Slashdot Mirror
New Gravity Theory Dispenses with Dark Matter
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."
Plain wrong : From TFA "critics point out that MOND cannot explain the observed masses of clusters of galaxies without invoking dark matter"
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?
While the graviton has never been observered, it's not as wildly unlikely as that article seems to suggest. Of the four fundemental forces (strong/weak nuclear forces, E/M and gravity) only gravity hasn't had a "force mediator" particle (one which "carries" the force, for example photons for electricity and magnetism) observed in lab experiments. However, as gravity is the weakest force (by an order of I believe around 10^-28 times, or similar), this is not unlikely. However, it is extremely unlikely there ISN'T a particle which mediates gravity, ergo the (pretty reasonable) assumption of the existance of the graviton. Assuming it exists, further things can be predicted about it from other laws of Physics, hence we have a particle we've never seen but are pretty sure exists, with certain properties.
The way the post is worded, I think there is some misunderstanding what a "graviton" is. The graviton is the force-carrying particle of gravity, in a similar way to the photon being the force-carrying particle for electromagnetic phenomena. Although the graviton has not yet been directly observed, there is little doubt among physicists that it does exist. The current best theories we have (standard model of particle physics, etc.) strongly suggest that it exists.
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). 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.
Now we're going to see the inevitable parade of, "See, I knew dark matter was a stupid idea all along".
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 (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 of the three papers (other two: here, here) 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.
This is the problem when you read dumbed-down explanations of science. It really does sound like fantasy and BS. However, we have not moved into "the fantasy realm". According to our current best theories of particle physics (the standard model, etc.), virtual particles can indeed appear in the vacuum, exist for a short time, then disappear. All of this comes out of a quantum treatemnt of fields (quantum field theory, etc.), where Heisenberg indeterminacy (also called the Heisenberg uncertainty principle) leads to these quantum fluctuations on small scales.
That virtual particles appear and disappear from a vacuum is actually well established. It gives rise, for instance, to Hawking radiation, and one can even measure Casimir forces.
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.
The Higg's Boson (or God Particle) mediates the Higg's Field, the Graviton mediates the gravitational field - they are different things, although are related.
The Higg's Boson creates mass and the graviton turns mass into gravity, or something like that - it's not my area...
Sorry to reply to my own post, but here are, possibly, the scientific papers in question. Doing a search on arXiv for the names of the authors (Joel Brownstein John Moffat) provides a paper entitled "Gravitational solution to the Pioneer 10/11 anomaly" (warning: PDF).
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, 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.
I'm not an expert in particle physics either, but here's what I know:
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. 90 Astronomical Units is 12 light-hours. 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, 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.
No, that's incorrect. Quantum mechanics has nothing to do with it, it's just relativity. Even in classical 19th century electromagnetism, effects propagate at the speed of light, because Maxwell's theory is a relativistic field theory. In fact, it was this fact that prompted Einstein to invent relativity, because he was trying to come up with a mechanics that was compatible with electromagnetism (which he thought beautiful). And it was the finite propagation speed of electromagnetic effects, at light speed, that led Maxwell to first postulate that light is an electromagnetic wave.
We have already indirectly measured the speed of gravity by observing the rate at which a binary star system is losing energy due to gravitational radiation (it's indirect because we infer the energy loss due to their orbital decay, not due to detecting the radiation itself). It works out to be equal to the speed of light, to within a few percent. The 1993 Nobel Prize was awarded for this work.
As you say, if gravitational waves are observed (which they may be within the next decade; we're looking now), we will be able to measure their speed directly.
It is technically complicated to explain how attractive forces work in quantum field theory by particle exchange. Part of the answer is that quantum particles have phases associated with them, and you sum over all possible ways for a particles to get there via different paths with different phases. Suffice to say, they don't act like little billiard balls running into each other.
There isn't one; see my correction.
No.
Technically, you're right; the nuclear force is "infinite ranged", but its strength drops off exponentially instead of inverse square (Yukawa potential, roughly), so it has an "effective cutoff distance" beyond which it drops substantially.
P.S. To the other poster who asked about gravitons and black holes, see this FAQ.
That's wrong. It comes from a naive treatment of gravitons as classical particles that bounce off each other, rather than as virtual particles whose quantum phases superimpose after taking all possible paths to their destination. See here.
That's also wrong. Gravitons are affected by everything they interact with. However, their interaction is very weak.
Gravitons have absolutely no problem explaining this, just like the photon theory doesn't have any problem with explaining why the electric field isn't "cut off" when a charge passes in front of another charge.
No it would not. Just as photons don't have negative momentum just because they pull together oppositely charged objects.
One of the most significant pieces of evidence for dark matter is the rotation curve of galaxies. If the Universe functioned like we thought it did, the rotation curve of a galaxy should be a downward sloping curve - the further out a star is from the galactic center (where the mass of the galaxy is concentrated), the slower its orbit should be. This is what Kepler's Law tells us - that the orbital speed of an object decreases inversely with the square root of the orbital radius.
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.
However, we're still left with the age old question: If gravity is manifest as a particle, why can't we shield against it?
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.
The strength plays the role of Newton's gravitational constant in GMm/r^2. The 1/r^2 is what matters the most in determining how quickly the force falls off, but the constant in front matters too. For a massive exchange particle, you get the Yukawa potential k exp(-r/R)/r where R is the "characteristic distance" determined by the mass, R = hbar/mc. (Larger masses mean shorter distances.) k is the strength of the interaction (actually the square of the coupling constant), but the overall function is dominated by how fast the exponential decays, which is governed by R (and thus m).
That's not true; the strength of an interaction determines how strongly two particles of a given energy influence each other, not how strong the particles themselves are. In fact, the strong nuclear force in quantum chromodynamics weakens at higher energies (leading to the phenomenon of asymptotic freedom and confinement).
That's true, but what governs their lifetime is not the strength of the interaction, but their mass(-energy), via a kind of energy-time uncertainty relation.
There is sort of a way like that to explain, say, the inverse square law: a sphere of radius r has an area that increases like r^2, explaining the 1/r^2 dropoff of the force: the virtual particles can go to more places and get "diluted". But the idea is that "fewer" virtual particles end up at any one spot, not that they "run out of gas" and can't travel that far. You sum over all virtual particle histories, of arbitrary lengths.
What's really going on is a million times weirder, of course.
If corporations are people, aren't stockholders guilty of slavery?
"rediculous"? For fuck's sake.
The photons from your light beam are real, not virtual. There is no limit to how long they can exist.
However, we're still left with the age old question: If gravity is manifest as a particle, why can't we shield against it?
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.
Blasphemy is a human right. Blasphemophobia kills.
"A theory is a generalization based on many observations and experiments; a well-tested, verified hypothesis that fits existing data and explains how processes or events are thought to occur."
Note there's observation and testing cycles involved, as well as accurate predictions of behavior based on the testing/observation/revision of the hypothesis graduation process. It's not a simple matter of consensus or grouped hypothesis. You make it sound like "Because a bunch of us have the same hypothesis, it can now be called a theory." That is blatantly untrue and entirely contrary to the scientific method.
I really do get tired of posters insinuating that scientists are some kind of priest class that pulls all of this stuff out of their ass. Just because you read a Popular Science magazine article about it or you have your doubts about the way some random slashdot-linked article doesn't make you a student of that field. I only say this because of the cursory nature of understanding that these articles present. They've been overviewed in common parlance because of their complex nature, and the situation here is analogous to someone here walking barefoot through a small puddle and claiming they have the ability to compete with an Olympic swimmer based on that puddle experience. The reason we have the ideas of how things work is not arbitrary conjecture, but really hard work and a LOT of failure. It's generally not a matter of pride or authority because there are many scientists that have different ideas about the same subject, and these ideas are subject to criticisms by their peers as well as the practical universe as a whole.
There have been MANY proposed explanations of gravity by "scientists" and "non-scientists". (The reason I put the word scientist in quotations above is because we're all really scientists to a degree) Most of those explanations have fallen by the wayside because they fail to work in real-life cases of testing and observation. If that wasn't true then technology as it is today wouldn't work at all.
That's not to imply that all the "answers" have been found, but it does mean that we're obviously doing something correct because we can make your hair-dryer work, or your computer, or fly to the Moon, or any number of examples you could cite. There is much progress to be made, and some of it (like quantum mechanics) might seem counter-intuitive and fantastical but come up with a better, testable, verifiable explanation (rather than just blatant criticism) as to why it works and your explanation will be accepted as the baseline.
I guess the point of my ramblings here is: The scientific community is not some amorphous blob of single-minded authority, and it shouldn't be treated as such.
Just read J Moffats paper, http://lanl.arxiv.org/abs/gr-qc/0506021, and so i'm not supprised he can
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.
That's not correct (from the same thread). Even at the time of that story, a rebuttal had appeared, and now a few more have as well. Astrophysicists aren't stupid; general relativity effects are nowhere near strong enough on galactic scales to account for galactic rotation curves. The authors should have known better.
Much like this story, you need to take Slashdot reports of breakthrough physics with a grain of salt, especially unpublished papers reported by New Scientist. Real breakthrough physics takes time to work its way through scientific debates before it can be accepted.