Slashdot Mirror
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."
is too!
is not!
is too!
...
Great minds think alike; fools seldom differ.
Dark Matter is the 21st century's ether.
P226
I am not sure that photons 'have' to have mass. I would indeed suspect that they are 'forbidden' from having mass, due to the fact that they are traveling at the speed of light. If they did have any intrinsic mass, traveling at that speed would cause the mass to move towards an infinite value, esencialy meaning that light would not be able to travel at the speed of light.
Of course relitivity could be wrong, or light could travel slower than the 'speed of light', if that makes any sense.
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.
"Dark matter," also known as "adolescent matter" is so known because of its inherent moodiness. It spends a lot of its time wearing dark makeup and brooding.
Um.
Google "electron volt in amu":
That's five whole orders of magnitude lighter than an electron. That sounds like a good reason they don't interact; it'd be like saying a dust cloud should interact with a chain-link fence.1 electron volt = 1.07354412 × 10-9 atomic mass units
110100 1101000 1101000 1100110 0 1101111 1101000 1100011 1
Photons lack mass but (since they move at 1,0 c) they do have momentum. This is wat makes solar sails work.
Hey, maybe that's the answer: substitute momentum for mass in all gravity calculations and see if that makes it all work.
Error: password can't contain reverse spelling of ancient Chinese emperor
Enh.
Neutrinos *do* have mass, and this fact is accepted by pretty much all physicists. The argument for this comes from discovery that they change states over the course of their lives, which means that they experience time, which means that they cannot travel at the speed of light, which means they must have a small mass. (This explains the apparently deficiency of solar neutrinos which was a problem in the 70s) Pinning down the exact value of this mass is more troublesome, though - for now, we know only that it's small, but positive.
What more puzzles me about this statement is that neutrinos have generally been counted as *part* of dark matter - in particular, they are proposed to constitute some of those so-called Weakly Interacting Massive Particles (WIMPs) which is one of two possible models for dark matter. I don't see how changing the details of these particles would change how neccessary they are, unless these guys are trying a bait and switch by redefining dark matter to be unneccessary. (Which would be a very dirty trick.)
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.
For me, dark matter is like religion. Made up to explain what we can't understand, and wrong.
Interesting observation, if a bit off. The difference being, of course, that we will eventually have a factual basis for dark matter ( whether it exists or not ), where as we will never know if $deity exists.
This is true for all supernatural values of $deity.
Mod me down with all of your hatred and your journey towards the dark side will be complete!
Comment removed based on user account deletion
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.
The famous equation: E=MC^2 converts to M=E/C^2. For truely tiny masses, that's the easiest way to measure and specify them.
'Sensible' is a curse word.
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.
I think I'd regret responding to the complete misunderstanding of forces and neutrinos in the body of your post. That would take pages.
Let me just respond to your title. That is completely wrong as well. Now, I think the alternative gravity guys are probably wrong and at this point I think they are stretching their theories to their limits. Dark matter is the "easiest" explanation. But, what they are doing is science. They are coming up with an alternate theory that makes predictions and testing them. The are countering circumstantial evidence for DM with another theory. They are not picking just one small thing, saying "Well that can't be true because of [insert some non-science babble like you just posted] so clearly God created everything." in contradiction to vast bodies of scientific evidence. And the alternative gravity people are publishing in peer-reviewed journals.
ID can't say any of those things. While the motivations may be similar (not wanting to give up on old ways of thinking about things) the methodology is completely different.
Slashdot Burying Stories About Slashdot Media Owned
I don't think anyone has a huge amount of "faith" in dark matter. The problem is that there is a conflict between theory and observation; gravity as we understand it doesn't predict the shape of the universe that we see, so we try to figure out what it is that we don't understand. That is how science progresses.
Our theories of gravity have held up well under testing many times, though it's fair to say that we don't know as much about it as we would like. Alternate gravity is also a matter of uncertainty, though, as we don't have any solid data showing that our gravity theories are wrong. Quantum physics has shown that there are many types of particles and many different interactions, suggesting that not all matter is structured the same way, so it's not unreasonable to suggest that there might be a type of matter that we don't understand.
Naturally there are physicists exploring both possibilities and they can be fans of one idea or the other, but that doesn't mean that they are acting on "faith". It's just how science has always progressed. There was a time before relativity was tested when it was controversial, and to some degree it still is.
I have seen the future, and it is inconvenient.
Here's Newton's law of gravity:
d mV/dt = - G M m/R^2 R_hat
It doesn't work for galaxies, it doesn't work for the big bang, it is broken for almost anything BIG. It also has a tiny bit of error that GR corrects, but that is minor. The problems with this law are HUGE. So we have two schools of thought. One wants to stuff the big M box with dark matter:
d mV/dt = - G (M + Dark_M) m/R^2 R_hat
These folks get to put Dark_M wherever it needs to go to get the answer right. Then there the MOND folks who want to mess with the R:
d mV/dt = - G (M + Dark_M) m/R^2 or if dV/dt is small, d m V/dt = - a_0 sqrt(G M/R^2) m R_hat
where a_0 is a new constant in nature that changes the form of gravity's law if tiny. I got my own proposal. Remember the chain rule from calculus?
d mV/dt = m dV/dt + V dm/dt
That V dm/dt is the stuff of rocket science. We know it is not relevant for stars cause those big star things and galaxies don't change. But we could, just for the fun of it, do a relativistic swap-out, and consider:
d mV/dt = m dV/dt + V dm/dt + V c dm/dR
Force is a change in momentum, which can be seen either as the usual acceleration, the rocket-ship effect, or as where stuff is distributed in space. That sounds like what is going on. So my proposed modification is this one:
d mV/dt = m dV/dt + V dm/dt + V c dm/dR = - G M m/R^2 (R_hat + V_hat)
Too bad I suck at numerical integration or I'd try and see if it could match real data sets. I like it because it uses stuff we know is true (the chain rule) with a fun twist to make an old law point in a new direction.
doug
Working on new views of old physics at http://VisualPhysics.org
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 sweaty fat kid at college had loads of mass and no one interacted with him. :-)
Perhaps neutinos are similar.
KATRIN experiment homepage URL
http://www-ik.fzk.de/~katrin/
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)
I try to be conservative, but not closed minded, in my accepting of new ideas. Concerning a concept as heavy as gravity, I think that scientists are just throwing us around, and i'm not falling for it.
This is incorrect. Theory exists regardless of the existance of any one theorist who believes that the theory must be true or is the only explanation available.
To re-state: dark matter is a theory becuase it was a hypothesis which has endured the gathering of some experimental data, but there is not yet enough experimental data to exclude other possibilities. This is, in no way, a matter of faith. It's certainly a matter of speculation and experimentation, and anyone who tells you "dark matter exists" is over-simplifying to the point of error.
Now, this hypothesis that we're discussing is a different beast. It's a mathematical model that may or may not preclude dark matter by chaning the rules slightly. Changing the rules of gravity isn't that much of a big deal (we assume that the unification of gravity with the other forces will probably come with some surprises), but one does not speculate about those changes lightly. To wit, this theory is being greated with skepticism, not because it offends some faith in dark matter, but because it requires some heavy thinking about existing mechanics.
This is what science is all about. You build a model, and then you tear it down. You repeat this process until you have a model for which the difference between "sturdy" and "unassailable" is indistiguishable. At that point, you refer to the model as a "law". That is, "a very sturdy model". Then you move on to the implications of that model, and start building new models.
I am fine with the possibility that there is a lot of normal matter which is not detectable from earth. I am also fine with the idea that more exotic forms of matter and energy might exist. The current dark energy models are the best matches for astronomical observations thus far. And when it is all said and done, if dark energy continues to be the best description, it will prevail, but that doesn't mean we shouldn't stop testing it.
At one time, all the scientists thought there was this stuff called ether, and it was the best explanation for the observations we had. Then people did more tests, and discovered incongruities. In the end it was proven an incorrect idea, and was supplanted with a better model.
Einstein spent many years trying to find a deterministic alternative to quantum mechanics. There were many respectable scientists that felt that QM was merely a useful approximation, but after years of testing, a the consensus finally turned, and the community accepted that the non-deterministic aspects of QM were real.
Should we have blindly accepted Ether or QM, just because preliminary results showed promise with the ideas? No - we continued to question them and test them until they were disproved or time had shown them to be solid ideas. Dark Matter is in the same place as these theories once were. I don't know whether it will turn out to be correct or not, but I do know we should continue to challenge it, to think of new ways to test it, and to think of alternative explanations, because that is what science is about and that is how we take good ideas and turn them into a rigorous and well-established understanding of the universe.
You would call these people pseudo-scientists, and yet your only argument an application of Occam's Razor (and as others pointed out, faulty understanding of principles). But that's the funny thing about Occam's Razor - it is dependant on one's personal opinion of what is the most likely, or most simple explanation. Some would consider making up new particles that we have never observed a real stretch, others consider tweaking the existing rules a hack. That someone has a different view of what is elegant than you, does not make their ideas pseudo-science. What matters is if they are predictive and falsifiable, which these are.
Honestly, if you can't tell the difference between people that present testable alternative hypothesis, and people whose best "theory" that they could present amounts to "does this not appear irreducible", then you are the one that needs a refresher on what is and is not science.
"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.
The real result of Clowe et al's fascinating work was to show that the missing mass in the bullet cluster must be COLLISIONLESS, whatever gravity looks like (a purely baryonic bullet cluster has been *falsified*). However, a big misconception about it was to think it was a direct *confirmation* of the Lambda-CDM concordance model that everybody is supposed to believe (may I recall that real science is about *falsifying* things, not "proving" them right), or that it was falsifying MOND. Actually, it is known for years that MOND is UNABLE to fit the temperature profiles of X-ray emitting clusters from their pure baryonic content. The fix, for MOND to stay in the game, was to propose that neutrinos have a 2eV mass and can then make up for the missing mass, in clusters ONLY, because they are too light to cluster on the galaxy scales (incidentally they are also too light to form structure in GR, but this is not a problem for structure formation in MOND). However, if dark matter is indeed cold as the lambda-CDM guys tend to take for granted, and even more since Clowe's work, why does the 2eV neutrino combined with MOND seem to work in ALL clusters??? The bullet cluster being a totally new kind of constraint for MOND on the galaxy cluster scale (constraint coming from gravitational lensing instead of temperature profiles), it was mandatory to check if 2eV neutrinos were excluded even in MOND, which would have *falsified* MOND indeed! This is what those guys wanted to do, to *falsify* MOND once and for all, but the surprising result is that they didn't manage to do so, because the SAME neutrino mass as the one needed to fit temperature profiles of other clusters ACTUALLY WORKS in the bullet cluster too. Their conclusion is thus just that MOND is *not excluded* by Clowe's data. One will thus have to wait for particle physics experiments to rule out massive neutrinos to rule out MOND. Until then, place your bets...
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.
You're trying to draw a binary distinction where none exists - that if we "catch" some dark matter that would mean we know for sure it exists, but until then it has the same status as ID.
That's simply nonsense - a direct detection of DM would mean you built a detector and it registered some hits. Assuming the particles detected had the correct properties, that would be taken by most as confirmation of the existence of DM - but you could assail it on precisely the same grounds, that scientists interpreted that as evidence for DM only because the can't imagine other explanations, etc.
The point is, all you can ever do is accumulate evidence for or against theories. At some point the evidence becomes convincing and the theory generally accepted, but it's not an either/or situation - it's gradual. There is massive evidence for DM, from all sorts of different observations. Direct detection would be additional very strong evidence, but the theory is already on a pretty firm foundation (and people have thought of MANY other possibilities, by the way - it's just that none of them are consistent with the data).
Attention, parent poster and everyone who agrees with him: please immediately cease enjoying benefits of government-funded science. That means logging off /., getting rid of your computer -- in fact, not owning any personal electrical devices whatsoever -- refusing any medical diagnostic procedure developed after the invention of X-rays, and generally living life ca. 1900.
For those who say, "that's technology, not science!" I will note that the examples I gave were based largely on previously abstract, largely government-funded scientific research whose applications were not immediately obvious, but which have since transformed the way we live. If you don't understand the research, that's fine; you don't have to in order to take advantage of it. But just because you don't give a shit about the way things work doesn't mean that you get to stand in the way of people who do, and whose work will benefit you and your children's lives, no matter how little you deserve it.
The correlation between ignorance of statistics and using "correlation is not causation" as an argument is close to 1.
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.
Photons travel at the speed of light and so by definition have zero mass. What they do have is an energy density, which our friend Dr. Einstein made sure to include in his GR equation. So if the universe was filled with nothing but photons, their combined energy density would create its own gravity. That being said, their combined gravity is predicted to be far less then what you need explain dark matter since the average energy of the photons in the universe is in the radio and microwave wavelengths.
The real question is "Is the converse true" As light propagates through the universe, does it warp space/time as well, allowing it to attract other bodies. Would high intensity light warp more than low intensity? Would high frequency light warp more than low frequency?
I wouldn't consider the mad hatter mad. Just reality impaired. He sure can make a mean cup of tea.
Photons lack mass but (since they move at 1,0 c) they do have momentum. This is wat makes solar sails work.
Nope.
Quoth wikipedia:
A solar wind is a stream of charged particles (i.e., a plasma) which are ejected from the upper atmosphere of a star. When originating from stars other than the Earth's Sun, it is sometimes called a stellar wind.
It consists mostly of high-energy electrons and protons (about 1 keV) that are able to escape the star's gravity in part because of the high temperature of the corona and the high kinetic energy particles gain through a process that is not well understood at this time.
http://en.wikipedia.org/wiki/Light_sail
There, see? He was right about solar sails.
You were thinking of a magnetic sail:
http://en.wikipedia.org/wiki/Magnetic_sail
Which is something completely different. Magnetic sails do use solar wind; solar sails use sunlight. Big big difference between the two.
Erotic is when you use a feather. Exotic is when you use the whole chicken.
Except that solar sails do not depend on "solar wind", ie. particles. The main thrust (when above a certain distance form the sun at least) is delivered by massless photons, ie. light. Hence the more correct term "light sail".
I choose to remain celibate, like my father and his father before him.
That's correct. That's the inertial mass of the photon; the photon has gravity proportional to that mass. It doesn't change when the speed of light is "slowed down", either. What happens there is complicated, as the mass is temporarily transferred to the electrons of the atoms and then re-emitted later in an identical form. But the energy is neither lost nor gained anywhere in the process.