Holographic Dualities and the Potential Misdirection of String Theory

In summary: Aristotelian physics hindered Galilean physics from being created.This is a bit more complicated. The assertion that string theory has not helped us in our understanding of quantum gravity is not true in and of itself. What bcrowell is saying is that, if we look at the progress of string theory objectively, it may have hindered our progress in understanding quantum gravity. This is a valid assertion, but it is not the only possible interpretation of the progress of string theory.In summary, bcrowell believes that if we look at the progress of string theory objectively, it may have hindered our progress in understanding quantum gravity.
  • #1
inflector
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In another thread: https://www.physicsforums.com/showthread.php?t=436080" a discussion came up relating to the validity of String Theory and the specific thought that, if nothing else, it has helped us to better understand the issues of quantum gravity. I wanted to start a new thread to adress this specific point so as not to take the other thread off topic.

Some background. First we have atyy:
atyy said:
Also, it is known that our best theories which do work break down at a high enough energy, and some theory must replace them. So by mathematical consistency alone, we have to search for a new theory. At present, string theory is without doubt an approach that has taught us a lot about whatever the true theory of quantum gravity is, even if it ultimately turns out that string theory does not model nature.

Then bcrowell replied:
bcrowell said:
I'm not convinced that this is true. It may have taught us absolutely nothing about the true theory of quantum gravity. It may have hindered us from finding a true theory of quantum gravity, in the same way that Aristotelian physics hindered Galilean physics from being created.

I'd take this a bit further. It is possible that the progress of string theory has hindered us by pointing us in the exact opposite direction from where we'd need to go to solve quantum gravity.

To illustrate what I mean, I need the next reply of atty:
atyy said:
Do you think these statements are false?

http://arxiv.org/abs/0809.4266
"These dualities become especially powerful when combined with string theory [1]. It is an occasional misconception, however, that the existence of holographic dualities is contingent on the validity of string theory. This is not the case."

Or that this approach is misguided?

http://arxiv.org/abs/1006.1902
"Ever since the seminal work of Bekenstein and Hawking, it has been clear that there is a deep and fundamental relation between gravity, thermodynamics and quantum mechanics, while its detailed form and origin was and is largely mysterious. ... It seems likely that the basic triangular relationships transcend string theory and AdS/CFT, although lessons from string theory are likely useful guides for unraveling the more general picture. It is our hope that the attempt here to generalize fluid/gravity duality away from the stringy context to its most essential ingredients may be useful in understanding this triangle."

(emphasis mine)

Both of these papers cover issues related to holographic dualities and their role in quantum gravity.

The general idea of a holographic duality for quantum gravity is that there is a correspondence between a theory with gravity in a particular dimension and a quantum field theory of one less dimension without gravity. The most famous example being the AdS/CFT correspondence originally proposed by Juan Maldacena. The AdS/CFT correspondence specifically relates to the correspondence of a string theory with gravity to a quantum field theory without gravity.

In http://homepage.mac.com/photomorphose/documents/qpdf.pdf" :
More precisely, the theories predict that the number of dimensions in reality could be a matter of perspective: physicists could choose to describe reality as obeying one set of laws (including gravity) in three dimensions or, equivalently, as obeying a different set of laws that operates in two dimensions (in the absence of gravity). Despite the radically different descriptions, both theories would describe everything that we see and all the data we could gather about how the universe works. We would have no way to determine which theory was “really” true.

Such a scenario strains the imagination. Yet an analogous phenomenon occurs in everyday life. A hologram is a two-dimensional object, but when viewed under the correct lighting conditions it produces a fully three-dimensional image. All the information describing the three-dimensional image is in essence encoded in the two-dimensional hologram. Similarly, according to the new physics theories, the entire universe could be a kind of a hologram.

It is an interesting and mathematically clever idea and one that ties into String Theory, though it is not exclusive to it. The implication of this holographic duality and way of thinking that I wish to address here is that it implies that gravity and even the perception of the extra dimension are emergent. The duality allows one to work in a lower-dimension quantum field theory and then the extra dimension and gravity emerge in the higher dimension through the duality.

The mathematics involved is by Maldacena's own admission difficult as thus far the conjecture itself has not been proved:
Since [the introduction of the AdS/CFT correspondence conjecture in 1997], many researchers have contributed to exploring the conjecture and generalizing it to other dimensions and other chromodynamics theories, providing mounting evidence that it is correct. So far, however, no example has been rigorously proved— the mathematics is too difficult.

Now, back to the statement by bcrowell:
bcrowell said:
[String Theory] may have taught us absolutely nothing about the true theory of quantum gravity. It may have hindered us from finding a true theory of quantum gravity, in the same way that Aristotelian physics hindered Galilean physics from being created.

The idea that gravity emerges from a theory operating at a lower dimension has two specific implications with respect to the complexity of the task of developing a theory of quantum gravity:

1) That one should be searching for potential correspondences which may map onto what we believe represents our actual space-time rather than the easier to work with AdS. This is very complicated.

2) That one should search for the ways in which gravity and dimensions can emerge from the lower-dimensional quantum field theories. This too is complicated because the number of ways in which something can emerge from a complicated theory is itself necessarily complicated. Emergence is not an easy phenomena to derive or explain a priori. Its very essence is that it is an unexpected complex outcome from a set of simpler rules. Emergence is complexity to some power greater than one.

Now it is possible that a holographic duality underlies quantum gravity and that it is indeed emergent. There are lots of physicists betting that way. But this is necessarily a complicated path and one in which it will be difficult to make headway.

It is also possible that it is not emergent but, in fact, is fundamental. Not too many physicists, by comparison, are exploring this option.

If this proves to be true at some future date and gravity ends up being fundamental, then one will then be able to argue that this offshoot of String Theory—the holographic duality which atyy used as his example of a contribution of String Theory—had led physics down the wrong path, and even the opposite path from the one required to reach a theory of quantum gravity.

Time will tell.

It seems to me that it makes sense to explore all options.
 
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  • #2
inflector said:
It is also possible that it is not emergent but, in fact, is fundamental. Not too many physicists, by comparison, are exploring this option.

If this proves to be true at some future date and gravity ends up being fundamental, then one will then be able to argue that this offshoot of String Theory—the holographic duality which atty used as his example of a contribution of String Theory—had led physics down the wrong path, and even the opposite path from the one required to reach a theory of quantum gravity.

Time will tell.

It seems to me that it makes sense to explore all options.

Yes. The other option is called "Asymptotic Safety". Polchinski states both options at the start of his string theory textbook, and says the other one is not ruled out: "There are two possible resolutions. The first is that the divergence disappears ... when the theory is treated exactly. In the language of the renormalization group, this would be a nontrivial UV fixed point. The second is that the extrapolation of the theory to arbitrarily high energies is incorrect." http://books.google.com/books?id=k4...esnum=4&ved=0CBsQ6AEwAzgK#v=onepage&q&f=false

Weinberg has advocated the study of both "Asymptotic Safety" and string theory.

Polchinski is a major contributor to the understanding of renormalization, and his equation is a key tool in Asymptotic Safety.

http://relativity.livingreviews.org/Articles/lrr-2006-5/
 
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  • #3
Have they reconciled asymptotic safety with black hole entropy? Haven't heard on this in a while but I haven't checked either. If not then asymptotic safety just can't work. How do you match the degrees of freedom?

Also, there's no such thing as a "wrong" or "right" "path" in physics. The results are either correct or not.
 
  • #4
negru said:
Have they reconciled asymptotic safety with black hole entropy? Haven't heard on this in a while but I haven't checked either. If not then asymptotic safety just can't work. How do you match the degrees of freedom?

Also, there's no such thing as a "wrong" or "right" "path" in physics. The results are either correct or not.

And string theory is proven to work?
 
  • #5
To a certain extent yes, Asymptotic Safety does too."1. Gravity. Every consistent String Theory must contain a masless spin-2 [vibrational] state, whose interactions reduce at low energy to general relativity.

2. A consistent theory of quantum gravity, at least in perturbation theory. As we have noted, this is in contrast to all known quantum field theories of gravity.

3. Grand unification. String theories lead to gauge groups large enough to include the Standard Model. Some of the simplest string theories lead to the same gauge groups an fermion representations that arise in the unification of the Standard Model.

4. Extra dimensions. String theory requires a definite number of space-time dimensions, ten [or 11 in M-Theory]. The field equations have solutions with four large flat and six small curved dimensions, with four dimensional physics that resembles the Standard Model.

5. Supersymmetry. Consistent String Theories require space-time supersymmetry, as either a manifest or a spontaneously broken symmetry

6. Chiral gauge couplings. The gauge interactions in nature are parity asymmetric (chiral). This has been a stumbling block for a number of previous unifying ideas: they required parity symmetric gauge couplings. String theory allows chiral gauge couplings.

7. No free parameters. String theory has no adjustable constants.

8. Uniqueness. Not only are there no continuous parameters, but there is no discrete freedom analogous to the choice of gauge group and representations in field theory: there is a unique string theory." - (String Theory, Volume 1: An Introduction to Bosonic String, Polchinski)

The Holographic Principle also provides insight into the nature of space-time and gauge-gravity duality has proven to be an important feature of understanding the structure of space time and Quantum Field Theory.
 
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  • #6
As far as I know Asymptotic Safety is not a theory, but a hypothesis of a mathematical property. Like a big elephant in the small room that was ignored and may make several theories that were non renormalizable normalizable.
 
  • #7
MTd2 said:
As far as I know Asymptotic Safety is not a theory, but a hypothesis of a mathematical property. Like a big elephant in the small room that was ignored and may make several theories that were non renormalizable normalizable.

Yes, I see that's how Percacci is using the term. So I should have said Asymptotic Safety of Einstein gravity.
 
  • #8
The holographic principle comes in different forms. Firstly it relates entropy with area instead volume; secondly it relates the surface degrees of freedom with the volume degrees of freedom.

I now that some people don't like it, but we have to mention LQG from which an two-dim. "horizon" theory emerges when applied to black holes. So it seems that neither the first nor the second step are specific to string theory.

For me this means that all theories incorporating the holographic principle in some way or the other are candidates for a theory of quantum gravity.
 
  • #9
I personally still consider the "holographic principle" to be far from even understood and properly formulated, and wether it's somehow universally fixed or somehow emergent as an equilibrium condition.

But it's clear that all the hints, and similarities suggest that there IS something interesting here. But we don't yet understand it, and as I see it the main hints doesn't come from ST.

So this general quest has IMO nothing at all specific to do with ST. So I agree with Tom.

Probably what we need to do, is to understand the functional meaning of information(and entropy) and what the connection between entropy and action is, and what this means in a picture of interacting quantum observers. I don't see how ST framework is anywhere near giving an acceptable handle on these questions.

/Fredrik
 
  • #10
negru said:
Have they reconciled asymptotic safety with black hole entropy? Haven't heard on this in a while but I haven't checked either. If not then asymptotic safety just can't work. How do you match the degrees of freedom?.

This idea seems wrong for two reasons:

1) The lack of a reconciliation today does not mean that someone won't come up with one in the near future. There are scientists working on every single one of the major holes in any major potential new theory for gravity.

2) The very idea of the need for any theory of quantum gravity to reconcile specifically with black hole entropy seems to me to be completely flawed in an almost self-parodic circular manner. I mean, isn't one of the reasons that we are looking for a new theory because we specifically don't know what happens at the singularities like the big bang and black holes since GR and quantum mechanics conflict there? So why would we require a theory that is supposed to contain the equations defining the behavior of a black hole to conform to predictions of models that were built before we had a valid theory and equations defining behavior of a black hole? Don't we know we can't trust our predictions in a regime where at least one of the two most tested theories we have must be wrong, or both? The math won't have it any other way. Not only that, we don't even know that black holes exist as we currently conceive them. The requirement that a theory of quantum gravity needs to reconcile itself with our current broken theory doesn't make any sense to me at all.

What am I missing?
 
  • #11
It's called consistency check. Whatever new revolutionary theory you find that we never thought about still has to be perfectly consistent with what we already know.

It doesn't even matter if black holes exist or not.
 
  • #12
AS is not a theory. Just like renormalizaton and regularization aren't. Those are just methods. But AS needs more work.
 
  • #13
inflector said:
The very idea of the need for any theory of quantum gravity to reconcile specifically with black hole entropy seems to me to be completely flawed in an almost self-parodic circular manner. I mean, isn't one of the reasons that we are looking for a new theory because we specifically don't know what happens at the singularities like the big bang and black holes since GR and quantum mechanics conflict there?

...

What am I missing?

I think you are missing the fact that both string theory and loop quantum gravity are theories of quantum gravity which are mathematically well defined and which allow in certain regimes (approximations) to calculate what happens inside a black hole.

In string theory the microstates of a black hole can be calculated for extremal black holes (BPS state) exactly. In LQG there is the symmetry-reduced LQC theory which shows that the singularity is resolved. In addition full LQG provides a means to understand the microstates as well.
 
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  • #14
negru said:
It's called consistency check. Whatever new revolutionary theory you find that we never thought about still has to be perfectly consistent with what we already know.

It doesn't even matter if black holes exist or not.

If it doesn't matter if black holes exist then how can you say that if asymptotic safety cannot be reconciled with black hole entropy then it can't work. How does the particular traits of a potentially non-existent entity constrain quantum gravity theory?

Yes, any new theory still has to be consistent with what we already know.

But we don't know anything about black holes.

Nothing at all. We don't even know they exist. We think they exist. We have seen lots of evidence that there is something really massive that isn't a star at the center of our galaxy and of others we have studied. We have seen signs that fit theories we have about how black holes work assuming GR holds.

But we do know that GR doesn't hold at extreme energies confined to extremely tiny space as is predicted for a black hole. We need a quantum gravity theory to tell us about that regime. That's why so much effort is going into discovering one. So we don't know anything about that regime. We have a theory that we know doesn't work there, so we know our theorizing about what happens there is likely to contain at least some error.

The new theory might not have the same prediction for black hole entropy. The new theory might even show that black holes as we think of them right now cannot exist and it may provide an even better explanation for what is happening at the center of our galaxies than the current supermassive black hole hypothesis.
 
  • #15
tom.stoer said:
I think you are missing the fact that both string theory and loop quantum gravity are theories of quantum gravity which are mathematically well defined and which allow in certain regimes (approximations) to calculate what happens inside a black hole.

No, I'm not missing this fact. I just don't find it at all relevant. String theory needs to be consistent with itself. Loop quantum gravity needs to be consistent with itself.

Another different theory of gravity does not need to be consistent with string theory or loop quantum gravity because it is a different theory, this different theory of gravity only needs to be mathematically consistent with itself and known experimental observation. What string theory calculates to be the behavior of black holes does not matter if string theory is not correct.

tom.stoer said:
In string theory the microstates of a black hole can be calculated for extremal black holes (BPS state) exactly. In LQG there is the symmetry-reduced LQC theory which shows that the singularity is resolved. In addition full LQG provides a means to understand the microstates as well.

That's wonderful for string theory and LQG, but again, irrelevant for another theory that does not rely or or extend either one of those theories.

Why do you think that string theory's description of black hole properties is relevant to another theory that is not derivative of string theory?

You may assume that string theory is correct but that doesn't mean that someone who is working on another theory is going to make the same assumption. In fact, they'd be foolish to develop a theory that they didn't think was even possibly correct.
 
  • #16
It's like in the early days of quantum mechanics: the Bohr-Sommerfeld model of atom was - strictly speaking - wrong. Nevertheless it told us some truth about atoms, energy levels etc. Sometimes theories make reasonable predictions beyond their regime of applicability.

Another thing: LQG and ST have nearly NOTHING in common. Nevertheless in some regimes they arrive at nearly identical predictions. For BH entropy the predictions fit to the Bekenstein-Hawking result which has nearly nothing in common with both LQG and ST. I would say that these fascinating results are good indications that LQG and ST are good approximations of some underlying truth of nature.
 
  • #17
tom.stoer said:
It's like in the early days of quantum mechanics: the Bohr-Sommerfeld model of atom was - strictly speaking - wrong. Nevertheless it told us some truth about atoms, energy levels etc. Sometimes theories make reasonable predictions beyond their regime of applicability.

Yes, they do. Physicists, astronomers, and mathematicians are generally very smart people so even if their ideas are in error it is likely that they are not completely without merit. For this reason, successful new theories don't generally completely replace old ones, they usually extend them into new domains and clarify them.

tom.stoer said:
Another thing: LQG and ST have nearly NOTHING in common. Nevertheless in some regimes they arrive at nearly identical predictions.

This might be due to the fact that LQG and ST theorists have been trying to match the prior predictions because, all else being equal, that makes the theory easier to accept. It also makes it less falsifiable.

tom.stoer said:
For BH entropy the predictions fit to the Bekenstein-Hawking result which has nearly nothing in common with both LQG and ST. I would say that these fascinating results are good indications that LQG and ST are good approximations of some underlying truth of nature.

Sure, they might be. But they might also be simply evidence that physicists can be mighty clever when they need to be. Holographic dualities are one great example. The idea was specifically developed to match the results of Bekenstein and Hawking. Should we be surprised that it solves the problem for which it was created?

Perhaps it is indicative of some underlying truth of nature. I think most physicists would bet that way. But it might just be indicative of the cleverness of 't Hoof, Susskind, Maldacena, et al.

So until we have a theory of quantum gravity that we understand and have tested we can't be sure that this is the case.
 
  • #18
inflector said:
This might be due to the fact that LQG and ST theorists have been trying to match the prior predictions because, all else being equal, that makes the theory easier to accept. It also makes it less falsifiable.
I don't think it's up to you to question the integrity of research groups!

Especially for LQG and ST contrasting results would have been welcome in both "factions" over the years as it would allow for a clearer assessment regarding the validity of both theories.

inflector said:
Holographic dualities are one great example. The idea was specifically developed to match the results of Bekenstein and Hawking. Should we be surprised that it solves the problem for which it was created?
The idea of ART was developed to solve the problem of perihelion precession - and it worked. What do you want to insinuate?

inflector said:
... it might just be indicative of the cleverness of 't Hoof, Susskind, Maldacena, et al.
You can be sure about that!

inflector said:
So until we have a theory of quantum gravity that we understand and have tested we can't be sure that this is the case.
As I just wanted to point out we have mathematically consistent theories of quantum gravity - and we understand them to some extend. About testability you are right, that's a major problem. But you can't blame the theoretists for the smallness of the Planck length.

What we are facing is a paradigm shift how to do physics (I am not talking about landscapes here). There is a shift from experimental guidance towards mathematical principles.
 
  • #19
I still don't understand why so many are surprised that string theory (or QG in general) is failing to produce experimental predictions. You think the Planck length was discovered to be so small in the last few years? No, this was known since people first started thinking about the issue many decades ago. It's not news to anyone what the situation is.

If black holes don't exist, you're going to need a reason for why they shouldn't exist. They're a solution of GR, so unless GR is wrong even in its usual regime, any QG theory will need to reproduce them, including their entropy.
 
  • #20
I partly symphatise with the general scepsis that new hypothesis should be tested against somewhat verified experiments and not necessarily against pure expectations based on competing hypothesis that may or many not hold true.

As an example, I for example personally think that at some point we we may need to rework the basic structure of QM. QM is extremely well tested, but as we also konw, only for specific domains! In particular those where we are studying small subsystems in a highly controlled much more massive context (ie. particle/atomic physics) Some people think that this success means it's extrapolation to arbitrary cases and scales must be true as well. This isn't so. Even if it turns out to be, it's far from obvious. In particular is it not obvious that the abstration making sense for studying a subsystem makes sense in a cosmological perspective.

So if we a new theory does not reproduct the exactr structure of QM or GR in all domains, but only in the respective domains where they are experimentally verified that's fine as well. It's just that I think a lot of people find it hard to imagine how a new theory can comply to QM predictions to the extreme accuract it's known to hold, and still violated it's general principles in other domains.

But to get back to holography, it seems inflector thinks something is wrong about the holographic principle; then I would guess the maybe you have some own idea of why or how? Maybe it's easier to accept your arguments if there is a good alternative? Somehow if there is not other options, using the "extrapolations or expectation" from semiclassical theory seems like the only rational guide. In my example above, I wouldn't personally think QM needed to change if I didn't have the slightest clue how. It's somehow not rational to throw away the only clues you have, until you have something that appears more promising.

Maybe it would be interesting to just discuss some alterantives to the holographic principles? or maybe equivalently, alternative meanings of it? Inflector?

/Fredrik
 
  • #21
tom.stoer said:
I don't think it's up to you to question the integrity of research groups!
:bugeye:

How was I in any way impugning the integrity of research groups? What I stated with reference to ST and LQG is that they are doing what I think any rational scientist or group of scientists would do. You look at data and you develop theories which match the data that you know already exists. If you can't do that, the theory must be thrown out.

So to me, the fact that two different groups match in one aspect is simply a function of their separately trying to develop theories that match one particular aspect of what is perceived as current best thinking of what reality is. Why does that make their actions bad in any way? What did I say that makes you think I "questioned their integrity?"

tom.stoer said:
The idea of ART was developed to solve the problem of perihelion precession - and it worked. What do you want to insinuate?

I couldn't find ART defined as a physics acronym. What does it mean? Needless to say searching for ART finds too many pages about art as an English word.

I'm not trying to insinuate anything. I'm not even saying that the holographic principle is wrong or that holographic dualities won't end up being a key part of what we end up believing to be the best theory in 200 years.

My main point is that we should be very clear about what we actually know and what we are currently guessing at. They might be good educated well-informed intelligent guesses but they are still guesses.

tom.stoer said:
As I just wanted to point out we have mathematically consistent theories of quantum gravity - and we understand them to some extend. About testability you are right, that's a major problem. But you can't blame the theoretists for the smallness of the Planck length.

To me this seems like a step down the slippery slope where we'll no longer be able to distinguish science from religion. After all, you can't blame Christians because God hides in the 11th dimension and we can't see any evidence of him here can you?

Look, I don't want you to mistake what I'm trying to say here. I know that this is a difficult problem. Our bests minds have been working on this for 80 years or more. If it were easy, it would be done by now.

I just think it makes good sense to understand the bright line between our hard data and the theory which we have built to explain that data which has not yet been experimentally proven.

tom.stoer said:
What we are facing is a paradigm shift how to do physics (I am not talking about landscapes here). There is a shift from experimental guidance towards mathematical principles.

When you are blind you need to rely on your other sense more. That makes sense. I'm not arguing that this isn't a rational idea.

I do think we need to understand, however, that this is not as firm a basis for theory as experimental guidance was. So we should allow for greater possibility of error or misguided assumptions that end up causing a major portion of our theory to eventually fail.
 
  • #22
negru said:
I still don't understand why so many are surprised that string theory (or QG in general) is failing to produce experimental predictions. You think the Planck length was discovered to be so small in the last few years? No, this was known since people first started thinking about the issue many decades ago. It's not news to anyone what the situation is.

I don't see anyone arguing that Planck length experiments are currently feasible or that quantum gravity is easy or should have produced experimental predictions necessarily.

I can't speak for others, but what I am arguing is nothing of the sort.

I am simply arguing that until we have experimental confirmation or at least observational confirmation of new phenomena at the cosmological scale (which we can reasonably expect a theory of quantum gravity would predict because both the big bang and black holes should have some cosmological effects a more accurate theory will clarify). Until that point, then we don't have a theory that has observational and experimental support. So for that reason, we should be more open to the idea that the theory itself and ideas which depend upon it might actually be wrong.

It's a simple scale:

Experimentally tested theory: best confirmation of theory within the regime tested, for GR, the Pound-Rebka experiment is one example of this.

Observationally tested theory: second-best confirmation of theory, Eddington's 1919 observation of starlight bending in an eclipse due to the effect of the sun's gravitation was an example of this type of confirmation

Mathematically consistent theory: while this is important, in and of itself, this is not as good, not even close

negru said:
If black holes don't exist, you're going to need a reason for why they shouldn't exist. They're a solution of GR, so unless GR is wrong even in its usual regime, any QG theory will need to reproduce them, including their entropy.

Clearly, any theory of quantum gravity that conflicts with GR's predictions with respect to black holes will have to have a good reason or science won't accept it.

The existence of black holes is, fortunately, an idea that we might be able to test at the LHC. If they find mini-black holes, for example.

But I don't understand why you think that black holes are GR's usual regime. Isn't the singularity of ultra high energy in ultra tiny space undefined/infinite in GR and quantum mechanics? Isn't this one of the main reasons that science is looking for a theory of quantum gravity because we know our theory doesn't cover this regime?

We don't have any direct observational confirmation for black holes. We have plenty of circumstantial evidence. We are close here, from my read, and the vast majority of astronomers believe that they exist, but they are not yet proven. The discovery of absolute proof will probably win someone or some group a Nobel, for this reason.
 
  • #23
inflector said:
How was I in any way impugning the integrity of research groups? What I stated with reference to ST and LQG is that they are doing what I think any rational scientist or group of scientists would do.
You wrote that "this might be due to the fact that LQG and ST theorists have been trying to match the prior predictions because, all else being equal, that makes the theory easier to accept." That was certainly not their intention. They tried to derive a result, nothing else. Unfortunately they have only some hints - Bekenstein-Hawking is one of them - where they can try to derive GQ effects some orders of magnitude below the Planck scale. So it's natural that they focus on similar problems, but that does not mean that they "try to match" and "fiddle their calculations" in order to make them easier to accept.

inflector said:
You look at data and you develop theories which match the data that you know already exists. If you can't do that, the theory must be thrown out.
No, that's unfortunately no longer true. In the QG regime we are hardly able to test theories against experiments (even so this is desired). That means the "can't do ..." does no longer work.

inflector said:
I couldn't find ART defined as a physics acronym. What does it mean?
Sorry; I meant GR; ART is "Allgemeine Relativitätstheorie" = "General Relativity".

inflector said:
My main point is that we should be very clear about what we actually know and what we are currently guessing at. They might be good educated well-informed intelligent guesses but they are still guesses.
I agree that the holographic principle is not a proven fact. But I think that it's more than a guess.

inflector said:
To me this seems like a step down the slippery slope where we'll no longer be able to distinguish science from religion.
Both science and religion have something in common. In science this is what we call axioms, principles etc. In GR we believe that c ist constant, we believe that the world is a four-dim-manifold, we believe in the equivalence principle. And the theory produces correct and experimentally testable results which sustains our belief in these principles. So the basis of science is no longer science, it's meta-physics (as the ancient greeks called it). Unfortunately we are doing more meta-physics in QG than we were used to from other physical theories over the last decades. We unlearned to ask these deep questions as since the dawn of QM everything was so easy: ...- experiment - calculation - experiment - calculation - ... If you read Heisenberg, Weizsäcker, Bohr etc. you will find that they were working very hard even on the meta-physical aspects of QM.I think the same applies to Sussking, Rovelli and Smolin to name a few.

inflector said:
I just think it makes good sense to understand the bright line between our hard data and the theory which we have built to explain that data which has not yet been experimentally proven.
As I said: I agree in principle, but I don't think that this is the way we can make progress in QG.
 
  • #24
inflector said:
We don't have any direct observational confirmation for black holes.

They way I personally elaborate these holographic connections has nothing directly to do with black holes ONLY. One can consider arbitrary boundaries and subsets and generic observer horizons. So IMO, there are (in the future) possible implications of this that may be tested in different ways, not necessarily ONLY for BH horizons. At least that's how I think of it.

This is why I think a reformulation where you define "area" of a more abstract observer horizon in terms of more intrinsic properties of the system, such as maybe a complexity bound of the inference machinery etc. This is more close to the computer science meaning of the bound, in terms of computation capacity etc.

Then, the holographic principle MAYBE could be understand from first principlces that doesn't a priori have anything to do with say black holes, and that instead "a black hole" could be defined as certain extermal systems.

Here I think a lot more research is needed.

/Fredrik
 
  • #25
I think we are to vague when sayig "black hole". What is a black hole? This depends on the theory.

In GR a black hole is a curvature singularity with an event horizon hiding the singularity from the outside oberver. One can calculate its temperature and entropy using classical GR with a semiclassical approx. for QFT. In LQC the curvature singularity is replaced by a mathematically well-behaved region w/o any singularity, whereas in full LQG the event horizon enclosing the singularity seems to stay intact. So the interior which no longer contains a singularity is still hidden from the outside observer.

For black holes in the GR sense we have astrophysical support (not evidence).
 
  • #26
negru said:
If black holes don't exist, you're going to need a reason for why they shouldn't exist. They're a solution of GR, so unless GR is wrong even in its usual regime, any QG theory will need to reproduce them, including their entropy.

This is another point that will probably be raised in the next years. If no particles that are significantly responsible for Dark Matter are found, the validity of GR will be questioned too. That's quite a limbo for fundamental science.
 
  • #27
Fra said:
As an example, I for example personally think that at some point we we may need to rework the basic structure of QM. QM is extremely well tested, but as we also konw, only for specific domains! In particular those where we are studying small subsystems in a highly controlled much more massive context (ie. particle/atomic physics) Some people think that this success means it's extrapolation to arbitrary cases and scales must be true as well. This isn't so. Even if it turns out to be, it's far from obvious. In particular is it not obvious that the abstration making sense for studying a subsystem makes sense in a cosmological perspective.

So if a new theory does not reproduct the exact structure of QM or GR in all domains, but only in the respective domains where they are experimentally verified that's fine as well. It's just that I think a lot of people find it hard to imagine how a new theory can comply to QM predictions to the extreme accuract it's known to hold, and still violated it's general principles in other domains.
(emphasis mine)

This last paragraph gets to the point I am trying to make. In particular the sentence I bolded.

I also strongly agree with the last sentence. A lot of people do find it hard to imagine this.

Fra said:
But to get back to holography, it seems inflector thinks something is wrong about the holographic principle; then I would guess the maybe you have some own idea of why or how?

No, actually I don't have a strong opinion of the holographic principle. It is an idea that is clever that has some intuitive validity. It solves a problem.

I was using it as an example of something that COULD BE WRONG, and therefore that COULD BE LEADING US DOWN THE WRONG PATH. I am NOT PROPOSING specifically that I have some reason to believe THAT IT ACTUALLY IS WRONG. I don't know.

The difference between my thinking and many others here (or at least my read on their thinking) is that I tend to look at most ideas tentatively, even ones that are well accepted, because I believe that there is some set of assumptions, or perhaps only one, that science currently holds which is in error. And that this error in assumption is keeping us from seeing the key to the solution to quantum gravity. I believe it is probable that at least one cherished idea is wrong or only partially correct and that we can't see the solution because our assumptions cloud our perception of what is possible.

That is why I have been trying to differentiate very intentionally between what we actually know through experiment and direct observation and the interpretation of those experiments and observations that we currently hold.

I also believe that it is likely that the theory of quantum gravity that science ends up accepting will be able to be experimentally and observationally confirmed.

Fra said:
Maybe it's easier to accept your arguments if there is a good alternative? Somehow if there is not other options, using the "extrapolations or expectation" from semiclassical theory seems like the only rational guide.

This is not an unreasonable initial approach. However, after as many years as we have been searching for a theory of quantum gravity, it makes sense to consider alternatives where our extrapolations might not hold.

Fra said:
In my example above, I wouldn't personally think QM needed to change if I didn't have the slightest clue how. It's somehow not rational to throw away the only clues you have, until you have something that appears more promising.

I find that it is often helpful after one has carefully considered a given assumption for a long time to imagine what it would be like if the assumption were not true or even if the opposite were true. If you can imagine scenarios which are just as likely in the event your assumption does not hold or that its opposite holds, this is an indication that the assumption should not be assumed but held open as only a possibility.

Fra said:
Maybe it would be interesting to just discuss some alternatives to the holographic principles? or maybe equivalently, alternative meanings of it? Inflector

Consider the goal for which the holographic principle was developed: to match the entropy of black holes which is theorized to scale as [tex]r^2[/tex] rather than as [tex]r^3[/tex] as one might expect.

Here are the relevant points with respect to the holographic principle as it stands:

  • There is a correspondence between a lower dimensional theory without gravity and a higher dimension
  • Gravity is emergent in this higher dimension
Another possibility is that the idea of a holographic projection holds but from the exact opposite perspective:

  • Gravity is fundamental in a higher dimension where black hole entropy scales as [tex]r^3[/tex] say for example because a black hole is a 3D object in 4D space
  • In one lower dimension entropy scales as [tex]r^2[/tex]
Another possibility is that:

  • The idea of a holographic projection between dimensions doesn't hold
  • Entropy scales as [tex]r^2[/tex] for another theoretical reason
Finally, it could be true that:
  • Our theory of black holes is not correct, they don't exist as we currently conceive them
  • Entropy for the black hole replacement in the new theory scales as [tex]r^3[/tex]

I believe that anyone of these possibilities could be true.

For this reason, I don't believe that one should assume that gravity is emergent when trying to build a theory of quantum gravity. That is one possibility. The other alternative, that it is fundamental, should also be considered and developed too.
 
  • #28
tom.stoer said:
So it's natural that they focus on similar problems, but that does not mean that they "try to match" and "fiddle their calculations" in order to make them easier to accept.

I should have chosen different words.

I didn't mean that they were trying to fiddle with their results, and I didn't intend the wording of "easier to accept" to imply that they were doing anything nefarious or for marketing purposes only.

I think it is quite natural when developing a new theory that one tries to derive existing theoretical expectations. That is the whole reason that the correspondence principle is important. A theory that can show that it reproduces the results of existing theory in well tested domains will be more easily accepted. That's all I meant. I think this is a good thing, not a bad one.

tom.stoer said:
No, that's unfortunately no longer true. In the QG regime we are hardly able to test theories against experiments (even so this is desired). That means the "can't do ..." does no longer work.

It may be that they can't be tested, that's different than what I was trying to point out. I said: "match the data that you know already exists." If a theory covers a domain that can't be tested, say the Planck scale, then there is no data for that domain.

I meant matching experimental results that we already have, not new experiments that we can't run because of limits to our current technology. So, for example, if you have a theory that can't explain the Bell's Inequalities test results, that theory doesn't work and must be discarded or fixed.
 
  • #29
inflector said:
The difference between my thinking and many others here ... is that I tend to look at most ideas tentatively ... because I believe that there is some set of assumptions ... that science currently holds which is in error. And that this error in assumption is keeping us from seeing the key to the solution to quantum gravity.
I don't think so. Up to now there is reason to believe that any fundamental principle is wrong. It's just that QG is currently neither verifiable nor falsifiable in the QG domain.

As long as there are no hints that something goes wrong experimentally, I can't see any reason why we shouldn't use these well-established principles. Changing some basic principle doesn't make QG falsifiable / experimentally testable!

But changing some principle may spoil correct predictions of the QG theories in the experimentally testable domain (were we do not need them, but only check their consistency); we do not want this to happen.
 
  • #30
Fra said:
They way I personally elaborate these holographic connections has nothing directly to do with black holes ONLY. One can consider arbitrary boundaries and subsets and generic observer horizons. So IMO, there are (in the future) possible implications of this that may be tested in different ways, not necessarily ONLY for BH horizons. At least that's how I think of it.

This makes good sense. If the holographic principle holds then we should see other evidence of it and other ways it affects our 4D world. In fact, it should be possible to find areas where one could make a prediction which could be confirmed by experiment, one would think.
 
  • #31
Nice turn!

This is indeed how many people today think about holography. The volume degrees of freedom of a certain system are represented by a boundary Hilbert space living on the surface. These boundaries need not be physical; they can be introduced rather artificially, e.g. as boundary between a "quantum system" and an "observer". Then the Hilbert space structure encodes naturally what we (= the observer) can know about the system, as we (the observer) have defined what this system really _is_ - namely in drawing the boundary.
 
  • #32
tom.stoer said:
I don't think so. Up to now there is reason to believe that any fundamental principle is wrong. It's just that QG is currently neither verifiable nor falsifiable in the QG domain.

As long as there are no hints that something goes wrong experimentally, I can't see any reason why we shouldn't use these well-established principles.

I understand your position, and most scientists probably hold the same view that you do.

I see hints all over the place that "something is wrong experimentally.":

  • Galaxy rotation curves and the need for dark matter which we can't yet detect
  • The large-scale structure of the universe
  • The pioneer anomaly
  • The flyby anomaly
  • Dark energy
  • Ultra high energy cosmic rays (OMG particles)
  • The recent detection of changes in radioactive emission tied to the sun and solar flares

These unexplained phenomena tell me that some part of current theory is wrong or incomplete.

tom.stoer said:
Changing some basic principle doesn't make QG falsifiable / experimentally testable!

Not by itself it doesn't you are right. Just changing widely-held assumptions doesn't guarantee success or a testable theory. Nevertheless, a theory that changes one of our unproven but still widely held assumptions might have features that are unexpected but testable.

All of the above list of "hints" could potentially be explained by some sort of bizarre gravity scenario and this idea would be testable. For example, consider the pioneer anomaly, if we have observed effects using probes we have sent already using early 1970s technology, it stands to reason that we'd see the same effects if we sent new probes specifically to test a given theory if the new bizarre theory is valid.

Suppose this new theory of gravity predicts anisotropies in gravitational mass and that gravity is a statistical phenomena on the large scale but anisotropic on the level of an individual particle. Suppose, again that it is posited that this new theory can explain the pioneer anomaly, ultra-high-energy-cosmic rays, and the existence of dark matter. The ability to tie the predictions of this new theory to specific observational anomalies also would imply the ability to test it further through observation and experiment. Anisotropy in gravity is something we could test using current technology if that is what is behind the acceleration anomalies I listed above.

This is just one very hypothetical example of how discarding a widely held assumption, in this case gravitational isotropy, could lead to a testable theory.

This is admittedly, not the path most would take. Nevertheless, I do think it represents one approach that should be developed.
 
  • #33
> Nice turn!

I agree :) now we're closing in.

> In fact, it should be possible to find areas where one could make a prediction which could be confirmed by experiment, one would think.

This is my conviction as well, but before we get there, the exact formulation and meaning is still unclear.

For example

> Then the Hilbert space structure encodes naturally what we (= the observer) can know about the system

One may ask, what DIFFERENCE does it make, what the observer knows? ;-) Unless we just thinkg of knowledg or "information" in a realist sense, we need to have some ideas to make sense of this.

The idea I have is the idea of rational action, which means that this has implications for how the observer ACTS onto it's environment. This can (with some work) possibly give plenty of predictions, that does not need black holes produced in labs.

/Fredirk
 
  • #34
If black hole entropy is not proportional to area, then QFT is wrong as well. You can do the computation when the curvature is not very big, and that's what you get. Higher order corrections are not going to change area to volume.

You are expecting way too many things to be wrong just in order to accommodate nothing in return.
 
  • #35
negru said:
If black hole entropy is not proportional to area, then QFT is wrong as well. You can do the computation when the curvature is not very big, and that's what you get. Higher order corrections are not going to change area to volume.

You are expecting way too many things to be wrong just in order to accommodate nothing in return.

Who and what specific assertion are you addressing?
 

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