Source: <a href="https:&#x2F;&#x2F;www.nist.gov&#x2F;news-events&#x2F;news&#x2F;2026&#x2F;04&#x2F;nist-weighs-mystery-gravitational-constant" rel="nofollow">https:&#x2F;&#x2F;www.nist.gov&#x2F;news-events&#x2F;news&#x2F;2026&#x2F;04&#x2F;nist-weighs-my...</a> (4 points, 12 days ago) <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=47796892">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=47796892</a>

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.

Laying out the math (assuming earth is an homogeneous sphere) just in case it&#x27;s not clear:F_gravitational = G m1 m2 &#x2F;r^2g = G Mass_earth &#x2F; r_earth^2Mass_earth = r_earth^2 * g&#x2F;GDensity_earth = r_earth^2 * g&#x2F;G &#x2F; V_earthDensity_earth = 3*g &#x2F; (4*Pi*G*r_earth)Prior to Cavendish we already new g and r_earth, just missing G.

Yup, that&#x27;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 &quot;g&quot; gravitational acceleration value)- then from the mass and the otherwise measured size of Earth the density pops outMore details in good ol&#x27; Wikipedia: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Cavendish_experiment#Derivation_of_G_and_the_Earth&#x27;s_mass" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Cavendish_experiment#Derivatio...</a>

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?

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.

&gt; 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&#x27;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&#x27;t Cavendish&#x27;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&#x27;m probably missing?

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&#x2F;mass*time and the speed of light is 1e9 length&#x2F;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&#x2F;pi units of conductance and the inverse conductance quantum is pi units of resistance. Which is somewhat funny.

The paper is <a href="https:&#x2F;&#x2F;iopscience.iop.org&#x2F;article&#x2F;10.1088&#x2F;1681-7575&#x2F;ae570f" rel="nofollow">https:&#x2F;&#x2F;iopscience.iop.org&#x2F;article&#x2F;10.1088&#x2F;1681-7575&#x2F;ae570f</a>

Figure 1 in the paper helps contextualize the numbers better.

&quot;Please don&#x27;t fulminate.&quot;<a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;newsguidelines.html">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;newsguidelines.html</a>

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.

&gt; he says he just takes it on faith that it has been the same foreverIt’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.

&gt; Nobody can ever know an ultimate why, for obvious and well established philosophical reasonsYes 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

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.

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 &quot;our greatest societal ills&quot;. I think there is a difference between confident communication and being listened to. I have many a times said confidently &quot;I don&#x27;t know&quot;, 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.

&gt; 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] <a href="https:&#x2F;&#x2F;pdg.lbl.gov&#x2F;2025&#x2F;reviews&#x2F;rpp2025-rev-history-plots.pdf" rel="nofollow">https:&#x2F;&#x2F;pdg.lbl.gov&#x2F;2025&#x2F;reviews&#x2F;rpp2025-rev-history-plots.p...</a>

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&#x27;re very closely related.

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 &quot;failing&quot; 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 &quot;Ha! It is not accurate!&quot; 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. &quot;Ha! You didn&#x27;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!&quot;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.

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

<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Big_Bang" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Big_Bang</a>&gt; 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. ...&gt; 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&#x27;s law.&gt; 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.

It might also be nice if cosmologists stopped claiming their Big Bang &quot;Theory&quot; wasn&#x27;t more accurately termed a mere Hypothesis. IIRC, 12 out of 13 predictions failing and necessitating &quot;model&quot; &quot;tweaks&quot; is not a fantastic track record for a Theory, which are supposed to robustly survive investigation.

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.

We still don't have a more precise value for "Big G"