The Third Road to Quantum Gravity

[Careful]

Looks like hand-waving to me. I see more mud than cement.

Dear Chronos,

I would kindly request you to take a look at the Ambjorn - Durhuus book about ``quantum gravity´´ (especially the Euclidean part is very useful) and then read up a bit more on the Lorentzian ``quantum gravity´´ formulation within dynamical triangulations. You might also want to study the Sorkin-Rideout classical dynamics for causal sets. Then, you might notice that only in CDT, the *dimension* comes out right (even a ridiculous observable like dimension poses great difficulties - and so far there are not more substantial results). In the spin foam, I am not even aware that *any* such results exists (not even a negative one). Next, you might want to gain some understanding of WHY this is so difficult and then see if your opinion deviates from mine.

To repeat myself: category theory is a beautiful abstraction (I was once seduced by medusa too ), but anyone who is announcing that it could very well be successful is either ignorant of these results, or is not willing to consider them. Ignorance is not bad at all (happens to me every day), however unwillingness (to listen) is stupid. I am still willing to listen if Kea comes up with a concrete example (she basically just said: ``string theory´´).

Cheers,

Careful

 

[Tonko]

the quantum computational approach is currently the most fruitful and promissing thing to happen to cosmology since the discovery of the Big Bang

Absolutely fascinating reading! Thank you so much for this list of articles. Excuse my lazyness, if there already is one and I haven't spotted it, but is there a thread dedicated to quantum computation? If not, can somebody qualified start it with a decent introduction for nonexperts?

I have just listened to a few lectures from the Perimeter Institute and I am in awe of what has develeped out of EPR and quantum entanglement.

It is not that I have completely missed on developments, I read about the quantum teleportation, for example, but I have never connected the dots.

I find Seth Lloyd's approach very appealing. Time to forget those almost century old images of clocks and rods in space, with spaceship explosions as the events

Tony

 

[Careful]

**
I have just listened to a few lectures from the Perimeter Institute and I am in awe of what has develeped out of EPR and quantum entanglement. **
NOTHING PRACTICAL has been developped so far (that is main critique people have about the research on entanglement - quantum computers are a dream until now). Microsoft reported recently some sucess AFAIR, but I am not at all convinced that this is QUANTUM (people tend to forget that classical correlations can be as high (as quantum ones) on sales of 1/10'th of a meter where you cannot exclude spacetime locality for realistic measurements).


** I find Seth Lloyd's approach very appealing. **

Read the section about spacetime reconstruction from the quantum computing process and tell me wether this is mathematically correct or not (I could equally ask you how many mistakes it contains).


**Time to forget those almost century old images of clocks and rods in space, with spaceship explosions as the events
Tony **

Really What do we have to believe in then? In infinite dimensional ghost spaces where god is playing dice but only allows us to see a four dimensional projection of this russian roulette ?

 

[Kea]

The String Coffee Table http://golem.ph.utexas.edu/string/index.shtml is a blog (maintained these days mainly by Urs Schreiber) that discusses the categorical aspects of String Theory. It is a great resource for those interested in the technical side of this story.

 

[Kea]

What do we have to believe in then? In infinite dimensional ghost spaces where god is playing dice but only allows us to see a four dimensional projection of this russian roulette?

Not a bad way of putting it! But a little better: in what we can measure and in what we can compute.

 

[Careful]

Not a bad way of putting it! But a little better: in what we can measure and in what we can compute.

Sorry, but a local realist would agree with that as well (so that is not a good characterization of QM) ! The question is whether the framework *underlying* the ``operational´´ machinery (i.e. the wavefunction in QM) as well as the measurement itself (the reduction in QM) is locally causal in spacetime or not. I was simply making a bit of fun of how huge Plato's quantum mechanical cave is in comparison to seize of the shadowworld.

 

[jarek]

NOTHING PRACTICAL has been developped so far (that is main critique people have about the research on entanglement - quantum computers are a dream until now).

Putting aside already aviable commercial quantum crypthographical devices (IDQuantique of N. Gisin and MagiQ), if you consider knowledge alone to be practical (at least in principle) then that's a bit of b-s what you are saying here, Careful. During the study of quantum information and entanglement people did learn quite a few new things about quantum mechanics: there is something in quantum states that bears some signs of "reality" and, for the lack of a better name, is labeled quantum information. I can understand that the language they speak in QIT sounds weird, much like engineering (my background is in GR)- convex sums, entropies, information content,etc - but no theory is born already in its final form. Also, I don't claim quantum info is *the way* (I fully agree with your view on Lloyd's paper but he is pretty isolated in his aspirations).
As for classical realism and stuff, my personal view on LHV and Bell-like theorems is that it simply states that quantum statistics cannot be simulated by local classical statistics. That's it. People try to inferr too much from this fact. I referr to Werner & Wolf's excellent paper on Bell stuff: quant-ph/0107093.
-jarek

 

[Careful]

**Putting aside already aviable commercial quantum crypthographical devices (IDQuantique of N. Gisin and MagiQ), if you consider knowledge alone to be practical (at least in principle) then that's a bit of b-s what you are saying here, Careful. **

Quantum cryptography is not equal to quantum computing, and again the question is wether this is QUANTUM or classical (and that is not clear at all).


**During the study of quantum information and entanglement people did learn quite a few new things about quantum mechanics: there is something in quantum states that bears some signs of "reality" and, for the lack of a better name, is labeled quantum information.**
I can understand that the language they speak in QIT sounds weird, much like engineering (my background is in GR)- convex sums, entropies, information content,etc - but no theory is born already in its final form. **

No, this language is not weird at all (I have been doing some quantum entropy myself - in my masters time - before I went to do real physics, ie. GR ). I deliberatly reacted so sharp because people seem to already have decided that the correlations obtained in such processes cannot be obtained by any *classical* means (on the appropriate distance scales). So, the question wether this quantum information is really so quantum in the sense that no underlying realistic theory can account for the correlations (or wether these correlations really can be obtained in practical situations) remains to be seen. Nevertheless, I acknowledge that gaining further insight is valuable when you later apply it to the relevant distance scales.


**Also, I don't claim quantum info is *the way* (I fully agree with your view on Lloyd's paper but he is pretty isolated in his aspirations). **

Good, that is sensible.

**As for classical realism and stuff, my personal view on LHV and Bell-like theorems is that it simply states that quantum statistics cannot be simulated by local classical statistics. **

Well, that depends upon the distance scales you consider. For example local classical statistics can be exactly the same as quantum statistics on those distance scales where the separability assumption cannot be made. That is one reason why I have good hopes for local realism.

Cheers,

Careful

 

[jarek]

Quantum cryptography is not equal to quantum computing, and again the question is wether this is QUANTUM or classical (and that is not clear at all).

Not equal, but cryptography (in the modern incarnation) *heavily* uses entanglement (like evesdropping detectiion, NOT POSSIBLE classically) and your objection concerned practical benefits from studying entanglement. It is QUANTUM ...

No, this language is not weird at all (I have been doing some quantum entropy myself - in my masters time - before I went to do real physics, ie. GR ).

I moved in the other direction

I deliberatly reacted so sharp because people seem to already have decided that the correlations obtained in such processes cannot be obtained by any *classical* means (on the appropriate distance scales). So, the question wether this quantum information is really so quantum in the sense that no underlying realistic theory can account for the correlations (or wether these correlations really can be obtained in practical situations) remains to be seen.

Well, that depends upon the distance scales you consider. For example local classical statistics can be exactly the same as quantum statistics on those distance scales where the separability assumption cannot be made. That is one reason why I have good hopes for local realism.
Cheers,
Careful

Not really understand what do you mean by your "scales" and "separability assumption", but if you referr to Bell-type experiments then let me just comment that they have been succesively closing the remaining locality and efficiency loopholes (ions give 95+% eff. and they change setups fast enough to exclude causal interaction). The subject seems to be still quite active though...but that belongs to another thread.

-jarek

 

[Careful]

**Not equal, but cryptography (in the modern incarnation) *heavily* uses entanglement (like evesdropping detectiion, NOT POSSIBLE classically) and your objection concerned practical benefits from studying entanglement. It is QUANTUM ... **

In theory, yes, but in practice probably not

**
I moved in the other direction
**

Ah, mistakes are there to be forgiven

**
Not really understand what do you mean by your "scales" and "separability assumption", **

The separability assumption indeed means that no causal contact is possible and that both ``particles´´ can be assumed to move independently which is not possible in experiments where both detectors are less than 1/10'th of a metre apart (because detection times are like 3 nanoseconds).


**but if you referr to Bell-type experiments then let me just comment that they have been succesively closing the remaining locality and efficiency loopholes (ions give 95+% eff. and they change setups fast enough to exclude causal interaction).**

And that in ONE experiment ?? If that were true then there would be no reason for Bell type experiments anymore so I guess it is false. In the other case, please give me the reference which must necessarily date from 2005 :-) ).

Cheers,

Careful

 

[jarek]

Ah, mistakes are there to be forgiven

The time will show

And that in ONE experiment ?? If that were true then there would be no reason for Bell type experiments anymore so I guess it is false. In the other case, please give me the reference which must necessarily date from 2005 :-) ).

I didn't say that in one experiment, that's why the field is still active. There has been a proposal of Fry to do such an experiment (with Hg+ ions), but as far as I know he hasn't done it yet (I heard him in 2001, but I think the idea is much older).
There are also another experimental methods of detecting entanglement (if we speak just of entanglement and not realism stuff), like entanglement witnesses. Wineland and Blatt groups have been generating multi-ion GHZ- and W-states (the record is 8-ion W) with 70%+ fidelities. But of course producing entanglement and experimental breaking Bell ineq. are two different things.


But there are also indirect evidences for genuine QUANTUM corellations (I'm not talking about realism now). One of them is precise spectrometry, see e.g. Leibfried at al, Science 304, 1476 (2004). The idea is that they get sensitivity beyond the one predicted by in an ideal case with non-entangled particles. It is also practical, Careful, as people are working on application of entanglement enhanced spectrometers to gravity wave detectors what an irony, eh?.


Geee...I have never written so much on experiments since my high school (puke)
-jarek

 

[Careful]

**
I didn't say that in one experiment, that's why the field is still active. There has been a proposal of Fry to do such an experiment (with Hg+ ions), but as far as I know he hasn't done it yet (I heard him in 2001, but I think the idea is much older).**

Perhaps it has been done already, but the result might not have been what one hoped...

**There are also another experimental methods of detecting entanglement (if we speak just of entanglement and not realism stuff), like entanglement witnesses. Wineland and Blatt groups have been generating multi-ion GHZ- and W-states (the record is 8-ion W) with 70%+ fidelities. But of course producing entanglement and experimental breaking Bell ineq. are two different things.**

Indeed...

**
But there are also indirect evidences for genuine QUANTUM corellations (I'm not talking about realism now). One of them is precise spectrometry, see e.g. Leibfried at al, Science 304, 1476 (2004). The idea is that they get sensitivity beyond the one predicted by in an ideal case with non-entangled particles. **

Thanks, I shall figure that out, but I feel pretty confident that other explanations are possible Let me stress that I agree that *standard* QM needs entanglement since measurement is supposed to be something instantaneous; but I am afraid people want to push it to distance scales where this consideration is not appropriate anymore.

**It is also practical, Careful, as people are working on application of entanglement enhanced spectrometers to gravity wave detectors what an irony, eh?. **

Same comment, don't worry I like some good irony

**
Geee...I have never written so much on experiments since my high school (puke) **

Nah, it is good to do real physics from time to time . I am a theorist myself but it is my sacred duty to figure out what those lab people are messing around with. Let's quit this discussion about entanglement: the outcome is always the same (make a local realist theory !). Like our friend Vanesch, I am of the opinion that one should not look for a case against local realism (it is extremely difficult and probably impossible to rule out), rather local realists should look for a unified theory (stepwise of course).

One can only learn from this.

Cheers,

Careful

 

[vanesch]

What do we have to believe in then? In infinite dimensional ghost spaces where god is playing dice but only allows us to see a four dimensional projection of this russian roulette ?

Admit at least, that *this* at last, has some panache ! Physics finally relieved of its century-long dusty boredom
I really hope it stays that way !

 

[Careful]

Admit at least, that *this* at last, has some panache ! Physics finally relieved of its century-long dusty boredom
I really hope it stays that way !

Bah, you just like too much science fiction and conscious ants It is clear you never calculated Feynman diagrams :-) By the way, the local realist picture is also pretty exiting (involves fast spinning clouds and so on); exit point particles, hello microscopic explosions.

Grandpa

 

[vanesch]

Bah, you just like too much science fiction and conscious ants It is clear you never calculated Feynman diagrams :-) By the way, the local realist picture is also pretty exiting (involves fast spinning clouds and so on); exit point particles, hello microscopic explosions.
Grandpa

Pouh! Fast spinning clouds against conscious ants in superposition... What do you think will sell best ? Your stuff is way not crazy enough.

 

[Kea]

There appears to be a rush of papers before the holidays, such as this little one:

Gerbes and Heisenberg's Uncertainty Principle
J.M. Isidro
http://www.arxiv.org/abs/hep-th/0512241

I thought this might appeal to Careful as it looks at a classical characterisation of QM, although of course it does this via a kind of categorification.
Must go! Lots to do.

 

[Kea]

This change in viewpoint is demanded by two facts, one observational and one theoretical. The first is that the expansion of the universe is accelerating...The second fact is that some recent progress has been made in exploring the landscape.

Now if one were forced to question the validity of the first 'fact', would the landscape be so compelling?

 

[selfAdjoint]

Via Woit's comments I found this paper:

A Garrett Lisi

Clifford bundle formulation of BF gravity generalized to the Standard Model

http://arxiv.org/gr-qc/0511120


Classical gravity plus standard model, with an opening toward LQG. Looks good to me!

 

[marcus]

Via Woit's comments I found this paper:
A Garrett Lisi
Clifford bundle formulation of BF gravity generalized to the Standard Model
http://arxiv.org/gr-qc/0511120
Classical gravity plus standard model, with an opening toward LQG. Looks good to me!

Our PF thread started 21 November about this paper
https://www.physicsforums.com/showthread.php?t=100984

we began talking about Garrett's but at post #7 Torsten Helge was introduced and we veered off onto that.

However Garrett's paper was the OP topic, so we could resume it if you want

 

[selfAdjoint]

Thanks for the heads-up, Marcus. Sorry about the mixup. I'm away from home, and won't have an opportunity to digest the paper till next week. I'll post something on the thread then.

 

[Kea]

John Baez is currently speaking at a conference on the Geometry of Computation http://iml.univ-mrs.fr/geocal06/ on

Universal Algebra and Diagrammatic Reasoning
http://math.ucr.edu/home/baez/universal/

These slides end with the quote: ... to get closer to reality we should climb the n-categorical ladder, and learn to love the quantum universe.


P.S. I had a wonderful time at a conference on the Poincare conjecture, which turned into a conference mostly on TFTs and quantum computation (the talk titles were not advertised so this took some people by surprise!).

 

[Kea]

Category Theory Blog

What a pleasant surprise to discover yet another Category Theory blog:
David Corfield's
Philosophy of Real Mathematics
http://www.dcorfield.pwp.blueyonder.co.uk/blog.html

From today's comments:
...it wouldn't surprise me if much of the category theoretic 'metaphysics' gets done by computer science people and physicists.

What a nice mathematician! He might be right about the computer scientists but I'm not so sure about the physicists. A philosopher once sighed and said to me that philosophers were about 100 years behind the physicists, but even though he meant it I can't help feeling that deep down he knew that the opposite was true. After all, physicists have to worry about formulating the concepts in some very concrete way and can't spend too much time trying to express their most elusive thoughts. Quite an excuse, heh?

 

[Kea]

Over at The String Coffee Table http://golem.ph.utexas.edu/string/index.shtml
Urs Schreiber has started talking about renormalisation and operads!

 

[Chronos]

You're on the cutting edge, Kea, and don't even know it yet! [or maybe you do and don't want to show your hand]. Treating the universe as a quantum computer is a very sound approach, IMO.

 

[Kea]

Treating the universe as a quantum computer is a very sound approach, IMO.

 

[GOsuchessplayer]

Wow i just find all of this so interesting. I hate the fact that my school (high school) does not offer AP physics. But hey these forums are teaching me alot. Since i know most of the basics. Anyways thanks guys keep posting.

 

[marcus]

Where is setAI when we need him?

Once Kea thus bewailed the absence of setAI---when another wildman It-from-Bit spacetime-is-a-computer theory made its appearance.

this one also seems choice and right down the alley of some here:

http://arxiv.org/abs/astro-ph/0603084
On the similarity of Information Energy to Dark Energy
M.P.Gough, T.D.Carozzi, A.M.Buckley
5 pages, no figures, no tables

"Information energy is shown here to have properties similar to those of dark energy. The energy associated with each information bit of the universe is found to be defined identically to the characteristic energy of a cosmological constant and the universe information content of 10^90 bits provides a total energy comparable with the high dark energy value. Information energy is also found to have a significantly negative equation of state parameter, w < -0.4, and thus exerts a negative pressure, similar to dark energy."

It cites several Seth Lloyd papers, and with their help finds the information density of the universe. Then DARK ENERGY turns out to be comparable to the energy associated with the estimated information.

 

[setAI]

Once Kea thus bewailed the absence of setAI---when another wildman It-from-Bit spacetime-is-a-computer theory made its appearance.

I am always lurking in the shadows- and read all the new papers

BTW- only one week to go until https://www.amazon.com/gp/product/1400040922/?tag=pfamazon01-20 ]Seth Lloyd's book [/url] comes out! have you pre-oprdered yet?

 

[marcus]

I am always lurking in the shadows- and read all the new papers

that is reassuring to know

BTW- only one week to go until [u rl=[URL]https://www.amazon.com/gp/product/1400040922/?tag=pfamazon01-20[/URL] ]Seth Lloyd's book [/u rl] comes out! have you pre-ordered yet?

no, I have not pre-ordered----I am naturally frugal and extremely patient. do you have any other links that might provide sample chapters or other clues as to what's in the book?

 

[setAI]

...do you have any other links that might provide sample chapters or other clues as to what's in the book?

no- but I would assume it is an expansion of his original paper on the subject: http://arxiv.org/abs/quant-ph/0501135

there are some things I am hoping for- in the paper Seth wrote:

This paper showed that any computation, including, for example, one that calculates
the digits of π, corresponds to a class of spacetimes that obeys the Einstein-Regge equations.
Which computation corresponds to the world that we see around us? As noted,
quantum cellular automata and random computations are both reasonable candidates for
the ‘universal’ computation. Cellular automata possesses a built-in regular structure which
simplifies the analysis of their behavior. A quantum cellular automaton is a natural choice
for a computational substrate...

I am hoping there has been some work on this- perhaps some ideas or examples of quantum cellular automata that could simulate different approaches to Quantum Gravity-

edit: on a personal note- you may know that I am an experimental/abstract computer musician- lately I have been playing around with the idea of somehow utilizing classical simulations of very simple quantum logic gates as sound-modulation sources and/or for algorithmic music composition- I am hoping that the book might contain more detailed quantum logic gate descriptions and quantum algorithms that I might be able to kludge for my work-