Source: https://www.nist.gov/news-events/news/2026/04/nist-weighs-my... (4 points, 12 days ago) https://news.ycombinator.com/item?id=47796892

> Cavendish painstakingly recorded those oscillations to measure the gravitational force of the larger spheres on the smaller ones, and from that he could infer Earth’s density.

I can't wrap my head around this sentence. How is the force between two objects, none of which is the earth, related to any property of the earth (its mass or density)? Wouldn't Cavendish's experiment have worked - even better - in zero g and given the same result? There is something more to his setup beyond two big weights pulling on each other that I'm probably missing?

• Retric

  today at 9:25 AM

  Force of gravity for spherical objects of constant density is calculated from 2 masses + 1 distance + a constant.

  Before the experiment you can measure the mass of both objects. In the experiment you measure the force and distance to calculate the constant.

  The weight either object gives you the force between that object and earth (adjusting for atmospheric buoyancy). Altitude at your location + size and shape of earth gives distance between object and center of earth, you just learned the constant. So you know 4 out of five variables in an equation and can thus calculate the mass of the earth.

  Technically that excludes the weight of the atmosphere above your altitude, but you can get that from the air pressure. Similarly the density of the earth isn’t constant but it is very close to symmetrical so you can get a reasonable estimate.

• ricksunny

  today at 8:06 AM

  By ascertaining an approxiamte value of G , perhaps? After that, you know M_earth, and already knowing Earth’s geometry, one arrives at average density rho.

  • phh

    today at 8:38 AM

    Laying out the math (assuming earth is an homogeneous sphere) just in case it's not clear:

    F_gravitational = G m1 m2 /r^2

    g = G Mass_earth / r_earth^2

    Mass_earth = r_earth^2 * g/G

    Density_earth = r_earth^2 * g/G / V_earth

    Density_earth = 3*g / (4*Pi*G*r_earth)

    Prior to Cavendish we already new g and r_earth, just missing G.

  • imrehg

    today at 8:16 AM

    Yup, that's exactly it:

    - get the gravitational constant with these two known masses

    - then can deduct the mass of the unknown Earth by its interaction with other masses (say the "g" gravitational acceleration value)

    - then from the mass and the otherwise measured size of Earth the density pops out

    More details in good ol' Wikipedia: https://en.wikipedia.org/wiki/Cavendish_experiment#Derivatio...

    • UncleSlacky

      today at 10:38 AM

      It's also a notoriously difficult experiment to perform. When I did it at university, the value of G I got was out by an order of magnitude - and that was considered a good result!

  • alkonaut

    today at 8:41 AM

    Oh so the earth density is merely the motivation for the experiment? I read it as the earth mass actually being used somewhere in the formulas within the setup itself which was what confused me.

    He uses his experiment to calculate G based only on the test masses and spring and then the _result_ of the calculation was just used as a final step to calculate the mass of the earth, and then from that the density?

If you use a unit of length that is 1e33 plank lengths, and a unit of time that is 1e42 plank time, and a unit of mass that is 1e6 plank masses then big G is just 1e-9 length^3/mass*time and the speed of light is 1e9 length/time. Which would be convenient.

Along with a unit of charge that is related to the elementary charge by the square root of the fine structure constant then the conductance quantum is 1/pi units of conductance and the inverse conductance quantum is pi units of resistance. Which is somewhat funny.

Figure 1 in the paper helps contextualize the numbers better.

• nine_k

  today at 4:58 AM

  The paper is https://iopscience.iop.org/article/10.1088/1681-7575/ae570f

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[dead]

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• lordgrenville

  today at 6:06 AM

  Are you sure these are actually issues, or are you just trusting that the AI criticism is correct?

• thrance

  today at 7:19 AM

  There's a new-ish rule on HN:

  > Don't post generated comments or AI-edited comments. HN is for conversation between humans.

  No one cares about what ChatGPT had to say to you on TFA. What do you have to say about it?

• grey-area

  today at 7:16 AM

  Apology not accepted.

• antiloper

  today at 7:22 AM

  You must feel so smart.

[flagged]

• dang

  today at 4:43 AM

  "Please don't fulminate."

  https://news.ycombinator.com/newsguidelines.html

• contact9879

  today at 4:40 AM

  how is this at all related to TFA

  • i_think_so

    today at 7:26 AM

    Gravity experiments? Easier to do accurately in lower gravity environments, like away from the Earth? And we just so happen to have a nearly permanent laboratory in orbit, which was built and is maintained at tremendous taxpayer expense?

    I thought this would be obvious from TFA. I should have been more explicit.

    • rcxdude

      today at 11:34 AM

      Are you confident this would be easier to do in space? It's not entirely obvious to me that the tradeoffs would be worth it.

I appreciate the scientists’ honesty. When asked about big G and time invariance, he says he just takes it on faith that it has been the same forever. If more people would admit their leaps I think the theistic schism would be far more shallow.

• kergonath

  today at 8:51 AM

  > he says he just takes it on faith that it has been the same forever

  It’s not faith, it’s a working assumption. There are in facts Physicists working on figuring out what would change if some fundamental constants changed with time. Our best measurements and understanding right now is that they do not. If we show tomorrow that it’s wrong, then we’ll build better theories and move on. There is absolutely no faith involved.

• mattclarkdotnet

  today at 7:27 AM

  The theistic schism? I had to look it up, and was not cleverer after. Nobody can ever know an ultimate why, for obvious and well established philosophical reasons. At least the scientists are trying to squeeze the knowledge gap down as small as possible instead of making up stories.

  • bowsamic

    today at 7:51 AM

    > Nobody can ever know an ultimate why, for obvious and well established philosophical reasons

    Yes we can, you are just presupposing that philosophy is ultimately ineffective. For example Hegel gave a presuppositionless development of all metaphysics among other things. It’s not some kind of philosophical consensus that ultimate justification is impossible

    • mattclarkdotnet

      today at 8:35 AM

      Philosophy can be perfectly effective as a tool of thought while still being unable to resolve self evidently unsolvable “ultimate questions”

      • bowsamic

        today at 9:36 AM

        It can resolve them though

    • kergonath

      today at 8:48 AM

      Interesting word soup. Ultimately, no, you cannot build a valid representation of the universe from nothing and you need observation and validation. You can presupposition whatever you want when you are talking about unproveable models, but it says more about you than the universe. Until we have a reason to think that there is a "why", discussing what it is is completely unnecessary and futile because 1) it does not change anything about our understanding or the predictions we can make, and 2) it is not something we can observe, measure or prove.

      • bowsamic

        today at 9:37 AM

        You don’t need any observations at all to build up a complete knowledge of the entire universe. Hegel showed this

        • hilariously

          today at 9:52 AM

          Quantum physics from no observations? With your monkey brain? Yeah right.

          • bowsamic

            today at 10:57 AM

            This is why I don’t use this site. Low IQ discussions and insults from morons

            • hilariously

              today at 11:12 AM

              Ah yes, that is a good answer to how you would derive quantum physics from rationality alone.

• today at 11:39 AM

• mananaysiempre

  today at 5:52 AM

  > If more people would admit their leaps I think the theistic schism would be far more shallow.

  There’s an important gap here between science as practiced and science communication.

  Working scientists will absolutely admit their ignorance, shaky foundations, etc. This is especially important in astronomy and cosmology, as the field is relatively young and experiments are impossible, outside of those that nature has already done for us. (Both evolutionary biology and linguistics have similar problems but cosmology has it especially hard.)

  This, however, is a losing strategy for communication. Most people equate confidence with credibility (and by high school we’ve beat children down enough that they do so as well), so if you do not sound confident people will not listen to you. (I could pontificate on how this is one of the greatest societal ills of our time, science or no science, but I won’t.) Even outside social situations, most people frankly cannot deal with holding a position and simultaneously not being confident in it, and absolutely cannot deal with holding an entire network of mutually-supporting positions and different degrees of confidence in each, while also having multiple alternatives with different degrees of plausibility for some of them. (This is somewhat more advanced than the programmer’s skill of relying on a deep stack of supporting services and debugging tools while keeping in mind that any given subset of them could be lying, which I’m sure you’re aware is also fairly difficult to communicate the experience of.)

  Then there’s the active (if not always successful) effort towards never ever reasoning backwards from things you would prefer to be true or that would make the world nicer for you. (The “History Plots” section[1] of the biennial Review of Particle Physics is there solely as an admonishment never to go with the herd. And that’s for things that have no implications for anybody’s worldview, morality, or livelihood!) It is very uncomfortable to genuinely not know where you are going and also not be able to aim anywhere in particular. (It might among other things imply that the entirety of your life’s work only serves to seal off a dead end and you might not even live long enough to learn that. And either way you’re consigning yourself to a very lonely sort of life if you veer away from the mainstream.)

  On the flip side from the vagueness, there’s the experience of doing everything you can to break something and failing, of your forefathers doing the same at their most imaginative and still failing. (The aforementioned RPP has pages and pages of tests for frickin’ energy conservation, without which most of physics and engineering just falls apart. And cosmologists can only dream of doing the same on the scales that are relevant to them, and indeed they do keep things like “modified Newtonian dynamics” around. Note that time invariance [as much as there is such a thing in general relativity] is energy conservation [ditto].) It is a sort of confidence that few others have justifiably had in their lives. (Few other things will infuriate a physicist more than offhand quoting a number with six significant digits. They know—in some cases from direct experience—that this sort of precision takes generations. And a well-established theory needs multiple times the effort.)

  So when, say, a cosmologist says that cosmic inflation is a bit of a speculative crapshoot but probably true, the Big Bang is likely true, general relativity they’re fairly sure is true but it sure would be nice to find some cracks, the Standard Model is true despite everybody doing their level best to break it because the foundational issues are quite serious, the mass of a free electron is nearly certain, and the inability to surpass the speed of light is pretty much absolute—this is a dynamic range of confidence that none of us can adequately feel. Now take one of those statements in isolation and try to make your listener understand what the apparent equivocation in it really means.

  (I do not believe the typical theist in a debate is on more than an advanced amateur level in all of this.)

  Then you get into the cursed philosophical issues, like the (weak) anthropic principle (a class of “why” questions don’t and can’t actually have much of a meaningful answer) or nonexceptionalism in cosmology (it is possible that everything we can or will ever be able to see around us is in fact wildly atypical as a great cosmic joke, but if so we couldn’t ever know enough to join in and any science we do would be completely meaningless, so we might as well proceed on the assumption that it is not, and happily enough it’s been working out thus far.)

  [1] https://pdg.lbl.gov/2025/reviews/rpp2025-rev-history-plots.p...

  • dingdongditchme

    today at 8:07 AM

    Thank you for the nice read. I empathize with many of your points, we are standing on the shoulders of giants. I refute on the claim around "our greatest societal ills". I think there is a difference between confident communication and being listened to. I have many a times said confidently "I don't know", as I have made decisions on low confident bets but leant into them with all my heart. Sometimes it paid of and sometimes it did not. It has served me well in my career and in life. As a scientist at heart I still agree that too often confidence is given too much weight and the quiet voice in the room should also be heard. However, we should teach everybody to communicate confidently even if they sometimes communicate wrongly. Of course we should not confuse confidence with credibility and accept that we know little for sure and are all just trying our best with the very limited understanding we have of our universe.

• today at 5:25 AM

• i_think_so

  today at 4:39 AM

  It might also be nice if cosmologists stopped claiming their Big Bang "Theory" wasn't more accurately termed a mere Hypothesis. IIRC, 12 out of 13 predictions failing and necessitating "model" "tweaks" is not a fantastic track record for a Theory, which are supposed to robustly survive investigation.

  • dudisubekti

    today at 4:53 AM

    We can literally observe cosmic microwave background and it fits our prediction that the universe was denser and hotter. It is a scientific theory.

    You might be confusing the established big bang with the more speculative cosmic inflation model. They're very closely related.

    • i_think_so

      today at 9:18 AM

      > We can literally observe cosmic microwave background and it fits our prediction that the universe was denser and hotter.

      I can't observe that, because I don't have the gear. (Nor the time, budget, inclination nor training, for that matter. :-) But I am happy to admit the possibility that some of those observations, as reported in the literature, are correct.

      However, unlike a depressingly large percentage of my former scientific colleagues, I also appreciate just how much of what gets reported in the literature, from the conclusions all the way back to the raw data, is anything from sloppily wrong to flat out lies. Witness the decades-long fiasco in genetics that is only this past month being corrected:

      Before: https://www.nature.com/articles/437047a

      After: https://www.nature.com/articles/s41586-025-08816

      TLDR: The original work by the CSAC reported only a fraction of the actually relevant data and hid the remainder where nobody was going to look. This was not the kind of Reproducibility Crisis mess, where an undergrad isn't paying attention when he grabs the electrophoresis gel off the shelf and then writes down the wrong brand name in his lab book. This was fraud. They intentionally misrepresented the data and hence the conclusions by an order of magnitude, which allowed them to delude the whole world for decades that "humans are 98.8% the same as chimps!"

      Many people had their entire worldview swayed by this pronouncement, myself included. I don't like being lied to.

      So yeah, you'll have to forgive me if I'm a bit skeptical when it comes to scientific observations and reportage that I'm a few $million shy for confirming myself. And I'll continue to think poorly of those who have been making lucrative careers out of doing "well-established" physics that "everybody" "accepts", only to have to quietly admit under scrutiny that their predictions didn't work out quite as nicely as the popular press has told us.

      Furthermore,

      > It is a scientific theory.

      It is a scientific hypothesis. It has not been subjected to repeated experimental trials or observations and found to be correct.

      A hypothesis does not become "well-established" simply because every college professor whose salary depends upon supporting the grant authority's narrative repeats it.

      I am fully aware that some people (present company excluded; I'm not placing any blame here) have watered down the definition of these terms. They are wrong. I do not consent to and will not be bullied into accepting changes to my language. Especially nothing as important as the language of science.

      > You might be confusing the established big bang with the more speculative cosmic inflation model. They're very closely related.

      Perhaps. I was never too terribly interested in things "smaller than an electron"[1] or larger than a whale.

      Lerner's arguments[2], particularly on relative elemental abundances, are persuasive to me. That may be because during my formative years I was a bit preoccupied with H vs D, because deuterated compounds for the NMR were too expensive for me to just play around with as I liked, so I had to tinker with spectroscopy/spectrophotometry instead. In any case, he's right. You can't have a cosmological constant be one value to account for the D and another value to account for the He3.

      As for the CMB, he addresses that as well, though once again I haven't done the work to confirm either side myself.

      Lerner has a whole basket full of other arguments as well, but I'm not a fan of lazy people posting Youtube links to hour long videos and saying "watch this!", so I shan't be a hypocrite. I believe that pdf should give a good flavor of it. It's been a while since I read it and I only skimmed it now, but I believe it a good representative sample of his other work.

      [1] PS: Yes, I know, I know. Stop being pedantic. This site is a hobby and I'm not about to cheat and get a chatbot to write me a 12 page essay every time I want to save a few words. I get to abuse quotation marks when I'm feeling lazy.

      [2] See sections II and IV in particular: https://web.archive.org/web/20260429053749/https://www.resea...

      • dudisubekti

        today at 10:27 AM

        I think you have too high an expectation of the scientific community.

        People work there, and it will have people's dramas and problems, like everywhere else: fraud, crime, jealousy, simple mistakes, etc.

        Despite their imperfections, the reason people with power trust their consensus more is because they are a lot more useful than other groups of people.

        If you reject this statement, you can start by joining the Amish, since virtually all modern technology is built on top of the scientific community's consensus and work.

        • i_think_so

          today at 11:44 AM

          > I think you have too high an expectation of the scientific community.

          Indeed, I did. I joined expecting it to be above mere politics. I paid for my folly.

          > People work there, and it will have people's dramas and problems, like everywhere else: fraud, crime, jealousy, simple mistakes, etc.

          Yes, but it's far worse in the scientific communities. In the "real world" the average person is way better at doing their average job than the average scientist publishing their work.

          Imagine even a civilizationally incompetent modern society today (without naming any names). Now imagine what it would be like if >50% of the time you got into a taxi something far worse than the expected result occurred. You got: taken to the wrong destination; or cheated by the driver; or woke up in a bathtub missing your kidneys; etc. Extend that behavior to even a tiny fraction of the whole. That society would have collapsed already.

          Compare that to any journal you please and let's see what percentage of their published works can be verified. Even for the "better" fields their rates are shockingly bad. Some are below 50%.

          I don't know about you, but my standards for the behavior of scientists are considerably higher than that for taxi drivers.

          > Despite their imperfections, the reason people with power trust their consensus more is because they are a lot more useful than other groups of people.

          Hard disagree. The reason people with power "trust" scientific consensus is because they manufacture that consensus by controlling the funding. This is fact. It's not pleasant, and is far from uncontroversial. But it is what it is.

          The people with power today who are telling you that <topic X> is "settled science" are the spiritual (and in some cases genetic!) descendants of the people who were telling Columbus that he was going to sail off the edge of the Earth and locking up Galileo. Eppur si muove!

          If you want to naively believe that calling one's self a big-s Scientist makes one, if not immune then perhaps we could say resistant, to that fraud, crime, corruption, etc. then I suppose you're entitled to that opinion. I look at the data on their output and my vote is to trash the lot of them and start over. They've fallen that far from grace.

          I'm inclined to trust the ancient Greeks, sure. Modern scientists, not so much.

          > If you reject this statement, you can start by joining the Amish, since virtually all modern technology is built on top of the scientific community's consensus and work.

          Unfortunately the Amish are automatically suspicious of the academically tainted such as myself. Otherwise I'd love to chill with them. Their lives are blissfully stress-free compared to ours.

      • suuuuuuuu

        today at 11:14 AM

        Your reference here is a 33 year old paper whose quoted observations and theoretical claims are totally out of date. The measured light element abundances are now consistent (and have been for decades).

        The black body distribution of the CMB is the (confirmed, of course) prediction of the Big Bang. The structure, age, etc. all depend on the cosmological model, and the claims that no such model can explain observations is ridiculous, given the counterexample of the \Lambda CDM model, the cornerstone of the field for decades now, that explains them all.

        It's almost impressive how obstinately you've convinced yourself of something so blatantly wrong and out of date, using only a reference predating the entire modern era of cosmology that you even admitted to not having read "for a while." A far, far cry from engaging seriously in a topic.

        Like with the frontier LLMs, seeing commentary on this site on topics that I'm an expert in makes me seriously doubt whether I should lend any credence at all to what's said about those that I'm not.

  • atoav

    today at 4:58 AM

    The model failing is a question of how accurately you want it to model the world.

    Many laypersons have absolutely no conception of how accurate those "failing" models were.

    A good example is Newtonian physics. Strictly speaking it is a failing model, after all, under certain conditions and if you look very closely ot falls apart. Yet, every bridge you ever walked on and the most precise mechanical watches ever made were all only calculated using newtonian physics. It is still accurate enough for most tasks on earth.

    A model can still be useful despite its limitations, you just need to know those. People who are like "Ha! It is not accurate!" often have their own mental models of the world which are magnitudes worse, miss key bits or get other parts completely wrong (despite clear evidence to the opposite). As if a morbidly obese person for whom even walking presents a challenge made fun of an Olympic silver medalist for only getting second place. "Ha! You didn't get it 100% right so now my fringe theory that fails to even explain the most basic observations must be seen as equally valid!"

    So if you say it fails, consider how many digits after the comma it was accurate before it failed and how many digits your own theory would manage.

    • pfdietz

      today at 11:37 AM

      Also, models are most interesting on the boundaries where they fail.

      Take for example Lord Kelvin's model of thermal conduction in a solid Earth. He used it to incorrectly predict the age of the Earth, but if he had taken that failure to heart he could have used it to predict mantle convection and plate tectonics.

    • dingdongditchme

      today at 8:14 AM

      This is what has always made it hard for me to go beyond the Newtonian physics. The only thing I know and use daily that relies on relativity is GPS and having looked into the equations on how it accounts for this it seemed to me that I could not discount that the equations account for some arbitrary consistent (or random) error, not relativity specifically. All experiments I have run never needed precision beyond Newtonian physics, but I am not at the end of my career yet so maybe relativity will become relevant some day. I will be looking forward to it if that is the case...

      • kergonath

        today at 9:07 AM

        You could well live your whole life without needing anything more than Newtonian Physics. For most of us, relativity is a fun thought experiment. If you want to grapple with it, special relativity is the answer to "how can the speed of light be constant regardless of the speed of whoever is measuring it?" In his "vulgarisation" books, Einstein explains it with nothing more sophisticated than trains and stopwatches.

        General relativity is more complex and quickly goes in complicated mathematical weeds but is just as profound from a philosophical point of view, which is that things do not merely affect other things around them, but instead change space-time itself. You can see with a couple of clicks observations of phenomena predicted by it, like black holes and gravitational lenses. It’s interesting to think about even if you are not directly affected.

      • eesmith

        today at 9:55 AM

        Once upon a time most households had a small particle accelerator, used daily. While the the electrons in the cathode ray tube (CRT) traveled at relativistic speeds (something like 0.1-0.3 c, from what I can tell), people did not need need to know about special relativity to change the channel on their TV.

        That said, those effects would have been small, and likely handled in practice as "some arbitrary consistent (or random) error."

    • kergonath

      today at 8:57 AM

      See the relativity of wrong by Asimov for a simple development of this theme.

  • mr_mitm

    today at 9:07 AM

    What are the 13 predictions? Can you list them or provide a link to that list?

    • i_think_so

      today at 9:28 AM

      Whew. This is work! :-)

      As I said in my effortpost above, the pdf I linked is a sample. There's more in his book, which I can't post here. And the videos he has put out are long, slow and some might find tedious, so I didn't bother to link them. (I didn't see your response while I was writing. Now I feel bad so I'm going to have to take a look and see what I can find that will post well.)

      A simple search for "big bang predictions" will find plenty of even mainstream press discussing them, albeit in a positive light, usually along the lines of "oh look, some scientists are talking about how they discovered something really interesting!" when what they really ought to be saying is "some scientists discovered that their hypotheses were wrong, their models failed to predict observable reality, and they were forced to make corrections that they shouldn't have to, if their hypotheses were actually a correct theory."

      As in so many different kinds of scientific endeavors, if your "theory" is based on a "model" and you have to keep correcting your "constants", they aren't constants, they are variables. And you don't have a theory.

  • eesmith

    today at 5:04 AM

    https://en.wikipedia.org/wiki/Big_Bang

    > The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. .. A wide range of empirical evidence strongly favors the Big Bang event, which is now widely accepted. ...

    > The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the light elements, the cosmic microwave background, large-scale structure, and Hubble's law.

    > Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations. ... Viable, quantitative explanations for such phenomena are still being sought. These are unsolved problems in physics.

    • fc417fc802

      today at 5:20 AM

      In reality it's just that the output of the procedural generation routines doesn't quite match that of the primary simulation loop. A classic worldbuilding inconsistency.

      • i_think_so

        today at 9:30 AM

        +1

        I got your joke and I appreciate the effort you put in to make it. :-)

      • eesmith

        today at 8:57 AM

        Even if your observation were 100% correct, it's also 100% irrelevant to the point.

        We still refer to Maxwell's theory of electromagnetism as a theory even though we know quantum electrodynamics is a more precise match to the primary simulation loop.

        I suppose a few might have decided to rename it Maxwell's hypothesis of electromagnetism, but I would consider them crackpots or dilettantes with little understanding of the meaning underlying those terms.