One explanation, they say, is that three extra dimensions, in addition to the three spatial ones to which we are accustomed, are altering the effects of gravity over very short distances of about a nanometre1.
The team argues that such astronomical observations of dark matter provide the first potential evidence for extra dimensions. Others are supportive, but unconvinced. Lisa Randall, a Harvard physicist who has explored the possibility of extra spatial dimensions, says "Even if their idea works, which it probably does, it may be an overstatement to use these observations as evidence of extra dimensions."
Silk himself acknowledges that the proposal is "extremely speculative".
Garth said:From that Nature article
SimonA said:You can invent 'crumbly' forms of dark matter that could explain it but then you are inventing something purely to fit the data, with no fundamental basis for doing so.
My point exactlySimonA said:Any more speculative than WIMPS etc. ?
Until it is discovered in a laboratory exotic non-interacting DM is just "pixie dust" invented purely to fit the dataAnd why the missing dense dark matter halos at galaxy cores ? You can invent 'crumbly' forms of dark matter that could explain it but then you are inventing something purely to fit the data, with no fundamental basis for doing so.
Check out Self Creation Cosmology - An Alternative Gravitational Theory .On the other hand there are plenty of fundamental mathematical reasons for considering extra dimensions.
Of course its not by itself "evidence for" extra dimensions. But the current trend of the multiplication of entities approach to physics will keep having to plaster up all the different invented entities to get them to fit the data - which gives you a "standard model" thats less 'speculative' - but is it more realistic ? 'Where' does gravity 'leak' to (heirarchy problem) ? How can a superposition of states collapse instantly across 3D space ? And what about quantum gravity ? ( http://www.calphysics.org/articles/gravity_arxiv.pdf )
There's no point simplifying what can't be simplified any further. But if there is a chance that a more fundamental look at the basics across all these 'anomalies' can resolve them all at a fundamental level, what a grand prize that would be :)
ohwilleke said:God forbid that theory could ever follow empirical evidence instead of the other way around!
Why does it need to be a particle? Could it not be a field of particle/antiparticle pairs that is difficult to detect except locally (quantum vacuum field)? This would entail a new model of gravitational interaction (a polarized self attractive vacuuum field), but is this a bad thing? The Newtonian inverse-square relationship works well for simple systems, and Einstein's improvement works well in some domains. Is it not appropriate to explore the question of whether we need an even better theory of gravitation, to explain the flat rotation curves of galaxies, the excess lensing of clusters, and the excess binding energy of clusters?SpaceTiger said:If you're whining about why we haven't detected a particle that could be the dark matter, you need to stop and think about why it is that the particle would be dark and what implications that has for its detectability.
turbo-1 said:Why does it need to be a particle?
Could it not be a field of particle/antiparticle pairs that is difficult to detect except locally (quantum vacuum field)?
Is it not appropriate to explore the question of whether we need an even better theory of gravitation, to explain the flat rotation curves of galaxies, the excess lensing of clusters, and the excess binding energy of clusters?
SpaceTiger said:I would say that it's considerably less ad hoc to postulate a weakly-interacting particle than it is to postulate an alternative theory of gravity and/or extra dimensions.
SpaceTiger said:I said it before and I'll say it again. If you're whining about why we haven't detected a particle that could be the dark matter, you need to stop and think about why it is that the particle would be dark and what implications that has for its detectability.
SimonA said:ad hoc
For the special purpose or end at hand; also, by extension, improvised or impromptu. The term, Latin for "to this," is most often used for committees established for a specific purpose, as in The committee was formed ad hoc to address health insurance problems. The term is also used as an adjective (An ad hoc committee was formed), and has given rise to the noun adhocism for the tendency to use temporary, provisional, or improvised methods to deal with a particular problem.
Errr ... whining ? Why do I need to stop and think about something so inately and implicitly obvious ?
SpaceTiger said:It's perfectly appropriate, but it makes no sense to attack the conventional theories for "making things up to fit the data" when the alternative gravity models do so in an even more egregious fashion.
SimonA said:An "even more egregious fashion" ? Strange - I have no idea where you are coming from. Which gravity model are you talking about exactly ? Einsteins stretched-by-mass concept of spacetime ? I think we all can agree to accept that. So is M Theory the "alternative gravity model" you have a problem with ? It does have all those 'messy' extra dimensions that no one claims to understand properly. So I guess you consider the likes of Ed Witten and Lisa Randal to be the kind of people who willingly spend their time on "egregious" theories ?
What is this "attack the conventionial theories" comment refering to ? The idea of science is to question. In fact its the heart of the scientific process.
So tell me how making things up purely to fit the data in cosmology, things that have no direct evidence whatsoever...
let alone any kind of fundamental significant basis for their derivation, compare with a concept based on the broad horizon of cutting edge theoretical physics, derived from well established maths ? How do you weigh such things ?
Your comments are as dubious as Garth's paper.
You say "It makes no sense to attack the conventionial theories" ??? Where would science be if we all followed your lead ?
Of course you did qualify that..."
If you're such a SpaceTiger then I assume that along with you gutteral roar that shakes the ground - as all good tigers do, you will provide us all with a relatavisticaly invariant description of youngs slit experiment ?
SpaceTiger said:I don't "have a problem" with any of the above theories, I just think they're more ad hoc than the dark matter theory, seeing that there's no observational support for them. Perhaps you've forgotten that this was what we were discussing...
SpaceTiger said:When did I say one shouldn't question? That's rather nasty of you to put words in my mouth.
I said it before and I'll say it again. If you're whining about why we haven't detected a particle that could be the dark matter, you need to stop and think about why it is that the particle would be dark and what implications that has for its detectability.
SpaceTiger said:So yeah, that's where I'll stop you. You see, the alternative gravity theory you're referring to in the post up top is not only lacking direct evidence, it's lacking any evidence at all other than that for which it was created to explain. Dark matter has made many successful predictions, including the CMB power spectrum, large scale structure, and lensing results. Meanwhile, alternative theories (like MOND), have done no such thing.
SpaceTiger said:I'll grant that we're not yet at the stage where we can rule them out, but it seems to me to be considerably more ad hoc to "invent" a theory that has no basis other than...
SpaceTiger said:As a scientist (that is, not a philosopher), I would give those things little or no weight. Math and the physical world are two different things and, as much as we'd like to evaluate our theories based on their "beauty", I'm strongly inclined to say that observational support should always be the trump card.
SpaceTiger said:Now that was just mean. :tongue2:
SpaceTiger said:Wow, that's one of the most egregious (in italics so that you can plug it into Webster's) examples of taking a quote out of context that I've ever seen. The quote was:
"...it makes no sense to attack the conventionial theories for "making things up to fit the data" when the alternative gravity models do so in an even more egregious fashion."
SpaceTiger said:Oh, this is fun. Let me try. You said:
"Of course...conventionial theories...are as dubious as Garth's paper."
Mr. SimonA, it's so foolish of you to say that. You must not be very smart. Of course, you did qualify those things...
SpaceTiger said:I think I pushed a button...
SimonA said:Why are extra dimensions "more ad hoc than the dark matter theory, seeing that there's no observational support for them" ? What is the "observational support" to suggest that the motion of galaxies can be explained by a four dimensional universe ?
Well I was questioning and you replied;
Que ? You tell me - what is the basis for extra dimensions ? Is it all a new concept lacking any kind of scientific rigour ? Did Einstein have the final world by considering a 4D reality ?
Yes and it rained yesterday and I need rain to survive and so it must be me that causes it to rain... "Observational support" relies on correct interpretation.
Yes and thats the one I was replying to. I did say that you qualified it and dealt with that as well...
Mmm ... it must be that I'm not very smart. But even in my half-witted and troubled with the whole concept of reality state, I can still see that this is fun :)
SpaceTiger said:I'm not sure what you're asking for here. Einstein's theory (specifically, GR) is well supported by observation, but I'm sure you know that. If you're asking how we can be sure that there aren't extra dimensions outside of the limits of our experiments, then we can't be...but the burden of proof is not on the negative...
SpaceTiger said:It is lacking observational/experimental support, which relegates it to the status of an untested possibility.
Mathematically, I don't doubt its rigor -- what I doubt is its correspondence to reality. But that's fine, prove me wrong. I have no objection to extra dimensions as a concept and if the observations turn out to support it, then I will as well. Until then, however, I will not lend it support simply based on a philosophical prejudice.
Absolutely right, but I hope that you can see how the two are intertwined. The beauty of, say, relativity is certainly amazing in of itself, but it was not that alone that made it one of the foundations of modern physics. The important thing was that it survived rigorous experimental testing. If, as in your example, I postulated that you caused it to rain yesterday, what testable predictions could I draw from that? How does that contribute to the general acceptance or rejection of the theory?
The point is that it makes no sense to argue based on something that was taken out of context. No rational person would conclude that this quote:
"...it makes no sense to attack the conventionial theories for "making things up to fit the data" when the alternative gravity models do so in an even more egregious fashion."
means that I think science should be done without challenges to tradition.
Yeah, I've been copying and pasting this into some of my AIM conversations. I love your bit about the "guttural roar". :rofl:
There are more than a few reasons to suspect the existence of dark matter. While direct detection is still lacking, there is plenty of 'empirical' evidence favoring DM:SimonA said:Okay so you notice that the motion of galaxies do not fit Newtonian predictions, because there is not enough mass to hold it all together at that velocity in Newtonian terms. So, following what was missing from Newtons predictions - namely mass - invent a kind of mass that interacts 'weakly' with baryonic (normal) matter. Then you look a little closer and all kinds of things do not match predictions. Then you spend millions to try to detect this invented concept and fail to do so. So in effect the theory of non baryonic matter contained within 3D space has so far failed to find empirical support.
I disagree. Science is all about following empirical evidence to its logical conclusion.SimonA said:Science is not always about "follow[ing] empirical evidence". Ideally its about understanding empirical evidence.
That is a gross oversimplification, at best.SimonA said:Einstein opened up a new level of understanding by considering time as a dimension.
It means you do not understand basic relativity.SimonA said:Surely there is room in your empirical world to consider there may be even more to it all ? According to E=mc^2 mass travelling at c becomes "energy". What does that mean ?
It obeyed the rules of quantum physics.SimonA said:If you where 'watching' an electron, specifically in the orbital shell of an atom, and that electron recieved enough energy to jump to another shell, what would you say had happened to that electron ?
SimonA said:Sure - of course the "burden of proof" in science must admit at least the same space to less fundamental issues that don't even provide any hope of providing answers across the broad reach of enquiry - such as "how can we be sure there are weakly interacting particles that exist within 3D space" ?
Now wait just one second (or more) there. How can you say that the heirarchy issue, the motion of galaxies, the accelerating expansion of the universe, quantum entanglement and inertia are "lacking observational/experimental support" ?
Where is the "philosophical prejudice" ? The observations support extra dimensions far better than the standard 4D cosmology
Well it doesn't contribute anything because its nonsense - that was my point.
So what exactly is the non-alternative gravity model you are implicitly refering to ? Stretched spacetime
Chronos said:SpaceTigers reasoning is quite sound, IMO. I have some doubts about this There are more than a few reasons to suspect the existence of dark matter. While direct detection is still lacking, there is plenty of 'empirical' evidence favoring DM:
The Dark Side of the Universe
Chronos said:Cosmological Parameters and the case for Cold Dark Matter
Most dark matter searches rely on a substantial fraction of the dark halo of our Galaxy being made up of cold dark matter, and that this cold dark matter consists of the neutralino.
In this paper I review the current evidence on cosmological parameters and assess their relevance to cold dark matter searches.
it is likely that most of the dark matter in the halo of our Galaxy is in the form of cold dark matter.
Chronos said:More Evidence that Dark Matter Rules the Universe
Most astronomers already view dark matter as the only logical way to explain the orbits of stars and shapes of galaxies. Nobody has ever seen dark matter, and scientists dont know exactly what it is, but without it galaxies would fly apart.
Chronos said:I disagree. Science is all about following empirical evidence to its logical conclusion.
Chronos said:That is a gross oversimplification, at best.
Chronos said:It means you do not understand basic relativity.
Chronos said:It obeyed the rules of quantum physics.
SpaceTiger said:You either ignored the part of my post where I gave you examples of evidence for the dark matter theory or hold much higher standards for those theories that don't align with your beliefs.
SpaceTiger said:Those things were not predicted by your "extra dimensions" theory.
SpaceTiger said:In fact, extra dimensions are instead being used to try to explain those phenomena.
SpaceTiger said:Oh really? Please, link away.
SpaceTiger said:Apparently, you missed mine. Theories need to make predictions to be useful. The theory that you caused it to rain yesterday might lead to some predictions -- for example, a correlation between rain and your presence -- but they would be very quickly refuted by experiment. If theories are going to be of any scientific value, they must make predictions as well. It's not enough to go back to your unsolved problems and say, "Hey, if we introduce this length scale into the theory, we can explain this old observation!" Theories invoking extra dimensions are notoriously bad in this regard. Oh, and please don't quote me in your next post as having said, "Theories invoking extra dimensions are notoriously bad."
SpaceTiger said:It's called general relativity.
turbo-1 said:Could it not be a field of particle/antiparticle pairs that is difficult to detect except locally (quantum vacuum field)?
Wow! All of space is suffused with a vacuum field that is projected variously to hold 120 OOM expansive energy AND 120 OOM more gravitational equivalence than observed (that's some fine balance!), and somehow you know that this field cannot be gravitationally dynamic. Apparently, you are aware of some research that I have not been able to dig up. The authors must have avoided the use of words like "vacuum", "energy", "gravity", etc, or published in a language other than English.ST said:No.
SimonA said:There is no "evidence for the dark matter theory". The "dark matter theory" as you call it is a theory invented to explain anomalies that do not fit established and proven laws governing gravity related dynamics.
Hey ? What exactly is my "extra dimensions theory" ? I was stating a couple of anomalies that could potentially be explained by extra dimensions. Are you suggesting that WIMPs and MACHOs where derived from first principles rather than being a proposed plaster (come educated guess) for gaping holes that are the anomalies themselves.
Yes exactly and they make a far better job of doing so than inventing imaginary particles like WIMPs or MACHOs.
But a significant point is that current dark matter theories don't really make any specific predictions other than the anomalies they where designed to explain. I say 'really' because of course there is 'clumpiness' in the universe and galaxies are 'seeded' in some way etc. But those are models retro-fitted onto dark matter because its the current theory and so many smart people are using it as a basis for their models. That doesn't mean its the accurate description of what causes these anomalies or that extra dimensions could not describe those phenomena just as well. Certainly they are not of the calibre that relativity is when it makes predictions.
Hey ? When have I disputed GR ???
SpaceTiger said:What you fail to understand is that those problems are solved consistently -- that is, all of the alternative gravity models have so far failed to explain all of these things at once without invoking a significant amount of dark matter. The initial introduction of dark matter was completely ad hoc, I agree, but from the "invented" solution to the rotation curve problem, we can construct models to predict other phenomena, such as the CMB power spectrum, and the distribution of large scale structure. There are still some kinks (such as the halo cusps mentioned in the "extra dimensions" paper), but they're in regimes that are hard to model, so they're not particularly surprising.
SpaceTiger said:The only thing the "extra dimensions" add, according to the paper you link, is a possible solution to the halo cuspiness problem. In every other respect, it is a dark matter theory.
SpaceTiger said:Unless we're defining the bounds of Einstein's theory differently, adding extra dimensions (beyond 4-D spacetime) would be "disputing" GR.
SimonA said:I do understand this but I think you're missing my argument. "Dark matter" used as a kind of like a place holder for 'something' that affects large scale gravitational dynamics in a way that has a weaker effect than baryonic matter is something I accept. What I'm contesting is this is non-baryonic matter that exists within 3D space. The same predictions, such as large scale structure, could equally be obtained from considerng that "dark matter" is in fact an 'equivalent' of matter in extra dimensions. Its just that non-baryonic dark matter within 3D space ('above' the 'zero point') is the asociation being made with this unknown element and so gets the credit for what is in effect a place holder for something that in not understood.
Well I'd prefer to suggest GR is relevant to a subsection of reality. Its kind of like considering the solar system as an isolated unit in terms of thermodynamics/entropy. It works most of the time but if you want to understand the full picture you really need to consider the input of gamma rays, the ouput of radiation from the sun etc...
SpaceTiger said:Well, if you're not disputing dark matter, then your argument makes even less sense. The paper you link supports one of the popular non-baryonic particle dark matter candidates -- which was no less a "place-holder" than the others -- and then simply adds another detail, extra dimensions, to the dark matter theory. In fact, this particle is even weakly interacting. The only reason it's not called a WIMP is that it's not massive.
More recently, with Karch, I studied a
theory that is in some ways even more surprising
(13). In that theory, space looks four
dimensional on the brane and at some distance
away from the brane. This is because
there is again a mode that looks like a fourdimensional
graviton. However, the majority
of the space is not sensitive to the force
mediated by this four-dimensional graviton,
because it only couples in a small portion of
the space. The part of the space where the
trapped mode does not couple sees itself as
five dimensional. This leads one to consider
the possibility of gravitational “islands”; the
dimensionality of space you think you see
depends on where you are in the bulk. The
brane can be considered to be a four-dimensional
sinkhole. This is truly a possibility of
nature; we only ever see a finite region of
space, even with cosmological observations.