[HN Gopher] A new tool for finding dark matter digs up nothing
___________________________________________________________________
A new tool for finding dark matter digs up nothing
Author : rbanffy
Score : 84 points
Date : 2022-03-22 13:52 UTC (9 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| adriangrigore wrote:
| > A new tool for finding unicorns digs up nothing
|
| :D
| jrm4 wrote:
| Complete armchair novice here, but I always found "dark matter"
| as I have seen it to be pretty implausible.
|
| 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."
| yk wrote:
| The thing is, your proposed change to the scale has to work
| exactly like "invisible stuff on the scale." So, we can observe
| dark matter in the early universe with the CMB, we can observe
| it in the large scale structure, and we can observe it in
| rotation curves. And the results of these observations is, that
| dark matter has to behave like matter, just dark. So your
| proposed new theory would need to deviate from standard theory
| in precisely the way an additional matter component would.
| veilrap wrote:
| You're describing a
| https://en.wikipedia.org/wiki/Tautology_(logic)
|
| 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.
| MadcapJake wrote:
| This is not a tautology because it's not being used as a
| final explanation. It's called "matter" because the
| observational gaps appear to fit what would be some mass
| that we're missing and we think of matter as "stuff with
| mass". It's called "dark" because we can only observe it's
| gravitational impact and by all other observables, it
| doesn't show up.
|
| > 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.
| nickpinkston wrote:
| Yea, I'm with you and have been really interested the MOND
| (Modified Newtonian Dynamics) research, as their charts seem to
| fit the data better.
|
| 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)
| Scarblac wrote:
| But there are also galaxies that do agree with what our
| gravity laws say, and therefore don't seem to have much dark
| matter. That's a big problem for theories that want to change
| the laws of physics, I believe?
| tzs wrote:
| A good recent look at MOND and how it compares to dark matter
| theories is the recent episode "What If Our Understanding of
| Gravity is Wrong?" [1] on PBS Space Time.
|
| [1] https://www.youtube.com/watch?v=0sTBZ2G4vow
| pdonis wrote:
| _> the idea is that the gravitational constant actually
| changes at very high rotational speeds / centripetal
| accelerations_
|
| 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).
| nickpinkston wrote:
| Thanks for chiming in to clarify!
|
| 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.
| pdonis wrote:
| _> I 'm unsure the differences between "same reference
| frame" vs. "can't reproduce the required conditions"_
|
| "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".
| russdill wrote:
| It'd be a much different problem if everything it was like
| this. This is more of a "most things we measure" give a heavier
| reading, but not by any consistent amount. But not all. And
| very often the scale shows a number when there's absolutely
| nothing to see.
| mohaine wrote:
| Except the scale is right locally, but not on the galaxy scale.
| dboreham wrote:
| The sim may be running a different version of code out there.
| specialp wrote:
| This could be true, but it could also be true that there is
| something else at play that we are unaware of. In science there
| have been cases of things that did not fit the model, but the
| model was not wrong. One example is the precession of
| Mercury[1]. Newtonian models were right that this anomaly does
| not agree with our understanding and calculations of gravity.
| But later on it was shown by Einstein that this anomaly was
| caused by relativistic effects. It wasn't that there was a
| large miscalculation, or our understanding of gravity was
| completely wrong, it was that the model did not count on a
| previously undiscovered phenomenon.
|
| https://aether.lbl.gov/www/classes/p10/gr/Precessionperiheli...
| Aachen wrote:
| Yeah this comment is posted whenever dark matter comes up. I'm
| a similar novice and I feel the same way. However, I also know
| that the experts out there don't say this for no reason and
| that, even if "dark matter" just a construct/illusion that
| turns out to be something else (not a new kind of matter), it
| can still be a useful placeholder to work with (like how we
| still model gravity as a force).
|
| 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.
| nodja wrote:
| Another novice here, but every time the matter of dark matter
| comes up I remember this story.
|
| 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.
|
| https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
| wongarsu wrote:
| Another important part of the story is that a central
| figure in this was Urbain Le Verrier, who just a couple of
| years earlier had predicted the orbit of a planet based on
| the orbit of Uranus being a bit weird. That planet was
| found and named Neptune.
|
| 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.
|
| https://en.wikipedia.org/wiki/Discovery_of_Neptune
| gpderetta wrote:
| Sometimes you look for a missing planet (Vulcan) and turns
| out the theory is wrong. Sometimes you look for a missing
| particle to carry away momentum and indeed it is there (the
| Neutrino).
| bostonsre wrote:
| I kind of had the same intuition as the op, but you are most
| likely correct. A lot of ridiculously smart people all seem
| to think it's out there somewhere. Are there any popular
| theories that posit that gravity behaves differently at these
| large scales? Or are all the smart people in the same camp of
| the being dark matter somewhere?
| hnuser123456 wrote:
| Yes,
| https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
| jrm4 wrote:
| Yep, there's probably a big difference in how real scientists
| talk about it vs. what we end up seeing in casual magazines,
| etc.
| dcolkitt wrote:
| The difference is the existence and proportion of dark matter
| has been confirmed across multiple different scales and
| context, from galaxy rotation curves to the cosmic microwave
| background to gravitational lensing. We've also found galaxies
| where dark matter doesn't exist in any measurable quantities.
|
| 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.
| thereddaikon wrote:
| Part of the problem is bad PR. Its induced by 1: a terrible
| name, dark matter implies a lot. and 2: Pop science doing an
| utterly awful job of explaining the topic.
|
| 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.
| hannob wrote:
| I am a novice as well, but as far as I understand, "Dark
| Matter" ultimately is really just a term for "there's a
| phenomena we don't understand yet, and we call it Dark Matter".
| FrenchAmerican wrote:
| I heard a podcast with top scientists about this (in French,
| still available online) about this: it's indeed a nickname
| for the phenomenon - but the scientists using it know that,
| of course.
|
| 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 Godel, mathematicians have kept
| going despite his proof that the mathematics are based on a
| quite fragile foundation.
|
| I never quite understood that Godel thing. Anyone to try and
| explain us that "dark" theorem?
| pdonis wrote:
| _> The planets in the solar system move all accordingly
| with Newton 's equations, except Mercury._
|
| 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.
| kbelder wrote:
| And our own satellites.
| pdonis wrote:
| Yes, good point.
| jfengel wrote:
| The Godel thing isn't quite as a big a problem as it seems.
| One way of looking at it is that there will always exist
| true theorems that you cannot prove.
|
| 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.
| pdonis wrote:
| _> Many think that Einstein 's relativity has a more
| limited domain of validity than we thought._
|
| 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.
| Someone wrote:
| You're missing an important part: _"and extra matter that
| doesn't interact with the electromagnetic field would explain
| it."_
|
| 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.
| fknorangesite wrote:
| > Complete armchair novice here
|
| 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.
| rootusrootus wrote:
| Welcome to HN, and every other forum on the Internet. It's a
| conversation forum, not a debate forum.
|
| Speaking of which, shall we start a discussion on climate
| science? ;-)
| fknorangesite wrote:
| > 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?
| kortex wrote:
| I think dark matter (and dark energy, for the same reason)
| invite lots of armchair discussion by curious non-experts
| precisely because they are basically fudge factors. There's
| no ontological baggage and the theories are fairly
| "decoupled" from other aspects of physics. It's basically
| data, a fitted curve, and some X factor (or Lambda for dark
| energy) added on to make the data fit the curve. That X does
| not interact with any other models outside of cosmology in
| any currently observable way, and not for lack of searching.
|
| 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.
| [deleted]
| nimish wrote:
| There needs to be a real Kuhnian paradigm shift IMO; it's
| embarrassing to have modern-day epicycles used as a fudge
| whenever a model breaks down vs. questioning the core assumptions
| of the model.
|
| There are more interesting and less implausible hypotheses out
| there than the "just so" dark matter one.
| [deleted]
| alangibson wrote:
| People are trying to shift the paradigm all the time, but so
| far nothing has worked. Replacing General Relativity and the
| Standard Model is the mother of all hard problems.
|
| 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.
| huachimingo wrote:
| Systems Methodology in a nutshell.
| kurthr wrote:
| There are more interesting and less implausible TESTABLE
| hypotheses out there?
|
| 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.
| uoaei wrote:
| This isn't an argument so much for science as it is for
| funding of scientific experiments. I don't know about you but
| I feel like this public, cultural, collective investment of
| time and energy toward dark-matter-seeking projects is not
| bearing out. It's worth testing other theories more
| completely.
|
| 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...
| zmgsabst wrote:
| I think the goal is a paradigm shift via ADS-CFT
| correspondences:
|
| 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.
| WithinReason wrote:
| It's not quite single parameter, since each galaxy needs a
| parameter to describe the dark matter contained in it to
| explain the observations about it.
| AnimalMuppet wrote:
| Yeah. It's more like a single parameter _at each point in
| space_. That adds up to a lot of parameters.
| bsedlm wrote:
| but the odds that somebody with a personality profile who'd
| think creatively enough to do this, then make it out through a
| modern scientific programme, and also sticks to academia are
| from low to zero.
|
| most people creative enough are driven to the arts, which in
| modern society are only good for entertainment.
| bsder wrote:
| > it's embarrassing to have modern-day epicycles
|
| 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.
| user-the-name wrote:
| No, there are not. Pretty much all the popular contenders are
| less plausible, and most still need dark matter to explain all
| observations, so they only add more complexity.
|
| Dark matter is a simple theory that explain a wide variety of
| unrelated observations.
| aisengard wrote:
| I mean, until we actually prove that dark matter is
| "something", we need to stop calling it that. All it is, is
| some constant that we apply to make sense of the motion of
| galactic-scale objects. And we use "dark energy" as a
| constant to explain why the universe keeps apparently
| accelerating its expanse beyond what we would have predicted
| given what we know about the universe. There is no reason to
| believe that these phenomena are caused by some singular
| thing. These latest development only strengthens that
| conclusion.
|
| 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.
| russdill wrote:
| You've described the search for dark matter as it is
| performed. There's already a percentage that's been filled
| in and most theories try to fill in additional chunks
| rather than as you say "some singular thing".
| LocalPCGuy wrote:
| I think it helps to give a name to the concept, and then do
| a better job of educating people that this is basically a
| placeholder name until it's better understood. (and yes, I
| realize that is not an easy task, might be easier to find
| the solutions and figure out the best name)
| aisengard wrote:
| Right, I'm not sure there's one unifying theory that you
| could bundle it under, not sure what the right term is
| for a general concept rather than a hard testable theory.
| But that just further proves the need to stop calling it
| one thing in particular, because it leads the vast
| majority of people (including scientists!) to think that
| "dark matter" is some big unifying theory, when there is
| no such thing as some singular substance called "dark
| matter".
| mcdonje wrote:
| Correct me if I'm wrong, but we call it dark because we can't
| directly observe it. Us not being able to observe something
| seems quite plausible. We spent most of our existence as a
| species with no way to observe viruses.
| axg11 wrote:
| We can observe viruses though? Through electronic microscopy,
| by sequencing their genomes and also by observing their
| downstream effects.
|
| In the case of dark matter, we only observe their downstream
| effects but have no other direct evidence of its existence.
| mcdonje wrote:
| We can observe viruses _now_. We cannot _currently_ observe
| dark matter.
| willis936 wrote:
| What qualifies as direct evidence? Photon emission? What if
| a phenomenon does not interact with matter via most
| forces/fields?
| AnimalMuppet wrote:
| We call it dark because it doesn't interact with
| electromagnetic radiation, i.e., light. (At least, as far as
| we can tell, it doesn't.)
| robocat wrote:
| Pedantic: dark matter does affect electromagnetic radiation
| via gravity.
|
| "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?
| AnimalMuppet wrote:
| I did know that, but it's not amiss to point it out.
|
| 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.
| pmontra wrote:
| Yep, I'm with you. I'm afraid this is another misunderstanding
| and a new quest for ether. But this is a good thing.
|
| 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.
| russdill wrote:
| I'm really not sure why so many people have such a strong
| reaction to dark matter theories.
|
| 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.
| kloch wrote:
| > Yes, dark matter is the best way we currently have for
| fitting the data
|
| 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.
| dTal wrote:
| The fact that there's _any_ "amount" - a single scalar
| quantity of a thing we already have all the equations for -
| that neatly explains multidimensional data, is highly
| suggestive by itself, no?
|
| It's not just a fudge factor, it's a fudge factor that
| behaves _exactly like mass_.
| veilrap wrote:
| Any fudge factor with a continuous effect on the output
| that covers the range of interest can be used to adjust a
| given output to an output of choice.
|
| 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.
| russdill wrote:
| There's actually an embarrassingly large number of theories
| for what makes up dark matter. I doubt most people have
| ever even heard of the form that was ruled out in this
| article. There's not just a ton of theories, a great number
| of them are falsibiable with experiments we can plausibly
| perform.
|
| 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".
| akvadrako wrote:
| Having a large number of theories is embarrassing! That
| means you have no idea. Gravity and QM are so great
| because they are clear winners - they are not easy to
| modify a little bit and nothing approaches them in terms
| of practical power.
|
| Falsifiability comes from having specific predictions,
| which means one theory that's an island, not a bunch of
| approximations.
| Splendor wrote:
| > There are more interesting and less implausible hypotheses
| out there than the "just so" dark matter one.
|
| I'd love to hear about all of the less implausible hypotheses
| you're referring to.
| alecst wrote:
| As a grad student in High-Energy Physics I worked at the XENON1T
| dark matter detector in Italy. I left feeling disenchanted about
| the search for dark matter, and I felt like a lot of us were just
| doing the best we could at the time, which was build a bigger and
| bigger detector to get more and more sensitivity.
|
| 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...
| Maursault wrote:
| The reasons given for Dark Matter _always_ begin with galaxy
| rotation curves (the galaxy rotation problem and the galaxy
| winding problem). But the problem with this "evidence" is that
| the observations were made of galaxies _in isolation,_ but even
| galaxies are not gravitationally isolated.[1] I can not speak
| to the veracity of the rest of the evidence for Dark Matter,
| but galaxy rotation curves can no longer be counted as evidence
| of Dark Matter because they are more simply explained by a
| gravitational phenomenon caused by the out of frame mass of
| other nearby galaxies.
|
| [1] https://www.youtube.com/watch?v=PL0ewiwqoTw&t=4m5s
| russdill wrote:
| Explanations always begin with Galaxy rotation curves because
| that's how it was first notified that something about our
| understanding of the universe was off.
|
| It's been a long time since then and there's been a ton more
| data collected about wide ranging phenomenon since.
| pdonis wrote:
| _> a gravitational phenomenon caused by the out of frame mass
| of other nearby galaxies_
|
| 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.
| Maursault wrote:
| > 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.
| alecst wrote:
| There are many theories of dark matter given at conferences
| like this. Around that time (2013-2014) I remember hearing
| about axion theories too. I don't know anything about this
| person's presentation so I can't say how it displaces the
| problem of galaxy rotation curves. If the math checks out, I
| think it would have, but you never know. I would like to see
| a reaction from cosmologists.
|
| 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.
| lil_dispaches wrote:
| I know how to build a dream catcher.
| Melatonic wrote:
| Is it possible that Dark Matter is simply not possible to
| observe from our frame of reference?
| wongarsu wrote:
| But we are only talking about it because we can observe it.
| Or rather observe how it interacts with other things, but
| that's just semantics: all observation happens via
| interactions.
|
| 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.
| Melatonic wrote:
| Thats more along the lines of what I was thinking - we can
| observe this specific interaction but without also
| observing other interactions it seems very difficult to
| define exactly what it is. Or maybe it is just that our
| current frame of reference (both physically and in terms of
| time) is just a bad vantage point to do so.
| russdill wrote:
| An important property of dark matter need to fit what we
| are seeing is that it also doesn't interact with itself.
| It would seem weird if it can't interact with itself
| because of some innate property, but we can't interact
| with it because we are somehow out of phase with it.
| dave333 wrote:
| Dark matter is possibly the best evidence in support of Dr.
| Mills' Hydrino theory https://brilliantlightpower.com/theory/
| which deserves a fair reading. There are about 20 experimental
| results supporting this theory that models electron orbits as
| spherical shells instead of probability clouds. He has machines
| that produce 100s of kW from small quantities of hydrogen - 200
| times the energy produced by burning hydrogen.
| https://brilliantlightpower.com/tpv-suncell-rectanguloid-cav...
| tux3 wrote:
| 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]
|
| Source: https://en.wikipedia.org/wiki/Brilliant_Light_Power
|
| Every submission you've made seems to be about this company
| (https://news.ycombinator.com/submitted?id=dave333).
| dave333 wrote:
| So how do these Nobel prize winners explain dark matter? Do
| "known physics laws" not evolve as new discoveries are
| made? Wikipedia is policed by skeptics that only support
| the status quo. IMO that article is not a fair description
| of Brilliant Light Power. As for my submissions, if as I
| believe hydrinos turn out to be real, it will be the
| greatest scientific discovery since the genome, so worth
| harping on about.
| zvrba wrote:
| A naive question from a hobbyist with over-average interest in
| particle physics: quantum theory is based on complex numbers.
| Has anyone considered that mass/gravity may also have an
| imaginary component that manifests itself as "dark matter"?
| (I.e., it's not "matter" at all, just an incomplete
| understanding of gravity.)
| alecst wrote:
| That's exactly the kind of question a curious physics student
| would ask. This might be what you're looking for:
| https://en.wikipedia.org/wiki/Tachyonic_field
|
| > In physics, a tachyonic field, or simply tachyon, is a
| quantum field with an imaginary mass.
| henriquecm8 wrote:
| I hear the word Tachyon so much in science fiction that I
| was under the impression it was a sci-fi word.
| Someone wrote:
| It almost is.
| https://en.wikipedia.org/wiki/Tachyon#History:
|
| _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)
| wyager wrote:
| Good question. Imaginary mass -> negative 4-momentum ->
| momentum exceeds energy -> object is moving faster than light
| (backwards in time in some reference frames).
|
| 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.
| SkyBelow wrote:
| As someone who is similarly and over-interested hobbyist, I
| wonder what happens when you throw in even more, for lack of
| the proper term, deviant math systems. The cousins of
| imaginary numbers using either epsilon (where epsilon^2 = 0,
| epsilon not a member of R) or j (where j^2 = 1, j not a
| member of R). Or quaternions. Or the family of quaternions
| similar to the family of complex numbers. And so on.
|
| 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.
| at_a_remove wrote:
| Most people do not understand "dark matter" because they do not
| understand astronomy. Astronomy would be _better_ defined as the
| study of celestial objects by the collection of photons, and
| inferential calculations made upon those collections. Astronomers
| collect photons from stars, which is to say "matter that gives
| off light." In a practical sense, the Earth is made of dark
| matter, because it does not shine by its own light (okay, if you
| are near enough, shielded from the Sun, and looking in infrared,
| if you like splitting hairs). Dark matter need not be
| particularly exotic, just ... almost hard to find by definition.
|
| 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.
| breezeTrowel wrote:
| This is incorrect. Yes, Dark Matter is matter that does not
| emit light but it does not absorb nor reflect light as well. In
| other words, it does not interact with the electromagnetic
| force. That makes it _extremely_ exotic.
|
| And, no, the Earth is not made of dark matter.
| at_a_remove wrote:
| That's a rather newer definition. The concept has been
| _around_ longer than the name, which is why I mentioned
| Neptune. The original definition was literally anything where
| the presence is puzzled out from its gravitational attraction
| instead of than its luminosity. Hence why MACHOs (MAssive
| Compact Halo Objects) were under consideration for being dark
| matter. They would consist of brown dwarfs, planets, and the
| like. They are currently out of vogue for not supplying
| enough gravitation, but that matter had no special properties
| to it. Not exotic at all. You can look this up.
|
| I sort of feel the frustration here of people on Wikipedia
| trying to correct something they actually know about.
| kbelder wrote:
| It's interesting the way language works, and specifically
| naming. It's almost magic.
|
| 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.)
| changoplatanero wrote:
| missed opportunity to call it stealth matter instead of dark
| matter
| Cd00d wrote:
| With a molten core I'd be surprised if the earth didn't emit
| it's own light.
| vecter wrote:
| Anything with a positive temperature radiates photons. The
| Earth probably does in the infrared spectrum.
| perardi wrote:
| Haha, it would be a shock if it didn't. That whole "black
| body radiation" thing.
|
| And we are imaging the Earth in infrared. Well, infrared
| from non-geologic sources, but oh man are we sure imaging
| in infrared.
|
| https://en.wikipedia.org/wiki/Suomi_NPP
| at_a_remove wrote:
| Precisely why I said "(okay, if you are near enough,
| shielded from the Sun, and looking in infrared, if you like
| splitting hairs)" ...
| Melatonic wrote:
| We have known about Neptune far longer than Dark Matter - no
| idea what you are talking about here. I mean we have great
| photographs of the planet itself - obviously it reflects light
| !
| amanaplanacanal wrote:
| How sure are we that dark matter isn't neutrinos?
| russdill wrote:
| Everything we know about neutrinos says they move too fast to
| account for what we see. Doesn't mean that there couldn't be
| some discovery they completely changes our understanding of
| neutrinos though.
| matja wrote:
| Indeed, neutrinos appear to not be completely understood, htt
| ps://en.wikipedia.org/wiki/List_of_unsolved_problems_in_p...
| -> ctrl-F
| kloch wrote:
| Yes. We have a pretty good understanding of how neutrinos are
| created and destroyed and there should not be enough of them to
| explain the anomalous gravitation.
| yk wrote:
| Pretty sure. The problem is, that neutrinos are too light, and
| when they are produced in the early universe they are too fast.
| That means galaxies do not form like we observe them.
| Basically, neutrino dark matter would "orbit the outskirts" of
| the galaxy instead of building a defined center.
| tzs wrote:
| As others have said, they are too fast. But that just raises
| the question of why do we expect that if dark matter is
| particles they must be slow?
|
| 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.
|
| [1] https://www.youtube.com/watch?v=fidzLZQyaJE
| jfengel wrote:
| As other replies have said, neutrinos move too fast. They seem
| to move almost exclusively at the speed of light. (So close
| that we can only infer indirectly that they don't ... and it's
| possible that one kind still might.)
|
| 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.
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