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New Gravity Theory Dispenses with Dark Matter
Posted by
ScuttleMonkey
on Wed Jan 25, 2006 10:12 AM
from the making-quantum-theorists-happy dept.
from the making-quantum-theorists-happy dept.
Darkness Matters writes "According to New Scientist, a theory of modified gravity, which has no need of dark matter, has just explained why the Pioneer 10 probe is 400,000 miles off its expected course as it leaves the solar system. It sounds pretty convincing, although in dispensing with dark matter, they've had to utilize the theoretical particle, called a graviton, which appears from the vacuum of space wherever stars are densely packed, making gravity stronger."
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Vast improvement (Score:5, Funny)
"See I told you guys it wasn't flying monkeys! Turns out it's flying Unicorns!"
Re:Vast improvement (Score:3, Funny)
Your language is giving it too much credibility.
Ahh, that's better. I move that the name "Dark Matter" be forever changed to "Government Funded Flying Space Monkeys from Beyond the Moon."
Re:Vast improvement (Score:4, Funny)
Seriously for all we know planets could still be held together in the solor system by invisible angels in invisible golden chariots.
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Re:de rigueur (Score:5, Funny)
Because that's the noodle-string theory.
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so... (Score:3, Funny)
Re:so... (Score:5, Funny)
Parent
at first I saw... (Score:4, Funny)
My invisible friend (Score:5, Funny)
Re:My invisible friend (Score:5, Funny)
I suspect your invisible things weren't of the 'no, really, hang around and watch, you'll see, it's about to do it again!' kind. More of the 'uh... well, it went away when you came in. It's only around when you're not looking. And it knocks things over when I'm the only one in the room' variety, if you were a kid making excuses.
That, I think, makes you a budding theologian :)
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Actually... (Score:5, Insightful)
This sounds more like the work of a quantum physicist.
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Gravitons are not a new concept (Score:4, Insightful)
Besides, didn't we use to shoot gravitons at that loud squiggly thing in Yar's Revenge?
Re:Gravitons are not a new concept (Score:5, Interesting)
However, we're still left with the age old question: If gravity is manifest as a particle, why can't we shield against it?
Until that question is answered, the graviton theory is going nowhere, fast.
Parent
Re:Gravitons are not a new concept (Score:5, Insightful)
There could be lots of reasons for this. The mechanism certainly isn't the same, but as an example of a particle which cannot be shielded, you need look no further than a neutrino, which can pass through the entire Earth.
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Re:Gravitons are not a new concept (Score:5, Funny)
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Re:Gravitons are not a new concept (Score:3, Interesting)
We can hardly shield against neutrinos, right?
Re:Gravitons are not a new concept (Score:4, Informative)
You typically think about stopping "particles" with other particles - like a wall. That's a very classical idea. But in order for a particle to stop at a wall, it needs to interact with that wall - in fact, it needs to either be "absorbed" by the wall, or it needs to be totally deflected by the wall. But in either case, it needs to interact with the wall.
Neutrinos, for instance, don't interact with much, since they only interact via the weak interaction. So we really can't shield from neutrinos that well, although you could build weird gadolinium-doped materials which would probably cut down on the flux of neutrinos more than others. Thankfully, neutrinos interact just like normal matter when they *do* interact, and so you could conceivably shield against them - just not easily.
As for gravitons, though, the situation changes - now you have to ask "can we build a material that interacts with gravitons?" Well, yes - all matter does. But annoyingly, that material itself would produce gravitons as well, and in terms of the SVTG theory, it sounds like it "conducts" them through, too. It's a lot like magnetic shielding - putting a material that interacts with magnetic fields isn't enough to shield a field from you. You need a high-permeability material - that is, one that makes it easier for a magnetic field to flow around you rather than through you.
You can even realize this based on the spin that the graviton has: a graviton would be a spin-2 particle, and any interaction with a spin-2 particle as its mediator must be an attractive potential. Without the possibility of repelling a graviton, you can see that you can't build a shield.
Note that we don't have any fundamental spin-2 particles other than a graviton, so it's understandable that naive ideas don't work.
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Re:Gravitons are not a new concept (Score:5, Informative)
Because gravitons are spin 2. There's no nice "intuitive" way of explaining it, but within the mathematical framework of modern quantum field theory spin 2 particles always produce an attractive force between things that those particles interact with. This means that there is no possibility of "anti-gravity", which is what is required to shield against gravity. Electromagentic shielding is possible because photons, being spin 1, can produce either attractive or repulsive forces. This is how it comes to be that there are two electrical charges of opposite sign, whose differential displacements in matter allow us to create electromagentic sheilding. The spin-2 nature of gravitons means there is only one "gravitational" charge, called "mass".
The most one would be able to do with gravity is cancel out gravity waves in a small region by generating out-of-phase waves. The static field cannot be cancelled in this manner, so there is no way of shielding it.
All of this, of course, depends on the imperfect mathematics of quantum field theory, which may or may not be an accurate description of the universe.
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Re:Gravitons are not a new concept (Score:4, Insightful)
So, it won't be the theory you are after until some more time.
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Please be real! :D (Score:3, Insightful)
Anyway, these "Gavitons".. I think I've had them in computer games for a while now, its about time we 'discovered' them. Aethe-- I mean Dark Matter was such a cranks idea anyway... anything has to be better then "OOoooh! There must be... some.. uh, invisible undetectable matter.. that uh, has mass. But you can't see it, because.. its dark! yeah thats the ticket." Given an unlimited choice of possibilities I could have came up with something better, and it probably would have been about as scientifically valid too.
Hurray for gavitons! Prepare the graviton pulse cannons!
--SD
Sure, it's off course (Score:5, Funny)
Oh...graviton.
scalar-tensor-vector gravity ? (Score:3, Funny)
"Hypothetical particle" (Score:5, Informative)
Re:"Hypothetical particle" (Score:5, Informative)
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Some clarification (Score:5, Informative)
The post makes it sound like suggesting that gravitons exist is outlandish... but this is rather accepted. Instead, it seems that their theory is a particular attempt to quantize gravity (there have been many attempts over time, with all ultimately being unsatisfactory). Whether or not their new theory is useful remains to be seen.
Also, in TFA, they say: "In this case, a hypothetical particle called a graviton - which mediates gravity - appears in large numbers out of the vacuum of space in regions crowded with massive objects such as stars." Again, it is generally accepted that in any reasonable theory of quantum gravity, gravitons will be the force-carrying particle for gravity. Where there is a large gravitational field, virtual gravitons will be exchanged to mediate the force (more info on virtual particles [wikipedia.org]). This is nothing new. And in particle physics, virtual particles can always appear and disappear from the vacuum.
So again, I think we can't coment much on this theory without reading the actual paper (anyone have a link?). I would like to understand what is actually novel about their formulation. Also, they are not the first to try and reformulate the basic laws of gravity to get rid of the "dark matter anomaly" and none have been found to be consistent with all the experimental data.
links to paper... (Score:5, Informative)
I'm not an expert in gravitational theory, so I would appreciate others correcting any mistakes I make. The abstract to the paper says: "The theory allows for a variation with distance scales of the gravitational constant G, the fifth force skew symmetric field coupling strength (omega) and the mass of the skew symmetric field = 1/(lambda)."
I think this is quite a departure from what is conventional accepted about gravity. The gravitational constant, G, [wikipedia.org] sets the scale for the force of interaction of gravity. It is normally assumed that this value is constant throughout the entire universe. They seem to be allowing that this value changes with distance, so that the interaction of gravity is different at small and large length-scales. That they are able to come up with a fit to actual experimental data is quite amazing... although so many bits of astronomical data have been computed assuming a particular (and constant) value of G, so to compare with "established facts" they will have to reconsider all of these previous calculations.
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Re:links to paper... (Score:4, Informative)
This isn't a new idea. This idea has been around a long time - it comes from Mach's principle [wikipedia.org] (yes, that Mach) which essentially states that the inertial mass of an object only means anything in context of other objects. Taken with the equivalence principle, this means that gravity depends on the spatial distribution around it.
General relativity does not satisfy Mach's principle - you can create an "empty Universe" which solves Einstein's equations. There have been several modifications to GR to try to satisfy Mach's principle. The simplest one is Brans-Dicke [wikipedia.org] theory, which does exactly what SVTG does - allows G to vary. Brans-Dicke theory is essentially identical to general relativity if the coupling between the scalar field and the tensor field (gravity) becomes infinitely weak. Sounds like SVTG is an extension of Brans-Dicke, allowing a scalar, vector, and tensor component for gravity.
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Re:Uhh - Action at a Distance? (Score:5, Informative)
Is not one of the big problems with "gravitons" that gravity appears to act more or less instantaneously at great distances? And isn't that a little troubling from the "Action at a Distance is Big No-No" point of view?
No, according to the theories gravitons would travel at the speed of light. In fact, bear in mind that the exchange of virtual particles is what prevents "action at a distance", if you like. Instead of gravity (or magnetism) having an effect "just because", the theory explains that it is because virtual particles are flying back and forth between the two objects in question. In the case of gravity, it is virtual gravitons, and in the case of magnetism, it is virtual photons. Both travel at the speed of light, which explains why force effects (like gravity and magnetic fields) are not instantaneous: they propagate at the speed of light (this has been measured and is not in dispute).
Pioneer 10 is pretty damned far out there at this point.
Apparently Pioneer 10 is 89 AU from the sun [nasa.gov]. 90 Astronomical Units is 12 light-hours [google.ca]. Still, your point is well-taken... gravity operates over distances of millions and even billions of light-years... so how can these "virtual gravitons" cover such distances? After all, supposedly virtual particles exist only for a short time!
My apologies to the hard-core particle physicists for this simplistic explanation, but here goes: When you look at the Heisenberg Indeterminacy Principle [wikipedia.org], you find that there is a relation between space and momentum. We all know the famous "the more accurately you localize a particle, the more spread out its velocity is"... it turns out that this implies a similar relation for energy and time. What it means is that high-energy particles can "pop into existence" for very short periods of time... but low-energy particles could exist for longer times. This is what allows virtual particles to do their thing. Very strong forces (nuclear forces and electromagnetic) involve high-energy virtual particles, which can only travel short distances before "disappearing"... that's why those forces operate over short distances.
But gravity is very very weak (by comparison). So that means that a virtual graviton can pop into existence, and travel for a long distance and time (millions of years) before disappearing. That's what, in fact, causes gravity to operate over such vast distances. So in fact the distance-scales and force intensities are intrinsicaly related in quantum treatments. So "a short time" means something different for EM-forces and gravity-forces.
I hope this (simplistic) explanation is somewhat useful to someone.
Parent
Testable with planetary motion? (Score:4, Insightful)
Dark Matter Is Chuck Norris (Score:5, Funny)
Scientists have also not yet revealed the real reason behind the ban on human cloning. The real reason human cloning is outlawed is because scientists fear Chuck Norris being cloned. They theorize that two simultaneous Chuck Norris roundhouse kicks could possibly destroy the universe....
Nibbler (Score:5, Funny)
Here we go again (Score:5, Informative)
First off, dark matter, even if it ultimately turns out to be wrong, is not a stupid idea; it explains a wide variety of independent phenomena [slashdot.org] (and contrary to the eternal "it's just epicycles" cry among Slashdotters, it is testable, falsifiable, and predictive).
Second, this new work is, well, new. Only one [arxiv.org] of the three papers (other two: here [arxiv.org], here [arxiv.org]) has passed peer review so far. When a theory like dark matter has amassed evidence in its favor over a period of decades, it takes a lot to overturn it.
Even if their STV theory does ultimately pan out (and there have been many alternate proposals in the past that have ended up failing), it will take years to be hashed out in the literature and subjected to far more tests; so far they have only passed a few of the observational tests that dark matter does, even assuming that their papers are correct, which no one has checked — there are no followup studies by other authors at this point.
Basic lesson: for every revolutionary new theory that works, there are a hundred that don't, and it can take a long time to decide which is which. New Scientist is not doing anyone a service by jumping on the latest unpublished preprint of the month and hyping it as the revolution of the century, as they tend to do.
Re:Here we go again (Score:4, Interesting)
Epicycles are actually a valid description of planetary orbits, and are still used today to analyze perturbations in planetary and protoplanetary systems. (A particle in a circular orbit will, if perturbed a small amount, acquire both drift and epicyclic motion relative to its original path).
Parent
Heim Unified Field Theory (Score:3, Insightful)
I highly suggest you read up on it if you like physics.
Yoda knows about Dark Matter (Score:4, Funny)
From Wikipedia [wikipedia.org]:
Detecting a graviton, if it exists, would prove rather problematic. Because the gravitational force is so incredibly weak, as of today, physicists are not even able to directly verify the existence of gravitational waves, as predicted by general relativity.
Yoda: Hard to see, the dark side is.
It's only a theory (Score:5, Insightful)
(For the humor impaired I'll give you a few moments to let the words sink in)
Re:It's only a theory (Score:4, Insightful)
No, I do not think that is a fair accusation.
His ridicule did not target religious folk, and it certainly did not target anyone for for believing in God. His comment in no way chastied the MAJORITY of religious folk, nor did it chastize the majority of Christians, and it did not chastize the official position of the Vatican. His comment targeted a minority group who happen to also be religious, a minority group going counter to mainstream majority Christians, and it targeted them for very specific reason of bad acts that are independant of belief in God.
Seems science puts a lot of weight on things no one has ever seen or proved as fact.
Science puts an amount of weight on things based on the amount of testing and confirmed predictions.
For example Dark Matter Theory has some pretty hefty weight. It is by far the leading theory in its area because it has made quite a few predictions that have been extensively tested and confirmed. It has explained more and had more confirmed predictions and has fewer problems and unexplained issues than any competing theory in the field.
And while "leading theory" is a fairly hefty position, it still falls far short of earning the full weight of "established/accepted science" as accepted by over 99% of PhDs and professionals in its area, far short of the title "indisputed foundation of its field". That is a far weightier level, a level of weight that is only earned by theories that have undergone staggering amounts of testing and which have make a large number of predictions that have been exhaustively and conclusively confirmed, theories with no signifigant contrary evidence, theories where there is *NO* known usable countertheory.
For example Relativity and Quantum Mechanics and Evolution all fall in that catagory. Each one is accepted by over 99% of the PhD's and professionals in its area. Each one is considered the very foundation of its area of science. Each one has made many many predictions, and they have been exhastively tested and conclusively confirmed with staggering mountains of evidence supporting them. None of them has any known signifigant counter evidence. There is no known theory that even comes close to competing with them in their area.
If you think that Relativity or Quantum Mechanics or Evolution conflict with God, then you are a scientifically illiterate and religiously illiterate fool deserving of chastizement. The position of mainstream majority scientists, the position of mainstream majority Christians (two groups with substantial overlap by the way), the official position of the Vatican and as explicitly and repeatedly stated by multiple Popes, is that there is no conflict between science and religion, no conflict between any of those areas of science and God.
The people who rejected and attacked Galileo because they thought there was a conflict between his science and God, they were fools and they deserved chastizement. And anyone today who says that there is a conflict between some feild of science and God is equally a fool, and equally deserves chastizement. That goes equally for either side. Anyone who's a scientist and happens to be anti-religion is a fool if they claim there is a conflict. Anyone who is religious and happens to be anti-science is a fool if they claim there is a conflict.
If someone with no education in Quantume Mechanics and no understanding of what it actually says and means, if that person were to claim that Quantum Mechanics was wrong and that 99+% of expert PhD professional physicists were all wrong and stupid and that they somehow mysteriously overlooked all the simple and obvious "counter proofs" that they come up with to prove Quantum Mechanics wrong and impossible, what do you think the reaction would be? Someone with no education in the field having the delusion to claim that a half million or so genuine experts and PhDs and professionals rep
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This doesn't even seem to explain the data. (Score:5, Informative)
What we find, however, is that the rotation curves of galaxies are nearly flat, meaning that the mass distribution of galaxies must be nearly equal all the way through. This means there must be a large amount of matter that we don't see. There aren't enough dwarf stars, planets and other things like that to make up this mass. Of course we haven't counted or seen all of these, but if you do the math, there would have to be a ridiculous amount of these - more than is likely. Hence, we have dark matter.
This new theory says that the force of gravity should be stronger near the galactic core, where the stars are packed most densely. So the core is even more massive than we thought, meaning that the rotation curve of the galaxy should be even more skewed - far from flat. So either New Scientist seriously misrepresnted his theory, or it doesn't even deserve a cursory thought. MOND at least seems plausible.
Re:Nearly right... (Score:5, Informative)
The new theory is STVG, not MOND. MOND is ANOTHER alternate theory, which is being compared to STVG. Maybe instead of trying to rush to prove the submitter wrong and post early so you can get modded up, you should... I dunno, read the fucking article?
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I still don't buy it (Score:5, Interesting)
Altering the laws of physics so that they do not behave consistently (specifically, they just happen to do something different when far away from our local experiments) and giving no explanation why this should be so except to fit the previous data is really a terrible way to do things. And if new observations topple your carefully constructed modifications? What are you going to do - change your maths again?
The analogy is seeing a red ball, and declaring not that there is a red ball, but that there is a subtle effect with optics that creates circular red blobs in your vision from time to time.
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STVG, epicycles? (Score:5, Informative)
describe correctly the three problems: galaxies, galaxy clusters, and the pioneer anomally. With enough free parameters you can always fit a curve to a data, and STVG has got lots a parameters:
its got ordinary gravity as GR
plus a cosmological constant
plus a repulsive vector field
plus 3 scalar fields
The scale fields describing how the strength of each of the forces varies in space (and time). He then curve fits his new equations with different free parameters for each problem, which you have to do because the strength of the forces varies from place to place. With 3 problems and 3 free variables its not surprising he can fit a solution. This isn't to say STVG doesn't make sense, it does, and fits in well with string theory for example. The problem is with that many free parameters its easy to fit a solution to any problem, but hard to make acturate predictions or disproveable assertions.
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F for reading comprehension (Score:4, Insightful)
Nearly right : The theory posits that gravitons are created by all (massive) matter, it's just that near densely packed stars the effect is more significant.
Actually, no. The point the atricle is making is that the effect increases near large concentrations of mass at a rate greater the simple total mass would predict.From TFA: Plain wrong : From TFA "critics point out that MOND cannot explain the observed masses of clusters of galaxies without invoking dark matter"
The article isn't about MOND, it's about the scalar-tensor-vector gravity (STVG) theory. MOND was just mentioned as a competing theory...a theory that couldn't adequately explain the behavior of galatic clusters or the Pioneer spacecraft.
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Re:Gravitons?! (Score:3, Insightful)
Re:Gravitons?! (Score:5, Informative)
The Higg's Boson creates mass and the graviton turns mass into gravity, or something like that - it's not my area...
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Re:Gravitons?! (Score:5, Funny)
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Re:Gravitons?! (Score:3, Funny)
Re:Nothing (Score:5, Informative)
That virtual particles appear and disappear from a vacuum is actually well established. It gives rise, for instance, to Hawking radiation [wikipedia.org], and one can even measure Casimir forces [wikipedia.org].
This new theory is clearly speculative, but that whole "particles appearing out of the vacuum" thing is not the new and interesting part. That is a plainly accepted aspect of all modern quantum theories.
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Re:read your augustine (Score:3, Insightful)
1. God needs to exist without being caused (P, proposition)
2. Therefore, God exists without being caused (P, same proposition again)
You don't really even have an argument there, you're just making an arbitrary fact claim with no evidence, and no argument. You can expand this and make it more fancy, but it's always reducible to this fact claim. Arguably the greatest mistake is fact claim "Everything needs a c
Re:when will it stop (Score:3, Insightful)
To exerimentally verify this go and buy one copy of New Scientist and one copy of Scientific America. Compare.
Can't. Quit reading SA after they started publishing thinly vieled political hacks instead of science articles.
Re:Dark Matter? Gravitons? (Score:4, Insightful)
Parent
Re:Dark Matter... graviton.... God.. OH MY! (Score:3, Funny)