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PhilCo126
ASA Updates Plans for Hubble 'Ring Of Dark Matter' Briefing

GREENBELT, Md. - NASA will hold a media teleconference at 1 p.m. EDT on May 15 to discuss the strongest evidence to date that dark matter exists. This evidence was found in a ghostly ring of dark matter in the cluster CL0024+17, discovered using NASA's Hubble Space Telescope. The ring is the first detection of dark matter with a unique structure different from the distribution of both the galaxies and the hot gas in the cluster. The discovery will be featured in the June 20 issue of the Astrophysical Journal.
Littlebit
A dark matter ring? How/why would DM be corraled into a donut? Doesn't the shape imply a distribution of matter with electromagnetic properties?
Mongo
Colour me sceptical on this one -- which should be no surprise, given my stance on 'dark matter'. I do not know much about this latest claim, but I do know that there have been repeated claims of some observation 'proving' the existence of dark matter, which is invariably shown, a little later, to be equally explainable (and in fact, usually more easily explainable) without dark matter (usually under some form of MOND theory). A case in point is the most recent example before this one, the 'bullet cluster', which as you will recall was loudly trumpeted last November as 'proving' the existence of Dark Matter. Here are some results from later, more thorough analysis:

The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter

A detailed analysis of the November 15, 2006 data release (Clowe et al., 2006) X-ray surface density Sigma-map and the strong and weak gravitational lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density Sigma-model is computed using a King beta-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the two-dimensional convergence kappa-map data are obtained without non-baryonic dark matter, accounting for the 8-sigma spatial offset between the Sigma-map and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the kappa-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM) gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on average 76% dark matter (neglecting the indeterminant contribution due to the galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a statistically significant result when compared to the predicted Lambda-CDM cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al., 2006).

Timing and Lensing of the Colliding Bullet Clusters: barely enough time and gravity to accelerate the bullet

We present semi-analytical constraint on the amount of dark matter in the merging bullet galaxy cluster using the classical Local Group timing arguments. We consider particle orbits in potential models which fit the lensing data. {\it Marginally consistent} CDM models in Newtonian gravity are found with a total mass M_{CDM} = 1 x 10^{15}Msun of Cold DM: the bullet subhalo can move with V_{DM}=3000km/s, and the "bullet" X-ray gas can move with V_{gas}=4200km/s. These are nearly the {\it maximum speeds} that are accelerable by the gravity of two truncated CDM halos in a Hubble time even without the ram pressure. Consistency breaks down if one adopts higher end of the error bars for the bullet gas speed (5000-5400km/s), and the bullet gas would not be bound by the sub-cluster halo for the Hubble time. Models with V_{DM}~ 4500km/s ~ V_{gas} would invoke unrealistic large amount M_{CDM}=7x 10^{15}Msun of CDM for a cluster containing only ~ 10^{14}Msun of gas. Our results are generalisable beyond General Relativity, e.g., a speed of $4500\kms$ is easily obtained in the relativistic MONDian lensing model of Angus et al. (2007). However, MONDian model with little hot dark matter $M_{HDM} \le 0.6\times 10^{15}\msun$ and CDM model with a small halo mass $\le 1\times 10^{15}\msun$ are barely consistent with lensing and velocity data.

The collision velocity of the bullet cluster in conventional and modified dynamics

We consider the orbit of the bullet cluster 1E 0657-56 in both CDM and MOND using accurate mass models appropriate to each case in order to ascertain the maximum plausible collision velocity. Impact velocities consistent with the shock velocity (~ 4700km/s) occur naturally in MOND. CDM can generate collision velocities of at most ~ 3800km/s, and is only consistent with the data provided that the shock velocity has been substantially enhanced by hydrodynamical effects.

In other words, later analysis shows that MOND theories appear to fit the observations better than Dark Matter theories. This has happened again and again. How many times must this cycle repeat before Dark Matter is discarded as an implausible hypothesis?

Bill
Tman


From the Hubble site http://hubblesite.org/newscenter/archive/r...007/17/image/a/
The blue structure shows the possible 'dark matter' and is superimposed on a Hubble image of the cluster. A bit too detailed in this image I guess smile.gif

The PDF data release:
http://www.spacetelescope.org/news/science...70246.proof.pdf

If you like have a look at the big Hubble image of the cluster too... Wow, what a huge linse there! blink.gif

Edited: There is yet another interesting part of the release here. An animation (see Video 2) that zooms in from the naked eye view of the night sky to the small section image of the cluster. At the beginning, on the right is the asterism “Great Square of Pegasus”, and top middle, just out of view, the Andromeda galaxy. To the left is the constellation of Pisces, and leftmost the redish Mira.
Mongo
The wedding ring of MOND and non-exotic dark matter

The lensing mass reconstruction of the rich galaxy cluster Cl0024+17 has been argued to have revealed a ringlike dark matter structure that is offset from both the gas and the galaxies in the cluster. This has been claimed to be hard to explain in the framework of modified Newtonian dynamics (MOND). We aim to check that claim, taking into account the old-known mass discrepancy of galaxy clusters in MOND, which can be resolved by, e.g., adding a component of 2 eV neutrinos. For that purpose, we derive an upper limit to the density of matter in the ring, and compare it to the Tremaine-Gunn limit on the density of neutrinos. We conclude that the maximum density of matter in the ring is two sigmas below the Tremaine-Gunn limit, and that the ringlike structure in Cl0024+17 does not pose a new challenge to MOND.

No surprise here. As always, the overblown claims that the 'Dark Matter' theory has been proven correct by the latest observation are shown to be mistaken. I was expecting a paper like this to appear (as you can probably tell from my earlier posts on this, and other threads). Not that this refutation will be anywhere near as well-publicized as the original claim.

Bill
nprev
I'm with you, man. Dark matter has always been a very hard frog for me to swallow; seems much, much more likely that we just don't fully understand the properties of "light" matter, esp. neutrinos and other exotic little beasts...
dvandorn
QUOTE (nprev @ Jun 11 2007, 10:46 PM) *
...Dark matter has always been a very hard frog for me to swallow...

That's because your cosmology is faulty. It has nothing to do with frogs -- it's TURTLES. All the way down.

smile.gif

-the other Doug
Mongo
Using globular clusters to test gravity in the weak acceleration regime
Riccardo Scarpa, Gianni Marconi, Roberto Gilmozzi, Giovanni Carraro

http://xxx.lanl.gov/abs/0707.2459

We report on the results from an ongoing program aimed at testing Newton's law of gravity in the low acceleration regime using globular clusters. It is shown that all clusters studied so far do behave like galaxies, that is, their velocity dispersion profile flattens out at large radii where the acceleration of gravity goes below 1e-8 cm/s/s, instead of following the expected Keplerian fall off. In galaxies this behavior is ascribed to the existence of a dark matter halo. Globular clusters, however, do not contain dark matter, hence this result might indicate that our present understanding of gravity in the weak regime of accelerations is incomplete and somehow incorrect.
nprev
MOND + neutrino mass is looking more and more plausible all the time.
Mongo
Sorry to resurrect this long-dead thread, but a paper just appeared on ARXIV that bears on the possible physics behind MOND.

As background, in recent months a new theory of gravity has received a lot of attention. A Dutch physicist named Erik Verlinde appears to have made the crucial connections that allow for a physical explanation of gravity (both Newtonian and relativistic) as well as related concepts like Newtonian mechanics, inertia and indeed space and time itself. This is a bold claim, but top physicists like Lee Smolin, George Smoot and Gerard 't Hooft have jumped on board in recent weeks. This is no crackpot idea.

On the Origin of Gravity and the Laws of Newton -- Erik Verlinde

QUOTE
Starting from first principles and general assumptions Newton's law of gravitation is shown to arise naturally and unavoidably in a theory in which space is emergent through a holographic scenario. Gravity is explained as an entropic force caused by changes in the information associated with the positions of material bodies. A relativistic generalization of the presented arguments directly leads to the Einstein equations. When space is emergent even Newton's law of inertia needs to be explained. The equivalence principle leads us to conclude that it is actually this law of inertia whose origin is entropic.


What Verlinde found is that gravity can be explained as an entropic force -- a pseudo-force that results from the universal tendency to maximize entropy (the Second Law of Thermodynamics). Other examples of entropic forces include osmosis, and the 'force' causing large polypeptide molecules suspended in a fluid to curl up into a roughly spherical shape, instead of stretching out in a linear fashion. (This is NOT due to intra-molecular bonding forces, but instead is due to the fact that there are many more internal degrees of freedom in a spherical configuration than in a linear one, and hence a 'random walk' between states, driven by random collisions with the fluid molecules, will almost inevitably result in a spherical configuration.)

In Verlinde's theory, the most fundamental quality is information, which is stored in what are called 'holographic screens' that separate and define every point in space-time. These holographic screens are not new, they have become an indispensable concept in black hole physics over the past decade or so. They have the effect of 'smearing' information over a large area, rather than having it concentrated at a single point -- resulting in quantum mechanics. The amount of information associated with matter and its location is measured in terms of entropy. Changes in entropy associated with changes in location result in an entropic force, which when the math is done exactly matches gravity, both Newtonian and relativistic. The theory also automatically generates an additional force that exactly matches the effects of so-called 'dark energy' which had been recently postulated in order to explain the observed changes in the Hubble constant at extreme distances. In this theory it is a consequence of information being stored at the 'universal horizon', covering every point in the universe. Additionally, it provides an easy and natural explanation of inertia, which had always been a big puzzle.

Over the past two months a great many additional people have written papers, filling in and extending the theory. So far, it has been found to accurately describe both Newtonian and relativistic gravity, Newtonian mechanics including inertia, the early period of cosmological inflation, the current cosmological acceleration commonly ascribed to 'dark energy', and much of electrodynamics (!) including the Coulomb law, the Poisson equation and indeed all the Maxwell equations -- raising the possibility that electromagnetism is also a non-fundamental entropic force, with the photon perhaps being akin to the phonon (soliton) as a derived 'particle'. The newest such result concerns MOND:

Entropic corrections to Newton's law -- Leonardo Modesto, Andrew Randono

QUOTE
It has been known for some time that there is a deep connection between thermodynamics and gravity, with perhaps the most dramatic implication that the Einstein equations can be viewed as a thermodynamic equation of state. Recently Verlinde has proposed a model for gravity with a simple statistical mechanical interpretation that is applicable in the non-relativistic regime. After critically analyzing the construction, we present a strong consistency check of the model. Specifically, we consider two well-motivated corrections to the area-entropy relation, the log correction and the volume correction, and follow Verlinde's construction to derive corrections to Newton's law of gravitation. We show that the deviations from Newton's law stemming from the log correction have the same form as the lowest order quantum effects of perturbative quantum gravity, and the deviations stemming from the volume correction have the same form as some modified Newtonian gravity models designed to explain the anomalous galactic rotation curves.

[...]

Let us now focus on the corrections to Newton's laws stemming from the volume correction to the area-entropy relation. We notice here that the form of the potential is of the same functional form as the correction to the Newtonian potential posited by Modified Newtonian Dynamics (MOND) in order to model the anomalously flat galactic rotation curves.


Of course this does not prove that some form of MOND is correct, but it does raise the likelyhood a good deal, in my opinion.

p.s. -- if anybody wants to see links to other papers regarding aspects of this theory of gravity, please let me know on this thread and I will post links to the more important papers.
Explorer1
So in layman's terms, this is a potential 'theory of everything' if it's proven? Fascinating stuff.
Greg Hullender
QUOTE (Explorer1 @ Mar 10 2010, 08:46 PM) *
So in layman's terms, this is a potential 'theory of everything' if it's proven? Fascinating stuff.

No, it's just a new way to look at gravity. But it's a very, very exciting new way to look at gravity. I'm not a physicist myself, but my college friends who became physicists are pretty excited about it, and that's good enough for me.

--Greg
Mongo
Just to give a taste of how (some) physicists are viewing this idea, here are some extracts from some of the involved papers:

Part 1 of 2

Notes Concerning "On the Origin of Gravity and the Laws of Newton" by E. Verlinde -- Jarmo Mäkelä

QUOTE
If the idea of gravity as an emergent effect, rather than as a fundamental force turns out to be correct, we may currently be in a somewhat similar position as were the founders of quantum mechanics and atomic physics about 100 years ago. Instead of attempting to understand the microstructure of matter, however, we should, in this time, attempt to understand the microstructure of spacetime itself.


The Coulomb Force as an Entropic Force -- Tower Wang

QUOTE
If one accepts the holographic principle as well as our generalizations and reinterpretations, then the Coulomb's law, the Poisson's equation and the Maxwell's equations can be derived smoothly. Our attempt can be regarded as a new way to unify the electromagnetic force with gravity, from the entropic origin.


Gravity from the Entropy of Light -- Alessandro Pesci:

QUOTE
This suggests a deeper description of what we have discussed. Indeed, holography can be stated to mean that the allowed number of elementary bits of information in layers per unit bit of thickness, at given sum of the energy per bit and pressure, is limited (by an actual value). To the extent that concepts like bit of information, energy associated with a bit and pressure of a collection of bits can be regarded as primeval and, as such, meaningful even in absence of space, holography is pre-existing to space. In this perspective, when space is introduced as the information on ‘where’ information is, the energy in the bit should spread to keep unchanged the elementary amount of information for the bit, and this would be quantum mechanics. When expressed in terms of this space of information, holography would then become the metric theory which describes gravity.


Newtonian Gravity in Loop Quantum Gravity -- Lee Smolin

QUOTE
The idea that the unification of quantum theory with gravity is essentially thermodynamic has been on the table since the discovery of the laws of black hole thermodynamics and Bekenstein’s discovery of black hole entropy. The discoveries of the Unruh temperature and Hawking radiation strengthened the reason for hoping for a deep relationship between gravity, quantum physics and thermodynamics.

Very early in this history, Bekenstein hypothesized that the entropy of any isolated system is bounded by its area. In 1994 ’t Hooft extended this to the bold conjecture that the degrees of freedom needed to describe an isolated system in nature can be considered to live on a two-surface surrounding the region, with the number of degrees of freedom finite and proportional to the area in Planck units. He called this the holographic principle and since then we have come to call any application of the relationship between area and entropy as “holographic.” As developed by Susskind and then Maldacena and many others this led, in the context of string theory and supersymmetric quantum gauge theory, to the AdS/CFT correspondence.

The furthest realization of this idea to date, in the context of gravitational theory, is the discovery by Jacobson that the Einstein equations can be derived from the laws of thermodynamics, assuming only that Bekenstein’s proportionality between area and entropy is universal. This idea has been studied also by Padmanabhan and others, but there has remained the feeling that there was a further discovery, just over the horizon.

In a remarkable paper, Erik Verlinde has provided the next step, which is a non-relativistic analogue of Jacobson’s argument, in which he derives Newton’s law of gravity from thermodynamics plus the relationship between area and entropy. A different argument to the same conclusion has also been provided by Padmanabhan.
Mongo
Part 2 of 2

Modified gravity emerging from thermodynamics and holographic principle -- Yi Zhang, Yun-gui Gong, Zong-Hong Zhu

QUOTE
We discuss three different corrections to the area law of entropy. The number of bits N is then modified according to the form of entropy, and the law of gravity is obtained by using the method developed by Verlinde. We also discuss the effect of the entropy correction on the Friedmann equation, and we find that the Friedmann equation got an extra term H^n, which can be used to explain the current accelerating expansion of the universe. The result suggests that dark energy may be an emergent phenomenon based on the holographic principle and thermodynamics.


Towards a Holographic Description of Inflation and Generation of Fluctuations from Thermodynamics -- Yi Wang

QUOTE
Recently, Verlinde conjectured that gravity may be an entropic force, arising from thermodynamics on the holographic screen. We investigate the implications of the entropic force formalism for inflationary cosmology. We find the background dynamics of inflation can be dually described in the holographic language. At the perturbation level, two kinds of novel scale invariant scalar fluctuations arise from thermal fluctuations on the holographic screen. These fluctuations can be responsible for CMB anisotropy and structure formation.


Is Holographic Entropy and Gravity the result of Quantum Mechanics? -- Joakim Munkhammar

QUOTE
In a remarkable paper Verlinde recently proposed a framework for gravity as an entropic force. This theory, while related to Jacobsson's approach, showed that Newtonian gravity easily could be obtained by using entropic and holographic arguments. The assumption was that space is emergent and that the holographic principle holds. Bekenstein entropy was also a key component in his approach. He thus reversed the line of research, assuming that the holographic principle was underlying Newtonian physics. The change of entropy was linked to the change of the Newtonian potential, this led to the conclusion that inertia might be equivalent to the lack of entropy gradients. As Verlinde states, the holographic principle has not been easy to extract from the laws of Newton and Einstein because it is deeply hidden within them. His paper attracted quite some attention and several papers from various fields of theoretical physics, including Loop Quantum Gravity, have been published relating to its topic. A shortcoming of the theory was the unknown origin of the coupling constant h~. This coupling constant was added by Bekenstein in the 1970s mainly for dimensional reasons and has since remained a mystery. We will suggest an origin of this constant in this paper.

In Verlinde's view space is mainly a storage place of information, which is associated with positions, movements and mass of matter. This information is displayed to us on a surface, a screen in front of us. The information is stored in discrete bits on the screen and since the number of bits is limited we get holographic effects. This means that if there is more information on the inside than the amount of information accessible on the screen then information will be hidden from us as we observe the dynamics. This is the holographic principle. Thus the dynamics on the screen is governed by some unknown rules which then only can utilize the information on the screen. Since information is stored on a screen this means that space is emergent in the normal direction of the screen. The microstates may be thought of having all sorts of physical attributes such as energy, temperature etc. This is then related, via entropy, to the information associated with the system.

[...]

The great connections between matter and information made by Bekenstein, Hawking and others in the 1970s has turned out to have very interesting consequences. Verlinde's framework for the origin of the laws of Newton including gravitation based on entropy is perhaps one of the greatest consequences of this. In what way space is emergent and how the holographic principle holds is starting to fall into place. In this paper we have proposed an entropy arising from quantum mechanics and we have investigated its relation to Verlinde's theory. This was then applied to generalize the Newtonian potential arising in Verlinde's theory. There are many open problems remaining as this is a theory in progress. The study of multiple particle situations in the quantum mechanical approach should be interesting. A relativistic approach to quantum entropic gravity also needs to be established and investigated. The construction of various quantum field theories in curved spacetimes based on this approach should be of particular interest. Generally the connection to quantized gravity theories, if there are valid such, is left for further investigation. The nature of space and time as derived concepts, as spoken of by Verlinde, is not addressed in this paper and are features in need of investigation. The relation to AdS/CFT correspondence is also left open for further investigation. In conclusion, this paper, guided by a pure speculation, suggests that the gravitational attraction perhaps could be the result of a particular type of entropy arising in quantum mechanics.

Verlinde proposed a theory of gravity where gravity no longer was a fundamental force, but rather an effect of entropy. His theory, as a reversal of physics research, is a remarkable framework that has many open questions and interesting consequences still to be uncovered. This is why the greatest achievement of this paper is that it provides Verlinde's remarkable theory with a possible physical explanation for the factor h~, which had previously been added by Bekenstein mainly for dimensional reasons.
nprev
There's definitely a deterministic flavor to this line of thinking, which seems to be a bit of a flashback to 19th Century physics. I suppose that you could consider Newtonian physics a deterministic sum of random effects @ the quantum level, though.
stevesliva
I find it interesting that physicists have been considering inertia to be a "puzzling" concept. It has never even occurred to me to think, huh, it takes a force to impart velocity to an object? Why would that be? Of course I can't understand why it would be more "natural" to understand this as adding entropy rather than giving it a kick, but whatever! Yay physics.
Mongo
From Inertia Theory -- Paul Davies:

QUOTE
Fill a bucket with water, grab it by the handle and whirl it in an arc above your head. If you do it right, you will stay dry. A mysterious force seems to glue the water into the upside down bucket. Scientists are still unsure about where this force comes from.

Newton believed that inertia is an innate property of matter manifesting itself whenever matter accelerates (this includes rotation) relative to absolute space. You could think of the space in the vicinity of the accelerating body as somehow reacting to its motion to produce inertial forces. Newton never explained how, but took it to be a law of nature.

Newton's arch rival, Gottfried Leibniz, rejected this. Space, being empty, provides no reference against which a body can be said to accelerate. How can something move with respect to nothingness? We can judge motion, claimed Leibniz, only relative to other material bodies. Take the Earth's rotation. We observe the daily progression of the sun and stars across the sky. Our ancestors believed it was the heavens that turned, not the Earth. But suppose there were no sun and stars? Suppose Earth were alone in infinite space? Would it then make any sense to say it was rotating?

The debate raged on. Newton's hypothesis of absolute space predominated, but champions of the alternative "relative" view fought back, first in the guise of the Irish philosopher Bishop George Berkeley, then Austrian physicist-philosopher Ernst Mach - he of the Mach numbers. Mach, whose ideas greatly influenced Einstein when formulating his theory of relativity at the start of the century, insisted that acceleration can be defined only relative to the distant stars, a statement that came to be dignified with the name of "Mach's principle".

Mach's principle faced a thorny problem. Acceleration produces inertial forces. How can the distant stars be responsible for those? Could it really be that the child riding the roundabout is being tugged at by far-flung galaxies?

Einstein and others sought a mechanism to explain how a rotating body might experience a centrifugal force as a result of some sort of interaction with all the distant matter in the universe. A clue came from the theory of gravitation: after all, centrifugal force is sometimes even called artificial gravity.

Viewed from the roundabout, it is the rest of the universe that is rotating. We know that when electric charges circulate around a loop the resulting electric current produces a magnetic field. Could it be that the apparent rotation of the universe produces a gravitational version of a magnetic force that plucks at the clinging child? To test the idea, Einstein considered a small body at rest inside a rotating shell of material in otherwise empty space. Using his theory of relativity, he calculated what would happen. It turns out that the body should indeed feel a tiny gravito-magnetic force.

Further evidence in favour of Mach's principle comes from cosmology. If rotational motion is purely relative, then it is clearly nonsensical to talk about the rotation of the universe as a whole, for with respect to what would it rotate? In Newton's theory, it is entirely possible for the entire cosmos to spin about some axis. Given that almost all astronomical systems are observed to rotate to some extent, we might expect, if Newton is right, to observe a universal rotation too.

Astronomers find no evidence for a systematic rotation of the universe. Their observations imply that the universe cannot have turned by even one degree since the big bang. If rotation is absolute, the absence of a universal rotation seems to be a very special and contrived state of affairs, but if as Mach claimed it is relative, then the observations are explained.
djellison
Is it just me, or is this a catastrophically screwed up analogy?

"Fill a bucket with water, grab it by the handle and whirl it in an arc above your head. If you do it right, you will stay dry. A mysterious force seems to glue the water into the upside down bucket. Scientists are still unsure about where this force comes from"

Errr - F=MA and A=V^2 / R

Nothin mysterious about it.
Mongo
That was my first thought too, but how does the water 'know' that it is experiencing circular acceleration? The magnitude of the 'centrifugal' (or conversely, centripetal) force is easily calculated, but the reason it exists is apparently much harder to understand. Einstein never could. As the example stated, if the Earth were rotating in an otherwise empty universe, would it still be considered to be rotating? And would it still experience polar flattening as a result? If it does, how does it 'know' how quickly it is rotating (to generate that degree of flattening) with nothing to act as an external reference frame, and if it does not, why does the presence of some other random object(s) in the universe affect the shape of the earth, to a far larger degree than those caused by tidal effects? And is one other physical object in the universe enough to establish a reference frame, or is an entire universe equivalent to ours necessary? If so, why?

Inertia is very easy to handle mathematically, it's practically the first thing taught in high school physics, but its origin is apparently much more difficult to explain. It appears that this new theory of gravity by Verlinde has inertia fall out almost automatically, in terms of entropy gradients.
djellison
So it'snot a question about a bucket at all, it's just asking WHY F=MA
centsworth_II
QUOTE (djellison @ Mar 11 2010, 01:25 PM) *
...it's just asking WHY F=MA
And, why mass? Hence the search for the Higgs.
Shaka
If Earth were in an otherwise empty Universe, why would it have formed into a sphere? blink.gif

(Just when I had it figured out that a 'string' broke when the apple hit Newton.)
stevesliva
QUOTE (Mongo @ Mar 11 2010, 01:04 PM) *
Inertia is very easy to handle mathematically, it's practically the first thing taught in high school physics, but its origin is apparently much more difficult to explain. It appears that this new theory of gravity by Verlinde has inertia fall out almost automatically, in terms of entropy gradients.


Yes, but I don't understand why an balanced entropy gradients are so much more gee-whiz than balanced forces. Someone will be unsatisfied with not knowing the origin for the 2nd law of thermodynamics, right?

I understand the elegance of describing inertia in terms of entropy, but in my mind, it's just saying that this thing we think is fundamental is just like this other thing that is fundamental. It's still fundamental.

I would think that if some young einstein asked his physics teacher "why is there inertia?" the answer would be "it's fundamental." Upon reaching university, a physics professor might say, "actually, it can be expressed as a manifestation of entropy." At which point the young einstein says, "why is there entropy?" Does the physics professor say, "it's fundamental?" tongue.gif

Is the reason it's so gee-whiz because entropy is a fundamental property of everything, not just matter and energy, but also information? We're better linking in the concepts of data and uncertainty with matter and energy?
Mongo
The following is just my own understanding of what this theory of gravity is saying.

It looks to me that it says that the only truly fundamental 'things' in our universe are the holographic screens that separate and define every point in (two-dimensional) spacetime, and the information / entropy associated with each such screen (plus whatever micro-level theory actually coordinates the individual points and screens, i.e. string theory or its big brother M-theory, loop quantum qravity, twistor theory, etc.), everything else emerging from that.

(Actually, the current word is that this theory is incompatible with string theory or M-theory, since it goes directly from 2D spacetime to our observed spacetime with no intervening extra dimensions. This is no surprise to me, since I have always been sceptical of string theory -- it must have the highest ratio of effort invested to usable results, of any physical theory ever.)

I have heard nothing about the strong and weak nuclear forces so far, but gravity, and very possibly electromagnetism, are already being described as derived forces, so I would not be surprised if the other two 'fundamental' forces are as well, in the full theory.
Greg Hullender
QUOTE (djellison @ Mar 11 2010, 10:25 AM) *
So it'snot a question about a bucket at all, it's just asking WHY F=MA

This has been a question for a long time. I remember my Physics professor at Caltech making a point of it when I was a freshman back in 1977.

When I was a kid, protons and neutrons were thought to be fundamenal particles, although there was a bewildering array of other, short-lived subatomic particles. It was exciting to witness the discoveries that replaced all that with the much more elegant system of quarks and leptons, and it's even exciting to think we may be witnessing another major refinement.

It'll be nice when someone has worked out an easier way to explain it to the educated layman. Meanwhile, I'm still waiting for someone to explain how they plan to test this hypothesis.

--Greg
Marz
QUOTE (Greg Hullender @ Mar 11 2010, 04:25 PM) *
This has been a question for a long time.


lol - who would've thought a spinning bucket is at the heart of modern physics?

I think the essence of the Bucket is to understand what aspects of gravity are mathematical abstractions or a natural phenomenon. From Greene's layman book "Fabric of the Cosmos" Chp 2, The Universe and the Bucket:

"If velocity is something that only makes sense by comparisons ... how is it that changes in velocity are somehow different, and don't also require comparisons to give them meaning? ... Could it be that there is some implicit or hidden comparison that is actually at work every time we refer to our experiences of accelerated motion? This is a central question .... it touches on the deepest issues surrounding the meaning of space and time."

Meanwhile... I'll have to decide what feels less silly:

Dark Matter (and perhaps Dark Energy) or holographic screens. Planetary geology is so lovingly concrete in comparison.
Mongo
QUOTE (Marz @ Mar 11 2010, 11:56 PM) *
Meanwhile... I'll have to decide what feels less silly:

Dark Matter (and perhaps Dark Energy) or holographic screens. Planetary geology is so lovingly concrete in comparison.


The term 'holographic screen' is perhaps a little unfortunate. We are not talking about some special movie screen on which to watch Avatar in glorious 3D! It is simply a way of describing the information associated with any volume of spacetime as being encoded on the surface boundary of that volume (when observed from outside that volume), hence the term 'screen', and since there is always more information contained in that screen than can be observed from outside, it is also 'holographic'. The term was originally applied to the event horizens of black holes, where it made perfect sense to refer to a 'screen' which screened the interior from outside observation, but it was found that the concept could be extended to any (continuous) 2D surface that fully enclosed some volume of spacetime.

In the case of this theory, they are taking the holographic screens to be the smallest possible, each one surrounding a single point of spacetime, which leads to gravity by means of thermodynamic arguments. At the other end of the scale, by taking a holographic screen that encompasses the entire 'universal horizon' of this universe, it is possible to generate a force that matches that of so-called 'dark energy', which is the cause for the cosmological acceleration at very large distance scales.
monty python
I'm really loving this thread, but for some reason I find comfort knowing that theories are just usefull tools we humans use to figure out how stuff works. They don't always need to be spot on correct.
AndyG
QUOTE (Marz @ Mar 11 2010, 10:56 PM) *
lol - who would've thought a spinning bucket is at the heart of modern physics?


smile.gif I prefer the late Douglas Adams' prescient insight into this. A nice cup of tea lies at the heart of Arthur Dent's well-being and, of course, stirring tea and then momentarily rotating the cup provides a demonstrably easier, tastier (and potentially less wet) appreciation of inertia and Mach's Principle.

Andy
SteveM
I've noticed that none of the arXiv papers are listed as having been accepted by (or even submitted to) a peer-reviewed journal. They may be interesting developments of a theoretical speculation, but caveat lector.

Steve M
Floyd
Mongo
"As the example stated, if the Earth were rotating in an otherwise empty universe, would it still be considered to be rotating?"
Absolutely. Just set up a Foucault's pendulum, or play pool on a frictionless table and see if your ball goes on a straight or on a curved path (we will assume somone on earth for this experiment is not on the equator). If I recall correctly, this was a problem in one of my first year of physics class.
Greg Hullender
That begs the question, Floyd.

--Greg
stevesliva
The NY Times covers Verlinde, and does the newspaperly thing of gleaning quotes from lots of other physicists:
http://www.nytimes.com/2010/07/13/science/13gravity.html

What Verlinde actually said in his paper is not covered terribly well.
nprev
No, it's not. But I liked the cartoon. laugh.gif
AndyG
QUOTE (stevesliva @ Jul 13 2010, 01:46 AM) *
What Verlinde actually said in his paper is not covered terribly well.


I hope so, because the article contains this line:

QUOTE
...gravity is simply a byproduct of nature’s propensity to maximize disorder.


Which seems counter-intuitive to me. Surely the impact of gravity is order?

Andy
Hungry4info
If I'm not mistaken, "order", in reference to entropy, referrs to how basic or primordial the Universe can get. A Universe of even density, comprised entirely of hydrogen gas would be considered more orderly (less entropic) than a Universe where gravity has shaped it into galaxies, stars, etc.

[prepares to be shown completely wrong...]
Juramike
Entropy would be maximum for an evenly diffused but chaotically mixing bunch of atoms. There would be transient local concentrations and emptier zones as atoms collide and zoom apart, but these would be transient and random.

(If all the atoms universe was prefectly rigidly ordered and perfectly spaced, then the atoms would be technically a solid lattice and would have lower entropy due to the ordered arrangement.)

http://en.wikipedia.org/wiki/Entropy_of_mixing
Greg Hullender
Mike: if any of the separated atoms/particles can stick together, either chemically or gravitationally, how can the entropy already be maximized? Shouldn't there also be no more energy to liberate? Otherwise, the new compound or the new, larger object will be hotter than the things around them.

--Greg

Juramike
Full inline quote removed - Mike, you should know better. - ADMIN

I'm assuming the atoms don't stick together, they just bounce off. If they stick, then your are going against entropy and need to have a lower energy state (delta H) to make it worthwhile...which would release heat into the environment.

* I think *

[scratches head]
Greg Hullender
Which means the maximum entropy ought to be when all the particles clump together into a single mass and that mass cools to the background temperature of the universe. (Assuming no black hole.) If there's angular momentum, I think the system can get trapped in a local minimum, where there are multiple bodies orbiting one another; entropy would be lower if they could collide, coalesce, and cool down, but there's no mechanism for them to do so.

Back to the paper, I read through it more carefully, and what he seems to be saying is that if we assume that entropy somehow (means unspecified) increases when one mass approaches another, then, given certain choice of parameters, he can derive the usual laws of gravity. He gives no clue what entropy actually means in this case. Far as I can tell, anyway.

--Greg
stevesliva
Experiments would seem to indicate gravity is not emergent:
http://www.technologyreview.com/blog/arxiv/27102/
nprev
Actually, seems that interpretation of the theories indicate that, but there also seems to be a means to obtain experimental evidence.

Understanding the origin of gravity is one of the things I REALLY want to see happen before I take a dirt nap. Just fascinating.
Mongo
This paper just came out on arXiv, in response to the paper linked to in post #41

On entropic gravity: the entropy postulate, entropy content of screens and relation to quantum mechanics

We consider the controversial hypothesis that gravity is an entropic force that has its origin in the thermodynamics of holographic screens. Several key aspects of entropic gravity are discussed. In particular, we revisit and elaborate on our criticism of the recent claim that entropic gravity fails to explain observations involving gravitationally-bound quantum states of neutrons in the GRANIT experiment and gravitationally induced quantum interference. We argue that the analysis leading to this claim is troubled by a misinterpretation concerning the relation between the microstates of a holographic screen and the state of a particle in the emergent space, engendering inconsistencies. A point of view that could resolve the inconsistencies is presented. We expound the general idea of the aforementioned critical analysis of entropic gravity in such a consistent setting. This enables us to clarify the problem and to identify a premise whose validity will decide the faith of the criticism against entropic gravity. It is argued that in order to reach a sensible conclusion we need more detailed knowledge on entropic gravity. These arguments are relevant to any theory of emergent space, where the entropy of the microscopic system depends on the distribution of matter in the emergent space.
Mongo
No surprise at all to me. Maybe now the Dark Matter theory of galactic rotation curves will finally go into the trashcan of science where it belongs, along with the Phlogiston theory of combustion and the Epicycle theory of planetary motion.

Serious Blow to Dark Matter Theories? — New study finds mysterious lack of dark matter in Sun’s neighbourhood

A team using the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory, along with other telescopes, has mapped the motions of more than 400 stars up to 13 000 light-years from the Sun. From this new data they have calculated the mass of material in the vicinity of the Sun, in a volume four times larger than ever considered before.

"The amount of mass that we derive matches very well with what we see — stars, dust and gas — in the region around the Sun," says team leader Christian Moni Bidin (Departamento de Astronomia, Universidad de Concepcion, Chile). "But this leaves no room for the extra material — dark matter — that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there!"



This artist’s impression shows the Milky Way galaxy. The blue halo of material surrounding the galaxy indicates the expected distribution of the mysterious dark matter, which was first introduced by astronomers to explain the rotation properties of the galaxy and is now also an essential ingredient in current theories of the formation and evolution of galaxies. New measurements show that the amount of dark matter in a large region around the Sun is far smaller than predicted and have indicated that there is no significant dark matter at all in our neighbourhood. Credit: ESO/L. Calçada
Mongo
Dark Matter gone missing in many places: a crisis of modern physics?

This week, two additional studies report that even more seems to be missing (when your expectations are based on what LCDM predicts, that is). They both point at a serious lack in the amount of expected dark matter on two very different size-scales: the local universe and our immediate neighborhood within the Milky Way.

In the work titled "Missing Dark Matter in the Local Universe", Igor D. Karachentsev has looked at a sample of 11,000 galaxies in the local Universe around the MW. He has summed up the masses of individual galaxies and galaxy-groups and used this to test a very fundamental prediction of LCDM.

The idea is as simple as it is brilliant: cosmology has precise predictions as to what is the content of our universe. In particular, it predicts the density of matter to be Ωm,glob = 0.28 +- 0.03 (83 per cent of this in dark, 17 per cent in luminous matter). Now, to test this, all you have to do is to sum up all the mass within a certain volume of space, and you can estimate the actual density of mass within that volume. To be sure that your volume is representative, it needs to be large. If you only sum over, say, a sphere of 100 kpc in diameter, the density strongly depends on whether you have a galaxy in this volume or not. Karachentsev chose to use a volume of 50 Mpc around the MW. On this size-scale, the density is expected to fluctuate by only 10 percent, a reasonably low value in astronomy. The scale can thus be assumed to be representative and you should observe the mass density predicted by LCDM.

Except that you do not.

Karachentsev reports that the average mass density is only Ωm,loc = 0.08 +- 0.02, a factor of 3-4 lower than predicted and can not be explained by the uncertainties in the data or prediction. As most of the mass-content in the Universe is supposed to be dark matter, this means that most dark matter is missing in this volume.

<snip>

Indeed, a 50 page review of the observational tests of the standard model has been compiled by Pavel Kroupa in "The dark matter crisis: falsification of the current standard model of cosmology" and will appear in the Publications of the Astronomical Society of Australia (PASA-CSIRO publishing). Using a huge number of different data, Pavel Kroupa performs a strict logical falsification of the currently standard cosmological model, which is based on Einstein's theory of general relativity, concluding that cold or warm dark matter cannot exist.

Not surprising then, that the above studies have found dark matter to be missing....
Mongo
Beating an increasingly dead horse here:

Vast Structure of Satellite Galaxies Discovered: Do the Milky Way’s Companions Spell Trouble for Dark Matter?

QUOTE
Astronomers from the University of Bonn in Germany have discovered a vast structure of satellite galaxies and clusters of stars surrounding our Galaxy, stretching out across a million light years. The work challenges the existence of dark matter, part of the standard model for the evolution of the universe.


QUOTE
In their effort to understand exactly what surrounds our Galaxy, the scientists used a range of sources from twentieth century photographic plates to images from the robotic telescope of the Sloan Deep Sky Survey. Using all these data they assembled a picture that includes bright 'classical' satellite galaxies, more recently detected fainter satellites and the younger globular clusters.

"Once we had completed our analysis, a new picture of our cosmic neighbourhood emerged," says Pawlowski. The astronomers found that all the different objects are distributed in a plane at right angles to the galactic disk. The newly-discovered structure is huge, extending from as close as 33,000 light years to as far away as one million light years from the centre of the Galaxy.


QUOTE
The various dark matter models struggle to explain this arrangement. "In the standard theories, the satellite galaxies would have formed as individual objects before being captured by the Milky Way," explains Kroupa. "As they would have come from many directions, it is next to impossible for them to end up distributed in such a thin plane structure."


QUOTE
Kroupa concludes by highlighting the wider significance of the new work. "Our model appears to rule out the presence of dark matter in the universe, threatening a central pillar of current cosmological theory. We see this as the beginning of a paradigm shift, one that will ultimately lead us to a new understanding of the universe we inhabit."


Link to original article: The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way

QUOTE
These findings demonstrate that a near-isotropic infall of cosmological sub-structure components onto the MW is essentially ruled out because a large number of infalling objects would have had to be highly correlated, to a degree not natural for dark matter sub-structures. The majority of satellites, streams and YH GCs had to be formed as a correlated population. This is possible in tidal tails consisting of material expelled from interacting galaxies. We discuss the tidal scenario for the formation of the VPOS, including successes and possible challenges. The potential consequences of the MW satellites being tidal dwarf galaxies are severe. If all the satellite galaxies and YH GCs have been formed in an encounter between the young MW and another gas-rich galaxy about 10-11 Gyr ago, then the MW does not have any luminous dark-matter substructures and the missing satellites problem becomes a catastrophic failure of the standard cosmological model.
stevesliva
Does make you wonder if "dark matter" will go the way of planet Vulcan-- explained by a nifty new theory. 'course I've always wondered that.
Mongo
The Bullet Cluster as Evidence against Dark Matter

QUOTE
It might sound like a story from a parallel universe – but it’s true. The Bullet Cluster isn’t the incontrovertible evidence for particle dark matter that you have been told it is. It’s possible to explain the Bullet Cluster with models of modified gravity. And it’s difficult to explain it with particle dark matter.

How come we so rarely read about the difficulties the Bullet Cluster poses for particle dark matter? It’s because the pop sci media doesn’t like anything better than a simple explanation that comes with an image that has “scientific consensus” written all over it. Isn’t it obvious the visible stuff is separated from the center of the gravitational pull?

But modifying gravity works by introducing additional fields that are coupled to gravity. There’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is. Indeed, one would expect that modified gravity too should have a path dependence that leads to such a delocalization as is observed in this, and other, cluster collisions. And never mind that when they pointed at the image of the Bullet Cluster nobody told you how rarely such an event occurs in models with particle dark matter.
Gerald
I'd think, the vast majority of astrophysicists sees it the other way round: Modified Newtonian Dynamics (MOND) and the Bullet Cluster (1E 0657-558)
QUOTE
We present herein an overview of the theory of MOND and its success at explaining certain observations as well as the Bullet Cluster and its case against MOND

-- I'd think, with MOND we scratch along rule 1.9. Does there exist a peer-reviewed paper supporting the Bullet Cluster ruling out WIMPs?
fredk
I agree with Gerald. I'm familiar with the blog post, and somewhat with dark matter. Nonlinear astrophysics is very messy. For gory details and subtleties that the blogger (not a specialist in dark matter) missed check out some of the comments, especially those from Peter Erwin.
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