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."

Vast improvement

So we finally get rid of this ad-hoc and hypothetical construct by replacing it with... oh.

"See I told you guys it wasn't flying monkeys! Turns out it's flying Unicorns!"

Re:de rigueur

Why not flying spaghetti monsters?

Because that's the noodle-string theory.

My invisible friend

You know as a child I made up invisible things to blame and was told that was a bad thing to do. Only now I find out I was really a budding scientist...

Re:My invisible friend

You know as a child I made up invisible things to blame and was told that was a bad thing to do. Only now I find out I was really a budding scientist...

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 :)

Actually...

"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..."

This sounds more like the work of a quantum physicist.

Sure, it's off course

If you get Gravitrons [wikipedia.org] spinning fast enough, you can unpredictably launch things anywhere, especially smaller objects such as probes or children.

Oh...graviton.

"Hypothetical particle"

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.

Re:"Hypothetical particle"

Gravitons [bookrags.com] are supposed to be the exchange particle [gsu.edu] for gravitation, as the photon is for electromagnetic force. The graviton and photon are chargeless, massless particles, differentiated by their spin. The strong and weak nuclear forces are also mediated by exchange particles (W & Z for the weak, Gluons for strong quark interactions, Pions for strong nucleon interactions). The graviton's importance is in serving as a moderator of the gravitational force; if this theory is correct, then it won't be long before someone will come up with an idea for exposing the graviton to the light of day.

Some clarification

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 [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...

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) [arxiv.org] provides a paper entitled "Gravitational solution to the Pioneer 10/11 anomaly" [arxiv.org] (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, [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.

Re:Uhh - Action at a Distance?

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 [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.

Dark Matter Is Chuck Norris

Scientists have actually calculated that Dark Matter is, in actuality, Chuck Norris. He recently flew to the west coast, and this threw off the Pioneer 10 probe by 400,000 miles.

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

If there's no dark matter, what's that stuff coming out of Nibbler?

Here we go again

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 [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.

It's only a theory

It's only a theory folks. We shouldn't be teaching it because it hasn't been proven. It's not a fact.

(For the humor impaired I'll give you a few moments to let the words sink in)

This doesn't even seem to explain the data.

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.

Re:Gravitons are not a new concept

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. :-/

Re:Gravitons are not a new concept

However, we're still left with the age old question: If gravity is manifest as a particle, why can't we shield against it?

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.

Re:Gravitons are not a new concept

Sure, they can pass through Earth, but I'll be really impressed when they can pass through New Jersey without paying a toll.

Re:Gravitons are not a new concept

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.

Re:Nearly right...

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?

Re:Gravitons?!

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...

Re:Gravitons?!

... and then there's the crouton, which mediates salads.

Re:so...

I think it's fair to say that the matter of whether or not dark matter really matters is a matter of some gravity... :P

Re:Nothing

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 [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.