0
$\begingroup$

EDIT: I apparently need to rewrite this from the RE's, but I'll leave it for continuity sake. Please read my replies for my point/question clarifications.

There are several threads regarding big G and constants, though none address the fundamental question... Why do universal constants have the values they do?, Have the values of constants ever changed before?, Gravitational Constant in Newtonian Gravity vs. General Relativity

The closest is this, though its old and I am disallowed from commenting. What is the proof that the universal constants ($G$, $\hbar$, $\ldots$) are really constant in time and space? In that thread the OP has essentially the same (I think) question, though he slightly mis-worded it. The confusion in that thread was people responding to 'what is a constant / how is it determined'or 'do constants change in time/space' which I believe was NOT the question, NOR is my question.

Simply: how do we know / why are we assuming Newtons formula is universal, and not just describing gravity reasonably well on our frame? In other words, perhaps big G is itself a function, or even the whole formula needs tweaking. It stands to reason there may be alot more going on w gravity than what we empirically measure on our scale. We cannot use the orbits/behaviors of bodies, since those masses are estimated by the Newton equation and that is recursive.

EG/thought experiment: conduct an ideal Cavendish type experiment with neutron stars instead of lead balls. Repeat with individual atoms then mixed. Is G the same? If so, how is this proven?

$\endgroup$
  • $\begingroup$ But we don't assume that Newton's formula is universal. We know that it's approximately correct, but less accurate than general relativity. How do you propose doing a Cavendish type experiment with neutron stars, getting around the problem that we can only determine their masses from their orbital motions? $\endgroup$ – PM 2Ring Feb 6 at 3:04
  • $\begingroup$ @RoninISC It's fairly straightforward to assemble a ball with a mass of 1 kg without using any gravitational measurements. We know the mass of a silicon atom, because we have measured its charge-to-mass ratio in mass spectrometers and we know the charge on its nucleus. We know the crystal structure of silicon from X-ray diffraction, so we therefore know its density. Knowing the density, it's easy to compute the volume of a ball of mass 1 kg. Now make a ball with that volume. $\endgroup$ – probably_someone Feb 6 at 4:22
  • $\begingroup$ To the second half, a rhetorical re-- How did they determine the Cavendish balls' (or similar) masses where G was derived? To the first half-- G is considered an immutable Universal Constant. But then as you say the formula is not as accurate as GR, so then.... it/and or the formula is not very universal nor constant is it? This is the crux. $\endgroup$ – RoninISC Feb 6 at 4:54
  • $\begingroup$ Again, this is not about finding a more accurate measurement for G on our local scale. Im looking for something along the lines of various-- as greatly different as is feasible (hence the star and atom analogy)-- masses being tested w same test under as identical conditions as possible. Has/Is this done? Something equivalent / better? $\endgroup$ – RoninISC Feb 6 at 4:54
  • $\begingroup$ @probobly Awesome. And? $\endgroup$ – RoninISC Feb 6 at 6:02
3
$\begingroup$

The period of the Hulse-Taylor binary, a system 21,000 light-years away, is decreasing at just the rate predicted based on gravitational waves carrying away energy, using the standard value of $G$ that we measure in Earth laboratories.

Now, you might argue, “But we calculate the masses of the two stars in this system using the observed period and our standard value of $G$.” That’s true, but then we turn around and put these masses, and $G$, into the equation for the power radiated as gravitational waves, which depends on these parameters in a different way than the period does. So if the value if $G$ at this system were different from what it is on Earth, I don’t think the predicted rate of period change would match the observed rate.

This is good evidence that $G$ is the same throughout our galaxy.

I think similar arguments can be made that $G$ is the same a billion light years away (and therefore a billion years ago) based on LIGO’s initial detection of a gravitational waveform from merging black holes that matches theoretical predictions from General Relativity. The $G$ used in these models is of course the standard Earth-measured $G$, and the nonlinear dynamics of the inspiral and ringdown depend on $G$ and the masses in a complicated way which I think precludes $G$ having a different value there and then compared with here and now.

This system is a substantial fraction of the way across the expanse and history of the observable universe, and therefore suggests that $G$ is constant over a large swath of spacetime.

Finally, Wikipedia has this to say: “Under the assumption that the physics of type Ia supernovae are universal, analysis of observations of 580 type Ia supernovae has shown that the gravitational constant has varied by less than one part in ten billion per year over the last nine billion years.”

$\endgroup$
  • $\begingroup$ Starting to get somewhere. Again though, not talking about G variance with distance or 'history', only masses. So the relevant part is mass of HT Binary, which you say uses standard G. But I'm assuming you mean used in GR (yes its in there), which actually speaks to my point. The classic equation is off, just like every extreme gravity case, therefore G is only a constant to define units. $\endgroup$ – RoninISC Feb 6 at 19:41
  • $\begingroup$ SO can and shouldn't G, or the equation, be adjusted to match empirical data? Is it that hard or did people just stop trying after GR? I understand GR works better at extreme mass, but I think alot of baby got thrown out w bathwater. $\endgroup$ – RoninISC Feb 6 at 19:41
  • $\begingroup$ "put these masses, and G, into the equation for the power radiated as gravitational waves, which depends on these parameters in a different way than the period does." Can you expand on this a bit? mostly w focus on how its not still recursive ie: how this 'different way' is independent. $\endgroup$ – RoninISC Feb 6 at 20:02
  • 1
    $\begingroup$ The classic Newtonan equation $F=Gm_1m_2/r^2$ is accurate enough for use in the Cavendish experiment to measure $G$. $G$ is not “only a constant to define units”. It expresses the measured strength of gravity. Our units for mass, length, and time are all defined independent of $G$. $G$ gets “adjusted to match empirical data” on Earth when we do Cavendish and equivalent experiments. It does not get adjusted based on astronomical data because that data is consistent with $G$ being the same at all places, at all times, and (using GR equations) at all mass scales (until, perhaps, the Planck scale) $\endgroup$ – G. Smith Feb 6 at 21:43
2
$\begingroup$

There are a few different possibilities for the idea that "$G$ is a function": it could be a function of space and time, it could be a function of mass, or it could be a function of separation between the two masses.

This answer will exclude the case of extremely high masses, because we already know Newtonian gravity fails in that regime, and general relativity is needed. This answer will also exclude the case of extremely small separations, because we already know an as-yet-undeveloped theory of quantum gravity is needed to describe that regime. (This restriction actually unfortunately excludes one of the greatest tests we have of the constancy of $G$, namely, the slowing of the rotation rates of pulsars as they emit gravitational waves, but you seem to specifically be asking about Newtonian gravity, whereas that is purely a result of general relativity.)

It's also important to note that any measurement can only impose limits on how much that $G$ could change, and it's always technically possible that there is a subtle enough variation in $G$ that current measurements will not be able to detect it. This is why the concept of a "proof" that $G$ is constant everywhere is largely not something that can be reasonably expected. There is no evidence that it is not constant, and we have observed that it's constant to within certain limits, but asking for a proof that it's exactly constant under all circumstances is asking for a measurement with infinite precision. The constancy of $G$ is something that can only be disproved, if it is someday observed to vary. As of right now, we assume that it is constant because 1) predictions of the theories that assume this match observations to an extremely precise degree, 2) the symmetry that this uniformity provides is integral to the fundamental nature of how we think about reality (namely, translational symmetry of the laws of physics gives us conservation of energy via Noether's theorem), and 3) there isn't any evidence against it, despite there being a very large number of tests.

As such, this answer will mainly focus on which measurements would be different if there was a substantial change to $G$ under different circumstances.

Is $G$ a function of space and/or time?

If Newtonian gravity worked differently at other locations in our Solar System, then we would have seen it in the orbital trajectories of the objects of known mass that we have sent through the Solar System (like the New Horizons probe, for instance). This doesn't run into any recursive problems because we already know the mass of these objects, since we were the ones who made them.

If gravity worked substantially differently in other star systems within our galaxy, then stellar astrophysics wouldn't work in the same way that it does nearby ("nearby" meaning "within a few hundred light-years"). The life of a star is a constant battle between a few different forces: radiation pressure from nuclear fusion at the core and hydrostatic pressure from the fluid dynamics of the stellar interior push outward, and gravity pulls inward (in older stars, there is also electron degeneracy pressure, but we will consider only youngish, main-sequence stars here). If gravity was stronger, then stars of a given mass would be smaller and more dense; likewise, if gravity was weaker, then stars of a given mass would be bigger and less dense.

The luminosity of a star (the total radiation output) is directly related by the Stefan-Boltzmann law to two quantities: the temperature of the star and its size. We can measure the temperature of a star by observing its color, and we can measure the luminosity of a star by measuring its apparent brightness and distance. Both of these things would change for a star of a given mass. It turns out that we have plotted the luminosity vs. temperature of tens of thousands of stars on the Hertzsprung-Russell diagram, and this plot contains large empty regions. If the luminosity and temperature change in the right way due to this (stellar modeling is complicated, so I don't know precisely how they would change), you might see a bunch of stars from a particular location/time in an otherwise-empty region of the Hertzsprung-Russell diagram, which would indicate that there's something weird going on in that region of space/time. We don't currently see anything like this.

There is still the possibility that the luminosity and temperature would change in just the right way to keep this population of stars within the already-filled regions of the Hertzsprung-Russell diagram. In that case, we have another tool at our disposal: stellar spectra, namely, the width of the spectral lines in different populations of main-sequence stars. It turns out that for stars along the main sequence, we have a pretty good idea what their mass is, given their luminosity, from the appropriately-named mass-luminosity relation. This is important because, given the mass, temperature, and size of a star (where the size is derived from luminosity and temperature), you can determine the average pressure inside the star. A star with higher internal pressure has broader spectral lines - the atoms within it are perturbed more by their neighbors. So if gravity was stronger in a particular region of space/interval of time, we would see a population of stars that would have abnormally high pressures given their luminosity and temperature, and therefore would have abnormally broad spectral lines given their position in the Hertzsprung-Russell diagram. Once again, we have not seen any evidence of this.

If Newtonian gravity was substantially different in other galaxies, then there's a very important, quite visible event that might change: the type Ia supernova, which occurs when the electron degeneracy pressure in a white dwarf is insufficient to combat gravitational collapse. Due to the nature of electron degeneracy pressure, this basically always occurs once a white dwarf reaches a specific mass, called the Chandrasekhar limit. If gravity is stronger, this limit gets smaller, and white dwarfs explode with less energy. Importantly, we can see these supernovae from our galaxy, and we can monitor their apparent brightness with time; in fact, this is one of the ways we can calculate the distance to galaxies. If we can determine the distance by another means, like using redshift measurements, then we could easily see that the Type Ia supernovae from a particular region of space would seem to be abnormally dim, which would mean that the white dwarfs in that region could not get as massive before exploding, which means that the Chandrasekhar limit is different there and $G$ is higher (only making this conclusion after having accounted for other effects, of course). Even if we can't determine the distance by other means, it turns out that the more luminous the Type Ia supernova is, the slower its luminosity declines over time, so we would notice that supernovae from a particular region get dimmer abnormally quickly. Once again, we have not seen any evidence of this as of yet.

Is $G$ a function of the masses?

We have tested the gravitational attraction between two large bodies by examining the orbital motion of the Earth and Moon. We know the mass of the Earth because seismology and geology tell us the density of various parts of it, and we know the volume of those parts. Using the mass of the Earth, we then calibrated scales that we took to the Moon, which means we can also measure the mass of the Moon independent of the orbital motion. We have also tested the gravitational attraction between a large object and a very small one; ultracold neutrons are often kept in open-topped magnetic bowls for experiments, and gravity prevents them from escaping. If we were wrong about the value of $G$ in that experiment, we would notice something odd about the distribution of neutrons in the bowl. We have not yet measured the gravitational interaction between two extremely small objects, but that likely requires a theory of quantum gravity anyway. So, in all cases that we're able to measure, we haven't seen any difference in $G$ as a function of mass.

Is $G$ a function of separation?

Measurements of $G$ have been done at quite close separations, in various iterations of the Cavendish experiment. Measurements have also been done at medium ranges, by again examining the orbital motion of the Earth-Moon system. Measurements have also been done at the interstellar scale, since we are able to discern the luminosity, temperature, spectra, and hence masses of several nearby binary-star systems. None of these measurements seem to be inconsistent with a constant $G$ as a function of separation. At the intergalactic scale, things get a bit muddled due to the presence of dark matter; there are almost surely still a few modified-Newtonian-dynamics theories out there that haven't been completely ruled out by experimental evidence. That said, Newtonian gravity with a constant $G$ and dark matter explains the current experimental evidence very well, especially the evidence in the Bullet Cluster of a direct observation of an invisible lump of mass that produced gravitational lensing, which was the final nail in the coffin for many modified-gravity theories.

In general, these are far from the only tests that have been done; I merely wanted to provide a relatively straightforward example in each case.

$\endgroup$
  • $\begingroup$ Thanks for writing such a detailed post. "This answer will exclude the case of extremely high masses, because we already know Newtonian gravity fails in that regime, and general relativity is needed." True, and the crux of my post: G is a constant only with 'normal' masses. Cannot reworking of the terms/G (something more exponential with mass) match the empirical observation at all scales? Even if the number(s) seems meaningless now, at least it would work, and perhaps later an association would be found. $\endgroup$ – RoninISC Feb 6 at 20:56
  • $\begingroup$ Everything else in your re is variance w time/space (which I never questioned) or cases where masses are necessarily not extreme enough. $\endgroup$ – RoninISC Feb 6 at 20:56
  • $\begingroup$ @RoninISC General relativity matches all known empirical observations at all scales, including those without "normal" masses. It also uses a constant $G$. If you want to call that a "reworking of the terms," then you have your answer. $\endgroup$ – probably_someone Feb 7 at 1:17
  • $\begingroup$ @someone Useful info again, thanks. The point is to do so outside of GR. $\endgroup$ – RoninISC Feb 7 at 2:22
  • $\begingroup$ @RoninISC By "outside of GR" do you mean "as a small modification to GR", or do you mean "based on different fundamental postulates than GR"? $\endgroup$ – probably_someone Feb 7 at 2:43
1
$\begingroup$

1) I'm fairly certain we don't know about gravity on the atomic scale, because to understand this would need a correct theory of quantum gravity

2) In terms of large scale masses, I'm assuming you don't mean to actually reproduce the Cavendish experiment, as this experiment depends on suspension of the apparatus itself in a gravitational field, and I'm not sure we can get neutron stars here on Earth

3) I think the fairly accurate validation/prediction of the planets' orbits (with Newton's law of gravity doing fairly well and deviations corrected by GR) is a validation of $G$ using large masses on large scales.

$\endgroup$

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.