Thanks for the link (+1) but it's very far from being an explanation for the layman.
> In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime.
It doesn't explain how gravity emerges from bits of information. I'd love to understand how those bits make apples move from trees to the ground, the equivalence between gravity and acceleration and the weightless free fall experience. That should be one paragraph for the general principle and one for each example. Then it's for the layman, and me too :-)
Thanks for the layman link... I started reading and encountered a concept I didn't understand so I googled it. Then, I encountered another concept I didn't understand and google that, again and again, until I was several pages deep and only on the second sentence of the paper's description. TIL I don't know as much as I thought.
There is a great deal of observational evidence favoring Lambda-Cold-Dark-Matter, ie the status quo of cosmology with dark matter. The bullet cluster [1] is evidence for two colliding galaxy clusters where the luminous matter ran into itself but the dark matter kept going. The total mass (luminous plus dark) can be mapped out with the gravitational lensing of background galaxies and can be done very accurately. This was essentially the nail in the coffin for the leading alternative theory modified newtonian dynamics (MOND).
Dark matter is so deeply embedded into cosmology and modern astrophysics, you would have to explain a host of solidly observed phenomena to be a compelling alternative (baryon acoustic oscillations, the cosmic microwave background, primordial abundances, the evolution of structure or the "cosmic web"). Comparing to those things would probably take man-decades of work and also require computationally expensive cosmological simulations on super computers, so I'm skeptical we will see the kind of theoretical/observational comparison that would make you sit up and take notice.
I love this, it feels cleaner and more elegant a solution than dark matter particles. However, nature doesn't care what we like, so that doesn't make it more likely to be true.
I agree that it sounds elegant, especially since it puts entropy/information into the core ontology so as to explain the arrow of time and thermodynamic behavior as part of the same theory that can account for Einsteinian space-time.
I just wish I understood enough physics to tell this paper from elegant-sounding word salad!
If you're wondering how this isn't some weird form of MOND, which is basically dead as an explanation, here's an important paragraph from the paper:
> We like to emphasize that we have not derived the theory of modified Newtonian
dynamics as proposed by Milgrom. In our description there is no modification of the
law of inertia, nor is our result (7.43) to be interpreted as a modified gravitational field
equation. It is derived from an estimate of an effect induced by the displacement of the
free energy of the underlying microscopic state of de Sitter space due to matter. This
elastic response is then reformulated as an estimate of the gravitational self-energy
due to the apparent dark matter in the form of the integral relation (7.40). Hence,
although we derived the same relation as modified Newtonian dynamics, the physics
is very different. For this reason we referred to the relation (7.43) as a fitting formula,
since it is important to make a clear separation between an empirical relation and
a proposed law of nature. There is little dispute about the observed scaling relation
(7.43), but the disagreement in the scientific community has mainly been about whether
it represents a new law of physics. In our description it does not.
> The central idea of this paper is that the volume law contribution to the entanglement
entropy, associated with the positive dark energy, turns the otherwise “stiff” geometry
of spacetime into an elastic medium. We find that the elastic response of this ‘dark
energy’ medium takes the form of an extra ‘dark’ gravitational force that appears to
be due to ‘dark matter’.
I think it helps a lot to try to understand the universe from an engineering perspective, thus by posing the question "How would I build a universe?"
The apparent obvious answer is : as an N dimensional matrix where each cell executes some code.
With such a setup, effects like gravity, are more probable to be emergent effects.
Another consideration is that in such a setup the only thing that matters is the state of the matrix, in other words there is no time, just iterations.
So my 2 cents are: gravity should be explained as an emergent phenomenon, and time should be removed from all equations that try to explain the underlying fabric of reality.
> According to the holographic principle, all the information in the entire universe can be described on a giant imaginary sphere around it. Verlinde now shows that this idea is not quite correct: part of the information in our universe is contained in space itself.
I thought the holographic principle just said the amount of information in any physical structure (up to the whole universe) was such that it could fit on a 2D surface the size of the event horizon of the structure, not that it actually is contained on the surface. Does this theory change that?
Gravitons always struck me as a goofy half-baked concoction (even though we've supposedly observed them now), and so did dark matter. Emergent gravity theories are very interesting.
Gravitons appear in several different families of theory.
We have not observed any of them; the interaction cross-section is so small (because gravity is so weak) that we have no hope of observing a single graviton, or failing to observe an expected one, any time soon.
Gravitational waves have spin 2 symmetry and are massless. A straightforward quantization of a gravitational wave leads to gravitons as a spin 2 massless gauge boson. Conversely, in almost any gauge theory that admits a massless spin 2 particle, that particle mediates a force almost exactly like gravitation, and is amenable to analysis under the Paramaterized Post-Newtonian Formalism.
Indeed, when gravity is weak, such theories essentially exactly reproduce General Relativity. It's only in strong gravity that non-negligible differences appear, and in this case strong gravity is when the quantum uncertainty in curvature distorts distances in ways that can't be worked around e.g by a change of basis or system of coordinates. Alternatively, gravity is strong when you have multiple loops of gravitons in a Feynman diagram.
So in the type of quantization above, the problem is that the momentum carried by an individual graviton is much much much much smaller than that carried by an individual low frequency photon. For example, a near-infrared photon has a momentum-energy on the order of about 1 eV/c^2, a 100 MHz photon has an energy on the order of 10^-7 eV/c^2, and a graviton will be less than 10^-22 eV/c^2 (LIGO collab., https://arxiv.org/abs/1602.03837, about six paragraphs before section VIII).
In other words, even the extremely weak signals LIGO's detected represent waves comprising huuuuuuge numbers of gravitons. But each graviton is so weak (or equilvalent long-wavelengthed) that we have no hope of detecting it individually. Think of how much fun you would have trying to isolate with your fingers an individual molecule of water in a surfers-beware ocean wave -- that'd be easier than isolating a graviton from a presently-detectable GW.
Observing gravitational waves is not really the same as observing individual gravitons. It's unlikely we can observe gravitons with any equipment we can actually build.
Gravitons are basically unavoidable. They may not be fundamental particles, but they'll always show up in the long-wavelength approximation of any theory of quantum gravity that extends general relativity.
So I too love the idea that we don't need dark matter to explain all the gravitational effects we observe in the universe. However, is it just me or does the falsification of this seem a bit sketchy: if we find a dark matter particle...then my theory is false. That's a bit like saying...oh yeah if we find a different way to unify general relativity and quantum mechanics...then String Theory is false. It seems like there should be a 'smaller' way to falsify it.
I think you will find all the models are false to a degree. A model is only an approximation of the real thing. Quantum mechanics and General relatively cannot both be right. The obvious answer is that they are both wrong.
Remember that these people use a language that might sound similar to common english, but words have different meaning and it is much more precise. No reading between the lines.
He gave an example of what would falsify his theory, that is all.
In the summary, it says that these theories are "best understood in Anti de Sitter space...". I thought current observations ran against a spacetime that had a negative curvature. Are they just saying the math is easier in that geometry, but that it still applies to flat (or ever-so-slightly-positively curved) spacetime? Or are they arguing in favor of a "saddle-shaped" universe?
Spacetime has a positive cosmological constant, so it's closer to de-Sitter than Anti de-Sitter. It doesn't really have to do with curvature exactly, which is given by the Riemann tensor (so it doesn't make sense to say it's positive/negative).
The ADS in 'ADS/CFT correspondence' stands for 'anti-de Sitter'. Any time you read someone talking about 'information on the boundary' of a bulk space, that's usually what they're working with.
I'm sure there's a way to transform everything to a de Sitter space, but i'm neither a mathematician nor a physicist, so I don't know how it's done...
Imagining gravitational waves inevitably leads me to believe that gravity is a force that varies (in a cycle) over spacetime. But.. what exactly does it mean to propagate over a cycle in spacetime? (particularly the time aspect confuses me since something propagating in space remains constant in time (either accelerating or decelerating)
I do not understand this stuff, but the connection between gravity and horizont sounds like this [quantized acceleration theory](http://physicsfromtheedge.blogspot.de/)
I'm currently reading The Confusion [0] and just got through Leibniz's explanation of monadology [1][2]. Summarized very very briefly, the idea is that every particle in the universe is a sort of computational device, and their actions/reactions are determined by their "perception" of all the other particles in the universe.
Does this research suggest that he may have been semi-correct??
Frankly, I'd be surprised if anything ever suggested that Leibniz's colorful metaphysics was correct, even or incorrect.
Leibnizian monads are unextended and unable to act on one another (from §1 and §7 of the Monadology), which is to say that they're non-physical. On the other hand, as they're somehow supposed each to have a complete picture to the world (§56), they imply some form of non-locality. Which is to say that they're some very spooky things.
The thing that comes closest to Leibniz's monadology is the idea that the world is a simulation running inside a computer. But Leibniz's idea was more like that there is an arbitrarily large number of computers running the same simulation, while some of them also run artificial intelligences which just watch the simulation unfold.
Which is an idea I think can be fairly described as "completely nuts".
It does not contradict Einstein's theory of gravity, just as Einstein did not contradict Newton. All three are different models explaining progressively more thorough levels of observation.
Don't think of a theory so much as "what is true" but rather as a set of propositions and equations which are designed to model the way physical processes are observed to occur.
Theories exist within their own provenance of observation. Both Einstein and Newton's theories reflect reality within certain limits, but outside those limits they do not. To allow Einstein's theory to apply at larger scales, we proposed dark matter because Einstein could not account for the behavior of galaxies unless there was some hidden element involved. Thus we proposed dark matter. It is a reasonable accommodation, but one that has defied confirmation.
The emergent theory embraces both Newton and Einstein, eliminates any need for dark matter, and attempts to explain all current observations within a single framework.
Einstein's theory made testable predictions which were later proven to be true. The emergent theory lacks such confirmation, and so time will tell whether it reflects reality.
[+] [-] supermatou|9 years ago|reply
[+] [-] pmontra|9 years ago|reply
> In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime.
It doesn't explain how gravity emerges from bits of information. I'd love to understand how those bits make apples move from trees to the ground, the equivalence between gravity and acceleration and the weightless free fall experience. That should be one paragraph for the general principle and one for each example. Then it's for the layman, and me too :-)
[+] [-] geo101101|9 years ago|reply
[+] [-] nealabq|9 years ago|reply
[+] [-] freelancezombie|9 years ago|reply
[+] [-] mrreelmo|9 years ago|reply
[+] [-] autocorr|9 years ago|reply
Dark matter is so deeply embedded into cosmology and modern astrophysics, you would have to explain a host of solidly observed phenomena to be a compelling alternative (baryon acoustic oscillations, the cosmic microwave background, primordial abundances, the evolution of structure or the "cosmic web"). Comparing to those things would probably take man-decades of work and also require computationally expensive cosmological simulations on super computers, so I'm skeptical we will see the kind of theoretical/observational comparison that would make you sit up and take notice.
[1] https://en.wikipedia.org/wiki/Bullet_Cluster
[+] [-] eloff|9 years ago|reply
[+] [-] eli_gottlieb|9 years ago|reply
I just wish I understood enough physics to tell this paper from elegant-sounding word salad!
[+] [-] Avshalom|9 years ago|reply
It's literally just
"Hey you know how there's like a bunch of particles that don't interact with EM forces?"
"Yeah"
"Well what if there's like a BUNCH of particles that don't interact with EM forces?"
"...oh hey that makes this math way easier"
[+] [-] taliesinb|9 years ago|reply
> We like to emphasize that we have not derived the theory of modified Newtonian dynamics as proposed by Milgrom. In our description there is no modification of the law of inertia, nor is our result (7.43) to be interpreted as a modified gravitational field equation. It is derived from an estimate of an effect induced by the displacement of the free energy of the underlying microscopic state of de Sitter space due to matter. This elastic response is then reformulated as an estimate of the gravitational self-energy due to the apparent dark matter in the form of the integral relation (7.40). Hence, although we derived the same relation as modified Newtonian dynamics, the physics is very different. For this reason we referred to the relation (7.43) as a fitting formula, since it is important to make a clear separation between an empirical relation and a proposed law of nature. There is little dispute about the observed scaling relation (7.43), but the disagreement in the scientific community has mainly been about whether it represents a new law of physics. In our description it does not.
[+] [-] slicktux|9 years ago|reply
[+] [-] MrQuincle|9 years ago|reply
+ Kobakhidze explaining neutron experiments: https://arxiv.org/abs/1108.4161
+ http://physics.stackexchange.com/questions/4289/is-gravity-a...
+ Lubos Motl: http://motls.blogspot.nl/2010/01/erik-verlinde-why-gravity-c... and http://motls.blogspot.nl/2010/01/erik-verlinde-comments-abou...
It's nice to see that also experts in these highly specified fields quarrel about what it means to be "entropic" and "irreversible", etc.
[+] [-] 323454|9 years ago|reply
[+] [-] plank|9 years ago|reply
[+] [-] zer0gravity|9 years ago|reply
With such a setup, effects like gravity, are more probable to be emergent effects.
Another consideration is that in such a setup the only thing that matters is the state of the matrix, in other words there is no time, just iterations.
So my 2 cents are: gravity should be explained as an emergent phenomenon, and time should be removed from all equations that try to explain the underlying fabric of reality.
[+] [-] mcbits|9 years ago|reply
I thought the holographic principle just said the amount of information in any physical structure (up to the whole universe) was such that it could fit on a 2D surface the size of the event horizon of the structure, not that it actually is contained on the surface. Does this theory change that?
[+] [-] hammock|9 years ago|reply
[+] [-] raattgift|9 years ago|reply
We have not observed any of them; the interaction cross-section is so small (because gravity is so weak) that we have no hope of observing a single graviton, or failing to observe an expected one, any time soon.
Gravitational waves have spin 2 symmetry and are massless. A straightforward quantization of a gravitational wave leads to gravitons as a spin 2 massless gauge boson. Conversely, in almost any gauge theory that admits a massless spin 2 particle, that particle mediates a force almost exactly like gravitation, and is amenable to analysis under the Paramaterized Post-Newtonian Formalism.
Indeed, when gravity is weak, such theories essentially exactly reproduce General Relativity. It's only in strong gravity that non-negligible differences appear, and in this case strong gravity is when the quantum uncertainty in curvature distorts distances in ways that can't be worked around e.g by a change of basis or system of coordinates. Alternatively, gravity is strong when you have multiple loops of gravitons in a Feynman diagram.
So in the type of quantization above, the problem is that the momentum carried by an individual graviton is much much much much smaller than that carried by an individual low frequency photon. For example, a near-infrared photon has a momentum-energy on the order of about 1 eV/c^2, a 100 MHz photon has an energy on the order of 10^-7 eV/c^2, and a graviton will be less than 10^-22 eV/c^2 (LIGO collab., https://arxiv.org/abs/1602.03837, about six paragraphs before section VIII).
In other words, even the extremely weak signals LIGO's detected represent waves comprising huuuuuuge numbers of gravitons. But each graviton is so weak (or equilvalent long-wavelengthed) that we have no hope of detecting it individually. Think of how much fun you would have trying to isolate with your fingers an individual molecule of water in a surfers-beware ocean wave -- that'd be easier than isolating a graviton from a presently-detectable GW.
[+] [-] wbhart|9 years ago|reply
[+] [-] maverick_iceman|9 years ago|reply
[+] [-] wanda|9 years ago|reply
Source? Or are you saying that the existence of gravitational waves constitute a proof of the existence of gravitons, due to wave/particle duality?
I'm not a physicist, but all I thought we had detected were gravitational waves.
[+] [-] theptip|9 years ago|reply
[+] [-] auntienomen|9 years ago|reply
[+] [-] cshreve|9 years ago|reply
[+] [-] jcoffland|9 years ago|reply
[+] [-] npgatech|9 years ago|reply
It is an attempt to explain. Just like an attempt to explain (prediction) of Higgs Boson. It was only true once we observed it via experimentation.
Falsification makes sense if the theory has been experimentally validated. You can't falsify something that has not been proven.
There is no one claiming String theory or the theory discussed here is true - no experimental evidence has been produced.
[+] [-] maikb|9 years ago|reply
He gave an example of what would falsify his theory, that is all.
[+] [-] slowmovintarget|9 years ago|reply
[+] [-] maverick_iceman|9 years ago|reply
[+] [-] empath75|9 years ago|reply
I'm sure there's a way to transform everything to a de Sitter space, but i'm neither a mathematician nor a physicist, so I don't know how it's done...
[+] [-] spynxic|9 years ago|reply
[+] [-] novalis78|9 years ago|reply
[+] [-] benibela|9 years ago|reply
[+] [-] jpm_sd|9 years ago|reply
Does this research suggest that he may have been semi-correct??
[0] https://en.wikipedia.org/wiki/The_Confusion [1] https://en.wikipedia.org/wiki/Monadology [2] https://quantummoxie.wordpress.com/2014/09/09/neal-stephenso...
[+] [-] tpeo|9 years ago|reply
Leibnizian monads are unextended and unable to act on one another (from §1 and §7 of the Monadology), which is to say that they're non-physical. On the other hand, as they're somehow supposed each to have a complete picture to the world (§56), they imply some form of non-locality. Which is to say that they're some very spooky things.
The thing that comes closest to Leibniz's monadology is the idea that the world is a simulation running inside a computer. But Leibniz's idea was more like that there is an arbitrarily large number of computers running the same simulation, while some of them also run artificial intelligences which just watch the simulation unfold.
Which is an idea I think can be fairly described as "completely nuts".
[+] [-] zmanian|9 years ago|reply
[+] [-] rudolf0|9 years ago|reply
[+] [-] teilo|9 years ago|reply
Don't think of a theory so much as "what is true" but rather as a set of propositions and equations which are designed to model the way physical processes are observed to occur.
Theories exist within their own provenance of observation. Both Einstein and Newton's theories reflect reality within certain limits, but outside those limits they do not. To allow Einstein's theory to apply at larger scales, we proposed dark matter because Einstein could not account for the behavior of galaxies unless there was some hidden element involved. Thus we proposed dark matter. It is a reasonable accommodation, but one that has defied confirmation.
The emergent theory embraces both Newton and Einstein, eliminates any need for dark matter, and attempts to explain all current observations within a single framework.
Einstein's theory made testable predictions which were later proven to be true. The emergent theory lacks such confirmation, and so time will tell whether it reflects reality.