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."

So We Must Wait.

[Mikkeles]

Basically, then, until the mass of the neutrino has been tested, dark matter or alternate gravity are just speculations with the arguments being:

is too!
is not!
is too!

...

Re:

[God'sDuck]

no they're not!

It's the Ether

[Fysiks Wurks]

Dark Matter is the 21st century's ether.

Re:

[RangerRick98]

Does that mean I can take dark matter to recover my MP?

Black magic woman

[tepples]

Does that mean I can take dark matter to recover my MP?

Only if your black magic comes from the dark side of the Force.

(Got a female illusionist of African descent?)

Re:

[Fordiman]

^_^

Very good analogy. Much better than BadAnalogyGuy's.

Re:

[SixByNineUK]

No, Gravity waves are the 21st century ether, Dark matter is the 21st century phlogiston

Now that's really confusing

[H3lldr0p]

Are you saying that Dark Matter is why we have fire?

Re:

[Lord_Slepnir]

I like, know what you mean. Last night I was huffing Dark Matter, it was like I was on FIRE man. I went tango dancing with Stephen Hawking and then we watched Charles Darwin and Al Sharpton settle the whole Intelligent Design vs. Evolution thing once and for all by having a hot dog eating contest.

That dark matter stuff is better than ether, man.

Re:

[Stalyn]

You mean aether. The big difference between the luminiferous aether and dark matter is that there was never any evidence of the aether however there is lots of evidence for dark matter, albeit indirect.

Re:It's the Ether

[dmatos]

To the scientific understanding at the time, there was evidence of the aether. It had been observed that light exhibited wave-like characteristics, and could, in fact, be understood as a wave. At that time, all waves were known to travel through a medium. There were no waves that could travel without one. There was no other medium in the vacuum of space, so it was decided that there must be an aether.

A perfectly valid scientific theory, as it was also falsifiable - as demonstrated by Michelson and Morley. When it was falsified, it required a major change in how the scientific communtiy thought about light. It is entirely possible that we'll see something similar with dark matter. Sure, an unobserved WIMP could explain things like the rotation of galaxies at their current rates. But, what happens when we get out there and don't find any? What then? Well, maybe it will require a major change in how we think about gravity. Maybe there's an entirely new force out there, that's weak enough that we can't see it on terrestrial or even solar scales. Who knows?

Re:

[Stalyn]

The categorical difference is that the aether was created to fit into a model, 'all waves were known to travel through a medium'. Dark matter was created out of a need to explain empirical evidence. There is no real model for Dark matter and we don't know why it exists. Some say that the Standard Model could explain dark matter but we really don't know.

WIMPs are just one possible candidate for cold non-baryonic dark matter. WIMPs could be wrong. Like you said it could be something where we have to change th

Re:

[lubricated]

Dark matter was created out of a need to explain empirical evidence because the motion of the galaxy did not fit our model of gravity. Same thing really.

Re:

[ceoyoyo]

We talk about the aether all the time today, just in relation to gravity, not electromagnetism. We usually call it "the fabric of space" or some such today.

Technically, neutrinos are dark matter

[Anonymous Coward]

They're just proposing that there is no "exotic", new kind of dark matter.

Incidentally, I'd watch the Cosmic Variance [cosmicvariance.com] 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.

Re:

[phirzcol]

but,,, when looking at electron even with their small mass i ask what is the mass of every electron ever traveling in space since the start of the universe also what about the expansion of the universe is their a crest of mass at the "edge" of the universe, perhapse the extra mass is only reflected to us as a result of the distortion that exists as a result of the expantion of the universe, ie light that we see is old so a possible explanation is that there is a fraction diffrence between the speed of light

Mod Up

[spun]

From reading the summary, this is my understanding of what these guys are saying, too. Call it modified MOND.

Dark Matter Lite!

[Rob T Firefly]

This article presents an alternative to dark matter

Just as dark as your regular matter, but with only 1/3 the calories!

Re:

[Chacham]

>>This article presents an alternative to dark matter
>Just as dark as your regular matter, but with only 1/3 the calories!

Heh.

Of course, there's alma [m-w.com] matter, which is just plain wet, eh. But, being full of DiHydrogen Monoxide (DHMO) [dhmo.org] which some want to ban [netreach.net] but other defend [armory.com].

Use the chain rule Luke

[sweetser]

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

Re:

[ObsessiveMathsFreak]

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

What the hell?! Where did that d/dR come from? "Relativistic swap-out"? And why are those total derivatives? I fear your methods are .... unsound.

Re:Use the chain rule Luke

[sweetser]

The relativistic 4-force law is d mV^u/dtau, where V^u has four places, and tau is the spacetime interval, sqrt(t^2 - R^2). Nothing is going fast, so we get classical laws. In the case of gravity, the road from d/dtau goes to d/dt. Simple, and standard enough.

d/dtau is asking about changes with respect to spacetime intervals. We know the changes with respect to time work for our little solar system. What I am suggesting is that a change in spacetime may in the classical limit also be seen as a change in space. That would require c*d/dR to have the same units.

There are limits to what can be done in ASCII, so they appear as derivatives.

There is nothing radical about the V dm/dt, which people sometimes mention does not amount to squat. There is nothing radical about saying the "truer" force law must be a 4-force law. There is nothing wrong with the units in the switch from d/dt to c*d/dR. Don't worry, I do think it is a darn strange thing to do, but the data is forcing us in an odd direction, and at least the math here is far more constrained, as there are NO new factors or mass distributions, just relativistic rocket science.

Clarification: dark matter is STILL real!

[jpflip]

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...".

Re:

[radtea]

Many people (particularly those who do not understand the evidence) dislike the idea of dark matter, thinking it sounds too much like epicycles

Some of us were skeptical because we did understand the evidence, and understood that extraordinary claims require extraordinary evidence.

Galactic dark matter is something (almost) everyone believes in, because the evidence is compelling and it does not require additional, and thus far quite undetectable, particles.

Dark matter on larger scales, which is completely un

Re:

[sholden]

Please show me some.

And no just because there's a gravitational lensing picture that implies there is mass where we can't see it isn't good enough. Maybe we don't understand gravitational lensing? Maybe we are completely misinterpreting the images and there's no lensing at all? Maybe those galaxies didn't collide at all and hence there is no "drag"?

Anytime your theories result in the need for magical invisible stuff in larger quantities than the stuff we can observe/interact with it might be time to questio

Re:Clarification: dark matter is STILL real!

[jpflip]

I completely agree - as a previous poster said, the extraordinary claim of invisible stuff requires extraordinary evidence. We definitely need to question our assumptions about gravity, since they are the foundation of our reasoning about dark matter.

The thing is, people HAVE been questioning those assumptions for decades. Even with a lot of fancy theoretical footwork, no one has yet managed to explain our observations without assuming that the bulk of the universe's mass is invisible (including the work described in this article!). It's not like everyone has gotten brainwashed by the dark matter gospel and it never occurred to any of them to question gravity. EVERYONE has thought of questioning gravity! EVERYONE has a revulsion for the idea of invisible matter dominating our galaxies. They just haven't had much success in the ultimate test of science - explaining observations.

- Gravitational lensing and rotation curves agree that a galaxy (or cluster) has much more matter than can be accounted for by visible baryons (or even less-visible hot gas), and that the distribution of that matter is much larger than the visible structure indicates.
- Studies of big bang nucleosynthesis and the cosmic microwave background agree that the vast majority of the universe's gravitating matter is non-baryonic.
- The Bullet cluster shows a situation where the dark matter and baryonic matter are segregated from one another, in a way that makes perfect sense with dark matter and stymies MOND-only theories.

Any one of these observations can be explained by modifications to gravity, but it turns out to be very hard to make them ALL work out. I obviously can't say it's impossible, and maybe someday someone will come along and show how it all works. But right now the SIMPLEST theory which fits the facts extraordinarily well says that the bulk of the universe's matter is not visible and interacts weakly (if at all) with ordinary matter.

At a certain point you get so beaten over the head with evidence that you have to (at least tentatively) accept something that sounds crazy at first. Common sense isn't always right...

Re:

[RKBA]

Naive question: Have the astronomers taken into account the mass equivalent of the background radiation left over from the "Big Bang"?

P.S.
You can send my Nobel prize to ... ;-)

Re:

[jpflip]

Yes, they have. We know the temperature of the background radiation filling the universe, and so we know the energy density this represents. It turns out to be completely negligible at the present day, a few orders of magnitude less than the matter density.

Re:

[ByteSlicer]

Nice try :P
Of course, since background radiation are photons which move at the speed of light, they have no rest mass, and their mass equivalent doesn't interact gravitationally with other masses. Also, measurements show that there is very little variation in cosmic background radiation, meaning that these photons don't clump together, and thus don't interact gravitationally.

Re:

[thePig]

IANAP, so just wondering;
Why should we consider that all the gas should be hot?
My question is such - Suppose the galaxy contains the remnants of lot of white dwarfs etc, wherein most of the elemental hydrogen has been converted to iron. Now, there is no more fusion possible, and because of which the gas, even though grouped together - stays as that - just a group of iron particles grouped by gravity, which is not hot enough to emit radiations.
Since there is no fusion, it wouldnt emit any rays, and if it is

Re:Clarification: dark matter is STILL real!

[jpflip]

That's an excellent question! You've just described the MACHO model of dark matter (Massive Compact Halo Objects). The idea is that there could be cold, compact objects made of ordinary matter filling our dark matter halo and giving us all the extra mass. In this theory, the invisible mass exists but is still "ordinary" - no modification to gravity and no fancy new particles. It contrasts with the more exotic WIMP model of dark matter - Weakly Interacting Massive Particles.

This theory was extremely popular for many years, but has fallen out of favor for two main reasons:

(1) Using studies of the cosmic background radiation and light element abundances, you can conclude that the bulk of the matter in the hot early universe was not made up of baryons ("ordinary matter"). If it were, you would expect very different abundances of deuterium in the universe today and a very different spectrum of fluctuations in the microwave background. So we need most of the universe's matter to be non-baryonic anyway (e.g. WIMPs), and baryonic MACHOs cannot make up all of the missing mass

(2) Every now and then a MACHO should pass in front of a distant star (say, in the Large Magellanic Cloud), producing a "micro-lensing" event. Many collaborations around the world studied the skies for years looking for such events, and did find a few. The number and kind of lensing events they observed, however, was insufficient to account for all of the missing mass.

For these and other reasons, the cosmological community has rejected the MACHO hypothesis. There are objects like that out there, but the bulk of the dark matter must be something else.

Re:

[amliebsch]

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.

Oh my, yes. You have your ordinary gravitons, which are mostly harmless, and then you have your graviolis, a neutrino on the outside with a rich, meaty graviton embedded inside of it.

Re:

[jpflip]

Amusing, since I still look for dark matter in a mine :) People are flowing into the field in general, though I think they may be flowing out of it in Britain due to various difficulties there. They are difficult experiments and the reward may be long in the future (if at all).

There is enough evidence now that I would call dark matter the simplest explanation for observed phenomena, not just an epicycle (though it will take a long while before we can answer that question for dark energy). I agree that we

KATRIN experiment homepage URL

[tobyvoss]

KATRIN experiment homepage URL
http://www-ik.fzk.de/~katrin/ [www-ik.fzk.de]

Prudence

[Digitus1337]

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.

simple definitions

[peter303]

"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.

From someone who just finished *reading* the paper

[Anonymous Coward]

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...

Re:

[DoubleEdd]

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.

Re:

[SixByNineUK]

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.

Re:why would matter be dark

[SigILL]

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.

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.

Re:

[sco08y]

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 t

Re:why would matter be dark

[RsG]

Quoth wikipedia right back at you:
http://en.wikipedia.org/wiki/Light_sail [wikipedia.org]

The spacecraft deploys a large membrane mirror which reflects light from the Sun or some other source. The radiation pressure on the mirror provides a minuscule amount of thrust by reflecting photons.

There, see? He was right about solar sails.

You were thinking of a magnetic sail:
http://en.wikipedia.org/wiki/Magnetic_sail [wikipedia.org]

Which is something completely different. Magnetic sails do use solar wind; solar sails use sunlight. Big big difference between the two.

Re:why would matter be dark

[Jhan]

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...

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".

No mass for photons

[jpflip]

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.

Re:

[roystgnr]

In our current understanding, photons are forbidden from having mass

Technically aren't they just forbidden from having a non-zero "rest mass"? They still have energy, and that energy is still convertible to mass and still generates a gravitational field like mass. There are still reasons why photons can't directly account for the effects of dark matter, but it's not an intrinsic impossibility.

(I'd say photons can't indirectly account for the effects of dark matter either, but that would be tempting fate;

Re:

[gilroy]

Blockquoth the poster:

Technically aren't they just forbidden from having a non-zero "rest mass"?

Modern formulations recognize only rest mass. The concept of "relativistic mass" (that increases as you speed up) leads to a minefield of misconceptions stemming from the application of Newtonian connotations of "mass".

Of course, for any amount of energy, you can compute the equivalent mass by dividing by c^2. But that is, at best, a shorthand. The real issue is, gravity is generated by energy (of any type) an

Re:

[eggoeater]

...or that something has to be massive to feel gravity, both of which are false.

I agree with you in that photons have no mass and you don't have to have mass to "feel" gravity.

However, I've always heard/read that the reason gravity "attracts" non-massive objects (eg photons), is because gravity is bending space itself.
i.e. You can say that a photon is traveling in a "curved path" because it is "attracted" or "pulled" to a near-by massive object.
But isn't it more correct (in terms of general relativit

Re:

[jpflip]

However, I've always heard/read that the reason gravity "attracts" non-massive objects (eg photons), is because gravity is bending space itself. i.e. You can say that a photon is traveling in a "curved path" because it is "attracted" or "pulled" to a near-by massive object. But isn't it more correct (in terms of general relativity) to say that the photon is traveling in a straight line and it's space that's "curved"?

Agreed - that's exactly how general relativity works. Particles (including photons) foll

Re:

[Fordiman]

"If objects lose mass when they emit photons, where does it go?"

Well that's not always true.

In a star, it loses mass which is converted to the energy of the photon. In a lightbulb, the photon's energy comes from the covering of a potential difference in voltage.

The thing is, while E=mc^2 and E=hw (E is Energy; m is mass; c is speed of light; h is planck's constant; w is angular frequency [similar to frequency, but in radians per time unit]) state that more energetic photons have more apparent mass than non

Re:

[roemcke]

When you shut the light off, the light bouncing back and forth would keep pushing.

After each "push" the photons will not be reflectet with the same frequency (the sails are moving away from the light. hint: doppler effect)

Re:

[Austerity Empowers]

Ah, let the transportation safety board try to stop be from boarding a plane and giving photons mass! JUST LET THEM TRY! MUAHAHAHAHA!

Re:

[jpflip]

OK, so this is an issue of nomenclature. When a physicist says "mass", he essentially always means "rest mass".

The distinction comes from the full version of Einstein's famous equation: E^2 = (p*c)^2 + (m*c^2)^2, where p is the momentum of the particle and m is its "rest mass". This means that a particle can get energy from two places: energy of motion (the momentum term) and an intrinsic minimal energy (the particle's rest mass). A massless particle (like a photon) still has energy, but its energy is ju

Re:why would matter be dark

[SixByNineUK]

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.

Re:

[eonlabs]

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?

Re:why would matter be dark

[Claws Of Doom]

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.)

Re:

[trewornan]

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.

Re:

[Fordiman]

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?

Re:why would matter be dark

[Jerf]

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?

The formulas work for all mass-energy, which photons possess. Photons thus do technically interact via gravitation. However, if you do the math, you'll find that the interaction is very, very small, to put it lightly, so while it is technically wrong to say "photons don't interact with each other", it isn't very wrong.

Somewhere in the great online book Reflections on Relativity [mathpages.com], 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.

Re:

[Physics Dude]

Do high-energy (ie: high mass) photons have a gravitational effect? Or do the formulae only work given a rest mass?

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.

Re:

[Gilmoure]

So...when people slow down light [msnbc.com], does it gain mass?

Re:why would matter be dark

[jfengel]

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.

Re:

[jfengel]

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.

Re:

[Orange Crush]

Gravity bends spacetime so mass isn't required to affect light's path around a gravity source.

Re:

[eln]

"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.

Re:

[dantastic3000]

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.

Re:Anti-dark-matter scientists are like ID scienti

[denominateur]

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.

Re:Anti-dark-matter scientists are like ID scienti

[timster]

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.

Re:

[geoffspear]

Hydrogen and helium molecules also produce a "truly tiny" amount of gravity. Therefore, it's impossible that the Sun could actually be exerting enough pull on the Earth to keep it in orbit.

Wait, there might be a flaw in the logic here somewhere.

Re:Anti-dark-matter scientists are like ID scienti

[cp.tar]

I would have replied to all the inconsistencies... then I noticed the name. Not a bad troll.

Re:Anti-dark-matter scientists are like ID scienti

[SteveAyre]

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.

Re:

[SteveAyre]

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.

Re:

[SteveAyre]

(of the electron variety anyway)

Re:Anti-dark-matter scientists are like ID scienti

[Fordiman]

Um.

Google "electron volt in amu":
1 electron volt = 1.07354412 × 10-9 atomic mass units

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.

Re:Anti-dark-matter scientists are like ID scienti

[FhnuZoag]

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.)

Re:

[spun]

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.

Re:

[account_deleted]

Comment removed based on user account deletion

Re:

[timster]

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,

Re:

[dc29A]

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 [wikipedia.org] 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 [wikipedia.org].

Dismissing dark matter as purely theoretical is shortsighted IMO.

Important Note

[dmatos]

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 ther

Re:

[ajs]

Dark matter is a theory not because we are sure it's there, but because some scientist can't imagine any other explanation

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

Re:

[account_deleted]

Comment removed based on user account deletion

Re:

[opaqueice]

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 t

Re:Anti-dark-matter scientists are like ID scienti

[kestasjk]

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.

Re:Anti-dark-matter scientists are like ID scienti

[vondo]

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.

Re:Anti-dark-matter scientists are like ID scienti

[stiggle]

The sweaty fat kid at college had loads of mass and no one interacted with him.
Perhaps neutinos are similar. :-)

Occam's Razor != Science

[pavon]

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.

Re:Anti-dark-matter scientists are like ID scienti

[ajs]

But these guys would have you change the Theory of Gravitation because they can't grasp that maybe there are weird states of matter that exist just outside our physical grasp.

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 i

Re:

[grasshoppa]

For me, dark matter is like religion. Made up to explain what we can't understand, and wrong.

Interesting observation, if a bit off. The difference being, of course, that we will eventually have a factual basis for dark matter ( whether it exists or not ), where as we will never know if $deity exists.

This is true for all supernatural values of $deity.

Re:

[Daniel_Staal]

The famous equation: E=MC^2 converts to M=E/C^2. For truely tiny masses, that's the easiest way to measure and specify them.

Re:weighs 2eV?

[FhnuZoag]

Mass, man, mass. It's not weight, but mass.

After relativity and E=Mc^2, physicists have preferred to measure mass in terms of energy rather than silly units like grams and ounces. In short, we give the energy equivalent for the particle if it was somehow completely annihilated. 1 electron volt refers to the energy of one electron under an electric field at a point of 1 volt of potential difference. It doesn't have anything to do with the actual mass of the electron, but rather the electrical potential energy such an electron would hold.

Re:

[Fordiman]

Electron Volt [wikipedia.org]

Re:

[kakapo]

The electron volt is a measure of energy -- the amount of energy needed to move an electron through a potential difference of one volt in an electric field. (Think of it as moving a small ball up a hill). Thanks to "E=mc^2" this is also a measure of an equivalent mass -- and it is frequeuntly used to specify the masses of subatomic particles. (For comparison, an electron "weighs" about 500,000 eV -- even by particle standards, 2eV is very small)

Re:

[trongey]

I stand corrected. Made a fool ... *departs in shame*

Don't feel bad, you were incorrectly corrected.
If eV is being used as a unit of mass then a neutrino still can't "weigh" 2eV. It would have a mass of 2eV. So you were right after all.

Re:

[ceoyoyo]

The eV is not a measure of charge. The eV is a measure of energy, which is equivalent to mass. Charge is measured in units of proton charge: electron = -1, proton = 1. That used to be considered the fundamental unit until we discovered quarks, which have charges that are multiples of +-1/3.

Re:

[PakProtector]

Can we get a -1, Laypersonspeakify mod?

Re:

[caryw]

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! [fairfaxunderground.com]

Re:

[ThePhilips]

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.

Re:Useful research

[Daniel Dvorkin]

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.

Re:

[ThePhilips]

> 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

Re:

[ThePhilips]

Hear about those scientists doing research into this electricity thing?

Read on. [wikipedia.org] 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. arguin