The difference is that there are NOT many different studies confirming ESP happens. In fact, there are many studies arguing the contrary (particularly if you focus on studies from reputable sources). There are plenty of people who WANT ESP to be true, but I don't think there are many who have been convinced by the evidence.
One big take home point about dark matter and dark energy is that physicists didn't want them to be true! It took an enormous amount of evidence, with countless independent confirmations over decades to convince the community that they were real. Real evidence can do that - convince reasonable people who begin as non-believers.
As I understand it, luminosity is one major reason why this technology is not yet ready for prime time (i.e. not in time for the proposed ILC). You can't just accelerate a few particles to high energies and say you are done. You're looking for rare processes, so you need to create many consistent particle collisions per second in a tiny area. This means you need to have a tight, "bright" beam. The Tevatron has a luminosity of ~2e+32 interactions/cm^2/s now, the LHC may eventually reach 1e+34, and the goal for the ILC is more like 2e+34. Plasma wakefield systems are now demonstrating large increases in energy over short distances, but it's very difficult to daisy-chain them together to reach high total energies with any significant luminosity.
I'm a physicist as well, and I'd say that Wikipedia's science articles are generally quite good, though not always very pedagogical. I find that Wikipedia is among the best places to get an up-to-date introduction to (or at least the basic gist of) to some topic that I'm not fully familiar with, even a very technical one. I agree that far more work is needed to make Wikipedia's science articles as complete and pedagogical as they should be and that authors sometimes get a little too pedantic or sidetracked. Nonetheless extensive contributions from experts make it a surprisingly good starting point for real science. Again, in general - there are certainly plenty of exceptions.
It is law/business/medicine oriented in that it has famous and excellent schools in those fields. However it is also more science-oriented than most of the Ivy league. Harvard physics, for example, is substantially more well-regarded than, say, Yale physics.
It turns out that the axion can have a wide range of properties, depending on its mass and its coupling to ordinary matter. There are regions of parameter space in which the axion is heavy enough and strongly-coupled enough to decay rapidly. Professor Jain is claiming to have detected such a short-lived version of the axion (or, at least, some sort of short-lived neutral particle).
Most models for axions are much lighter and have much weaker interactions, giving them much longer lifespans. That's what's being described in the article you cite. An axion with those properties would be an ideal candidate for dark matter - tons of them would fill the universe, and they'd be nearly undetectable due to their weak interactions.
Most searches for axions focus on the longer-lived possibilities for this reason, so far with no success. I'm intrigued if this claim is true, but I'll wait to see what other physicists think.
First off, the problem dark energy tries to solve isn't the expansion of the universe, it's the acceleration of that expansion over time. This is not something the Lawler piece tries to address, so it's not relevant to this discussion.
In general, the Lawler piece looks like an exercise in numerology and formula hunting. In chapter 2, for example, he appears to use his composite photon model to explain the spectral lines of halogens, and suggests that this relationship is startling. But this is just the 1/r law of the electrostatic potential in action - it's not news, and has nothing to do with whether photons are composite. Each of these gases also has countless other spectral lines at many frequencies, from ultraviolet through radio waves. I can predict them all accurately to numerous decimal places using quantum mechanics - can his theory predict more than just the "easy" one?
Further, some of his computations are somewhat tautological. For example, in chapter 2 he also claims to derive the proton/electron mass ratio. The derives (actually sort of makes up) a formula relating this ratio to his photon scale factor, but of course this photon scale factor is just asserted at the top of the page - it was chosen to make things come out right.
A composite photon at the scale he describes would also be apparent in precision experiments at accelerators and in labs. If a photon has two charged objects in it, then it should be possible to affect its frequency by passing it between two charged plates.
That's true, and thanks for reminding us of it - too many people get the erroneous idea that Einstein predicted all of this 90 years ago.
Nonetheless, Einstein's cosmological constant is not just a fudge factor he introduced. The equations of general relativity are the most general equations you can write down consistent with certain principles (the equivalence of gravitational and inertial mass, among others). The main terms relate the curvature of space to the local matter/energy distribution, but there is one more term which is consistent with the principles and should be included - the cosmological constant. The constant may be zero, of course, but a priori it's something you need to understand. Einstein chose a particular value of the constant to produce a static universe - a blunder, since he could have realized that almost any value of the constant gives an expanding or contracting universe.
I couldn't disagree more. This work is eminently Nobel-worthy. In fact, we cosmologists have been expecting this Prize for a long time. I'd say that the COBE announcement in 1992 is widely seen as the beginning of modern "precision" cosmology.
I agree that in 1992 the Big Bang was extremely well established, if only because we already knew the CMB existed (it had been awarded the Prize in the 1960s, after all). Most physicists also expected that we would see anisotropies at the level of 1 part in 100,000 - that's why COBE was built, of course. So in that sense I agree that we saw things we already expected (but didn't know!) we'd see.
It wasn't clear, however, that we'd see coherent structure in the CMB on scales larger than 1 degree. Inflation predicts this, and many people liked the inflationary idea, but there were other models for structure formation (notably cosmic strings) which don't. The COBE spectrum basically proved that inflation was more than just speculation, which was astonishing. Overnight a world of theories died and a world of others were born. This was a monumental achievement!
Good question - sufficiently small objects would not produce observable microlensing events, and if there are enough of them we can still produce the same total mass. Conversely, if the individual particles are large enough and rare enough they might produce huge microlensing events but they would be so rare we might have missed them.
I think that the issue with small particles of ordinary matter (e.g. gas or dust) is that they should affect the passage of light through our galaxy. They should literally blot out the light of distant sources, or at least distort the spectrum by absorbing some of that light. We can measure the amount of gas (especially neutral gas) and dust by studying its effect on light and conclude that there probably can't be enough to account for all of the dark matter. I don't know the details of this argument, but I think that's the idea.
For sufficiently massive particles you run into clumpiness issues. It would distort the shape of the galaxy if there were a handful of huge masses drifting through it, rather than a fine mist of smaller particles.
There may be a couple of windows in allowed masses that are still possible, but enough of the reasonable parameter space has been ruled out that people aren't particularly convinced by this model anymore. And regardless there are still BBN and CMB arguments to tell us that most of the universe's matter is non-baryonic anyway.
OK, so this is an issue of nomenclature. When a physicist says "mass", he essentially always means "rest mass".
The distinction comes from the full version of Einstein's famous equation: E^2 = (p*c)^2 + (m*c^2)^2, where p is the momentum of the particle and m is its "rest mass". This means that a particle can get energy from two places: energy of motion (the momentum term) and an intrinsic minimal energy (the particle's rest mass). A massless particle (like a photon) still has energy, but its energy is just proportional to its momentum - it has no "intrinsic" energy, and E=p*c. A massive particle requires a certain minimum amount of energy just to exist, independent of its motion.
It's still true that we can interconvert energy and mass. A massive particle can decay into two massless photons, as long as the total amounts of energy and momentum are conserved. We just wouldn't use this relationship to say that a photon has mass - it has no rest mass, and this is an important distinction (particles with no rest mass travel at the speed of light!).
In general relativity, gravity depends on both energy and momentum. For non-relativistic massive particles the mass is by far the dominant term, but for relativistic particles the momentum is also an important contribution to the gravitational field equations.
That's an excellent question! You've just described the MACHO model of dark matter (Massive Compact Halo Objects). The idea is that there could be cold, compact objects made of ordinary matter filling our dark matter halo and giving us all the extra mass. In this theory, the invisible mass exists but is still "ordinary" - no modification to gravity and no fancy new particles. It contrasts with the more exotic WIMP model of dark matter - Weakly Interacting Massive Particles.
This theory was extremely popular for many years, but has fallen out of favor for two main reasons:
(1) Using studies of the cosmic background radiation and light element abundances, you can conclude that the bulk of the matter in the hot early universe was not made up of baryons ("ordinary matter"). If it were, you would expect very different abundances of deuterium in the universe today and a very different spectrum of fluctuations in the microwave background. So we need most of the universe's matter to be non-baryonic anyway (e.g. WIMPs), and baryonic MACHOs cannot make up all of the missing mass
(2) Every now and then a MACHO should pass in front of a distant star (say, in the Large Magellanic Cloud), producing a "micro-lensing" event. Many collaborations around the world studied the skies for years looking for such events, and did find a few. The number and kind of lensing events they observed, however, was insufficient to account for all of the missing mass.
For these and other reasons, the cosmological community has rejected the MACHO hypothesis. There are objects like that out there, but the bulk of the dark matter must be something else.
Yes, they have. We know the temperature of the background radiation filling the universe, and so we know the energy density this represents. It turns out to be completely negligible at the present day, a few orders of magnitude less than the matter density.
However, I've always heard/read that the reason gravity "attracts" non-massive objects (eg photons), is because gravity is bending space itself.
i.e. You can say that a photon is traveling in a "curved path" because it is "attracted" or "pulled" to a near-by massive object.
But isn't it more correct (in terms of general relativity) to say that the photon is traveling in a straight line and it's space that's "curved"?
Agreed - that's exactly how general relativity works. Particles (including photons) follow the "straightest" lines on curved spacetime - the geodesics. The rest mass of a particle determines which sort of path it will follow - massless photons follow paths called "null geodesics", while massive particles follow "timelike geodesics". All forms of energy distort the space around them, which in turn affects how that energy moves through space. We call it all "gravity".
I completely agree - as a previous poster said, the extraordinary claim of invisible stuff requires extraordinary evidence. We definitely need to question our assumptions about gravity, since they are the foundation of our reasoning about dark matter.
The thing is, people HAVE been questioning those assumptions for decades. Even with a lot of fancy theoretical footwork, no one has yet managed to explain our observations without assuming that the bulk of the universe's mass is invisible (including the work described in this article!). It's not like everyone has gotten brainwashed by the dark matter gospel and it never occurred to any of them to question gravity. EVERYONE has thought of questioning gravity! EVERYONE has a revulsion for the idea of invisible matter dominating our galaxies. They just haven't had much success in the ultimate test of science - explaining observations.
- Gravitational lensing and rotation curves agree that a galaxy (or cluster) has much more matter than can be accounted for by visible baryons (or even less-visible hot gas), and that the distribution of that matter is much larger than the visible structure indicates. - Studies of big bang nucleosynthesis and the cosmic microwave background agree that the vast majority of the universe's gravitating matter is non-baryonic. - The Bullet cluster shows a situation where the dark matter and baryonic matter are segregated from one another, in a way that makes perfect sense with dark matter and stymies MOND-only theories.
Any one of these observations can be explained by modifications to gravity, but it turns out to be very hard to make them ALL work out. I obviously can't say it's impossible, and maybe someday someone will come along and show how it all works. But right now the SIMPLEST theory which fits the facts extraordinarily well says that the bulk of the universe's matter is not visible and interacts weakly (if at all) with ordinary matter.
At a certain point you get so beaten over the head with evidence that you have to (at least tentatively) accept something that sounds crazy at first. Common sense isn't always right...
Amusing, since I still look for dark matter in a mine:) People are flowing into the field in general, though I think they may be flowing out of it in Britain due to various difficulties there. They are difficult experiments and the reward may be long in the future (if at all).
There is enough evidence now that I would call dark matter the simplest explanation for observed phenomena, not just an epicycle (though it will take a long while before we can answer that question for dark energy). I agree that we need critical experiments, but we've moved way beyond the idle speculation stage and into real quantitative work.
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...".
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.
I'm a physicist myself, so in fact I tried to list items with cosmological appeal, rather than mere cataloguing (which interests me less). Redshift surveys are the bread-and-butter of mapping the structure of our universe (and thus the cosmology that generated it), supernova surveys led to the discovery of the accelerating universe, and variable stars are the bedrock of the interlocking distance measurements that allow us to determine large distances in the universe. My own work interests tend more toward the particle end of things, but the results that come out of this sort of work should be very interesting to anyone with a taste for cosmology.
I guess my confusion is that I can't think of a telescope that would be much more "multi-purpose" than this one, unless you built a similar apparatus in space. The focus would certainly be on lensing at first, but once this sort of data set is sitting around people will do all sorts of analysis on it.
I'm not sure I'd call the study of the origin and structure of the entire universe "narrow", but be that as it may... The data set that will come out of this instrument (if it's ever built) will be on an entirely different scale than anything astronomers have had to deal with. There are lots of things that can be done with such an instrument - lensing surveys, redshift surveys, variable stars, supernova searches... Pretty much anything requiring a wide search where you don't know the exact locations of the interesting bits.
The Hubble (for example) will always be better if you want to look at a specific spot very closely, but a high resolution survey of the entire Southern sky every few nights is hardly of limited interest! My only concern is that it's too much - a few days of data could keep people busy for a very long time!
It's true that being educated does not necessarily make one a good researcher, nor does being uneducated mean one can't have good ideas. I'm not someone who would say that the current system is perfect or that the right people always get opportunities - it's not and they don't. I think the wiki is a great idea and I wish it luck, but I worry that in practice it will get bogged down and neglected.
Ramanujan was a genius who did not have the opportunity for an advanced education. There may be people like that, but it's not so clear that they will (1) work on math problems (most people don't have the time to devote to such things) or (2) have extensive access to the internet and the wiki.
I expect that this wiki will be mostly filled with postings from people who have both time and a good internet connection: people in the industrialized nations, not Ramanujans. My feeling is that the vast bulk of the postings by amateurs will be honest attempts to get up to speed or crackpot theories. Experts will attempt to describe things to the newbies and respond to the crackpots, but they'll eventually get tired. Crackpots have astonishing amounts of time to promote their views and an incredible resistance to seeing their errors.
The site is unlikely to be able to discover the next Ramanujan because (as other posters have pointed out) the signal-to-noise in the entries is likely to be low enough that experts will stop reading it in detail. It may, however, turn out to be a great resource for understandable descriptions of current research on these problems.
... that Einstein had advanced training in physics. He was working as a patent clerk because professorships were hard to come by.
I often wonder if the "Einstein was a patent clerk who had difficulty with math" mythos has empowered far too many crackpots who don't understand the problems they write about.
A massless particle (like the photon) should move at exactly the speed of light, while a massive particle should always move slower than light. We always used to say that neutrinos move at the speed of light because we assumed they had no mass. Now that we know they are massive, they must be moving slower. They are so incredibly light, however, that we expect them to be moving extremely close to that speed - it takes very little force to accelerate them, so anything energetic enough to make them would make them go very fast.
If photons (quanta of light) had mass, the world around us would be very different. Photons mediate the electromagnetic force, which is responsible for light, the pull of magnets, the fact that electrons stay in their orbits, etc. If the photon were massive this force would become short-range - its strength would decay exponentially with distance (like the weak nuclear force), rather than as an inverse-square law. We have done ridiculously precise tests of the inverse-square law, which translates into very tight constraints on photon mass.
Dark matter (mass we can't see) has several components: ordinary (protons, neutrons, electrons) matter we happen to be unable to see, exotic matter that we do understand, and exotic matter that we don't understand. You could go into a Rumsfeld-esque discussion of "known unknowns" and "unknown unknowns" at this point.
When people talk about dark matter, they usually mean the exotic stuff, since there is a lot of evidence that the bulk of the universe's matter is exotic (look up "big bang nucleosynthesis" for details).
Neutrinos make up some of the exotic stuff, and how much depends on their mass. It turns out that they can't make up nearly enough of it, however. Furthermore, neutrinos are light particles which move at speeds near that of light. This means they don't clump together under their own gravity very easily, and tend to disrupt the formation of galaxy clusters. From looking at the distribution of galaxies in the universe, we can argue that most of the exotic dark matter must be slow-moving and "clumpable". The bulk of what people mean by dark matter is this stuff, which can't be neutrinos.
I'm glad to see people excited about this result! Super-K and others had discovered neutrino masses first, but this was the most controlled experiment to date - they made the neutrinos, examined them when they left the accelerator, and examined them again 700 km away. Any modifications to the Standard Model are very exciting.
One thing I feel obligated to point out, however: this has nothing to do with string theory. String theory is a framework for thinking about how to unify the known Standard Model with general relativity. It's incredibly interesting, both from a physics point of view and as a purely mathematical construct. However, it has no prediction about neutrino mass, or indeed about anything remotely accessible to experiment (except, perhaps, that supersymmetry should be true at some level), and has little prospect of making such predictions anytime soon.
Many posters seem to jump to the conclusion that if something is new in physics (whether it be neutrino mass or supersolids) then it MUST somehow be confirming string theory. String theory is very pretty and I hope it's true, but not everything in physics points back to it.
Sorry for the physicists' rant, no offense intended.
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The difference is that there are NOT many different studies confirming ESP happens. In fact, there are many studies arguing the contrary (particularly if you focus on studies from reputable sources). There are plenty of people who WANT ESP to be true, but I don't think there are many who have been convinced by the evidence.
One big take home point about dark matter and dark energy is that physicists didn't want them to be true! It took an enormous amount of evidence, with countless independent confirmations over decades to convince the community that they were real. Real evidence can do that - convince reasonable people who begin as non-believers.
As I understand it, luminosity is one major reason why this technology is not yet ready for prime time (i.e. not in time for the proposed ILC). You can't just accelerate a few particles to high energies and say you are done. You're looking for rare processes, so you need to create many consistent particle collisions per second in a tiny area. This means you need to have a tight, "bright" beam. The Tevatron has a luminosity of ~2e+32 interactions/cm^2/s now, the LHC may eventually reach 1e+34, and the goal for the ILC is more like 2e+34. Plasma wakefield systems are now demonstrating large increases in energy over short distances, but it's very difficult to daisy-chain them together to reach high total energies with any significant luminosity.
I'm a physicist as well, and I'd say that Wikipedia's science articles are generally quite good, though not always very pedagogical. I find that Wikipedia is among the best places to get an up-to-date introduction to (or at least the basic gist of) to some topic that I'm not fully familiar with, even a very technical one. I agree that far more work is needed to make Wikipedia's science articles as complete and pedagogical as they should be and that authors sometimes get a little too pedantic or sidetracked. Nonetheless extensive contributions from experts make it a surprisingly good starting point for real science. Again, in general - there are certainly plenty of exceptions.
It is law/business/medicine oriented in that it has famous and excellent schools in those fields. However it is also more science-oriented than most of the Ivy league. Harvard physics, for example, is substantially more well-regarded than, say, Yale physics.
It turns out that the axion can have a wide range of properties, depending on its mass and its coupling to ordinary matter. There are regions of parameter space in which the axion is heavy enough and strongly-coupled enough to decay rapidly. Professor Jain is claiming to have detected such a short-lived version of the axion (or, at least, some sort of short-lived neutral particle).
Most models for axions are much lighter and have much weaker interactions, giving them much longer lifespans. That's what's being described in the article you cite. An axion with those properties would be an ideal candidate for dark matter - tons of them would fill the universe, and they'd be nearly undetectable due to their weak interactions.
Most searches for axions focus on the longer-lived possibilities for this reason, so far with no success. I'm intrigued if this claim is true, but I'll wait to see what other physicists think.
First off, the problem dark energy tries to solve isn't the expansion of the universe, it's the acceleration of that expansion over time. This is not something the Lawler piece tries to address, so it's not relevant to this discussion.
In general, the Lawler piece looks like an exercise in numerology and formula hunting. In chapter 2, for example, he appears to use his composite photon model to explain the spectral lines of halogens, and suggests that this relationship is startling. But this is just the 1/r law of the electrostatic potential in action - it's not news, and has nothing to do with whether photons are composite. Each of these gases also has countless other spectral lines at many frequencies, from ultraviolet through radio waves. I can predict them all accurately to numerous decimal places using quantum mechanics - can his theory predict more than just the "easy" one?
Further, some of his computations are somewhat tautological. For example, in chapter 2 he also claims to derive the proton/electron mass ratio. The derives (actually sort of makes up) a formula relating this ratio to his photon scale factor, but of course this photon scale factor is just asserted at the top of the page - it was chosen to make things come out right.
A composite photon at the scale he describes would also be apparent in precision experiments at accelerators and in labs. If a photon has two charged objects in it, then it should be possible to affect its frequency by passing it between two charged plates.
That's true, and thanks for reminding us of it - too many people get the erroneous idea that Einstein predicted all of this 90 years ago.
Nonetheless, Einstein's cosmological constant is not just a fudge factor he introduced. The equations of general relativity are the most general equations you can write down consistent with certain principles (the equivalence of gravitational and inertial mass, among others). The main terms relate the curvature of space to the local matter/energy distribution, but there is one more term which is consistent with the principles and should be included - the cosmological constant. The constant may be zero, of course, but a priori it's something you need to understand. Einstein chose a particular value of the constant to produce a static universe - a blunder, since he could have realized that almost any value of the constant gives an expanding or contracting universe.
I couldn't disagree more. This work is eminently Nobel-worthy. In fact, we cosmologists have been expecting this Prize for a long time. I'd say that the COBE announcement in 1992 is widely seen as the beginning of modern "precision" cosmology.
I agree that in 1992 the Big Bang was extremely well established, if only because we already knew the CMB existed (it had been awarded the Prize in the 1960s, after all). Most physicists also expected that we would see anisotropies at the level of 1 part in 100,000 - that's why COBE was built, of course. So in that sense I agree that we saw things we already expected (but didn't know!) we'd see.
It wasn't clear, however, that we'd see coherent structure in the CMB on scales larger than 1 degree. Inflation predicts this, and many people liked the inflationary idea, but there were other models for structure formation (notably cosmic strings) which don't. The COBE spectrum basically proved that inflation was more than just speculation, which was astonishing. Overnight a world of theories died and a world of others were born. This was a monumental achievement!
Good question - sufficiently small objects would not produce observable microlensing events, and if there are enough of them we can still produce the same total mass. Conversely, if the individual particles are large enough and rare enough they might produce huge microlensing events but they would be so rare we might have missed them.
I think that the issue with small particles of ordinary matter (e.g. gas or dust) is that they should affect the passage of light through our galaxy. They should literally blot out the light of distant sources, or at least distort the spectrum by absorbing some of that light. We can measure the amount of gas (especially neutral gas) and dust by studying its effect on light and conclude that there probably can't be enough to account for all of the dark matter. I don't know the details of this argument, but I think that's the idea.
For sufficiently massive particles you run into clumpiness issues. It would distort the shape of the galaxy if there were a handful of huge masses drifting through it, rather than a fine mist of smaller particles.
There may be a couple of windows in allowed masses that are still possible, but enough of the reasonable parameter space has been ruled out that people aren't particularly convinced by this model anymore. And regardless there are still BBN and CMB arguments to tell us that most of the universe's matter is non-baryonic anyway.
OK, so this is an issue of nomenclature. When a physicist says "mass", he essentially always means "rest mass".
The distinction comes from the full version of Einstein's famous equation: E^2 = (p*c)^2 + (m*c^2)^2, where p is the momentum of the particle and m is its "rest mass". This means that a particle can get energy from two places: energy of motion (the momentum term) and an intrinsic minimal energy (the particle's rest mass). A massless particle (like a photon) still has energy, but its energy is just proportional to its momentum - it has no "intrinsic" energy, and E=p*c. A massive particle requires a certain minimum amount of energy just to exist, independent of its motion.
It's still true that we can interconvert energy and mass. A massive particle can decay into two massless photons, as long as the total amounts of energy and momentum are conserved. We just wouldn't use this relationship to say that a photon has mass - it has no rest mass, and this is an important distinction (particles with no rest mass travel at the speed of light!).
In general relativity, gravity depends on both energy and momentum. For non-relativistic massive particles the mass is by far the dominant term, but for relativistic particles the momentum is also an important contribution to the gravitational field equations.
That's an excellent question! You've just described the MACHO model of dark matter (Massive Compact Halo Objects). The idea is that there could be cold, compact objects made of ordinary matter filling our dark matter halo and giving us all the extra mass. In this theory, the invisible mass exists but is still "ordinary" - no modification to gravity and no fancy new particles. It contrasts with the more exotic WIMP model of dark matter - Weakly Interacting Massive Particles.
This theory was extremely popular for many years, but has fallen out of favor for two main reasons:
(1) Using studies of the cosmic background radiation and light element abundances, you can conclude that the bulk of the matter in the hot early universe was not made up of baryons ("ordinary matter"). If it were, you would expect very different abundances of deuterium in the universe today and a very different spectrum of fluctuations in the microwave background. So we need most of the universe's matter to be non-baryonic anyway (e.g. WIMPs), and baryonic MACHOs cannot make up all of the missing mass
(2) Every now and then a MACHO should pass in front of a distant star (say, in the Large Magellanic Cloud), producing a "micro-lensing" event. Many collaborations around the world studied the skies for years looking for such events, and did find a few. The number and kind of lensing events they observed, however, was insufficient to account for all of the missing mass.
For these and other reasons, the cosmological community has rejected the MACHO hypothesis. There are objects like that out there, but the bulk of the dark matter must be something else.
Yes, they have. We know the temperature of the background radiation filling the universe, and so we know the energy density this represents. It turns out to be completely negligible at the present day, a few orders of magnitude less than the matter density.
Agreed - that's exactly how general relativity works. Particles (including photons) follow the "straightest" lines on curved spacetime - the geodesics. The rest mass of a particle determines which sort of path it will follow - massless photons follow paths called "null geodesics", while massive particles follow "timelike geodesics". All forms of energy distort the space around them, which in turn affects how that energy moves through space. We call it all "gravity".
I completely agree - as a previous poster said, the extraordinary claim of invisible stuff requires extraordinary evidence. We definitely need to question our assumptions about gravity, since they are the foundation of our reasoning about dark matter.
The thing is, people HAVE been questioning those assumptions for decades. Even with a lot of fancy theoretical footwork, no one has yet managed to explain our observations without assuming that the bulk of the universe's mass is invisible (including the work described in this article!). It's not like everyone has gotten brainwashed by the dark matter gospel and it never occurred to any of them to question gravity. EVERYONE has thought of questioning gravity! EVERYONE has a revulsion for the idea of invisible matter dominating our galaxies. They just haven't had much success in the ultimate test of science - explaining observations.
- Gravitational lensing and rotation curves agree that a galaxy (or cluster) has much more matter than can be accounted for by visible baryons (or even less-visible hot gas), and that the distribution of that matter is much larger than the visible structure indicates.
- Studies of big bang nucleosynthesis and the cosmic microwave background agree that the vast majority of the universe's gravitating matter is non-baryonic.
- The Bullet cluster shows a situation where the dark matter and baryonic matter are segregated from one another, in a way that makes perfect sense with dark matter and stymies MOND-only theories.
Any one of these observations can be explained by modifications to gravity, but it turns out to be very hard to make them ALL work out. I obviously can't say it's impossible, and maybe someday someone will come along and show how it all works. But right now the SIMPLEST theory which fits the facts extraordinarily well says that the bulk of the universe's matter is not visible and interacts weakly (if at all) with ordinary matter.
At a certain point you get so beaten over the head with evidence that you have to (at least tentatively) accept something that sounds crazy at first. Common sense isn't always right...
Amusing, since I still look for dark matter in a mine :) People are flowing into the field in general, though I think they may be flowing out of it in Britain due to various difficulties there. They are difficult experiments and the reward may be long in the future (if at all).
There is enough evidence now that I would call dark matter the simplest explanation for observed phenomena, not just an epicycle (though it will take a long while before we can answer that question for dark energy). I agree that we need critical experiments, but we've moved way beyond the idle speculation stage and into real quantitative work.
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...".
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.
I'm a physicist myself, so in fact I tried to list items with cosmological appeal, rather than mere cataloguing (which interests me less). Redshift surveys are the bread-and-butter of mapping the structure of our universe (and thus the cosmology that generated it), supernova surveys led to the discovery of the accelerating universe, and variable stars are the bedrock of the interlocking distance measurements that allow us to determine large distances in the universe. My own work interests tend more toward the particle end of things, but the results that come out of this sort of work should be very interesting to anyone with a taste for cosmology.
I guess my confusion is that I can't think of a telescope that would be much more "multi-purpose" than this one, unless you built a similar apparatus in space. The focus would certainly be on lensing at first, but once this sort of data set is sitting around people will do all sorts of analysis on it.
I'm not sure I'd call the study of the origin and structure of the entire universe "narrow", but be that as it may... The data set that will come out of this instrument (if it's ever built) will be on an entirely different scale than anything astronomers have had to deal with. There are lots of things that can be done with such an instrument - lensing surveys, redshift surveys, variable stars, supernova searches... Pretty much anything requiring a wide search where you don't know the exact locations of the interesting bits.
The Hubble (for example) will always be better if you want to look at a specific spot very closely, but a high resolution survey of the entire Southern sky every few nights is hardly of limited interest! My only concern is that it's too much - a few days of data could keep people busy for a very long time!
It's true that being educated does not necessarily make one a good researcher, nor does being uneducated mean one can't have good ideas. I'm not someone who would say that the current system is perfect or that the right people always get opportunities - it's not and they don't. I think the wiki is a great idea and I wish it luck, but I worry that in practice it will get bogged down and neglected. Ramanujan was a genius who did not have the opportunity for an advanced education. There may be people like that, but it's not so clear that they will (1) work on math problems (most people don't have the time to devote to such things) or (2) have extensive access to the internet and the wiki. I expect that this wiki will be mostly filled with postings from people who have both time and a good internet connection: people in the industrialized nations, not Ramanujans. My feeling is that the vast bulk of the postings by amateurs will be honest attempts to get up to speed or crackpot theories. Experts will attempt to describe things to the newbies and respond to the crackpots, but they'll eventually get tired. Crackpots have astonishing amounts of time to promote their views and an incredible resistance to seeing their errors. The site is unlikely to be able to discover the next Ramanujan because (as other posters have pointed out) the signal-to-noise in the entries is likely to be low enough that experts will stop reading it in detail. It may, however, turn out to be a great resource for understandable descriptions of current research on these problems.
... that Einstein had advanced training in physics. He was working as a patent clerk because professorships were hard to come by.
I often wonder if the "Einstein was a patent clerk who had difficulty with math" mythos has empowered far too many crackpots who don't understand the problems they write about.
A massless particle (like the photon) should move at exactly the speed of light, while a massive particle should always move slower than light. We always used to say that neutrinos move at the speed of light because we assumed they had no mass. Now that we know they are massive, they must be moving slower. They are so incredibly light, however, that we expect them to be moving extremely close to that speed - it takes very little force to accelerate them, so anything energetic enough to make them would make them go very fast.
If photons (quanta of light) had mass, the world around us would be very different. Photons mediate the electromagnetic force, which is responsible for light, the pull of magnets, the fact that electrons stay in their orbits, etc. If the photon were massive this force would become short-range - its strength would decay exponentially with distance (like the weak nuclear force), rather than as an inverse-square law. We have done ridiculously precise tests of the inverse-square law, which translates into very tight constraints on photon mass.
See this posting
Dark matter (mass we can't see) has several components: ordinary (protons, neutrons, electrons) matter we happen to be unable to see, exotic matter that we do understand, and exotic matter that we don't understand. You could go into a Rumsfeld-esque discussion of "known unknowns" and "unknown unknowns" at this point.
When people talk about dark matter, they usually mean the exotic stuff, since there is a lot of evidence that the bulk of the universe's matter is exotic (look up "big bang nucleosynthesis" for details).
Neutrinos make up some of the exotic stuff, and how much depends on their mass. It turns out that they can't make up nearly enough of it, however. Furthermore, neutrinos are light particles which move at speeds near that of light. This means they don't clump together under their own gravity very easily, and tend to disrupt the formation of galaxy clusters. From looking at the distribution of galaxies in the universe, we can argue that most of the exotic dark matter must be slow-moving and "clumpable". The bulk of what people mean by dark matter is this stuff, which can't be neutrinos.
I'm glad to see people excited about this result! Super-K and others had discovered neutrino masses first, but this was the most controlled experiment to date - they made the neutrinos, examined them when they left the accelerator, and examined them again 700 km away. Any modifications to the Standard Model are very exciting.
One thing I feel obligated to point out, however: this has nothing to do with string theory. String theory is a framework for thinking about how to unify the known Standard Model with general relativity. It's incredibly interesting, both from a physics point of view and as a purely mathematical construct. However, it has no prediction about neutrino mass, or indeed about anything remotely accessible to experiment (except, perhaps, that supersymmetry should be true at some level), and has little prospect of making such predictions anytime soon.
Many posters seem to jump to the conclusion that if something is new in physics (whether it be neutrino mass or supersolids) then it MUST somehow be confirming string theory. String theory is very pretty and I hope it's true, but not everything in physics points back to it.
Sorry for the physicists' rant, no offense intended.