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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."
If a hypothesis involving gravitons is explained by experimental evidence, then this hypothesis could be elevated to theory.
:-/
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.
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We can hardly shield against neutrinos, right?
No data, no cry
None of those claims have actually been examined or verified in the peer reviewed scientific literature. From a physicist's perspective, it's extremely difficult to find out what the Heim theory is (does it even have a Lagrangian? how is the theory quantized?), let alone what its predictions are and whether it's right. All you have to go by is a bunch of claims from an Internet "Heim appreciation society".
Try the book Gravity from the Ground Up, written by Bernard Schutz, one of the directors of the Albert Einstein Institute (the largest gravity research center in the world). You can also Google for information on "general relativity", our current best theory of gravity, and "quantum gravity" for the theories that may replace it.
However, it may not satisfy you, because I have often found that people who are frustrated by "How does X work?" in fundamental physics are really looking for an answer at a philosophical level that science cannot provide.
I thought Dark matter effectively functioned like negative gravity so as to account for the accelerated expanding universe? How does the graviton explain this phenomenon?
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).
Note: some of the replies to my posts have fixed mistakes I made in the explanations. For instance, it is more correct to specify that the maximum range over which a force acts has to do with the mass of the particle (which is related to particle energy, of course). Massless particles can operate over (in principle) infinite distances, so there is no upper bound to EM and gravity forces... however the "perceived distance" over which a force acts also has to do with how quickly it decays, and I believe this is related to what I was describing.
It's important to distinguish between virutal particles and real particles. Take E&M for a moment: an electric field exists because of the exchange of virtual particles between the (charged) objects in question. However, a beam of light (or radio waves etc.) is an E&M wave, and is carried by real photons. The terms 'real' and 'virtual' were used because you can directly measure real particles, whereas virtual particles are detected only indirectly (by their effect). So a beam of light is 'real' and its constituent particles will not "pop out of existence" ever (they may be absorbed or otherwise interact with other particles, mind you). So a beam of light will eventually reach the other end of the universe. However, the field "emanating" from an electrically charged stationary particle will be infinitely weak if you go infinitely far away (because only extremely low-energy virtual particles can reach out that far).
Similarly, the graviational field is made by exchanging virtual gravitons. In principle, an accelerating mass creates gravitational waves: REAL gravitons. Efforts are underway to detect these gravitational waves directly (LIGO).
The problem, however, is that Neutrinos don't interact with the Earth as they pass through. If they interact, then they are being shielded against. As TheRaven64 pointed out, methods do exist for shielding against neutrinos, even if such methods are only partly successful.
If it was not possible to shield against gravitons, then how do they interact with bodies such as the Earth? If they do interact with objects, and it is possible to shield against them, then why doesn't placing one object in front of another reduce the influence of the body that the second object is shielding against? Yet if you stack a wall of bricks to the moon, you would still see the same influence from the Earth on the top brick as you would if the top brick were suspended with no objects blocking its path.
Or at least, that's what we believe at the moment. If someone can show an actual gravitational difference between the two situations (even if it's minute, but reproducable), then the hypothesis of gravitons would begin to gain serious leverage in becoming a theory.
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So... basicly, what you're saying is that you're a theoretical physicist working on dark matter, and you don't want to see your research invalidated.
Go look up all the previous revolutionary theories from New Scientist that have appeared on Slashdot, and see for yourself how many have actually turned out to be correct after experiencing the test of time. The grandparent post's caution is correct.
then why doesn't placing one object in front of another reduce the influence of the body that the second object is shielding against?
Because gravitons don't "push", they only "pull". The only reason we can shield against, say, an electron, is because the electric field can "push" as well as "pull".
Since a gravitational interaction is always purely attractive, any material you put in the middle, to first order, doesn't have any effect. If you've got particle A, B, and C, if A exchanges with C (A==C), and then you put B in the middle, forcing A to exchange with B, then B to exchange that same graviton with C (A==B==C), the net effect is the same - B gets momentum towards A, and C gets momentum towards A because B is now heading towards A.
If you read the article, though, SVTG says it does have an effect - in exactly the wrong way you'd expect. That is, putting matter between doesn't shield gravity, but makes it stronger. And that's how they can explain galaxy rotations - gravity is stronger near the dense central region, and weak farther away. That's also how they explain the Pioneer anomaly - it's not an acceleration as much as gravity gets weaker as density decreases (so the farther you get, the weaker it gets).
The "scalar" part of scalar-vector-tensor gravity makes me think it's related to a Brans-Dicke theory, which was an attempt to satisfy Mach's principle - that is, the mass of an object only makes sense if there's something else around it, so gravity itself should be dependent upon the matter distribution. This theory, also, seems to satisfy Mach's principle.
For those who applaud the end of dark matter, this sort of law-modification theory should really be much more worrying. Dark matter at least has a plausible explanation, is predicted by various theories, makes predictions, and is somewhat intuitive. (come on, how likely is it that all the massive particles in the universe just happen to interact electromagnetically as well? We know from measurements that some, at least, do not - e.g. neutrinos.)
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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by Brian Greene that he thought it might be possible to detect a graviton using CERN's Large Hadron Collider when it is completed. This may also help to substantiate some of the ideas of string theory... namely that the energy of a particle (string) is inversely proportional to its "size". Gravitons may be "large" enough to be detected when the particle smashing fun begins! I really enjoyed reading this book BTW. Recommended reading!
I liked it better when nerds weren't cool.