The gravity of things

originally posted by: iamthevirus

originally posted by: BadFinger
a reply to: iamthevirus

Gravity is the curvature of space,

Yes it is a fictitious force according to Einsteinian physics. So what is space? What is it made of?

We don't know that's why we call it Dark matter... instead of saying "we have no freaking idea" in physics (science) we always use keyword such as "dark, black, bang, ect"

It makes it sound like we/they knownwhat they're taking about when they really haven't a clue... think of it as job security

Dark matter, dark energy, black holes and big bangs helps what they're trying to convey still sound scientific but if you ask me it's rather childish (specially Big Bang) that one gotta go too lol.

As usual for ATS, spoken with a degree of arrogance and a big dash of ignorance.

Perhaps you might want to educate yourself a little before making such dismissive and bold claims without showing you have any of the knowledge or done any of the ground work.

I say this because I am a particle physicist, and can maybe shine a light on exactly how compelling and complex the evidence is for dark matter. Also what i find quite interesting is how people are quick to state that its wrong, without giving any tangible replacement... just that they dont like it because X. and sometimes just... dont like it without a reason.

Physicist never actually claim they know what dark matter is, if you believe they do then once more it shows your ignorance of the subject. 'Dark Matter' can be explained in a manner that fits into slight model changes in the standard model of particle physics in probably 20-30 different models.

Astronomers look out at the night sky with their instruments and they obviously want to make predictions and measurements. The first of these was using light to mass conversion to make predictions on the size and luminosity of galaxies as a measurement of their mass, and looking at how movement in galaxy clusters fit together and do a zero order check on if their velocities and orbits make sense.

Turns out that basically no... that galaxies are not heavy enough and the clusters too, in order to gravitationally bind the movement to keep the galaxies in a cluster together. This is profound because this turns out to be true, on inspection for clusters observed at all distances from the earth, and as such suggests that galaxy clusters are long lived structures.

The issue could be 'fixed' by saying that there is some extra factor of gravitational effect that 'turns on' at larger distances, but the form that takes and how that fits in logically and physically is not so well motivated.

People say, maybe its because we cant see all of the dim stuff and extra stuff like compact massive objects and brown dwarfs, nonluminous gas clouds etc - But once again, studies of our own galaxy and estimates on what that missing factor is does give us a push in the right direction... but it isnt enough to fix things.

Moving forward, we also looked at the rotation of galaxies themselves, rather than clusters and all standard gravitational models for spiral galaxies predict that velocity should have a linear(ish) rise through the roughly spherical core, and then have a 1/r fall off when you move out into the spiral arms. Turns out... no... spiral galaxies universally maintain a constant rotation out to long distances. So this tells us that while the physics models work perfectly for the core... they completely mess up for the spiral arms... and in terms of visible light and studies... there is nothing that leaps out at us to say "you missed this"

Now thinking of models, what can fix this and importantly give a solution that predicts exactly what is observed is if galaxies exist within a spherical or ellipsoid distribution of... 'matter' that gives a regular, non-exotic contribution to the gravitational environment that binds the galaxy together. Thats by no means... a 'aha! fixed' but is an interesting prediction that is important later.

Now, one of the most rigorously tested models we have in physics is general relativity. So far those models have fit observables to a high level of precision and have come up good. One prediction they make is that of gravitational lensing, which if you look at that deep field image from the James Webb, you can clearly see a central object that is lensing background objects. Now when astronomers look at clusters and identify lensing of background objects, it allows them to calculate the shape of the lens, or shape of the gravitational field. Turns out... that galaxies do appear to have extensive gravitational fields that extend much wider than what you expect from the visible galaxy and much higher in intensity than what you'd expect from the visible matter.

So thats two predictions that cross over. It gives us an idea that we might be onto something.

So that observable informs us of a couple things, that what ever it is, gravitationally interacts with the galaxy as a whole, and that if it is caused by a particulate matter of some form, then it is non-relativistic (it produces and is bound by gravity, like the stars)

So astronomers go and look for objects of interest... they come up with several galaxies that are extremely dim, in that they have very few stars and gas clouds. They find several. These still have the same features of heavily populated galaxies... very fast rotation, which is odd given they are so less massive then their dense counter parts. Again this points at the presence of some other material that has helped to stablize a galaxy that has been stripped of its stars and gas (for the most part)

They then find the bullet cluster... looking at this cluster of galaxies it features extensive gravitational lensing. Observations are performed on it, there are actually two interacting clusters of galaxies that appear to have passed through or by one another. Performing measurements across the electromagnetic spectrum scientists can identify where all the hot gas is within the clusters, this is a measure of were the centre of mass of luminous material is located. The gravitational lensing gives an opportunity to show where the centre of mass of material producing the gravitational lens.
Interestingly, the gravitational lens has two blobs, one for each cluster which actually leads the clusters. It isn't centred on them. The hot luminous matter trails behind the clusters.
What this tells us is that, there is likely some component of this missing matter that is gravitationally interacting, but does not carry any charge and does not interact outside the realm of gravity.

There is other evidence too, but this is already too long and iv put in far more effort to try and make a compelling rebuttal to the throw away statements that basically 'physicists are idiots who are making stuff up' which so often gets said on ATS.

a reply to: ErosA433

A nice illustrative post. But I need to make some comments:

"The issue could be 'fixed' by saying that there is some extra factor of gravitational effect that 'turns on' at larger distances, but the form that takes and how that fits in logically and physically is not so well motivated."

This statement is a but biased. Relativistic theories on modified gravity and discrete spacetime are sufficiently motivated, and there are other well-constructed theories that could explain the effects some attribute to dark matter, without needing to postulate its existence. The problem with dark matter, as I see it, is that even accepting the undetected wimps, neutralinos, axions or CDM they still are not enough to account for the observational anomalies, while postulating large-scale modifications of spacetime depending on large distances does. However, I must admit testing for these large-scale anomalies is so far impossible. I therefore think the debate is still valid and the question remains open.

"if it is caused by a particulate matter of some form, then it is non-relativistic (it produces and is bound by gravity, like the stars)"

Can you expand on this? Whether a particle is or is not relativistic refers to the speed the particle is moving, not to its very nature. All particles are relativistic. It happens the relativistic effects only manifests when they move at relativistic speeds.

a reply to: mbkennel

By the way, the concept of true 'mass' in GR is actually extremely complicated and I think has only been solved (in some ways) by mathematicians fairly recently. This is because, in the general case, the warping of space caused by normal matter itself sort of behaves like mass so that has to be allocated to the mass itself as an effective mass.

I could imagine that this could explain my issue, but I wonder,
What's normal matter, and what isn't?

originally posted by: ErosA433
Now when astronomers look at clusters and identify lensing of background objects, it allows them to calculate the shape of the lens, or shape of the gravitational field. Turns out... that galaxies do appear to have extensive gravitational fields that extend much wider than what you expect from the visible galaxy and much higher in intensity than what you'd expect from the visible matter.

So thats two predictions that cross over. It gives us an idea that we might be onto something.

Great overall post and I think this point is where some alternatives like MOND seem weak in not addressing gravitational lensing observations.

originally posted by: Direne
This statement is a but biased. Relativistic theories on modified gravity and discrete spacetime are sufficiently motivated, and there are other well-constructed theories that could explain the effects some attribute to dark matter, without needing to postulate its existence.

Setting "motivations" aside, it's "observations" which concern me more. If you're referring to "relativistic theories on modified gravity" like TeVeS, the observations do not appear to support TeVeS as explained here:

Troubled Times for Alternatives to Einstein’s Theory of Gravity

Then there is TeVeS (tensor-vector-scalar), MOND’s relativistic cousin. While MOND is a modification of Newtonian gravity, TeVeS is an attempt to take the general idea of MOND and make it into a full mathematical theory that can be applied to the universe as a whole — not just to relatively small objects like solar systems and galaxies...

Consider the neutron-star merger. At the same time that the Laser Interferometer Gravitational-Wave Observatory (LIGO) spotted the gravitational waves emanating from the event, the space-based Fermi satellite spotted a gamma ray burst from the same location. The two signals had traveled across the universe for 130 million years before arriving at Earth just 1.7 seconds apart.

These nearly simultaneous observations “brutally and pitilessly murdered” TeVeS theories, said Paulo Freire, an astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn, Germany. “Gravity and gravitational waves propagate at the speed of light, with extremely high precision — which is not at all what was predicted by those [alternative] theories.”

originally posted by: Terpene
a reply to: Arbitrageur
I think to understand the geometry and how my gravity bends space to affect other masses, what eludes me is why, or how that geometry seemingly doesn't affect my own mass in the same inverse square way...

Gravity does affect your mass the same way, but I suspect your difficulty is you you seem to have these far-fetched thought experiments in your mind that are completely divorced from reality like putting a source of gravity in a tire to increase the air pressure, so maybe your expectations are divorced from the reality of how the universe actually works.

One way to resolve the discrepancy is through math. Use the math to make predictions of what you would expect, and compare to what you actually observe and I'm sure you will find they match, if you do the math correctly. The issue I suspect is your anticipated expectations are nowhere near what known models would predict.

An extreme example of flawed expecations is that if you get fat enough, you will have your own gravity like in this cartoon where balls and books orbit the fat man:


This of course is not what models predict, though it's funny. The models do predict the fat man will have more gravity or curved space time around him, but they do not predict such a man standing on earth to have objects orbiting him.

So if you wanted to re-phrase your question, what specifically do you expect the gravity models predict, and what specifically do you observe that you think contradicts those models?

a reply to: Arbitrageur

it's "observations" which concern me more. If you're referring to "relativistic theories on modified gravity" like TeVeS, the observations do not appear to support TeVeS

Yes, that's true. Postulating a change in the way gravity works depending on distance should have a clear observational: a correlation with how fast gravity propagates in that space. And so far there is no evidence of any anomalies in the speed at which gravity travels. But it is also the case that postulating different flavors of dark matter (massive neutrinos, wimps, axions, and so on) should also have led to a direct observation of such particles, something that has not happened.

Thus, assuming there is a deficit in the visible mass that could explain the observations, we are left with only two possibilities: new particles, or a modification of GR that could account for that effect. I abhor modifying my beloved GR and would really prefer some new particle being responsible, but honestly I think introducing new particles would have a disastrous consequence for the rest of particle physics.

Tweaking spacetime metric seems the way to go, as I feel it, but I'm fully aware a minute change in the metric could end up with the equations describing a Universe that bears no relation with ours. An interesting debate this one.

originally posted by: ErosA433
As usual for ATS, spoken with a degree of arrogance and a big dash of ignorance.

As usual for ATS, another walking around with a giant stick up their butt...

it's just an opinion about those silly words science uses, don't take it personally.

a reply to: iamthevirus

it's just an opinion about those silly words science uses, don't take it personally.

Kind of hard to talk about or attempt to explain the concept of gravity without utilizing words without scientific connotations all the same, so there is that.

originally posted by: Direne
Tweaking spacetime metric seems the way to go, as I feel it, but I'm fully aware a minute change in the metric could end up with the equations describing a Universe that bears no relation with ours. An interesting debate this one.

Sabine Hossenfelder's idea is interesting, which is maybe it's not either dark matter OR modified gravity, but maybe it could be some combination. I don't know if that's right but in at this point it's difficult to rule out.

originally posted by: iamthevirus
it's just an opinion about those silly words science uses, don't take it personally.

What you said is that talking about dark matter sounds childish to you:

originally posted by: iamthevirus
Dark matter, dark energy, black holes and big bangs helps what they're trying to convey still sound scientific but if you ask me it's rather childish (specially Big Bang) that one gotta go too lol.

So ErosA433 schools you on why he thinks otherwise and do you rebut any of his well considered points about why the dark matter topic is not childish? No, you say:

originally posted by: iamthevirus
As usual for ATS, another walking around with a giant stick up their butt...

You're entitled to your opinion, but you didn't refute any of the points made by ErosA433 arguing against "dark matter" being a childish notion, but instead made some comment about a stick up the butt. It then becomes apparent where the childish behavior actually resides, and it's not with people trying to explain or debate the topic rationally, but with people who respond to such not with more rational debate, but with ad hominem comments about a stick up the butt.

originally posted by: andy06shake
a reply to: iamthevirus

it's just an opinion about those silly words science uses, don't take it personally.

Kind of hard to talk about or attempt to explain the concept of gravity without utilizing words without scientific connotations all the same, so there is that.

I know right... I didn't even get past that first opening statement in their response, that's one way to get people to read your posts just insult them (because it's the scientific thing to do lol)

Like I said it was just an opinion... I am tired of hearing those silly words from top paid physicists talking about current scientific understanding of the cosmos.

a reply to: Arbitrageur

oh Dark Matter isn't "childish" per say... the title is.

We don't know so we'll just call it dark for now... I wonder if they'll rename it when/if we discover what it is or if it exists at all?

I don't know why I feel patronized or like I am in the 5th grade when I hear top physicists use words like Dark, Black, Bang, ect... maybe it's just me?

Sometimes I feel like I want a refund

a reply to: iamthevirus

By the looks of things ErosA433 is a particle physicist, he's not insulting you buddy, but educating us, regarding scientific understandings that are possibly somewhat above our paygrade.

If you have a better theory other than the existence of dark matter that can account for the galaxies of our universe holding together without sufficient mass i think you may find we are all ears?

Edit: Please don't let it be electric universe nonsense that fails to explain the observable universe never mind the rest.

originally posted by: iamthevirus
a reply to: Arbitrageur

oh Dark Matter isn't "childish" per say... the title is.

What would you prefer to call it?

We don't know so we'll just call it dark for now... I wonder if they'll rename it when/if we discover what it is or if it exists at all?

Seems to me like that's the idea of calling it dark for now, it's an admission we don't know what it is. If we figure out what it is I expect it will get a better name, or names, since it may be more than one thing.

originally posted by: andy06shake
a reply to: iamthevirus

By the looks of things ErosA433 is a particle physicist, he's not insulting you buddy, but educating us, regarding scientific understandings that are possibly somewhat above our paygrade.

If you have a better theory other than the existence of dark matter that can account for the galaxies of our universe holding together with our sufficient mass i think you may find we are all ears?

Well they're a poor educator...

Maybe dark matter is just gravity? doesn't take a particle physicist to ask that question. So many self proclaimed quantum physicists simply lack a clear understanding of GR as I've said earlier... it's taught incorrectly even at academic levels.

a reply to: iamthevirus

Maybe dark matter is just gravity?

With a modified theory of gravity?

Because you require such to dispense with the notion of dark matter in our universe.

Which parts are taught incorrectly at academic levels?

Galaxies don't fly apart because the spacetime around them is curved/distorted holding the mass in place

a reply to: iamthevirus

Yeh you require more mass than is available, or at least thats my understanding.

Without dark matter, modeling suggests they just all fly apart.

originally posted by: andy06shake

Which parts are taught incorrectly at academic levels?

they teach GR and basically gravity in 2D that's a huge problem (rolling a ball across a sheet) once a person sees the 2D representation it seems to take an epiphany to get them to visualize it three dimensional.

Gravity doesn't work like a ball sitting on the surface of the water, the ball is actually submerged in the water... which is difficult to represent and never even attempted... everyone just leaves class for that first time with the 2D representation locked in their head skewing every thought experiment they may have from that point forward.

that's a problem (imo)

a reply to: iamthevirus

I always thought the heavy ball on a sheet of rubber concept illustrated the fact that large bodies of mass warp the fabric of space-time somewhat eloquently, to be honest.

Yes it's somewhat of a two-dimensional representation, but nonetheless illustrates the concept imho.

originally posted by: andy06shake
Without dark matter, modeling suggests they just all fly apart.

correct... and there is a lot more empty space than there is clumped together mass, that's why it works.

maybe dark matter is just general relativity it's very simple I think because it's super hard to out-do Einstein haha.

a reply to: iamthevirus

I don't think Einstein's theory of general relativity predicted the existence of dark matter, it did however imply the existence of black holes.