@Aonrathon said in #37:
> Gravity doesn't actually work as a theory.
To the contrary! Even Newtonian gravity (which is only an approximation to Einstein's general theory of relativity), works exceptionally well. Newtonian gravity allowed astronomers to make sense of the entire solar system (save for the precession of the perihelion of mercury). Newton's theory of gravitation can beautifully explain Kepler's first and third laws of planetary motion (both of which are solidly supported by observations), namely that a planet orbits the sun on an ellipse with the sun located at one of the foci and that the square of the orbital period of a planet is proportional to the cube of the major axis of that planet's orbit.
The first law follows directly from setting up the appropriate (vectorial) equation of motion in heliocentric relative coordinates setting F = m*a equal to Newton's force of gravity and solving analytically. Differentiating the angular momentum vector with respect to time and using both the equation of motion and the properties of the vector product it's easy to show that the planet orbits in a well-defined, invariable plane (the planet's velocity vector is always perpendicular to its angular momentum vector which is constant). The general solution of the equation of motion (in polar coordinates (r, f) where r is the distance and f is the true anomaly) corresponds to the four conic sections (circle, ellipse, parabola, hyperbola):
r(f) = ([(L/m)^2]/[G*(m + M)])/(1 + e*cos(f))
The energy integral h = (1/2) v^2 - G(m + M)/r, incidentally another conserved quantity, then determines the eccentricity and thus shape of the orbit with bounded systems exhibiting either circular or more commonly elliptical orbits (orbits are only ever approximately circular in reality). In bounded systems the planet's kinetic energy is less than its gravitational potential energy, the total energy is negative. This is (approximately) true for all planets and asteroids and comets in the solar system. They all orbit the sun on ellipses. Parabolic orbits are theoretically assumed in the limiting case as h = 0 (which in reality is only ever approximately the case, e.g. for highly eccentric comet orbits). Unbounded systems where h is positive, i.e. the object's kinetic energy exceeds the gravitational potential energy in the sun's gravitational field, have positive total energy and hyperbolic trajectories. ʻOumuamua and the rogue comet 2I/Borisov are examples for such objects as are the man-made Voyager space probes. All of their trajectories are observed to be hyperbolae. Such objects escape the solar system.
Kepler's third law follows from the equation for the area velocity of the planet, separation of variables and integrating over an entire period. Using a few auxiliary relations the third law can be shown.
Kepler's second law is much more general and independent from gravity, it even holds for other central forces. The fact that a planet sweeps out equal areas in equal times is a consequence of the conservation of angular momentum (which itself follows from a continuous symmetry in the system's Lagrangian, more specifically from its symmetry under continuous rotation or – stated another way – from direction-wise isotropy of space). But that's not the point, is it?
The point is that gravitational theory explains most all (celestial mechanics) phenomena observed in the solar system. It can be used to accurately determine the geometrical shape of the orbits of eight planets, countless dwarf planets, thousands of discovered exoplanets and binary star systems, millions of asteroids and comets in our solar system, tens of thousands of man-made spacecraft and the occasional interstellar visitor like ʻOumuamua. It can predict where any object will be at a certain time and where the Earth will be a thousand years from now (and where it was when ancient Rome was founded). Pluto takes 248 years for a complete orbit around the sun. It was discovered in 1930, about 93 years ago, so humanity hasn't even seen a half of a Pluto orbit. Yet it conforms perfectly to the predictions of Newtonian gravity. Astronomers 90 years ago calculated where it would be now and they are right.
Everything so far has been a simplified account of celestial mechanics in the solar system of course. Only the two-body problem has been discussed where two bodies tug on each other gravitationally. Astronomers have long realised that this simplification is not accurate enough for real world use. Unfortunately the 3-body problem, let alone the n-body problem, has not been solved analytically. An arbitrary number of planets all tugging on each other, that gets pretty complicated. Nowadays numerical solutions can be found using computers. But in the days of old astronomers had to develop techniques to approximate the perturbative effects that other planets have on the two-body problem orbit of a certain planet of interest around the sun.
They came up with (classical) perturbation theory, a way to approximate the small effects of the slight gravitational pulls of the other planets (especially Jupiter) in ever more accurate power series expressions. Need extra precision? Just calculate the next order term in the power series. That way they could calculate how Jupiter and Saturn for example perturb and influence the trajectory of an outer planet like Uranus. In doing so several astronomers noted a discrepancy between the observed motion of Uranus and the one predicted by considering only the attraction of the sun and the perturbations caused by Jupiter (the most massive planet) and Saturn (the second most massive).
Some of them therefore concluded that there must be another planet – undiscovered as of yet – influencing the motion of planet Uranus. Some set out to calculate the position of that planet based on the orbital perturbations. French mathematician Urbain Le Verrier's calculation was the most accurate. The French academy of science wouldn't believe that he could predict the existence, current position, mass and orbit of an unknown planet and he therefore had difficulty in finding a French astronomer that would indulge him. Eventually he turned to Johann Gottfried Galle, an astronomer at the Berlin observatory. Upon receiving Le Verrier's letter Galle and an assistant started the search for this new planet right away. That same evening on the 24th of September 1846 after less than an hour of searching the sky they discovered Neptune, just 1º from the position Le Verrier had predicted it to be: en.wikipedia.org/wiki/Discovery_of_Neptune#Discovery_observation:_24_September_1846
This is physics at its best! Urbain Le Verrier used Newton's gravitational theory and perturbation theory in order to predict the future, in order to predict the unknown, a planet that no-one else knew of at the time. All based on a slight anomaly in the orbit of Uranus. A similar situation with the precession of Mercury's perihelion would later lead Le Verrier astray and even later allow Einstein to test his theory of general relativity.
> That's why dark matter had to be invented, with literally no evidence of it. That is the actual definition of so-called "dark matter" - there is nothing there. Nothing to see or observe in a scientific sense.
>
> "Dark matter" is nothing but the admission of % error in the theory of gravity.
I don't know where you've heard this but it's simply not true that there is no evidence for dark matter. There is plenty of observational evidence for dark matter. Of course there's no direct observational evidence for dark matter. Just as there wasn't any direct observational evidence for the faint and distant planet Neptune when Le Verrier made his predictions. There was only indirect evidence for its existence in the anomalies it caused in the orbit of a visible body, namely planet Uranus.
And in the same way there is plenty of indirect evidence for the existence of dark matter in the anomalies it causes on the orbits and distribution of visible matter.
Some of the oldest evidence is Fritz Zwicky's application of the Virial theorem to the Coma Cluster: en.wikipedia.org/wiki/Virial_theorem#Dark_matter
The same can be done with velocity dispersions of stars in elliptical galaxies. Rotation curves of galaxies also show anomalous behaviour that could be explained by invisible matter further out of the galaxy (just as the anomalous orbit of Uranus was explained by an unseen piece of matter further out in the solar system).
The Bullet Cluster provides very strong indirect evidence for dark matter as well.
And recently a galaxy (NGC 1052-DF2) has been discovered that is deficient in dark matter, i.e. has no or very little dark matter. This is not yet entirely certain, but it might help put a nail in the coffin of modified gravity models that try to account for anomalous rotation curves of galaxies by means of altered laws of gravitation on large scales instead of proposed dark matter halos. If the laws of gravitation really worked differently on larger scales than our solar system, this dark matter deficient galaxy should be no exception, it's certainly large scale so it should work the same. And assuming that the modified laws of gravity are laws of nature that do not apply everywhere in the universe may not only amount to special pleading but may also make physics as an enterprise absurd.
If however dark matter is real and does exist, it's at least plausible to find an oddball galaxy which has somehow lost all or most of its dark matter.
> Gravity does not explain anything in our observable universe.
Let me put it this way: Your phone's GPS would not work at all if that sentence were true. Gravitational time dilation is a real and measurable effect of general relativity and has to be accounted for when comparing clocks on the ground and on GPS satellites (which is necessary in order to calculate the round-trip time taken by the signal from which the distance can be accurately deduced and the position can be determined if several satellites are used).
> Gravity doesn't actually work as a theory.
To the contrary! Even Newtonian gravity (which is only an approximation to Einstein's general theory of relativity), works exceptionally well. Newtonian gravity allowed astronomers to make sense of the entire solar system (save for the precession of the perihelion of mercury). Newton's theory of gravitation can beautifully explain Kepler's first and third laws of planetary motion (both of which are solidly supported by observations), namely that a planet orbits the sun on an ellipse with the sun located at one of the foci and that the square of the orbital period of a planet is proportional to the cube of the major axis of that planet's orbit.
The first law follows directly from setting up the appropriate (vectorial) equation of motion in heliocentric relative coordinates setting F = m*a equal to Newton's force of gravity and solving analytically. Differentiating the angular momentum vector with respect to time and using both the equation of motion and the properties of the vector product it's easy to show that the planet orbits in a well-defined, invariable plane (the planet's velocity vector is always perpendicular to its angular momentum vector which is constant). The general solution of the equation of motion (in polar coordinates (r, f) where r is the distance and f is the true anomaly) corresponds to the four conic sections (circle, ellipse, parabola, hyperbola):
r(f) = ([(L/m)^2]/[G*(m + M)])/(1 + e*cos(f))
The energy integral h = (1/2) v^2 - G(m + M)/r, incidentally another conserved quantity, then determines the eccentricity and thus shape of the orbit with bounded systems exhibiting either circular or more commonly elliptical orbits (orbits are only ever approximately circular in reality). In bounded systems the planet's kinetic energy is less than its gravitational potential energy, the total energy is negative. This is (approximately) true for all planets and asteroids and comets in the solar system. They all orbit the sun on ellipses. Parabolic orbits are theoretically assumed in the limiting case as h = 0 (which in reality is only ever approximately the case, e.g. for highly eccentric comet orbits). Unbounded systems where h is positive, i.e. the object's kinetic energy exceeds the gravitational potential energy in the sun's gravitational field, have positive total energy and hyperbolic trajectories. ʻOumuamua and the rogue comet 2I/Borisov are examples for such objects as are the man-made Voyager space probes. All of their trajectories are observed to be hyperbolae. Such objects escape the solar system.
Kepler's third law follows from the equation for the area velocity of the planet, separation of variables and integrating over an entire period. Using a few auxiliary relations the third law can be shown.
Kepler's second law is much more general and independent from gravity, it even holds for other central forces. The fact that a planet sweeps out equal areas in equal times is a consequence of the conservation of angular momentum (which itself follows from a continuous symmetry in the system's Lagrangian, more specifically from its symmetry under continuous rotation or – stated another way – from direction-wise isotropy of space). But that's not the point, is it?
The point is that gravitational theory explains most all (celestial mechanics) phenomena observed in the solar system. It can be used to accurately determine the geometrical shape of the orbits of eight planets, countless dwarf planets, thousands of discovered exoplanets and binary star systems, millions of asteroids and comets in our solar system, tens of thousands of man-made spacecraft and the occasional interstellar visitor like ʻOumuamua. It can predict where any object will be at a certain time and where the Earth will be a thousand years from now (and where it was when ancient Rome was founded). Pluto takes 248 years for a complete orbit around the sun. It was discovered in 1930, about 93 years ago, so humanity hasn't even seen a half of a Pluto orbit. Yet it conforms perfectly to the predictions of Newtonian gravity. Astronomers 90 years ago calculated where it would be now and they are right.
Everything so far has been a simplified account of celestial mechanics in the solar system of course. Only the two-body problem has been discussed where two bodies tug on each other gravitationally. Astronomers have long realised that this simplification is not accurate enough for real world use. Unfortunately the 3-body problem, let alone the n-body problem, has not been solved analytically. An arbitrary number of planets all tugging on each other, that gets pretty complicated. Nowadays numerical solutions can be found using computers. But in the days of old astronomers had to develop techniques to approximate the perturbative effects that other planets have on the two-body problem orbit of a certain planet of interest around the sun.
They came up with (classical) perturbation theory, a way to approximate the small effects of the slight gravitational pulls of the other planets (especially Jupiter) in ever more accurate power series expressions. Need extra precision? Just calculate the next order term in the power series. That way they could calculate how Jupiter and Saturn for example perturb and influence the trajectory of an outer planet like Uranus. In doing so several astronomers noted a discrepancy between the observed motion of Uranus and the one predicted by considering only the attraction of the sun and the perturbations caused by Jupiter (the most massive planet) and Saturn (the second most massive).
Some of them therefore concluded that there must be another planet – undiscovered as of yet – influencing the motion of planet Uranus. Some set out to calculate the position of that planet based on the orbital perturbations. French mathematician Urbain Le Verrier's calculation was the most accurate. The French academy of science wouldn't believe that he could predict the existence, current position, mass and orbit of an unknown planet and he therefore had difficulty in finding a French astronomer that would indulge him. Eventually he turned to Johann Gottfried Galle, an astronomer at the Berlin observatory. Upon receiving Le Verrier's letter Galle and an assistant started the search for this new planet right away. That same evening on the 24th of September 1846 after less than an hour of searching the sky they discovered Neptune, just 1º from the position Le Verrier had predicted it to be: en.wikipedia.org/wiki/Discovery_of_Neptune#Discovery_observation:_24_September_1846
This is physics at its best! Urbain Le Verrier used Newton's gravitational theory and perturbation theory in order to predict the future, in order to predict the unknown, a planet that no-one else knew of at the time. All based on a slight anomaly in the orbit of Uranus. A similar situation with the precession of Mercury's perihelion would later lead Le Verrier astray and even later allow Einstein to test his theory of general relativity.
> That's why dark matter had to be invented, with literally no evidence of it. That is the actual definition of so-called "dark matter" - there is nothing there. Nothing to see or observe in a scientific sense.
>
> "Dark matter" is nothing but the admission of % error in the theory of gravity.
I don't know where you've heard this but it's simply not true that there is no evidence for dark matter. There is plenty of observational evidence for dark matter. Of course there's no direct observational evidence for dark matter. Just as there wasn't any direct observational evidence for the faint and distant planet Neptune when Le Verrier made his predictions. There was only indirect evidence for its existence in the anomalies it caused in the orbit of a visible body, namely planet Uranus.
And in the same way there is plenty of indirect evidence for the existence of dark matter in the anomalies it causes on the orbits and distribution of visible matter.
Some of the oldest evidence is Fritz Zwicky's application of the Virial theorem to the Coma Cluster: en.wikipedia.org/wiki/Virial_theorem#Dark_matter
The same can be done with velocity dispersions of stars in elliptical galaxies. Rotation curves of galaxies also show anomalous behaviour that could be explained by invisible matter further out of the galaxy (just as the anomalous orbit of Uranus was explained by an unseen piece of matter further out in the solar system).
The Bullet Cluster provides very strong indirect evidence for dark matter as well.
And recently a galaxy (NGC 1052-DF2) has been discovered that is deficient in dark matter, i.e. has no or very little dark matter. This is not yet entirely certain, but it might help put a nail in the coffin of modified gravity models that try to account for anomalous rotation curves of galaxies by means of altered laws of gravitation on large scales instead of proposed dark matter halos. If the laws of gravitation really worked differently on larger scales than our solar system, this dark matter deficient galaxy should be no exception, it's certainly large scale so it should work the same. And assuming that the modified laws of gravity are laws of nature that do not apply everywhere in the universe may not only amount to special pleading but may also make physics as an enterprise absurd.
If however dark matter is real and does exist, it's at least plausible to find an oddball galaxy which has somehow lost all or most of its dark matter.
> Gravity does not explain anything in our observable universe.
Let me put it this way: Your phone's GPS would not work at all if that sentence were true. Gravitational time dilation is a real and measurable effect of general relativity and has to be accounted for when comparing clocks on the ground and on GPS satellites (which is necessary in order to calculate the round-trip time taken by the signal from which the distance can be accurately deduced and the position can be determined if several satellites are used).
@Thalassokrator said in #42:
Your post is such a massive wall of coping BS it's unbelievable. You even admit:
> Of course there's no direct observational evidence for dark matter.
Gravity does not work as a theory. What we can observe in the universe does not align with the proposed theory of gravitation. Period. Something in the current scientific world view is completely wrong.
Inventing invisible matter that hold everything in place is NOT scientific. It's religious faith-based adherence that the proposed theory MUST be true for dogmatic reasons. That's the reason your post reads like someone text-dumping about their faith.
Your post is such a massive wall of coping BS it's unbelievable. You even admit:
> Of course there's no direct observational evidence for dark matter.
Gravity does not work as a theory. What we can observe in the universe does not align with the proposed theory of gravitation. Period. Something in the current scientific world view is completely wrong.
Inventing invisible matter that hold everything in place is NOT scientific. It's religious faith-based adherence that the proposed theory MUST be true for dogmatic reasons. That's the reason your post reads like someone text-dumping about their faith.
How much space are we permitted to use here? I have a rather long reply I am working on. I'm not done yet, but this is the first part.
The Theory of Ground
It began late one September many autumns ago, when we were young and easy under the apple boughs. Only in this case, it wasn’t an apple tree. No, that’s another, similar story with a different ending. I believe it was an oak, and it was an acorn, not an apple that fell. And fall it did, upon my young brother’s head, and so gave rise to a nasty bruise and a whole new field of physics, metaphysics, or religion, take your pick.
My younger brother was only about 5 or 6 at the time, and I a year older and not noticeably wiser. I was then occupied with other matters of personal significance, whilst my brother lounged and daydreamed under a tree. A tree which, I repeat, was probably an oak, and certainly not an apple tree. While he lay and pondered whatever came into his head at that time, a squirrel was gathering nuts high overhead. As squirrels will, when the high blue sky and brilliant sunshine of Indian summer promises a frosty November. The method of the squirrel, so far as I can tell, is not to gather as many nuts as it can and carry them to its nest, but rather to harvest as many as it can and let them drop to the ground. Then to go down and gather them up and cart them away to wherever. And so the squirrel was dropping acorns from the tree, and one dropped upon my young brother’s head. On which it bounced.
It bounced a couple of times, before it rolled off into the grass. My young brother gazed at the acorn as it rolled off into the gras, then up at the offending squirrel. He may have said a few choice words, and no doubt the little rodent answered back, and perhaps the two of them carried on a brief discussion, but after a few minutes the squirrel decided to leave the argument and scampered and leapt from branch to branch and tree to tree and out of this story. But before leaving entirely, it did add one footnote to this story. As it jumped from one branch to another, it’s hind claws tore one small twig and cluster of leaves from the tree, and that twig and cluster of leaves fell. And my brother watched that cluster as it fell.
A brief aside, which may or may not explain what followed. When I was a student at university, many years ago, I had a professor who claimed that that most unusual class of people known as geniuses were only know to be derived from 3 sources: lunatics, drunkards (he jerked a thumb at his own breast as he said this), and children. I raised a querulous eye for a moment, but when he asked me if I disagreed, I indicated my full agreement. I wasn’t going to tell the man who would in a few weeks be grading me that it takes more than just being a drunk to become a genius. However hard he was trying. Getting to the point however; my brother is not noticeably madder than most people, and certainly no fonder of strong drink. But he was at that time very much a child, which may or not explain his fascination with that falling cluster of leaves as it swirled and helicoptered its way to the ground.
I'll finish the story after work this evening.
The Theory of Ground
It began late one September many autumns ago, when we were young and easy under the apple boughs. Only in this case, it wasn’t an apple tree. No, that’s another, similar story with a different ending. I believe it was an oak, and it was an acorn, not an apple that fell. And fall it did, upon my young brother’s head, and so gave rise to a nasty bruise and a whole new field of physics, metaphysics, or religion, take your pick.
My younger brother was only about 5 or 6 at the time, and I a year older and not noticeably wiser. I was then occupied with other matters of personal significance, whilst my brother lounged and daydreamed under a tree. A tree which, I repeat, was probably an oak, and certainly not an apple tree. While he lay and pondered whatever came into his head at that time, a squirrel was gathering nuts high overhead. As squirrels will, when the high blue sky and brilliant sunshine of Indian summer promises a frosty November. The method of the squirrel, so far as I can tell, is not to gather as many nuts as it can and carry them to its nest, but rather to harvest as many as it can and let them drop to the ground. Then to go down and gather them up and cart them away to wherever. And so the squirrel was dropping acorns from the tree, and one dropped upon my young brother’s head. On which it bounced.
It bounced a couple of times, before it rolled off into the grass. My young brother gazed at the acorn as it rolled off into the gras, then up at the offending squirrel. He may have said a few choice words, and no doubt the little rodent answered back, and perhaps the two of them carried on a brief discussion, but after a few minutes the squirrel decided to leave the argument and scampered and leapt from branch to branch and tree to tree and out of this story. But before leaving entirely, it did add one footnote to this story. As it jumped from one branch to another, it’s hind claws tore one small twig and cluster of leaves from the tree, and that twig and cluster of leaves fell. And my brother watched that cluster as it fell.
A brief aside, which may or may not explain what followed. When I was a student at university, many years ago, I had a professor who claimed that that most unusual class of people known as geniuses were only know to be derived from 3 sources: lunatics, drunkards (he jerked a thumb at his own breast as he said this), and children. I raised a querulous eye for a moment, but when he asked me if I disagreed, I indicated my full agreement. I wasn’t going to tell the man who would in a few weeks be grading me that it takes more than just being a drunk to become a genius. However hard he was trying. Getting to the point however; my brother is not noticeably madder than most people, and certainly no fonder of strong drink. But he was at that time very much a child, which may or not explain his fascination with that falling cluster of leaves as it swirled and helicoptered its way to the ground.
I'll finish the story after work this evening.
@Aonrathon said in #43:
> Your post is such a massive wall of coping BS it's unbelievable. You even admit:
>
>> Of course there's no direct observational evidence for dark matter.
Well, of course I would admit that. It's true! Why didn't you include the next few sentences in the quote though? Especially:
> And in the same way there is plenty of indirect evidence for the existence of dark matter in the anomalies it causes on the orbits and distribution of visible matter.
Dark matter is not alone in being inferred from indirect evidence:
There's also no direct observational evidence for the atomic nucleus! It's too small to be seen by any means. The best optical microscopes do not even come close to being able to observe an individual atom, let alone a nucleus. And even electron microscopes cannot image a nucleus. Yet nuclei and their properties can be studied extensively via indirect means. Mostly via scattering experiments where an electron is repeatedly shot at a nucleus and the scattering angles and energies are observed. This is indirect evidence for the nucleus. It makes itself known through the deflection and momentum transfer to the electron which then hits a particle detector producing a macroscopic signal which can be read out.
Nuclear theory is very successful in predicting the phenomena of the microscopic world, so much so that it allows for nuclear power plants and nuclear weapons to be built. And they actually work. Even though nobody has ever directly observed a nucleus.
It's foolish to totally discount something due to a lack of direct observational evidence while ignoring heaps of indirect observational evidence. You do not have to observe something directly in order to infer that it must exist or in order to find out its properties. Planet Neptune's existence, mass and orbit was inferred from indirect evidence alone. The same with atomic nuclei.
> Gravity does not work as a theory.
You keep asserting that, yet you ignore the myriad of observational lines of evidence that support gravitational theory:
en.wikipedia.org/wiki/Tests_of_general_relativity
Physicists know that general relativity (GR) must be wrong in some way as it doesn't mix well with quantum mechanics. A quantum theory of gravity is needed. However whatever precision tests have been performed thus far, the result has always been the same: they have failed to falsify the predictions of general relativity. The theory works too well. No matter what you throw at it, orbits of all solar system objects, binary stars, exoplanets, stars around the supermassive black hole in the centre of the Milky Way, gravitational redshift (on scales ranging from thousands of kilometres to just a single centimetre), gravitational time dilation, orbit of mercury, gravitational lensing, frame dragging, gravitational waves, size and shape of a supermassive black hole's accretion disk. The predictions all fit perfectly within observational error. It's frustrating really. GR works splendidly for systems with little space-time curvature (like our solar system) AND for systems with extreme curvature (orbiting black holes).
Hold on, you will say: It fails when predicting rotation curves of galaxies or velocity dispersion of stars (etc.). Yes! But only if you assume that what we currently see with our telescopes is all there is in the universe. By the same token Le Verrier could have dismissed Newtonian gravity and perturbation theory since it didn't predict the orbit of Uranus based on what planets astronomers had thus far been able to see in our solar system.
Once you add dark matter, GR can describe all observed gravitational phenomena in the universe without any inconsistency, galaxy rotation and all. And given strange, extremely weakly interacting particles like neutrinos, a particle physics solution to the mystery of dark matter doesn't seem implausible at least.
There's of course doubt whether or not dark matter really exists, despite the great number of indirect lines of evidence. Le Verrier, after all, later also unsuccessfully predicted another planet, Vulcan, by the same means which he thought accounted for the anomalous precession of Mercury's perihelion. The reason you probably haven't heard of this planet is that it doesn't exist (it has not been discovered in the past 200 years, despite having been predicted to be very close to the sun and therefore easy to see). In this case it turned out that Einstein was needed since Newton's theory of gravitation didn't have sufficient accuracy to describe the strong gravitational attraction close to the sun (where Mercury is situated) adequately. It predicted a perihelion precession which was ever so slightly less than that observed. The tiny (but significant) anomaly was fully explained by general relativity which predicts the observed perihelion precession for Mercury's orbit. In that case no extra hidden mass was found, instead the Newtonian theory of gravitation failed ever so slightly (it was still approximately correct though).
In principle it seems possible that GR might fail similarly at large scales (instead of Vulcan-like additional dark matter being present). That's why there are theorists who try to find a modified theory of gravity which behaves the same as GR on small scales but differs significantly on large scales. This is a very tough ask and such models regularly have difficulty accounting for all purported dark matter phenomena (although it has to be said that their proponents would disagree).
In particular, if the odd galaxy called NGC 1052-DF2 despite indubitably being a large scale object really turns out not to contain any appreciable amount of dark matter whatsoever, i.e. it is NOT anomalous with regard to GR's naive predictions based on the observable matter (total mass of the stars) alone, this poses a significant problem for such modified gravity attempts. Dark matter might reasonably be absent from a few galaxies. But if gravity depends on the scale of the object, this galaxy should be as anomalous as every other galaxy of its size. But it appears not to be. We'll see.
> What we can observe in the universe does not align with the proposed theory of gravitation. Period. Something in the current scientific world view is completely wrong.
Nope. What we can DIRECTLY observe does not align with the proposed theory of gravitation. This is an important distinction as the difficulty of directly observing some natural phenomena (faint planets, minuscule atomic nuclei or subatomic particles, weakly interacting particles like neutrinos) cannot be overstated. Saying "something in the current scientific world view is completely wrong" is throwing the baby out with the bathwater. It's also not productive if you don't propose alternatives. Lastly it becomes a true statement if you throw out the "completely" since the current scientific world view is and will always remain slightly wrong. Every model is wrong, some are useful. General relativity is extremely useful.
> Inventing invisible matter that hold everything in place is NOT scientific. It's religious faith-based adherence that the proposed theory MUST be true for dogmatic reasons.
You might as well say: "Inventing invisible nuclei to explain radioactivity is NOT scientific." Something doesn't need to be directly observable if the evidence pertaining to it is. If there is enough indirect evidence, its existence can reasonably be inferred.
Furthermore, theories are never true and no scientist (including astronomers who favour dark matter) claims that they are. True is not an attribute of scientific theories. Truth value is a thing of mathematics. In empirical science there is no way to prove a hypothesis true. Hypotheses that make testable predictions can be compared to experimental findings. An experimental (or observational) test is an attempt to falsify the hypothesis. The best thing science has to offer are failed falsification attempts of hypotheses. When the experiment shows a result that is consistent with the hypothesis within observational error/uncertainty. This does not mean that the hypothesis is true. Theories are collections of hypotheses for which all falsification attempts have failed.
Science can show that a hypothesis is false. When the experiment's result is inconsistent with the hypothesis, it is falsified.
However this is again a simplified account of how the scientific method works. In reality you always have bundles of hypotheses (according to the Duhem–Quine thesis). That is to say the hypothesis of interest and a set of auxiliary hypotheses. While the entire bundle of hypotheses can be tested and falsified via empirical experiment (as described above), individual hypotheses cannot be tested and falsified in isolation (without the auxiliary hypotheses).
A simplified example:
The hypothesis of interest is: 1. General relativity describes the rotation curve of the Milky Way galaxy.
And an auxiliary hypothesis might be: 2a. A dark matter halo around the Milky Way galaxy of such and such a mass and geometry exists.
This bundle of hypotheses can be tested against observational evidence and indeed observations are consistent with this bundle of hypotheses. GR + dark matter can account for the Milky Way's rotation curve.
Likewise, a proponent of modified gravity theories might propose a different bundle of hypotheses:
The hypothesis of interest is: 1. General relativity describes the rotation curve of the Milky Way galaxy.
And an auxiliary hypothesis might be: 2b. All (or at least the vast majority of) matter in the universe is readily visible to the telescopes of humanity after 400 years of telescope development.
This bundle of hypotheses can be tested against observational evidence and indeed observations are inconsistent with this bundle of hypotheses. GR + visible matter cannot account for the Milky Way's rotation curve.
Proponents of dark matter credit this to the assumed incorrectness of the auxiliary hypothesis 2b. while proponents of modified gravity theories credit this to the assumed incorrectness of the hypothesis of interest 1. They say that GR is falsified by this observational test while proponents of dark matter disagree.
> That's the reason your post reads like someone text-dumping about their faith.
I wrote stuff like: "This is not yet entirely certain, but it might help put a nail in the coffin of modified gravity models that try to account for anomalous rotation curves of galaxies by means of altered laws of gravitation on large scales instead of proposed dark matter halos."
I expressed uncertainty, I used "might" to indicate tentative findings and pointed out competing theories (modified theories of gravity) which can also account for (some) observations generally accredited to dark matter.
Sure, my post clearly favoured dark matter over those competing theories and pointed out ways in which these might be falsified in future. But do I take the existence of dark matter on faith? Surely not. I want to see evidence for it. I want particle physicists to discover axions or sterile neutrinos or other proposed dark matter candidate particles (WIMPs or the like). I want astronomers to try to rule out or vindicate even the weirdest edge case from rogue planets, to primordial black holes and brown dwarfs. A lot of stuff has already been ruled out. Alternatively I want to see modified gravity people present a gravitational theory that can explain everything that GR can explain (which is an awful lot) in addition to deceptive dark matter phenomena. It would be nice if they could do that using a quantum theory of gravity while they are at it! We need one anyways. Otherwise they could also falsify the dark matter hypothesis by finding anomalous behaviour that cannot be explained by any distribution of matter, dark or not (which hasn't happened as of yet). I'm open towards all of these possibilities.
Given the indirect evidence at the current time some sort of dark matter seems to be the most promising approach, but it might even be mix of both. Who am I to know what strange effects a quantum theory of gravity would have on large scales?
But I refuse not to acknowledge the usefulness of the current best theory of gravity, namely general relativity. Everybody is aware it's not the ultimate theory (no theory ever is), but it's frustratingly accurate. Using atomic clocks and GR's gravitational redshift effect you can even measure differences in elevation (relative to Earth's gravitational field) of centimetres or millimetres. This might allow for earthquake forecasts (predicting when and where the next big earthquake will occur) in the years to come:
See for example this excellent talk
www.youtube.com/watch?v=ydZ9OsWoemk
or the one entitled "Making optical lattice clocks compact and useful for real-world applications" by Hidetoshi Katori on the same YouTube channel.
> Your post is such a massive wall of coping BS it's unbelievable. You even admit:
>
>> Of course there's no direct observational evidence for dark matter.
Well, of course I would admit that. It's true! Why didn't you include the next few sentences in the quote though? Especially:
> And in the same way there is plenty of indirect evidence for the existence of dark matter in the anomalies it causes on the orbits and distribution of visible matter.
Dark matter is not alone in being inferred from indirect evidence:
There's also no direct observational evidence for the atomic nucleus! It's too small to be seen by any means. The best optical microscopes do not even come close to being able to observe an individual atom, let alone a nucleus. And even electron microscopes cannot image a nucleus. Yet nuclei and their properties can be studied extensively via indirect means. Mostly via scattering experiments where an electron is repeatedly shot at a nucleus and the scattering angles and energies are observed. This is indirect evidence for the nucleus. It makes itself known through the deflection and momentum transfer to the electron which then hits a particle detector producing a macroscopic signal which can be read out.
Nuclear theory is very successful in predicting the phenomena of the microscopic world, so much so that it allows for nuclear power plants and nuclear weapons to be built. And they actually work. Even though nobody has ever directly observed a nucleus.
It's foolish to totally discount something due to a lack of direct observational evidence while ignoring heaps of indirect observational evidence. You do not have to observe something directly in order to infer that it must exist or in order to find out its properties. Planet Neptune's existence, mass and orbit was inferred from indirect evidence alone. The same with atomic nuclei.
> Gravity does not work as a theory.
You keep asserting that, yet you ignore the myriad of observational lines of evidence that support gravitational theory:
en.wikipedia.org/wiki/Tests_of_general_relativity
Physicists know that general relativity (GR) must be wrong in some way as it doesn't mix well with quantum mechanics. A quantum theory of gravity is needed. However whatever precision tests have been performed thus far, the result has always been the same: they have failed to falsify the predictions of general relativity. The theory works too well. No matter what you throw at it, orbits of all solar system objects, binary stars, exoplanets, stars around the supermassive black hole in the centre of the Milky Way, gravitational redshift (on scales ranging from thousands of kilometres to just a single centimetre), gravitational time dilation, orbit of mercury, gravitational lensing, frame dragging, gravitational waves, size and shape of a supermassive black hole's accretion disk. The predictions all fit perfectly within observational error. It's frustrating really. GR works splendidly for systems with little space-time curvature (like our solar system) AND for systems with extreme curvature (orbiting black holes).
Hold on, you will say: It fails when predicting rotation curves of galaxies or velocity dispersion of stars (etc.). Yes! But only if you assume that what we currently see with our telescopes is all there is in the universe. By the same token Le Verrier could have dismissed Newtonian gravity and perturbation theory since it didn't predict the orbit of Uranus based on what planets astronomers had thus far been able to see in our solar system.
Once you add dark matter, GR can describe all observed gravitational phenomena in the universe without any inconsistency, galaxy rotation and all. And given strange, extremely weakly interacting particles like neutrinos, a particle physics solution to the mystery of dark matter doesn't seem implausible at least.
There's of course doubt whether or not dark matter really exists, despite the great number of indirect lines of evidence. Le Verrier, after all, later also unsuccessfully predicted another planet, Vulcan, by the same means which he thought accounted for the anomalous precession of Mercury's perihelion. The reason you probably haven't heard of this planet is that it doesn't exist (it has not been discovered in the past 200 years, despite having been predicted to be very close to the sun and therefore easy to see). In this case it turned out that Einstein was needed since Newton's theory of gravitation didn't have sufficient accuracy to describe the strong gravitational attraction close to the sun (where Mercury is situated) adequately. It predicted a perihelion precession which was ever so slightly less than that observed. The tiny (but significant) anomaly was fully explained by general relativity which predicts the observed perihelion precession for Mercury's orbit. In that case no extra hidden mass was found, instead the Newtonian theory of gravitation failed ever so slightly (it was still approximately correct though).
In principle it seems possible that GR might fail similarly at large scales (instead of Vulcan-like additional dark matter being present). That's why there are theorists who try to find a modified theory of gravity which behaves the same as GR on small scales but differs significantly on large scales. This is a very tough ask and such models regularly have difficulty accounting for all purported dark matter phenomena (although it has to be said that their proponents would disagree).
In particular, if the odd galaxy called NGC 1052-DF2 despite indubitably being a large scale object really turns out not to contain any appreciable amount of dark matter whatsoever, i.e. it is NOT anomalous with regard to GR's naive predictions based on the observable matter (total mass of the stars) alone, this poses a significant problem for such modified gravity attempts. Dark matter might reasonably be absent from a few galaxies. But if gravity depends on the scale of the object, this galaxy should be as anomalous as every other galaxy of its size. But it appears not to be. We'll see.
> What we can observe in the universe does not align with the proposed theory of gravitation. Period. Something in the current scientific world view is completely wrong.
Nope. What we can DIRECTLY observe does not align with the proposed theory of gravitation. This is an important distinction as the difficulty of directly observing some natural phenomena (faint planets, minuscule atomic nuclei or subatomic particles, weakly interacting particles like neutrinos) cannot be overstated. Saying "something in the current scientific world view is completely wrong" is throwing the baby out with the bathwater. It's also not productive if you don't propose alternatives. Lastly it becomes a true statement if you throw out the "completely" since the current scientific world view is and will always remain slightly wrong. Every model is wrong, some are useful. General relativity is extremely useful.
> Inventing invisible matter that hold everything in place is NOT scientific. It's religious faith-based adherence that the proposed theory MUST be true for dogmatic reasons.
You might as well say: "Inventing invisible nuclei to explain radioactivity is NOT scientific." Something doesn't need to be directly observable if the evidence pertaining to it is. If there is enough indirect evidence, its existence can reasonably be inferred.
Furthermore, theories are never true and no scientist (including astronomers who favour dark matter) claims that they are. True is not an attribute of scientific theories. Truth value is a thing of mathematics. In empirical science there is no way to prove a hypothesis true. Hypotheses that make testable predictions can be compared to experimental findings. An experimental (or observational) test is an attempt to falsify the hypothesis. The best thing science has to offer are failed falsification attempts of hypotheses. When the experiment shows a result that is consistent with the hypothesis within observational error/uncertainty. This does not mean that the hypothesis is true. Theories are collections of hypotheses for which all falsification attempts have failed.
Science can show that a hypothesis is false. When the experiment's result is inconsistent with the hypothesis, it is falsified.
However this is again a simplified account of how the scientific method works. In reality you always have bundles of hypotheses (according to the Duhem–Quine thesis). That is to say the hypothesis of interest and a set of auxiliary hypotheses. While the entire bundle of hypotheses can be tested and falsified via empirical experiment (as described above), individual hypotheses cannot be tested and falsified in isolation (without the auxiliary hypotheses).
A simplified example:
The hypothesis of interest is: 1. General relativity describes the rotation curve of the Milky Way galaxy.
And an auxiliary hypothesis might be: 2a. A dark matter halo around the Milky Way galaxy of such and such a mass and geometry exists.
This bundle of hypotheses can be tested against observational evidence and indeed observations are consistent with this bundle of hypotheses. GR + dark matter can account for the Milky Way's rotation curve.
Likewise, a proponent of modified gravity theories might propose a different bundle of hypotheses:
The hypothesis of interest is: 1. General relativity describes the rotation curve of the Milky Way galaxy.
And an auxiliary hypothesis might be: 2b. All (or at least the vast majority of) matter in the universe is readily visible to the telescopes of humanity after 400 years of telescope development.
This bundle of hypotheses can be tested against observational evidence and indeed observations are inconsistent with this bundle of hypotheses. GR + visible matter cannot account for the Milky Way's rotation curve.
Proponents of dark matter credit this to the assumed incorrectness of the auxiliary hypothesis 2b. while proponents of modified gravity theories credit this to the assumed incorrectness of the hypothesis of interest 1. They say that GR is falsified by this observational test while proponents of dark matter disagree.
> That's the reason your post reads like someone text-dumping about their faith.
I wrote stuff like: "This is not yet entirely certain, but it might help put a nail in the coffin of modified gravity models that try to account for anomalous rotation curves of galaxies by means of altered laws of gravitation on large scales instead of proposed dark matter halos."
I expressed uncertainty, I used "might" to indicate tentative findings and pointed out competing theories (modified theories of gravity) which can also account for (some) observations generally accredited to dark matter.
Sure, my post clearly favoured dark matter over those competing theories and pointed out ways in which these might be falsified in future. But do I take the existence of dark matter on faith? Surely not. I want to see evidence for it. I want particle physicists to discover axions or sterile neutrinos or other proposed dark matter candidate particles (WIMPs or the like). I want astronomers to try to rule out or vindicate even the weirdest edge case from rogue planets, to primordial black holes and brown dwarfs. A lot of stuff has already been ruled out. Alternatively I want to see modified gravity people present a gravitational theory that can explain everything that GR can explain (which is an awful lot) in addition to deceptive dark matter phenomena. It would be nice if they could do that using a quantum theory of gravity while they are at it! We need one anyways. Otherwise they could also falsify the dark matter hypothesis by finding anomalous behaviour that cannot be explained by any distribution of matter, dark or not (which hasn't happened as of yet). I'm open towards all of these possibilities.
Given the indirect evidence at the current time some sort of dark matter seems to be the most promising approach, but it might even be mix of both. Who am I to know what strange effects a quantum theory of gravity would have on large scales?
But I refuse not to acknowledge the usefulness of the current best theory of gravity, namely general relativity. Everybody is aware it's not the ultimate theory (no theory ever is), but it's frustratingly accurate. Using atomic clocks and GR's gravitational redshift effect you can even measure differences in elevation (relative to Earth's gravitational field) of centimetres or millimetres. This might allow for earthquake forecasts (predicting when and where the next big earthquake will occur) in the years to come:
See for example this excellent talk
www.youtube.com/watch?v=ydZ9OsWoemk
or the one entitled "Making optical lattice clocks compact and useful for real-world applications" by Hidetoshi Katori on the same YouTube channel.
I try to rise up but it pulls me down :(
"Gravity" does not exist. Density, and buoyancy is all that exist. So believed Nikola Tesla. So called gravity wave measurement, is bs science.
@EnemyAce said in #47:
> "Gravity" does not exist. Density, and buoyancy is all that exist. So believed Nikola Tesla. So called gravity wave measurement, is bs science.
So, some guy in a lab created gravity?
> "Gravity" does not exist. Density, and buoyancy is all that exist. So believed Nikola Tesla. So called gravity wave measurement, is bs science.
So, some guy in a lab created gravity?
@Thalassokrator said in #45:
> literally, unironically, religious babble
Get a grip. It can't be observed. The "effects" of dark matter are literally, LITERALLY, just the evidence that gravity is not working.
Lol.
What could possibly falsify the theory of gravity if not "everything in observable universe is not conforming to it"?
> literally, unironically, religious babble
Get a grip. It can't be observed. The "effects" of dark matter are literally, LITERALLY, just the evidence that gravity is not working.
Lol.
What could possibly falsify the theory of gravity if not "everything in observable universe is not conforming to it"?
@LordSupremeChess said in #48:
> So, some guy in a lab created gravity?
Imagine when your teacher asks you to hand in your homework you give them a big handful of nothing and tell them it's "dark homework" that simply can't be observed.
And your teacher can know the dark homework is real because you stayed up late last night, just like you would if you did regular homework, and not unobservable "dark" homework.
> So, some guy in a lab created gravity?
Imagine when your teacher asks you to hand in your homework you give them a big handful of nothing and tell them it's "dark homework" that simply can't be observed.
And your teacher can know the dark homework is real because you stayed up late last night, just like you would if you did regular homework, and not unobservable "dark" homework.
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