So far my absolute layman understanding of dark matter is: "Well, the observations do not fit the theory. But if there was this huge amount of matter that we just cannot see, it would work out well." Whereas the alternative theories boil down to: "Well, we do not know how gravity works over there, so maybe it just works like this." Whereas the usual answer is: "No it cannot work like this, because [OBSERVATION/FORMAL ARGUMENT]."
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?
Physicist here. It's roughly right, but it's kind of the picture we had 50 years ago. 50 years ago, dark matter and modified gravity were both reasonably equally good hypothesis. Then we got a treasure trove of astrophysical, galactic, and cosmological data from nearly a hundred distinct experiments.
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.
> 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.
Slightly off-topic, but can I ask if dark energy has anything like this going for it?
>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.
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.
> That's practically the definition of what a good scientific theory should do. Put in a single parameter and explain a hundred observed results.
[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?
I haven't seen any explanation for Dark Matter that explains its distribution. It's always "hey, if there's some stuff with this distribution it will explain this observation". An example would be a halo that fixes a rotation curve. If it interacts with visible matter gravitationally then why doesn't it take on the same distribution? That is never explained.
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.
There have been observations where the observed additional matter 'lagged behind' colliding galaxies, because dark matter only interacts gravitationally. If gravity itself were to behave differently, and the additional mass would always come from the observed matter, the dark matter should have been observed right where the colliding galaxies were, but since it didn't, it's a good indicator for the actual existence of gravitationally-interacting dark matter.
In my humble opinion, the key to appreciating the advantage of dark matter over modified gravity (of any kind) is that there is at least one galaxy that has been observed to be consistent with no dark matter. That’s only possible if unexpected gravitational effects are due to dark matter, and that it be absent... it’s pretty difficult to think of a situation where modified gravity would re-modify itself to behave exactly as we expect it to.
A very bad analogy is air pressure. Because air pressure is really not a force in of itself, it's just the presence of air at certain measures, but we observe the effects of air pressure as we move through the atmosphere. That drag coefficient changes depending on air pressure, and influences the aerodynamics..... you get the idea. In orbit around earth the drag of upper atmosphere is enough to decay orbits, even though it's so minute it's practically a near vacuum.
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.
Fry: Usually on the show, they came up with a complicated plan, then explained it with a simple analogy.
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.
Quite happy to see entropic gravity discussed here. One thing I really like about EG is that it ought to be very testable. For example, there are a few observations of very diffuse galaxies that exhibit very little if any dark matter behavoir. These represent a direct challenge to EG, as gravity ought to emerge from any concentration of mass, whereas under DM we can say there simply aren't any DM particles there (of course why this is the case is still very important).
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.
I get the premise (gravity becomes linear at large distance), but I don't understand any of the terms used in the article, and their respective pages didn't do anything to help. Could someone give an ELI5 of entropic forces, anti de-Sitter space, and the rest of the opening paragraph?
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
Without endorsing it, and presenting it here without comment, http://physicsfromtheedge.blogspot.com/ has explanations of the theory preferred there (which I think can be labeled an entropic theory of inertia, or perhaps an information-theory-based theory of inertia) that might be an ELI5.
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.
Gravity is weird. On the face of it, it seems to directly contradict the second law of thermodynamics, that overall entropy in the universe must increase.
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?
Entropy is not just about position, but about temperature as well. Objects with higher temperature have higher entropy since you have more options with how velocities are set, hence entropy increases as things gets pulled together by gravity and energy gets spread out among the particles, at least with a classical interpretation of gravity.
> Objects with higher temperature have higher entropy
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)
So, I'm thinking about two asteroids in deep space. When they are drawn together in an orbit via gravity, this would seem to lower their entropy (a smaller number of possible microstates required to define their macrostate). So, gravity should produce some tidal forces that would heat them up to a greater extent than the entropy they lose for being brought closer together?
All the titans of physics, including Albert Einstein, believed in a zero energy universe. Electromagnetism represents positive energy while gravitation represents negative energy.
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.
Ok, so they are just as confused, that's helpful to know:
"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."
Stars aren't powered by gravitational energy - that would only give a lifetime 20 million years for the sun (Kelvin Helmholtz timescale). Fusion is what powers the sun.
That interpretation of the criticism text is at best wishful thinking and at worst willful ignorance. Take off the rose colored glasses and read it again - there are several fundamental problems brought up.
> 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.
>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...
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..
Can somebody explain to me how my "internal" imagination of gravity is wrong? For some reason I started imagining it as a "consequence" of different space densities. I.e. if some particle randomly moves a bit, it tends to automatically move towards where there is "more" space to be in.
Imagine a piece of paper with many small dots on, symbolising places where a particle could be. If a particle randomly jumps to neighbouring dots wouldn't it eventually get trapped in areas where there are a lot of dots? (Also somehow the presence of particles increases the amount of dots?)
I think that intuition would imply that objects could randomly escape even black holes, even if the sequence of random jumps required would be very unlikely.
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.
Readit News
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.
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.
Slightly off-topic, but can I ask if dark energy has anything like this going for 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.
[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.
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.
https://en.wikipedia.org/wiki/Bullet_Cluster
Deleted Comment
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.
Deleted Comment
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)
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.
++++++++++++++
++++++++++++++
https://en.wikipedia.org/wiki/Zero-energy_universe
https://en.wikipedia.org/wiki/Action_(physics)
https://en.wikipedia.org/wiki/Planck_constant
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".
"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."
https://en.wikipedia.org/wiki/Entropic_gravity#Criticism_and...
And a relevant xkcd:
https://www.xkcd.com/1758/
...
> 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.