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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."
is too!
is not!
is too!
...
Great minds think alike; fools seldom differ.
Dark Matter is the 21st century's ether.
P226
Dark matter has to be 'cold' - slow moving. If it goes too fast, like photons (or neutrinos for that matter) then structure can't form as it all gets washed out. It needs to be slow moving enough to clump. So even if photons did have a small mass that wouldn't work.
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.
Well, minor nitpick: I don't see why neutrinos would interact with other particles just because they had some small amount of mass. It's not as if they would produce more than a truly tiny amount of gravity, and they obviously don't interact electromagnetically with anything.
I have seen the future, and it is inconvenient.
I would have replied to all the inconsistencies... then I noticed the name. Not a bad troll.
Ignore this signature. By order.
"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.
For me, dark matter is like religion. Made up to explain what we can't understand, and wrong.
So..... you have no idea of physics, haven't you?
They do interact with matter, but because they're very small they simply pass straight through matter because the distance between particles in atoms / atoms are so great and only actually collide very rarely.
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
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Perhaps the clumpyness comes from standing waves, or if it is directly related to the baryonic matter.
Of course this assumes that the baryonic matter formed first. I also think that this kind of model would imply that a star would produce a constant stream of photons that 'weigh' 10 times its own mass. There is some conservation of energy problems there!
I don't think that photons could naturaly clump under their own mass as I imagine very strange physics would result.
Thats electron Volts right? I don't get it. How does one measure weight with volts? Or does it mean it weighs as much as two electrons, in which case why not just say that?
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.)
Moreover recent experiments have shown they do have mass (see Wikipedia).
They know their mass is >= 0 and 2.2 eV, hence why the proposed 2eV is possible.
the only time you can view photons is if they are traveling tward you. as far as the grouping of dark matter photons are probably more or less evenly distributed on a macro scale and uneven on a micro scale(remember scale is relative to the entire universe)
Technology will default in society to its most rudimentary level:::stupid computers for stupid users:::
Wait, there might be a flaw in the logic here somewhere.
Don't blame me; I'm never given mod points.
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.
(of the electron variety anyway)
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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 believe it's saying that when this experiment takes place in 2009, this data will be tested.
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.)
Neutrinos do have mass, that has been experimentally proven within experimental error. It was the first example my lecturer gave in Physics 101 of experimental uncertainty. We're just not sure how much mass.
I'm skeptical, but it would tidy things up nicely if it turned out right.
// MD_Update(&m,buf,j);
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.
Photons have mass only because they have energy and e=mc2. What the GP is referring to, is rest mass i.e. if a photon were to be at rest and therefore had no kinetic energy (ignoring for the moment that a photon cannot exist at rest) it would then have no mass.
Are you saying that Dark Matter is why we have fire?
Slashdot Burying Stories About Slashdot Media Owned
It sounds like they are going for a MMOND theory, modified modified newtonian dynamics. They admit the presence of dark matter. In fact, they reference another experiment as well as the bullet cluster observations that seems to show that MOND can't account for everything. So to save MOND, they are saying it doesn't have to account for everything. Massive neutrinos migth account for the rest.
- None can love freedom heartily, but good men; the rest love not freedom, but license. -- John Milton
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
From reading the summary, this is my understanding of what these guys are saying, too. Call it modified MOND.
- None can love freedom heartily, but good men; the rest love not freedom, but license. -- John Milton
There's a some evidence (see? gravity! DARK MATTER!) but it's theoretical,
IANAP (I am not a physicist), but isn't the The Great Attractor more than theoretical proof of something dark that is attracting all those galaxies? Also, why do stars on the outer rim of galaxies revolve so fast around the core? Spiral galaxies do not seem to spin around their core like expected.
Dismissing dark matter as purely theoretical is shortsighted IMO.
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.
As has been explained before, and will doubtless be explained again, ID is not a scientific theory, and does not hold the same value as the scientific theory of the existence of dark matter.
If dark matter exists (ie, the theory), it can explain certain observed phenomena. However, where it differs from ID is that the theory can be used to make falsifiable predictions about things we have not yet measured. Using the theory as a base point, we can predict what will happen in certain regions of space if there are WIMPs there. We can predict how the universe is going to expand or contract. If any of these predictions are wrong, then the theory that there is dark matter is also wrong.
On the other hand, intelligent design does not offer any falsifiable predictions. There is no way to test the "theory" (in this case, non-scientific) to determine if it is false. If you can devise a test that would prove ID to be false, then that would elevate it to the status of a scientific theory. It still wouldn't be a useful theory, however, as it does not offer any predictions as to how unobserved phenomena will react when we do get a chance to observe them.
Dark matter is there because the math behind it explains the current set of observations, and because there is a way to prove that the current set of observations is not due to dark matter. Intelligent design is there because Christians are upset about the removal of their god from the science classroom.
It may look like I'm doing nothing, but I'm actively waiting for my problems to go away.
--Scott Adams
And photons that are 'clumped', would thus have no mass.
On the other hand, query: Do high-energy (ie: high mass) photons have a gravitational effect? Or do the formulae only work given a rest mass?
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KATRIN experiment homepage URL
http://www-ik.fzk.de/~katrin/
Even if dark matter exists, there is no reason why it all would add up to (nearly) zero.
2 0Gravity would stand a chance.
For example, if dark matter exists, what if there is just a little more of it than expected? Then the theory of "pushing gravity" http://www.everything2.com/index.pl?node=Pushing%
Hey don't blame me, IANAB
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.
Look at the sun. Look at how bright it is.
It isn't difficult to look up at all the brightness from the sun and think that maybe there is something there that we cannot see.
That thing in the sun that we cannot see is the planet Vulcan (not to be confused with the planet from Star Trek.) It is clear that this planet is the reason why the orbit of Mercury is not what we would expect using Newtonian physics. All we need to do is find that planet and newton's laws will be validated
But now this Einstein guy would have us believe that we should change Newton's law of gravity just because he couldn't grasp that there is a strange planet out there that exists just outside our ability to measure it.
My point is that any theory may seem insane until it is validated using physical measurements.
Those who push a theory that is eventually proven wrong are doing an important job. They are helping the theory that will be proven right to become that much stronger. So no matter which side is eventually proven right we really need to thank both sides!
Is there dark matter or do we need to change our ideas about the laws of gravity? I for one am going to wait until all the measurements are in before I make up my mind!
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.
Photon's are counter intuitive, picture it as having mass only when it's at rest, but none when it's moving. On the same note energy, in large enough quantities can warp spacetime, this could only happen in concentrated regions. Dark Matter can easily be defined as the particle alot of scientists are looking to as an explanation, the Axion. The Axion is defined as a weakly interacting, low mass particle, almost invisible to normal matter. So something that fits the bill, such as a neutrino with a low mass, could also explain what dark matter is. Although IMHO a neutrino's tend to move to fast (~C) and do not concentrate around galaxies, as far as can be detected. It doesn't appear that these people are factoring Neutrino Oscillation either, but i could be wrong.
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.
Only if your black magic comes from the dark side of the Force.
(Got a female illusionist of African descent?)
Can we get a -1, Laypersonspeakify mod?
Edward@Tomato - /home/Edward/ man woman
man: no entry for woman in the manual.
"Qua!?"
"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.
So...when people slow down light, does it gain mass?
I drank what? -- Socrates
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.
I don't think there's anything approaching reasonable in that statement. This is a theory. It is either a good theory or a bad theory on its own merits, and you don't introduce a theory because you feel that no other competing theory could be correct, you introduce a theory because you can demonstrate that it could be correct.
Science is the process of breaking existing theory, and there is nothing wrong with attacking the existing model for gravitation. In fact, attacking existing theory is one of the most important tasks in the scientific method. How correct this theory turns out to be is still anyone's guess, and our speculation without either mathematical proof or experimentation is moot.
Gravity bends spacetime so mass isn't required to affect light's path around a gravity source.
The gravitational effect is always due to relativistic mass, not rest mass. You can think of it as the total energy. So, yes, photons have a gravitational effect based on their total energy.
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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).
Sad and ignorant. Don't you realize the grasp of how our world works defines our status of accomplishment as a species? In 1,000 years what will be more important to the race?
Nevermind, I shouln't have bothered. It IS an obvious troll.
--
Northern Virginia? Fairfax Underground!
That is a failed analogy.
Mostly becuase chainlink fences do not show a discernable gravity force on dust clouds...
Neither did the Dark Matter feel compelled to be effected by gravity...
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.
Hear about those scientists doing research into this electricity thing?
Thanks God it's not my taxes that are spent on this research.
I'd rather have my artist friends make another painting using gas light. Not like this electricity thing will ever lead to anything useful!
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.
IANAP, just a computer engineer.
But I was wondering, could it be that dark matter is just invisible to current methods of detection? Perhaps the energy it radiates is in frequencies above what our current technology can detect? Like...exohertz or something?
I also recall something from some of my signals classes that there is such a phenomenon as negative frequency. Maybe dark matter emits energy in negative frequencies?
I pulled all of the above straight out of my ass, but the point seems valid enough. Obviously we can't detect all electromagnetic phenomena at every frequency range.
:(){
Don't worry, your errors are ABSOLUTELY TRIVIAL compared to Mr. Grandparent Poster -_-.
"May the days be aimless. Let the seasons drift. Do not advance the action according to a plan."
True. I'm ignorant. I do not watch "Dr. Who" and "Lexx". Well, I do not have TV at all.
We have wars. We have starving people. We have sicknesses with no cure.
But all news channels are flooded with what? Right. Is Pluto a planet or not? Does black/white/red/whatever matter exists?
Who's ignorant? Me wonders.
All hope abandon ye who enter here.
I am glad somebody else recognizes this problem with the grandparent post!! Seriously, I am losing faith in Slashdot with this and a few of its children being modded Insightful. *Cries*.
"May the days be aimless. Let the seasons drift. Do not advance the action according to a plan."
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.
> logging off /.
The networks existed before Internet. Learn facts. And Internet at large is privately owned - as communication channels concerned. As well as standards used to back it are collective effort - not affiliated with any gov't. (Though it's true that DARPA sponsored protocol is the Internet Protocol)
> getting rid of your computer
Computers and algorithms were invented about 150 years ago - when science was still considered mostly private matter and not supported by gov'ts.
Also transistor was invented again by non-gov't related companies. Check here - http://www.pbs.org/transistor/album1/index.html
> X-rays
X-rays? Check your facts - http://en.wikipedia.org/wiki/X-ray - before posting.
> applications were not immediately obvious
Well, yes. I do not understand how existence of black/dark/light/whatever mater would help anyone. As well as all the theories about Universe origin: in the beginning there was nothing and then it exploded. I really do not understand.
P.S. What was first - nuclear reactor or nuclear bomb? Bombs were first - and sponsored by government.
All hope abandon ye who enter here.
Read on. Then post. Show me were the gov't was involved there. (*)
I have seen many researchers using state funds. And honestly all there were capable - research-wise - find a ways to get another trunk of money.
On other side, I have worked a lot with research institution sponsored by Intel: they do not keep there people who do only theorize. If you can improve something - that's achievement.
When you waste bandwidth e.g. arguing Pluto is planet/not planet (underline correct answer) - to me that what it is - waste of time and resources.
(*) Reminded me of Ronald Reagan saying: "When something starts moving - tax it. If it still moves after that - regulate it. If it starts dying - fund it."
All hope abandon ye who enter here.
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.
Alternative gravity? Alternative energy? What else are we running out of?
Don't blame me -- I voted for Roslin.
The comment: "In short, dark matter is still not a necessity, provided that neutrinos weigh 2eV." is contradicted by this statement in the abstract of the paper: "In agreement with Clowe et al. (2006) we show that a dominant component of non-baryonic matter is needed in the bullet cluster - in MOND as well as in GR."
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.
One will thus have to wait for particle physics experiments to rule out massive neutrinos to rule out MOND. Until then, place your bets...
Actually, my understanding is that we need only wait until experiments rule out 2 eV neutrinos - they can still be massive.
Photons do have mass, since they have energy. But they have zero rest mass. If you stop a photon, it has no remaining mass. When you accelerate something to the speed of light, its mass increases infinitely. So unless the photon starts with no mass at all, you'd never get to the speed of light.
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.
Look, GP was from BadAnalogyGuy. It was a bad analogy, OK? It's what the boy does. You might as well get annoyed at the Pope for being Catholic.
And the brethren went away edified.
I recall from baby quantum physics (so this could be (and probably is!) an oversimplication) that, while a photon of light truly does not have mass (violation of special relativity), it has an effective mass via the famous E=mc^2. Since a photon has an energy E=hv (planck's constant times it's frequency) or E=(h/2pi) omega, then a photon's "mass" is simply hv/c^2. hv = E = mc^2, so hv = mc^2, solving for m, you get hv/c^2
If it weren't for DARPA you would still have to worry about 30 different TCP-type protocols.
Please, for the good of Humanity, vote Obama.
Ether
by the Gang of Four
Trapped in heaven life style (locked in Lock Kesh)
Now looking out for pleasure (H-block torture)
It's at the end of the rainbow (White noise in)
The happy ever after (a white room)
Dirt behind the daydream
Dirt behind the daydream
The happy ever after
It's at the end of the rainbow
Dig at the root of the problem (Fly the flag on foreign soil)
It breaks your new dreams daily (H-block Lock Kesh)
Fathers contradictions (Censor six countries news)
And breaks your new dreams daily (each day more deaths)
Dirt behind the daydream
Dirt behind the daydream
The happy ever after
It's at the end of the rainbow
White noise in a white room
There may be oil under Rockall
The happy ever after
There may be oil under Rockall
It's corked up with the ether
RS
Shoes for Industry. Shoes for the Dead.
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.
Actually, the question is "does the GP really even know anything about relativity at all given that he can't even spell it?"
My blog. Good stuff (when I remember to update it). Read it.
The networks existed before Internet. Learn facts. And Internet at large is privately owned - as communication channels concerned. As well as standards used to back it are collective effort - not affiliated with any gov't. (Though it's true that DARPA sponsored protocol is the Internet Protocol)
... it wasn't until home internet access became widely available that any of them even tried to adapt by increasing access and lowering prices. And now they've almost all died a well-deserved death, with AOL marching steadily toward the grave shared by its forebears.
... until computers came along. Bernoulli's law? Great for explaining how birds fly, but not terribly practical until the invention of the internal combustion engine and this little thing called "the airplane." Quantum physics? Neat but useless stuff for the first fifty years or so ... Look, astrophysics doesn't exist in isolation; it's simply the study of how the universe works on the grand scale, and a great deal of our understanding of how the universe works at our scale stems quite directly from it.
Strictly speaking, of course, you're correct -- there were plenty of computer networks before anything that could reasonably be called "the internet" started to come into existence in the 1960's. But none of those networks became the internet; ARPAnet did. And more recently, we have perfectly good examples of what commercial networks for general use look like. Compuserve, GEnie, Prodigy, Delphi, AOL -- any of these ring a bell? Closed systems, not extensible by private users, refusing to communicate with each other, insanely expensive
Computers and algorithms were invented about 150 years ago - when science was still considered mostly private matter and not supported by gov'ts.
Babbage received a mix of private and government funding (the Crown has been funding science on a large scale a lot longer than the US government has -- actually, longer than the US government has been in existence.) And modern electronic computers were almost entirely developed, for the first critical decades of their existence, with government money.
> X-rays
X-rays? Check your facts - http://en.wikipedia.org/wiki/X-ray - before posting.
Nice one -- leave out four crucial words and completely change the meaning of what I wrote. I wrote "after the invention of X-rays," by which I meant newer imaging methods such as CT, MRI, and PET. Also, of course, modern genetic diagnostics, which depend almost entirely on research funded by the NIH and similar institutions.
Well, yes. I do not understand how existence of black/dark/light/whatever mater would help anyone. As well as all the theories about Universe origin: in the beginning there was nothing and then it exploded. I really do not understand.
You don't have to understand, and I don't have to understand, as long as someone, at some point down the line, somewhere, understands; we'll all benefit regardless. Linear algebra and numerical analysis? Mildly entertaining but utterly abstract branches of mathematics
P.S. What was first - nuclear reactor or nuclear bomb? Bombs were first - and sponsored by government.
No, Fermi's reactor came first. And IIRC, the Manhattan Project had to build several reactors (and thus lay the foundation for the nuclear power industry) in order to accomplish their primary mission.
In any case, this may not be an opinion with which you have much sympathy, but I'm inclined to say that on the whole, the existence of the Bomb is a good thing. Yes, it's a terrible weapon, the worst weapon ever made, in fact. But it is also the thing which kept the US and the USSR from fighting WW3 on a scale that would have dwarfed WW1 and WW2 combined, and which continues to hold the dreams of would-be Caesars and Napoleons and Hitlers in check. No one will ever try to conquer the world ag
The correlation between ignorance of statistics and using "correlation is not causation" as an argument is close to 1.
A great deal of teh darkness you see has nothing to do with dark matter -- it's light polution. In most Western countries, there are relatively few stars at night - say 200 - 500. But get out to a third world country where there is no light polution and you will see the entire sky lit up with 20,000-50,000 stars. Everyone knows that in between these stars is emptiness. There may be dark matter somewhere, but it certainly isn't between stars. Dark Matter would pull a galaxy appart. Therefore, if there is Dark Matter, it is speculated that it surrounds a galaxy.
Mike www.sharecube.com
Comment removed based on user account deletion
I'm no physicist, but I like to think my understanding of GR is solid for an amateur.
In my understanding, GR operates on energy only, so rest mass is irrelevant except for its contribution to energy. The full energy-mass equation is E^2 = m_0^2*c^4 + p^2*c^2. Electrons, even those at rest, have m_0 (rest mass) and thus produce gravity (warp spacetime). Photons, though they have no rest mass, always have p (momentum) and thus produce gravity (warp spacetime) as well. In GR, any particle which produces gravity must have energy (rest mass, momentum, or both) and any particle with energy must produce gravity. In practice, it follows that all particles produce gravity, since energy-free particles at c would never interact with the universe in any way, and energy-free particles at less than c cannot exist (both because v=c for massless particles, and because energy is relative to the observer, and thus the "no energy" statement is only true for an observer matching velocity and acceleration with the particle).
As stated in your post, gravity warps spacetime and thus would affect even energy-free particles if they existed. However, it's purely an academic question: there's no way to measure that a particle with E=0 has or has not been deflected by gravity, if such a particle existed.
Range Voting: preference intensity matters
Zeroeth, this is definitional. Dark matter is defined as all the matter that we don't see photons reflecting off of nor do we see producing photons. Photons do not reflect other photons nor do they produce photons (except possibly at absurdly high energies or if QED is ridculously more nonlinear than all available data would suggest). So photons are dark matter. So are neutrinos and almost all other particles that mediate forces. Almost all "non-dark matter" is atoms/ions and free electrons.
First, I'd recommend reading the Dark Matter article http://en.wikipedia.org/wiki/Missing_mass.
Second, there's about one baryon per 5 cubic meters in the universe and the ratio of photons to baryons is about 10^10. The 3 K background radiation has a peak wavelength of ~1 mm, so an energy of 1.2 meV. The energy equivalence (E=mc^2) of a baryon is ~1 MeV, about 10^9 times as great as that of the photon. So the the 10^10 photons have about 10-times as much mass equivalence as do the baryons in the Universe. However, photons are "hot particles", moving at the speed of light. Hot particles move too fast to be trapped in pedestrian gravitational wells, like those of stars and galaxies. So, where we expect to find that baryons are clumped together with densities several orders of magnitude higher in galaxies and stars, the photons should be evenly distributed. Thus, the huge photon mass provides a uniform, flat background that cannot explain anomalous galactic rotation data. Similarly, there are no good stories about other massless or light particles (like neutrinos) providing the additional mass to make galactic rotation curves work.
So, ..., where's the follow-up paper estimating the temperatures of the baryonic and non-baryonic components as they "relax" out into the shallower gravitational wells left after being separated?
I am not sure that photons 'have' to have mass.
Everything in the universe that exists, has mass. Einstein discovered this. The gravitational field of the sun has mass, and this mass exerts its own, secondary gravitational field. Although this secondary gravitational field is small, it is large enough to have a detectable effect on the orbit of Mercury. When this effect was experimentally verified, it was a huge win for the theory of relativity.
The mass of a photon can be computed by taking the photon's energy, E, plugging it into E = MC**2, and solving for M.
It is true that photons do not have a "rest mass", as they can never be at rest, as they always travel at the speed of light. If we replace the terms "mass" and "instrinsic mass" in your posting by "rest mass", then your posting becomes correct.
Doug Moen
I have written a truly remarkable program which this sig is too small to contain.
Off topic, but yes, I would say so.
I wouldn't consider the mad hatter mad. Just reality impaired. He sure can make a mean cup of tea.
That's 1 *G*eV for a baryon. So baryons dominate over the photon energy density by a large factor and the answer to the original question is yes, it has been thought of, and no, it doesn't make any difference (in the present-day universe). The rest of your argument is pretty much correct, though.