←back to thread

357 points jchanimal | 1 comments | | HN request time: 0.214s | source
Show context
samsartor ◴[] No.42158987[source]
My hangup with MOND is still general relativity. We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold. Any model of gravity based on an inverse law is simply wrong.

Another comment linked to https://tritonstation.com/new-blog-page/, which is an excellent read. It makes the case that GR has never been tested at low accelerations, that is might be wrong. But we know for a fact MOND is wrong at high accelerations. Unless your theory can cover both, I don't see how it can be pitched as an improvement to GR.

Edit: this sounds a bit hostile. to be clear, I think modified gravity is absolutely worth researching. but it isn't a silver bullet

replies(7): >>42159034 #>>42159161 #>>42159582 #>>42159774 #>>42160543 #>>42160861 #>>42165272 #
meindnoch ◴[] No.42159582[source]
>We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold

[citation needed]

The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places.

Emphasized part added because people in the replies thought that I literally think that General Relativity is somehow wrong. Don't be dense. All I'm saying is that gravity at galactic scales works as Newton described it. General Relativity has extremely tiny effect at those scales.

replies(7): >>42159646 #>>42159734 #>>42159753 #>>42159761 #>>42159764 #>>42159815 #>>42160651 #
samsartor ◴[] No.42159734[source]
https://en.wikipedia.org/wiki/Tests_of_general_relativity#Pe... is the example I learned in school. You don't need to be around a black hole for GR to suddenly switch on.

Newtonian gravity is an approximation. A perfectly acceptable one in many contexts, but still measurably incorrect.

replies(1): >>42159855 #
meindnoch ◴[] No.42159855[source]
Nobody said that general relativity is "switched on" around black holes.

But ok, let me put it this way: outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe.

replies(5): >>42159893 #>>42159925 #>>42159932 #>>42160112 #>>42160210 #
1. radishingr ◴[] No.42160112[source]
So spacetime (interactions between mass, space, and time) are required for any sort of precision explanation. If "extreme" means planet size masses, I guess, but I generally consider our solar system pretty normal. However we cannot explain the planetary motion of mercury without relativity, so define your extreme.

But sure, newton is good enough to handle most ground based scenarios where we only care about forces at low precision.