the GR picture a mere "map". 



#141
Feb1412, 09:47 AM

P: 381

Wait. You mean in the Wheeler Gravitation book the writers were referring to low energies only? I thought it includes high energies which if true means the strings were moving in flat background and these graviton modes giving all the curvature. Hence. The flat picture is the real "territory" and the GR picture a mere "map". Maybe Hobba just misunderstood this from Carlip and the Wheeler Gravitation.




#142
Feb1412, 10:05 AM

Sci Advisor
P: 8,004

Now what about the quantum versions? The first classical picture has no known quantization. The second classical picture has a quantum version, but the quantum version only works below the Planck scale. String theory tries to complete the quantum version of the second picture above the Planck scale by introducing new objects called strings. Loops tries to complete the quantum version of the second picture by quantizing the first classical picture. 



#143
Feb1412, 10:06 AM

P: 381

"Gravitons interact with all matterenergy. They interact in such a way as to make rulers and clocks behave as if spacetime had curvature. It is a semantic issue of zero scientific value if spacetime is thus curved or just appears curved. At this stage their is no way to experimentally distinguish between the two views." Good to understood now it is only low energies. Thanks. (Bhobba, who is a participant here, please comment if you don't agree). 



#144
Feb1412, 10:24 AM

Astronomy
Sci Advisor
PF Gold
P: 22,796

As far as I know CDT and Shape do not have any graviton papers as yet. It is not the main concern, at some point you want to see if you can handle the low energy nearly flat case and reproduce certain results. Loop has done this now to some extent, but those others not. I don't know if Hobba misunderstood or whether he knew better but was just goofing off. Attentiongetting? I can't say, because I've only a cursory glimpse. The whole thing with Hobba struck me as having a kind of geriatric flavor. Harking back to papers from the 1970s. Weinberg's *Gravitation and Cosmology* book from 1972 etc. Or something Carlip said at some point in the past. There was a temporary suspicion among particle theorists back then that you actually did not have to take GR seriously and maybe you could do everything with a fixed flat space. But you might want to look at Weinberg's NEW book (2008). You can browse the ToC and Index on Amazon. It is called *Cosmology*. You will not find much if anything about the perturbative representation of GR. Very little if any mention of "gravitons". http://www.amazon.com/CosmologyStev.../dp/0198526822 The Physics Today review said it would be a great help to "particle physicists tooling up for cosmology" All based on dynamic changing curved geometry. HEP theorists taking GR more seriously now than, say, in 1972. Think about a massive star collapsing to form a black hole. Are you going to model that whole process from beginning to end using a fixed unchanging flat space with ripples running around on it? Perturbative methods of calculation very good for some things. Not a full picture of reality. The full picture has to be able to handle extremes, highly dynamic changing geometry, extreme density, extreme moments of expansion. "Graviton" picture is inconvenient not to say unworkable. So (as Atyy indicates) the fashion among researchers has swung towards nonperturbative models. (which is where the relativists have been all along.) 


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