4 ms·

> My hangup with MOND is still general relativity. Fwiw, we know for a fact also that for edge cases GR is wrong because it doesn't agree with quantum mechanic

> My hangup with MOND is still general relativity. Fwiw, we know for a fact also that for edge cases GR is wrong because it doesn't agree with quantum mechanics (unless QM is wrong), so it's maybe not right to take GR as gospel, especially for a theory that only seems to also change GR in edge cases, and the only reason why "it doesn't agree" might amount to "the math is hard and the physicists haven't put enough work in yet" To wit, accepting a mond-ified GR is probably not going to change how GPS works so the claim that "GR has withstood the test of time and engineering" is not a totally solid refutation of MOND

mort96 2y ago
Well this doesn't seem like such a conundrum. We know for sure that ND is wrong because it predict things incorrectly which GR predicts correctly. We know GR is wrong because it is incompatible with any form of QM and we know some form of QM is more or less correct. Essentially, GR and ND are both wrong, but ND is more wrong than GR.
- codethief 2y ago
  > we know some form of QM is more or less correct This is a wild take, given all the issues QM and QFT have.
  - mort96 2y ago
    Sure, a better way of saying it is tat we observe natural phenomena which are explained by something like QM and for which we have no other explanation. I think people got the idea.
- RandomThoughts3 2y ago
  > We know GR is wrong because it is incompatible with any form of QM and we know some form of QM is more or less correct. It’s not really about one model being correct. GR is not a perfect model because its predictions don’t match what’s observed on the scale where QM gives predictions which do. "Wrong" is overall a poor way of thinking about models. People would like a model which is both general and elegant, not simply a model which is "right". A large and very general model with a lot of parameters which can be well tuned to fit all the observations we have would be "correct" but I am not convinced it would be very useful.
scotty79 2y ago
> because it doesn't agree with quantum mechanics I don't think it doesn't agree. It's just that we never managed to neither formulate quantum mechanics on 4 dimensional space time nor quantize gravitational force. So we simply have no idea what happens in small scale in significant gravitational fieldd.
- naasking 2y ago
  We absolutely know GR is wrong, at the very least because of its singularities.
  - scotty79 2y ago
    All we can tell from singularities is that GR might have a realm of applicability ... like every other theory ever. Not that its wrong.
    - anon84873628 2y ago
      "Has a realm of applicability" is what people mean by "wrong" here. As in, the equations we have don't fully generalize and explain all regimes. The "right" model will cover all regimes and still reduce to match what we already have.
      - scotty79 2y ago
        Then every physical (and nature) law is wrong. Only math is right. There's not a single law that covers full regime for at least one definition of "full".
        naasking 2y ago
        Possibly, but we're converging on something correct as we cover more regimes in this inductive process called science. That's the best we can do.
- throwawaymaths 2y ago
pdonis 2y ago
> for edge cases GR is wrong because it doesn't agree with quantum mechanics What "edge cases" are you talking about? I agree that GR is not a quantum theory, but it's not established that that has to be a problem, nor is it a matter of "edge cases".
- naasking 2y ago
  GR has singularities. It's definitely wrong in those regimes.
  - pdonis 2y ago
    > GR has singularities. More precisely, GR allows spacetime solutions which are geodesically incomplete. > It's definitely wrong in those regimes. No, that's too strong a claim. Most physicists believe that the solutions that are geodesically incomplete will turn out not to be valid in the regimes close enough to the endpoints of the incomplete geodesics. But that is a belief, not a proven fact. The solutions themselves are perfectly consistent mathematically.
    - naasking 2y ago
      > But that is a belief, not a proven fact. The solutions themselves are perfectly consistent mathematically. Every physical theory with singularities has has broken down in that regime. It's not even clear what it would mean for reality to permit singularities. That's a bit more than just a belief.
      - pdonis 2y ago
        > Every physical theory with singularities has has broken down in that regime. Can you give some examples? Note that GR has not even been tested anywhere close to the regime you are talking about. > It's not even clear what it would mean for reality to permit singularities. GR doesn't "permit singularities" in the sense I think you are using that phrase. "Singularity" in GR actually does not mean what I suspect you think it means, that things like spacetime curvature "become infinite". Notice that in my previous post I was careful to use the term "geodesic incompleteness", since that's what "singularity" actually means in the GR literature. And even in particular cases where there are invariants that increase without bound along incomplete geodesics, the limit points, such as r = 0 in Schwarzschild spacetime, are not actually part of the spacetime in GR. All invariants are finite at every point in the actual spacetime.

mort96 2y ago

Well this doesn't seem like such a conundrum. We know for sure that ND is wrong because it predict things incorrectly which GR predicts correctly. We know GR is wrong because it is incompatible with any form of QM and we know some form of QM is more or less correct. Essentially, GR and ND are both wrong, but ND is more wrong than GR.

codethief 2y ago
> we know some form of QM is more or less correct This is a wild take, given all the issues QM and QFT have.
- mort96 2y ago
  Sure, a better way of saying it is tat we observe natural phenomena which are explained by something like QM and for which we have no other explanation. I think people got the idea.
RandomThoughts3 2y ago
> We know GR is wrong because it is incompatible with any form of QM and we know some form of QM is more or less correct. It’s not really about one model being correct. GR is not a perfect model because its predictions don’t match what’s observed on the scale where QM gives predictions which do. "Wrong" is overall a poor way of thinking about models. People would like a model which is both general and elegant, not simply a model which is "right". A large and very general model with a lot of parameters which can be well tuned to fit all the observations we have would be "correct" but I am not convinced it would be very useful.

scotty79 2y ago

> because it doesn't agree with quantum mechanics I don't think it doesn't agree. It's just that we never managed to neither formulate quantum mechanics on 4 dimensional space time nor quantize gravitational force. So we simply have no idea what happens in small scale in significant gravitational fieldd.

naasking 2y ago
We absolutely know GR is wrong, at the very least because of its singularities.
- scotty79 2y ago
  All we can tell from singularities is that GR might have a realm of applicability ... like every other theory ever. Not that its wrong.
  - anon84873628 2y ago
    "Has a realm of applicability" is what people mean by "wrong" here. As in, the equations we have don't fully generalize and explain all regimes. The "right" model will cover all regimes and still reduce to match what we already have.
    - scotty79 2y ago
      Then every physical (and nature) law is wrong. Only math is right. There's not a single law that covers full regime for at least one definition of "full".
      - naasking 2y ago
        Possibly, but we're converging on something correct as we cover more regimes in this inductive process called science. That's the best we can do.
throwawaymaths 2y ago

pdonis 2y ago

> for edge cases GR is wrong because it doesn't agree with quantum mechanics What "edge cases" are you talking about? I agree that GR is not a quantum theory, but it's not established that that has to be a problem, nor is it a matter of "edge cases".

naasking 2y ago
GR has singularities. It's definitely wrong in those regimes.
- pdonis 2y ago
  > GR has singularities. More precisely, GR allows spacetime solutions which are geodesically incomplete. > It's definitely wrong in those regimes. No, that's too strong a claim. Most physicists believe that the solutions that are geodesically incomplete will turn out not to be valid in the regimes close enough to the endpoints of the incomplete geodesics. But that is a belief, not a proven fact. The solutions themselves are perfectly consistent mathematically.
  - naasking 2y ago
    > But that is a belief, not a proven fact. The solutions themselves are perfectly consistent mathematically. Every physical theory with singularities has has broken down in that regime. It's not even clear what it would mean for reality to permit singularities. That's a bit more than just a belief.
    - pdonis 2y ago
      > Every physical theory with singularities has has broken down in that regime. Can you give some examples? Note that GR has not even been tested anywhere close to the regime you are talking about. > It's not even clear what it would mean for reality to permit singularities. GR doesn't "permit singularities" in the sense I think you are using that phrase. "Singularity" in GR actually does not mean what I suspect you think it means, that things like spacetime curvature "become infinite". Notice that in my previous post I was careful to use the term "geodesic incompleteness", since that's what "singularity" actually means in the GR literature. And even in particular cases where there are invariants that increase without bound along incomplete geodesics, the limit points, such as r = 0 in Schwarzschild spacetime, are not actually part of the spacetime in GR. All invariants are finite at every point in the actual spacetime.