Is Gravity Just Entropy Rising? Long-Shot Idea Gets Another Look
(quantamagazine.org)77 points by pseudolus 11 hours ago
77 points by pseudolus 11 hours ago
"There’s some kind of gas or some thermal system out there that we can’t see directly" - The Ether is back on the menu boys
It has been back for a while in the form of quantum fields.
Isn't that how equations get solved?
Pretty much anything known entered through such placeholder, it's just that equations could be connected more easily.
It's not like Higgs field is something you can directly observe
Right, but you can push unknowns into tmp vars only so much before you have to introduce constraints, otherwise it's all downright undetermined. You have to inject a structure into the placeholder soup or you're just pushing ambiguity around with no real net gain.. which is also fun to play around, question is will you get a paper out of it or even paid if you play like that to no end.
Maybe, (I don't know), but it's easy to accidentally come up with a theory of "mysterious stuff" that appears to explain something, but neither constrains your expectation nor provides predictions.
Phlogiston is the classic example. https://www.lesswrong.com/posts/RgkqLqkg8vLhsYpfh/fake-causa...
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.
Please, how is the article related to MOND's theories?
I don't get it.
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.
If you want to only have one possible past (i.e. can't destroy information) then when you end up in one branch of quantum state you need to "store" enough information to separate you form other branches and you really do need to have multiple possible microstates to differentiate them. If you look post-factum obviously you did end up in a specific state, but statistics do their work otherwise.
Your perspective is incorrect.
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.
My take, for what it's worth,
Entropy isn’t always the driver of physical change, sometimes it’s just a map.
Sometimes that map is highly isomorphic to the physical process, like in gas diffusion or smoke dispersion. In those cases, entropy doesn't just describe what happened, it predicts it. The microstates and the probabilities align tightly with what’s physically unfolding. Entropy is the engine.
But other times, like when ice melts, entropy is a summary, not a cause. The real drivers are bond energies and phase thresholds. Entropy increases, yes, but only because the system overcame physical constraints that entropy alone can’t explain. In this case, entropy is the receipt, not the mechanism.
So the key idea is this: entropy’s usefulness depends on how well it “sees” the real degrees of freedom that matter. When it aligns closely with the substrate, it feels like a law. When it doesn't, it’s more like coarse bookkeeping after the fact.
The second law of thermodynamics is most “real” when entropy is the process. Otherwise, it’s a statistical summary of deeper physical causes.
> there are far more ways for a system to be disordered than ordered
I'm a complete layman when it comes to physics, so forgive me if this is naive — but aren't "ordered" and "disordered" concepts tied to human perception or cognition? It always seemed to me that we call something "ordered" when we can find a pattern in it, and "disordered" when we can't. Different people or cultures might be able to recognize patterns in different states. So while I agree that "there are more ways for a system to be disordered than ordered," I would have thought that's a property of how humans perceive the world, not necessarily a fundamental truth about the universe
You only hear these terms in layman explanations. Physics has precise definitions for these things. When we say "ordered", we mean that a particular macrostate has only few possible microstates.
Check this Wikipedia article for a quick overview: https://en.wikipedia.org/wiki/Microstate_(statistical_mechan...
Details can be found in any textbook on statistical mechanics.
Think minimum description length. Low entropy states require fewer terms to fully describe than high entropy states. This is an objective property of the system.
>Simple example: why ice melts in a warm room.
Ice melting is simply the water molecules gaining enough kinetic energy (from collisions with the surrounding air molecules) that they break the bonds that held them in the ice crystal lattice. But at the microscopic level it's still just water molecules acting according to Newton's laws of motion (forgetting about quantum effects of course).
Now, back on the topic of the article: consider a system of 2 particles separated by some distance. Do they experience gravity? Of course they do. They start falling towards the midpoint between them. But where is entropy in this picture? How do you even define entropy for a system of 2 particles?
> But where is entropy in this picture? How do you even define entropy for a system of 2 particles?
The answer is that this doesn't happen in a system with only 2 particles. The idea of gravity as an entropic phenomenon is that you introduce some other kind of particle that permeates spacetime, so there is no system that only contains 2 particles. You may use some idea like virtual particles from quantum field theory, or you may define "quanta of space time" as something that is not technically a particle but basically works like one in a handwavy sense.
But the basic point of these entropy based theories is to explain gravity, and typcilaly spacetime itself, as an emergent result of a collection of numerous objects of some kind. This necessarily means that they don't make sense if applied to idealized systems with very few objects - which is why they typically posit such isolated systems simply can't actually exist in reality.
Let me try to answer. Let's say the particles are experiencing gravity as a natural entropy phenomena. They will attract until they become so close that they are now seen as a single particle. The new system has a lower entropy and a higher gravity than before.
Explanation seems very rudimentary but that is the gist of the theory.
From my point of view, I might add the layer of information density. Every quantum fluctuation is an event and the more particles the more information is produced in a defined space volume. But there is no theory of information that is linked to the physics so ...that let me leave as that :).
I think original post is confused exactly because of “tangled chords” analogies. Something being “messy” in our daily lives can be a bit subjective, so using the same analogies for natural forces may seem a tad counterintuitive actually.
Maybe it would be more fitting to say that it just so happens that our human definition of “messy” aligns with entropy, and not that someone decided what messy atoms look like.
I’d say a bucket of water is more neat than a bucket of ice, macroscopically.
But "disordered" and "ordered" states are just what we define them to be: for example, cords are "tangled" only because we would prefer arrangements of cords with less knots, and knots form because someone didn't handle the cords carefully.
Physical processes are "real", but entropy is a figment.
I believe you are correct.
Entropy is not a physical quantity, it is a measure of how far a system is from equilibrium.
Lots of people talk about order/disorder or macro and micro states, not realizing these are things we've invented and aren't physical in nature.
The way we use the word 'entropy' in computer science is different from how its used in physics. Here is a really good explanation in a great talk! https://youtu.be/Kr_S-vXdu_I?si=1uNF2g9OhtlMAS-G&t=2213
It sounds like you're talking about information entropy which to my understanding is analogue to but not the same as entropy in physics?
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:
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.
Sure, I'm not denying that entropy exists as a concept, that can be used to explain things macroscopically. But like you said, it's origins are statistical. To me, temperature is also a similar "made up" concept. We can only talk about temperature, because a sufficiently large group of particles will converge to a single-parameter distribution with their velocities. A single particle in isolation doesn't have a temperature.
So if they say gravity might be an entropic effect, does that mean that they assume there's something more fundamental "underneath" spacetime that - in the statistical limit - produces the emergent phenomenon of gravity? So it isn't the entropy of matter that they talk about, but the entropy of something else, like the grains of spacetime of whatever.
Im an idiot, let's get that out of the way first. I think that your temperature analogy answered your own question.
I guess my question in turn is, if we imagine a universe at the end of time(?), one that maybe dominated by a few black holes and not much else. Would an observer experience gravity if place sufficiently far enough way? Or even further, if nothing is left in the universe at all. Assuming that doesn't cause a big crunch, rip, or whatever...
Even if we take that view, gravity is still basically a similar case. What we call "gravity" is really an apparent force, that isnt a force at all when seen from a full 4d pov.
Imagine sitting outside the whole universe from t=0,t=end and observing one whole block. Then the trajectories of matter, unaffected by any force at all, are those we call gravitational.
From this pov, it makes a lot more sense to connect gravity with some orderly or disorderly features of these trajectories.
Inertia, on this view, is just a kind of hysteresis the matter distribution of the universe has -- ie., a kind of remembered deformation that persists as the universe evolves.
> From this pov, it makes a lot more sense to connect gravity with some orderly or disorderly features of these trajectories.
On the contrary, entropic gravity works pretty well for the Newtonian view of gravity as a force, and not the GR view of gravity as a deformation of space time and analogous to acceleration. Acceleration is a very elementary concept, one you find even in microscopic descriptions. Gravity being essentially the same thing makes it far more elementary than a concept like entropy, which only applies to large groups of particles.
So, if the GR picture is the right one, if gravity and acceleration are essentially the same thing, its very hard to see how that aligns with gravity being an emergent phenomenon that only happens at large scales. However, if gravity is just a tendency for massive objects to come together, as in the Newtonian picture, that is perfectly easy to imagine as an entropic effect.
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.
If we knew the exact state of all particles in an enclosed system, we can calculate what future states will be exactly. No need to calculate possible states.
> Entropy isn't a function of imperfect knowledge. It's a function of the possible states of a system and their probability distributions.
There are no probability distributions over possible states when there is perfect knowledge of the state.
> Quantum mechanics
Entropy is also zero for a pure quantum state. You won’t have entropy without imperfect knowledge.
> There are no probability distributions over possible states when there is perfect knowledge of the state.
I know very little about physics but I thought that the leading interpretations of quantum physics say that the probability distribution is all we can know about a system. The entropy is not due to due to a lack of information about the quantum state, but because the outcomes are inherently stochastic?
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.
EDIT: not sure why this is downvoted, is the explanation Nobel Physics laureate Roger Penrose gives: https://g.co/gemini/share/bd9a55da02b6
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."
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?
I read the gravity explanation for the sun low entropy in the "Road to Reality" book from Roger Penrose. Asked Gemini to summarize the argument (scroll to end)
It's also in his previous book "The Emperor's New Mind: Concerning Computers, Minds and The Laws of Physics", along with a lot more. Strongly recommended (even though after reading a lot of Greg Egan, my views on consciousness somewhat shifted towards "classical computation can do it, too".)
Space exists around things with mass. Also, above-absolute-zero temperatures cause particles to jump around randomly.
Now if there is "more space" around particle A, particle B will have a slightly higher statistical chance of randomly jumping closer to it, than farther.
Rinse-repeat. Gravity as we know it.
>Also, above-absolute-zero temperatures cause particles to jump around randomly.
Does it? A single free particle won't "jump around randomly". Thermal motion is plain Newtonian motion with an extremely high rate of collisions. There's nothing random about it (let's put quantum things aside for now).
If space existed around things with mass, then what would you call the emptiness that replaces space the further you go away from things with mass?
It sounds a bit like Le Sage's theory of gravity:
> particle B will have a slightly higher statistical chance of randomly jumping closer to it,
Why?
Also how do you explain acceleration due to gravity with that model. How do you explain solid objects?
My guess would be the answer is right in the part before you quote? If theres more "space" (imagining more space coordinates possible) for me on the left than on the right, me jumping to a random location would statistically move me left.
Repeating results in movement, getting closer to the object intensifies this effect, results in acceleration.
Solid objects are products of electric charge preventing atoms/particles from hitting each other, I dont think that has to have to do anything with gravity in this example?
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."