17 ms·

Paper: https://arxiv.org/pdf/1712.07962.pdf https://arxiv.org/pdf/1712.07962.pdf Not a physicist, but my summary of the author's point would be: If you allow

Paper: https://arxiv.org/pdf/1712.07962.pdf https://arxiv.org/pdf/1712.07962.pdf Not a physicist, but my summary of the author's point would be: If you allow for matter with a negative mass and plug it into a n-body simulation. You seem to get what looks like dark matter from first principles. Namely, a halo of non interacting matter around galaxies in just the right "non-cuspy" shape. Their conclusion is that this is worth considering, as almost all mainstream cosmology makes the assumption that mass >= 0, despite the fact mass <=0 doesn't seem to violate any physical invariants we know. The idea is persuasively simple. Every other force besides gravity is polarised, so why not also gravity? It seems interesting to me, but I'm nowhere near knowledgeable enough to know whether it can be easily refuted.

aetherson 8y ago
Is it true that every other forces besides gravity is polarised? There's a repulsive nuclear strong force and an attractive nuclear weak force? EDIT: Partial answer is that there is a three-"color" polarization for strong force. I feel like it somewhat weakens the argument of "everything else is polarized, why not gravity" if the other forces are polarized in unique ways.
- walrus1066 8y ago
  Strong nuclear force is always attractive.
  - galadran 8y ago
    It most definitely isn't. https://en.wikipedia.org/wiki/Nuclear_force https://en.wikipedia.org/wiki/Nuclear_force
    - walrus1066 8y ago
      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!
      - dvh 8y ago
        Why strange, rubber also works like that.
mrfusion 8y ago
Are they pretty sure it’s different than antimatter? Last I heard we weren’t sure if it has negative gravity or not.
- shepardrtc 8y ago
  Matter-antimatter annihilation would produce gamma rays that are detectable. I believe there was a study that went looking for that but didn't find anything.
  - AnimalMuppet 8y ago
    To clarify (presuming I understand correctly): Antimatter might have negative gravity, but dark matter can't be antimatter, because we'd see the gamma rays from the collisions.
    - mrfusion 8y ago
      But if they’re repelling each other and for billions Of years. Why should there be any collisions?
      - AnimalMuppet 8y ago
        Because it's still in places where there's interstellar gas (if this theory is right). And repelling each other gravitationally doesn't work very well at the molecular level.
  - DonHopkins 8y ago
    Maybe there's a matter/antimatter Leidenfrost effect? https://en.wikipedia.org/wiki/Leidenfrost_effect https://en.wikipedia.org/wiki/Leidenfrost_effect
- Pxtl 8y ago
  If it's antimatter why isn't it coalesced into antigalaxies that we would be able to see visibly, and is instead diffused as a dark fluid?
duality 8y ago
"Every other force besides gravity is polarised, so why not also gravity?" Gravity is mediated by a spin-2 gauge boson, the graviton, whereas the other known fundamental forces are mediated by vector (spin-1) bosons. There is an exercise in Peskin & Schroeder (I believe) working out the lack of a gravitation "repulsion" resulting from this difference. That being said, I think this kind of phenomenology is excellent and raises lots of interesting questions. There may be reason to rule out this author's new idea, or maybe rethink an established tenet. That's why science is such an exciting field.
- zamalek 8y ago
  > 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, may be answer. Focusing all our energy on one explanation (that has been falsified more than once) seems like a waste.
  - acqq 8y ago
    “What’s most impressive is that the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made. And yet, even 35 years later, there are no modifications of gravity that achieve the galaxy-scale successes of MOND that also explain these other observations. The best tests of dark matter vs. MOND, which are on large, cosmic scales, have a clear winner and a clear loser.” https://medium.com/starts-with-a-bang/theres-a-debate-raging-over-whether-dark-matter-is-real-but-one-side-is-cheating-d25f28319d70 https://medium.com/starts-with-a-bang/theres-a-debate-raging... In short: looking at everything we can observe and can model, we do know that the “dark matter” is matter. Because we first made predictions (“it should behave as matter”), then confirmed them, many times. And MOND can’t even explain that what’s already observed.
autopoiesis 8y ago
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 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 https://www.youtube.com/channel/UC8ltFtaETXDphec0l-VxMsg -- or the publication history (in this case, a DOI is given).
- nerdponx 8y ago
  I actually started a Firefox extension that made a popup to redirect Arxiv PDF links to the abstract page, but never finished it for some reason.
- 317070 8y ago
  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.
  - FabHK 8y ago
    But from the abstract it's one click to the pdf, while from the pdf it's manual URL editing to get to the abstract.
andrepd 8y ago
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 take it for granted, it's weird that these two concepts coincide, and not something that would be necessarily true a priori. Asserting this equivalence as a principle is the starting point for Einstein's theory of General Relativity, which is one of the most beautiful and thoroughly tested theories in modern physics. 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). Therefore, if that equivalence stands, the gravitational mass also cannot be negative.
- baybal2 8y ago
  Well, even the possibility of inertial mass not exactly equaling gravitational mass on interstellar scale open a big can of worms for physics
- smaddox 8y ago
  What happens if the gravitational mass is the exact negative of the inertial mass for some fraction of matter, though? Does that break GR?
- dogma1138 8y ago
  I’m not sure this stands true when you think about fields. Think of charged particles in a magnetic field and how polarity and and strength of the charge impacts their interaction the strength would do the same thing for both particles while the polarity would mirror their interaction. I need to read the paper a few more times tho.
- thaumasiotes 8y ago
  > 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 hand. This is a positive feedback loop that immediately results in the object and your hand exerting infinite amounts of force on each other, because applying force just leads to applying more force.
monocasa 8y ago
Also not a physicist. AFIAIK the "dark matter & energy might be negtive mass" idea has popped up before but has been dismissed because our current observations show that it's gravitational effect isn't lessened by the expansion of the universe. He addresses this in section 2.3.1 by adding a "creation term" to balance universe expansion with the creation of new negative mass, but that just seems like a dirty hack to me. Albeit, the cosmological constant was a dirty hack too, so that doesn't really necessarily discount anything.
deleted 8y ago
[deleted]

aetherson 8y ago

Is it true that every other forces besides gravity is polarised? There's a repulsive nuclear strong force and an attractive nuclear weak force? EDIT: Partial answer is that there is a three-"color" polarization for strong force. I feel like it somewhat weakens the argument of "everything else is polarized, why not gravity" if the other forces are polarized in unique ways.

walrus1066 8y ago
Strong nuclear force is always attractive.
- galadran 8y ago
  It most definitely isn't. https://en.wikipedia.org/wiki/Nuclear_force https://en.wikipedia.org/wiki/Nuclear_force
  - walrus1066 8y ago
    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!
    - dvh 8y ago
      Why strange, rubber also works like that.

mrfusion 8y ago

Are they pretty sure it’s different than antimatter? Last I heard we weren’t sure if it has negative gravity or not.

shepardrtc 8y ago
Matter-antimatter annihilation would produce gamma rays that are detectable. I believe there was a study that went looking for that but didn't find anything.
- AnimalMuppet 8y ago
  To clarify (presuming I understand correctly): Antimatter might have negative gravity, but dark matter can't be antimatter, because we'd see the gamma rays from the collisions.
  - mrfusion 8y ago
    But if they’re repelling each other and for billions Of years. Why should there be any collisions?
    - AnimalMuppet 8y ago
      Because it's still in places where there's interstellar gas (if this theory is right). And repelling each other gravitationally doesn't work very well at the molecular level.
- DonHopkins 8y ago
  Maybe there's a matter/antimatter Leidenfrost effect? https://en.wikipedia.org/wiki/Leidenfrost_effect https://en.wikipedia.org/wiki/Leidenfrost_effect
Pxtl 8y ago
If it's antimatter why isn't it coalesced into antigalaxies that we would be able to see visibly, and is instead diffused as a dark fluid?

duality 8y ago

"Every other force besides gravity is polarised, so why not also gravity?" Gravity is mediated by a spin-2 gauge boson, the graviton, whereas the other known fundamental forces are mediated by vector (spin-1) bosons. There is an exercise in Peskin & Schroeder (I believe) working out the lack of a gravitation "repulsion" resulting from this difference. That being said, I think this kind of phenomenology is excellent and raises lots of interesting questions. There may be reason to rule out this author's new idea, or maybe rethink an established tenet. That's why science is such an exciting field.

zamalek 8y ago
> 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, may be answer. Focusing all our energy on one explanation (that has been falsified more than once) seems like a waste.
- acqq 8y ago
  “What’s most impressive is that the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later. This is not a case of tweaking the model to fit the data; this is a case of the best kind of science you hope for: where you make predictions, make the observations, and what you see validates and confirms the predictions you had made. And yet, even 35 years later, there are no modifications of gravity that achieve the galaxy-scale successes of MOND that also explain these other observations. The best tests of dark matter vs. MOND, which are on large, cosmic scales, have a clear winner and a clear loser.” https://medium.com/starts-with-a-bang/theres-a-debate-raging-over-whether-dark-matter-is-real-but-one-side-is-cheating-d25f28319d70 https://medium.com/starts-with-a-bang/theres-a-debate-raging... In short: looking at everything we can observe and can model, we do know that the “dark matter” is matter. Because we first made predictions (“it should behave as matter”), then confirmed them, many times. And MOND can’t even explain that what’s already observed.

autopoiesis 8y ago

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 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 https://www.youtube.com/channel/UC8ltFtaETXDphec0l-VxMsg -- or the publication history (in this case, a DOI is given).

nerdponx 8y ago
I actually started a Firefox extension that made a popup to redirect Arxiv PDF links to the abstract page, but never finished it for some reason.
317070 8y ago
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.
- FabHK 8y ago
  But from the abstract it's one click to the pdf, while from the pdf it's manual URL editing to get to the abstract.

andrepd 8y ago

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 take it for granted, it's weird that these two concepts coincide, and not something that would be necessarily true a priori. Asserting this equivalence as a principle is the starting point for Einstein's theory of General Relativity, which is one of the most beautiful and thoroughly tested theories in modern physics. 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). Therefore, if that equivalence stands, the gravitational mass also cannot be negative.

baybal2 8y ago
Well, even the possibility of inertial mass not exactly equaling gravitational mass on interstellar scale open a big can of worms for physics
smaddox 8y ago
What happens if the gravitational mass is the exact negative of the inertial mass for some fraction of matter, though? Does that break GR?
dogma1138 8y ago
I’m not sure this stands true when you think about fields. Think of charged particles in a magnetic field and how polarity and and strength of the charge impacts their interaction the strength would do the same thing for both particles while the polarity would mirror their interaction. I need to read the paper a few more times tho.
thaumasiotes 8y ago
> 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 hand. This is a positive feedback loop that immediately results in the object and your hand exerting infinite amounts of force on each other, because applying force just leads to applying more force.

monocasa 8y ago

Also not a physicist. AFIAIK the "dark matter & energy might be negtive mass" idea has popped up before but has been dismissed because our current observations show that it's gravitational effect isn't lessened by the expansion of the universe. He addresses this in section 2.3.1 by adding a "creation term" to balance universe expansion with the creation of new negative mass, but that just seems like a dirty hack to me. Albeit, the cosmological constant was a dirty hack too, so that doesn't really necessarily discount anything.

deleted 8y ago

[deleted]