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Dark Matter — "Alternative Gravity" Team Responds
An anonymous reader writes, "Following previous results, an international team of astronomers answers, defending the case for a modification of the theory of gravity. This article presents an alternative to dark matter and states constraints on the neutrino mass. In short, dark matter is still not a necessity, provided that neutrinos weigh 2eV. This is allowed by what we currently know and should be tested in the KATRIN experiment in 2009."
err, neutrinos do have mass, but not as much as stated in the paper. As far as current experiments go neutrinos come in three flavours and interchanging between them is only possible if they have mass. It has been shown in experiments that they change type and hence must have mass.
I do not beleve that this is true.
Newtonian mechanics implies that for gravity to affect an object it must have a mass, however General Relitivity does not impose this restriction.
I am pretty sure that the gravity effect is caused by the distortion of space time such that the 'shortest' path (That which the light must follow) is curved.
I am not an expert in GR, but perhaps someone here can verfy my claims.
I'd like to clear this up because there are very common misconception that photons are massive or that something has to be massive to feel gravity, both of which are false.
THEORY: In our current understanding, photons are forbidden from having mass because of the way quantum electrodynamics (the most precisely tested theory in the history of science) works. It's an exercise in field theory to show it, but the gist is that electromagnetism (light, charge conservation, electric and magnetic forces...) are a consequence of a symmetry of nature, and that symmetry only works if the associated carrier particle (the photon) has exactly zero mass.
EXPERIMENT: If the photon had even a very tiny mass, it would also mean that the electromagnetic interactions would become short range (just like the weak interactions, which are mediated by a massive carrier). The usual inverse square law would become an exponential falloff. This has been tested for in laboratories (and in astronomy!) very precisely, so there are ridiculously strict upper limits on the photon mass.
This doesn't mean photons don't feel gravity!! Gravity interacts with all energy, not just mass, and so the energy of a photon is enough to cause it to bend around massive objects.
Mass, man, mass. It's not weight, but mass.
After relativity and E=Mc^2, physicists have preferred to measure mass in terms of energy rather than silly units like grams and ounces. In short, we give the energy equivalent for the particle if it was somehow completely annihilated. 1 electron volt refers to the energy of one electron under an electric field at a point of 1 volt of potential difference. It doesn't have anything to do with the actual mass of the electron, but rather the electrical potential energy such an electron would hold.
Their paths don't bend - it is the paths themselves that are distorted in space-time by the gravity well. This distortion appears to be bent in three dimensions - to the photon it is perfectly straight...
(ok, ok, simply *massive* oversimplification here - to the point of error, but I hope you understand my motives.)
They're just proposing that there is no "exotic", new kind of dark matter.
Incidentally, I'd watch the Cosmic Variance blog in the coming days for a discussion of this point; Sean Carroll's post there on dark matter was linked to in the last Slashdot story.
Responding to other posters: the amount of photons in the universe can be estimated based on how many of them reach us, as well as from theoretical predictions on the emission of light from stars, the Big Bang, etc., and is woefully inadequate to produce the needed gravitational effects — not to mention it is too "hot" to be the kind of dark matter needed to explain early universe structure formation.
An eV, or electron volt, is a measure of energy: the amount of energy acquired when an electron is accelerated through a 1-volt electric potential difference. It is about 1.6 * 10^-19 joules. By E=mc^2, it also corresponds to a mass, about 1.8*10^-36 kilograms. An electron, by comparison, masses about 511,000 electron volts.
People should NOT take the impression from this article that there is doubt that dark matter exists. The only doubt being raised is over what form the dark matter takes. Let me clarify:
(Note: Baryons are protons and neutrons. "Non-baryonic" means not made up of the building blocks of ordinary atoms.)
The beauty of the Clowes work (the "proof that dark matter exists" from a couple of weeks ago) is that the colliding clusters they worked on give simple, clean evidence that galaxy clusters are really dominated by invisible, non-baryonic dark matter. At it's core, it's a very simple argument. Two clusters collided, and the baryonic clouds (hot gas, seen with X-rays) experienced drag and got a bit hung-up passing through one another. Most of the mass, however (seen with gravitational lensing), passed straight through with no drag. We see the X-rays and lensing in two different places on the sky - they really are two different kinds of stuff. This is VERY direct proof that most of the mass in galaxy clusters is not the ordinary matter we see on earth - it's something non-baryonic that does not interact with light and does not interact much with ordinary matter. In other words, dark matter is real, physical stuff!
This article argues only about what that dark matter might actually be. It's generally believed that it can't be neutrinos, because neutrinos are so light that they would mess up galaxy formation, and so must be some new, exotic kind of particle. The logic here is that very light particles move so fast that they don't clump together well under their own gravity, which would disrupt the formation of galaxies and smaller clusters of galaxies. All this paper argues is that the dark matter might not be a truly new particle - the combination of modified gravity and neutrinos can be made to work. They still conclude that the invisible neutrinos must outmass the baryons in the clusters by a factor of at least 2.5.
Many people (particularly those who do not understand the evidence) dislike the idea of dark matter, thinking it sounds too much like epicycles. That's understandable, and it's good to be very skeptical of such a weird idea (I know I was). The truth is that there is now enough evidence to say that it really does exist, no matter how strange it may seem to us. The future lies is figuring out what the dark matter is actually made of, not bland assertions that "that just can't be right...".
The electron volt is a measure of energy -- the amount of energy needed to move an electron through a potential difference of one volt in an electric field. (Think of it as moving a small ball up a hill). Thanks to "E=mc^2" this is also a measure of an equivalent mass -- and it is frequeuntly used to specify the masses of subatomic particles. (For comparison, an electron "weighs" about 500,000 eV -- even by particle standards, 2eV is very small)
"Dark matter": an invisible attractive force operating on galaxy-level distances (at million light years). Size: about 23% of the energy-mass of the observed universe. Evidence: Galaxies spinning faster than the number of visible stars justify. Gravitational lenses stronger than visible stars justify. Suspects: known low mass particles like neutrinos; unknown low or high mass particles like strings, wimps; a new phsyical force; non-r-squared term in Newton's equation of gravitation, observational error ...
...
"Dark energy": an invisible repulsive force operating on universe-size distances (at billion light years). Size: about 73% of the energy-mass of the observed universe. Evidence: Hubble expansion is accelerating over time when gravity would suggest eventual deceleration or collapse. Suspects: energy in fabric of space-time, unknown force, observational error
"Observed matter": stars, galaxies, gas clouds, neutrinos; Size: about 4% of the energy-mass of the universe.
Somewhere in the great online book Reflections on Relativity, there is a discussion of "kugelblitz"s, which is a theoretical black hole that consists entirely of energy, which could be just a lot of photons. The term isn't in much use in science (though I did find at least one arxiv.org reference) because it's not very useful; in practice, a photonic kugelblitz is impossible, and once such a black hole forms, it would be indistinguishable from any other black hole. But it is theoretically possible, because all mass-energy contributes to the gravitational field.
No. The photon never has any "rest mass". It has momentum without mass. That falls out of the equations. You can think of the speed of light as infinity, because under special relativity the equations have a term 1/sqrt(1-v^2/c^2). When v=c, the term goes to 1/0, and numbers pop out of nowhere.
The term "c" is the speed of light in a vacuum. Experiments like these don't happen in a vacuum. Light isn't really being slowed down per se; it's still moving at c when it's away from the atoms. These experiments are a very clever way to keep the light pulses intact while keeping the light itself from actually going very far. The net effect is to slow down the light pulse without actually slowing the light itself, so the mass of the light is unchanged, as is its net momentum. Inside the system the momentum of the light is bounced around and interacting with electrons, but at that level the light is just behaving as light does with nothing to affect its speed beyond plain old quantum juju, so there's no change in mass or momentum there, either.