The WLC/SC "Something From Nothing" Cosmology Thread

[_DrW]

What's your guess?

I'm not qualified to guess, but what was on my mind was how quantum fields aren't physical but D-branes are, if you say yes. and if no, then how would anything be physical-- why certain dimensions are "physical" and others aren't without being arbitrary.

String Theory and D-branes: As stated upthread, the strings of string theory do appear to be physical. They are said to have mass as well as charge. We are told that the strings themselves constitute particles, this depending upon their energy and vibrational modes. So I would think strings, and their associated branes, would qualify as physical.

We are further told that the string theory dimensions of which we are unaware (which is most of them) are somehow "wound up" or twisted so tightly that they are not detectable. They are supposed to be physical - we just can't detect them. In fact, we cannot detect the strings themselves without stupendous expenditures of energy - unachievable with present technology.

Again, I don't claim to understand string theory, or D-branes (or p-branes for that matter). I'm simply relating here what the string theorists tell us, and providing a best guess as to their physicality based on that.

LQG and Multiverse Theory: On the other hand, the multiverse theory that Sean Carroll describes is a natural consequence of cosmic inflation for which there is a great deal of physical evidence. In fact, this idea has developed into one of "eternal inflation". (Used in this sense eternal refers to the future.) Eternal inflation, and the multiverse it likely spawns, are a possible reality emerging from loop quantum gravity (LQG) theory.

LQG is "background independent". That is, LQG does not operate against a background of spacetime. (One might say that LQG needs no stage on which to perform.) It is the interaction of the quantum fields that actually create energy and matter which, in turn, create gravity and spacetime.

While they permeate all space, the non-excited quantum fields of LQG do not constitute a physical medium. For example LQG quantum fields do not interacts with light, as would an aether. This lack of interaction was demonstrated by Michaelson and Morley, and is consistent with special relativity.

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The references provided in the response to tana upthread should be sufficient for a good overview LQG. Again I would also recommend Carlo Rovelli's book entitled, "Reality is not what it seems - The journey to loop quantum gravity".

Sean Carroll's "The Big Picture" is also worthwhile, in no small measure because it also serves as an excellent manifesto for naturalism.

Of course, string theory has been described in the popular literature in more than one book by Brian Greene, including "The Elegant Universe".

[_Ceeboo]

Hey Mr. Ripley!

[MODERATOR NOTE: Ceeboo, please immediately stop telling people to go away, even if you're partially joking.

Oh there was nothing partial about Ceeboo's joking, so he's in the clear.

Thank God!

[_Choyo Chagas]

[MODERATOR NOTE: Ceeboo, please immediately stop telling people to go away, even if you're partially joking.

Oh there was nothing partial about Ceeboo's joking, so he's in the clear.

as clear as our screen; thanks for the calfy

[_Gadianton]

Your description makes me think I have the wrong kind of brane in mind. There was an episode of the Universe that showed two branes described as being like slices of bread clashing together and creating the universe. My understanding was that the universe was still somehow part of those branes, in this theory, and so sounded like the branes were "off stage" but maybe not really?

[_DrW]

Your description makes me think I have the wrong kind of brane in mind. There was an episode of the Universe that showed two branes described as being like slices of bread clashing together and creating the universe. My understanding was that the universe was still somehow part of those branes, in this theory, and so sounded like the branes were "off stage" but maybe not really?

You and I are referring to the same kind of branes. D-branes are simply a special class of p-branes. As described above, these D-branes are surfaces on which so called 'open strings' are localized.

Since the branes have mass, they will necessarily exist in spacetime. So, in that sense, they operate against a pre-existing background (or on a stage - my wording). This is the 'stage' of spacetime - a stage that the quantum fields in LQG are independent of because they can create spacetime for themselves.

You remember correctly regarding the idea that occasional (but apparently very rare) contacts between waves or undulations in at least two different branes are what cause massive releases of energy that lead to inflation and a Big Bang type event.

[_mentalgymnast]

...I don't claim to understand string theory, or D-branes (or p-branes for that matter).

When we were kids we knew who the p-branes were.

Thanks for this thread. Interesting stuff.

Regards,
MG

[_DrW]

...I don't claim to understand string theory, or D-branes (or p-branes for that matter).

When we were kids we knew who the p-branes were.

Thanks for this thread. Interesting stuff.

Regards,
MG

I had the same thought upon first encounter with this particular nomenclature. The "p" in p-branes is a dimension variable. And a D-brane is simply a p-brane that meets so-called Dirichlet boundary conditions.

So, for example, in one of the string theories there are 1, 5, and 9 dimensional D-branes (D1-branes, D5-branes and D9-branes, respectively).

Again, string theory is mainly math, and pretty esoteric math at that. In terms of practical applications, proponents claim that string theory has provided an effective tool for better understanding black holes (another class of esoteric phenomena that we will never be able to study directly).

IMHO, one would be hard pressed to think of much else in terms of practical application that has come from the decades of effort by more string theorists than the world probably needs.
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ETA: In his book, "Time Reborn: From the Crisis in Physics to the Future of the Universe", Lee Smolin pretty much agrees with this view of string theory.

[Guest]

I'm not an expert in loop quantum gravity, but I did read a thesis on it a couple of years ago as a second examiner, and my impression from that was that loop quantum gravity is not based on quantum fields at all. My understanding, indeed, is that the whole point of LQG is to avoid that. Instead the basic entities are mappings from the set of geometrical properties of closed loops in space, into the set of complex numbers, where (I think) the loops are not any loops in particular, but rather any arbitrary loops that one might in principle define. I have to say that I found this a good deal harder to grasp than string theory, so I might well be missing something here. Insofar as I'm on the right track about LQG, though, it has little to do with quantum field theory: it's a quantum theory, all right, but not a field theory.

What I do know is that LQG is a speculative theory that is still chugging along after string theory has boomed and busted, but that is still just a work in progress by a tiny minority of theoreticians. By no means is it accepted fact. I know that there is currently a whole little daughter field that calls itself "loop-inspired quantum cosmology". The "inspired" part is an abject confession that nobody really even knows what LQG implies for cosmology, so you can publish papers that are merely attempts to determine the implications of certain wild guesses as to what LQG might imply. Comparisons with data seem to be rare and inconclusive at best.

The only major theological issue that I can see in physical cosmology is whether the universe is eternal or had a beginning. Lots of observations and well-established theory make it hard to deny that the universe had much smaller volume than it has now, a finite time ago. What's not clear is whether we can extrapolate that right back to a singular beginning of time. That's the simplest available option, and to my mind that is worth something, but we can hardly rule out the possibility that the unknown rules that apply under those extreme conditions made the universe expand after first shrinking down for half of eternity ("Big Bounce"), or make a really big universe continually break out in Big Bangs like pimples, one of which happens to contain everything we've ever seen and more (the "multiverse"). Or something else, maybe—who knows?

Cosmology may be a science, but if the rest of science were much like cosmology, nobody would give a damn about science.

[_DrW]

I'm not an expert in loop quantum gravity, but I did read a thesis on it a couple of years ago as a second examiner, and my impression from that was that loop quantum gravity is not based on quantum fields at all. My understanding, indeed, is that the whole point of LQG is to avoid that. Instead the basic entities are mappings from the set of geometrical properties of closed loops in space, into the set of complex numbers, where (I think) the loops are not any loops in particular, but rather any arbitrary loops that one might in principle define. I have to say that I found this a good deal harder to grasp than string theory, so I might well be missing something here. Insofar as I'm on the right track about LQG, though, it has little to do with quantum field theory: it's a quantum theory, all right, but not a field theory.

Thanks for the clarification. It was not my intention to claim that LQG could be reconciled with, or incorporated into, QFT specifically rather than incorporating QM in general. I tried to separate discussion of the standard model from the mention of LQG.

I understand the fact that there is a renormalization problem when trying to reconcile GR and QFT or to incorporate GR (specifically gravitons) directly into QFT.

However, at the level of the discussion of the debate between Carroll and Craig, and at the level at which Carroll and Rovelli have discussed cosmology in their respective recent books (The Big Picture and Reality is Not What it Seems), I think that the explanations provided so far in this thread are probably adequate.

For example, Carroll talks specifically about quantum fields that permeate all of space 'prior' the Big Bang in his videos on cosmology, as does John Gribbin in his book Before the Big Bang. Craig referenced such fields in the debate video as well.

On page 192 of Rovelli's book cited above on Covariant LQG, there is a summary graphic that clearly and specifically shows "Covariant Quantum Fields" as the the basis for a ToE and indicates that Covariant LQG theory is a reconciliation of quantum fields and spacetime. You may want to have a look at Rovelli's book if you are interested and have not already done so.

In any case, I will go back over the thread when I have some time and clarify as needed. I will also redraw and post the graphic I mentioned from Rovelli's book. It provides an excellent overview for the lay reader of the major advances in understanding on the journey to what Rovelli considers to be a ToE, or at least an excellent candidate for such.

Thanks again for the clarification. It would have been great if you had engaged earlier in the discussion.
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I certainly do not claim to be an expert in LQG, or QFT or QM or in physics in general. However, I have had a love for physics since college. I earned an undergraduate degree in physics but could never really make a living at it. So I went over to the dark side. Nonetheless, I try to keep up basic skills and understanding when it comes to the force(s).

[Guest]

Physics Guy,

This is a re-draw of the graphic I mentioned upthread from Carlo Rovelli's book* on quantum gravity.

Note Rovelli's use of the term 'covariant quantum fields'.



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*Reality is not what it seems: the journey to quantum gravity.