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Yea, I'm with you and have been really interested the MOND (Modified Newtonian Dynamics) research, as their charts seem to fit the data better. For those who h

Yea, I'm with you and have been really interested the MOND (Modified Newtonian Dynamics) research, as their charts seem to fit the data better. For those who haven't heard of this, the idea is that the gravitational constant actually changes at very high rotational speeds / centripetal accelerations (such as those at the galactic scale), and this is why our Earth-based gravity experiments don't detect this because we're in roughly the same reference frame and don't see the effect. https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics (Also, I am really not a physicists, so I'll let them chime in)

Scarblac 5y ago
But there are also galaxies that do agree with what our gravity laws say, and therefore don't seem to have much dark matter. That's a big problem for theories that want to change the laws of physics, I believe?
pdonis 5y ago
> the idea is that the gravitational constant actually changes at very high rotational speeds / centripetal accelerations First, the MOND regime is not very high (large) accelerations, but very low (small) accelerations. Second, the MOND hypothesis is not quite that the gravitational constant changes; it's that the form of the gravitational force changes--it's no longer the Newtonian inverse square force. The key issue with this is that Newtonian gravity isn't our best current theory of gravity: General Relativity is. MOND as it was originally proposed is not a relativistic theory; there have been attempts to formulate a relativistic version, but none of them have resolved all of the open issues. > this is why our Earth-based gravity experiments don't detect this because we're in roughly the same reference frame and don't see the effect. No, that's not why MOND proponents say we don't detect the effect in Earth-based experiments. It's simply that we can't reproduce the required conditions on Earth (the net gravitational acceleration from all sources being small enough to get into the MOND regime).
- nickpinkston 5y ago
  Thanks for chiming in to clarify! I'm unsure the differences between "same reference frame" vs. "can't reproduce the required conditions" - ie Isn't it that in order to reproduce the very low accelerations we can't be in our current location with its high accelerations? I'm guessing there's some nuance between location / ref frame here that I'm missing.
  - pdonis 5y ago
    > I'm unsure the differences between "same reference frame" vs. "can't reproduce the required conditions" "Same reference frame" is just an abstraction (and quite often a meaningless one--according to the proper concept of "reference frame" in physics, there is no such thing as being "in" one reference frame but not another). "Can't reproduce the required conditions" is a straightforward statement about the limitations of what we can do with our current technology. > Isn't it that in order to reproduce the very low accelerations we can't be in our current location with its high accelerations? According to MOND, yes, in order to test it we would have to set up a lab out in deep space (i.e., not near any planet or star) far enough away from the center of our galaxy that the sum of all the "accelerations due to gravity" from all sources was less than the MOND threshold. Which of course is well beyond our current technical capability. But none of that has anything to do with "reference frames".
tzs 5y ago
A good recent look at MOND and how it compares to dark matter theories is the recent episode "What If Our Understanding of Gravity is Wrong?" [1] on PBS Space Time. [1] https://www.youtube.com/watch?v=0sTBZ2G4vow https://www.youtube.com/watch?v=0sTBZ2G4vow

Scarblac 5y ago

But there are also galaxies that do agree with what our gravity laws say, and therefore don't seem to have much dark matter. That's a big problem for theories that want to change the laws of physics, I believe?

pdonis 5y ago

> the idea is that the gravitational constant actually changes at very high rotational speeds / centripetal accelerations First, the MOND regime is not very high (large) accelerations, but very low (small) accelerations. Second, the MOND hypothesis is not quite that the gravitational constant changes; it's that the form of the gravitational force changes--it's no longer the Newtonian inverse square force. The key issue with this is that Newtonian gravity isn't our best current theory of gravity: General Relativity is. MOND as it was originally proposed is not a relativistic theory; there have been attempts to formulate a relativistic version, but none of them have resolved all of the open issues. > this is why our Earth-based gravity experiments don't detect this because we're in roughly the same reference frame and don't see the effect. No, that's not why MOND proponents say we don't detect the effect in Earth-based experiments. It's simply that we can't reproduce the required conditions on Earth (the net gravitational acceleration from all sources being small enough to get into the MOND regime).

nickpinkston 5y ago
Thanks for chiming in to clarify! I'm unsure the differences between "same reference frame" vs. "can't reproduce the required conditions" - ie Isn't it that in order to reproduce the very low accelerations we can't be in our current location with its high accelerations? I'm guessing there's some nuance between location / ref frame here that I'm missing.
- pdonis 5y ago
  > I'm unsure the differences between "same reference frame" vs. "can't reproduce the required conditions" "Same reference frame" is just an abstraction (and quite often a meaningless one--according to the proper concept of "reference frame" in physics, there is no such thing as being "in" one reference frame but not another). "Can't reproduce the required conditions" is a straightforward statement about the limitations of what we can do with our current technology. > Isn't it that in order to reproduce the very low accelerations we can't be in our current location with its high accelerations? According to MOND, yes, in order to test it we would have to set up a lab out in deep space (i.e., not near any planet or star) far enough away from the center of our galaxy that the sum of all the "accelerations due to gravity" from all sources was less than the MOND threshold. Which of course is well beyond our current technical capability. But none of that has anything to do with "reference frames".

tzs 5y ago

A good recent look at MOND and how it compares to dark matter theories is the recent episode "What If Our Understanding of Gravity is Wrong?" [1] on PBS Space Time. [1] https://www.youtube.com/watch?v=0sTBZ2G4vow https://www.youtube.com/watch?v=0sTBZ2G4vow