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> It made one small tweak to gravity, and all of a sudden we can successfully predict a range of galaxy properties MOND gets one or two properties of galaxies

> It made one small tweak to gravity, and all of a sudden we can successfully predict a range of galaxy properties MOND gets one or two properties of galaxies correct, at the cost of messing up huge numbers of other observed facts about the universe (such as the abundances of chemical elements or the CMB power spectrum). Also, you should realize that MOND in itself is not actually a theory of nature. You can't just "hack" Newton's 2nd law, because that law is a consequence of other, more fundamental principles. If you want to modify gravity, you have to modify General Relativity. There have been many attempts to modify GR in order to create a MOND-like theory, but they tend to fail in various ways (again, by predicting things that are empirically false). So far, GR passes every empirical test of gravity physicists have come up with.

naasking 4y ago
> Also, you should realize that MOND in itself is not actually a theory of nature. You can't just "hack" Newton's 2nd law, because that law is a consequence of other, more fundamental principles. That's not how science works. You absolutely can write a formula that matches observations without justifying it or deriving it from first principles, ie. "hacking" Newton's second law is totally fine if it works. If an argument from principles reaches different conclusions that don't match observations, then the argument is flawed or your understanding of the principles is wrong. If such a hack is empirically successful for gravity, then it suggests there's likely some kind of underlying property missing from your model that yields this emergent order. For instance, superfluid dark matter models reproduced MOND at galactic scales from a dark matter model. That's the novel kind of thinking we need, not this stubborn protectionism. > So far, GR passes every empirical test of gravity physicists have come up with. Don't pretend there aren't huge gaping holes you could drive a dump truck through [1]. GR is well-tested and well-motivated at solar system scales, but there are many open problems at larger scales that ultimately stem from assuming GR continues to hold at those scales. [1] https://www.nature.com/articles/s41550-022-01808-7 https://www.nature.com/articles/s41550-022-01808-7
- DiogenesKynikos 4y ago
  > You absolutely can write a formula that matches observations without justifying it or deriving it from first principles, ie. "hacking" Newton's second law is totally fine if it works. Physics is an interconnected whole. You cannot simply hack whatever formula you want. Newton's 2nd law is a consequence of Relativity, in the low-velocity limit. You cannot hack Newton's 2nd law without changing Relativity, which means changing basically everything in physics, including extremely precisely measured quantities like the energy levels of the hydrogen atom. That's why MOND itself isn't a theory of nature. You have to modify General Relativity to produce MOND-like effects. It's very difficult to do that without messing up other parts of the theory. > Don't pretend there aren't huge gaping holes you could drive a dump truck through Every precision test of gravity produces results that match GR to within experimental precision. No, these are not just tests within the Solar system. To give you one example, gravitational waves have now been verified to travel at light speed, which rules out large classes of modified GR theories.
  - naasking 4y ago
    > You cannot hack Newton's 2nd law without changing Relativity Yes, changing some aspects of relativity is what will ultimately happen. We already know relativity is technically incorrect because of its singularities. And yes, you absolutely can hack Newton's 2nd law if it's empirically supported. It's frankly bizarre that you keep insisting that you can't do this. > which means changing basically everything in physics Don't be ridiculous, GR absolutely does not have an impact on "everything in physics". > No, these are not just tests within the Solar system. I didn't we've only tested GR within the solar system, I said GR was only well tested within the solar system. There's a big difference and you seem to have a habit of attacking strawman like this.
    - naasking 4y ago
      > I didn't we've only tested GR within the solar system Err, I meant "I didn't say we've only tested..."
    - DiogenesKynikos 4y ago

naasking 4y ago

> Also, you should realize that MOND in itself is not actually a theory of nature. You can't just "hack" Newton's 2nd law, because that law is a consequence of other, more fundamental principles. That's not how science works. You absolutely can write a formula that matches observations without justifying it or deriving it from first principles, ie. "hacking" Newton's second law is totally fine if it works. If an argument from principles reaches different conclusions that don't match observations, then the argument is flawed or your understanding of the principles is wrong. If such a hack is empirically successful for gravity, then it suggests there's likely some kind of underlying property missing from your model that yields this emergent order. For instance, superfluid dark matter models reproduced MOND at galactic scales from a dark matter model. That's the novel kind of thinking we need, not this stubborn protectionism. > So far, GR passes every empirical test of gravity physicists have come up with. Don't pretend there aren't huge gaping holes you could drive a dump truck through [1]. GR is well-tested and well-motivated at solar system scales, but there are many open problems at larger scales that ultimately stem from assuming GR continues to hold at those scales. [1] https://www.nature.com/articles/s41550-022-01808-7 https://www.nature.com/articles/s41550-022-01808-7

DiogenesKynikos 4y ago
> You absolutely can write a formula that matches observations without justifying it or deriving it from first principles, ie. "hacking" Newton's second law is totally fine if it works. Physics is an interconnected whole. You cannot simply hack whatever formula you want. Newton's 2nd law is a consequence of Relativity, in the low-velocity limit. You cannot hack Newton's 2nd law without changing Relativity, which means changing basically everything in physics, including extremely precisely measured quantities like the energy levels of the hydrogen atom. That's why MOND itself isn't a theory of nature. You have to modify General Relativity to produce MOND-like effects. It's very difficult to do that without messing up other parts of the theory. > Don't pretend there aren't huge gaping holes you could drive a dump truck through Every precision test of gravity produces results that match GR to within experimental precision. No, these are not just tests within the Solar system. To give you one example, gravitational waves have now been verified to travel at light speed, which rules out large classes of modified GR theories.
- naasking 4y ago
  > You cannot hack Newton's 2nd law without changing Relativity Yes, changing some aspects of relativity is what will ultimately happen. We already know relativity is technically incorrect because of its singularities. And yes, you absolutely can hack Newton's 2nd law if it's empirically supported. It's frankly bizarre that you keep insisting that you can't do this. > which means changing basically everything in physics Don't be ridiculous, GR absolutely does not have an impact on "everything in physics". > No, these are not just tests within the Solar system. I didn't we've only tested GR within the solar system, I said GR was only well tested within the solar system. There's a big difference and you seem to have a habit of attacking strawman like this.
  - naasking 4y ago
    > I didn't we've only tested GR within the solar system Err, I meant "I didn't say we've only tested..."
  - DiogenesKynikos 4y ago