We still don't have a more precise value for "Big G" • Hacker News Board

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

#41

Post by Retric » Wed, Apr 29, 2026, 9:25 AM UTC

alkonaut wrote: post #26

> 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…

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.

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

#42

Post by i_think_so » Wed, Apr 29, 2026, 9:28 AM UTC

mr_mitm wrote: post #39

i_think_so wrote: post #8

Earlier quoted context omitted.

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.

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

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.

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

#43

Post by i_think_so » Wed, Apr 29, 2026, 9:30 AM UTC

fc417fc802 wrote: post #15

eesmith wrote: post #14

Earlier quoted context omitted.

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 elemen…

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.

+1

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

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

#44

Post by themafia » Wed, Apr 29, 2026, 9:32 AM UTC

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.

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

#45

Post by bowsamic » Wed, Apr 29, 2026, 9:36 AM UTC

mattclarkdotnet wrote: post #31

bowsamic wrote: post #25

Earlier quoted context omitted.

> 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

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

It can resolve them though

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

#46

Post by bowsamic » Wed, Apr 29, 2026, 9:37 AM UTC

kergonath wrote: post #34

bowsamic wrote: post #25

Earlier quoted context omitted.

> 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

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…

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

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

#47

Post by hilariously » Wed, Apr 29, 2026, 9:52 AM UTC

bowsamic wrote: post #46

kergonath wrote: post #34

Earlier quoted context omitted.

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…

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

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

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

#48

Post by eesmith » Wed, Apr 29, 2026, 9:55 AM UTC

dingdongditchme wrote: post #29

atoav wrote: post #12

Earlier quoted context omitted.

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 w…

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 Ne…

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."

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

#49

Post by dudisubekti » Wed, Apr 29, 2026, 10:27 AM UTC

i_think_so wrote: post #40

dudisubekti wrote: post #11

Earlier quoted context omitted.

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.

> 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 for…

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.

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

#50

Post by UncleSlacky » Wed, Apr 29, 2026, 10:38 AM UTC

imrehg wrote: post #30

ricksunny wrote: post #27

Earlier quoted context omitted.

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.

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...

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!