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355 points jchanimal | 5 comments | | HN request time: 1.058s | source
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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

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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.

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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.

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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.

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1. exe34 ◴[] No.42159893[source]
that's like saying the visible mass of the universe is 99% hydrogen and helium, so we don't need to learn about chemistry.
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2. meindnoch ◴[] No.42159945[source]
So you're saying we should model galaxies down to the level of individual protons? Lol.

Galactic dynamics is governed by gravity, which is Newtonian at those scales.

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3. exe34 ◴[] No.42160482[source]
No I did not say that.
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4. meindnoch ◴[] No.42160585{3}[source]
Ok, then how does your chemistry comment have anything to do with the motion of galaxies? Reminder: you're commenting on an article about MOND, which is a theory that stems from trying to explain the motion of galaxies.
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5. exe34 ◴[] No.42163461{4}[source]
> 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.

I meant it in the sense that "most of the cosmos runs on Newtonian gravity, therefore we can ignore GR" is similar to "most of the visible matter in the cosmos is hydrogen/helium, so we can ignore chemistry".

The interesting part is in the 0.0000001% that isn't like the others.