One night of telescope time rules out black hole/dark matter idea
arstechnica.com
arstechnica.com
Maybe a physicist can explain why, but as a mathematician my first quest would be to try to form a mathematical system that follows what we are observing, without introducing physical objects that we aren't actually seeing.
"Modified Newtonian dynamics (MOND) is a theory that proposes a modification of Newton's laws to account for observed properties of galaxies. It is an alternative to the theory of dark matter in terms of explaining why galaxies do not appear to obey the currently understood laws of physics." [1]
Seems like this is (one approach to) exactly the question I asked! Even if proven not to lead to any satisfactory cosmological model, these kind of studies will at least tell you why the alternatives (to r^2) don't hold.
[1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
Nobody really likes the idea of particles or other stuff that is impossible to measure.
But until now, General Relativity has passed every experimental test and most alternative approaches to dark matter other than "Some heavy stuff not interacting electricly" is basically ruled out by one or another observation.
There are solutions that fit the large scales pretty well (Cosmic Microwave Background, Galaxy Formation) but fail at the smaller scales (Orbital speeds) and vice versa.
Which part of GR requires that GR => r^2?
I can try to think of why, but I'd be guessing. I understand that many things in GR follows from when we translate reference frames that there is a light speed constancy requirement. My exposure to physics is via category theory and specifically self-dual theories that apply to the quantum sized world.
It's over my head, but if you want to see the math, I think this link derives Newton's law from GR: https://math.uchicago.edu/~may/VIGRE/VIGRE2010/REUPapers/Tol...
Well, there's the answer that it's what seems to fit the facts. GR may be one of the primo-grade "unreasonable effectiveness of mathematics" examples as it captures rather complex (to human minds) behavior in a handful of carefully-defined symbols, but technically it's still an empirical theory first and mathematical theory second.
Secondly, as others mention, there is the obvious answer that r^2 is how area changes as you increase the radius of a hypersphere in 3-D space; here I deliberately say "hypersphere" even though we're in 3-space here just to emphasize that it's particular to the the 3-D case. There is a theory that the reason gravity is so much weaker than the other forces is that it is "escaping" out in other dimensions we can't see, though that still leaves the stuff we can see expanding as inverse-square in what we can see.
(One of my favorite crazy dark-matter theories is that it lies in those other dimensions, and the reason we can't capture any is that it is literally not "in our dimension". That said, it still can't be conventional baryonic matter, because dark matter doesn't clump anything like it does. If there was a "parallel universe" or 10 just like ours, except they gravitationally interact, we'd see things like stars in orbit around "nothing", because the stars in the parallel universes are interacting. That's not what the universe looks like.)
Modern science has available modern technology to measure so much and with such a precision that any theory that attempts to be scientifically accepted has to match with the existing measurements.
The only approach that is not "dark mater" which is not completely disqualifying itself on all scales (by not matching anything) is MOND, which is still more "let's try to find some formulas that can match at least some observations" than something that even now can be consistent with all observations and measurements. Still, where MOND doesn't match the observations, it fails not in the orders of the magnitudes, it even results in the wrong shape of the curve:
https://arxiv.org/abs/1112.1320
"The biggest challenge facing MOND today is the shape of the matter power spectrum. The shape depicted in Fig. 1 is related to the acoustic oscillations observed in the CMB. If the Universe is dominated by dark matter, these matter oscillations, dubbed Baryon Acoustic Oscillations (BAO), are highly suppressed as the baryons fall into the potential wells created by dark matter, leaving only percent level traces of the primordial oscillations. In a no-dark matter model, on the other hand, the oscillations should be just as apparent in matter as they are in the radiation. Indeed, Fig. 1 illustrates that – even if a generalization such as TeVeS fixes the amplitude problem – the shape of the predicted spectrum is in violent disagreement with the observed shape."
Or to summarize:
https://medium.com/starts-with-a-bang/theres-a-debate-raging...
"On the scales of groups of galaxies, individual galaxy clusters, colliding galaxy clusters, the cosmic web, and the leftover radiation from the Big Bang, MOND’s predictions fail to match reality, whereas dark matter succeeds spectacularly. It’s possible, and perhaps even likely, that someday we will understand enough about dark matter to understand why and how the MOND phenomenon on the scales of individual galaxies arises. But when you look at the full suite of evidence, dark matter is practically a scientific certainty. It’s only if you ignore all of modern cosmology that the modified gravity alternative looks viable. Selectively ignoring the robust evidence that contradicts you may win you a debate in the eyes of the general public. But in the scientific realm, the evidence has already decided the matter, and 5/6ths of it is dark."
That said there are some fascinating alternative theories of gravity such as mond or the unruh effect which don't invoke an inverse swuare rule.
Why is the rule not r^3 though? And what would the rule be in a 2-dimensional world?
In 2 dimensions it would be just ~r.
The "better" theory must cover more, not less of the measurements. The one that at the moment certainly covers the most is dark matter, and the alternatives simply cover much less (or as I've posted even somewhere result in the wrong shapes). Once some alternative manages to cover the observations and bring even more predictive power, that one will eventually be accepted (although sometimes "one funeral at a time" was needed), even if it's less "pretty" and less "obvious."
More than what exactly? As far as I know, the https://en.wikipedia.org/wiki/Lambda-CDM_model is the best we have, i.e. with the minimal number of parameters that still covers most of the observations. If there were a better one, it would already be accepted.
But unless you try to inform yourself, you wouldn't even believe how good it is, compared to the alternatives. Really, really good, as in, it actually predicted the future measurement results of the oldest signals reachable to us, and then later the measurements did match perfectly.
> epistemologically
Ah... I'd guess then that you don't write about physics but some non-scientific belief system.
> If there were a better one, it would already be accepted.
I see you've never actually been a scientist.
Why do we look at the surface? Because every point at the surface has the same distance from the center, and so we expect the field to be the same on each point.
I imagine a theory that approaches r^2 on some galactic scales but not others would not very elegant.
The issue is that modifying how we think gravity and matter works is not just a change in a math equation.
Changing those equations would also mean a whole chunk of modern physics, including general relativity is wrong - yet all those theories can be observed and confirmed in many very different ways to be consistent and valid.
So if you change those equations to adjust for dark matter, they no longer give the result for many other phenomena that we observe.
The easiest to understand evidence against MOND is shown in the bullet cluster. The bullet cluster is a pair of galaxies in the process of colliding. They also happen to be gravitationally lensing galaxies behind them. While the xray and visible light observations show that gas clouds that make up the bulk of the normal matter in the galaxies stopped when they hit each other, (like two cars in a head on collision) the gravitational lensing shows that the overwhelming majority of the mass in the galaxies simply passed straight through, the way dark matter as particles would.
So in order to accommodate the bullet cluster, MOND will need to show that empty space has some sort of momentum and memory, which is going to be much less elegant than a second hierarchy of particles which do not interact via the electromagnetic or strong nuclear forces.
There are a very large number of other observations which also disprove MOND or variations on them, the bullet cluster is just the easiest to describe in a paragraph.
It's actually very, very common for laypeople to come up with MOND as a solution for dark matter. I'm pretty sure I did when I first heard about dark matter. I don't think I've ever seen a dark matter post on HN or reddit that didn't include at least one post positing MOND, even if (especially if) they don't that the theory they came up with has a name, an acronym, and had already been thoroughly disproven by the scientific community.
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Bullet_Cluster
Without commenting on the merits, I think the theory "traditional kinematics need to be adjusted somehow at large scales" is a little too vague to have been disproven.
Seems a bit lazy and not very innovative.
2- Theories that don't involve 1/r^2 (MOND) exist, but they are not sufficient to match observations without involving dark matter. And following Occam's razor, since MOND introduces another parameter (the behavior of gravity on long scales), but doesn't simplify anything (dark matter is still there), it is not preferred.
3- While we don't know the details about dark matter, we "see" it in a way. We see it affect the movement of galaxies, and the gravitational lensing effect it has. It is just that we can't observe it with our preferred methods, which involve electromagnetism. I mean, we don't need to see a glass panel to know that it's there. Sure, we'd like to know more, and evidence of dark matter is weaker than our glass pane, but we simply don't have anything better right now.
(Meanwhile, to a first approximation, intervening dense objects like planets or stars are rare enough to be negligible: even in a region like our solar system where such objects are most likely to be found, the available volume is overwhelmingly empty space.)
https://arxiv.org/pdf/astro-ph/0001272.pdf
https://arxiv.org/pdf/astro-ph/9904401.pdf
When you see figures like "1E-7 to 1E-2 M⊙ mass range" (from the above paper's introduction), note that a Jupiter mass is about 1E-3 M⊙---one thousandth or 0.1% the mass of the Sun. Earth is 3E-6 M⊙, the Moon is 3.7E-8 M⊙, and Pluto is 6.6E-9 M⊙; or 0.00037%, 0.000004%, and 0.00000066% the mass of the Sun respectively. Compared to stars, planets comprise a tiny fraction of the mass of a galaxy.
TL;DR - No, that's not how gravity works, but you can still ignore the planets in between because they're tiny.
The reason for the question, I think, is because of an intuition built up around EM. In the system
A B C
B can block and otherwise interfere with C's EM radiation so that its effect on A is different depending on B's location.
Thanks. So, in the analogy between gravity and light, you have to change all "massive bodies" to "fully transparent light sources" to make the analogy complete.
A nontransparent light source would not translate as a (straightforward) massive body.
That being said, general relativity predicts that there is in fact a non-linear scaling when gravitational fields are extremely strong. The reason for this is that the there is some energy associated with the gravitational field, and any energy produces its own gravitational field. So the gravitational field itself exerts its own gravitational field, leading to the non-linearity.
For a static 1/r^2 field, intervening matter has very little effect at all. (There are non-linearities in principle, but for all but the most extreme configurations they're negligible in practice.) Certainly when the Newtonian approximation holds you can just add up the separate 1/r^2 effects of all the separate sources to find the overall effect. (This, by the way, is very similar to the behavior of electromagnetic fields. The waves are attenuated or blocked by intervening matter fairly easily, but it takes a pretty special situation to actually block the 1/r^2 effects of a static electric or magnetic field: you'd need to surround your system in a conducting "cage" that would polarize in response to an external electric field to cancel out the effects of that field inside, for example. That isn't possible for gravity, since there doesn't exist negative mass to do the screening.)
Ultimately, the 1/r^2 rule really is intimately connected to the three dimensions of space in our universe. In theories with additional spatial dimensions, that rule changes. Many theories with extra dimensions follow the Kaluza-Klein model, where the extra dimensions are "curled up" very small (in the sense that if you were to travel a distance L in that extra dimension, you'd come back to where you started: like a little circle). In the case of one extra dimension like that, if you were to measure the behavior of gravity over distances much shorter than L, you'd find that the gravitational force fell off like 1/r^3, but if you measured its behavior over distances much longer than L you'd find the familiar 1/r^2. (And for that reason, we know that the L for any system like this must be very small: my memory is that direct gravity measurements can set a limit like L<1mm or maybe even L<1 micron, and indirect evidence from things like particle physics observations pushes the limit down to the nuclear scale or below: L<10^(-15) m or even much less. I probably ought to know the actual values of both of those limits off the top of my head, but it's been a while since I looked at it.)
I remember reading that Gauss's (and his contemporaries') foray into statistics was greatly motivated by the central limit theorem. Every now and then one sees a question about least squares, asking why we don't usually use least cubes. [1]
Abel said of Gauss: "He is like the fox who erases his tracks in the sand with his tail."
[1] In such a discussion, if we are lucky to have a healthy discusion, we see references such as https://en.wikipedia.org/wiki/Gauss%E2%80%93Markov_theorem.
I 100% agree that when the model doesn’t fit the data, making up some hitherto undetected kind of matter, distributed in just the right way to explain the discrepancies, is kinda odd... the QI author is constantly complaining about this.
That seems totally backwards to me. Matter, by definition, is what is distributed spatially in just the right way to explain our observations. Physical laws, by definition, are not; they are universal. So why keep trying to explain what appears to be matter with adjustments to physical laws?
It's like you kept seeing elephant footprints appear out of nowhere, but insisted that rather than believe in invisible elephants, the most parsimonious explanation is that they are a natural consequence of a corrected law of gravity. That is, to me, turning Occam's Razor inside out.
I'd start to wonder if the tracks could be explained by another phenomenon.
But the other reply to your comment is good too - at what point, after failing to detect any OTHER signs of these invisible elephants, do you wonder if there’s other explanations? What it would take to falsify dark matter, given that you can invent it in whatever distribution you like to fit the data?
There’s nothing that logically compels us to assume the existing model of gravity is correct, and its discrepancy with observation is explained by assuming the inputs aren’t what we can observe. It is also logical to consider that the existing model of gravity isn’t quite right. Of course any tweaks to gravity have to be compatible with a lot of other observations but it would be wrong to rule out the possibility of an alternate model.
What does it take to falsify any observation of any kind of matter? Isn't the chair you are sitting on just as unfalsifiable? In an effort to be consistent, you might concede that the belief your chair (or anything!) exists is not scientific, so then I ask if that's not a problem, why is dark matter a problem?
I’d say models are useful when they are simple and predictive. Dark matter isn’t really predictive, it’s what is hypothesized to compensate for the prediction error of our current model of gravity. (Doesn’t mean it’s wrong but am just pointing this out)
So I don’t think dark matter is inherently problematic, just that it’s not predictive and it’s worth considering whether the model of gravity we have needs tweaking instead (much like GR refined Newtonian gravity).
But Newton didn't take that for granted, he examined other laws and proved that there were no stable elliptical orbits under them.
Did they publish the images? I'm sure many astronomers would be happy to examine them.
Yes future analysis of the images could potentially turn up something else but that wasn't what was asked.
The experiment described here uses new equipment that presumably has never been used for this purpose before. But I don't see any mention of efforts to verify that it can actually be used to detect black holes. Hopefully the remaining 10 days of their experiment can confirm that it works...
If you can test each part of a system independently, and the parts connect in a reasonably trivial fashion, it’s about as good as testing the whole.
Not really, which is why integration tests are so important.
That said, software testing isn't really an apt analogy now that I give it some thought. Testing real world equipment has to contend with a lot more edge cases and environmental factors.
We have done it before. We believe there is one rule from smallest to the largest. God does not play dice with us and definitely not in the dark. We were wrong.
Do not know whether this dark material (and the total different case of dark energy). Just to say eliminate one more theory like dark hole may not be the end of non- or better the birth of Dark Matter. If it is just weak ... but dark. Still need convincing most positively.
* search for why Ethernet is called Ethernet you should find this :
“In 1973, Metcalfe changed the name to “Ethernet.” He did this to make it clear that the system he had created would support any computer, not just Alto’s.
He chose the name based on the word “ether” as a way of describing an essential feature of the system: the physical medium carrying bits to stations.
He thought this was much like the old luminiferous ether was once thought to propagate electromagnetic waves through space.”
>"[dark matter] seems to make up about 95% of the galactic mass...crushingly narrow passages between observation and tests of GR"
GR only matches observation because you added in 20x more stuff that is undetectable other than as a deviation from the predictions of GR.
On a more philosophical level, though, if experts aren't concerned about something, and you as an amateur are, you're almost certainly the one who is wrong. Assume you are, and look for why, rather than questioning their conclusions. It's like the old adage about compiler bugs: they exist, but if you think you've found one, you haven't.
* And indeed scientists don't treat it as though it is - there are several other theories, though it's the strongest contender.
Iirc about a hundred years ago when we were trying to measure electron diameter, several iterations of scientists bsed there results assuming the last guy was accurate. Dark matter has a lot of similar mysticism and I would not be at all surprised to see a metric shit ton of scientists just being sheep and going along with incorrect models. I'd also not be surprised if they were all spot on too.
For reference in grad school I worked in a neutrino lab right by a dark matter lab underground. So I'm not an expert but do have decent exposure.
Untrained amateurs are right from time to time, but given that they have almost no basis for their assumptions except gut feelings, it basically comes down to chance.
I'm not arguing that scientists can't be wrong, certainly not. I'm arguing that believing they are, or even believing you have a valid criticism, without understanding on what they base their arguments, because "it sounds reasonable" or "scientists are sheep" is deeply intellectually dishonest.
Certainly there is bias in physics but I don't recall any occasion where amateurs where correct and experts not (in fundamental physics, that is). This may occur in fringe fields but is highly unlikely in fundamental stuff.
I'm not sure if that's a good simile. I've found several compiler bugs in my time, and I proved that with assembly output from the compilers and I've received updated compilers from the vendor as a result of my reports.
I suspect that to find "bugs" in physical theories must be orders of magnitudes more unlikely.
Am I reading it wrong? Or why is the title so misleadingly?
Also, there's zero indication in the article that this invalidates the existence of black holes.
And about dark matter, the last sentence says: "But the study does slim down the already thin chances that they are the source of dark matter's effects."
Insinuating that primordial black hole were a hypothesis for the effects of dark matter. Not that dark matter theory is proven inconsistent by the lack of evidence for primordial black holes.
I'm looking for someone who knows better I guess, to clarify. I'm only interpreting this using my reading skills from the article itself.
But the title of the article insinuates so: "One night of telescope time rules out black hole/dark matter".
So I wasn't sure if I had misinterpreted the article, or if the title was just confusing.
Edit: Maybe I wasn't clear. From the title, I got the impression that the observations ruled out both black holes and dark matter. But from the article, I only got the impression it ruled out the hypothesis that black holes were the force behind dark matter.