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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

#251

Post by FiatLuxDave »

agapon wrote: post #229
agapon wrote: post #227

I recall reading article in a popular science magazine about hypothetical negative mass with exactly these properties. It was about 25 years ago and the magazine was Russian language. Unfortunately, I can't recall its name now.

Oh, thanks to the power of the internet search, I found a scan of that article: http://epizodsspace.airbase.ru/bibl/tm/1990/10/otrits-massa.... The journal is "Техника-молодежи" (a Soviet style name that I am incapable of translating to English) and the article is from 1990. And that article refers to a publication by Robert Forward in a journal with a name like Aerospace Technology or Aerospace Engineering. But I fo…

Thank you! The first thing I did when coming to this thread was search for posts containing "Robert Forward", as any discussion of negative inertial mass is incomplete without mentioning his work. So, kudos.

Anyone interested in the implications of negative mass should probably check out Timemaster, a sci-fi novel by Forward. Negative mass can hypothetically do some interesting things. Things such as allowing lightspeed travel by matching negative mass to positive mass in equal amounts, leaving a non-photon object with rest mass = 0, or producing energy by accelerating negative mass to high velocities.

Unfortunately, Forward is much better as an idea guy than he is as an author. The scene where the hero travels back in time and has a threesome with himself and his wife is a bit awkward...

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

#252

Post by btilly »

drjesusphd wrote: post #236
btilly wrote: post #182

Earlier quoted context omitted.

A different hand-wavy explanation. The places that an electron can be found are described by a wave pattern. If the electron is staying in place around a nucleus, that wave pattern has to be a standing wave that reinforces itself. To reinforce itself it has to wrap around the nucleus an integer number of times. This explanation doesn't just explain why it doesn't fall in, it also explains why there are discrete shell…

This is the Bohr model of the atom, but was replaced by a proper quantum theory.

Yes. Which is why I called it "a different handwavy theory". And also listed some of the things that it didn't explain.

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

#253

Post by zamalek »

dmurray wrote: post #165
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…

> 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. Or you're getting closer to finding out what the real parameters are? If you eliminate all possibilities and have to make up some new parameters, sure, that feels like yo…

> Or you're getting closer to finding out what the real parameters are?

By that metric, epicycles were rock-solid.

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

#255

Post by danbruc »

cbzbc wrote: post #247
raattgift wrote: post #244

Earlier quoted context omitted.

> Space expands everywhere, also right in front of you, it is just a very small effect over short distances ... .07 nanometers per year but two points one kilometer apart move away with 70 nanometers per year because there are a thousand meters in between them No, what you wrote is contradicted by evidence from within the inner solar system (MESSENGER's observations of solar mass loss were highly sensitive; we have e…

I don't think this is correct: "Grossly, 0.07 nanometers per metre per year expansion everywhere would be impossible to hide from solid state physics, and even several everyday polymers and ceramics; an expansion term would have to appear in accurate descriptions." On a small scale, the effects of the expansion are overcome by other forces -- this doesn't mean that the expansion doesn't occur - merely that its effect…

My model was the following. Take the classical rubber sheet, place two marbles on it, then stretch the rubber sheet. The marbles are unbound and therefore expand away from each other with the expanding rubber sheet. Now connect the two marbles with a spring and again stretch the rubber sheet. The rubber sheet expands everywhere just as before but now the spring prevents the marbles from being pulled apart and makes them slip across the rubber sheet.

I can see where this model might be misleading or go wrong, it requires some friction between the marbles and the rubber sheet because all massive things are bound together at least by a tiny force and without friction the weakest imaginable spring would still prevent the marbles from separating. I have to think about this more carefully and try to figure out how badly this affects the model, maybe it's actually a really bad model.

But as far as I can remember, whenever raattgift said I am wrong, I was wrong. So I will reserve some time this evening for trying to work through his answer. I also skimmed the Ask Ethan article and at least at first it sound like what I intended to say, especially the following paragraph. On the other hand I did not notice any of raattgift's points in this article on a first quick pass.

The reason for this is subtle, and is related to the fact that the expansion itself isn’t a force, but rather a rate. Space is really still expanding on all scales, but the expansion only affects things cumulatively. There’s a certain speed that space will expand at between any two points, but if that speed is less than the escape velocity between those two objects — if there’s a force binding them — there’s no increase in the distance between them. And if there’s no increase in distance, that impetus to expand has no effect. At any instant, it’s more than counteracted, and so it never gets the additive effect that shows up between the unbound objects. As a result, stable, bound objects can survive unchanged for eternity in an expanding Universe.

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

#256

Post by marcosdumay »

gpderetta wrote: post #242
starbeast wrote: post #233

Earlier quoted context omitted.

In which case, we seem back to having two things distorting spacetime. Could mass just be the momentum of massless particles going in really small circles such that you can pack them together and call them matter?

Space is distorted by the stress-energy tensor field. Mass is only a component of that.

If you use "mass" to mean the rest mass of something. If you use it meaning the relativistic mass, it's the entire value, not a component.

Yes, using it as the relativistic mass leads to confusing papers and equations, so it's better avoided. Yet, the press does that, what makes everything very confusing for lay people. I should have clarified that the OP usage of the term wasn't optimal (although common on the media he sees), but this following discussion about word definitions seems to be completely missing the point.

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

#257

Post by ravar »

andrepd wrote: post #201
ravar wrote: post #192

Earlier quoted context omitted.

Actually the lagrangian is not minimized, it is merely stationary. An example of this is the harmonic oscillator, the lagrangian is clearly not bounded from below.

Yes, I realised that but I couldn't edit the post anymore :) I mean the action, not the Lagrangian, must have a minimum.

The action of a trajectory is not minimized either, it is usually at a saddle point, maximal relative to some trajectories and minimal to others. The linked paper has a detailed discussion of this.

http://www.eftaylor.com/pub/Gray&TaylorAJP.pdf

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

#258

Post by EthanHeilman »

matte_black wrote: post #203
EthanHeilman wrote: post #191

Earlier quoted context omitted.

Take a net negative mass vehicle, and accelerate it by having it collide with its propellant. Would this be an anti-rocket?

How does it come to a stop, the vehicle gains too much mass?

The mass part of the vehicle could either detach from the negative mass component. Or the vehicle could eject mass to reduce velocity.

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

#259

Post by acqq »

nonbel wrote: post #248
acqq wrote: post #222

Earlier quoted context omitted.

> The claimed predictions from the 1970s and 1980s are not mentioned on that page. They are mentioned, they just aren't elaborated: you have to do your own homework if you apparently "understand" the subject enough to claim your own opinion. A begin of that homework, however, as I've already written, you even have on the initial page that you chose to post, specifically: "2013: ΛCDM provides an excellent fit to the i…

> "They are mentioned" No, they were not mentioned. If they were, you would be quoting it. > '"2013: ΛCDM provides an excellent fit to the improved CMB data" As I've said, that's the "item 3" from the 5 item list, and it's ultimately confirmed by ESA's Planck satellite results in 2013. That is a strong confirmation.' You have to read the entire quote. This "fit" was a post-diction, not a pre-diction: "in order to obt…

What is your support for that claim? Can you quote other sources? I can quote my sources: Here are documented two independent approaches in estimating baryonic density: one is calculations based on the Big Bang Nucleosynthesis D/H ratio and another one is the values measured by the satellites observing the SMB. Note the two decades of similar values using both approaches:

https://lambda.gsfc.nasa.gov/education/graphic_history/baryo...

"There is general good agreement between the recent CMB and D/H+BBN determinations shown in the figure."

I don't see the "double" anywhere there. And the values could be traced to all the sources mentioned there.

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

#260

Post by raattgift »

danbruc wrote: post #255
cbzbc wrote: post #247

Earlier quoted context omitted.

I don't think this is correct: "Grossly, 0.07 nanometers per metre per year expansion everywhere would be impossible to hide from solid state physics, and even several everyday polymers and ceramics; an expansion term would have to appear in accurate descriptions." On a small scale, the effects of the expansion are overcome by other forces -- this doesn't mean that the expansion doesn't occur - merely that its effect…

My model was the following. Take the classical rubber sheet, place two marbles on it, then stretch the rubber sheet. The marbles are unbound and therefore expand away from each other with the expanding rubber sheet. Now connect the two marbles with a spring and again stretch the rubber sheet. The rubber sheet expands everywhere just as before but now the spring prevents the marbles from being pulled apart and makes t…

Your italicized paragraph is a good clue.

I started a reply which was getting too long and technical, with the aim of looking at three prongs. Post-Newtonian elements are at work, and it is easy to fool oneself thinking of the underlying one: metric gravitation and how it manifests within extended objects. (It does; we can see that from the figure of the Earth and its internal structure, and from other roundish celestial bodies.) Where self-fooling is easiest is in thinking (1) in terms of forces, (2) using too large a set of local coordinates or inappropriate coordinates, and (3) studying an object that extends beyond the boundaries of "local".

I have some things to do but will return to your comment and its parent in a few hours with a more detailed comment that hopefully won't stray off into the weeds of technicalities. :D

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