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.
"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."
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.
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.
"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.
> 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.
> 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.
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.
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"?
Entropic gravity is a compelling framework. I think that most Physicists admit that it would be nice to believe that the yet unknown theory of everything is microscopic and quantum mechanical, and that the global and exquisitly weak force of gravity emerges from that theory as a sort of accounting error.
But there are so many potential assumptions baked into these theories that it's hard to believe when they claim, "look, Einstein's field equations."
As an experimental physicist, I refuse to get excited about a new theory until the proponent gets to an observable phenomenon that can fix the question.
This is why I'm skeptical of theories like Wolfram's: It feels like an overfit based on this: It produces all sorts of known theories (special relativity, parts of QM, gravity etc), but doesn't make new testable predictions, or new fundamentals. When I see 10 predictions emerge from the theory, and they all happen to be ones we already known of... Overfit.
The problem with emergent theories like this is that they _derive_ Newtonian gravity and General Relativity so it’s not clear there’s anything to test. If they are able to predict MOND without the need for an additional MOND field then they become falsifiable only insofar as MOND is.
Sometimes I wonder, imagine if our physics never allowed for Blackholes to exist. How would we know to stress test our theories? Blackholes are like standard candles in cosmology which allows us to make theoretical progress.
An observable is always the strongest evidence but there's also the improbability of a mathematical coincidence that can serve as almost as strong evidence. For example the fact that the Bekenstein-Hawking entropy of an Event Horizon comes out to exactly be the surface area divided by plank-length-square units, seems to me almost a proof that nothing really falls into EHs. They only make it to the surface. I'm not saying Holographic Theory is correct, but it's on the right track.
My favorite conjecture is that what happens is things effectively lose a dimension when they reach the EH surface, and become like a "flatlander" (2D) universe, having only two degrees of freedom on the surface. For such a 2D universe their special "orthogonal" dimension they'd experience as "time" is the surface normal vector. Possibly time only moves forward for them when something new "falls in" causing the sphere to expand.
This view of things also implies the Big Bang is wrong, if our universe is a 3D EH. Because if you roll back the clock on an EH back to when it "formed" you don't end up at some distant past singularity; you simply get back to a time where stuff began to clump together from higher dimensions. The universe isn't exploding from a point, it's merely expanding because it itself is a 3D version of what we know as Event Horizons.
Just like a fish can't tell it's in water, we can't tell we're on a 3D EH. But we can see 2D versions of them embedded in our space.
To me, entropy is not a physical thing, but a measure of our imperfect knowledge about a system. We can only measure the bulk properties of matter, so we've made up a number to quantify how imperfect the bulk properties describe the true microscopic state of the system. But if we had the ability to zoom into the microscopic level, entropy would make no sense.
So I don't see how gravity or any other fundamental physical interaction could follow from entropy. It's a made-up thing by humans.
Physical entropy governs real physical processes. Simple example: why ice melts in a warm room. More subtle example: why cords get tangled up over time.
Our measures of entropy can be seen as a way of summarizing, at a macro level, the state of a system such as that warm room containing ice, or a tangle of cables, but the measure is not the same thing as the phenomenon it describes.
Boltzmann's approach to entropy makes the second law pretty intuitive: there are far more ways for a system to be disordered than ordered, so over time it tends towards higher entropy. That’s why ice melts in a warm room.
Good question. You are absolutely right that entropy is always fundamentally a way to describe are our lack of perfect knowledge of the system [0].
Nevertheless there is a distinct "reality" to entropic forces, in the sense that it is something that can actually be measured in the lab. If you are not convinced then you can look at:
So when viewed in this way entropy is not just a "made-up thing", but an effective way to describe observed phenomena. That makes it useful for effective but not fundamental laws of physics. And indeed the wiki page says that entropic forces are an "emergent phenomenon".
Therefore, any reasonable person believing in entropic gravity will automatically call gravity an emergent phenomenon. They must conclude that there is a new, fundamental theory of gravity to be found, and this theory will "restore" the probabilistic interpretation of entropy.
The reason entropic gravity is exciting and exotic is that many other searches for this fundamental theory start with a (more or less) direct quantization of gravity, much like one can quantize classical mechanics to arrive at quantum mechanics. Entropic gravity posits that this is the wrong approach, in the same way that one does not try to directly quantize the ideal gas law.
[0] Let me stress this: there is no entropy without probability distributions, even in physics. Anyone claiming otherwise is stuck in the nineteenth century, perhaps because they learned only thermodynamics but not statistical mechanics.
Entropy isn't a function of imperfect knowledge. It's a function of the possible states of a system and their probability distributions. Quantum mechanics assumes, as the name implies, that reality at the smallest level can be quantised, so it's completely appropriate to apply entropy to describing things at the microscopic scale.
Entropy is certainly a physical “thing”, in the sense that it affects the development of the system. You can equally well apply your argument that it isn’t a physical thing because it doesn’t exist on a microscopic scale to temperature. Temperature doesn’t exist when you zoom in on single particles either.
There’s no reason to involve our knowledge of the system. Entropy is a measure of the number of possible micro states for a given system, and that number exists independently of us.
Something to consider is that entropy has units of measure. Why would a purely philosophical concept be given units of Joules per Kelvin?
I'm only reminded about this because, though I'm a physicist, I've been out of school for more than 3 decades, and decided that I owed myself a refresher on thermodynamics. This coincided with someone on HN recommending David Tong's textbook-quality lecture notes:
I think at least the first few pages are readable to a layperson, and address the issue of our imperfect knowledge of the precise configuration of a system containing, say, 1e23 particles.
But if we knew all of those relationships, the system would still have entropy.
I’ve been a believer in entropic gravity for a long time and believe it’s due to quantum foam. In a region of space with nothing in it the quantum foam in that space would be perfectly uniformly random. With mass and energy the state of the space would be biased and less random. This creates an entropic gradient. Further this doesn’t just explain gravity but explains why the space between galaxies seems to demonstrate negative energy and space expansion. I’m glad to see more research into the idea of entropic gravity as it’s IMO a more reasonable explanation than most other gravity theories I’ve heard.
We all know that life on Earth gets it's energy from the Sun.
But we also know that's an approximation we tell kids, really life gets low entropy photons from the Sun, does it's thing, and then emits high entropy infrared waste heat. Energy is conserved, while entropy increases.
But where did the Sun got it's low entropy photons to start with? From gravity, empty uniform space has low entropy, which got "scooped up" as the Sun formed.
Your question fascinated me. Googling "where did the Sun got its low entropy" I also came across these explanations:
"Solar energy at Earth is low-entropy because all of it comes from a region of the sky with a diameter of half a degree of arc."
also, from another reply:
"Sunlight is low entropy because the sun is very hot. Entropy is essentially a measure of how spread out energy is. If you consider two systems with the same amount of thermal energy, then the one where that energy is more concentrated (low entropy) will be hotter."
Probably it's a bit of both. I'm not sure I understand your hypothesis about the Sun scooping up empty, low-entropy space. Wasn't it formed from dusts and gases created by previous stellar explosions, i.e. the polar opposite of low entropy?
This is just a question about the origins of inhomogeneity in the universe. The prevailing theory is cosmic inflation, I believe: in the early universe a quantum field existed in a high entropy state and then the rapid expansion of space magnified small spatial inhomogeneities in the field into large-scale structures. What we see as "low entropy" structures like stars are actually just high entropy, uniform structures at a higher scale but viewed from up close so that we can see finer-scale structure.
But where did the Sun got it's low entropy photons to start with? From gravity, empty uniform space has low entropy, which got "scooped up" as the Sun formed.
From the Big Bang originally. We don’t know what caused the Big Bang.
It's a fascinating idea that gravity could be an emergent result of how information works in the universe. I feel like we still don't have that clear piece of evidence where this model predicts something different from general relativity. For now it is one of those theories that are fun to explore but still hard to fully accept.
the statistical mechanical definition of entropy relies on the number of possible arrangements of particles in a system. In a closed system entropy approaches an equilibrium which has been sensationally described as the 'heat death of the universe'. But since we know our universe is expanding, the number of possible arrangements is also increasing so entropy may never reach equilibrium. If the universe is expanding faster than its components redistribute, then entropy could be decreasing. With this in mind, any theory involving entropy as a component of gravity would suggest a changing gravity over time
This effect reminds me of the "hydrophobic interactions" used when modelling biological systems. E.g. The tendency for hydrophobic residudes to be on the inside of a protein.
really enjoyed this article from a few years ago. looks like the domain is no longer active but the content is a good intro and most of the external links still work
Wonder if this perspective is compatible with Wolframs physics model based on hypergraphs?
Gravity, in this framework, is an emergent property arising from the statistical behavior of the hypergraph's evolution, suggesting that gravity is an "entropic force" arising from the tendency of the system to minimize its computational complexity
Couldn’t it be a byproduct of frame dragging? Any massive object that spins is pulling stuff into it by forcing things to rotate in some kind of space time whirlpool?
This means if something massive doesn’t spin, it would have no gravity, but isn’t everything large enough to have gravity in space pretty much spinning?
TLDR: areas around treasure have higher entropy by a measure relevant primarily to stochastic movement of foxes. Since there are thus on average more ways for a fox to walk toward treasure than away, they tend to gravitate toward treasure.
[+] [-] abetusk|9 months ago|reply
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.
[0] https://en.wikipedia.org/wiki/Granular_convection
[1] https://www.youtube.com/watch?v=Incnv2CfGGM
[+] [-] hellohello2|9 months ago|reply
"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."
[+] [-] FilosofumRex|9 months ago|reply
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
[+] [-] raattgift|9 months ago|reply
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.
[+] [-] Someone|9 months ago|reply
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.
[+] [-] WalterBright|9 months ago|reply
I always thought it was because the smaller nuts can fall into smaller spaces, while the larger nuts cannot.
[+] [-] forgotoldacc|9 months ago|reply
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.
[+] [-] hcarvalhoalves|9 months ago|reply
[+] [-] hatsunearu|9 months ago|reply
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.
[+] [-] ndriscoll|9 months ago|reply
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"?
[+] [-] collaborative|9 months ago|reply
[+] [-] sim7c00|9 months ago|reply
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
[+] [-] MathMonkeyMan|9 months ago|reply
But there are so many potential assumptions baked into these theories that it's hard to believe when they claim, "look, Einstein's field equations."
[+] [-] pif|9 months ago|reply
[+] [-] the__alchemist|9 months ago|reply
[+] [-] lewdwig|9 months ago|reply
[+] [-] cantor_S_drug|9 months ago|reply
[+] [-] elyase|9 months ago|reply
[+] [-] nitwit005|9 months ago|reply
[+] [-] quantadev|9 months ago|reply
My favorite conjecture is that what happens is things effectively lose a dimension when they reach the EH surface, and become like a "flatlander" (2D) universe, having only two degrees of freedom on the surface. For such a 2D universe their special "orthogonal" dimension they'd experience as "time" is the surface normal vector. Possibly time only moves forward for them when something new "falls in" causing the sphere to expand.
This view of things also implies the Big Bang is wrong, if our universe is a 3D EH. Because if you roll back the clock on an EH back to when it "formed" you don't end up at some distant past singularity; you simply get back to a time where stuff began to clump together from higher dimensions. The universe isn't exploding from a point, it's merely expanding because it itself is a 3D version of what we know as Event Horizons.
Just like a fish can't tell it's in water, we can't tell we're on a 3D EH. But we can see 2D versions of them embedded in our space.
[+] [-] meindnoch|9 months ago|reply
To me, entropy is not a physical thing, but a measure of our imperfect knowledge about a system. We can only measure the bulk properties of matter, so we've made up a number to quantify how imperfect the bulk properties describe the true microscopic state of the system. But if we had the ability to zoom into the microscopic level, entropy would make no sense.
So I don't see how gravity or any other fundamental physical interaction could follow from entropy. It's a made-up thing by humans.
[+] [-] antonvs|9 months ago|reply
Physical entropy governs real physical processes. Simple example: why ice melts in a warm room. More subtle example: why cords get tangled up over time.
Our measures of entropy can be seen as a way of summarizing, at a macro level, the state of a system such as that warm room containing ice, or a tangle of cables, but the measure is not the same thing as the phenomenon it describes.
Boltzmann's approach to entropy makes the second law pretty intuitive: there are far more ways for a system to be disordered than ordered, so over time it tends towards higher entropy. That’s why ice melts in a warm room.
[+] [-] prof-dr-ir|9 months ago|reply
Nevertheless there is a distinct "reality" to entropic forces, in the sense that it is something that can actually be measured in the lab. If you are not convinced then you can look at:
https://en.wikipedia.org/wiki/Entropic_force
and in particular the example that is always used in a first class on this topic:
https://en.wikipedia.org/wiki/Ideal_chain
So when viewed in this way entropy is not just a "made-up thing", but an effective way to describe observed phenomena. That makes it useful for effective but not fundamental laws of physics. And indeed the wiki page says that entropic forces are an "emergent phenomenon".
Therefore, any reasonable person believing in entropic gravity will automatically call gravity an emergent phenomenon. They must conclude that there is a new, fundamental theory of gravity to be found, and this theory will "restore" the probabilistic interpretation of entropy.
The reason entropic gravity is exciting and exotic is that many other searches for this fundamental theory start with a (more or less) direct quantization of gravity, much like one can quantize classical mechanics to arrive at quantum mechanics. Entropic gravity posits that this is the wrong approach, in the same way that one does not try to directly quantize the ideal gas law.
[0] Let me stress this: there is no entropy without probability distributions, even in physics. Anyone claiming otherwise is stuck in the nineteenth century, perhaps because they learned only thermodynamics but not statistical mechanics.
[+] [-] logicchains|9 months ago|reply
[+] [-] willvarfar|9 months ago|reply
[+] [-] Ma8ee|9 months ago|reply
There’s no reason to involve our knowledge of the system. Entropy is a measure of the number of possible micro states for a given system, and that number exists independently of us.
[+] [-] analog31|9 months ago|reply
I'm only reminded about this because, though I'm a physicist, I've been out of school for more than 3 decades, and decided that I owed myself a refresher on thermodynamics. This coincided with someone on HN recommending David Tong's textbook-quality lecture notes:
https://www.damtp.cam.ac.uk/user/tong/statphys.html
I think at least the first few pages are readable to a layperson, and address the issue of our imperfect knowledge of the precise configuration of a system containing, say, 1e23 particles.
But if we knew all of those relationships, the system would still have entropy.
[+] [-] fnordpiglet|9 months ago|reply
[+] [-] dist-epoch|9 months ago|reply
But we also know that's an approximation we tell kids, really life gets low entropy photons from the Sun, does it's thing, and then emits high entropy infrared waste heat. Energy is conserved, while entropy increases.
But where did the Sun got it's low entropy photons to start with? From gravity, empty uniform space has low entropy, which got "scooped up" as the Sun formed.
EDIT: not sure why this is downvoted, is the explanation Nobel Physics laureate Roger Penrose gives: https://g.co/gemini/share/bd9a55da02b6
[+] [-] uncircle|9 months ago|reply
"Solar energy at Earth is low-entropy because all of it comes from a region of the sky with a diameter of half a degree of arc."
also, from another reply:
"Sunlight is low entropy because the sun is very hot. Entropy is essentially a measure of how spread out energy is. If you consider two systems with the same amount of thermal energy, then the one where that energy is more concentrated (low entropy) will be hotter."
https://physics.stackexchange.com/questions/796434/why-does-...
Probably it's a bit of both. I'm not sure I understand your hypothesis about the Sun scooping up empty, low-entropy space. Wasn't it formed from dusts and gases created by previous stellar explosions, i.e. the polar opposite of low entropy?
[+] [-] dawnofdusk|9 months ago|reply
[+] [-] mjanx123|9 months ago|reply
The photons from Sun are hot, the space around Sun is cold, the system has a low entropy.
If the space around Sun was as hot as the photons, the entropy would be high.
[+] [-] aurareturn|9 months ago|reply
[+] [-] Caelus9|9 months ago|reply
[+] [-] bawana|9 months ago|reply
[+] [-] the__alchemist|9 months ago|reply
[+] [-] uticus|9 months ago|reply
https://web.archive.org/web/20211215122133/https://an0maly.c...
[+] [-] raindeer2|9 months ago|reply
Gravity, in this framework, is an emergent property arising from the statistical behavior of the hypergraph's evolution, suggesting that gravity is an "entropic force" arising from the tendency of the system to minimize its computational complexity
[+] [-] deadbabe|9 months ago|reply
This means if something massive doesn’t spin, it would have no gravity, but isn’t everything large enough to have gravity in space pretty much spinning?
[+] [-] ang_cire|9 months ago|reply
[+] [-] vkou|9 months ago|reply
[+] [-] nathias|9 months ago|reply
[+] [-] hoseja|9 months ago|reply
[+] [-] colanderman|9 months ago|reply
TLDR: areas around treasure have higher entropy by a measure relevant primarily to stochastic movement of foxes. Since there are thus on average more ways for a fox to walk toward treasure than away, they tend to gravitate toward treasure.
[+] [-] fourthark|9 months ago|reply
[+] [-] MintNow|9 months ago|reply
[+] [-] omeysalvi|9 months ago|reply
[+] [-] metalman|9 months ago|reply
[+] [-] amai|9 months ago|reply
[+] [-] almosthere|9 months ago|reply