Examples of no gravity in quantum mechanics?

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  • #1
Hoku
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I seem to be missing the resources that describe how gravity is not found in quantum mechanics. What phenomena in quantum mechanics illustrate that gravity is [thus far] not a part of it?

This is not a homework question.

I know that gravity is incompatible with what we know about QM. What I DON'T know is WHY. I'm not interested in theories on quantum gravity. I have books about them and that's not what my question is about. My question is more about the reason we are trying to find quantum gravity in the first place. What is it SPECIFICALLY about QM that is not compatible with gravity? What QM behavior "defies" gravity?
 
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Answers and Replies

  • #2
Frame Dragger
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I seem to be missing the resources that describe how gravity is not found in quantum mechanics. What phenomena in quantum mechanics illustrate that gravity is [thus far] not a part of it?

It's not so much that a particular phenomena shows the lack of gravity at the quantum level... the opposite in fact. Rather, QM just doesn't describe gravitiational interactions well (or at all). It's the gap between Quantum Mechanics and Relativity.

EDIT: The reverse is true in terms of Relativity explaining the "world of the very small". That's why people are so excited about any possiblity of a working theory of quantum gravity.

Now, if this is a simple homework question, and you need help understanding why that is, we kind of need to know, and this should be in the help section. I will say that either way, examining what theories purporting to be a roadmap to quantum gravity claim to bring to the table is a roadmap to understanding what SR/GR and QM lack in terms of unity.

Of course, the NAME "QUANTUM GRAVITY" should be the big hint... what is gravity NOT, that all other forces ARE shown to be? What particle has no clear evidence of its existence, or for which a mechanism may or may not exist (not the Higgs)?
 
  • #3
humanino
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It is not true that "QM does not describe gravitational interaction". There is no difficulty in computing quantum corrections to general relativity at moderate energies. We obtain a non-renormalizable theory, which therefore is not valid at arbitrarily large energies (more precisely, it looses its predictivity as more and more counterterms must be included, while performing only a finite number of measurements), but it works at low energy and the first corrections are expected to be fine on general grounds.

It will be best to understand the problem if one looks at John Donoghue's contributions, such as :
Perturbative dynamics of quantum general relativity
 
  • #4
Frame Dragger
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It is not true that "QM does not describe gravitational interaction". There is no difficulty in computing quantum corrections to general relativity at moderate energies. We obtain a non-renormalizable theory, which therefore is not valid at arbitrarily large energies (more precisely, it looses its predictivity as more and more counterterms must be included, while performing only a finite number of measurements), but it works at low energy and the first corrections are expected to be fine on general grounds.

It will be best to understand the problem if one looks at John Donoghue's contributions, such as :
Perturbative dynamics of quantum general relativity

I suppose that's why I said it doesn't "describe graviational interactin well". I would call the complete breakdown of a theory a "not well" outcome for a description of an interaction. :grumpy:
 
  • #5
humanino
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I would call the complete breakdown of a theory a "not well" outcome for a description of an interaction. :grumpy:
You may call it that way if you please, however it is not a surprise if we make little progress in this field : there is no data. I do not expect data in the deep ultraviolet will come before we have data to test first order corrections, as in the above effective approach. All expectations for this theory suffering a "complete breakdown" is that, when we have data at intermediate energy, they will agree with first order corrections, and this will therefore constitute possibly the most beautiful success of fundamental physics in history, until we obtain data in the deep ultraviolet, which is so difficult it may never happen.

I believe it is important to know this approach well, before even embarking into any other speculation.
 
  • #6
Frame Dragger
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You may call it that way if you please, however it is not a surprise if we make little progress in this field : there is no data. I do not expect data in the deep ultraviolet will come before we have data to test first order corrections, as in the above effective approach. All expectations for this theory suffering a "complete breakdown" is that, when we have data at intermediate energy, they will agree with first order corrections, and this will therefore constitute possibly the most beautiful success of fundamental physics in history, until we obtain data in the deep ultraviolet, which is so difficult it may never happen.

I believe it is important to know this approach well, before even embarking into any other speculation.

I wasn't speculating, but noting an absence of a theory's predictive abilities. There IS a UV catastrophe, just as HR seems to break Unitarity. When it's solved I'll start to refer to things differently, but I'm not couching my every phrase in terms of what progress MIGHT be made and how beautifull it will be then.
 
  • #7
Hoku
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I appreciate your input so far. I'm still looking for clarification, though. I think the answer I'm looking for is easier than it seems. I think the answer I'm looking for is something like this:

Quantum particles cannot be predicted the way other things in the universe can. That's because quantum particles are always jumping around to different places, changing position, momentum and energy without following any classical rules for these changes. Consequently, quantum particles do not abide by the same spacetime laws that other things in the universe do. Since spacetime is fundamental to gravity, this means that quantum particles are not compatible with the laws of gravity.

Is this right? Because spacetime does not apply to quantum particles neither does gravity????
 
  • #8
humanino
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I'm not couching my every phrase in terms of what progress MIGHT be made and how beautifull it will be then.
What I am referring to only awaits measurement. The theory is available in the papers I quoted. Just as Fermi theory, it works effectively, and so much so that the so-called ultraviolet "catastrophe" can also be interpreted as an amazing prediction of the theory itself : its own failure signals new physics. This is an interpretation of the results which does not deserve the qualification of "catastrophe" I believe.

Hoku, you propose a qualitative line of reasoning which may or may not turn out to be true in an ultimate theory which we do not have. Again, I think if you want to understand the problem with naive quantum gravity, you need to study it. It's simply exposed in the papers above. Your reasoning is not valid in the above papers for instance. The theory is not renormalizable perturbatively, which means that as energy increases, you will have to include more and more correction terms in your theory, until you have more terms than the number of experimental observations, at which point all predictive power is lost, and we do not have a "theory" anymore. A century ago, that was considered a "bad catastrophe". Today, I dare say it's been a while we do not think like this anymore. Yes, it is possible that our picture of a spacetime with particles moving inside is only "effective", and so ? You will never experimentally prove anything more than a theory with a limited range of validity, even if you came up with a pertubatively renormalizable theory of quantum gravity, what good would it be for ? You would only be able to test at forever higher energies, until you get tired and quit. A non-renormalizable theory has this very desirable feature that it tells you how far you need to go (worse case) before you know you will find something new.

It is also possible that the above theory is renormalizable after all, non-perturbatively. It's a long story, but we have good hints, you may ask what Marcus thinks. So far, Nature always had more tricks in Her leaves when we thought we finished laying down the laws. It's only speculation, personally I prefer to keep my mind open.
 
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  • #9
humanino
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I think I'd better get out of this discussion, but before doing so, I will emphasize once again that I do not understand how one could know there is something wrong with canonical perturbation of gravity if one has not studied it. So I think the resources I provided are essential. From the above paper, one can read
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
This is all I am trying to say here.
 
  • #10
Hoku
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Humanino, I want to thank you again for your contributions. I have printed the papers you suggested and will spend time reading through them. Although the math is a little over my head, I think I can find something in it to grab onto and move forward with. HOWEVER... I realize that I need to be more clear with what I'm looking for. What I'm specifically looking for with this question is not "the facts" about gravity as it does or does not relate to QM. I'm looking for the history of our perspective on it.

If I asked why we thought the sun revolved around the Earth, I wouldn't expect the answer to be, "Well, the sun doesn't revolve around the earth. That's an outdated way of thinking". I would expect the answer to be more like, "Because it doesn't feel like we're moving and that's the way it looked to us as the sun rose and set." Do you understand where I'm coming from? I'm not looking for the "right way" to view the problem, I'm looking for a basic history of how we have viewed the problem.

That being said, would you say that my mock answer is a good representative of why people have viewed QM and gravity to be incompatible?
 
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  • #11
humanino
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Maybe I need to think a little while longer to provide an alternative answer. For me, it's essentially technical, both the history and the conceptual reasons. I am not especially a string proponent, but I so happen to be in the mood right now, so I will provide a link to a more historical discussion by Distler
http://golem.ph.utexas.edu/~distler/blog/archives/000639.html
 
  • #12
Frame Dragger
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What I am referring to only awaits measurement. The theory is available in the papers I quoted. Just as Fermi theory, it works effectively, and so much so that the so-called ultraviolet "catastrophe" can also be interpreted as an amazing prediction of the theory itself : its own failure signals new physics. This is an interpretation of the results which does not deserve the qualification of "catastrophe" I believe.

That is most definitely an opinion, and no more right or wrong for that, but still just an opinion. You may well be right, but for now the catastrophe exists, and until those measurements are made and MATCH predictions... well... every time you make an assumption, a virtual pair dies. :wink:
 
  • #15
humanino
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Well, I think I spent enough time justifying why your initial answer was neither helpful nor appropriate, and why Hoku, if he is serious about understanding the question he initially posted, should start by studying canonical perturbative quantum gravity. I have a hard time finding a constructive contribution of yours in this discussion indeed. It is a fact that the expectation is widespread, you may disagree, that does not change anything nor is it really interesting.
 
  • #16
jfy4
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One of the major incompatibilities, though I would not say the incompatibility with QM and Gravity, is time... Time is like the craziest thing ever...

Quantum uses a background time, which GR says does not exsist. I know you said you got QG books so this should be in there, but a Quantum theory of gravity will be gackground independent, which is different then current quantum theory which uses a background time to be successful, and it is very successful...

to the point, A quantum theory of gravity is needed to explain gravitational interactions inside of a plank distance becuase of quantum fluctucations in space-time at those scales.

The time issue is a big one, and maybe, just maybe, the coolest discrepency, but that is the reason why we need a QToG.
 
  • #17
Hoku
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Thanks jfy4 for your input. Although I was first excited by the idea, I've since become skeptical of the "background independant" issue, as was described in my post, "LQG and relational spacetime" posted in "beyond the standard model". I'm formulating an idea for a new post about the issue and I hope that it will bring more responses that my LQG post did. Unfortunately, there are so many different perspectives to all of this that people seem to loose their calm, objective science skills and become almost fanatical about the perspective they've chosen. I think that might be why its dificult for people answer the basic questions I have.

The question I raised in this post seems like a basic one, indeed. It should not be so difficult to answer. I even created a mock answer as a starting point (see reply #7). I think my mock answer is pretty good and don't see why it doesn't explain the historic view of incompatability. Maybe we can break it down:

"Quantum particles cannot be predicted the way other things in the universe can. That's because quantum particles are always jumping around to different places, changing position, momentum and energy without following any classical rules for these changes."

Has this statement not, at some point in our history, been viewed as true? I believe it has. Can we first nail down an agreement on this? If no agreement is possible, perhaps we can alter it to a more agreeable statement, without sacrificing simplicity and clarity...
 
  • #18
jfy4
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Well, first i would say that particles are things, and things are things! we have right now two different treatments for things, i guess three, a Quantum treatment, a classical treatment, and a semi-classical treatment... I would not be inclined to call a particle a "quantum particle", but thats what physics is, interpretation backed by observation, you can if youd like, but it might lead to misunderstanding because interpretation varies.

How we treat the particles seems to matter when it comes to observing results... which is what you were saying in a sense i think.

The last part

"That's because quantum particles are always jumping around to different places, changing position, momentum and energy without following any classical rules for these changes."

Is a interpretation, and i think you are right, it is shared by many. Interpretations matter, there are two parts if you will. First, we as a community have chosen math as the language of physics, so english is a fantastic start, but our math must match with experiment. Second, even our english must be scientific, which it seems you are wholly committed too IMO, so it must be correctable and testable.

these being said, I dont interpret, along with others, that say a quantum particle has a constantly changing position and momentum, and my meaning would obviously vary from circumstance to circumstance, but rather that these properties of a particle are probabilistic.

That is to say, i can imagine a situation where a momentum is changing, and that is some physics to be sure, but the quantum part is that that momentum is probabilistic. And our math should say that, along with our english i think...

so after all this i would say: "particles can be treated in various fashions, however, depending on the situation one should be more inclined to use a a treatment which yields consistent results with experiment. (here's the part i think you are interested in) The quantum treatment tells us that the various aspects of particles which are observable, are probabilistic, and so they follow the rules of probability and wave mechanics."

that would be how i would write, what i think is, your statement. I hope we make some ground here! :)
 
  • #19
qsa
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"Quantum particles cannot be predicted the way other things in the universe can. That's because quantum particles are always jumping around to different places, changing position, momentum and energy without following any classical rules for these changes."

QUOTE]

Your conclution is correct but your reasoning is not.First, the interpertation of particles jumping around is not correct in the standard QM, even in other interpretation it does not matter because the wave is the particle i.e. not only it is not jumping IT is everywhere at once. second, if you apply your reasonning even the hydrogen atom will not have a binding force and will disintegrate in no time.

but your conclusion is correct for a different reason. the same force is called gravity over large distances (low energy) as the scale gets shorter we see it as electromegnatic (binding atoms). Once we get to the size of the proton and less then we can call it gloun and so on. The same force really.
 
  • #20
jfy4
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"Quantum particles cannot be predicted the way other things in the universe can. That's because quantum particles are always jumping around to different places, changing position, momentum and energy without following any classical rules for these changes."

QUOTE]

Your conclution is correct but your reasoning is not.First, the interpertation of particles jumping around is not correct in the standard QM, even in other interpretation it does not matter because the wave is the particle i.e. not only it is not jumping IT is everywhere at once. second, if you apply your reasonning even the hydrogen atom will not have a binding force and will disintegrate in no time.

but your conclusion is correct for a different reason. the same force is called gravity over large distances (low energy) as the scale gets shorter we see it as electromegnatic (binding atoms). Once we get to the size of the proton and less then we can call it gloun and so on. The same force really.

Not so fast! electromagnetic interactions are not gravitational ones! While there are many similarities to E&M with linearized gravity, Electromagnetic theory is about densities and currents and such, just like gravity, but about a field which operates in space-time, as opposed to being space-time in the case of gravity! Granted, if there is a day when all four forces are united, we can call them the same force, but right now, this is not the case. Scale matters, and for sure various forces are more dominant at certain scales, but look at the magnetic field of the sun. Remarkably important and which plays a huge role in the dynamics of the solar system, but which is not strictly responsible to binding atoms.
 
  • #21
qsa
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Not so fast! electromagnetic interactions are not gravitational ones! While there are many similarities to E&M with linearized gravity, Electromagnetic theory is about densities and currents and such, just like gravity, but about a field which operates in space-time, as opposed to being space-time in the case of gravity! Granted, if there is a day when all four forces are united, we can call them the same force, but right now, this is not the case. Scale matters, and for sure various forces are more dominant at certain scales, but look at the magnetic field of the sun. Remarkably important and which plays a huge role in the dynamics of the solar system, but which is not strictly responsible to binding atoms.

Other than LQG and CDT the majority of QG theories work in normal space time. It is treated just like the rest, starting with some symmetry(even ST). My point is that gravity cannot be found at high energy because it is not there. Roughly speaking it is like the strong force(coupling) vanishes when quarks come close together , that is called asymptotic free . so one can say the coupling vanishes at very small scales. just an idea I got from my own research.(my profile)
 
  • #22
jfy4
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Other than LQG and CDT the majority of QG theories work in normal space time. It is treated just like the rest, starting with some symmetry(even ST). My point is that gravity cannot be found at high energy because it is not there. Roughly speaking it is like the strong force(coupling) vanishes when quarks come close together , that is called asymptotic free . so one can say the coupling vanishes at very small scales. just an idea I got from my own research.(my profile)

That's fine, But to be sure, are you suggesting that the sun's magnetic field is actually gravity, whether space-time or in QFT terms? There are many similarities between forces in this universe, which is why a search for force unification is underway, but similarities right now do not mean equivalence right now, and interpretations like:

"so one can say the coupling [gravity] vanishes at very small scales."

is still completely unknown. A fine interpretation, but quite a step to declare that one force only scales depending on size to manifest itself like four, especially without the ability to test a claim like that...

I am all for working through various interpretations and predicting results from your own work. But you should not enjoy interpretation as fact! that is scary. I hope we can see ye to eye on this.
 
  • #23
Frame Dragger
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@Hoku: It's not always fanaticism, but people don't often get second (big) chances in academia, so unless you're tenured somewhere cozy believing in a given theory = having a horse in that race. This becomes a MAJOR issue when people not working with QM begin to debate or cling to interpretations or too much "beyond the standard model" content.

There is a lot to be said for Positivism, with spicings of Skepticism and Instrumentalism in QM. Rarely found there however...
 
  • #24
Hoku
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Last I knew, no theories have yet unified gravity with the other 3 basic forces, which have had success in unification only amongst themselves. LQG has been making it's own progress towards it, but hasn't yet reached the "eureka!" moment.

As you point out qsa, sub-atomic particles are both waves and particles. When sub-atomic particles are waves they are everywhere at once, but when they are particles they have definable locations. From what I understand, when they are particles, they make arbitrary and unpredictable "quantum leaps" between different energy levels - which encompases position, momentum and energy. This has at least been an interpretation in the past, hasn't it? If so, then the interpretation of them leaping around is a correct history.

jfy4, I've been running your suggested interpretation over in my mind:

"The quantum treatment tells us that the various aspects of particles which are observable, are probabilistic, and so they follow the rules of probability and wave mechanics."

I agree with and understand what you're saying, however, I'm not immediately seeing how gravity, or a lack thereof, is implicit in this statement. The only connection I can make is that spacetime does not work with probabilities, it follows predictable laws of cause and effect. Right? Is this how I should be understanding this? Is this why we are searching for quantum gravity? To take quantum phenomena out of probability and into predictability?

Is the reason there is no perceived gravity on the quantum level because the quantum world is unpredictable?
 
  • #25
Frame Dragger
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Last I knew, no theories have yet unified gravity with the other 3 basic forces, which have had success in unification only amongst themselves. LQG has been making it's own progress towards it, but hasn't yet reached the "eureka!" moment.

As you point out qsa, sub-atomic particles are both waves and particles. When sub-atomic particles are waves they are everywhere at once, but when they are particles they have definable locations. From what I understand, when they are particles, they make arbitrary and unpredictable "quantum leaps" between different energy levels - which encompases position, momentum and energy. This has at least been an interpretation in the past, hasn't it? If so, then the interpretation of them leaping around is a correct history.

jfy4, I've been running your suggested interpretation over in my mind:

"The quantum treatment tells us that the various aspects of particles which are observable, are probabilistic, and so they follow the rules of probability and wave mechanics."

I agree with and understand what you're saying, however, I'm not immediately seeing how gravity, or a lack thereof, is implicit in this statement. The only connection I can make is that spacetime does not work with probabilities, it follows predictable laws of cause and effect. Right? Is this how I should be understanding this? Is this why we are searching for quantum gravity? To take quantum phenomena out of probability and into predictability?

Is the reason there is no perceived gravity on the quantum level because the quantum world is unpredictable?

Why search? Because the two best and most predictive and useful theories; one describing the quantum world, and the other the macroscopic world... DON'T AGREE, and disturbingly, there is no single explanation as to when, how, or why the macroscropic world emerges from quantum interactions.

The bottom line: QM does a great job of helping us make things, but many of its implications require that one either accept a view of reality which is contrary to one's experience, or believe that this wonderful and predictive theory is somehow (not just wrong as all theories are) right and wrong at the same time.

Electricity and Magnetism have been unified, and then again in theory with the weak force at high energy levels. There is no unification theory for any of the fundamental forces with gravity, which by the way is strangely and inexplicably weak compared to other forces. Weak, but it has a cumulative effect and it's always attractive (so it seems), and that also requires and explanation. Why is gravity weak when the strong nuclear force holds quarks together with immense "strength"? This is why string theorists need extra dimensions for gravity to "leak" into, to account for weakness we percieve. Presumably a true quantum theory of gravity would explain this, and the exact mechanism (is there a graviton?) by which gravity acts.
 
  • #26
Hoku
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Thank you Frame Dragger! You are absolutely correct and I hope I don't come across as cynical, because I have a tremendous amount of respect for everyone that is working to move science forward - whether they are right or not or whether I agree with them or not. But these topics do become very touchy for the very reasons you've mentioned. Researchers have been working themselves silly to get and maintain funding and to get and maintain a respectable standing in the community. Their lives have been devoted to their cause - lost sleep, hours of research, personal sacrifices, etc, etc. The pressures that these researchers face to deliver for investors and universities and their "following" is certainly enormous and I have true compassion for them. But the position of having so much money and prestige riding on all of these different theories I think threatens good science.

Part of my point in saying what I did revolved around the fact that I can't seem to find straight answers to my basic questions that certainly DO have answers! I'm wondering if people are scared to speak up because of the "borderline fanatical" (as I had probably mistakenly likened it to) controversies that may arise or what. I'm not trying to ask controvertial questions. I'm certain my questions are quite basic. But is it possible that my questions are flirting too much with controversy after all or am I just having bad luck? Not just with this post, but with my other as well!

Frame Dragger - you, Humanino, jfy4 and qsa have all jumped in to try and help with this question and I can't tell you how grateful I am for that! Still, even with the four of you, this question hasn't been answered. I know that none of you are stupid, and neither am I. So I'm beginning to wonder if maybe nobody really understands the answer.
 
  • #27
Frame Dragger
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Thank you Frame Dragger! You are absolutely correct and I hope I don't come across as cynical, because I have a tremendous amount of respect for everyone that is working to move science forward - whether they are right or not or whether I agree with them or not. But these topics do become very touchy for the very reasons you've mentioned. Researchers have been working themselves silly to get and maintain funding and to get and maintain a respectable standing in the community. Their lives have been devoted to their cause - lost sleep, hours of research, personal sacrifices, etc, etc. The pressures that these researchers face to deliver for investors and universities and their "following" is certainly enormous and I have true compassion for them. But the position of having so much money and prestige riding on all of these different theories I think threatens good science.

Part of my point in saying what I did revolved around the fact that I can't seem to find straight answers to my basic questions that certainly DO have answers! I'm wondering if people are scared to speak up because of the "borderline fanatical" (as I had probably mistakenly likened it to) controversies that may arise or what. I'm not trying to ask controvertial questions. I'm certain my questions are quite basic. But is it possible that my questions are flirting too much with controversy after all or am I just having bad luck? Not just with this post, but with my other as well!

Frame Dragger - you, Humanino, jfy4 and qsa have all jumped in to try and help with this question and I can't tell you how grateful I am for that! Still, even with the four of you, this question hasn't been answered. I know that none of you are stupid, and neither am I. So I'm beginning to wonder if maybe nobody really understands the answer.

This is why we need a theory of quantum gravity, to answer questions as well. :smile:
 
  • #28
Hoku
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I liked your last entry Frame Dragger. It used simple language to describe conflict between the quantum world and the classical world. I'm certain that the answer I'm looking for is nearby...

I understand fairly well the conflict between the quantum and classic worlds. I have a few books about it. What I still don't understand, though, is why or how gravity is the point of breakdown between them. Why do we pinpoint GRAVITY as the reason the two worlds have been viewed as incompatable?
 
  • #29
jfy4
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I liked your last entry Frame Dragger. It used simple language to describe conflict between the quantum world and the classical world. I'm certain that the answer I'm looking for is nearby...

I understand fairly well the conflict between the quantum and classic worlds. I have a few books about it. What I still don't understand, though, is why or how gravity is the point of breakdown between them. Why do we pinpoint GRAVITY as the reason the two worlds have been viewed as incompatable?

Here is one reason why gravity and QM are having trouble, this is extremely similar to what I said before however, it merits repeating.

QM was formulated using a background time, and a background non-dynamical space-time. This is not compatible with GR. GR is based on Riemannian geometry, which is a smooth and deterministic dynamical background. QM demands that any dynamical field must be quantized.

So there we have it... we have been told, from a good source that has had great success in his business (QM) that another amazing worker (GR) is not how it was formulated to be, smooth and dynamic. So reconciling these two great working theories, as stated before, is the task at hand, and the reason is that we would like to continue using such agreeable theories, but in a framework that makes them compatible, for epistemological reasons.

Im not sure how else to say this... Its true what the former poster said that there are "revamps" of GR which do not consider it being space-time. But I can say that a great many physicists are in agreement with that interpretation...

this is a good talk!!!
 
  • #30
Frame Dragger
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I liked your last entry Frame Dragger. It used simple language to describe conflict between the quantum world and the classical world. I'm certain that the answer I'm looking for is nearby...

I understand fairly well the conflict between the quantum and classic worlds. I have a few books about it. What I still don't understand, though, is why or how gravity is the point of breakdown between them. Why do we pinpoint GRAVITY as the reason the two worlds have been viewed as incompatable?

Well, one simple way of thinking about it is about how the two worlds were derived. Gravity is the issue because it's the "representative" of Relativity. Gravity is a result of the factors described in the SET, so it's the focus. Gravity is of interest, but above all it's what jfy4 siad; GR describes a continuum, and QM is... well... quantized, ultimately at the Planck scale. Quantum Gravity would let us describe what goes on below that scale, or rather, help you answer "what is space-time?"


The headache is as jf74 describes, 2 really amazing theories which seem COMPLETELY incompatible at their fundamental levels, and who's predictions ultimately break down at a certain scale.

Not to mention that GR is deterministic, and QM is probabilistic. That has nothing to do with gravity, but it is a conflict without a resultion. It has consensus, but not resultion.
 
  • #31
Hoku
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I don't have time for a full response now but it seems like it's all coming back to the issue of relational vs. absolute spacetime - just like my other post did. I'll give this more thought and come back tomorrow.
 
  • #32
Frame Dragger
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I don't have time for a full response now but it seems like it's all coming back to the issue of relational vs. absolute spacetime - just like my other post did. I'll give this more thought and come back tomorrow.

By all means, I feel as you do, that we're getting some traction here.
 
  • #33
Hoku
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Yes, I think we're making some headway here, thanks to you both. From this point, two roads have opened up that require some probing.

1) 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.


2) Ok, this is where things might start getting really confusing so I want to take the exploration of this second road slowly. The idea is to approach the issues of background dependence vs. background independence. Let's first clarify a couple of basic ideas.

* Is it true that we are contrasting two worlds? 1) the quantum world, which includes all sub-atomic particles and 2) the macroscopic world, which includes all animals, vegetables and minerals on earth as well as stars, galaxies and other things in the universe.

* Is it true that, before relativity theories, we never required a mathematical formula for gravity in order to find formulas for other events? For example, Hooke's Law, momentum of inertia, etc. etc. do not depend on a formula for gravity. Is this true?
 
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  • #34
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Hoku
(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.)


how do you explain that particles that have probabalistic characters generate gravity, gravity is a property of matter after all. have you heard of verlinde(entropic force). google it.
also do this do see QM GR relationhttp://www.google.com.kw/search?hl=en&safe=active&q=effective+field+theory+gravitation+arxiv&meta=&aq=f&oq=
 
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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.
 
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