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I don't understand why it wouldn't be? One of the most fundamental scientific facts is that quantum theory (the most precisely tested theory in human history)

I don't understand why it wouldn't be? One of the most fundamental scientific facts is that quantum theory (the most precisely tested theory in human history) and general relativity are incomplete. There must be a bridge. And we have no idea what that bridge is. It's been this way for over a century; the lifespan of string theory is not so long in comparison. Until we find a way to falsify it, we have to keep trying, don't we?

simcop2387 3y ago
Yep, or at least until we find something else that is explaining things better and testing better and looks like a better path forward. As much as I dislike ST because of the ambiguity/vagueness that means it's so hard to nail down as a single concrete theory, the alternatives so far still also have a lot of those same issues (e.g. too many parameters, effectively impossible to test, etc.) so there's no reason to drop ST just yet as useless or an invalid theory.
runarberg 3y ago
Why do you assume there is a bridge? Or even if there is one, why do you assume that descriptions of that bridge in a mathematical language is at all possible? We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent. Would it be a stretch to assume descriptions of our physical reality could have the same restriction? General relativity and Quantum mechanics are relatively complete in describing our physical reality, however they are not consistent with each other. Perhaps if we ever find a description that is consistent, it won’t be complete. Perhaps grand unified theories are simply a mathematical impossibility.
- mr_toad 3y ago
  > We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent. It would be troubling to think that physical reality was inconsistent.
- GoblinSlayer 3y ago
  Mathematical completeness is a mathematical concern and is irrelevant for physics, because physics and mathematics have different notions of existence.
- jakeinspace 3y ago
  It’s sort of incomprehensible that there wouldn’t be, even if that bridge is forever incomprehensible to us. There are boundaries between where GR and QM are predictive, so presumably, unless there’s some strange smooth transition of equations between those two regimes (which would itself lend itself to a mathematical theory), then there must be a consistent set of equations which explain both regimes consistently and simultaneously.
  - runarberg 3y ago
    Coming from psychology this feels alien to me. In psychology there is a definite boundary between individual behavioral dynamics and population behavioral dynamics. There is no smooth transition between the two, either you describe the individual or you describe a group, you cannot do both (even thought debates about IQ here on HN will have you believe otherwise) and there is certainly no smooth transition. How is the boundary between GR and QM different from the boundary of psychology and sociology?
    - jakeinspace 3y ago
      Im neither a psychologist nor a physicist, but I think one can analogize population vs individual behaviour to condensed matter physics versus particle physics. Condensed matter physics finds emergent behaviour in large clumps of stuff that would, in principle, be totally predictable from first principles (the standard model), but which in practice are quite difficult to guess a priori. Different scales lend themselves to different tools, since nonlinear dynamics (chaos) makes it intractable to apply bedrock reductionist formulae to large systems. The higher order behaviour of complex systems is in no way less interesting or true, I would argue, than the seemingly simpler behaviour of very small systems. In contrast to condensed matter vs the standard theory (QM basically), QM vs GR has fundamental incongruities, since both theories make claims about what happens at the same scale. Only one (or most likely neither) can be correct at the event horizon and center of a black hole.
    - PopePompus 3y ago
      I would say it is completely different. In physics, quantum mechanics gives extremely precise and verifiable predictions of the outcomes of experiments within a certain range of physical conditions. So does general relativity. In contrast, psychology has no mathematical model that will, for example, accurately predict what I am going to eat for lunch, nor is there a model that predicts the exact outcomes of elections. Since physics has two models which are exquisitely precise in different size regimes, but which are mutually incompatible, you have a definite puzzle about what happens in situations where the effects of both theories should be important. There are no exquisitely precise and accurate mathematical models of anything in the social sciences, as far as I am aware.
- eouwt 3y ago
  > We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent. I take it you're referencing Godel's theorems here, but "consistent" and "complete" have rather technical (and somewhat limited) meanings within that context, so it's not clear to me how they'd usefully map onto the potential relationship between QM and GR?
  - simiones 3y ago
    That's a very good point. In particular, "complete" refers to the ability of the mathematical-logical system to prove every statement that is true within that system, in terms of the system. This property is completely irrelevant to a theory like QM or GR - it is only relevant for a system that aims to be a universal foundation for mathematics (a formal language in which any mathematical statement whatsoever could be precisely formally encoded, and then proven or disproven).
- simiones 3y ago
  The problem with this line of thinking comes from the way the problem is posed. The reality is worse for both GR and QM. QM is not a theory about small particles: it is a theory that describes the evolution of any object whatsoever. It just happens to be completely wrong for large objects. And even if we accepted that there is some objective cutoff points between "small objects" and "large objects" (the "objective collapse" interpretation), it would still be wrong, because it predicts effects like gravitational lensing don't exist. Conversely, GR is also a theory about any size of object; and it is also completely wrong when tested on small objects, as it predicts effects like the self-interference of particles don't exist. And again, even if some kind of objective boundary existed between the domains of GR and QM existed, that would need to be incorporated into the maths of both of them, and questions about behaviors close to that margin would arise.
  - cthalupa 3y ago
    >because it predicts effects like gravitational lensing don't exist. But we use gravitational lensing, and the amount of lensing is predicted by GR? I don't understanding this statement. Is there some additional affect around lensing we should see if quantum gravity is a thing?
    - simiones 3y ago
      No no; I'm saying that if you apply QM as it exists today to describe the motion of light beams around the sun, you will not get any effect similar to gravitational lensing. Since gravitational lensing is a real effect that we have clearly seen, it means that QM as it exists today is just wrong (in its predictions about how light moves around the sun). GR does of course predict gravitational lensing. However, if you used the formulae of GR to compute the motion of photons passing through a double-slit experiment, the solution would show two different bright spots, corresponding to the two slits; in reality, we see an interference pattern. So GR is also wrong. By definition, a (correct) theory of quantum gravity would predict both gravitational lensing and the way photons behave in a dobule-slit experiment (otherwise, we would say that either the theory is wrong, or it is not a theory of quantum gravity). However, no such theory exists today, at least none that doesn't contradict other observations.
joaorico 3y ago
Opportunity costs. The real debate has been whether it makes sense for string theory (whatever the prevailing definition is) to dominate funding for theoretical research of the "bridge". There are alternatives besides strings for the bridge, and there should be even more, in theory...
- digging 3y ago
  I don't know enough about the history of funding theoretical physics research to comment on that one way or another. However, neither did the comment I was replying to reference any actual facts about the distribution of funding that might suggest any of them had been wasted. The fact is, we have no falsifiable theories that can unite GR and QM. Should every theory be abandoned that doesn't quickly lead to a resolution? No, clearly not. So the question is what kind of criteria we could use to determine that string theory is a dead end or is otherwise stifling true progress. And that's pretty much what I was trying to ask previously... is ST actually sucking all the air out of the room? I'm a layperson and not just going to assume that hundreds of experts have blown their careers doing pointless calculations on a theory that "obviously" isn't worth the resources put into it. But the comment I replied to seemed to be making that assumtpion,
simiones 3y ago
> Until we find a way to falsify it, we have to keep trying, don't we? By that same token, we should keep looking for the fountain of eternal youth in El Dorado until it can be conclusively proven it doesn't exist. A theory which is not falsifiable is not a scientific theory, at least in principle, and it is hard to tell why it should be entertained for quite so long.
- atoav 3y ago
  While I agree with the general premise, don't you think that if we were to find a simple explaination that elegantly explains many things at once, it would be at least worth a try to find ways of falsifying that explaination? Not at any cost of course, but the falsifications gathered e.g. in trying to disprove the string theory might also help us figure out what is actually going on.
  - edgyquant 3y ago
    Yes, but we shouldn’t pull that theory out our mathematical asses. We should always be falsifying things not trying to make an unfalsifiable theory work because the math is interesting.
  - simiones 3y ago
    Sure, but once it keeps failing for a few decades, and given that ST is significantly more complex than either QM or GR, not simpler, there has to come a time where it simply is abandoned, even if it hasn't been disproven.
    - senttoschool 3y ago
      A few decades… How long did it take to go from Principia to General Relativity?
      - simiones 3y ago
        Principia was proven right at every turn, and explained all of the observed mechanics experiments, all the way up until Maxwell's equations were formalized and their effects tested out, some 200 years after it was published. It's also important the theories it replaced were massively more complex. Maxwell's equations were first formulated in 1865, the "patch" to Newton's theories was formulated soon after (the luminiferous aether), the Michelson-Morley experiment proving the patch did not work was run in 1887, and the theory of special relativity was proposed in 1905 - so it took about 20 years at most between Newtonian mechanics being conclusively proven to contradict experiments and a new theory becoming proeminent. And special relativity was quickly replaced with general relativity (just 10 years later), because, despite its success, its limitations were immediately apparent.
scotty79 3y ago
The bridge is "simply" doing the math of quantum mechanics in the curved spacetime of general relativity instead of the flat one.

simcop2387 3y ago

Yep, or at least until we find something else that is explaining things better and testing better and looks like a better path forward. As much as I dislike ST because of the ambiguity/vagueness that means it's so hard to nail down as a single concrete theory, the alternatives so far still also have a lot of those same issues (e.g. too many parameters, effectively impossible to test, etc.) so there's no reason to drop ST just yet as useless or an invalid theory.

runarberg 3y ago

Why do you assume there is a bridge? Or even if there is one, why do you assume that descriptions of that bridge in a mathematical language is at all possible? We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent. Would it be a stretch to assume descriptions of our physical reality could have the same restriction? General relativity and Quantum mechanics are relatively complete in describing our physical reality, however they are not consistent with each other. Perhaps if we ever find a description that is consistent, it won’t be complete. Perhaps grand unified theories are simply a mathematical impossibility.

mr_toad 3y ago
> We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent. It would be troubling to think that physical reality was inconsistent.
GoblinSlayer 3y ago
Mathematical completeness is a mathematical concern and is irrelevant for physics, because physics and mathematics have different notions of existence.
jakeinspace 3y ago
It’s sort of incomprehensible that there wouldn’t be, even if that bridge is forever incomprehensible to us. There are boundaries between where GR and QM are predictive, so presumably, unless there’s some strange smooth transition of equations between those two regimes (which would itself lend itself to a mathematical theory), then there must be a consistent set of equations which explain both regimes consistently and simultaneously.
- runarberg 3y ago
  Coming from psychology this feels alien to me. In psychology there is a definite boundary between individual behavioral dynamics and population behavioral dynamics. There is no smooth transition between the two, either you describe the individual or you describe a group, you cannot do both (even thought debates about IQ here on HN will have you believe otherwise) and there is certainly no smooth transition. How is the boundary between GR and QM different from the boundary of psychology and sociology?
  - jakeinspace 3y ago
    Im neither a psychologist nor a physicist, but I think one can analogize population vs individual behaviour to condensed matter physics versus particle physics. Condensed matter physics finds emergent behaviour in large clumps of stuff that would, in principle, be totally predictable from first principles (the standard model), but which in practice are quite difficult to guess a priori. Different scales lend themselves to different tools, since nonlinear dynamics (chaos) makes it intractable to apply bedrock reductionist formulae to large systems. The higher order behaviour of complex systems is in no way less interesting or true, I would argue, than the seemingly simpler behaviour of very small systems. In contrast to condensed matter vs the standard theory (QM basically), QM vs GR has fundamental incongruities, since both theories make claims about what happens at the same scale. Only one (or most likely neither) can be correct at the event horizon and center of a black hole.
  - PopePompus 3y ago
    I would say it is completely different. In physics, quantum mechanics gives extremely precise and verifiable predictions of the outcomes of experiments within a certain range of physical conditions. So does general relativity. In contrast, psychology has no mathematical model that will, for example, accurately predict what I am going to eat for lunch, nor is there a model that predicts the exact outcomes of elections. Since physics has two models which are exquisitely precise in different size regimes, but which are mutually incompatible, you have a definite puzzle about what happens in situations where the effects of both theories should be important. There are no exquisitely precise and accurate mathematical models of anything in the social sciences, as far as I am aware.
eouwt 3y ago
> We do know that mathematical frameworks cannot be at the same time totally complete and internally consistent. I take it you're referencing Godel's theorems here, but "consistent" and "complete" have rather technical (and somewhat limited) meanings within that context, so it's not clear to me how they'd usefully map onto the potential relationship between QM and GR?
- simiones 3y ago
  That's a very good point. In particular, "complete" refers to the ability of the mathematical-logical system to prove every statement that is true within that system, in terms of the system. This property is completely irrelevant to a theory like QM or GR - it is only relevant for a system that aims to be a universal foundation for mathematics (a formal language in which any mathematical statement whatsoever could be precisely formally encoded, and then proven or disproven).
simiones 3y ago
The problem with this line of thinking comes from the way the problem is posed. The reality is worse for both GR and QM. QM is not a theory about small particles: it is a theory that describes the evolution of any object whatsoever. It just happens to be completely wrong for large objects. And even if we accepted that there is some objective cutoff points between "small objects" and "large objects" (the "objective collapse" interpretation), it would still be wrong, because it predicts effects like gravitational lensing don't exist. Conversely, GR is also a theory about any size of object; and it is also completely wrong when tested on small objects, as it predicts effects like the self-interference of particles don't exist. And again, even if some kind of objective boundary existed between the domains of GR and QM existed, that would need to be incorporated into the maths of both of them, and questions about behaviors close to that margin would arise.
- cthalupa 3y ago
  >because it predicts effects like gravitational lensing don't exist. But we use gravitational lensing, and the amount of lensing is predicted by GR? I don't understanding this statement. Is there some additional affect around lensing we should see if quantum gravity is a thing?
  - simiones 3y ago
    No no; I'm saying that if you apply QM as it exists today to describe the motion of light beams around the sun, you will not get any effect similar to gravitational lensing. Since gravitational lensing is a real effect that we have clearly seen, it means that QM as it exists today is just wrong (in its predictions about how light moves around the sun). GR does of course predict gravitational lensing. However, if you used the formulae of GR to compute the motion of photons passing through a double-slit experiment, the solution would show two different bright spots, corresponding to the two slits; in reality, we see an interference pattern. So GR is also wrong. By definition, a (correct) theory of quantum gravity would predict both gravitational lensing and the way photons behave in a dobule-slit experiment (otherwise, we would say that either the theory is wrong, or it is not a theory of quantum gravity). However, no such theory exists today, at least none that doesn't contradict other observations.

joaorico 3y ago

Opportunity costs. The real debate has been whether it makes sense for string theory (whatever the prevailing definition is) to dominate funding for theoretical research of the "bridge". There are alternatives besides strings for the bridge, and there should be even more, in theory...

digging 3y ago
I don't know enough about the history of funding theoretical physics research to comment on that one way or another. However, neither did the comment I was replying to reference any actual facts about the distribution of funding that might suggest any of them had been wasted. The fact is, we have no falsifiable theories that can unite GR and QM. Should every theory be abandoned that doesn't quickly lead to a resolution? No, clearly not. So the question is what kind of criteria we could use to determine that string theory is a dead end or is otherwise stifling true progress. And that's pretty much what I was trying to ask previously... is ST actually sucking all the air out of the room? I'm a layperson and not just going to assume that hundreds of experts have blown their careers doing pointless calculations on a theory that "obviously" isn't worth the resources put into it. But the comment I replied to seemed to be making that assumtpion,

simiones 3y ago

> Until we find a way to falsify it, we have to keep trying, don't we? By that same token, we should keep looking for the fountain of eternal youth in El Dorado until it can be conclusively proven it doesn't exist. A theory which is not falsifiable is not a scientific theory, at least in principle, and it is hard to tell why it should be entertained for quite so long.

atoav 3y ago
While I agree with the general premise, don't you think that if we were to find a simple explaination that elegantly explains many things at once, it would be at least worth a try to find ways of falsifying that explaination? Not at any cost of course, but the falsifications gathered e.g. in trying to disprove the string theory might also help us figure out what is actually going on.
- edgyquant 3y ago
  Yes, but we shouldn’t pull that theory out our mathematical asses. We should always be falsifying things not trying to make an unfalsifiable theory work because the math is interesting.
- simiones 3y ago
  Sure, but once it keeps failing for a few decades, and given that ST is significantly more complex than either QM or GR, not simpler, there has to come a time where it simply is abandoned, even if it hasn't been disproven.
  - senttoschool 3y ago
    A few decades… How long did it take to go from Principia to General Relativity?
    - simiones 3y ago
      Principia was proven right at every turn, and explained all of the observed mechanics experiments, all the way up until Maxwell's equations were formalized and their effects tested out, some 200 years after it was published. It's also important the theories it replaced were massively more complex. Maxwell's equations were first formulated in 1865, the "patch" to Newton's theories was formulated soon after (the luminiferous aether), the Michelson-Morley experiment proving the patch did not work was run in 1887, and the theory of special relativity was proposed in 1905 - so it took about 20 years at most between Newtonian mechanics being conclusively proven to contradict experiments and a new theory becoming proeminent. And special relativity was quickly replaced with general relativity (just 10 years later), because, despite its success, its limitations were immediately apparent.

scotty79 3y ago

The bridge is "simply" doing the math of quantum mechanics in the curved spacetime of general relativity instead of the flat one.