new theory of gravity might explain dark matter

[mr friendly guy]

http://phys.org/news/2016-11-theory-gravity-dark.html

New theory of gravity might explain dark matter
November 8, 2016

A new theory of gravity might explain the curious motions of stars in galaxies. Emergent gravity, as the new theory is called, predicts the exact same deviation of motions that is usually explained by invoking dark matter. Prof. Erik Verlinde, renowned expert in string theory at the University of Amsterdam and the Delta Institute for Theoretical Physics, published a new research paper today in which he expands his groundbreaking views on the nature of gravity.

In 2010, Erik Verlinde surprised the world with a completely new theory of gravity. According to Verlinde, gravity is not a fundamental force of nature, but an emergent phenomenon. 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.
Newton's law from information
In his 2010 article (On the origin of gravity and the laws of Newton), Verlinde showed how Newton's famous second law, which describes how apples fall from trees and satellites stay in orbit, can be derived from these underlying microscopic building blocks. Extending his previous work and work done by others, Verlinde now shows how to understand the curious behaviour of stars in galaxies without adding the puzzling dark matter.
The outer regions of galaxies, like our own Milky Way, rotate much faster around the centre than can be accounted for by the quantity of ordinary matter like stars, planets and interstellar gasses. Something else has to produce the required amount of gravitational force, so physicists proposed the existence of dark matter. Dark matter seems to dominate our universe, comprising more than 80 percent of all matter. Hitherto, the alleged dark matter particles have never been observed, despite many efforts to detect them.
No need for dark matter
According to Erik Verlinde, there is no need to add a mysterious dark matter particle to the theory. In a new paper, which appeared today on the ArXiv preprint server, Verlinde shows how his theory of gravity accurately predicts the velocities by which the stars rotate around the center of the Milky Way, as well as the motion of stars inside other galaxies.
"We have evidence that this new view of gravity actually agrees with the observations, " says Verlinde. "At large scales, it seems, gravity just doesn't behave the way Einstein's theory predicts."
At first glance, Verlinde's theory presents features similar to modified theories of gravity like MOND (modified Newtonian Dynamics, Mordehai Milgrom (1983)). However, where MOND tunes the theory to match the observations, Verlinde's theory starts from first principles. "A totally different starting point," according to Verlinde.
Adapting the holographic principle
One of the ingredients in Verlinde's theory is an adaptation of the holographic principle, introduced by his tutor Gerard 't Hooft (Nobel Prize 1999, Utrecht University) and Leonard Susskind (Stanford University). 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.
This extra information is required to describe that other dark component of the universe: Dark energy, which is believed to be responsible for the accelerated expansion of the universe. Investigating the effects of this additional information on ordinary matter, Verlinde comes to a stunning conclusion. Whereas ordinary gravity can be encoded using the information on the imaginary sphere around the universe, as he showed in his 2010 work, the result of the additional information in the bulk of space is a force that nicely matches that attributed to dark matter.
On the brink of a scientific revolution
Gravity is in dire need of new approaches like the one by Verlinde, since it doesn't combine well with quantum physics. Both theories, crown jewels of 20th century physics, cannot be true at the same time. The problems arise in extreme conditions: near black holes, or during the Big Bang. Verlinde says, "Many theoretical physicists like me are working on a revision of the theory, and some major advancements have been made. We might be standing on the brink of a new scientific revolution that will radically change our views on the very nature of space, time and gravity."

Thoughts from those more versed in physics.

[Simon_Jester]

Articles written in this tone almost invariably turn out to be a lot of noise about nothing whatsoever.

[Esquire]

I'd want to read a real paper - if nothing else, what (if anything) does this theory not accurately predict?

[Zeropoint]

My understanding is that the whole array of "different theory of gravity" approaches to the the problem of dark matter are incorrect because they fail to take into account all of the observed behaviors of dark matter. For instance, in the case of colliding galaxies like the Bullet Cluster, the galaxies' ordinary matter and dark matter can get separated. We can see where the ordinary matter is by looking at it, and we can see where the dark matter is by looking at the gravitational distortions it causes in the background--and they're not in the same place.

This is still bleeding-edge science, and it's too early to make any definite statements (one of the defining characteristics of "dark matter" is that we have no bloody clue what it is) . . . but (to my understanding) we have enough observational evidence to believe, for now, that dark matter IS some form of "stuff".

[SolarpunkFan]

Articles written in this tone almost invariably turn out to be a lot of noise about nothing whatsoever.

My hypothesis is that dark matter is actually unicorn farts.

Phys.org will you publish me now pleeeaasssee!?

[Lord Revan]

Well the problem with these kinds of articles are that they're more often then not either a)exagrated b)faked. When I hear "this new discovery will totally change how we view 'x'" I tend to be skeptical as that generally doesn't happen.

[Simon_Jester]

Well, to be fair, it's not like we shouldn't try to explain dark matter by saying "maybe gravity doesn't work the way we thought."

Astronomers used to think planets ran on rails in the sky and that the rails were necessarily circular. However, the observed motions of the planets deviated slowly from those expected if you were looking at a glowy sky-rock rolling around a circular track. To explain this in terms of the circular track model required that the planets be on circular tracks that were nailed to and moving along other circular tracks, sometimes out to multiple layers of circular tracks- known as "epicycles."

Finally, Johannes Kepler came along and proposed an elegant alternative solution: maybe the tracks were not, in point of fact, circular! This changed everything, greatly simplified the science of astronomy, and laid the foundation work necessary for Newton to devise a theory of gravity- which could never have been useful and probably never even thought of in the context of epicycles and circular motion.

The analogy to dark matter is close enough that it SHOULD be philosophically troubling. Dark matter is a thing we have 'invented,' not directly observed, to explain a large number of subtle deviations from the way we expect the universe to behave. Maybe this time, the epicycles we're adding to our model of the universe are real... but maybe they're not. Until someone works out a way to directly observe dark matter particles, there is room for doubt.

And it is fitting and proper that we go on at least trying to come up with alternate theories of gravity that might somehow explain the things we now use dark matter to explain.

Maybe the alternate theories of gravity are wrong, but they're not outright stupid, the way attributing the same phenomena to unicorn farts would be.

[Zeropoint]

Oh, I absolutely agree. In science, NO ideas are above questioning. Proving well-established ideas to be wrong is the achievement that every scientist dreams of.

I'm having a hard time coming up with anything meaningful to add here. This modified theory of gravity will compete in the empirical marketplace of ideas just like any other scientific hypothesis, and stand or fall based on its ability to make testable predictions that turn out to be correct.

[Sea Skimmer]

You can get the actual paper here
https://arxiv.org/abs/1611.02269

The problem with the article is it's reporting a a theory which is just published online through the guy's own university, and has simply not yet undergone any peer review that I can see. You cannot trust any one persons opinion, ever, in this kind of field at this point in history. It's simply too hard for that, or else we've have solved physics now by.

But yeah to what's being said here, theories like this usually turn out to only work for certain situations, but not all situations we can observe. Still we need fresh theories like this to stimulate yet further theories in turn, and this will get reviewed sooner then later. One may note that his acknowledgement includes many other people, and that Verlinde himself isn't a random crazy.

[SolarpunkFan]

Maybe the alternate theories of gravity are wrong, but they're not outright stupid, the way attributing the same phenomena to unicorn farts would be.

Point taken.

Of course the fact that we can't currently reconcile gravity with quantum theory would be reason to as well, no?

[Lord Revan]

Well, to be fair, it's not like we shouldn't try to explain dark matter by saying "maybe gravity doesn't work the way we thought."

Astronomers used to think planets ran on rails in the sky and that the rails were necessarily circular. However, the observed motions of the planets deviated slowly from those expected if you were looking at a glowy sky-rock rolling around a circular track. To explain this in terms of the circular track model required that the planets be on circular tracks that were nailed to and moving along other circular tracks, sometimes out to multiple layers of circular tracks- known as "epicycles."

Finally, Johannes Kepler came along and proposed an elegant alternative solution: maybe the tracks were not, in point of fact, circular! This changed everything, greatly simplified the science of astronomy, and laid the foundation work necessary for Newton to devise a theory of gravity- which could never have been useful and probably never even thought of in the context of epicycles and circular motion.

The analogy to dark matter is close enough that it SHOULD be philosophically troubling. Dark matter is a thing we have 'invented,' not directly observed, to explain a large number of subtle deviations from the way we expect the universe to behave. Maybe this time, the epicycles we're adding to our model of the universe are real... but maybe they're not. Until someone works out a way to directly observe dark matter particles, there is room for doubt.

And it is fitting and proper that we go on at least trying to come up with alternate theories of gravity that might somehow explain the things we now use dark matter to explain.

Maybe the alternate theories of gravity are wrong, but they're not outright stupid, the way attributing the same phenomena to unicorn farts would be.

well I'm not saying it's impossible, what I am saying however that extra-orginary claims need extra-orginary proof. After all scientist are humans so they have the desire to gain fame and "glory" for inventing something that's a major change in how we think of things, that means everyone who claims to have done so shouldn't be belived without a dout.

[Simon_Jester]

Maybe the alternate theories of gravity are wrong, but they're not outright stupid, the way attributing the same phenomena to unicorn farts would be.

Point taken.

Of course the fact that we can't currently reconcile gravity with quantum theory would be reason to as well, no?

The biggest problem with reconciling quantum mechanics and gravity is that attempting to model gravity by quantum-mechanical principles results in mathematical chaos and inherently incalculable, unpredictable results. This does indeed suggest that there must be at least one deep principle, unknown to today's science, by which that chaos could be resolved into comprehensible, measurable order.

[Sea Skimmer]

It does suggest that, I hope. We might figure that out in our lifetimes if so. All the more so since we are now past that pesky period in which leaded gasoline made the entire developed planet a couple net IQ points dumber and starting to push human development yet higher then ever.

Course it could also mean the universe is super non deterministic, which would be unlikely but can't be ruled out completely. Also if some missing property does exist, it could be impossible to comprehend or define it in a literal sense. Math in this universe may be a property of the universe such that as a natural frame of reference it precludes explaining the universe, and if so we'd never be able to prove it. We may wallow in ignorance of a few final things forever, which would suck. Far out...but no more so then any theory of multiple universes.

[SpottedKitty]

<nod> Remember that old quote from JBS Haldane; "The Universe is not only queerer than we imagine, but queerer than we can imagine."

Come to think of it, that sounds like some sort of reverse-anthropic principle — the act of existing in this universe means we can never completely define or understand it. Did that make sense...?

[Simon_Jester]

The thing is, gravity works, and is predictable. This is an observed empirical fact. Gravity does not degenerate into random chaos. The answer to "what is the gravitational force on this thing in this place" is not "undefined" or "reply hazy, ask again later."

Therefore, there must be some mathematical explanation, and that explanation must be correct and (in principle, deterministic.

Either there is a deep principle that resolves the chaos resulting from quantum-mechanical methodology as applied to gravity, or there is something fundamentally wrong with quantum-mechanical methodology, as applied to gravity.

[Terralthra]

It does suggest that, I hope. We might figure that out in our lifetimes if so. All the more so since we are now past that pesky period in which leaded gasoline made the entire developed planet a couple net IQ points dumber and starting to push human development yet higher then ever.

Course it could also mean the universe is super non deterministic, which would be unlikely but can't be ruled out completely. Also if some missing property does exist, it could be impossible to comprehend or define it in a literal sense. Math in this universe may be a property of the universe such that as a natural frame of reference it precludes explaining the universe, and if so we'd never be able to prove it. We may wallow in ignorance of a few final things forever, which would suck. Far out...but no more so then any theory of multiple universes.

Or it could be perfectly understandable and explainable, but humans are too stupid. I'm not sure which would be more sad.

[SolarpunkFan]

Either there is a deep principle that resolves the chaos resulting from quantum-mechanical methodology as applied to gravity, or there is something fundamentally wrong with quantum-mechanical methodology, as applied to gravity.

Not surprising really. Our understanding of physics is pretty hazy starting at the zeptometer scale (that is: 10-21 meters range). And that isn't even close to the Planck length which is 1.6x10-35 meters, almost ten quadrillion (~10-16) times smaller than a zeptometer!

So we don't even have a firm grasp on a scale that's far larger than the smallest meaningful unit of length. Is it any wonder a theory of quantum gravity is so elusive to us at this time?

[Simon_Jester]

Distances being short is really, really not the problem here.

[SolarpunkFan]

Distances being short is really, really not the problem here.

I'm not sure what to say to that. I thought I was trying to get that we still don't know much about small scales where quantum gravity would be apparent.

I... huh.

[Simon_Jester]

Well, the reason we don't know much isn't just that the distances are short and that there's a wealth of phenomena that we can't detect "hiding" down there.

It's just that these are very small things. You could equally well say "the problem is the forces are weak" or something like that.

The real problem isn't even an experimental one at all, it's that the theory of quantum mechanics is profoundly incompatible with the way gravity actually, observably works. Either there's something wrong with even trying to use quantum mechanics to describe gravity, or there is a hidden principle at work here. Better experimental data (on higher-energy particles, on more subtle gravitational forces like gravitational waves, and so on) could help us narrow down the problem, but it wouldn't solve the problem as such. Just give us a hint.

[SolarpunkFan]

Well, the reason we don't know much isn't just that the distances are short and that there's a wealth of phenomena that we can't detect "hiding" down there.

It's just that these are very small things. You could equally well say "the problem is the forces are weak" or something like that.

The real problem isn't even an experimental one at all, it's that the theory of quantum mechanics is profoundly incompatible with the way gravity actually, observably works. Either there's something wrong with even trying to use quantum mechanics to describe gravity, or there is a hidden principle at work here. Better experimental data (on higher-energy particles, on more subtle gravitational forces like gravitational waves, and so on) could help us narrow down the problem, but it wouldn't solve the problem as such. Just give us a hint.

Ah, I see now. Thanks for clearing that up!