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As an aside, the fact that the problem is with step (2), and not step (1), is one of the reasons this subject is so technical and difficult to explain at a "non expert physics" level. The issue isn't some grand philosophical question about what kind of guess makes sense. We have a specific theory -- general relativity -- and we have a specific procedure to follow -- quantization - the issue is that when carrying out teh procedure in detail, we don't arrive at useful predictions at high energies. On some level, the "right answer" to this is that we're doing something wrong, and the obvious way to write an "easy non-expert" answer would be to simply tell you what the wrong thing is and how to fix it, but we don't know how to fix the problem. So, that maybe gives some context for why it's so hard to give a non-technical explanation; all we can really say for sure is what goes wrong in the calculation if we try to follow the usual rules.

As an aside, the fact that the problem is with step (2), and not step (1), is one of the reasons this subject is so technical and difficult to explain at a "non expert physics" level. The issue isn't some grand philosophical question about what kind of guess makes sense. We have a specific theory -- general relativity -- and we have a specific procedure to follow -- quantization - the issue is that when carrying out the procedure in detail, we don't arrive at useful predictions at high energies. On some level, the "right answer" to this is that we're doing something wrong, and the obvious way to write an "easy non-expert" answer would be to simply tell you what the wrong thing is and how to fix it, but we don't know how to fix the problem. So, that maybe gives some context for why it's so hard to give a non-technical explanation; all we can really say for sure is what goes wrong in the calculation if we try to follow the usual rules.

That may be unsatisfying, because I'm not really saying what the issue with (2) is, so I will try to go one level deeper (still only just scratching the surface though).

As an aside, the fact that the problem is with step (2), and not step (1), is one of the reasons this subject is so technical and difficult to explain at a "non expert physics" level. The issue isn't some grand philosophical question about what kind of guess makes sense. We have a specific theory -- general relativity -- and we have a specific procedure to follow -- quantization - the issue is that when carrying out teh procedure in detail, we don't arrive at useful predictions at high energies. On some level, the "right answer" to this is that we're doing something wrong, and the obvious way to write an "easy non-expert" answer would be to simply tell you what the wrong thing is and how to fix it, but we don't know how to fix the problem. So, that maybe gives some context for why it's so hard to give a non-technical explanation; all we can really say for sure is what goes wrong in the calculation if we try to follow the usual rules.

That may be unsatisfying, because I'm not really saying what the issue with (2) is, so I will try to go one level deeper (still only just scratching the surface though).

• "Quantization" is a series of heuristic rules (aka, guesses) that we apply to define a quantum theory that is guaranteed to reduce to a given classical theory in the limit that quantum effects are small.
• These rules have been very successful when applied to theories of particles with spin-0, 1/2, and 1, at relativistic energies (meaning when special relativity is relevant). The Standard Model is an example theory like this.
• The rules can be applied to gravity, which is described by a spin-2 particle coupled to other matter. When we apply the rules, we find that we can make predictions in the sense of computing the first few terms of a Taylor series of the energy scale of the process divided by Planck scale.
• However, for energies at or above this energy scale, it is not enough to compute a few terms. This causes several problems.
  • First, most obviously, we don't know how to compute the infinite number of terms and "resum" them into something useful.
  • Second, we need to account for effects that don't appear in Taylor series at all. This might include effects of quantum black holes, for example.
  • Additionally (I'm not saying "finally" because this subject is so complicated that I'm sure someone can come up with more problems), according to our normal rules, we expect that for each term in the series, we need to introduce a new parameter to make that term well defined. The details of how you introduce these parameters are what people call "renormalization" -- a "renormalizable" theory is one where you only need to introduce a finite number of parameters. Renormalizable theories are predictive because we can do a finite number of experiments to measure those parameters, then the results of all other experiments are predicted by the theory. Non-renormalizable theories, on the other hand, require an infinite number of parameters, so no matter how many measurements you do, you can always adjust a parameter in the theory to fit that experiment. Therefore, non-renormalizable theories are not predictive, so not useful for science. (More precisely, they are not predictive in the regime when all the terms in the Taylor series are important.)

• "Quantization" is a series of heuristic rules (aka, guesses) that we apply to define a quantum theory that is guaranteed to reduce to a given classical theory in the limit that quantum effects are small.
• These rules have been very successful when applied to theories of particles with spin-0, 1/2, and 1, at relativistic energies (meaning when special relativity is relevant). The Standard Model is an example theory like this.
• The rules can be applied to gravity, which is described by a spin-2 particle coupled to other matter. When we apply the rules, we find that we can make predictions in the sense of computing the first few terms of a Taylor series of the energy scale of the process divided by Planck scale. (Technically the Taylor series I'm referring to is called the "Wilson action" at scale $\Lambda$, and I'm supposing the cutoff scale $\Lambda$ is around the Planck scale).
• However, for energies at or above this energy scale, it is not enough to compute a few terms. This causes several problems.
  • First, most obviously, we don't know how to compute the infinite number of terms and "resum" them into something useful.
  • Second, we need to account for effects that don't appear in Taylor series at all. This might include effects of quantum black holes, for example.
  • Additionally (I'm not saying "finally" because this subject is so complicated that I'm sure someone can come up with more problems), according to our normal rules, we expect that for each term in the series, we need to introduce a new parameter to make that term well defined. The details of how you introduce these parameters are what people call "renormalization" -- a "renormalizable" theory is one where you only need to introduce a finite number of parameters. Renormalizable theories are predictive because we can do a finite number of experiments to measure those parameters, then the results of all other experiments are predicted by the theory. Non-renormalizable theories, on the other hand, require an infinite number of parameters, so no matter how many measurements you do, you can always adjust a parameter in the theory to fit that experiment. Therefore, non-renormalizable theories are not predictive, so not useful for science. (More precisely, they are not predictive in the regime when all the terms in the Taylor series are important.)