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Emergent Gravity and the Dark Universe

arxiv.org

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Re: Emergent Gravity and the Dark Universe

#71

Post by wallace_f »

M_Grey wrote: post #66
SomeStupidPoint wrote: post #59

Earlier quoted context omitted.

I mean, one could argue that perhaps EM is really a special kind of gravitational ripple (eg, geons might work out in quantum gravity to model electrons and photons, and then the question arises that if all EM is gravity induced and modeled, is EM not part of gravity?), but I don't know that I've heard it the other way. It's also certainly not the mainstream view, as classical geons are extremely unstable.

I'm not sure how EM and Gravity could be unified as a single force, especially since the best theory of gravity that's out there describes gravity as curvature in spacetime, not a force.

If this is true, the literature I've read--as a layman myself reading from sources written for the layman on the subject--is wildly misleading.

i.e. from the Wikipedia article: https://en.wikipedia.org/wiki/Fundamental_interaction

>Other theorists seek to quantize the gravitational field by the modelling behaviour of its hypothetical force carrier, the graviton and achieve quantum gravity (QG).

Re: Emergent Gravity and the Dark Universe

#72

Post by justifier »

pmontra wrote: post #42
supermatou wrote: post #5

Also, a simple explanation for the layman: http://www.alphagalileo.org/ViewItem.aspx?ItemId=169788&Cult...

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…

i think at the scale of a tree and an apple such insight would be a clear revolution stead essentially a whitepaper

but from the perspective of the paper i think this answers your initial question:

> It doesn't explain how gravity emerges from bits of information.

resides on large scales, infored bu flattening of rotational curves, and entanglement entropy(o)

there is a known relation, inequality, between entanglement entropy, a subsequent of quantum information, and large scale gravitational considerations

should one interpret the relation as 'emergent' or 'coincidence' it would seem a viable avenue of investigation

from the paper:

> 1.3 Hints from observations: the missing mass problem

> ...The observed flattening of rotation curves, as well as many other observations of dark matter phenomena, indicate that they are controlled by the Hubble acceleration scale a0, as first pointed out by Milgrom...

> ...Consider a spherical region with boundary area A(r) = 4πr2 that contains matter with total mass M near its center...

> ...The appearance of the cosmological acceleration scale a0 in galactic dynamics is striking and gives a strong hint towards an explanation in terms of emergent gravity, as envisaged in [38].

> To make this point more clear let us rewrite the above inequality as SM = 2πM/ha0 > The quantity on the left hand side represents the change in the de Sitter entropy caused by adding the mass M, while the right hand side is the entropy of a black hole that would fit inside the region bounded by the area A.

> Our goal is to give a theoretical explanation for why the emergent laws of gravity differ from those of general relativity precisely when the inequality (1.4) is obeyed.

> We will find that this criterion is directly related to the presence of the volume law contribution to the entanglement entropy.

> At scales much smaller than the Hubble radius gravity is in most situations well described by general relativity, because the entanglement entropy is still dominated by the area law of the vacuum.

that last quote seems to reference the apple

(o) https://en.wikipedia.org/wiki/Entropy_of_entanglement

Re: Emergent Gravity and the Dark Universe

#73

Post by Avshalom »

eloff wrote: post #7

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.

Dark matter is hilariously clean.

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"

Re: Emergent Gravity and the Dark Universe

#74

Post by gus_massa »

ars wrote: post #48
maverick_iceman wrote: post #36

Earlier quoted context omitted.

Gravitational wave doesn't carry mass. It travels at the speed of light, so it has to be massless. In fact, in 4D gravitational waves can arise even without any mass/energy source.

See here: https://en.wikipedia.org/wiki/Sticky_bead_argument I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard. And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not.

> I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard.

This sentence is wrong. I'm not sure what you mind with "photon which can disappear into nothing, leaving just the energy", but gravitons [1] can do the same thing.

A gravitational wave is made of a huge amount of gravitons, it's like the electromagnetic wave produced by radio transmitter. I´m not sure if with your classification the electromagnetic wave produced by radio transmitter "can't ever vanish entirely".

[1] If they exist. I'm almost almost sure gravitons exist. There is still not a good quantum gravity theory, but I strongly believe that we will discover it in the future an it will include gravitons.

Re: Emergent Gravity and the Dark Universe

#75

Post by spynxic »

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)

Re: Emergent Gravity and the Dark Universe

#76

Post by andrepd »

ajcarpy2005 wrote: post #57
pmontra wrote: post #42

Earlier quoted context omitted.

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…

It's funny how scientists always reference time in relation to gravity and yet separate out gravity from other forces of motion when the implicate order would be to have gravity always playing a role in motion whatever the axis or motion of travel. The other thing to know is that for a wave, there are more motions of travel than upon a triune 3-dimensional axis. The common thought process of a pondering individual is…

I don't have time to go over all your post, but you seem to be very very confused about several things. I suggest you try to read a science communication book, like Carl Sagan or the sort.

Re: Emergent Gravity and the Dark Universe

#77

Post by mcbits »

> 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?

Re: Emergent Gravity and the Dark Universe

#78

Post by autocorr »

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.

[1] https://en.wikipedia.org/wiki/Bullet_Cluster

Re: Emergent Gravity and the Dark Universe

#79

Post by raattgift »

codethief wrote: post #61
empath75 wrote: post #30

Earlier quoted context omitted.

In a quantum field theory, a wave implies a force carrying particle, does it not?

To be precise, particles (with precise momentum) are certain modes or excitations of a quantum field which is often governed by something like a wave equation. However, not all quantum fields (and their associated particles) are force carriers. These are usually just the gauge bosons. In any case, though, noone has managed to quantize gravity yet (i.e. describe gravity as a quantum field theory with gravitons as forc…

Well, we've quantized gravitation extremely well - it's just perturbative quantum gravity, of which there are a couple flavours. In weak gravity it makes predictions identical to General Relativity, but is taken to be an effective field theory (in the Charles Wilson sense of "effective) in that we do not know how to deal with Feynman diagrams with more than a loop or two of gravitons, which only happens in strong gravity.

In these theories, gravitons are quantizations of the weak-field perturbations.

The non-renormalizability by power-set counting of gravity because it is a long-range force is a good result of perturbatively quantized gravity. Who knows if there is a workable way of renormalization by other methods? Not me.

Since we don't get strong gravity except close to a black hole singularity or in the extremely early universe, it's unfair and premature to say that the EFT approach has been unsuccessful.

Re: Emergent Gravity and the Dark Universe

#80

Post by zer0gravity »

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.

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