Examples of no gravity in quantum mechanics?

[jfy4]

"Frame Dragger, you may be right about the deterministic/probabilistic qualitites not being related to gravity, but I'm not yet convinced of that due to gravity's involvement with spacetime. Something that is deterministic is predictable. One event "determines" the next in a continuous, thus predictable, fashion. We have spacetime to thank for this because the order of space and time is what allows any kind of predictability in the first place. Spacetime, although relative, is still very ordered. Even if we change velocity, thus warping the experience of space and time, the way in which the warping happens remains predictable. Consequently, deterministic"ness" cannot be separated from spacetime or gravity. In contrast, probabilistic events are not predictable, although they become more predictable as the number of sub-atomic particles/events increase. Something that lacks predictability defies space and time. This defiance is what makes it probabilistic in the first place. This is why I believe gravity and a lack thereof are entirely essential to deterministic/probabilistic qualities."

I realize you were not talking to me but "i am confuse".

Your use of the word predictable scares me. When we use QM we predict what will happen using wave mechanics, and this matches very well with experiment. it is predicted... prediction is present in both quantum and GR and both predict very well and in accordance with experiment. it is not the case that we can predict in GR and not in Quantum, that would mean Quantum is not a scientific theory! I hope I am being clear here.

I would consider rewriting the quoted paragraph for this reason, including the conclusion at the end.

 

[Hoku]

Thank you for voicing your objection! I believe I can clarify this:

From what I've gathered, and as I touched on in my confusing paragraph, quantum mechanics is predictable only on a "grand" scale. One of the books I have, I forget which one, compares it to a horse race track. Someone builds a horse race track with the intention of making money. They don't know what horses will have problems on any given day or who will come in betting what. In other words, there's a lot of chaos and unpredictability that make up the details of the day. But those details don't matter because, at the end of the day, one way or another, the race track, predictably, earned money.

The predictability of QM is an "end of the day" predictability. But when you get down to examine the "details of the day" - the individual quantum particles and events that make up the end of the day - then you loose that predictability. That's what I'm talking about; the individual quantum jumps, the wave/particle duality and all of the strange phenomena that happen in the quantum world that are unpredictable and conflict with classical physics. These phenomena underlie the predictability of QM that we put to use. It is the unpredictable phenomena that collectively give us something predictable.

Does this make sense? More importantly, am I mistaken?

 

[qsa]

From what I understand, Verlinde's entropic force is dependant on an emergent, discrete space. Emergent space is congruent with relational spacetime and background independance - the very things that, it seems, I'm trying to get to the bottom of.

The holographic principle that is essential to Verlinde's idea is derived from the thermodynamic/black hole connection. But does Verlinde's idea address the reconciliation between an emergent space with the "medium" onto which the holographic principle applies? The thermodynamic/black hole connection is also used to help justify the idea of a discrete space. Three Roads to Quantum Gravity addresses this and containes information that is disturbing to me. I addressed this in my other thread and pasted a quote from it here:

"From pages 100-105 [in Three Roads], [Smolin] is desribing one of the reasons they believe space is discrete in the first place. He says that a discrete space preserves the second law of thermodynamics as it relates to black holes. But on page 106, he says that 'if you half a volume of discrete space it creates two new regions that, together, give you more volume than you started with'. Well, wait a minute, isn't this contradicting the FIRST law of thermodynamics??"

Based on what Smolin is saying here, we are sacrificing one law in order to preserve another. For me, that is one good reason to doubt a discrete space and question Verlinde's idea.

Although first excited about it, I have developed doubts about relational, emergent space/spacetime. Since it seems to be the key to understanding the incompatability of the quantum and classical worlds, I need to probe into it deeper. That's why I'm here.

Leaving emergent concept aside I was trying to know if you get what verlinde was saying as regards to your question. The esence is that the entropy of the system (both masses) is nothing but the probability of the microstates of the particles which lead to gravity. hence your "unpredictable" probabilities is found out to be the reason for gravity. entropy is the log of probability.

 

[Hoku]

qsa, I appreciate you sticking with me and you've thrown out some interesting ideas. Thank you! Unfortunately, Verlinde's entropic idea really isn't relevant to my question. Let me refresh exactly what I'm looking for:

I'm looking for the reasons we believe(d) that gravity is incompatable with QM.

Entropic gravity does not answer this. It is one of different ways to describe how gravity IS compatable with QM. Although entropic gravity seems to have a strong following, it also faces lots of opposition. Theories for quantum gravity, including Verlinde's, are interesting but they all face some controversy and none of them have yet proved to be the holy grail of QG.

What I'm looking for is facts on the history of QM/gravitational incompatibility - not theories on the "truth" of QG.

Do you understand this difference? That's why I don't think this thread belongs in "beyond the standard model". I think we have the greatest chance of success by continuing the momentum from replies #36 and #37. Sadly, however, I'm beginning to wonder if any progress will be made, even though I know the answer is there...

 

[tom.stoer]

I'm looking for the reasons we believe(d) that gravity is incompatable with QM.

Let me correct this question as follows: "You looking for the reasons we believe(d) that gravity is incompatible with the well-known framework of QFT". It has become clear over the years that all known interactions (w/o gravity) can be described by (relativistic) quantum field theories. There are many hints that these QFTs are incomplete or even "not fully consistent", but nevertheless QFT is the only known framework able to describe nature (w/o gravity).

Now as this is clear one can ask why QFT does not work for gravity. There are a couple of technical reasons. The basic one is related to the above mentioned fact of incompleteness. As QFT has been understood over a couple of decades, perturbation theory is not only a calculational tool but in many cases a definition of QFT. The starting point of QFTs (e.g. a path integral) is formal; in many cases there is no rigorous mathematical definition. One problem is that this perturbative quantization fails for gravity. But it should be clear that this is not only a problem for gravity but for the framework of QFT framework.

Even if one is able to overcome the difficulty of the perturbative approach, there is the general problem that the definition of a QFT relies on classical, pre-existing spacetime structures. That means that spacetime serves as the stage for QFTs. As long as this holds, the QFT approach forbids spacetime becoming quantized. But on the other hand if one looks at the Einstein equations (formalyy G = T) the left side (spacetime) staying classical and the right side (matter) being quantized seems to be inconsistent.

Then there are rather general arguments, that the Planck scale is a limit for the applicability of QM/QFT methods: from gravity we know that objects which are compressed to fit into their Schwarzschild horizons will form a black hole. As in QFT there is no intrinsic limit for quantum fluctuations one argues that fluctuations will eventually collaps into micro-black holes as soon as the Compton wave length [energy] of a lump of energy becomes smaller [larger] than the Planck length [energy].

My conclusion is that gravity is not necessarily incompatible with the quantum nature, but with the well-know framework of QFT. So it could very well be that it's rather a lack of appropriate tools than a full incompatibility. Most approaches to quantum gravity do exactly this: they try to enlarge the scope of QFT, try to improve the tools, try to overcome the above mentioned technical difficulties. In parallel they find that spacetime as we know it from GR fails to exist at the quantum level; it is replaced by something more fundamental from which classical spacetime will emergy at lower energies as an effictive theory (unfortunately different theories do agree on this "something").

Examples:
1) string theory uses standard QFT methods, but they are not applied to spacetime but to the string world sheet
2) supergravity adds local supersymmetry in order to improve the behavior of the perturbation expansion; in addition the attempts to show finiteness of the perturbation expansion rely on on-shell methods (instead of standard off-shell Greens functions)
3) loop quantum gravity uses an approach known as canonical quantization, but it goes through a formalism which is manifestly inequivalent to the standard fock space construction
4) causal dynamical triangulation (and related approaches) uses a kind of discretization of spacetime which sounds familiar if one knows lattice gauge theories, but again differs in some fundamental aspects.

 

[qsa]

What I'm looking for is facts on the history of QM/gravitational incompatibility - not theories on the "truth" of QG.

...

Tom has given a nice overall "History" . The truth of of QG does have a bearing on the link with QM. here is a recent paper that explains the story very well with minimum technical math as possible.



http://arxiv.org/PS_cache/gr-qc/pdf/9712/9712070v1.pdfPerturbative Dynamics of Quantum General
Relativity
John Donoghue
Department of Physics and Astronomy,
University of Massachusetts, Amherst, MA 01003 U.S.A.
Abstract
The quantum theory of General Relativity at low energy exists and
is of the form called ”effective field theory”. In this talk I describe the
ideas of effective field theory and its application to General Relativity

comment from the paper

The outcome of this is that we need to stop spreading the falsehood
that General Relativity and Quantum Mechanics are incompatible. They go
together quite nicely at ordinary energies. Rather, a more correct statement
is that we do not yet know the ultimate high energy theory in Nature. This
change in view is important for the gravity community to recognize, because
it carries the implication that the ultimate theory is likely to be something
new, not just a blind continuation of General Relativity beyond the Planck
scale.

 

[tom.stoer]

Tom has given a nice overall "History" .

Thanks!

... we need to stop spreading the falsehood
that General Relativity and Quantum Mechanics are incompatible. ... we do not yet know the ultimate high energy theory in Nature. This
change in view is important for the gravity community to recognize ...

I haven't checked the paper but that sounds strange. Who on Earth's dares to propose GR as the ultimate theory w/o the requirement of a suitable UV completion? There are a (still growing) number of different research programs; they certainly do not agree on the right approach, but they all agree that we have to look for the UV completion or even unification (I would even say that string and loop guys agree that spacetime is not fundamental).

 

[humanino]

I haven't checked the paper but that sounds strange.

You have not read this very discussion either, since this paper was link in message #3 To me there is no doubt that this must be the starting point to answer the question with intellectual honesty. This is exactly the approach in survey papers such as

How Far Are We from the Quantum Theory of Gravity?

Difficult questions such as this cannot be seriously undertaken without a minimal amount of formalism. I take the opportunity to clarify what I mean by quoting from the above paper

The tale I have to tell is of necessity a complex one, requiring many digressive explanations. However, there is no need for the exposition to transcend the knowledge one expects of any physics graduate student. Because every one of the basic issues behind the problem of quantum gravity has a counterpart in either electrodynamics or introductory quantum mechanics, I shall use those subjects as paradigms. This is not condescension; even experts can benefit from occasionally viewing a tough problem in a general way, without becoming lost in technical details.

Looking into perturbative quantum general relativity is not necessarily about finding an ultimate UV finite theory. It is about understanding why we worry about UV finiteness in the first place. It is not even a given that the theory is not UV finite, non-perturbatively, as I am sure you are aware.

Effective Field Theory, Past and Future

http://www.worldscinet.com/ijmpd/17/1703n04/free-access/S021827180801236X.pdf

papers on asymptotic safety

videos on asymptotic safety

 

[Hoku]

Obviously, one way or another, the quantum world IS compatible with gravity. Our bodies are made up of quantum particles and our bodies are affected by gravity, right? Obviously compatible. The problem really isn't whether the two are compatible, it's HOW they're compatible. I did print out the papers by Donoghue that Humanino, then later qsa, recommended. It seems to be another way of unifying gravity with the quantum world, but its approach is to say that there was never a problem to begin with. Correct me if I'm wrong, but PQGR still faces the same problems that other theories are facing, only at a different place. To me,then, it's kind of like arguing semantics. Here's what I mean:

While most people say the conflict between QM and gravity arises when we get to the sub-atomic level, PQGR just seems to change the point at which the conflict arises. From what I'm gathering, PQGR says we can enter the quantum world without loosing the integrity of gravity... as long as we only look at low energies. But the problem STILL arises when we get to higher energies. Isn't that right? I stopped reading it for that reason; because, ultimately, it seems to be taking us back to the same problem. So, some people say that the point of separation is at the plank scale, PQGR is saying it is at the high energy "scale". It's a different approach that may or may not work in the end, but I cheer it on! Who knows, it may be the approach that will lead to greater answers.

I don't mind this thread being used to argue about PQGR, as long as we can also remember the original intention of it. Speaking of which...

tom.stoer, thank you for your response! The way you re-phrased my question works perfectly. That's exactly what I was asking. I've been researching perturbation theory today. Wikipedia (I know, I know, not the most reliable source) says it's a technique to find answers to an incomplete problem by looking at simpler, similar problems. Page 152 of Three Roads to Quantum Gravity says, "But we do not actually know whether the procedure is consistent or not, or whether it accurately reflects what a real solution to the theory would predict".

So I get a little confused by your statement that, "*perturbative quantization fails for gravity*". Based on Smolin's remark, the problem may be with perturbative quantization as a calculating tool. Is this what you meant when you said, "But it should be clear that this is not only a problem for gravity but for the framework of QFT framework."? At any rate, since that method is still experimental, it is a weak example of "no gravity" in QM.

To me, the greatest contribution of your answer is in three sentences, "...the definition of a QFT relies on classical, pre-existing spacetime structures. That means that spacetime serves as the stage for QFTs. As long as this holds, the *QFT approach forbids spacetime becoming quantized*.

BAM!

This takes us right back to what jfy4 and Fame Dragger were saying:

1) spacetime is continuous while QM quantizes space(time?).
But the quantization of space is an interpretation, right? It was designed (by Schrodinger?) as a way of understanding quantum jumps, right?

2) spacetime is relational while QM uses an absolute space(time?).
This is the one I have the greates issue with. I've read reasons why RT says spacetime is relational, but it's not jiving with me. understand the "relative" parts, but I'm not convinced how that makes space an emergent phenomena. I need to start a thread in the "relativity" section to get to the bottom of that one.

3)?
But what about spacetime being deterministic and QM being probabalistic? What are your thoughts on answers #36 and #37?

 

[humanino]

Obviously, one way or another, the quantum world IS compatible with gravity. Our bodies are made up of quantum particles and our bodies are affected by gravity, right? Obviously compatible. The problem really isn't whether the two are compatible, it's HOW they're compatible. I did print out the papers by Donoghue that Humanino, then later qsa, recommended. It seems to be another way of unifying gravity with the quantum world, but its approach is to say that there was never a problem to begin with. Correct me if I'm wrong, but PQGR still faces the same problems that other theories are facing, only at a different place. To me,then, it's kind of like arguing semantics.

Hoku, I thought you wanted an historical answer. PGQG is the natural and historical first attempt to quantize gravity. It is the failure of this approach in the UV which triggered the research for alternatives.

If you want more modern approaches, there are many. I'd vote for non-commutative geometry, by taste. The paper by Connes and Rovelli in 1994 is my all time favorite.

 

[Frame Dragger]

Hi Hoku, I got your PM, and I wasn't ignoring you! Anyway, from what I can tell, as a historical answer Humanino has it perfectly. jfy4 and I were more concerned with generalizations and current issues, whereas Humanino has given you a truly historically relevant answer.

Deterministic vs. Probabilstic is a question that is MOSTLY answered, and while it has historical significance, it isn't the historic view of nature.

 

[Fra]

I didn't follow the entire thread so this is just an interjection.

1) spacetime is continuous while QM quantizes space(time?).
But the quantization of space is an interpretation, right? It was designed (by Schrodinger?) as a way of understanding quantum jumps, right?

2) spacetime is relational while QM uses an absolute space(time?).
This is the one I have the greates issue with. I've read reasons why RT says spacetime is relational, but it's not jiving with me. understand the "relative" parts, but I'm not convinced how that makes space an emergent phenomena. I need to start a thread in the "relativity" section to get to the bottom of that one.

3)?
But what about spacetime being deterministic and QM being probabalistic? What are your thoughts on answers #36 and #37?

I think the three issues are related by the common question on what you actually measure and what are "observables" in QM vs GR.

One constructing principle behind both SR and GR is that the laws of physics should be invariant with respect to the choice of observer, and in relativity the set of observers are associated with the set of reference frames.

So the statement of GR and SR are statements of the symmetries only, or the equivalence classes of all observers.

This is partly very sound because what use would it be with observer dependent laws of physics? But it's partly unsound because the information about these laws are not subject to a measurement process - they are realist type constructions.

In QM, the measurement process is taken a little more serious in, but there is no clear understanding of what the set of all observers in QM is. And how objective symmetries of relativity is to emerge from it. Ie. how to find an observer invariant description of QM.

There is also a conceptual clash between the frameworks of GM and QM in terms of realism.

Edit: Like has already been noted this is of course a conceptual clash between the abstraction frameworks we currently have for GR and QM - not between the physical phenomenon gravity and subatomic physics itself. But the abstraction frameworks is what we're discussing.

Relativism and non-realism are not the same thing. Relativity is still a realist theory IMO. And the ideal behind a measurement theory that is conflicting this design principle is that we should not speak of what nature is, but only what we can infer about nature.

So one possible description of the confligt is howto combine the two demands

1. the laws of physics are observer invariant
2. the laws of physics are a result of an interaction/inference/observation process

I've seen two schools, either you have a structural realist view, and try to actually find a relational but still realist formulation of QM (I think advocated by rovelli and others).

Or you try to take the observation process more seriously and take the view that in all this incompletness the laws of phyiscs are emergent and evolving (sniffing in this direction are smolins evolving law ideas).

See.
"Relational Quantum Mechanics" http://arxiv.org/abs/quant-ph/9609002
"On the reality of time and the evolution of laws" http://pirsa.org/08100049/

/Fredrik

 

[Hoku]

Thanks for you sensitivity Frame Dragger. Glad to see you haven't abandoned the discussion.

The PQGR topic has me a little confused because, when I read the paper, written in Dec. 1997, it speaks as though this is a current issue. Both arguments that arose on this thread from it support the idea that this is a current issue. It's seems odd to me that a paper as late as 1997 should prove to be the best historical description of why we recognize(d) gravity/QM to have incompatibility issues. At any rate, I'm confident that the answers I was looking for are summerized at the end of reply #44.

My final satisfaction would come from critiques of my replies #33 and #37. That should help clear up the third answer ( 3) ? ) from reply #44 and allow me to move forward.

fra, I just saw your response. Thanks! I'll have a chance to read it tomorrow. I need to get to bed now.

 

[Fra]

But what about spacetime being deterministic and QM being probabalistic? What are your thoughts on answers #36 and #37?

Probability and indeterminism are after all not quite the same thing so and probability and determinism are not in contradiction. If the set of distinguishable probability distributions make up the state space, instead of individual events, QM is still deterministic.

The probability distributions in QM are ideally evolving deterministically. That's how the loss of determinism, is recovered at a higher level.

Similarly the loss of absolute notions in relativity, is recovered in the RELATIONS between the frames. Observations in relativity are not absolute, but the relation(transformation) between any two observers are still sort of absolute.

The connection between distributions and single events, is like single data points and conclusions in statistical inference. It's not possible to make a confident inference based on a single datapoint unless this single data point for some reason is assigne a massive weight. To get something statistically significant that's distinguishable from noise you need many events.

That something is distributed as per a decidable distribution is far from void in information. A probability distribution is in fact a lot of information. This information still has to be inferred from somewhere. Usually information about distributions follows from patterns observed in histories. So I think the best view of probability is to see it as acquired information about the probability of of events, in the subjective bayesian sense.

Not sure if this helps but here is a contemplation of what a sensible view of probability is.
http://math.ucr.edu/home/baez/bayes.html

The event space and the priors are part of the baggage in a statistical model, unless you somehow envision a hierarchy where the eventspace is somehow evolving in a way that given enough complexity could be explained by a more complete theory.

But I think this is conceptually related to the comments in the previous post. For example the symmetrys of nature, may it be symmetries of spacetime, symmetries of the weak and strong interactions, the question one might raise, which current models don't is if these symmetries are to be thought of in a realist way, that RULES nature, or wether they are merely emergent historic patterns that indirectly rules nature by a mechanism analogous to collective expectations, because all observers are acting as to reinforce the emergent symmetries. So the symmetries is then a result, rather than a constraint.

/Fredrik

 

[qsa]

3)?
But what about spacetime being deterministic and QM being probabalistic? What are your thoughts on answers #36 and #37?

this was posted recently on PF


http://arxiv.org/abs/1003.1262

Is Holographic Entropy and Gravity the result of Quantum Mechanics?

Joakim Munkhammar
(Submitted on 5 Mar 2010)

In this paper we suggest a connection between quantum mechanics and Verlinde's recently proposed entropic force theory of Newtons laws. We propose an entropy based on the quantum mechanical probability density distribution. With the assumption that the holographic principle holds we propose that our suggested quantum entropy generalizes the Bekenstein entropy used by Verlinde in his approach. Based on this assumption we suggest that Verlinde's entropic theory of gravity has a quantum mechanical origin. We establish a reformulation of the Newtonian potential for gravity based on this quantum mechanical entropy. We also discuss the notion of observation and the correspondence to classical physics. Finally we give a discussion, a number of open problems and some concluding remarks.

this is a post I made long time ago

https://www.physicsforums.com/showthread.php?p=2564949#post2564949

a quote from that post

"ok, susskind does use light rays(to produce holography), but he uses light rays to represent a parton(particle) on the screen. Not far enough. I propose a ray from every point in space-time to every other point in space-time. The number of connections(two way) per two points(A,B) will represent the entropy(information) that passes between those two points. the entropy at those points is related to the probability of finding a particle at those points. The entropy at A will affect B and vis=versa in such way to change their probabilities(entropies) to indicate attraction(by lowering the pobabilities at those points, forcing an increase in probabilties in the neighbouring points). This technique works for all forces"