131–140 of 302 posts
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The word 'dark' is used in titles because the articles are about the nature of dark matter and dark energy, which are widely accepted terms. You would have to go out of your way not to use the word dark when talking about these subjects and it would make titles less descriptive.
Well, isn't that just the most helpful, rational explanation? Thank you so much for disagreeing, and almost changing my mind.
Forgive the naive take on this, but it's a weird point to me... if air bubbles in water can be modelled as having negative mass, but clearly do not (they are made of particles with positive mass), then they are only "negative" in relation to the material surrounding them. So even if they can be modelled this way successfully, clearly the model doesn't imply for certain that they do have negative mass. So by analogy, just because a negative-mass model can explain the observations of dark matter, doesn't it still leave the question of whether anything "real" is implied by it? Perhaps it's just a reflection of a negative relative to some unknown bias (field), like the Higgs field, but in "absolute" terms is not actually negative.
(Not sure "absolute" has any meaning here..)
Reminds me of this (very old) theory of gravity: https://en.m.wikipedia.org/wiki/Le_Sage's_theory_of_gravitat...
If you put a negative mass and positive mass side by side and release from rest, they will accelerate indefinitely and break conservation of energy. Of course science should be open to any possibility, no matter how strange. But I feel the author understates its strangeness. And requiring negative mass to be continually created makes this theory not all that much "simpler" than the standard dark matter+dark energy.
"Firstly, the theory of positive–negative mass particle pairs provides clear rules that govern such interactions. The mechanics of these interactions are governed by the usual physical laws: the conservation of energy and momentum remain fundamental, and hence it is unclear why we should object to this potentially physical law of nature on grounds of aversion alone. Secondly, and more importantly, observations provide evidence for significant numbers of ultra-high-energy cosmic rays which are known to be extragalactic in origin, although the mechanism of their production remains a mystery (Pierre Auger Collaboration 2017). From this perspective, runaway motion is not a challenge for negative mass models, but is rather a useful observational constraint. The idea that all negative masses in a universe should form gravitational dipoles and accelerate to high energies is not supported by the simulations presented here (which have a limited number of particles), in which no runaway particles can be identified. While runaway motion is a legitimate physical facet of negative mass particle interactions, the simulations indicate that this behaviour is only common for idealised particle pairs and occurs more rarely as a bulk behaviour within a negative mass fluid. This is likely as the particles in such a fluid are subject to numerous counteracting forces from the surrounding medium. One can assume that some amount of runaway particles must still exist, although these would likely be highly scattered by Brownian motion (e.g. Landis 1991)."
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We haven't observed any of the predicted WIMPs at the LHC, as one example. They were subsequently relegated to a higher energy level.
That doesn’t disprove the “matter-like” observed behavior, it just shows that the particular assumptions under which exactly these non-found WIMPs were imagined were false.
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> raises lots of interesting questions Exactly. What I liked about MOND was that it basically amounted to "we don't know." The populist notion that dark matter is conclusively matter, and dark energy is energy seems a bold claim for such a young species. Crazy theories like non-universal constants are probably crazy, but they do explore unconventional lines of thought. One of those, most likely an unknown unknown, ma…
> that has been falsified more than once What has been falsified more than once?
Our math says the universe comes up short on mass by over 3/4ths? Must be invisible matter we can't see or interact with see, the models still good!
Over simplification in the extreme, but also a far cry from the rock solid foundations it claims to be.
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Here's an argument: If you think about it, "mass" actually means two different things. One one hand, there is mass as the "charge" of gravitational interaction, that is, the gravitational field generated by a body is proportional to its mass. Let's call this concept "gravitational mass". On the other hand, you have mass in the sense of "intertial mass", a body's resistance to change of velocity. Although we kinda tak…
> 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…
"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 closer "just leads to applying more force."
Thus, there are no electrons.
So, I don't think the thought experiment rules out negative mass. In fact, the article explicitly says "if a negative mass was pushed, it would accelerate towards you rather than away from you.", and I assume that if that's an obvious contradiction, the author would have caught it.
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It most definitely isn't. https://en.wikipedia.org/wiki/Nuclear_force
Apologies, you are correct, jumped the gun a bit there. It's repulsive at short distances, attractive at longer distances. And oddly, the longer the distance, the stronger the force, it doesn't follow the inverse square law!
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> 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…
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..
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Please post links to the arXiv abstract pages, everyone: we don't always want to download a PDF just to read the abstract! https://arxiv.org/abs/1712.07962 Edit to add: the abstract pages also often contain useful extra information, such as (in this case) a link to YouTube videos of the simulations -- https://www.youtube.com/channel/UC8ltFtaETXDphec0l-VxMsg -- or the publication history (in this case, a DOI is given)…
I am of the opposite opinion. Don't post links to the abstracts as the pdf's contain them too. But the pdf's also contain figures and other things which make it easier to browse, whereas the arXiv abstract page contains a lot of unnecessary fluff.