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Einstein's Theory Improved?
skaet writes to tell us that A Chinese astronomer from the University of St Andrews claims to have fine-tuned Einstein's theory of gravity. Dr Hong Sheng Zhao has created a 'simple' theory which could "solve a dark mystery that has baffled astrophysicists for three-quarters of a century." This new law seeks to discover whether Einstein's theory was correct and if dark matter actually exists.
Even if that is the worst graphic accompanying a well-written, intelligent article I've seen in my lifetime.
Ex nihilo nihil fit.
Would people please stop using the word "Law" when referring to scientific theories. It confuses the creationists.
I'm sure that there are lots of people that are far more clued up on this than I am that can find holes in what I am about to say but I always felt like dark matter was a bit of a fudge because we don't understand what is happening.
My problem with dark matter is that it's almost as difficult to believe in as God. The only real proof we have is that the universe doesn't appear to move correctly without it. If that's as good as we can do then we might as well say God (or the FSM) is holding the universe together. To my mind it is a big leap from "the universe isn't moving as we expect" to "90% of the universe is made of something we can't see". Surely if the universe was full of this stuff we would be able to detect it because it would block radiation from distant galaxies - or is dark matter conveniently transparent?
I used to have a better sig but it broke.
Won't it be ready until April? Stranger things have happened.
http://michaelsmith.id.au
What a clear and well-written article. And what a pleasant, unassuming statement from Dr Zhao:
"A non-Newtonian gravity theory is now fully specified on all scales by a smooth continuous function. It is ready for fellow scientists to falsify. It is time to keep an open mind for new fields predicted in our formula while we continue our search for Dark Matter particles."
Even if the theory turns out not to stand up, words like this take us back to what makes science interesting and important. That "falsify" is worlds away from the publicity hounds and egomaniacs who so often represent science to the lay reader.
Las qué passoun
tournoun pas maï
Name something that doesn't!
My problem with dark matter is that it's almost as difficult to believe in as God. The only real proof we have is that the universe doesn't appear to move correctly without it..... or is dark matter conveniently transparent
I have to disagree with that, I have no problem believing in the existance of dark matter. In fact I don't have to 'believe' in the existence of dark matter at all, I found some between my toes this morning and it was most certainly not transparent.
Only to idiots, are orders laws.
-- Henning von Tresckow
Although it is not the theory that has been improved, it is the model. It takes a simple function to interpolate between the dark matter area (which is non Newtonian - Modified Newtonian Dynamics or MOND) and the Newtonian area, where baryonic matter seems to reign. Despite a simple continuation function for the two areas, the authors find a nice agreement with rotation curves of galaxies including our own, and some external ones. The theory which has been used is the TeVeS (Tensor Vector Scalar) theory by Bekenstein. The scalar part of the theory could explain the dark matter behaviour.
Unfortunately, like Cambridge, St. Andrews has suffered from negative publicity as a result of its taking occasional pupils from failing schools and admitting them with A level scores which would not normally allow a student to be admitted. But at least it meant that some of the Windsors got access to higher education, so perhaps the policy is defensible.
Anyway, I'm very pleased that the astrophysics tradition is continuing. But I'm still left with a question: Why are the nicest British Universities (Cambridge, Durham, St Andrews) in such bloody cold places?
Pining for the fjords
As other people have pointed out, the word 'law' is not used (anymore) in science; if a hypothesis is experimentally verified, its status becomes a 'theory'. The word 'law' would indicate that it's results are set in stone; this is never the case in science.
I am not an astronomer. I just tend to apply logic to everything I collide with.
And when I can't collide or interact with it, it kinda ruffles me the wrong way. What kinda magical stuff is this supposed to be. Doesn't interact, doesn't shine, doesn't emit, doesn't absorb, all it does is offer some convenient gravity to explain a few things that don't make sense otherwise.
It kinda reminds me how about 500 years ago astronomers came up with double and triple rotations of planets around an imaginary point to explain why the planets move the way they move since they believed the Earth and not the sun is the center of our system. And if they rotated around earth, they had to jump through a few hoops to explain that odd orbits they showed. Instead of abandoning the system that didn't work and accept one that does, they religiously clinged to it and tried to explain what could not be explained.
Maybe we're at that point again?
Maybe, just maybe, it's not dark matter but some of our "laws" are simply wrong. Or, if not wrong, they maybe don't extrapolate well into the larger scale, what works and makes sense in the (comparably) small scale of our solar system doesn't make sense and doesn't work on a galactic scale.
I do hope this is a step into the right direction. Science is all about not setting stuff in stone. Everything has to be questioned, everything has to be tested, even the most holy scriptures from the most revered astronomers of all times should be ripped if they showed an error.
If not, science is no better than religion.
We used to have a Bill of Rights. Now, with the rights gone, all we have left is the bill.
f you check the equations, you'll find that light from a star causes its gravitational field to fall off as 1/r, whereas its mass causes it to fall off as 1/r^2.
Where on Earth you found that light has so much gravitational field? And why would be constant: shouldn't it vary with the luminosity of the star (which goes like mass^4, so it's highly nonlinear)??
Galaxy spanning in fact.
Ah, I see. You are off by four orders of magnitude. Come back when your astronomy is a little better.
I've heared once that the original hebrew text reads "Thou shalt not murder." If that's true, the contradiction is easy to resolve: Just define that killing ordered by god is no murder.
My understanding was similar but different. I had heard that the original ancient language of the bible did not have a rich engouh vocabulary to distingiuish between kill (e.g. an enemy) and murder (e.g. one in your own society), but the next most recent translation of the bible used the word "murder", not "kill".
The point is, when Moses was taking his tribe around the desert with their new commandments, they were to preserve their own society (which is what the 10 commandments promote), but if they had to kill competing tribes to survive, they could do so because it would be *killing*, not *murder*. Any society that condones unbridled murder within itself will quickly commit suicide.
The link at the bottom of the page labelled "About PPARC" obviously goes too close to a singularity and ends up in a parallel universe with a quantum difference of this address is a 404 page. They've got me convinced.
It's defintely not his observation ;-), Einstein himself presented the famous formula in the for m = E/c^2.
There is a nice lecture by Frank Wilczek, http://mitworld.mit.edu/video/204/, elaborating on this subject.
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It was Kepler who realised that ellipses could be the correct model for orbits, and even there, to try and keep the Church happy, he tried to fit the major and minor axes into the shapes of the "Platonic solids".
History suggests that the example you are quoting is the opposite of what you want to show. It is better to let scientists come up with initially ad hoc explanations because they lead to the truth. Making initial unscientific assumptions and treating them as dogma suppresses and delays progress. Scientists are ambitious and a good way to become important is to replace someone else's theory - so scientists can be relied on to do that. For every established Dark Matter theorist there are probably several PhD students who would love to annihilate Dark Matter.
The line of argument in the parent annoys me because it tries to suggest that scientists left to themselves will produce ridiculous non-explanatory theories and then cling to them forever. It's the anti-scientific agenda of the Creationists who want to discredit science. Creationists and their like want to confuse the public as to the explanatory status of different scientific theories so they can claim their snake oil is on an explanatory par with plate tectonics, quantum electrodynamics or evolutionary biology.
Pining for the fjords
(1) Jobs where if you goof up, some money goes down the drain, or you're embarrassed or, somebody gets hurt or dies. You know, like being a doctor or lawyer or engineer.
(2) Jobs where it doesnt matter one whit if you're wrong. Jobs like theoretical physicist in a field where there isnt the slightest possibility of carrying out an experiment. Such as dabbling in the theory of gravity.
Like an idiot, I'm in category #1. What a dope.
The equivalent rest mass of the light in our Galaxy is about a thousand solar masses, compared to 10^12 solar masses in matter.
So it's a cute idea, but it doesn't work in practice...
[TMB]
The statement that "science is/isn't better than religion" is not scientific, it's rather religious.
Yesterday was the time to do it right. Are we having a REVOLUTION yet?
http://arxiv.org/abs/astro-ph/0512425 The phenomena customly called Dark Matter or Modified Newtonian Dynamics (MOND) have been argued by Bekenstein (2004) to be the consequences of a covariant scalar field, controlled by a free function (related to the MOND interpolating function) in its Lagrangian density. In the context of this relativistic MOND theory (TeVeS), we examine critically the interpolating function in the transition zone between weak and strong gravity. Bekenstein's toy model produces too gradually varying functions and fits rotation curves less well than the standard MOND interpolating function. However, the latter varies too sharply and implies an implausible external field effect (EFE). These constraints on opposite sides have not yet excluded TeVeS, but made the zone of acceptable interpolating functions narrower. An acceptable "toy" Lagrangian density function with simple analytical properties is singled out for future studies of TeVeS in galaxies. We also suggest how to extend the model to solar system dynamics and cosmology, and compare with strong lensing data (see also astro-ph/0509590).
The map is not the territory. It doesn't matter if the theory invokes a state of matter which cannot be directly interacted with: if it fits the existing data and makes testable predictions regarding new data, then it's valid. Occam's razor (which is to do with removing theories found to be practically indistinguishable from other, simpler theories) doesn't come into it at this stage because the various theories make different predictions.
No kidding!!! What do you say at this point?
Although I share your skepticism about dark matter, I couldn't help thinking about the neutrino, a "hidden" particle that filled another gap in physics. It took 25 years for physicists to finally detect the neutrino.
Mea navis aericumbens anguillis abundat
This is not an improvement to Einstein's theory of gravity. It is, however, an improvement to Milgrom's Modified Newtonian Dynamics (MOND): MOND is merely an empirical correction to Newtonian gravity, so this is an improved 'empirical' gravity (well, it's got to survive a few more tests before we know if it's an improvement to MOND).
One big difference between Newton's theory of gravity and Einstein's theory of gravity is that Newton's theory says what gravity does (ie. it gives us the magnitude and direction of the force of gravity between two objects) whereas Einstein's theory says that and how as well (i.e. mass curves space-time). Einstein's theory improves on Newton's in that it is more accurate and actually provides insight (testable) into how gravity actually works.
It is likely that Einstein's theory will be improved upon at some time, and be replaced by a more enlightening theory (quantum gravity? string theory?), but for now it is the best theory of gravity we've got....
#include "cunning_plan.h"
"If something doesn't work first time, invent something imaginary"
This is not the first time in which an existing theory has had a part added (a fudge factor if you will...) to explain an anomalous phenomenon.
Such example include,
1. God
2. The aether
3. The cosmological constant
Each of these ideas have been used at some point to ensure that an existing theory (or foundationless preconceptions) coincide nicely with observation. In each case, they have been refuted at some point in the future.
The idea of modifying the rate of gravitational fall off with distance is not a new idea - back in the 1800s, Airy (If I remember correctly) discovered that if instead of gravity obeying an inverse square relationship, it obeyed an inverse relationship to a different power, the predicted orbit of mercury would fit the observed data. If this was proposed however, there would have been a lesser incentive to look for the more accurate theory that General Relativity provides.
I can't help but think that very rarely does true progress come from simple modifications to existing theory. When theory does not match observation, it is often a new idea entirely that is needed to resolve the problem. A modification to an otherwise elegant idea usually obscures the truth.
If this new theory really does provide highly accurate results, we should ask why and look for the underlying cause of gravity falling off faster than expected, rather be complacent with the introduction of a new constant.
Nothing sucks like a Vax, nothing blows like a PowerMac G4
First off, I want to say a few words about dark matter. I think it's kind of irritating when people rant on about how dark matter is ad hoc fudging, etc. etc. Well, "fiddling around with the laws of gravity" isn't any better on that account. The fact of the matter is that all of theoretical physics is creating new models that fit our observations, and both dark matter and MOND fall into that category. The very existence of MOND as a theory shows that it is not easy to distinguish "matter that primarily interacts gravitationally" from "modifications to the laws of gravity". Historically, both "unseen matter" and "modifications to gravity" have been valid solutions to anomalous gravitational behavior (in the cases of Neptune's orbit and the perihelion precession of Mercury, respectively).
As it stands, dark matter models can pass many experimental tests, and they're still the way to bet. That being said, MOND is not a bad idea either. It's not as well supported by dark matter, and it has serious problems with galaxy clusters, but it can still account for a surprising amount of data (for a nonrelativistic theory!). The flaw of non-relativistic has been "corrected" by Bekenstein's TeVeS theory (the one that Zhao and Famaey's work is based on).
Unfortunately, TeVeS appears to be rather ad hoc (even compared to dark matter). Z&F's work does not appear to be much better in this regard. In addition, solar system observations appear to place serious constraints on such MOND-like theories, leading to anomalous non-inverse square forces in the outer solar system (and no, it doesn't seem to be of a form that can be attributed to the Pioneer anomaly, though the jury is still out).
The TeVeS/dark matter debate should be definitively resolved by the Planck mission, which will be capable of resolving the third acoustic peak in the the cosmic microwave background radiation power spectrum. TeVeS and dark matter make very different predictions for the structure of this peak. Of course, if TeVeS fails this test, maybe some other MOND-like theory could be put forward (if the entire class of theories hasn't already been ruled out by other means, such as solar system dynamics, by then).
No, that's not what happens. Laws say what happens, theories say why and/or how it happens. Laws don't try to explain behaviour, they just state it. Hence the laws of thermodynamics are laws, while the theory of relativity is a theory and always will be.
:-)
And the law of gravity? Observations say what happens. Theories say why and/or how it happense. Laws are what we call theories we think will never be falsified, and it's probably a word that should be dropped from any kind of scientific discussion, since we all should have learned by now that even the most basic assumptions and most obvious conclusions drawn from the most irrefutable of observations have a way of requiring revision from time to time, as better observations are made (Newton couldn't look at gravitational motion, and we cannot yet see into the higher folded dimensions of string theory, assuming such in fact exist).
The "laws" of thermodynamics are as theoretical as relativity. Both have been observed, both are mathematically modelled to great precision, both make useful predictions, both are falsifiable, and no one outside of a few religious wackos expects either to be falsified. That doesn't mean they won't be.
Someday we might find conditions in which entropy in a closed system decreases (candidates for something like this include the time leading up to the big bang--if such is found to have existed--and certain theories of the internal workings of black holes, etc.). Not that I or anyone else realistically expects this (but then, who expected the anomalies that would lead to the dark matter/energy vs. non-newtonian gravity debate, either), but the "laws" of thermodynamics are as falsifiable as the theory of relativity and, as it turns out, the "law" of gravity.
Theories do have a habit of becoming "laws" when they are basically considered irrefutable. They shouldn't--we should probably refer to gravity as the theory of gravity, and the laws of thermodynamics as the theories of thermodynamics. It might stop the "big bang theory" and "theory of evolution" rhetorical nonsense we've all been subjected to by communications majors coasting through college with a "C" average only to become network anchors...and help all of us to think clearer. That having been said, I imagine my calls to refer to the laws of thermodynamics as the "theories of thermodynamics" would fall on my old physics professor's deaf ears. Most of us like keeping our language the way it is, no matter how cumbersome or confusing it becomes--but that's a rant for another day.
The Future of Human Evolution: Autonomy
For those that are interested in this, Jacob Bekenstein (the author of the first relativistic MOND paper ~2 years ago) has a paper on the preprint server today about the possible measurable effects of MOND in the solar system.
I couldn't tell if you were experimenting with poor-man's cryogenics or looking for the orange sherbet.
You yourself seem to think that killing an enemy soldier is murder, but also agree that murder is "The unlawful killing of one human by another, especially with premeditated malice". The whole idea of a soldier being an enemy soldier is that you are at war, and therefore are legally allowed to kill the enemy (unless they surrender)
Nonsense. Murder is wrong, unless we call it a "war", and makes it okay? No. It is *always* wrong to initiate the use of force against another person. The only time that the use of force is justified is to defend oneself or another against a person who has initiated the use of force.
Downmodding is the refuge of the weak. Don't downmod, make a better argument!
Not quite right. It's not that gravity is more long-range - both electromagnetism and gravitation have theoretically infinite range. The chief reason why the Universe is dominated by gravity and not by electromagnetism is because there's no such thing as a South Gravitational Pole, or a Negative Gravitational Charge.
Electromagnetic forces, taken as a whole, tend to cancel out because of this - although electromagnetism is enormously stronger than gravity, the attraction of one charge tends to cancel out the repulsion of the other, for a net force of pretty near sod all. Gravitational forces add up, because gravity is always attractive.
Real Daleks don't climb stairs - they level the building.
The problem is that as a result physicists really, really like very elegant theories when there's no particular reason to believe that the Universe itself has the same bias. Similarly, we like to take theories that work on scales and locations that we know and can easily interact with, and assume that they smoothly apply in the places that we can't get to know quite so easily. It's reasonable even if it isn't logical - we have to go with what we already have. It's a decision born of practicality.
In Cosmology, there's even a phrase for this: we assume homogeneity and isotropy. That is, that there's nothing special about where and when we are, and that the universe is pretty much the same (in physical laws) everywhere. The first time I heard about "dark matter," it was in the context of closure of the Universe. Physicsts really really wanted the universe to have enough mass/energy in order to be "closed," but we simply weren't finding enough matter. There was no reason to believe that the universe is closed (curvature 1.0), but it just seemed more elegant. So, they started to look for the "missing mass."
These are not logical assumptions, they're just assumptions that we have to make in order to get anywhere. Again, there's no reason that the universe will cooperate on this matter.
My own bias is to reject dark matter in favor of a revised theory of gravity, but that's just my own love of elegance - a different gravity feels more elegant than dark matter and dark energy, and in fact would hint at much more interesting cosmologies. But that's just how I am seduced by elegance...
"It is our blasphemy which has made us great, and will sustain us, and which the gods secretly admire in us." - Zelazny
'Dark Matter' really describes the problem more than anything. I don't think that when the solution is found anyone will refer to it as 'Dark Matter' any more.
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The article seems a good general summary of the subject area. If you want to get at the technical details of the new theory, now you know about it you can go and read the paper itself, which they've handily linked to in the second paragraph.
Why don't the theorists stick to explaining what can actually be observed and measured, instead of making up stuff in order to prop up theories that have more likely found their limits.
Um, yeah, that's what they're doing. We have [b]observed[/b] and [b]measured[/b] things that the existing theory of gravity doesn't explain, so the theorists are trying to develop new theories to explain it. The ultimate goal being to craft a new theory that can make predictions which further [b]observations[/b] and [b]measurements[/b] can either falsify or verify. If the data verifies the predictions, then we have a good theory -- until we make an observation that the theory does not predict.
If you think "making up" dark matter to explain observations within the constraints of Einstein's theory is silly, what would you have thought about "making up" the concept of masses warping space-time to fix problems with Newton's theory?
The enemies of Democracy are
I agree with the grandparent, and disagree with you.
There's no governing body of scientific terms, but I've seen many proposed laws with no prior history of being called a theory. In my physics experience, laws are almost always a mathematical model of observed behavior with no attempt to explain the underlying reasons or mechanics of said behavior.
Laws are theories as they fit all the definitions of a theory, but they don't become laws by extra proof, rather by their initial limited nature. For example, there is a law of gravity ( F = G Ma Mb / r^2 ) and there are separately various theories of gravity such as general relativity.
-Ryan C.
Dark Matter exists, and in my opinion it is here for good. The attempts to come up with alternative theories of gravity are quite noble, but they only work on certain scales, and the proponents of these theories sometimes neglect examples that invalidate their theory. It would be quite elegant to be able to account for dark matter via a modification of gravity alone, but I am afraid that it will not be possible.
One of the most compelling pieces of evidence for the existence of dark matter is the "bullet cluster of galaxies" discovered by Maxim Markevitch and collaborators. Their 2004 peer reviewed article shows a small cluster of galaxies passing through much more massive one. As the cluster passes through, its gas is stripped, but the dark matter stays behind, detected via weak gravitational lensing. This effect is impossible to reproduce using alternative theories of gravity, because there is a visible separation between the total mass peak and the observable mass peak.
There are dozens of other peer-reviewed articles that argue against these alternative theories of gravity. What about the cosmic microwave background? The CMB is one of the underpinnings of modern cosmology and basically made the big bang the widely accepted theory that it is today. This recent analysis of the CMB show that the kind of alternative gravity proposed here is strongly disfavored by the CMB spectrum, and that it would imply too high a neutrino mass.
I challenge you to look through the literature for yourself. Here is a list of papers discussing modified newtonian gravity and its derivatives... You will find that yes, these alternative theories do work quite well at describing the rotation curves of galaxies, as TFA suggest. But on larger scales, such as in cluster of galaxies and the cosmic microwave background, they seem to fail convincingly.
First, and glaringly....you said:
about scales, from TFA:
A non-Newtonian gravity theory is now fully specified on all scales by a smooth continuous function.
so, this yet to be reviewed theory claims to have overcome your first objection, and you cannot prove them wrong until April.
you said:
ok, so no theory that you have seen can explain gravity better than dark matter without being REALLY contradictory to observations. Yeah, you know what I'm going to say...it is possible this new theory can do what you say it can't...which brings me to:
overall, i think you're wrong when you say dark matter absolutely must exist. Supposedly, this theory can explain gravity in a way that somehow changes predictably on different scales.
IANAA, but judging from the new kuiper belt object xena, I think the Oort Cloud may be the beginning of a new understanding of what it is exactly that lies between us and our nearest neighbors...on all scales. I think it's possible we will eventually observe many more such objects. While it may sound as if I'm supporting a dark matter theory, no...I am merely stating that neither dark matter nor this new theory will be the last, simplest theory of gravity. Dr Fameay from TFA would agree:
It is possible that neither the modified gravity theory, nor the Dark Matter theory, as they are formulated today, will solve all the problems of galactic dynamics or cosmology. The truth could in principle lie in between, but it is very plausible that we are missing something fundamental about gravity, and that a radically new theoretical approach will be needed to solve all these problems. Nevertheless, our formula is so attractively simple that it is tempting to see it as part of a yet unknown fundamental theory. All galaxy data seem to be explained effortlessly
Thank you Dave Raggett