A new tool for finding dark matter digs up nothing
quantamagazine.org
quantamagazine.org
One talk started with a lecture on the lamppost problem. It asked: why exactly are we looking for WIMPs? Because we know how to build a WIMP detector.
I'm not equipped to answer questions about dark matter anymore, but I will say that a lot of the knee-jerk comments in this thread underestimate how smart physicists are. Some of the most brilliant minds in science have contributed to dark matter cosmology. They are particle physicists, cosmologists, fluid dynamicists, and simulationists (reaching a little bit with the terminology here) all working together.
Science is progressing in the natural way of excluding theories from easiest to hardest. The fact that there are so many people working to exclude the "obvious" (or even "obviously wrong") theories should be evidence of how difficult this problem really is.
It may turn out that dark matter is explained by a non-particle phenomenon, or that it is matter that simply acts purely via the gravitational force and nothing else. I don't know what the implications of this would be. But none of that is obvious or easy to prove at the moment.
Nor is it easy to come up with a dark matter model which satisfies all the necessary constraints. You cannot just say, "there's some stuff out there" without collateral effects on (say) the distribution of matter in the universe.
For why we believe dark matter exists, see here: https://kids.frontiersin.org/articles/10.3389/frym.2021.5760...
I would hesitate leaning too much on the always part. Fritz Zwicky was the first to notice the anomalous galaxy rotation curves and propose dark matter (is the story that I remember, anyway.) This is probably why it's presented first. It does not mean that it's the most important piece of evidence.
I'm intentionally writing with a light touch here because I don't consider myself an expert in the field.
It's been a long time since then and there's been a ton more data collected about wide ranging phenomenon since.
There have been papers published on this, but it is still speculative, so it's premature to say that this kind of alternative theory rules out dark matter.
No, it only eliminates the need for Dark Matter to explain galaxy rotation curves.
> In physics, a tachyonic field, or simply tachyon, is a quantum field with an imaginary mass.
The term tachyon was coined by Gerald Feinberg in a 1967 paper titled "Possibility of faster-than-light particles". He had been inspired by the science-fiction story "Beep" by James Blish.“
I don’t think “Beep” introduced the word, though (https://archive.org/details/galaxymagazine-1954-02)
There are some reasons to suspect/hope such things might not exist, such as protecting causality.
In general, asking "what if <simple field type> was <more complex field type>" can actually produce super fruitful lines of inquiry. You can use "complex numbers", "quaternions", "finite fields", whatever you like. There's plenty of unexplored ground here.
I'm not aware of a mechanism by which causality would arise from known QM, so I wouldn't expect a priori planck scale to play into it. There would also need to be an energy term for h to make sense as the relevant constant.
The most compelling source of temporal direction I'm familiar with is that it's a regional cosmological entropy gradient rather than a fundamental aspect of physics, although it strikes me as suspicious that the time direction lies inside the 4-cone generated by the odd-dimension-out in our spacetime metric. That certainly lends some credence to theories which would predict the direction of time as fundamental to physics.
My guess is that it falls back to Occam's Razor and there has yet to be any physics to use it thus it isn't a serious contender for current problems until much more work has been done on all the threads already being followed.
Dark matter could be a neighboring universe interacting with ours or something like that where the actual particle causing it is fundamentally out of reach (if it even is a particle), but just the fact that it interacts with our universe means we should in principle be able to interact with it and characterize it with experiments.
"Dark" matter refers to something that does not interact at all with photons - not that it scatters too little light.
In scientific study you want and need failure. Knowing what doesn't work is as important as knowing what does. This way we can make lists of dangerous or useless things to be avoided. As long as things are documented and reproducible to the best of your abilities, you've won.
In 1999, the Nobel prize winning physicist Philip Warren Anderson said he is "sure that it's a fraud",[12] and in the same year another Nobel prize winning physicist, Steven Chu, called it "extremely unlikely".[23] The following year, a 2000 patent based on its hydrino-related technology[24][25] was later withdrawn by the United States Patent and Trademark Office (USPTO) due to contradictions with known physics laws and other concerns about the viability of the described processes, citing Park and others.[26]
Every submission you've made seems to be about this company (https://news.ycombinator.com/submitted?id=dave333).
I've also always wondered about how we account for the physical presence of the astronomical objects now versus the observed position of those objects as they were, since light has taken however long to reach us...
Unless the galaxy were a black hole, they'd escape. Since it's not, it can't be them.
It would be great if there were something producing slow-moving neutrinos, which would not only solve this but give us a huge leg up in understanding what's going on with neutrinos. But it doesn't look like that's the case.
The episode "Is Dark Matter Made of Particles?" [1] of PBS Space Time covers that.
They have several other dark matter episodes looking into other possibilities, such as black holes, axions, gravity not working the way we think it does, and more. If you are interested in this topic doing a search for "dark matter" in the video list on the YouTube channel would be a great place to start.
:D
https://www.cosmology.info/media/open-letter-on-cosmology.ht...
https://web.archive.org/web/20170624042109/http://www.nytime...
There are more interesting and less implausible hypotheses out there than the "just so" dark matter one.
Yes, dark matter is the best way we currently have for fitting the data within most of our existing frameworks for understanding the universe. I really don't see the relation to epicycles.
There are people out there working very hard on modified gravity theories. I'm not aware of any that are able to explain the current data. If anything, trying to modify gravity to fit the dark matter data would be most similar to epicycles, it's devilishly difficult to come up with increasingly complex theories that fit more and more data that's coming in.
This is the key problem. Since there is no theory of what dark matter is made of or how it is created and destroyed, you can always have as much or as little of it as you need to fit the data.
As for gravity, we don't have anything close to a working theory of quantum gravity. We can't even measure the classical gravitational constant to more than about 4 digits. The reproducibility by different experiments has not demonstrably improved since the 1940's! Clearly there is a lot more we have to learn about gravity.
And yes, quantum gravity is also something we don't understand. But the domain where we are missing that understanding doesn't overlap with the dark matter "problem".
Falsifiability comes from having specific predictions, which means one theory that's an island, not a bunch of approximations.
It's not just a fudge factor, it's a fudge factor that behaves exactly like mass.
For dark matter, one of the problems is that any given instance is allowed to have whatever amount of dark matter gives the output we want. This makes the math work out great, but weakens its predictive power.
Dark matter and dark energy are the simplest (literally single parameter) fits to the cosmological data we see. Why we see data consistent with these parameters (e.g. what is the non-interacting massive particle) is up in the air, but adding multi-parameter new forces without more justification seems a little too interesting.
For instance, I've seen a convincing argument[1] that dark matter researchers consistently underestimate galactic inclination, which in turn results in poor fits between model and data, leading to the epicycle-like fudge inclusion of dark matter as "everywhere we would expect mass to be, given xyz".
[1] https://tritonstation.com/2022/01/10/the-curious-case-of-agc...
Einstein would be happy!
Eg, if we’re looking at pseudo-tangles in shadow projections, that would solve why QM looks statistical while relativity looks continuous.
But that would raise deep questions about “warp” in the macro and whether galactic scale tangles would generate additional binding energy — ie, dark matter. Large scale tangles hiding mass would explain why we can’t interact with it, for instance.
Why are "epicycles" embarrassing?
"Epicycles" worked fine for a very long time. They are predictive. And accurate! And can be translated to a physical compuation mechanism (see: Antikythera mechanism).
In addition, "epicycles" are a manifestation of Fourier decomposition, which is a perfectly fine mathematical tool. The errors are due to the fact that reality isn't purely mathematical.
"Epicycles" were quite fine until we got both 1) much better clocks and 2) much better astronomical observations.
People forget that physics was in pretty close to this same state about 150 years ago (roughly) right before quantum mechanics. Physics was pretty much buttoned down except for that really strange Black Body Radiation problem (Ultraviolet Catastrophe), but they'd have that sorted in a couple of years. Certainly no later than 1900--the end of the century. No big deal.
HAH! We know how that turned out.
One interesting approach I've heard of is abandoning reductionism and looking at how the macroscopic might actually determin the microscopic. We'll know how it pans out in half a century or so.
“The most successful technique with which to investigate [dark matter] has so far been the effect of gravitational lensing. The curvature of space-time near any gravitating mass (including dark matter) deflects passing rays of light - observably shifting, distorting and magnifying the images of background galaxies”.
“Dark matter appears not to interact via the electromagnetic force, and therefore neither emits nor reflects light.”
I am sure you know that, but perhaps not everyone does?
I must confess, when you said "pedantic", I expected something more annoying and less worthwhile. That's the best "pedantic" I've seen in quite a while.
In the case of dark matter, we only observe their downstream effects but have no other direct evidence of its existence.
I'd love to hear about all of the less implausible hypotheses you're referring to.
I bet one coffee that there is no dark matter in the form it's been looked for in the last decades. I bet two coffees that when somebody will discover the cause of the measured effects we'll be able to build some great new stuff, much like general relativity eventually gave us GPS and then navigation, etc.
Dark matter is a simple theory that explain a wide variety of unrelated observations.
In any case, until we actually are able to test that this is one "thing" rather than a bunch of things we still have no idea about, we should stop calling it "dark <foo>". Because I think that a lot of people are under the impression that there is some discrete substance out there causing this discrepancy, and there really is no reason to believe that's the case.
most people creative enough are driven to the arts, which in modern society are only good for entertainment.
It just kind of feels like there's a scale with weight on it that, by everything we now observe, should only weigh 200lbs and instead is reading 300 -- it just seems that "broken scale" is a better explanation than "invisible stuff on the scale."
Dismissing it as an incorrect measurement ignores the depth people have long gone into to verify this discrepancy between predictions and reality is not a sensor problem.
Almost 2 centuries ago astronomers found that the orbit of Mercury didn't make sense according to Newton's laws, laws which were at the time considered infallible. The astronomers only explanation was that there was a planet closer to the sun that we couldn't see, they believed this so much that they even named the planet Vulcan and spent many years searching for it.
Turns out that there was no Vulcan and general relativity explains the discrepancy in the orbit of Mercury.
That discovery made both Le Verrier and the world at large very excited, and the race to repeat that feat lead to everyone jumping to conclusions about Vulcan.
So it'd be more like a couch that's supposed to weight 200 lbs reads 300 lbs on a scale. But we've tested it on a half dozen different scales from different makers and keep getting 300 lbs. In addition we've tested this other couch that's suppose to read 100 lbs on some of these scales, and it correctly reads 100 lbs.
Dark matter works well because it's designed to fill in the gaps in our current understanding of astrophysics data. Given it's origin it of course can perfectly explain the data, because that's what it was designed to do.
However, that doesn't necessarily make dark matter particularly useful for understanding how nature actually works.
> Dark matter works well because it's designed to fill in the gaps in our current understanding of astrophysics data.
"Works well" seems like you are looking at it the wrong way. Nobody is touting "dark matter" as a solution. It's the problem itself.
It’s called dark because it doesn’t interact with the electromagnetic field (i.e. we cannot see it) and matter because it’s otherwise assumed to behave just like the matter we can see.
It is a bit of a “We don’t like it, but it’s the best we can do for now”, but it is not like physicists invented something completely out of thin air to make the theory explain observations.
However, this nickname is not absurd, in the sense that the gravity equations for the movement of stars in galaxies don't match the observation.
In order for the stars to move inside their galaxies at their observed speed, these galaxies should have much larger mass.
Hence the "dark matter".
Scientists do not all agree that there is an "invisible" matter to be found. Many think that Einstein's relativity has a more limited domain of validity than we thought.
But then the issue is as dark (in the sense of mysterious), since many of Einstein's predictions - based on his equations - have been observed in experimentations. The gravitational waves (LIGO experiment and other replications) appear to prove that the relativity equations are right. The precision of this instruments are extraordinary.
It's a terrible situation for Physics as a discipline: the equations are confirmed in all know situations, except this case (well, as far as I understood).
There are plenty of theories for explaining the phenomenon, but they lack an feasible experiment to see if their predictions match the observation.
Today, all the theories with feasible experiments have proven to be false. Physicists are stuck.
Many think that we need a new Einstein or a new Newton, meaning a genius among the geniuses, capable of think a whole new paradigm for gravity.
Gravity is a "law" and not a "force". It's as well the nickname for a phenomenon ; that phenomenon is sensible in the domain of validity of Newton, e.g. in our daily life.
The planets in the solar system move all accordingly with Newton's equations, except Mercury. Einstein had to totally put upside down the way we represent the world we live in, from a 3D world with a totally independent time to a 4D world, where everything and everyone live in its own spacetime.
Let's imagine two atomic clock set next to each other and synchronized. If we elevate one by 1 cm, then the clocks have a super slight desynchronizarion. And the tiny tiny difference match Einstein's equations.
Newton and Einstein are a few centuries apart. When will such genius reveal itself? Some physicists point that both were outsiders in their times. Nowadays, the knowledge is so vast that it takes ten years to master a fraction of it: by that time, a brain is so entrenched in the current representations that it's difficult to break the paradigm.
Others think likely that one of the theories already published is right but we'll need to wait a very long time, if ever, before being able to make the experiment that will prove it.
Maybe we will have to accept not being able to explain our universe. After all, since Gödel, mathematicians have kept going despite his proof that the mathematics are based on a quite fragile foundation.
I never quite understood that Gödel thing. Anyone to try and explain us that "dark" theorem?
That may seem surprising, but I don't think it is. There are infinite sets and infinite things you can say about them. It's not really a shock that there might be some properties about that set you can only prove by inspecting all of the elements, but you can't actually do that in finite time -- even though it's true. The sets are infinite, the theorems and their proofs are (by definition) not.
That doesn't mean that the foundations of math are fragile. It just means that there are facts you don't know. But you already knew that, too.
The specific theorems that can't be proven are just a subject of your axiom set. You can prove, or disprove, any specific thing you want, by taking it as axiomatic. It's just a question of whether anything interesting follows. That's a judgment for mathematicians, not a fact of mathematics.
I am, of course, handwaving like crazy, and probably driving the mathematicians nuts. Even though I said that the theorem is less surprising than it might seem, the fact that you can prove the theorem itself strikes me as very surprising.
This is not correct. Our measurements are accurate enough now that we have detected relativistic corrections in the motion of, IIRC, at least all the planets out to Jupiter or Saturn.
I'm not aware of any significant number of scientists who think this. Even proponents of MOND acknowledge that they need to come up with a viable relativistic formulation of their theory.
I basically came into the comments here just to see all the complete armchair novices come out of the woodwork to explain how they think they're smarter than the actual physicists.
That makes it really tempting to think "well maybe it's the model/equation that's wrong", which has been exactly the case throughout scientific history: planet Vulkan vs Relativity, phlogiston vs oxygen, luminiferous ether vs QED.
Speaking of which, shall we start a discussion on climate science? ;-)
Here's a Monty Hall explainer. Will a plane on a treadmill take off? When you shower, do you explicitly wash your legs? Did you know that 0.999...=1?
https://aether.lbl.gov/www/classes/p10/gr/Precessionperiheli...
For those who haven't heard of this, the idea is that the gravitational constant actually changes at very high rotational speeds / centripetal accelerations (such as those at the galactic scale), and this is why our Earth-based gravity experiments don't detect this because we're in roughly the same reference frame and don't see the effect.
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
(Also, I am really not a physicists, so I'll let them chime in)
First, the MOND regime is not very high (large) accelerations, but very low (small) accelerations.
Second, the MOND hypothesis is not quite that the gravitational constant changes; it's that the form of the gravitational force changes--it's no longer the Newtonian inverse square force.
The key issue with this is that Newtonian gravity isn't our best current theory of gravity: General Relativity is. MOND as it was originally proposed is not a relativistic theory; there have been attempts to formulate a relativistic version, but none of them have resolved all of the open issues.
> this is why our Earth-based gravity experiments don't detect this because we're in roughly the same reference frame and don't see the effect.
No, that's not why MOND proponents say we don't detect the effect in Earth-based experiments. It's simply that we can't reproduce the required conditions on Earth (the net gravitational acceleration from all sources being small enough to get into the MOND regime).
I'm unsure the differences between "same reference frame" vs. "can't reproduce the required conditions" - ie Isn't it that in order to reproduce the very low accelerations we can't be in our current location with its high accelerations? I'm guessing there's some nuance between location / ref frame here that I'm missing.
"Same reference frame" is just an abstraction (and quite often a meaningless one--according to the proper concept of "reference frame" in physics, there is no such thing as being "in" one reference frame but not another).
"Can't reproduce the required conditions" is a straightforward statement about the limitations of what we can do with our current technology.
> Isn't it that in order to reproduce the very low accelerations we can't be in our current location with its high accelerations?
According to MOND, yes, in order to test it we would have to set up a lab out in deep space (i.e., not near any planet or star) far enough away from the center of our galaxy that the sum of all the "accelerations due to gravity" from all sources was less than the MOND threshold. Which of course is well beyond our current technical capability. But none of that has anything to do with "reference frames".
Many people walk away with the assumption that dark matter is literally massive lumps of stuff out there that is just really hard to see. Kind of analogous to stealth aircraft or something. This sounds absurd and it is. And its why a common reaction is to consider it absurd and that we probably just have gravity wrong.
The reality is that dark matter is just the name given to a set of observations. It wasn't intended to be descriptive. We are talking about scientists here, not public relations. So they are liable to fumble in the PR bit. Its not what they are good at. I consider the Neil DeGrasse Tysons and Michio Kakus of the world more to blame than the community at large. They have chosen to become the popular face of science and with that have a responsibility to properly communicate it. Their failure contributes to why people distrust academia to some degree.
Dark matter has already had some success. Just as an example, the discovery of Neptune -- made entirely through dark matter. We couldn't see it obviously, too much space to search over, but some kind of gravitational disturbance gave suggestions as to where to look. Boom, a whole planet. That is quite literally search via dark matter: it doesn't shine, so we must look for its knock-on effects.
Easy enough in the local neighborhood, but at a galactic scale, it is much much harder. And I wouldn't expect anything quick, either. If you want a fascinating journey through astronomy, look up the history of the various estimations of the "standard candle," used as a way to gauge distance. This took quite a lot of time to bounce back and forth between numbers.
And, no, the Earth is not made of dark matter.
I sort of feel the frustration here of people on Wikipedia trying to correct something they actually know about.
There's various places where gravitational effects don't seem to line up with the distribution of mass that we see or expect. Something is generating gravity, that we can't see, in places we didn't expect. Call it 'dark' matter, because it's not shiny.
But now that it has a name, it's treated a bit like a proper noun. Dark Matter. We do some observations that probably rule out simple nonluminous planetoids or dark stars. Whatever is causing the galactic rotation discrepancies doesn't seem to be normal matter. That translates into: 'Dark Matter' isn't normal matter. As if it is a certain thing, and not a reference to an unknown set.
By naming it, and talking about it, even though we don't understand it, we run the risk of attaching all sorts of presumptions to its identity. We're prematurely normalizing a certain way of thinking about it. We may have done a similar thing to quantum mechanics. (I may not be making much sense here, it's just a thought that struck me and I'm working through it.)
And we are imaging the Earth in infrared. Well, infrared from non-geologic sources, but oh man are we sure imaging in infrared.
Bu before the name "dark matter" existed, the practice of it was evident. How did they discover Neptune? Gravitationally, not by seeing it first via a telescope. Its existence was inferred by the orbits of other planets, namely a gravitational disturbance suggesting a big glob of matter we had not collected photons from.
But we did not discover Neptune by collecting its photons. We had to infer that some glob of matter was present by gravitation. Then we pointed telescopes in the vicinity of prediction until this was confirmed.
Going further, we use these collected to infer all kinds of things -- a consistent dimming of a star in a certain period might suggest a planet occluding the star in an orbit coplanar to ours, which is our current method of locating exoplanets, but that wouldn't count.
You might recall how the neutrino was originally simply a placeholder, and actual detection came later. Similarly, the coining of the phase dark matter was simply as a contrast and a placeholder: we know something is there, but it ain't stars (light matter). Historically, many suggestions were made, and quite a few of them weren't exotic: MACHOs (Massive Compact Halo Objects), intergalactic gas, and so on. WIMPs would classify as a candidate for dark matter that would be somewhat exotic, so would axions.
Frustratingly, as the article suggests, we can do a lot of things to rule out (or more typically, set a maximum contribution as a fraction from each candidate), but we have little ruled in. We don't even have detection of axions yet, and those have been in the running for decades.
This is the sort of stuff that lured me into getting a physics degree.