This is an easier one than dark matter vs. modified gravity. As mentioned, I’m going to be on Science Friday today, and they asked me to contribute a guest blog post, which I’m cross-posting below. Old news, I’m sure, for longtime CV readers, but here you go.
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Probably the biggest single misconception I come across in popular discussions of dark matter and dark energy is the accusation that these concepts are a return to the discredited idea of the aether. They are not — in fact, they are precisely the opposite.
Back in the later years of the 19th century, physicists had put together an incredibly successful synthesis of electricity and magnetism, topped by the work of James Clerk Maxwell. They had managed to show that these two apparently distinct phenomena were different manifestations of a single underlying “electromagnetism.” One of Maxwell’s personal triumphs was to show that this new theory implied the existence of waves traveling at the speed of light — indeed, these waves are light, not to mention radio waves and X-rays and the rest of the electromagnetic radiation spectrum.
The puzzle was that waves were supposed to represent oscillations in some underlying substance, like water waves on an ocean. If light was an electromagnetic wave, what was “waving”? The proposed answer was the aether, sometimes called the “luminiferous aether” to distinguish it from the classical element. This idea had a direct implication: that Maxwell’s description of electromagnetism would be appropriate as long as we were at rest with respect to the aether, but that its predictions (for the speed of light, for example) would change as we moved through the aether. The hunt was to find experimental evidence for this idea, but attempts came up short. The Michelson-Morley experiment, in particular, implied that the speed of light did not change as the Earth moved through space, in apparent contradiction with the aether idea.
So the aether was a theoretical idea that never found experimental support. In 1905 Einstein pointed out how to preserve the symmetries of Maxwell’s equations without referring to aether at all, in the special theory of relativity, and the idea was relegated to the trash bin of scientific history.
Aether was a concept introduced by physicists for theoretical reasons, which died because its experimental predictions were ruled out by observation. Dark matter and dark energy are the opposite: they are concepts that theoretical physicists never wanted, but which are forced on us by the observations.
Dark matter, in particular, is nothing at all like the aether. It’s something that seems to behave exactly like an ordinary particle of matter, just one with no electric charge or strong interaction with known matter particles. Those aren’t hard to invent; particle physicists have approximately a billion different candidate ideas, and experiments are making great progress in trying to detect them directly. But the idea didn’t come along because theorists had all sorts of irresistible ideas; we were dragged kicking and screaming into accepting dark matter after decades of observations of galaxies and clusters convinced people that regular matter simply wasn’t enough. And once that idea is accepted, you can go out and make new predictions based on the dark matter model, and they keep coming true — for example in studies of gravitational lensing and the cosmic microwave background. If the aether had this much experimental support, it would have been enshrined in textbooks years ago.
Dark energy is conceptually closer to the aether idea — like the aether, it’s not a particle, it’s a smooth component that fills space. Unlike the aether, it does not have a “frame of rest” (as far as we can tell); the dark energy looks the same no matter how you move through it. (Not to mention that it has nothing to do with electromagnetic radiation — it’s dark!) And of course, it was forced on us by observations, especially the 1998 discovery that the universe is accelerating, which ended up winning the Nobel Prize in 2011. That discovery took theoretical physicists around the world by surprise — we knew it was possible in principle, but almost nobody actually believed it was true. But when the data speak, a smart scientist listens. Subsequent to that amazing finding, cosmologists have made other predictions based on the dark energy idea, which (as with dark matter) keep coming true: for the cosmic microwave background again, as well as for the distribution of large-scale structure in the universe.
There is still much we don’t know about dark matter and dark energy; in particular, we certainly haven’t nailed down what exactly they are (although we have many plausible ideas), and the only way we’ve detected them is indirectly, through their effects on gravitational fields in the universe. But they are not arbitrary; both ideas make very specific predictions for what those gravitational effects should be, which astronomers have tested and verified. Unlike the aether, which shrunk and eventually disappeared under experimental scrutiny, the case for dark matter and dark energy continues to grow stronger.
@Valatan
I do not hate them, I just critize them. There is no proof, no concrete empirical evidence and you expect me to come up with a better model? For a myth? You are pathetic. In fact I have come up with an other model. It is revolutionary and very controversial, but it is original , it is mine. I may be completely wrong, but at least I try to come up with something new. I invite you to try and do the same or stay put on the bandwagon and end up in the land of nowhere. I also invite you to try and find my model. You might learn something from it. You are welcome to try and critize me.
@ s johnson Dark matter should decay (which means it is unstable!), but slowly for the same reasons that it gains energy slowly. There’s a recent gamma ray line in the hundreds of GeV region (Higgs / Top mass area) that is a candidate for dark matter decay, but could turn out to be astrophysical. We hypothesize (nearly) stable dark matter because the observations indicate (nearly) stable dark matter. Supersymmetry, which is now basically dead but can serve as a representative case, hypothesized that dark matter was the Lightest Supersymmetric Particle, that Supersymmetric number was conserved or nearly conserved, and that this particle therefore could not decay, or at least not easily.
Dark matter does not collide with itself for the same reason that neutrinos do not readily collide with each other: to collide, they need to get within the interaction distance of the weak force, or something comparably weak, which is very, very, very, very small. Weak force cross sections are in the picobarn range, which is 10^-36 cm^2, which is equivalent to a circle of roughly 10^-20 m. Compare this to the atomic nucleus size of ~10^-14 m. So, dark matter would collide with itself (perhaps annihilate if it’s its own antiparticle) very occasionally, but not often.
Baryonic matter IS composed almost solely of stable particles, or particles so nearly stable that they may as well be (e.g., if the proton is unstable, its half life is > 10^34 years). Sure, strange quarks, muons, pions, etc can appear sporadically in particle detectors, but when we look out into the Universe, we don’t include strange quarks in our estimates of how much mass we should see. To create any possible hypothetical less stable versions of dark matter would be even harder than creating unstable versions of baryonic matter, since dark matter is much heavier and more energy, presumably, is necessary to reach the next highest particles (10s of GeV at least compared to a few MeV for baryonic particles).
@ martinvandijk
I am wondering, what qualifies as concrete empirical evidence? Detection of the particle at the LHC or bust? We have as strong a circumstantial case for dark matter as it is possible to have. In fact, dare I say it, we have a stronger case for dark matter than we have for gluons if you accept nothing less than direct detection of the particle as proof. We have never directly detected a gluon and never will due to confinement, but have plenty of indirect evidence consistent with their existence. You will find gluons extensively discussed in introductory textbooks nonetheless.
Meh, you can read about the quantum vacuum on Wikipedia or in many textbooks.
@ Entropy
Concrete empirical evidence is what you need to be able to take DM for granted. Do you take DM for granted?
What does it mean “to take DM for granted?” The evidence is strongly in favor of the existence of DM, by which I mean that it is far, far more difficult and implausible to explain observations in favor of it away than to accept its existence. If the only observation in favor of dark matter were galactic rotation curves, I would say we need to rethink our models, but those are by now a minor footnote on the dark matter theory’s explanatory power, although they were its original inspiration. Even if anomalous gravitational lensing is added in, I would still not prefer DM to an altered theory of gravity, but again, the theory explains more phenomena than overly massive galaxies. My question stands, what, by your definition, is sufficient evidence? Am I absolutely sure DM exists? Well, no, but I am as sure DM exists as I am sure quarks exist.
@ Entropy
I am not going to repeat my answer. “Sufficient evidence” are your words, not mine.You are relatively sure about the existence of DM. That answers my question. Thank you.
Um, no. And this is specifically what the OP is trying to explain.
Dark matter and dark energy are labels for something that was observed, which theory did not predict.
Aether is something theory predicted, but was never observed. Aether was NEVER used to “save” a theory. When aether failed to be detected, the theory that predicted it was killed.
Dark matter and dark energy also are not used to “save” a theory. When they were discovered, they forced the existing theory to be changed to accommodate them.
And now we are working on trying to figure out exactly what dark matter and dark energy are, and are testing hypotheses about them.
And trying to say that there is no direct empirical evidence for dark matter is like saying that watching you fall off a cliff is not direct empirical evidence for the existence of the earth.
There’s been no direct empirical observation of dark matter. Observation is not the only kind of direct evidence there is.
Amphiox #33, Man, Dark Matter is certainly a hot button topic. It’s such a big topic because it proves that we are missing something. For those who sit on the fringe with radically different ideas, it presents the opportunity to say “HA! I told you so!”, which drives physicists crazy. The reason is because this one single thing that we admit we need more evidence to understand, brings out the physics hobbyist who thinks they have the answer in their internet posted theory of everything. So in summation, I totally agree with you, though I’m sure our opinions of what exactly dark matter is may vary.
Roger #29, OH! you mean Hawking Radiation? The cosmic microwave background radiation? Was it really that hard to type that? And you’re statement is pretty inaccurate. The ‘aether-like substance’ you’re talking about in which electromagnetic radiation is produced…is electromagnetic radiation…I wouldn’t classify Electromagnetism as an aether-like substance because that would be totally wrong.
Meh, just click on the link. It does not say anything about Hawking radiation. Or just google it.
@Meh #34, If you truely understand where Hawking radiation comes from, then you know about the aether-like-stuff Roger is talking about. My understanding (and I’m SO not a physicist) is that in the vacuum of space, there’s actually a rippling sea of “things” (again, not a physicist – not sure whether to use the term “particles” or “quarks” or what here) and paired “anti-things” popping into existence together, then annihilating each other (thus popping back out of existence). Hawking radiation occurs when, near a black hole, one of this pair gets diverted into the blackhole, leaving the other of the pair to carry on its merry way – the remaining of the pair being Hawking Radiation.
The “rippling sea” I mentioned is Roger’s aether-like-stuff. It’s not the same as Hawking Radiation (and definitely not Cosmic Microwave Background Radiation) but “Aether-like-stuff + Black-Hole produces Hawking Radiation”.
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In #13 Bill actually asked two questions.
The first one, “If dark matter feels gravity, then why doesn it clump?”, was addressed in #18 by Brad, but I’m not sure I find the answer satisfying.
I don’t because of Bill’s second question: “And if it doesn clump, then shouldn’t it be distributed uniformly?” He mentions that in the Bullet Cluster picture, some parts of the dark matter region seem more concentrated than others. It looks like that to me too.
Also, if dark matter doesn’t clump, why doesn’t it spread uniformly all over the universe?
So the question still strikes me as a good one. I’m probably even less of a physicist than Tony above, so there’s no way I’m even close to being able to answer. Anybody want to take a crack at it?
‘Was the universe born spinning?’
http://physicsworld.com/cws/article/news/46688
“The universe was born spinning and continues to do so around a preferred axis”
The Universe spins around a preferred axis because the Universe is, or the local Universe we exist in is in, a jet; analogous to the polar jet of a black hole.
‘Mysterious Cosmic ‘Dark Flow’ Tracked Deeper into Universe’
http://www.nasa.gov/centers/goddard/news/releases/2010/10-023.html
‘The clusters appear to be moving along a line extending from our solar system toward Centaurus/Hydra, but the direction of this motion is less certain. Evidence indicates that the clusters are headed outward along this path, away from Earth, but the team cannot yet rule out the opposite flow. “We detect motion along this axis, but right now our data cannot state as strongly as we’d like whether the clusters are coming or going,” Kashlinsky said.’
The clusters are headed along this path because the Universe is, or the local Universe we exist in is in, a jet.
The following is an image analogous of the Universal jet.
http://aether.lbl.gov/image_all.html
The reason for the ‘expansion’ of the universe is the continual emission of aether into the Universal jet. Three dimensional space associated with the Universe itself is not expanding. What we see in our telescopes is the matter associated with the Universe moving outward and away from the Universal jet emission point. In the image above, ‘1st Stars’ is where aether condenses into matter.
The following is an image analogous of the Universe, or the local Universe, we exist in.
http://www.astro.ucla.edu/planetarium/graphics/st_images/BlackHole.jpg
The following is an image analogous of the Universal spin.
http://i.space.com/images/i/612/i02/040817_quasar_illo_02.jpg?1292259454
Dark energy is aether emitted into the Universal jet.
It’s not the Big Bang. It’s the Big Ongoing.
What is presently postulated as non-baryonic dark matter is aether. Aether has mass. Aether physically occupies three dimensional space. Aether is physically displaced by matter.
Non-baryonic dark matter does not travel with matter. Matter moves through and displaces the aether.
It is the aether which is displaced external to the matter which the Milky Way consists of which is pushing back and exerting inward pressure toward the Milky Way which causes the Milky Way to rotate at a speed which can not be accounted for by the matter the Milky Way consists of itself.
Yes, Tony, the cosmic background radiation is not the same as the luminiferous aether. But the CBR does give a rest frame for the universe. Some physicists in the 1800s thought that the aether might provide such a rest frame, until Lorentz showed in 1895 that such an assumption is unnecessary.
The new “Critical Opalescence” blog at Scientific American has an interesting piece on dark matter, MOND, and Erik Verlinde’s work.
http://blogs.scientificamerican.com/critical-opalescence/
The important question isn’t whether Dark Matter exists, it’s what can we use Dark Matter for if it does?
Consider a hypothetical super-dense material, laid out in a thin sheet with a thickness of one or two molecules. This material would be dense enough to reflect a non-negligible amount of Dark Matter hitting it. Since the Dark Matter in the galaxy must have some angular momentum in the direction of the rotation of the galaxy, locally there is a “Dark Matter Wind” in a single direction.
This super-dense material can then be used as a “Dark Matter Sail” and can be used for cheap interstellar travel. Even better, since it makes no difference if there is an atmosphere around, when the wind is blowing away from the surface of the Earth, this sail could be used as a super cheap way to achieve orbit.
The only drawback I can think of this is that excessive use would slow the rotation of the galaxy, but that would require an awful lot of excessive use.
We always seem to assume that matter preceeds gravity: Without matter to generate it, the assumption goes, there would be no gravity. Should we not question that assumption? Is it not possible that gravity waves (including their extreme version, singularities) existed in the moments before matter did?
In the Dark Matter vs. Modified Gravity trialogue from the other day, I made an argument that before matter precipitated into existence, the Big Bang might simply have generated more gravity in some regions of spacetime than there would come to exist matter with which to fill that spacetime.
That “excess gravity” is what we see today as Dark Matter.
Here’s the post: http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/#comment-240711
@ Alex
In order to stop a dark matter particle, the equivalent of 10000 light years of lead (assuming dark matter interacts with strength comparable to the neutrino) is required. Compressing 10000 light years of lead down into a sail would create a black hole many times over. In short, collecting any reasonable fraction of a dark matter wind is utterly impossible without somehow altering the strength of the weak force (or whatever dark matter couples through).
@Entropy,
yea, I was afraid that the density required would create a black hole. I was just entertaining a wacky possibility. A more serious question would be: is there any potential use for DM? Or is the difficulty in detecting it too high, even for hypothetical future technologies?
It’s difficult to conceive of a direct use for dark matter. Its density is too low (sure, it dominates the mass of galaxies, but that’s only because it fills space relatively evenly while baryonic matter is packed into tiny clumps of high density). Even 100% efficient collection wouldn’t do much. That said, dark matter could be the clue to new physics that is more useful, and understanding the formation of the Universe’s large scale structure can’t hurt.
Always remember with these things that, for instance, Maxwell discovered the light was an electromagnetic wave 34 years before the first practical radio was built using his discoveries. Maxwell’s discovery at the time could fairly have been greeted with the same “so what?” attitude as some greet modern discoveries. Who cares if light is an electromagnetic wave? That’s great trivia but not *useful*. đ
On the flip side, we discovered that the Universe is expanding in the 20s, and that hasn’t led to any direct benefits (plenty of spinoffs, but no direct applications). The point is, we just don’t know, the knowledge is benefit enough, and useful applications are bonuses.
Don’t get me wrong, I do think that the search for how the universe works and even possibly how it came to be is something that is more than worthwhile.
I read this blog because I find these sorts of questions fascinating, despite what I said in my first post, which was written a bit tongue in cheek.
When I think about how much information we have gleaned about the universe from our tiny vantage point here on Planet Earth, I am awestruck, to put it mildly.
I simply also enjoy thinking about what we could possibly do with these discoveries.
Q. Why is the particle always detected entering, traveling through and exiting a single slit in a double slit experiment?
A. The particle always enters, travels through and exits a single slit. It is the associated aether wave which passes through both.
What ripples when galaxy clusters collide is what waves in a double slit experiment; the aether. The ripple is a gravitational wave.
Einstein’s gravitational wave is de Broglie’s pilot-wave.
They are both aether displacement waves.
‘Interpretation of quantum mechanics by the double solution theory – Louis de BROGLIE’
http://aflb.ensmp.fr/AFLB-classiques/aflb124p001.pdf
âWhen in 1923-1924 I had my first ideas about Wave Mechanics I was looking for a truly concrete physical image, valid for all particles, of the wave and particle coexistence discovered by Albert Einstein in his “Theory of light quanta”. I had no doubt whatsoever about the physical reality of waves and particles.â
âany particle, even isolated, has to be imagined as in continuous âenergetic contactâ with a hidden mediumâ
The hidden medium of de Broglie wave mechanics is the aether. The âenergetic contactâ is the state of displacement of the aether.
A moving particle has an associated aether displacement wave.
In a double slit experiment the particle travels a well defined path which takes it through one slit. The associated aether wave passes through both. As the aether wave exits the slits it creates wave interference. As the particle exits a single slit the direction it travels is altered by the wave interference. This is the wave piloting the particle of pilot-wave theory. Detecting the particle strongly exiting a single slit turns the associated aether wave into chop. The aether waves exiting the slits interact with the detectors and become many short waves with irregular motion. The waves become disorderly. The waves are disorganized. There is no wave interference. The particle pitches and rolls through the chop. The particle gets knocked around by the chop and it no longer creates an interference pattern.
‘Surprise! IBEX Finds No Bow âShockâ Outside our Solar System’
http://www.universetoday.com/95094/surprise-ibex-finds-no-bow-shock-outside-our-solar-system/
‘âWhile bow shocks certainly exist ahead of many other stars, weâre finding that our Sunâs interaction doesnât reach the critical threshold to form a shock,â said Dr. David McComas, principal investigator of the IBEX mission, âso a wave is a more accurate depiction of whatâs happening ahead of our heliosphere â much like the wave made by the bow of a boat as it glides through the water.â’
The wave ahead of our heliosphere is an aether displacement wave. This is evidence of a moving ‘particle’, the solar system, having an associated aether wave.
‘Hubble Finds Ghostly Ring of Dark Matter’
http://www.nasa.gov/mission_pages/hubble/news/dark_matter_ring_feature.html
“Astronomers using NASA’s Hubble Space Telescope got a first-hand view of how dark matter behaves during a titanic collision between two galaxy clusters. The wreck created a ripple of dark mater, which is somewhat similar to a ripple formed in a pond when a rock hits the water.”
The ‘pond’ consists of aether. The moving ‘particles’ are the galaxy clusters. The ripple is an aether displacement wave. The ripple is a gravitational wave. This is also evidence of a moving ‘particle’, the galaxy clusters, having an associated aether wave.
‘Giant black hole kicked out of home galaxy’
http://www.astronomy.com/en/News-Observing/News/2012/06/Giant%20black%20hole%20kicked%20out%20of%20home%20galaxy.aspx
“But these new data support the idea that gravitational waves â ripples in the fabric of space first predicted by Albert Einstein but never detected directly â can exert an extremely powerful force.”
The fabric of space is the aether.
Gravitational waves are ripples in the aether.
What ripples when galaxy clusters collide is what waves in a double slit experiment; the aether.
Einstein’s gravitational wave is de Broglie’s pilot-wave.
They are both aether displacement waves.