Entropic gravity is like the "brazil nut effect" [0] [1]. The idea is that if you shake a glass full of different sized nuts, the large ones will rise to the top.
From what I understand, this is because larger objects have more mass, moving slower when shaked, so as the larger (brazil nuts) don't move as much relative to the smaller ones (peanuts), and because of gravity, there's a cavity left under the brazil nut which gets filled in with peanuts.
For entropic gravity, the idea is that there's a base density of something (particles? sub-atomic particles?) hitting objects in random ways from all directions. When two large massive objects get near each other, their middle region will have lower density thus being attracted to each other from particles hit with less frequency from the lower density region. They sort of cast a "shadow".
I'm no physicist but last time I looked into it there were assumptions about the density of whatever particle was "hitting" larger massive objects and that density was hard to justify. Would love to hear about someone more knowledgeable than myself that can correct or enlighten me.
As an aside, the brazil nut effect is a very real effect. To get the raisins, you shake the raisin bran. To get gifts left from your cat, you shake the kitty litter. It works surprisingly well.
Not a physicist either but this passage from the Feynman lectures seem related to what you are describing: https://www.feynmanlectures.caltech.edu/I_07.html
"Many mechanisms for gravitation have been suggested. It is interesting to consider one of these, which many people have thought of from time to time. At first, one is quite excited and happy when he “discovers” it, but he soon finds that it is not correct. It was first discovered about 1750. Suppose there were many particles moving in space at a very high speed in all directions and being only slightly absorbed in going through matter. When they are absorbed, they give an impulse to the earth. However, since there are as many going one way as another, the impulses all balance. But when the sun is nearby, the particles coming toward the earth through the sun are partially absorbed, so fewer of them are coming from the sun than are coming from the other side. Therefore, the earth feels a net impulse toward the sun and it does not take one long to see that it is inversely as the square of the distance—because of the variation of the solid angle that the sun subtends as we vary the distance. What is wrong with that machinery? It involves some new consequences which are not true. This particular idea has the following trouble: the earth, in moving around the sun, would impinge on more particles which are coming from its forward side than from its hind side (when you run in the rain, the rain in your face is stronger than that on the back of your head!). Therefore there would be more impulse given the earth from the front, and the earth would feel a resistance to motion and would be slowing up in its orbit. One can calculate how long it would take for the earth to stop as a result of this resistance, and it would not take long enough for the earth to still be in its orbit, so this mechanism does not work. No machinery has ever been invented that “explains” gravity without also predicting some other phenomenon that does not exist."
It also doesn't account for time dilation in a gravity well however i still think the general idea has some merit if you think of it as being bombarded by massless ‘action potentials’ on all sides with mass absorbing that field to some to enable translation in space time.
I get this is vague spitballing but essentially an ‘action potential’ would allow mass to move. Higher temperature mass interacts more, lower temperature interacts less. Mass with momentum would be biased to absorb more from one side so it travels in a specific direction in space more than others (the idea i’m getting at is that all movement in space only occurs with interaction with this field), this also would counteract issues with moving mass interacting more on a specific side - the very bias of mass with momentum to absorb more on one side means that from that masses point of view it has the same action potentials interacting from all sides. Mass shielded behind mass receives fewer action potentials so experiences exactly the effect that you can call time dilation. Mass shielding other mass from action potentials also means that mass accelerates towards other mass.
Essentially its the above but instead of a massive particle hitting other mass from all sides it’s a field that allows mass to experience a unit of time.
> earth, in moving around the sun, would impinge on more particles which are coming from its forward side than from its hind side
Would this be true if the speed of particles is constant in all frames of reference?
This is a better YouTube video describing granular physics and shows the speed (amplitude) of vibrations can cause counterintuitive arrangements of particles.
At lower speeds you get something akin to Newtonian gravity but at higher velocities you get something resembling MOND gravity where galaxies clusters and large voids appear - no dark matter needed.
https://www.youtube.com/watch?v=HKvc5yDhy_4
Thanks for the link, very interesting. I'll have to check out the paper but just watching the video it seems all these counter intuitive effects can be described from the oscillations being related to the size of the chamber.
For example if I were to roll the chamber at a very low frequency, I would expect the particles to clump on one side, then the other and so on. This is not really so surprising and the frequency will depend on the chamber dimensions.
No, here "entropic" is as in the entropic force that returns a stretched rubber band to its unstretched condition, which (as it tends to be scrunched a bit) is at a higher entropy.
https://en.wikipedia.org/wiki/Rubber_band_experiment
"The stretching of the rubber band is an isobaric expansion (A → B) that increases the energy but reduces the entropy"
[apologies for any reversed signs below, I think I caught them all]
In Verlinde' entropic gravity, there is a gravitational interaction that "unstretches" the connection between a pair of masses. When they are closer together they are at higher entropy than when they are further apart. There is a sort of tension that drags separated objects together. In Carney et al's approach there is a "pressure mediated by a microscopic system which is driven towards extremization of its free energy", which means that when objects are far apart there is a lower entropy condition than when they are closer together, and this entropy arises from a gas with a pressure which is lower when objects are closer together than when objects are further apart. Pressure is just the inverse of tension, so at a high enough level, in both entropic gravity theories, you just have a universal law -- comparable to Newton's -- where objects are driven (whether "pulled" or "pushed") together by an entropic force.
This entropic force is not fundamental - it arises from the statistical behaviour of quantum (or otherwise microscopic) degrees of freedom in a holographic setting (i.e., with more dimensions than 3+1). It's a very string-theory idea.
The approach is very hard to make it work unless the entropic force is strictly radial, and so it's hard to see how General Relativity (in the regime where it has been very well tested) can emerge.
The local theory part of the Carney et al paper (preprint <https://arxiv.org/abs/2502.17575>) is interesting in that it isn't obviously related to string theory / holographic entropic gravity. Instead masses induce a spin polarization near them which is a lower entropy state. Two masses with two polarized spin-clouds will attract each other as the system tries to thermalize to a higher-entropy state. With careful choices of parameters, they can generate any central force, and they explore a particular choice which corresponds to Newtons 1/r^2 mutual attraction.
The paper cannot deal with fast-moving masses at all: it's not just the relativstic regime (where speeds are significant fractions of c) but rather the masses must move more slowly than the thermalization. This is hugely restrictive.
Finally, comparing themselves to the traditional approach of quantizing perturbations (e.g. turning classical (General Relativity) gravitational waves into lots of spin-2 gravitons) the authors write:
They also say that while their starting point was being very different from the holographic picture: Some of this will necessarily by driven by the need to be compatible with General Relativity in the weak field limit. They are not compatible with strong gravity in General Relativity at present.So while the idea is kinda interesting, I think they are putting the cart before the horse in asking what their model says about things like the interaction between gravitation and entanglement. That's simply unmeasurable by experiment right now whereas the very-well-understood relativistic precession of Mercury's perihelion is completely out of scope for this initial paper.
> From what I understand, this is because larger objects have more mass, moving slower when shaked, so as the larger (brazil nuts) don't move as much relative to the smaller ones (peanuts)
That doesn’t make sense to me. If larger objects move slower, don’t they move faster relative to the (accelerating) reference frame of the container?
Also, conventional wisdom has it that shaking (temporarily) creates empty spaces, and smaller objects ‘need’ smaller such spaces to fall down, and thus are more likely to fall down into such a space.
> That doesn’t make sense to me. If larger objects move slower, don’t they move faster relative to the (accelerating) reference frame of the container?
Yes? But so what? The relevant interaction is between the peanuts and the Brazil it's.
> Also, conventional wisdom has it that shaking (temporarily) creates empty spaces, and smaller objects ‘need’ smaller such spaces to fall down, and thus are more likely to fall down into such a space.
Right, but preferentially under the larger Brazil nuts.
> From what I understand, this is because larger objects have more mass, moving slower when shaked, so as the larger (brazil nuts) don't move as much relative to the smaller ones (peanuts), and because of gravity, there's a cavity left under the brazil nut which gets filled in with peanuts.
I always thought it was because the smaller nuts can fall into smaller spaces, while the larger nuts cannot.
This sounds really dumb, so forgive me. But one thing that's always felt weird to me about gravity is how we consider things to be one body.
Like yes, when we look at earth from incredibly far away, it's a pale blue dot. But all those oceans on it are flowing and separate from the solid ground underneath. Those large boulders on earth that would have their own (tiny) gravitational pull on their own in space are just part of earth's single gravitational force. All the airplanes in the sky are subject to the pull of the earth, but they're also a part of the gravitational pull that pulls other things to earth.
When shaking cereal, the big flakes rise to the top, but tiny bits of dust from each flake also separates and settle at the bottom. But earth, as a whole, has big bits and little bits everywhere all flowing freely. And gravity seemingly treats all those bits as a single object. But with sufficient distance between objects (e.g. different planets), it treats them separately. And with greater distance (e.g. galactic scale), it treats them as one again.
> And gravity seemingly treats all those bits as a single object.
It does not; but when you learned about gravity in school or wherever, you were only presented with scenarios in which it was valid to do so. Specifically, for Newtonian gravity, an object whose density is spherically symmetrical may be treated as a point mass at its center (w/r/t other point masses farther from the center than any point in the first object); this can be seen by integrating Newton's equation.
There's no reason to attribute this to special behavior by gravity with respect to "objects" which you identify. You could decompose a (spherically symmetrical) mass into several different spherically symmetrical "objects" and sum up their influence on a test mass, and the result is the same as if you had treated the original mass as a point mass. The hypothesis that gravity is "treating" the object one way or another now fails to distinguish all these possibilities; it's no longer physically meaningful.
At sufficiently large distances, or for sufficiently large ratio of large to small mass, the difference between, say, a planet and a spherically symmetrical mass is so small with respect to the main effect that it can simply be ignored.
> gravity seemingly treats all those bits as a single object
Understanding the tides depends very much on disabusing oneself of this scaffolding.
There’s a bit of math you can do where you calculate the gravitational effect of standing on a sphere. You can redistribute the mass within the sphere and it won’t affect the gravitational pull, assuming it’s radially symmetrical.
One of those unintuitive results.
It probably only works in classical gravity but it’s still neat.
The bit of math is the Shell Theorem <https://en.wikipedia.org/wiki/Shell_theorem>.
That actually sounds really intuitive to me. If you start with a uniform mass and then redistribute mass symmetrically it makes sense that the change in the two sides would cancel out.
It works for some types of symmetry but not others!
Interesting! Now, that sounds like a more unintuitive result to me. Can you give examples of symmetries for which it doesn't work?
The (Newtonian) Shell Theorem is fairly sensitive to spherical symmetry. In General Relativity one can write down a metric wherein inside any boundary surface there is flat spacetime. It's easiest to do this for a spherical boundary, but one can work out a metric which is axisymmetric (e.g. oblate and spinning or prolate and tidally deformed) and probably all sorts of other weird shapes following ideas from Gauss's Law for Gravitation. Writing down a metric for that is hard though -- really hard if the idea is to make it time-independent, and really really hard if the idea is to make it time-dependent but static (as in a complex Gaussian surface doesn't relax into a more spherical shell). For example, bumps raised on each other by binary black holes will vanish after merger (or if they fly away on hyperbolic trajectories, having "grazed" each other), leaving you with a spherical horizon (if nonspinnning) or an oblate one (if spinning).
Essentially to break spherical symmetry (or axisymmetry where there's spin) and keep it broken you have to introduce something like a dark energy. One can do that outside (retaining flat space inside) or inside (leading to the equivalent direction-dependent attraction of outside objects).
In other words, gravity would be explainable by statistical mechanics (like heat)?
That's the allure, that gravity is a derived effect from statistical mechanics. Thus the name 'entropic attraction'.
My interpretation of entropy is that if you have X states that are equally probable, but not all states are distinct from each other in some sense, then the next state will likely be one where the states satisfying that condition is most numerous.
For example, if you flip N coins, there are 2^N states available once the flip is done. Each outcome has an 1/2^N probability of outcome. There's only one state where all of the states show all heads. While there's only one state where coins numbers 1-N/2 are heads, and N/2-N are tails, so that particular outcome is 1/2^N, if all we care is the macroscopic behavior of "how many heads did we get"--we'll see that we got "roughly" N/2 heads especially as N gets larger.
Entropy is simply saying there's a tendency towards these macroscopically likely groups of states.
A great video and practical demonstration of the concept for anyone interested: https://youtu.be/VCXqELB3UPg?si=lwRxmXLctY1ENu0U
Aren't more massive particles smaller though (in terms of de Broglie wavelength, at least), so they'd have a smaller "shadow"? Or do different forces have different cross-sections with different relationships to mass, so a particle's "size" is different for different interactions (and could be proportional to mass for gravity)?
Actually this is currently blowing my mind: does the (usual intro QM) wavefunction only describe the probability amplitude for the position of a particle when using photon interaction to measure, and actually a particle's "position" would be different if we used e.g. interaction with a Z boson to define "position measurement"?
The momentum wavefunction (or more properly, the wavefunction in the momentum basis) completely determines the position wavefunction (wavefunction in the position basis). And we can probe the momentum wavefunction with any particle at all, by setting up identical (say) electrons and seeing the momentum they impart on a variety of test particles. That is to say, the probability distribution of momentum of a particle does not depend on what we use to probe it.
As the position wavefunction is now completely determined in a probe-agnostic matter, it would be hard to justify calling a probe that didn't yield the corresponding probability distribution a “position measurement”.
You had lost me until you mentioned the kitty litter. I am now enlightened, thanks
but these nuts move by gravity do they not? and what in the universe is exactly up and down? and why would that matter?
are all celestial bodies then a local up and 'away from them' down?
this analogy hurts my brain. please tell me how to make the hurting stop
No you are right. You can't invoke gravity in an analogy trying to explain gravity.
The same effect could be replicated in a zero-gravity environment using an alternative background force (centrifugal force, vacuum suction, electromagnetism, etc.)
Wouldn't that mean that if gravity was like this analogy, that gravity would still need an external background force to work? And how would you explain the presence of that force?
Not without a bridge for the metaphor to generalize. What parameters map to the nutjar’s selective force, size, and mass?
You need to reread the middle only. It's a kind of "vacuum" effect.
but where in nature is then this vacuum? is it just higher and lower energy density or something? like high and low pressure systems in weather pushing and pulling?
that would or might imply gravity is electromagnetic?
(sorry if this seems really off the charts / missing your point!)
This problem of explaining gravity with gravity is a bit pervasive, and it frustrated the heck out of me as a kid.