firsttimesissybris t1_ixlky55 wrote on November 24, 2022 at 10:28 AM

Reply to comment by billfitz24 in Lopsided star cluster may disprove Newton and Einstein, controversial new study claims. An uneven distribution of stars in several nearby clusters may offer evidence of MOND — a controversial theory of gravity that disputes Newton and rejects the existence of dark matter. by nimobo

Because disproving newtonian gravity and Einstein's general rel and basically synonymous in this regime. Its sufficient to apply the Newtonian limit to GR, and so if the system doesn't obey Newtonian grav, and you can rule out other weird effects, then that is evidence against GR

billfitz24 t1_ixlmcek wrote on November 24, 2022 at 10:48 AM

AFAIK MOND isn’t going to disprove Newtonian gravity, it’s simply going to modify it in certain circumstances similar to what Einstein did.

firsttimesissybris t1_ixln125 wrote on November 24, 2022 at 10:58 AM

Newtonian gravity is wrong, we already know that. But we have no evidence (this paper says we do) that it is wrong in this situation. And due to the fact GR barely changes the answers in this situation, it would also mean GR is wrong also. These modifications are quite harsh, they basically apply to all length scales of galaxy or larger, while the difference between Newtonian and GR is almost always small.

The only reason MOND is modified Newtonian Dynamics and not modified GR is because it turns out to be very hard to modify GR in a consistent way. But it must actually be a modification of GR, that reduces to MOND in the Newtonian limit. If you include QFT, it turns out it's basically impossible to modify GR without introducing new particles, completely undermining the arguement that modifying gravity is neater than introducing new dark matter particles.

billfitz24 t1_ixlp20a wrote on November 24, 2022 at 11:26 AM

I can’t agree with much of what you’ve written here. Newtonian is fine for 99% of what we experience in our daily lives. Einstein is mostly needed when relative velocities are very high. MOND is thought to apply only on galactic and larger objects.

To say that GR changes the Newtonian numbers by only a small amount is simply not true at high velocities. With time & length dilation the difference in numbers can be quite large.

I don’t see how MOND can make Einstein wrong, since it’s trying to be an explanation of gravity on truly massive objects, and I’m not aware that relative velocities are even a consideration.

Saying that MOND must modify GR and in doing so must introduce new particles is not something I’ve ever read when MOND is being discussed.

I could be completely wrong, but it seems to me you’re presenting several false arguments in your dismissal of MOND.

firsttimesissybris t1_ixlr5ve wrote on November 24, 2022 at 11:55 AM

>I can’t agree with much of what you’ve written here. Newtonian is fine for 99% of what we experience in our daily lives. Einstein is mostly needed when relative velocities are very high. MOND is thought to apply only on galactic and larger objects.

That is exactly what I said, so yes we don't disagree on that. My point is, that Einstein's modifications are very small in most parts of the universe due to the fact relative velocities and curvature are often quite small when considering massive objects, bar compact objects like BH or neutron stars. However, MOND corrections don't disappear in most parts of the universe.

>To say that GR changes the Newtonian numbers by only a small amount is simply not true at high velocities. With time & length dilation the difference in numbers can be quite large.

True, but as I've said, high velocities and large curvatures are rare. Most galactic simulations completely ignore GR as it is a massive complexity just to account for very small corrections.

>I don’t see how MOND can make Einstein wrong, since it’s trying to be an explanation of gravity on truly massive objects, and I’m not aware that relative velocities are even a consideration.

MOND absolutely must supplant GR if it is to be correct, as GR makes predictions that have to be also made by any replacement to it, and that aren't included in MOND (lensing, blackholes, anything relativistic). I'm not sure what you mean by relative velocities aren't a consideration.

>Saying that MOND must modify GR and in doing so must introduce new particles is not something I’ve ever read when MOND is being discussed.

If you can find a relativistic modified Newtonian Dynamics type theory that includes GR as a limit (this is a requirement as GR is able to predict all strong gravity phenomena we've seen), and doesnt introduce new dynamical fields, then I'll retract my statement. I'm not aware of any such theories, and it is a very common criticism of MOND type theories. The first such rel version seems to be TeVeS, which introduces two new dynamical fields. QFT implies these will almost certainly be quantised, and either quanta will be new particles. Even if QFT is wrong in this, it's introduced two new fields, which is at least as ad-hoc as introducing dark matter.