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Could the reason Gravity is not compatible with QM be

  1. Jul 6, 2014 #1
    because of the probabilities located in the equations of QM, wave functions and uncertainty principles?

    Instead of exact values for particles in QM there are potential ranges of values, these ranges over greater 'theoretic energy value difference' (like a range instead of an exact value of 5, a value of 1-10, offers greater leeway in exchange for exactitude).

    Could it be that by including those ranges of value leeway, 'knocks out' the exactitude of Gravity from being necessary for the equations?

    In other words, because QM equations are self consistent and equal, using the probabilities and ranges, there is no room for Gravity; is it possible that the room for gravity is in those probabilities and ranges?

    That by admitting it may be impossible to know the exact value of a particle, but also admitting that the particle might have an exact value, one can think theoretically if the exact value was increasingly known over a time of trials, meaning the odds of probabilities would be smaller and smaller focusing in on exactitude, that this would give greater and greater room and possibility to input values of gravity?

    This is just a musing, sorry if everyone hates me.
  2. jcsd
  3. Jul 6, 2014 #2
    Hi Imafungi, you are correct that to describe a free particle you need a sum of wave functions over a range of energy states. I don't think that more measurements would result in a reduced uncertainty (greater exactitude).
  4. Jul 6, 2014 #3
    I understand and agree. But consider it in this way, over the progressions of years, since the beginning of subatomic particle physics, have we gotten better at all with measuring our probabilities? For rough example, if at the very beginning because of our measuring equipment in equations we used for particle values our probabilities were 1-100, and than 20 years later we discovered a smaller band of probability say even 1-90 range. Even with that small of a trend, or any trend at all, of a honing in, if my notion has any possible validity, regarding the relation of gravity to the existence of these probability ranges, that can potentially be used to plug in gravity values to the equations, with the differences of wave functions over a range of energy states, to see which equation becomes more balanced and utilizable. I think I am merely saying, in theory, the closer one got to the probability of a particles value, the smaller the range, the greater potential to plug in gravity values, the more opportunity to see which values for gravity and energy would check out to give you a QM and gravity compatible theory.

    And did you understand the sentiments of the OP at all, regarding the possibility of gravity's exclusion from QM being directly due to the existence of probabilities in QM? What I was mainly wondering, if this questioning has any substance to it?
  5. Jul 6, 2014 #4
    In QFT space is something different than the field. There is a classical, smooth, infinitely divisible space and an operator field defined on it. The field is quantized, space is not. Rather the quantized field depends on the point of classical time-space.

    On contrary in GR the field is the space. The metric tensor field describes the shape of the time-space itself. If the metric were quantized, it would mean the time-space is itself quantized and no one knows how to do it. Similarily any matter field would need to be quantized and defined on a quantized time-space. No one knows how to do it.

    To sum it up once again:
    - In QFT field is dependent on space, but space is independent. Field is quantized, space is not.
    - In GR field is dependent on space and space is dependent on field. Field is not quantized, space is not quantized.
    - In hypotetical quantum gravity both field and space would have to be quantized and this quantization is not a simple generalization of QFT.

    However there exists a weak-field approximation quantized gravity theory. It works under assumption that the quantum fields are so weak that the time-space is not curved enough for the quantum aspects to matter. That means it can not explain black holes, however it predicts gravitons.
    Its equations work that way:
    - There is a classical smooth time-space.
    - A quantum field is defined on time-space.
    - Mean value of the field is taken, which is again classical.
    - Time-space depends on that mean value.
  6. Jul 6, 2014 #5
    Thanks for the insightful response, I think I follow some what.

    To try to understand what you have expressed using my own meager and simplified terms.

    Well, thinking about the situation by myself, I come to the thought that; Either the substance (matter/energy/particles/fields) of the universe exists 'in/on' an infinite area of pure nothingness (meaning absent of matter/energy/particles/fields/anything/any quality other than the absence of all quality), and it is then possible for areas of pure nothingness (however small, even one planck length would qualify) to exist within the universe, in between matter/energy/particles/fields.

    Or, the substance of the universe exists 'in/on' an infinite area of pure nothingness and it is not possible for areas of pure nothingness to exist within the universe. Which would mean that not even a planck length of the universe is completely devoid of matter/energy/particles/fields.

    I think in either of these scenarios, there might be (might definitely be) an importance in regards to the binding nature of matter to matter. In the second scenario it would appear as the binding energy of matter/energy/particles/fields is so supreme that it doesnt not even allow a pocket of the all outer pervading nothing to enter the completely taught and closed and dense system that would than be the universe.

    In the first scenario, where there are pockets, even a planck length, of true nothingness, there is still the inherent binding energy that causes all particles to not be infinitely far away from each other. In this scenario, if this is closest to prevalent models and understanding of the universe, than I suppose the space between stars, and more so, the space between galaxies, would be the greatest expressions of this fact, that areas of nothingness pervade the inner confines of the universe at large.

    So now, is 'that which besides mass itself, is responsible for the phenomenon of gravity' a field of particles (like all other material and energetic things are a field of particles)?

    If so, how dense is it thought this field of particles is? If it is the scenario with pockets of true nothing space in the universe, what is the average area of nothing space between each gravity particle? In other words, how dense is the gravity field, compared with I suppose, how possibly dense can it be? Is the altering of its density what causes gravity?

    Can these particles transform into other particles? Do they fundamentally share an inherent nature with all other matter and energy?

    I know I did an awful job at following your example, but this is just how I had to start thinking about this at first. Hopefully you will be able to follow what I was getting at, and not be too mad that I apparently ignored the terms you used. I tried to put it into my own words so you can see where I am at in my understanding, but I hope I can excel my understanding and we can potentially have a nice discussion about these topics, eventually using more so your lingo.
  7. Jul 7, 2014 #6

    Even though they haven't been observed yet, gravitons are bosonian particals with no mass (obviously), 0e charge, and a spin of 2. Gravitons alow General Relativity to exist (to a limit) in the quantum field. Though they have calculated the 11 demention in the M-theory, the sum of all of the string theries, I read in an artical [could't find it sorry] that the 11th demention was caused by supergravity, superparticals (sparticals). That "spartical" could be a graviton or sgraviton (this is just a theory I read but it makes sense). So if gravitons do exist, then gravity can work in QM.
  8. Jul 7, 2014 #7


    Staff: Mentor

    It is compatible:

    The issue is, exactly the same as QED etc etc is its only valid to some kind of cut-off.

  9. Jul 8, 2014 #8
    What is the theorized reason its not compatible at the cut off?

    Could it be that the gravity particles and particles causing the gravity become one at certain energies or something?

    What I was mainly wondering in the OP, was if theoretically there was no need for probabilities in the QM equations, that is to say position and momentum could be known, then would gravity be compatible with no need for cutoffs?
  10. Jul 8, 2014 #9


    Staff: Mentor

    Its inherent in the effective field theory approach to renomalisation.

    To fully understand it you need to understand renormalisation a bit:

    QFT in general, not just quantum gravity, but in general, has a dirty little secret. Infinities crop up all over the place eg the energy of the vacuum turns out to be infinite. Obviously its wrong. A careful analysis shows the cause is our theories, from dimensional analysis, often are missing a parameter. The most reasonable one is a cutoff at some energy. Low and behold when you put in the cutoff everything is finite. But what cutoff?

    The interesting thing about renormalisable theories is it turns out that parameters like charge, mass etc depend on the cutoff - these are called the bare quantities. But from measurements at low energy we know their value and can calculate what it would be at various cutoffs - this is called the renormalisation group flow. We don't know the cutoff, but regardless of what it is we can assign a value and hence compute anything we like.

    The thing about gravity though is its not renormalisable - it turns out the number of parameters that need to be fixed is in a sense infinite so you cant do it. Drats. But it also turns out if you take the cutoff seriously its actually possible to construct a theory that is for all practical purposes the same as quantum gravity below the cutoff - and that theory allows things to be computed. This is the effective field theory approach to theories that are not renormalisable.

    So all QFT's basically cry out for cutoffs, but some theories have a slightly different take on the meaning of the cutoff.

    That's possible - but we simply do not know right now.

    You seem to be stuck on this probability thing in QM.

    I have posted my view, and the view of a number of researchers into the foundations of QM, many times.

    Its simply the most reasonable extension of probability theory that allows continuous transformations between pure states. That's what is required if say a system can change to something in one second, then it should in the process of that change to something in half a second.

    Why is nature probabilistic? Why is nature anything - nature is just - well nature - it is what it is.

  11. Jul 8, 2014 #10
    How did they determine what cut off to put in? And when then do they replace the infinite energies of the vacuum with, what energies of the vacuum?

    Could it be the energies are infinite because 'time' is infinite? In that, matter/energy cannot be created or destroyed, only transformed, so energy can interact with itself infinitely?

    Or is the vacuum values really the gravity values? That is to say the vacuum is the gravity field? And the reason it comes up as infinite energy, is because a mass in the vacuum or gravity field, can impart energy onto the gravity field and have energy imparted on it by the gravity field in a non exactable, there for, non finite way? Or in the gravity way, they dont know how many quanta per area of the gravity field, so they dont know the mass of the gravity particle, or its average density? Or the fact that the density of the gravity field, or vacuum, can be compressed and displaced, which gives it in the very least a non finite value, but a range of potential values?

    Hm, so are you suggesting all the mass values may be wrong? Or we have narrowed them down to such a potentially certain range that if they were wrong they would be wrong by such a small amount? Or its just that the values for mass and such are self consistent, like the values relate to themselves as a self referential system, like a yard stick, the value system of mass is a created yardstick, that the particles qualities can be measured up against, but you are suggesting we may not know exactly how the yard stick relates to a fundamental way to reality?

    The thing about gravity though is its not renormalisable - it turns out the number of parameters that need to be fixed is in a sense infinite so you cant do it. Drats. But it also turns out if you take the cutoff seriously its actually possible to construct a theory that is for all practical purposes the same as quantum gravity below the cutoff - and that theory allows things to be computed. This is the effective field theory approach to theories that are not renormalisable.

    I was only stuck on it, because I am just coming into an objective comprehension of the state of modern physics. I have been steadfast and skeptical and learning, and had heard mixed expressions, and with due diligence and the most careful thought performances which I know myself to have, I have found the most appropriate stance to take. I had heard a lot about the QM 'brand' of dazzling and spooky and bizarre and 'no one understands it' features, and have heard that people believe that QM is an exact reflection or perfect encapsulation of reality, and so from hearing that I was forced to consider their, and what appears to be your view. Though now after the past 2 years or so of armchairly learning physics in some free time, I am forced to take the side of Einstein and the materialist rationalist realists and say that QM is not a perfect map of reality as any ultimate Truth of Truth would be, but mans best yet tool of organization to perform different activities with the smallest known constituents of reality.

    I appreciate that, truly.

    I have no problem with models and theories and equations. I only have a problem with someone claiming that a model, theory or equation is Truth with the most capital T, when arguments can be made for a more, ironically, probable interpretation of those statements. (Truth in this circumstance, and all really, referring to the difficult/impossible notion to challenge that 'Reality = Reality', and that that is what Truth is, and if your model and theory and equation is not an equal part of Reality = Reality, then the model or theory or equation is not Truth, not the essence of reality.

    Certainly anyway we discover these things will be symbolic. I can say, a worm bit a red apple that was laying in a field. And you can imagine those things in your head clearly. These words are symbols, the images in your head are symbols, but to a pretty high degree they correlate to something, greatly arguably to exist and be real components of reality. An apple exists, we may not know everything there is to know about an apple, but 'something is there', that 'something' whatever it is, equals itself. Reality is tautological.

    The problem I have with QM and the probabilities in it, is they are trying to make reality their equations. They are saying, 'we can only use probabilities to do experiments and make predictions, that means the universe can only use probabilities to exist, therefore our equation, with its probabilities, is equal to the universe, the equations have probabilities, so the universe most be made of probabilities'.

    Now one of the most beautiful and startling things about the universe, is the human mind, because there is a case that does appear to defy determinism truly. Its severely complex, and I suppose part of the reason is because it creates a fake reality within it! That is composed of its own symbolic language and logic, which it can compute together, so to other observers, the unique nature of this symbolic mind language, is unpredictable. But however, not that I believe in human determinism, I believe in it to an extent of course (you must eat to live, you must have been born by your parents to live, its determined that your parents must be born before you) but I also believe in free will. I dont believe that the fundamental constituents of the universe have free will and for them to have probability they must. And in this case, just because we can not decipher the determinacy of fundamental constituents, doesnt mean they are not determined by reasons. QM and the probability interpretation, suggests that everything happens for no reason, in a physical sense of course, that is to say, I drop an ice cube and it breaks on the floor, the ice cube fell for a physical reason, the ice cube broke for a physical reason, the ice cube froze in the first place for a physical reason. Probability interpretation is saying, like an ancient man who didnt know how ice froze, like an ancient man that didnt know why physically ice fell, like an ancient man that didnt know how or why ice broke when it fell to the floor, that because he doesnt know these things, there must be know reason. Because you dont know a particles location and momentum at the same time, it must not have them. Because your equations can not know exact things about particles you must utilize probabilities, therefore the universe must utilize probabilities.

    Now that I can agree with!
  12. Jul 9, 2014 #11


    Staff: Mentor

    For renormaliseable theories they didn't. It doest matter what cutoff you use, as long as its well above the region you are interested in you can assign a value at some cutoff and everything is fine. For non renormalisable theories it depends on the theory they come up with that's a low energy approximation. For gravity it turns out the cutoff is about the plank scale.

    It depends on the cutoff. But the energy of the vacuum is not directly measurable so it doesn't matter.

    You have asked a lot of highly speculative questions.

    Sorry but I am going to wimp out. All I can suggest is you go through the technical detail of the link I posted:

    Its highly technical but unfortunately in physics some areas are not amenable to lay explanation. With renormalisation the jig is well and truly up. I have an honours degree in applied math and it stretches me, so I cant really 'dumb' it down so to speak because I can 'just' follow it myself.

    Physical theories are mathematical models. The relation of that to truth with a capital T is philosophy pure and simple and is off topic here. The philosophy forums is the place for that. However its exactly the same as the relation of good old Euclidean geometry you leant about at school and the truth of points and lines. Figure that one out and you have your answer. However most physicists, including me, would ascribe to the view it describes reality without delving too deeply into what reality is. In fact I would go as far as to say, and I certainly fall into this camp - such a description of reality is the best we as humans can get so for all practical purposes accept it as the actual reality. Some like Penrose go further than for all practical purposes and say it is the reality.

    This feeling that the math is the rock bottom essence of what's going on was examined in a famous essay by Wigner:

    Murray Gell Mann's answer would probably be what most people these days say - its my personal view:

    But again if you want to pursue it here is not the appropriate place.

    Last edited: Jul 9, 2014
  13. Jul 9, 2014 #12


    Staff: Mentor

    Closed pending moderation.

    EDIT: The thread will remain closed.
    Last edited: Jul 9, 2014
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