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Newton's law of gravity passes its biggest test

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Re: Newton's law of gravity passes its biggest test

#41

Post by rappatic »

When I see dark matter in the news I'm always reminded of the story of Vulcan.

In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors.

Le Verrier, the astronomer who made these observations, wondered how to explain the difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.

Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.

When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.

Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?

MOND may not be the solution, but I'm still skeptical about dark matter.

Re: Newton's law of gravity passes its biggest test

#42

Post by _ZeD_ »

htx80nerd wrote: post #37
GuB-42 wrote: post #2

If you follow Sabine Hossenfelder's channel, she has a MONDOmeter. With MOND (modified Newtonian gravity) on one side and dark matter on the other side. As new papers come out the needle goes back and forth, and I guess that she will make a new video if she hasn't already, with the needle moving one step towards dark matter. I find it interesting how it doesn't seem to settle. Dark matter is still the favorite, but t…

Dark Matter : supposedly makes up a big amount of the mass of the universe, but cant be seen, does not emit, absorb, or reflect light. Also it can 'pass through' other normal matter, and other dark matter. It's basically magic aka not actually real, just something in vogue to pretend is real at the present moment.

Or, you know, as aether.

It's a scientific theory. It's the best that we have right now to model the real world and be able to do prediction on its behavior.

Does it seems to be kept together by duct tape? Maybe.

Is it yet useful? Yep.

Will it be discarded if anything more fitting will came up? You can be sure of it.

Re: Newton's law of gravity passes its biggest test

#43

Post by magicalhippo »

NewEntryHN wrote: post #34
magicalhippo wrote: post #10

Earlier quoted context omitted.

General Relativity reduces to Newtonian gravity as the curvature goes to zero, that is when you're very far away from objects relative to their masses, for slow non-relativistic objects like stars and galaxies. Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well. However, inside galaxies things do not behave as expect…

Why is the article titled "Newton's law of gravity passes its biggest test" if it doesn't explain the movement more than MOND?

Other way around. Newtonian dynamics explains the data very well, MOND did not.

In particular, Newtons law of gravity says the effect of gravity falls off as 1/r^2 where r is the distance from the mass. MOND modifies the standard equations so that gravity starts like 1/r^2 when r is small, and acceleration is large, but for greater distances, when the acceleration is low, instead falls off like 1/r.

MOND explains the movement of the stars in (most) galaxies very well. However this result showed that MOND was not consistent with the motion of the galaxies in the cluster. On the other hand the motion was consistent with plain Newtonian dynamics. Hence Newtons law of gravity (and by extension GR) passed the test.

Re: Newton's law of gravity passes its biggest test

#44

Post by vonneumannstan »

GuB-42 wrote: post #2

If you follow Sabine Hossenfelder's channel, she has a MONDOmeter. With MOND (modified Newtonian gravity) on one side and dark matter on the other side. As new papers come out the needle goes back and forth, and I guess that she will make a new video if she hasn't already, with the needle moving one step towards dark matter. I find it interesting how it doesn't seem to settle. Dark matter is still the favorite, but t…

Until you definitively rule out all the dark matter candidates or get a direct detection the controversy will remain

Re: Newton's law of gravity passes its biggest test

#45

Post by messe »

fooker wrote: post #30
wongarsu wrote: post #29

Earlier quoted context omitted.

But at least the experimental results disproving these incremental fixes should be exactly the kind of thing the next Einstein should need for coming up with an entirely new way of looking at things

Interestingly, more often than not it happens the other way. Some once-in-a-generation scientist has an intuition that turns out to be true and mathematically elegant. It gets proven experimentally years or decades later. Relativity was exactly like this.

It was far from exactly like that. GR was in part prompted by the precession of the perihelion of mercury for which there was plenty of data.

Re: Newton's law of gravity passes its biggest test

#46

Post by messe »

fooker wrote: post #20
sebzim4500 wrote: post #6

Earlier quoted context omitted.

Yeah until you get to quantum computing and then it seems as if the universe is doing enormously more work than you would think necessary.

Not necessarily. You'd be correct given hidden variables. But we know pretty convincingly that quantum anything does not have hidden variables. https://en.wikipedia.org/wiki/Bell%27s_theorem

It doesn't have local hidden variables. That's an important distinction.

I'm not sure a non-local hidden variable explanation of QM is any distinct from superdeterminism though.

Re: Newton's law of gravity passes its biggest test

#47

Post by GuB-42 »

NewEntryHN wrote: post #35
GuB-42 wrote: post #9

Earlier quoted context omitted.

At these scales (entire galaxies, very weak forces), it doesn't make a significant difference. There are ways of adapting MOND to match general relativity, should it turn to be correct at explaining what it is supposed to explain (like the movement of galaxies).

I think OP's question is more how could Newton's law "pass" a test any more than General Relativity would, considering that it's merely an edge case of GR?

Oh yes, I think the title is a bit misleading.

It is an argument against MOND, a theory that says that gravity has to be modified to account for some observations. But for these particular observations, general relatively and Newton's laws gives the same results in practice, the difference is negligible, so showing that these observations can be explained by Newton's laws implicitly mean that they can also be explained by general relativity. No need for modifications.

Re: Newton's law of gravity passes its biggest test

#48

Post by Joker_vD »

_ZeD_ wrote: post #42
htx80nerd wrote: post #37

Earlier quoted context omitted.

Dark Matter : supposedly makes up a big amount of the mass of the universe, but cant be seen, does not emit, absorb, or reflect light. Also it can 'pass through' other normal matter, and other dark matter. It's basically magic aka not actually real, just something in vogue to pretend is real at the present moment.

Or, you know, as aether. It's a scientific theory. It's the best that we have right now to model the real world and be able to do prediction on its behavior. Does it seems to be kept together by duct tape? Maybe. Is it yet useful? Yep. Will it be discarded if anything more fitting will came up? You can be sure of it.

Yeah, pretty much, which is why this adherence to dark matter seems even more puzzling: we already had a mysterious substance with nonsensical mechanical properties (perfectly solid, but has zero collision) that turned out to be completely superfluous; the actual answer was the different shape of the physical laws. Now we again have a mysterious substance with nonsensical properties (has gravitational pull, doesn't interact with normal matter in any other way) — could it be that it simply doesn't exist?

And it's not like the concept of aether itself was really all that useful for anything. The physicists wanted the light to have some mechanical medium to propagate through instead of being a thing of itself, that actual itself shaped mechanical media, not the other way around (mechanical properties arise from the E-M interaction, not the other way around), simply because all other known waves phenomena existed in mechanical media.

Re: Newton's law of gravity passes its biggest test

#50

Post by gwynforthewyn »

rappatic wrote: post #41

When I see dark matter in the news I'm always reminded of the story of Vulcan. In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors. Le Verrier, the astronomer who made these observations, wondered how to explain the di…

Of course it can be that this is the case. There are predictions from the dark matter hypothesis, though, that have been testable. For example, there're some of the asymmetries in the cosmic microwave background that are explained presently by the dark matter hypothesis.

Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.

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