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'Dark fluid' with negative mass could dominate the universe

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Re: 'Dark fluid' with negative mass could dominate the universe

#151

Post by nonbel »

acqq wrote: post #141
dnautics wrote: post #135

Earlier quoted context omitted.

If you have an infinite space of particle parameters and a series of tests keeps eliminating large swaths of this space, at some point it feels like you're playing a god of inbetweens game a la creationists arguing using gaps in the fossil record. There is a categorical difference, of course, but as a practical matter the parallels are useful, at what point do we table (for now) the search for the populist position a…

It's not "populist" anything. It's the best science that can be done: https://medium.com/starts-with-a-bang/five-reasons-we-think-... "Five Reasons We Think Dark Matter Exists: No other idea explains even two of these." "1.) Galaxy Clusters" "2.) Galactic Rotation Curves" "3.) The Cosmic Microwave Background" "4.) The Bullet Cluster" "5.) Large-Scale Structure Formation" From these 5, only dark matter matches all . W…

>"only dark matter matches all."

Dark matter can be placed arbitrarily so it can match pretty much anything. I remember recently there was a galaxy with negligible deviation of the GR-predicted rotation curve and that was taken as evidence for dark matter too (even though MOND was still the better fit for that case).

"Dark Matter" started out ok but has become a "god of the gaps" argument, I expect eventually MOND or similar solutions will be developed for all those issues.

Also, I gather that while MOND does seem not predict these observations exactly, it is not fully developed yet and often the presence of some "normal" cold gas (undetectable via light) could make it fit. Also, to get dark matter to fit the observations requires post-hoc assumptions and parameter adjustments. Eg:

"The amplitude of the third peak observed by WMAP merely falsifies the simple ansatz I used to make the prediction, not MOND itself. Indeed, I pointed out in the original papers that the ansatz must fail at some level, so I am hardly surprised that it does. All this means is that there are degrees of freedom (like a scalar field) that can oscillate separately from baryons in whatever the relavistic parent theory of MOND might turn out to be. The real test was for ΛCDM: the third peak had to be higher than predicted by pure baryonic damping. ΛCDM survives this test. It "wins ugly" in that, in order to obtain a fit, we have had to nearly double the baryon density over what it was so confidently known to be before it wasn't. That, and the no-CDM prediction for the second peak is still bang on. Amazing coincidence, that." http://astroweb.case.edu/ssm/mond/

>"the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later."

I'm afraid I have never seen this although I have looked into this topic (as a layperson). Can you give an example of this? It would make the dark matter idea much more convincing.

Re: 'Dark fluid' with negative mass could dominate the universe

#153

Post by eutropia »

Nasrudith wrote: post #97
skykooler wrote: post #74

Earlier quoted context omitted.

Wouldn't that allow for a "perpetual motion" setup where a blob of normal mass is in front of a blob of negative mass? The negative mass would push the positive mass forward, and then be attracted to it and follow. What am I misunderstanding?

Screwily to our intuitions negative mass goes in the opposite direction of its acceleration. It still goes away but the acceleration is in the same direction. It is weird and easy to be confused.

I think you meant to say that the mass accelerates opposite to forces acting upon it.

An idealized -M + M = 0 pair of particles (one normal, one negative mass) could experience runaway acceleration. Author touches on this in the paper as a potential source for the incredibly high-energy extra-galactic cosmic rays.

Granted, the paper describes a model in which something that acts like a particle with negative mass exerts this force upon things, but has no claims as to what a physical manifestation of this would look like.

Re: 'Dark fluid' with negative mass could dominate the universe

#154

Post by danbruc »

dragonwriter wrote: post #124
arduinomancer wrote: post #123

Earlier quoted context omitted.

Think of if space were just two dimensions and it was on the surface of a balloon. Space is expanding in the sense of inflating the balloon. Every point on the balloon is getting farther away from every other point. If you stand on the balloon it doesn't make sense to ask "where is the balloon expanding from?". Its expanding in every direction. Every observer on the balloon sees all points around them getting farther…

> If you stand on the balloon it doesn't make sense to ask "where is the balloon expanding from?". Sure it does, and there is a well-defined answer, it's just the answer does not lie on the surface of the balloon. (And for a perfectly spherical balloon, is equidistant from every point on the surface.)

That of course only works because the surfaces of balloons we are used to are always embedded in spacetime. There is however no [good][known] reason to assume that spacetime itself is embedded in any space.

Re: 'Dark fluid' with negative mass could dominate the universe

#155

Post by thaumasiotes »

FabHK wrote: post #137
thaumasiotes wrote: post #102

Earlier quoted context omitted.

> On the other hand, you can show that the inertial mass cannot be negative (else the Lagrangian would not be bounded from below and therefore you could not minimise it; the whole of physics then comes crumbling down). Here's a purely thought-experiment argument for inertial mass not being negative: If you had an object with negative inertial mass, then pushing on that object would cause it to accelerate into your ha…

Here's a thought experiment that shows that there can't be electrons: "If you had an object with negative charge, then putting that object near a positive charge would cause it to accelerate towards the positive charge." Getting closer increases the force. "This is a positive feedback loop that immediately results in the object and the positive charge exerting infinite amounts of force on each other, because" getting…

So, electrons are attracted to protons by the electromagnetic force, but they don't make it all the way there because they are repelled by some other force. You don't have a paradox because our model of the electromagnetic force doesn't specify that it's the only force there is.

But if a negative mass is moving towards you and encounters a repulsive force, that repulsive force will, by definition, move the negative mass further toward you. It would need to encounter an attractive force to reach equilibrium.

Note that, unlike in the electron example, it doesn't matter what kind of force is being applied; the object's negative mass tells us that the response to any repulsive force is acceleration towards the force.

Re: 'Dark fluid' with negative mass could dominate the universe

#156

Post by acqq »

nonbel wrote: post #151
acqq wrote: post #141

Earlier quoted context omitted.

It's not "populist" anything. It's the best science that can be done: https://medium.com/starts-with-a-bang/five-reasons-we-think-... "Five Reasons We Think Dark Matter Exists: No other idea explains even two of these." "1.) Galaxy Clusters" "2.) Galactic Rotation Curves" "3.) The Cosmic Microwave Background" "4.) The Bullet Cluster" "5.) Large-Scale Structure Formation" From these 5, only dark matter matches all . W…

> "only dark matter matches all." Dark matter can be placed arbitrarily so it can match pretty much anything. I remember recently there was a galaxy with negligible deviation of the GR-predicted rotation curve and that was taken as evidence for dark matter too (even though MOND was still the better fit for that case). "Dark Matter" started out ok but has become a "god of the gaps" argument, I expect eventually MOND o…

> Dark matter can be placed arbitrarily so it can match pretty much anything.

Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact. That's why the alternative theories can't come even near: once the same rigor which was applied to test dark matter is applied to the alternatives, the alternative theories simply break.

> I'm afraid I have never seen this ... Can you give an example of this?

I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more. For example, even on the MOND page that you linked you can read:

"2013: ΛCDM provides an excellent fit to the improved CMB data just as MOND provides an excellent fit to improved rotation curve data. As before, one's interpretation depends entriely upon which you find more impressive."

Note: "rotation curve" from that quote is the "item 2" from my posts before. So, no, still the match of the single "item 2" is definitely not "more impressive." The "CMB" mentioned on your page is the "item 3" from all 5 which are explainable by ΛCDM (and specifically, ΛCDM is the model part of which the "dark matter" is: https://lambda.gsfc.nasa.gov/education/graphic_history/univ_... see there 4 and 5).

Edit: response to your later response below:

> The claimed predictions from the 1970s and 1980s are not mentioned on that page.

They are mentioned even on the very page that you linked to: that page, among other points, followed the history of the tests and effectively concludes that, yes, finally, in 2013 ΛCDM wins the "item 3". Read carefully. Learn what ΛCDM is. MOND can still only claim "item 2." Which actual "authority" can show anything else? Also, behind "darkmattercrisis" blog are just a few guys with very fringe positions.

Re: 'Dark fluid' with negative mass could dominate the universe

#158

Post by FabHK »

I've personally verified this part of the paper:

> Observations clearly indicate that the Universe is not empty.

EDIT to add: Another nice, slightly Douglas-Adams-y quote:

> This implies that our Universe is just one of those things that happen on occasion, and we can simply think of its existence as being illustrated by a 1 billion-σ statistical event.

Re: 'Dark fluid' with negative mass could dominate the universe

#160

Post by thaumasiotes »

wallace_f wrote: post #139
staplers wrote: post #127

Earlier quoted context omitted.

That's assuming your hand has negative inertial mass.. which I'm assuming it doesn't. Your scenario could create a black hole if I'm understanding correctly. Enough 'negative inertial mass' trapped in a feedback loop..

If inertia is resistance to change then negative inertia would be promotion of change? I really dont know, the idea of negative inertia seems wildly counterintuitive.

Inertia isn't "resistance to change". It's defined by Newton's second law, F = ma. Force and acceleration are directional vectors, and mass is scalar.

In the form of the equation favored by my high school physics teacher, a = F/m, we see that when a force is exerted on an object with positive mass, the object will accelerate in the same direction as the force, in inverse proportion to its own mass.

But it's very easy to put a negative number in for the mass, and then you see that such an object would accelerate in response to force just as much as an object of equal positive mass, but in the opposite direction.

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