Entropic Gravity
en.wikipedia.org
en.wikipedia.org
Did I get this (roughly) right?
So from my perspective: Is there any prediction that would prove the existence of dark matter? Preferably something that could be invoked repeatedly?
The results of _all_ of these experiments do not fit with the naive theory. And they each can be fit perfectly by adding in dark matter. But that's not important; the crucial point is that all of them can be _simultaneously_ fit perfectly by adding in the _same_ amount of dark matter. That's practically the definition of what a good scientific theory should do. Put in a single parameter and explain a hundred observed results.
Meanwhile, modified gravity theories have fared extremely poorly -- they were originally designed specifically to fit galaxy rotation curves, and accordingly it is very difficult to massage them into fitting anything else. Usually if you get the rotation curves right, the astrophysics and cosmology come out disastrously wrong. You could probably get it right if you added a pile of ad hoc parameters, but that would be bad science.
Unfortunately every discussion of this subject ever just hyperfocuses on the nearly 100 year old galaxy rotation curve observations... probably because it's easy to understand.
> Is there any prediction that would prove the existence of dark matter?
If dark matter interacts in a non-gravitational way, and is present near the Earth, then the smoking gun would be directly detecting it in an terrestrial experiment. (People also work on indirect detection, by looking at possible products of its decay or annihilation elsewhere in the galaxy, but this is less definite because such products could be made by something else.) Of course in all these cases, the specific kinds of predictions depend on what you think dark matter is made of. Unfortunately, its good scientific properties (i.e. fitting a lot of data with remarkably little input) also mean that we have very little to go on here. Practically any kind of new "stuff" that interacts weakly electromagnetically could work.
Short version:
* Distribution of stellar velocities in elliptical galaxies implies DM
* Mass distribution in clusters of galaxies implies DM
* Gravitational Lensing observations imply DM
* CMS anisotropy power spectrum matches DM
* Structure formation in early universe appears to require DM
* Bullet cluster observations imply DM
* Flatness of the universe implies DM
plus some other statistical measures which also imply DM.
It's a great resource for people with basic undestanding of fundamental physics and some experience with math, along with John Rennie's
Nice to see you here. Keep up great job popularizing scince on a bit higher level. Kudos.
Now WIMPs are basically what it says on tin. We know they don't interact much, with themselves or anything else, because the rotation curves of the galaxies implies there are large halos. This means there's little to no friction, as otherwise the majority would fall down to the galactic cores, like normal matter does.
We assume they're a separate type of particle from anything we know, as otherwise that shouldn't be possible. We know they're massive, as otherwise we'd see them in particle accelerators... well, we'd see the missing mass at least.
Then, what are they? Well...
Probably a lot of things. The "dark matter sector" outweighs visible matter by a lot, and there's no reason that should be just a single particle.
But anyway, think of graph theory. You can map particles to nodes on a graph, and the fundamental forces to edges.
Not all particles are connected by way of all forces. Electrons lack color charge; neutrinos lack basically everything. (Even when there's a connection, connections can be of varying strengths.)
Dark matter is matter that lacks any connection to the other particles, except gravity.
(Could there be particles that are unconnected by gravity? That gets speculative, but probably not. Gravity is special.)
Ah, but-- there might be more than one dark matter particle, and forces connecting them that won't touch our type. For the most part we know this isn't true, since we know they don't interact with themselves either, but this is only to say that most dark matter can't be doing that.
There absolutely could be dark-matter stars, or stranger things.
... This is purely speculative, though. Good for fiction, but the only kind of dark matter we know exists is the WIMPs.
Dark Matter is a bit of a misnomer... it’s invisible, transparent.
What I'm suggesting is just that there might be multiple dark matter particles, some of which might have structure. At present that is basically untestable, of course.
http://backreaction.blogspot.com/2018/10/dark-matter-or-what...
[edit] bellow is wrong (dark matter uses free params), misread from wikipedia see child comment...
Conversely entropic gravity uses free parameters, which according to the crushing wikipedia definition is likely to be a product of wishful thinking.
A stark difference, although i'm definitely not suggesting this to be some absolute measure of truth - perhaps it is useful to consider the historical context this good scientific quality (which is generally good) emerged from, it's possible it may not be a good fit for theories grounded in emergence and chaos of the very small?
disclaimer - not a physicist, obvs.
> Importantly, [entropic gravity] also explains (without invoking the existence of dark matter and its accompanying math featuring new free parameters that are tweaked to obtain the desired outcome) why galactic rotation curves differ from the profile expected with visible matter.
I get that entropic gravity doesn't use free parameters.
Also, I don't think that free parameters are necessarily so damning. I happen to have more hammocks than usual in my house. Any theory about how hammocks are distributed will either fail to explain the concentration, or will predict that hammocks are more likely to be found near people who like them, and then will subsequently fail to provide a way to derive the location or distribution of those people. That failure would be a free parameter. It's not necessarily an indicator of wishful thinking. It could just be that the theory knows its limits.
You are correct, my error in skim reading too fast.
> It could just be that the theory knows its limits.
Yes that's essentially what I meant by chaos, if you could know enough information you could predict where all your hammocks end up, but that requires vast and subtle knowledge about how you think and minutiae of your environment and how you interact with it that caused you to make arbitrary decisions... the interesting thing about chaos is that it _can_ be deterministic, in other-words, given initial conditions it can be computed - but even then in the context of such large systems that computation is infeasibly and irreducibly costly, in which case (i guess) you use free parameters in combination with what conceptually similar to statistical mechanics?
Free parameters which significantly outpower the power of the observations are damning because there is no way to falsify the theory: if the theory can explain an extremely wide range of observations by tweaking the free parameters then it has much less predictive power and should be viewed extremely suspicously.
I think the trouble with dark-matter-as-WIMPs is that the free parameter (the location of those WIMPs) is overpowered:
You can say that the extra gravitation is caused by the presence of these particles and leave the details of how those particles got where they are out of your theory. But then when you add "oh, and they can't be detected by any other means" it becomes a problematic free parameter because wait a second, that's the very problem we were trying to solve in the first place. If you can't connect the causal agent to anything besides the problem that necessitated it, then it might as well be a ghost.
Slightly off-topic, but can I ask if dark energy has anything like this going for it?
My other issue is that in discussions of rotation curves, I keep seeing reference to Kepler, which simply shouldn't apply. Where can I see the math behind the "expected" curve - I suspect an error.
In a large-scale, statistical sense, the answer is: initial quantum fluctuations (as seen in the Cosmic Background Radiation) + gravitational collapse and fragmentation in an expanding universe. Simulations done under these assumptions have done an increasingly good job of describing the general distribution of dark matter, and the typical statistical distribution within galaxies.
This is why it's good to do independent fits of dark matter distributions to galaxies: it provides something you test the simulations against. After all, if the individual-galaxy fits say dark matter generally has distribution X, but the simulation say that gravitational collapse and mixing should almost always produce something different, then you know you've got a problem.
"If it interacts with visible matter gravitationally then why doesn't it take on the same distribution?" Because visible matter interacts with itself via radiation and hydrodynamics (e.g., gas pressure). So it gets pushed around in ways the dark matter can't. These forces can be much stronger than gravity in some circumstances (gravity, after all, is the weakest fundamental force), so the gas atoms, ions, electrons, etc. experience forces the dark-matter particles do not.
"Keplerian" in this context is an approximate term. It refers to the fact that most of a galaxy's visible mass is centrally concentrated, and so as you get further and further away, with virtually all of the mass inside whatever distance you're at, the rotation curve should converge on a true Keplerian one, because the difference between the effect of the true mass distribution and one where all the galaxy's (visible) mass is concentrated in a point at the center gets smaller and smaller.
Actual published fits to galaxy rotation curves always use the measured visible-mass distribution for a given galaxy to compute the non-dark-matter curve. No one working in this field is confused about this.
Note that this person explicitly asks for the dataset, and the computation of the expected curve, but hardly ever does anyone help such a person in such a direction, it is always taken as a suspected insult on the mental state of one camp of interpretation.
I too would love some kind of central register or portal for the most widely accepted anomalies (anomalous.physics/dark-matter/...), where people can get and inspect observation datasets, and competing models to fit the datasets.
Imagine Brahe & Kepler's, data & interpretation to be widely popular, but whenever someone asked for data or computations to compare circular orbits with elliptical ones, nobody would point them where to find such data and computations?
http://letmegooglethat.com/?q=data+for+galaxy+rotation+curve...
(The fourth link, for example -- http://astroweb.cwru.edu/SPARC/ -- has both observed rotation-curve data and computations of expected rotation curves.)
If, on the other hand, you assume they must all be stupider than you are and say things like "That is exactly one of the mistakes I suspect keeps being made", then you're basically saying, "I'll bet none of the hundreds or thousands of people working in this field for decades have ever thought of this one point that just occurred to me!" The latter is, shall we say, rather unlikely.
(In point of fact, Newton's shell theorem generalizes to the case of axisymmetric, flattened spheroids with homeoidal density distributions, a result that was derived by Laplace and others in the 18th and 19th Centuries. Which means that disks do behave somewhat like spheres.)
To get a sense of what's really involved, you could look at something like Brandt's 1960 paper, and then some of the papers that cite it (including some of the classic early papers by Vera Rubin and collaborators), to get an idea of how much more sophisticated than an simple Keplerian rotation curve:
https://ui.adsabs.harvard.edu/abs/1960ApJ...131..293B/abstra...
Because unlike visible matter, it does not interact in non-gravitational ways, even with itself.
Visible matter can clump to form planets and stars because when two particles of it are attracted enough to hit each other there is some interaction other than gravity, which helps eat some of their kinetic energy. In contrast, when that happens with two particles of dark matter, they just fly through each other, and if they were moving fast enough, never meet again.
There are non-gravitational interactions even at the galactic scale, even though they are very weak indeed compared to gravity. As our sun plows through space, the particles it's sphere of influence hits have a preferred range of velocities and directions, the "galactic rest". Over billions of years, this does influence where in the galaxy our sun is. Dark matter has no such influence on it.
Is that true? There are claims of galaxies with no dark matter, or tons of dark matter. It seems like dark matter is a parameterized value with lots of local anisotropy across the universe.
It's often said that mind has difficulty explaining bizzare galaxies but my understanding is that's not true. I thought the only thing that MOND cannot explain is the matter distribution in the early universe, which requires an anisotropy that MOND does not "give for free" like dark matter does.
That is something like gravity's inverse square law, or put another ways we have been obsessing on the measured effects of gravity, but not truely understanding.... like we used to not understand air pressure centuries ago.
Anyways, it goes without saying air pressure emerges from lots of air concentrated in a space, just like gravity emerges from the presence of mass in a concentrated space.
Gravity is the air of the universe.
Leela: Hmmm... If we can re-route engine power through the primary weapons and configure them to Melllvar's frequency, that should overload his electro-quantum structure.
Bender: Like putting too much air in a balloon!
Fry: Of course! It's all so simple!
https://johncarlosbaez.wordpress.com/2012/02/01/entropic-for...
Similarly, with EG there should be testable pedictions at smaller scales. Microscopically under EG, there should be no gravity ever between fundamental particles, no matter how massive, for the same reason that a single atom cannot be warm.
1) Two point-masses at a distance subtend a sphere with the radius equal to the distance
2) That sphere has a radius and a maximum capacity for holding information (in bits) depending on the area and the Planck constant
3) If we treat the bits as particles in a gas, we can derive the temperature of that gas based on the mass of one of the two point-masses and Einstein's equivalence
4) Assuming that the temperature is due to Unruh effect, we can calculate the acceleration that would cause it
5) Lo and behold, this is the same acceleration due to Newton's law of gravitation
The following is my attempt at casting McColloch's idea as an ELI5. McColloch's idea is that inertia is a result of asymmetric Unruh radiation pressure resulting from acceleration causing the Hubble horizion sphere around the accelerated body to not be symmetrically spherical. The idea is that when you accelerate, then the Hubble horizon "behind" you gets much closer to you, which then disallows some Unruh radiation behind you, thus causing there to be more Unruh radiation pressure in front of you. A more fundamental idea underlying this is that Unruh radiation emanating from the Hubble (and, indeed, any) horizon has every possible discrete wavelength that would fit between the horizon and the particle surrounded by that horizon, so as the distance to the horizon changes, so does the longest wavelength that could be visible -that could exist-. Now, as accelerations tend to zero, the idea is that the number of Unruh waves that are disallowed by the horizon behind is much smaller than at higher accelerations, and therefore inertia drops at lower accelerations, leading to gravity feeling stronger.
I believe all entropic gravity/inertia theories are at least as controversial as dark matter theories or more so. McColloch's has the benefit of having a physical explanation that can be explained, if not to a 5-year old, then to a 10-year old. However, keep in mind that it could be a simple explanation that is just wrong.
Entropy generally involves "spreading out." It eliminates gradients, it doesn't fill them up. It is weird that there is a way to increase overall entropy by bringing diffuse parts together.
Despite the term "entropic gravity", I still don't understand how this or other versions of gravity do not violate the second law. Does anyone here?
That surely must be a bit too oversimplified, since that would imply that the beginning of the observable universe had more entropy than the current one (given that it was way hotter than the present)
But, coming from a high entropy state does not fit with the world we see, so we're pretty sure that the past hypothesis is true.
One of the leading theories about the early universe and how it lead to the world we know today, is inflation. In that theory "stuff" is basically created out of nothing, meaning that the early universe was small, hot, and with (relatively) few particles in a specific configuration. The inflation and reheating dramatically increased the entropy. In other words - there was much less stuff, and it needed to be in a particular configuration to expand into the universe as we know it. That is low entropy.
Note: I am not a theoretical physicist, so the above is my layman understanding of the topic.
Additionally, the Big Bang is theorised to have passed through "absolute hot" https://en.wikipedia.org/wiki/Absolute_hot
What do you mean by macrostate? If the macrostate is related to the geometry of the asteroids considered as solid bodies (and the changes in their positions and orientations) I don't see why the entropy would be lower.
In the case of the asteroids, I guess I'm thinking of number bits required to accurately model the system accurately. Is that an appropriate way to think about it?
I see it as lower because fewer cells in a matrix are needed to model the solid bodies...
But I fail to see the analogy with the asteroids. If the microstate is the atomic configuration it doesn’t seem to depend (at least significantly) on the macroscopic description of the position, momentum and angular momentum of the asteroids. The number of bits (?) is not a physical quantity (and why would you need less bits to describe the microstate?).
I hope I'm not expressing myself too foolishly, but I really don't understand it. After much effort, I feel like I have a good understanding of entropy, though. Which is what makes gravity so weird to me.
But you may not be looking at the whole picture. Just because a system occupies a smaller volume doesn’t mean the entropy is lower. In classical thermodynamics a gas can be compressed while keeping the entropy constant (at least in theory).
Edit: after a look at the first paper by Verlinde I still don’t know what are the microstates in this theory (they are related to string theory and intentionally left mostly undefined). But in any case your assumption that entropy decreases seems wrong by definition because an entropic force acts in the direction of increasing entropy.
In the story of the match, who lights the fire is far more interesting, to me, than what brand of match is. Honestly stellar evolution is far too much chemistry for me, just give me a well calibrated redshift and I'm happy.
"You can imagine life without the chemistry, nuclear physics, condensed matter physics we know. You can imagine intelligence made of plasma or computers made of dancing stars in clusters. If you have a powerful enough imagination, every cog in the architecture of life is replaceable. Except for one: gravity. Only gravity can reduce entropy and create complexity where it doesn't already exist. It is, ultimately, the origin of anything worthwhile in the Universe."
Energy is related to change of state, which brings us to action. Action is measured in J⋅s. The famous Planck's constant, h, is 6.62607015×10−34 J⋅s.
Energy = Action / Time = (Energy x Time) / Time = Energy
Essentially, a given amount of energy causes a certain amount of state changes per second.
Since the total energy of the universe is 0, once the universe reaches equilibrium, it should be back at the Big Bang, with no change to its initial state.
If one believes entropy is actually only increasing, probably largely informed by the electromagnetic field, then we should end in the so-called Heat Death instead of the aforementioned Cyclic Universe.
A lot of the eventual informing will come from knowing more about the magnitude of expansion of spacetime - its history of values through the past, present, and future.
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https://en.wikipedia.org/wiki/Zero-energy_universe
The statement is, roughly: in appropriate units, the sum of normal entropy and the area of black hole horizons is non-decreasing.
See, for example, https://inspirehep.net/record/91796 and literature surrounding "Generalized Second Law".
https://en.wikipedia.org/wiki/Entropic_gravity#Criticism_and...
And a relevant xkcd:
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> 3) this predicts missing masses in specific places
Not quite... It predicts that either
1. the observations were incomplete (so, not enough data was fed into the model, hence the model's predictions are off), or
2. The model itself is wrong.
Dark matter is about option 1, modified Newtonian dynamics is about option 2.
And is great that both are being investigated. Imagine dark matter had been proposed in 1900: the deviations in Mercury's orbit (from Newton's model of gravity) could have been attributed to dark matter. No need for general relativity...
Hence, both aspects should be researched: improving our observational capabilities to detect the oossible unknowns (including putting limits on their mass etc); and attempting new models that address these issues directly.
You should have a look [here](https://io9.gizmodo.com/the-200-year-old-mystery-of-mercurys...): TLDR: they looked for a 'Vulcain' planet which could cause Mercury's precession, but they didn't found one.
And for the dark matter, you're right that both aspects should be researched, and both ARE researched but dark matter currently fits better the experimental data as already said. Of course until we know exactly what is dark matter, there will be room for other theories..
Also it would mean that matter somehow affects the distribution of those dots. If the transition between dots is a quantum event, it sort of sounds like what imagined quantum gravity could look like, but I have no idea how you'd translate the thought experiment into actual maths.
I attended a talk by Verlinde and he is well aware of them, but I liked his answer. He basically said that even if this theory was not a valid alternative to dark matter and was eventually proven wrong, some of their results looked very interesting and could develop into different theories, so he thought that it was worth to keep going further with this line of research.
Nevertheless, you could feel that he still had some hope they could fix these problems in some way. But it looked to me like this feeling was more a personal desire than real scientific expectations.
A better answer is that dark matter is itself not free of problems either, so maybe some effort should be devoted to not-dark-matter theories. The entropic gravity theory space costs very little to fund by comparison to dark matter, so it's a reasonable thing to continue funding some of it for now.
Because things are not black or white.
It is never clear what we get from the advancement in physics theories, but many times we get more profit from the journey than from the final results. If Einstein equations can be derived from thermodynamic laws, this may have some very deep meaning, even if we still need dark matter to make cosmology models work.
At this point, we have no perfect theory, you could say that all they have been proven wrong in some way or another, and yet, many of them are useful.
I do not know enough about physics to know if this is the most practical way to spend funding, but from the personal viewpoint of Verlinde, I think it makes sense to keep working on his theory until a totally unsolvable problem is found. But this is only my impression after attending a layman talk, I am far from being expert.
Logically the maximum entropy possible would be with each particle placed randomly with random momentum, but gravity makes that impossible on cosmic scales, as such a system would soon collapse because of gravity pulling the particles together.
We could see gravity as an entropic limiting force, with entropy increasing statistical forces increasing entropy on the microscopic scale being contradicted by entropy limiting gravitational effects on the super-macrostopic scale.
Space is a substance - I can't imagine it any different due to existence of gravitational waves. If something vibrates it's substantial.
My logic skills are good enough to predict down-voting of my little story ;) Anyway, I think logically it is quite coherent.
It might make for some decent sci-fi, but don't confuse a cool story with understanding.
I would rather claim that due to epistemological reasons we cannot "understand" reality. And i think that "The problem with having "opinions" about reality is that reality doesn't care what your opinion is, at all." - is a great example of opinion (in my opinion) :)
Speaking of opinions in the context of reality implies that all (or at least most) alternatives are somehow equally valid, which is plainly not true. A justified belief is better than an unjustified one.
You have opinions about food. You may have opinions about what scientific hypotheses are well-justified and which aren't. You can't have opinions about how reality is.
You made a claim that black holes emit space. To justify it, you would have to come up with a model that incorporates this mechanism and then see if that model agrees with known facts before it has any practical value. If you have no such model, please at least go a bit further and explore the implications of your thought experiment.
As for epistemological reasons why understanding reality is impossible, that's just meaningless wordplay. For any practical definition of "understanding reality", it should be self-evident why opinions are irrelevant: reality is not going to just change itself to accommodate your thoughts.
I think it's important to pay attention to how you communicate your ideas, because it's very easy to make completely baseless conjecture sound convincing to yourself, let alone other people, if you're not careful.
Ironic, how does that fit with "advocating intellectual honesty and clarity"?
Saying that the sky is green is not a valid opinion, because the sky is not green, and it can be shown not to be so. Some things are not a matter of opinion.