How do we know for sure that DeepSeek is not actually trained on Nvidia chips? Did someone outside of China replicated the training from scratch (Spending $6M)?
They themselves said it was trained on NVIDIA chips, so I’m not sure where you got that it wasn’t. It was trained on the less capable versions sold for the Chinese market.
I see, thank you for pointing that out. Then I’d rephrase, how do we know for sure that it wasn’t trained on the most advanced Nvidia chips? Did anyone outside of China replicated the training?
Researchers are not sure about the thickness of the ice on Europa. Some scientists believe on a thick shell of 20Km (like Pappalardo) others on a much thinner of even 1Km or less.
"We’re not a life search mission. We’re a habitability mission,” says Robert Pappalardo, Clipper’s project scientist at the Jet Propulsion Laboratory (JPL), which manages the mission."
So we are going to wait 7 years, and spend $5B to actually not search for life on Europa? Why? Sometimes it feels like Nasa is doing everything possible for not searching for life outside earth, which would be a game changing discovery for humanity, several orders of magnitude more important than the habitability of Europa.
I'm not sure you truly appreciate just what a massive engineering challenge that would be. Try to do too many new things at once and you end up with the JWST being 2 decades late and costing 10x as much as originally projected.
The obstacles are numerous. Even Jupiter's magnetic field is a huge problem. There was recent talk that this missions electronics may not be sufficiently hardened. Typically, space probes to the gas giants will have a highly elliptical orbit to mitigate potential radiation damage.
So just surviving in Europa's orbit is a problem. Landing on Europa is another huge problem. There's no atmosphere to brake into. An icy surface may have crevasses and such and you could potentially immediately lose your probe. So how do you land safely on ice when you don't know how much weight that surface will support? A solution might be to do a burn to slow down and do a stationary land but that's also complex and adds a lot of weight. Also the engines and the fuel need to survive for 7 years until they're used.
Conquer all those obstacles and you're now on the surface. Now what? The ocea is under kilometers of ice so you can't really reach it. You really have to look for a volcano/geyser and you have to get to what that produces without being destroyed or damaged. Does the ice thin? Is there heat that means the ice thins and there's (heated) liquid water underneath? We really have no idea.
Finally you get a sample of subsurface ocean water and now what? What does life look like? How do you detect that? What signatures are you looking for? How do you avoid contamination from EArth-based life? That's not as easy as you might think.
The contingencies and redundancies required are jus tmind-bogglingly complex.
> An icy surface may have crevasses and such and you could potentially immediately lose your probe.
I wonder, would it be viable to send multiple probes? What cost effect would it have on the mission to build and launch an extra one?
I know that e.g. for the Curiosity mission they've built a second rover that they've kept on Earth for potential troubleshooting. How much more expensive would it be to build yet another one and launch two of them?
Jesus the orbital injection alone wasn’t something I would have thought about. We rely a lot on the atmosphere to break our probes. Without that you need to burn just as much fuel slowing down as you did speeding up. Well, actually that isn’t true because your mass is way different so your fuel requirements are much, much less than that initial launch but still a non trivial amount.
I’m not a space probe engineer but I sometimes wonder if we go overboard on specialized compute hardware. I kinda wonder if that made more sense “back in the day”…. Ingenuity only rad hardened microchip is its flight controller. The rest is commercial off the shelf “normal hardware”.
I dunno… all I know is most people including myself ask the same questions as the parent. What the hell are we waiting for? Send some shit over there! Let’s do this.
> We rely a lot on the atmosphere to break our probes.
Yes and no. The atmosphere on Mars is a great example of the worst of both worlds. It's actually worse than having no atmosphere at all. It's not enough for aero braking. But it's enough to blow corrosive dust all over your solar panels and instruments and generally make your life miserable.
Of course, aero braking works exceptionally well on Venus but it has... other issues.
It did help on Titan though with the Cassini-Huygens probe.
> Without that you need to burn just as much fuel slowing down as you did speeding up
Not really. It's... complicated. If you were going between two points in the same inertial frame of reference then yes you need equal delta-V to slow down at the other end but, as you point out, that takes less fuel because your weight is lower (although part of your initial delta-V comes from the launch vehicle you disposed of).
But the EArth is going around the Sun at ~30km/s. Jupiter is going around ~15km/s. Europa is going around Jupiter at ~13km/s. So we have to speed up to escape EArth's orbit (around the Sun) and the EArth's gravityh well but also slow down to match Jupiter's velocity and also avoid speeding up too much as Jupiter's gravity well captures you.
But the lower orbital speeds of the outer planets is why we have never done an orbital insertion on Uranus or Neptune. This distance and delta-V requirements put flight times at like 10-30 years, depending. Heck, we haven't even done a flyby of each and that was back in the 1980s. Saturn is kinda of our practical limit for orbital insertion currently. And that's expensive and takes a long time.
But Europa having an icy surface is just a huge complication. Even if you do a burn to slow down, what's the heat on those thrusters going to do once you land? Is it going to melt ice and then you immediately drown? How thick is the ice? I don't mean overall thickness. I mean there may be crevasses and such. Just look at how dangerous it is to walk across glaciers.
How will you get traction on ice in relatively low gravity?
> I’m not a space probe engineer but I sometimes wonder if we go overboard on specialized compute hardware. I kinda wonder if that made more sense “back in the day”
You seem to have a very naive understanding of the dynamics here. Making it a life finder mission would have taken two times longer and cost three times more, assuming it wouldn't have been cancelled long before that.
NASA is not in charge of its own budget. Neither it is, ultimately, in charge of what missions get greenlit. Sure, NASA is an inefficient organization in many ways, including planning and management practices that never seem to get better despite numerous reviews, but honestly it's incredibly difficult to be efficient when your bosses sit in the Congress. You don't want to know what NASA's Planetary Science division could have achieved in the last twenty years with all the billions that have gone to the boondoggle that's the Senate Launch System and its earlier incarnations.
Because a close fly-by probe is something we know how to do, and is a much more affordable and achievable goal than a mission that would have the true goal of confirming life on Europa.
Such a mission would involve landing on an outer solar system body with no atmosphere, penetrating 10-15 miles of ice, and directly sampling liquid water for microbes. That's a huge undertaking that would cost a lot more than $5B. If any part of that failed, it would be a pretty bad look for NASA and those who voted to fund it, and a negative result still wouldn't mean there is no life.
I certainly think that's something we should be attempting to do in the future. But an initial close flyby mission is something we know we can do with a high probability of success, and data gathered from such a mission could build support for a more extensive follow-up in the future. The data gathered might even make that future mission less expensive and more likely to succeed by mapping likely places where the ice is thinner, or where tectonic activity pushes water to the surface.
And hey if a microbe in a plume of water happens to land in a collection receptacle on this mission, that's just an incredible bonus without setting the mission up for disappointment.
NASA is absolutely looking for life in the Solar System. Many of the Mars missions have looked for signs of life.
What NASA is not doing is looking for communications from intelligent aliens. Why? Because Congress decided in the 1990s that that would be a waste of money, and banned NASA from doing it.
Searching for life to confirm would only be useful if you have lots of pocket change. We all know life exists outside the solar system, only the biggest ego maniacs will truly believe humans are the only life in the universe
I understand the view that life likely exists elsewhere in the universe, and I agree that, given the vast number of planets and galaxies, it's certainly a plausible idea. The sheer scale of the universe, coupled with our growing knowledge of exoplanets and extremophiles—organisms that thrive in conditions once thought inhospitable to life—makes it reasonable to think life could exist beyond Earth.
That being said, I have no logical reason to know life exists anywhere else except on this planet. I think it's important to differentiate between the likelihood of something and claiming certainty about it. While the possibility of extraterrestrial life is exciting and worth exploring, until we have direct evidence, we can't confidently say it’s out there.
In fact, we can't even answer the philosophical question, "Do other people aside from me even actually exist?" with 100% certainty. This brings us to the ironic part: sometimes, claiming we know life exists elsewhere can be a reflection of the same kind of ego that leads others to believe humanity is uniquely special in the universe. Both positions can, in a way, stem from an overestimation of our ability to know the unknowable.
I think it's great to remain curious and open to discovery, but also humble about the limits of our current knowledge. :)
in math they use a lot of approximations to do calculations like limits -> infinity. It's a good enough approximation that is almost unrefutable. Also, who has more ego, we are the winner of 1 in 10e30, or there is way more winners.
The mass in the observable universe is considered to be 10^53 kg. So nothing is going to infinity when it comes to life made from matter (or energy).
I am not sure how to talk about things outside the observable universe. If light speed provides the ultimate limit for causality, this outside might as well not exist, from our perspective.
We have a lot of really useful things to learn about life away from Earth even if you assume that life exists elsewhere.
How common is it? In what environments does it occur?
Does it start the same way everywhere? Does it end up going the same directions the same way everywhere? Does it use the same metabolic pathways and the same genetic material?
Even just a confirmation without taking samples or deeper analysis is enough to start on these questions. Right now we can't even really start.
yes but do they have DNA or something like it? Are we their descendants? Are they ours? How much of that life is “intelligent” and how much of it is just microbes and stuff?
You are right in that it seems pretty “obvious” that we aren’t alone… the math is overwhelmingly in favor of it being everywhere. But there is a huge distance between the math saying it exists and actually looking at it with your own eyes.
The origin of life remains a profound mystery. The more we investigate, the further we push back its beginnings, suggesting that we may eventually need to search beyond Earth for answers. Since the Miller-Urey experiment, which successfully synthesized amino acids, we have made little progress in understanding the exact processes that led to the emergence of life. How were RNA and DNA formed? How did the first cell come together? What were the necessary timelines, energy sources, and chemical conditions? Is Earth uniquely capable of generating life, or is life, in its most basic form, a common feature of the universe? The formation of water involves processes akin to supernovae; perhaps the genesis of life requires a similarly extraordinary scale of events, or even more? We need to find out.
This sounds a bit defeatist, and seems to ignore a large swathe of work that has happened since 1952 (the Miller-Urey experiment).
For example - this experiment was conducted a year before the structure of DNA was discovered / published - so it's quite a bold claim to say 'origin of life' research stalled in 1952.
To your first, second, and third questions - I can highly recommend Nick Lane's book 'The Vital Question' [0]. To some extent, I think he also spoke to your fourth question in that book - but I read 3 or 4 of his books around the same time, so my memory is fuzzy.
Either way, his hypothesis around alkaline thermal vents is hugely persuasive & compelling.
I'm really struggling to understand what you mean by the claim 'formation of water involves processes akin to supernovae' - do you just mean you need a star to explode before you get heavier elements (including Oxygen)?
I completely understand your struggle and wonder why this fact isn't more widely discussed. The vast majority of water on our planet (and in the solar system) was formed through either stellar formation or supernova events. During these events, an outward wind of particles collides with nearby material, forming new molecules such as water. Any other process of water formation is negligible in comparison.
There's no reason to think other solar systems wouldn't have water, in that case, as there's little that's unique about our garden - or, from a slightly different angle, we shouldn't be surprised to find ourselves living in a place that has water.
On the contrary, I don’t want to sound defeatist. We’ll figure this out sooner or later, and it’s actually quite exciting, after all, we didn’t just come from a little pond! The discovery of DNA, RNA, and many other cellular mechanisms has greatly improved our understanding of how life works. This knowledge helps us formulate theories about the origin of life. However, as of today, no one has, for example, taken a Miller-Urey like experiment and spontaneously generated DNA.
Well if RNA was a (necessary) precursor to DNA, some experiments that attempt to replicate aspects of a deep alkaline thermal vent have already been conducted - you can get, albeit very short, strings. But the obvious problem is in trying to replicate a chemical soup and structure under high pressure, that doesn't exist any longer, and then run experiments to try to replicate outcomes of a natural experiment that lasted hundreds of thousands of years.
The idea that DNA needs to be spontaneously generated in order for us to be confident that we understand how DNA came to be is misplaced.
The point is that until we understood stellar mechanics and observed the formation of water in stellar events, we didn’t know where water originated. This suggests that we should not limit our thinking to Earth-based mechanisms when considering the formation of RNA and DNA, which are molecules orders of magnitude more complex than water.
Another problem is that the earliest fossils are 4.3 billion years old, while Earth itself is 4.6 billion years old. This means that life must have originated within the first 300 million years of Earth’s existence. What was this unique environment that enabled the creation of RNA, DNA, and cells? And why did this magical environment disappear in the subsequent 4.3 billion years?
We don't need to generate it spontaneously, but we do need to understand the mechanism precisely. Filaments alone (can you share the relevant research on this?) are not enough.
We need to understand exactly how DNA, RNA, and cells came into existence, just as we sought to understand the formation of water. Until then, we won't have the answer to the origin of life.
Sure, the problem of having a sample set of precisely 1 makes it dangerous to extrapolate too wildly ... but similarly, the absence of any other type of life on this planet, and our understanding of - and ability to postulate on - other building blocks viability, should not be dismissed because a carbon / water / oxygen arrangement is known to work.
I think you'll really enjoy The Vital Question, as it covers a lot of what you seem to be interested in knowing more about.
> Another problem is that the earliest fossils are 4.3 billion years old ...
I thought the figure was a bit less that that - and referred to (disputed) indicators of life, not fossils per se, but the generally agreed upon earliest is dated at ~3.5bya.
Nonetheless your point is understood -- some basic building blocks appeared 'spontaneously' over the space of 800,000 years, give or take.
Conditions at the time are broadly understood, though of course not the details - and it's that ~ 800,000 years, plus the unknowable details of the environment, that make me think we'll never know for sure.
At least not in the level of confidence that you appear to need - where the precise chemicals, with the right ratios, in the right solution, at the right pressure/depth, making whatever the first life form was (we don't know what that was, and likely can't ever know).
Ultimately I don't think we need to know exactly how DNA, RNA, and cells came into existence - it'd be nice, sure, but there's no requirement for that likely unobtainable goal.
Regarding filaments - I just searched on 'hydrothermal vents rna' and found a few likely links, including this one:
One of the hardest parts of training models is avoiding overfitting, so "more parameters are better" should be more like "more parameters are better given you're using those parameters in the right way, which can get hard and complicated".
Also LLMs just straight up do overfit, which makes them function as a database, but a really bad one. So while more parameters might just be better, that feels like a cop-out to the real problem. TBD what scaling issues we hit in the future.
I love the ironic side of the article. Perhaps they should add the reason for it, from Fermi's and Neumann's. When you are building a model of reality in Physics, If something doesn’t fit the experiments, you can’t just add a parameter (or more) variate it and fit the data. The model should have zero parameters, ideally, or the least possible, or, even at a more deeper level, the parameters should emerge naturally from some simple assumptions. With 4 parameters you don’t know whether you are really capturing a true aspect of reality of just fitting the data of some experiment.
This was mentioned in the first paragraph of the paper. The paper is mostly humoristic.
That said, the wisdom of the quip has been widely lost in many fields. In many fields data is "modeled" with huge regression models with dozens of parameters or even neural networks with billions of parameters.
> In 1953, Enrico Fermi criticized Dyson’s model by quoting Johnny von Neumann: “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.”[1]. This quote is intended to tell Dyson that while his model may appear complex and precise, merely increasing the number of parameters to fit the data does not necessarily imply that the model has real physical significance.
> > In 1953, Enrico Fermi criticized Dyson’s model by quoting Johnny von Neumann: “With four parameters I can fit an elephant, and with five I can make him wiggle his trunk.”
For those who are interested, you can watch Freeman Dyson recount this conversation in his own words in an interview: https://youtu.be/hV41QEKiMlM
That's how I feel about dark matter. Oh this galaxy is slower than this other similar one. The first one must have less dark matter then.
What can't be fit by declaring the amount of dark matter that must be present fits the data? It's unfalsifiable, just because we haven't found it, doesn't mean it doesn't exist. Even worse than string/M-theory which at least has math.
The dark matter theory is falsifiable. Sure we can't see dark matter (it doesn't interact electromagnetically), but we can see its effects, and it has to follow the laws of physics as we understand them today.
It is actually a satisfying theory with regard to the Occam razor. We don't have to change our laws of physics to explain the abnormal rotations of galaxy, we just need "stuff" that we can't see and yet interact gravitationally. When we have stuff like neutrinos, it is not that far fetched. In fact, though unlikely given our current understanding of physics, dark matter could be neutrinos.
If, as it turn out, the invisible stuff we call dark matter doesn't follow the laws of physics as we know them, then the dark matter theory is falsified and we need a new one (or at least some tweaks). And it may actually be the case as a recent paper claims that gravitational lensing doesn't match the predictions of the dark matter theory.
The main competitor to dark matter is modified gravity, which calls for no new stuff, but changes the equations for gravity. For the Occam razor, adding some random term to an equation is not really better than adding some invisible but well characterized stuff, especially when we consider that the equation in question is extremely well tested. It is, of course, also falsifiable.
The problem right now is not that these theories are unfalsifiable, it is that they are already pretty much falsified in their current form (dark matter less than modified gravity), and some rework is needed.
'dark matter' is not a theory, it is the name of an observational problem.
There are many theories to explain dark matter observations. MOND is not a competitor with 'dark matter', because MOND is a theory and it tries to explain some aspects (spiral galaxy rotation) of what is observed as the dark matter problem, which consists of many more observations. There is no competition here. There are other theories to explain dark matter, like dark matter particle theories involving neutrinos or whatever, and these may be called competitors, but dark matter itself is not a theory, but a problem statement.
Yes and no...MOND's core proposition is that dark matter doesn't exist, and instead modified gravity does.
Whereas you can have many proposals for what dark matter is, provided it is capable of being almost entirely only gravitationally interacting, and there's enough of it.
MOND has had the problem that depending which MOND you're talking about, it still doesn't explain all the dark matter (so now you're pulling free parameters on top of free parameters).
I've seen this opinion before, but can't seem to square it with any reliable source.
Wikipedia has: "dark matter is a hypothetical form of matter that appears not to interact with light or the electromagnetic field ... Although the astrophysics community generally accepts dark matter's existence, a minority of astrophysicists, intrigued by specific observations that are not well-explained by ordinary dark matter, argue for various modifications of the standard laws of general relativity. These include modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity."
Even its name, "dark matter", sort of strongly implies this. If someone were just trying to refer to the observations, rather than a specific explanation for the observations, wouldn't they just say "abnormal galaxy rotation curves" rather than "dark matter"?
I'm not saying Wikipedia is an end-all-be-all source on this, I'm just asking where you're getting this alternate definition. If it is somewhere reliable then perhaps the article needs to be rephrased.
As far as I know it's from here: https://www.youtube.com/watch?v=PbmJkMhmrVI As I said elsewhere in this thread, I think she's trying to make some meta point but ends up just muddying the water.
Adding any finite number of parameters is strictly better than adding an infinity of parameters (i.e. an arbitrary distribution of dark matter chosen to match the observations).
The distribution has to be consistent forward and backwards in time. It's a lot less arbitrary than you're implying, and adding a hundred parameters (or similar finite number) to gravity is not better.
If we add an arbitrary amount of dark matter everywhere, to match the observed motions of the celestial bodies, that adds an infinity of parameters, and not even a enumerable one.
This obviously can match almost anything and it has extremely low predictive power (many future observations may differ from predictions, which can be accounted by some dark matter whose distribution was previously unknown), so it is a much worse explanation than a modified theory of gravity that would have only a finite number of additional parameters.
the reason this isn't true is that by the hypothesis of dark matter, it follows gravity but not electromagnetism. as such it only fits distributions recoverable from evolving gravity. e.g. if we require a certain distribution today, it fixes the distribution at all other points in time, and we can use light speed delay to look into the past to verify whether the distributions have evolved according to gravity.
All observations of individual galaxies occur at a specific point in time. We can’t use light speed delay to see the evolution of individual galaxies only completely different galaxies at some other point in time. As such each galaxy gets its own value for the amount of dark matter.
At minimum this is a ~200 billion parameter model, and more if you’re looking at smaller structures.
That's equally true of the distribution of baryonic matter. We have to assess each galaxy individually to figure out what it's made of? What a crime against science. Never mind that they're still all made of a small handful of types of parts, which can nevertheless combine to form lots of possible histories and shapes for individual objects. Just like literally everything else in the observable universe. Seriously, what part of this argument is different for computing the amount of visible mass in each galaxy?
The availability to detect visible light from stars or detect that light being blocked by baryonic matter.
With dark matter it’s two steps removed where we’re inferring the behavior of baryonic matter and then inferring the amount of baryonic matter we aren’t observing and then calculating the existence of dark matter to get that behavior after accounting for undetected baryonic matter.
Yeah, that's a pain, but calculating mass from photons is still pretty indirect. More importantly, and independently of "directness", no one pretends that galaxies having different masses introduces two billion parameters into our models of cosmology. Because that's not what a model of cosmology is.
Calculating the percentage of the universe’s observable mass is dark matter adds 200+ billion parameters because the mass fraction of each galaxy varies.
So there’s no simple way to calculate it from say looking at the Milky Way alone and extrapolating from the baryonic mass of the rest of the universe. Trying to approximate things from a representative sample is its own problem.
You're still confusing a physics model with a map of the universe. That said, it's sure a heck of a coincidence that the number they get from adding up estimated dark matter in galaxies lines up with the number they get from other cosmological measurements, isn't it? Almost like galaxy rotation curves aren't the only evidence for dark matter and haven't been for a long time. https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
Gravitational lensing, velocity dispersions, etc circles back to the total mass of galaxies. So it shows up on many of the ways we calculate the total mass fraction not just rotation anomalies.
Something being consistent with a model is different than something being sufficient evidence on its own to support a model.
If the observed dark matter fractions of all known galaxies were 0% but the CMB was unchanged we wouldn’t assume dark matter exists. Thus your #2 is false. There’s infinite models consistent with any observation so finding something after a model was created for other reasons is useful as validation, but the chain of logic is still dependent on the prior observations not the model.
In a meaningfully different cosmos different observations would have happened and different models would exist. Trying to pick out specific experiments as sufficient on their own glosses over that particular limitation.
> If the observed dark matter fractions of all known galaxies were 0% but the CMB was unchanged we wouldn’t assume dark matter exists.
No, astrophysicists would eventually figure out something was up when they couldn't replicate the actual spectrum with dark-matter-free simulations. Why would you assume otherwise? Unless you want to dig into the assumptions of the scenario, in which case you're probably proposing a self-inconsistent universe so of course you can draw whatever conclusions you want from it.
> There’s infinite models consistent with any observation...
You can't actually believe this and still believe in science. If observations don't constrain models, then there is no point in observing. And in the long run, there's asymptotically no difference between "prior observations" and later observations. They're just observations that all go into the same model-constraining mill. Scientists are not fools, and are capable of realizing when an initial observation put them on a wrong trail.
You're still barely touching the real point. This all just sounds like rationalizations to avoid the fact that dark matter, for now at least, and for all that it genuinely sucks, is the Occam's razor explanation for the full suite of observations. Why is this so hard to accept?
> to match the observed motions of the celestial bodies
The point is that even with current observational data there's no reasonable distribution of dark matter that correctly explains all evidence that we have.
Your intuition that "if I have an infinite number of degrees of freedom anything at all can be fit" is leading you astray here.
> Sure we can't see dark matter (it doesn't interact electromagnetically), but we can see its effects
Even this is granting too much: "seeing it" and "seeing its effects" are the same thing. No one has ever "directly seen", in the sense that internet DM skepticism demands, anything other than a photon.
The problem with dark matter is that there does not exist any second relationship from which to verify its existence, like in the case of normal matter, which takes part in a variety of interactions that lead to measurable effects, which can be compared.
The amount and the location of dark matter is computed from the gravitational forces that explain the observed movements of the bodies, but there are no additional relationships with any other data, which could corroborate the computed distribution of dark matter. That is what some people mean by "seeing".
All major DM candidates also have multiple interactions: that's the WI in WIMP, for instance. In fact I don't know that anyone is seriously proposing that dark matter is just bare mass with no other properties - aside from the practical problems, that would be a pretty radical departure from the last century of particle physics.
No interactions have been found, despite a lot of resources put into the search. So currently all dark matter particle theories apart from "non-interacting" have been falsified. And non-interacting theories are probably unfalsifiable.
Radical departure may well be needed, for other reasons too.
> The problem with dark matter is that there does not exist any second relationship from which to verify its existence.
This is exactly it! Dark matter is strictly defined by its effects. The only 'theory' part is a belief that it's caused by yet to be found particle that's distributed to fit observations. Take all the gravitational anomalies that we can't explain with ordinary matter, then arbitrarily distribute an imaginary 'particle' that solves them: that's DM.
The problem is that the language used to talk about DM is wrong. It's not that DM doesn't interact with EM, or the presence of DM is causing the galaxies to rotate faster than by observed mass. These are all putting the cart before the horse. What we have is unexplained gravitational effects being attributed to a hypothetical particle. If we discovered a new unexplained gravitational property, we would merely add that to the list of DM's attributes rather than say "oh then it can't be DM".
All physical entities are defined by their effects! Suppose we found axions and they had the right mass to be dark matter. Would that mean we now "really knew" what dark matter was, in your sense? No, it would just push the defining effects further back - because all an axion is is a quantum of the (strong CP-violation term promoted to a field).
Just like the electromagnetic field is the one that acts on charged particles in such and such a way, and a particle is charged if the electromagnetic field acts on it in that way. There's no deeper essence, no intuitive "substance" with some sort of intrinsic nature. All physical properties are relational.
I used to think this, but dark matter does make useful predictions, that are hard to explain otherwise.
This is partially because there are two ways to detect dark-matter. The first is gravitational lensing. The second is the rotatinal speed of galaxies. There are some galaxies that need less Dark Matter to explain their rotational speed. We can then cross check whether those galaxies cause less gravitational lensing.
Besides that, the gravitational lensing of galaxies being stronger than the bright matter in the galaxies can justify is hard to explain without dark matter.
The problem with dark matter is that there's no (working) theory on how the dark matter is distributed. It's really easy to "explain" gravitational effects if you can postulate extra mass ad-hoc to fit the observations.
I dunno if this is the correct way of thinking about it, but I just imagine it as a particle that has mass but does not interact with other particles (except at big-bang like energy levels?). So essentially a galaxy would be full of these particles zipping around never colliding with anything. And over time, some/most of these particles would have stable orbits (as the ones in unstable orbits would have flown off by now) around the galactic core. And to an observer, it would look like a gravitational tractor ahead of the rest of the physical mass of the galaxy (which is slower because it is affected by things like friction and collisions?). And so you'd see galaxies where the arms are spinning faster than they should be?
> I dunno if this is the correct way of thinking about it, but I just imagine it as a particle that has mass but does not interact with other particles (except at big-bang like energy levels?).
Not even anything that extreme. What's ruled out is interaction via electromagnetism (or if you want to get really nit-picky, electromagnetic interaction with a strength above some extremely low threshold).
If there are two different types of observations, and one parameter can explain both, that is pretty strong evidence. Put differently, dark matter is falsifyable, and experiments have tried to falsify it without success.
Besides the idea 'not all mass can be seen optically' is not that surprising. The many theories on what that mass might be are all speculation, but they are treated as such.
It's worth noting that one dark matter explanation is just: it's cold matter we just can't see through telescopes. Or black holes without accretion disks.
Both of these are pretty much ruled out though: you can't plausibly add enough brown dwarfs, and if it's black holes then you should see more lensing events towards nearby stars given how many you'd need.
But they're both concrete predictions which are falsifiable (or boundable such that they can't be the dominant contributors).
Dark matter is constrained by, among other things, dynamical simulations. For instance, here's an example of reproducing real world observations, that previously didn't have great explanations, using simulations with dark matter: https://www.youtube.com/live/8rok8E_tz8k?si=Q7vmQYpZr_6K7--m. And that's not even getting into the cosmology that has to (and mostly does) fit together.
Interesting that you should link that video. Its title card says "Angela Collier". Here's a more recent video by the physicist[0].
Re: where it says "using simulations with dark matter", we can't simulate DM because it doesn't have any properties beyond our observations. All we do is distribute amounts of it to match observations. It could be "Dyson spheres with EM shields" and the results would be the same.
Yes, and I think that video is stupid. She doesn't use the term that way in her own talk, and neither does any scientist I've ever heard. I think she's trying to make some abstract point about science in general and muddying the water in the process. Her takes on terminology are often bad IMO.
That doesn't take away the fact that when you work with the slightly more specific theory of "particle dark matter" it produces real results. And I believe there's a lot more work over the years in similar areas. It doesn't get talked about because it's not sexy, so people who only follow cosmology when there's drama don't hear about it. That was just the example at the top of my mind because I'd seen it recently, and the result is really quite spectacular. Did you watch it through?
> What can't be fit by declaring the amount of dark matter that must be present fits the data?
Tons of things - just like there are tons of things that can't be fit by declaring the amount of electromagnetically-interacting matter that must be present fits the data.
You can fit anything you like by positing new and more complicated laws of physics, but that's not what's going on here. Dark matter is ordinary mass gravitating in an ordinary way: the observed gravitational lensing needs to match up with the rotation curves needs to match up with the velocity distributions of galaxies in clusters; you don't strictly need large scale homogeneity and isotropy but you really really want it, etc. Lambda-CDM doesn't handle everything perfectly (which in itself demonstrates that it's not mindless overfitting) but neither does anything else.
There are modified gravity theories that are compatible/extensions to GR, e.g the f(R) gravity theories.
Nobody probably believes MOND as such is some fundamental theory, rather as a "theory" it's sort of a stepping stone. Also MOND is used often interchangeably (and confusingly) with modified gravity theories in general.
> Dark matter is ordinary mass gravitating in an ordinary way: the observed gravitational lensing needs to match up with the rotation curves needs to match up with the velocity distributions of galaxies in clusters
Those are all the same thing, the shape of spacetime. The only thing DM adds is a backstory that this shaping comes from hypothetical undiscovered particles with properties that match observations.
Well, the funny thing is Copernicus posits just about as many epicycles in his theory as previous geocentric theories. Only Kepler’s discovery of the equal area law and elliptical orbits successfully banishes epicycles.
The history of these discoveries is fascinating and shows that Kuhn’s scientific revolutions idea is wrong but it’s always rounded off to “Copernicus and Galileo” and doesn’t even get them right
There will be no Copernicus if everybody just studies epicycles. E.g. there are massive resources put into the desperate WIMP hunt that could be used for finding new theories.
I think a model with zero parameters belongs more to math, because it can be derived from first principles. E.g. The surface area of a sphere is 4/3 * pi * r^3, assuming Euclidean space. Physics begins when we have at least one constant of nature to measure, like the actual curvature of space or the attraction due to gravity.
Hodgin and Huxley did ground-breaking work on squid's giant axon and modelled neural activity. They had multiple parameters extracted from 'curve fitting' of recorded potential and injected currents which were much later mapped to sodium channels. Similarly, another process to potassium channels.
I woudnt worry too much having multiple parameters -- even four when 3 can't just explain the model.
Neuron anatomy is the product of hundreds of millions of years of brute contingency. There are reasons why it can't be certain ways (organisms that were that way [would have] died or failed to reproduce) but no reason whatsoever why it had to be exactly this way. It didn't, there are plenty of other ways that nerves could have worked, this is just the way they actually do.
The physics equivalent is something like eternal inflation as an explanation for apparent fine-tuning - except that even if it's correct it's still absolutely nowhere near as complex or as contingent as biology.
This is why I think that modeling elementary physics is nothing else than fitting data. We might end up with something that we perceive as "simple", or not. But in any case all the fitting has been hidden in the process of ruling out models. It's just that a lot of the fitting process is (implicitly) being done by theorists; we come up with new models and that are then being falsified.
For example, how many parameters does the Standard Model have? It's not clear what you count as a parameter. Do you count the group structure, the other mathematical structure that has been "fitted" through decades of comparisons with experiments?
You are using the word "fitting" rather loosely. We usually "fit" models of fixed function form and fixed number of parameters.
You are also glossing over centuries of precedent that predate high-energy physics, namely quantum field theory, special relativity, and foundational principles such as conservation of energy and momentum.
It tends to be a parameter that can be derived from rrasoning and assumptions. This contrasts to free parameters where you say "and we have no idea what this value should be, so we'll measure it"
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