Nov 20, 2024 07:53 PM
https://arstechnica.com/science/2024/11/...a-bad-one/
EXCERPTS: . . . One way to view nature is as a vast hierarchy. At the “bottom” of the hierarchy are the quantum fields, which we use quantum field theory to understand. On top of that are all the myriad subatomic and atomic interactions, also governed by quantum mechanics. Above that is chemistry, where the quantum starts mattering less. And on top of all that, far, far removed from quantum fields, are all the wonderful branches of science and their various tools that describe all manner of phenomena: astrophysics, oceanography, geology, sociology, and so on.
It’s technically true that “underneath” everything [...] is quantum field theory. But good luck using quantum field theory to describe those systems. That’s because these higher-order systems are emergent—they have new properties, new laws, and new behaviors that emerge from countless interactions operating at deeper levels.
Most of that time, we can't ever hope to make connections between lower and higher layers, and some physicists and philosophers argue that it might just be impossible to do so consistently. But in some cases, we can tie together low-level principles with higher-order emergent behavior. The best example of this is the relationship between thermodynamics and statistical physics.
Thermodynamics is the study of familiar everyday properties of systems like temperature, pressure, volume, entropy, and all their friends. We have examples of relationships between these properties, like the ideal gas law. Amazingly, you can derive, test, and use the ideal gas law while having no idea what a “gas” is made of (atoms and molecules) and what those components of the gas are doing (bouncing around a lot). So we don’t need a connection between those properties and any underlying rules.
But in an amazing feat of 19th-century physics that doesn’t get nearly enough airtime, we made exactly that connection. Through a set of techniques known as statistical mechanics, we can take the behaviors of individual gas molecules—their kinetic energy and momentum, for example—and use those to derive the emergent properties of temperature, pressure, and entropy of a gas that consists of a whole bunch of molecules working together.
[...] The roots of the idea that gravity might be emergent go all the way back to the funky '70s ... Maybe there is a deeper connection between the laws of thermodynamics and the laws of gravity. And it could be that if thermodynamics is really an emergent property of some deeper set of physics, then maybe ... the fact that black holes look kind of like warm glowing objects is telling us that gravity is also an emergent property of some deeper physics.
[...] In 2009, Dutch physicist Erik Verlinde guessed that the deeper physics might be some quantum information encoded on the surface of the Universe. This isn’t just some random idea plucked out of the Hat of Magical Physics; it's grounded in the very real observation that a black hole’s surface is much more important than its volume.
[...] Verlinde combined this holographic approach with the concept of black hole thermodynamics to rewrite Newton’s laws, and eventually general relativity, in terms of statistical relationships that give rise to gravity, making it an emergent force.
[...] Ideas can be beautiful, elegant, captivating… and dead wrong. Nature is the ultimate arbiter of ideas in physics, so it’s always up to the evidence to determine which theories we embrace and which we discard.
Our currently accepted paradigm for explaining the large-scale behavior of the Universe is rooted in general relativity, which has been put through its paces with over a century of successful experimental tests. Yet mysteries abound in the Universe, such as the apparent need for matter that’s invisible to us, known as dark matter, to explain the behaviors of stars and galaxies.
[...] In the end, these are pretty limited tests. There’s a lot more to the Universe that dark matter can explain, like the growth of large structures over cosmic time or the fluctuations in the appearance of the cosmic microwave background. Any theory that hopes to replace dark matter must run the full gamut, not just limit itself to galaxies and clusters. So far, nobody has attempted to approach these larger questions through the lens of emergent gravity.
On the theoretical front, emergent gravity has run into issues as well. The journey from vanilla general relativity to emergent gravity takes more than a few assumptions, leaps of faith, detours, and approximations. We don’t know if the holographic approach to physics is valid. We don’t know if the relationship between thermodynamics and black holes is nothing more than a coincidence. We don’t know the underlying quantum physics that might give rise to gravity in this picture.
[...] Is emergent gravity a dead idea? Right now, it seems likely that it is. But is emergent gravity a bad idea? Absolutely not... (MORE - missing details)
EXCERPTS: . . . One way to view nature is as a vast hierarchy. At the “bottom” of the hierarchy are the quantum fields, which we use quantum field theory to understand. On top of that are all the myriad subatomic and atomic interactions, also governed by quantum mechanics. Above that is chemistry, where the quantum starts mattering less. And on top of all that, far, far removed from quantum fields, are all the wonderful branches of science and their various tools that describe all manner of phenomena: astrophysics, oceanography, geology, sociology, and so on.
It’s technically true that “underneath” everything [...] is quantum field theory. But good luck using quantum field theory to describe those systems. That’s because these higher-order systems are emergent—they have new properties, new laws, and new behaviors that emerge from countless interactions operating at deeper levels.
Most of that time, we can't ever hope to make connections between lower and higher layers, and some physicists and philosophers argue that it might just be impossible to do so consistently. But in some cases, we can tie together low-level principles with higher-order emergent behavior. The best example of this is the relationship between thermodynamics and statistical physics.
Thermodynamics is the study of familiar everyday properties of systems like temperature, pressure, volume, entropy, and all their friends. We have examples of relationships between these properties, like the ideal gas law. Amazingly, you can derive, test, and use the ideal gas law while having no idea what a “gas” is made of (atoms and molecules) and what those components of the gas are doing (bouncing around a lot). So we don’t need a connection between those properties and any underlying rules.
But in an amazing feat of 19th-century physics that doesn’t get nearly enough airtime, we made exactly that connection. Through a set of techniques known as statistical mechanics, we can take the behaviors of individual gas molecules—their kinetic energy and momentum, for example—and use those to derive the emergent properties of temperature, pressure, and entropy of a gas that consists of a whole bunch of molecules working together.
[...] The roots of the idea that gravity might be emergent go all the way back to the funky '70s ... Maybe there is a deeper connection between the laws of thermodynamics and the laws of gravity. And it could be that if thermodynamics is really an emergent property of some deeper set of physics, then maybe ... the fact that black holes look kind of like warm glowing objects is telling us that gravity is also an emergent property of some deeper physics.
[...] In 2009, Dutch physicist Erik Verlinde guessed that the deeper physics might be some quantum information encoded on the surface of the Universe. This isn’t just some random idea plucked out of the Hat of Magical Physics; it's grounded in the very real observation that a black hole’s surface is much more important than its volume.
[...] Verlinde combined this holographic approach with the concept of black hole thermodynamics to rewrite Newton’s laws, and eventually general relativity, in terms of statistical relationships that give rise to gravity, making it an emergent force.
[...] Ideas can be beautiful, elegant, captivating… and dead wrong. Nature is the ultimate arbiter of ideas in physics, so it’s always up to the evidence to determine which theories we embrace and which we discard.
Our currently accepted paradigm for explaining the large-scale behavior of the Universe is rooted in general relativity, which has been put through its paces with over a century of successful experimental tests. Yet mysteries abound in the Universe, such as the apparent need for matter that’s invisible to us, known as dark matter, to explain the behaviors of stars and galaxies.
[...] In the end, these are pretty limited tests. There’s a lot more to the Universe that dark matter can explain, like the growth of large structures over cosmic time or the fluctuations in the appearance of the cosmic microwave background. Any theory that hopes to replace dark matter must run the full gamut, not just limit itself to galaxies and clusters. So far, nobody has attempted to approach these larger questions through the lens of emergent gravity.
On the theoretical front, emergent gravity has run into issues as well. The journey from vanilla general relativity to emergent gravity takes more than a few assumptions, leaps of faith, detours, and approximations. We don’t know if the holographic approach to physics is valid. We don’t know if the relationship between thermodynamics and black holes is nothing more than a coincidence. We don’t know the underlying quantum physics that might give rise to gravity in this picture.
[...] Is emergent gravity a dead idea? Right now, it seems likely that it is. But is emergent gravity a bad idea? Absolutely not... (MORE - missing details)
