Gravity without mass ?

Killjoy

Killjoy

Propelling The Farce!!
Valued Senior Member
Evidently, a physicist has devised a theory which he proposes eliminates the need for dark matter to explain the "missing gravity" in our universe.

Physicist's Gravity Revelation May Finish 'Endless Search' For Dark Matter

Richard Lieu, a professor of physics and astronomy at the University of Alabama in Huntsville, has a new theory that could lessen the need for dark matter to exist.

Lieu suggests that the additional gravity needed to hold a galaxy or cluster together could instead come from shell-like, topological defects in structures commonly found throughout the cosmos.

These defects were most likely created during the early universe when a cosmological phase transition took place. This is a physical process where the overall state of matter changes together across the entire universe.

In a statement, Lieu said: "Topological effects are very compact regions of space with a very high density of matter, usually in the form of linear structures known as cosmic strings, although 2-D structures such as spherical shells are also possible.

"The shells in my paper consist of a thin inner layer of positive mass and a thin outer layer of negative mass; the total mass of both layers—which is all one could measure, mass-wise—is exactly zero, but when a star lies on this shell it experiences a large gravitational force pulling it towards the center of the shell."
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This concept seems a bit funky to me - especially the "negative mass" business. Then again, most aspects of astro- and quantum physics often appear to be the province of some manner of wizards to this simple kibitzing curmudgeon.

The above quoted article is admittedly short on the specifics of Lieu's theory, but it did offer a link to the detailed version of his paper - complete with "rune" filled equations and spiff-a-riffic jargon:

The binding of cosmological structures by massless topological defects

My question to those with greater knowledge of such things is - does it appear that he's on to something ?
 
Yes. I scanned this article. And I raised my eyebrow at the same spot: "Huh, gravity without mass eh? Cool! How does that work?"

"Oh, negative mass." In other words, substitute one unevidenced, hypothesized exotic particle for a different unevidenced, hypothesized exotic particle.

And it doesn't even bother - as far as I could read - to hypothesize why it would gather in halos around galaxies (as it would have to, to explain the galactic rotation curve anomaly).
 
DaveC426913 said:
Yes. I scanned this article. And I raised my eyebrow at the same spot: "Huh, gravity without mass eh? Cool! How does that work?"

"Oh, negative mass." In other words, substitute one unevidenced, hypothesized exotic particle for a different unevidenced, hypothesized exotic particle.

And it doesn't even bother - as far as I could read - to hypothesize why it would gather in halos around galaxies (as it would have to, to explain the galactic rotation curve anomaly).
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Perhaps the idea is that these topological defects are what causes galaxies to coalesce where they do in the first place. Could that work?

This is not the only group working along these lines. A UCL prof, Jonathan Oppenheim, is exploring an idea that spacetime is not "classically" smooth and that this could lead to anomalous gravitational effects: https://www.theguardian.com/science...ory-of-gravity-rules-out-need-for-dark-matter

Though I see Carlo Rovelli is not impressed.
 
Killjoy said:
[...] does it appear that he's on to something?
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The latest pop-sci attention at bottom seems to echo the same thing I gleaned from it here, back on Monday (sans the sarcasm):
https://www.sciforums.com/threads/science-stories-of-the-week.164004/post-3729445

I expect it's just another "dark matter alternative" blip that will fade away in a year or so, rather than a MOND-level fad that garners a club following. Whether that will be due to forecasts of little chance of undisputed evidence, someone finding flaws in its abstract descriptive "proof", or from just the usual apathy about alt-DM candidates (in the extended sense).
- - - - - - - - - - - -

(June 13, 2024) Wild new study suggests gravity can exist without mass
www.sciencealert.com

Wild New Study Suggests Gravity Can Exist Without Mass

What is gravity without mass? Both Newton's revolutionary laws describing its universal effect and Einstein's proposal of a dimpled spacetime, we've thought of gravity as exclusively within the domain of matter.
www.sciencealert.com www.sciencealert.com

EXCERPT: . . . this curvature of space-time is what Einstein described as gravity [...] This gravity seems inextricably linked to mass. Not so, posits Lieu.

[...] Lieu's solution consists of shell-shaped topological defects that might occur in very compact regions of space with a very high density of matter.

These sets of concentric shells contain a thin layer of positive mass tucked inside an outer layer of negative mass. The two masses cancel each other out, so the total mass of the two layers is exactly zero. But when a star lies on this shell, it experiences a large gravitational force dragging it towards the center of the shell.

"The contention of my paper is that at least the shells it posits are massless," Lieu says. If those contentious suggestions bear any weight, "there is then no need to perpetuate this seemingly endless search for dark matter," Lieu adds... (MORE - details)
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Pinball1970 said:
No I meant I also posted a paper.

academic.oup.com

The binding of cosmological structures by massless topological defects

ABSTRACT. Assuming spherical symmetry and weak field, it is shown that if one solves the Poisson equation or the Einstein field equations sourced by a topo
academic.oup.com academic.oup.com
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Apparently these spherical topological defects (related to cosmic strings) rotate or something. So that he's maybe appealing to centrifugal force. These structures thereby replacing the proposed effects of dark matter spherical halos. From the paper:

  • Thus, the specific topological defect proposed in this paper is reasonably corroborated by the unusually large number of observations of organized shell-like or (as a projection effect) ring-like manifestations of stars and galaxies on kpc scales and beyond. Such defects can in principle provide the necessary centrifugal force without enlisting ‘missing’ mass [of dark matter].

    [...] we shall argue that the properties of the massless singular shell sources capable of driving an attractive central force field are specified by several parameters having values to be determined observationally; moreover, the increasing frequency of sightings of ring and shell like formation of galaxies in the Universe lends evidence to the type of source being proposed here. Beyond that, we will also show in Section 5 that, in respect of (c), the proposed model can reproduce the bending of light, hence the gravitational lensing by a DM isothermal sphere without necessarily enlisting DM.

And again... although one-dimensional topological defects like cosmic strings have mass, the organization of these shell-like structures "cancel" that out. (Figuratively? I.e., that's surely not quite right, but a quicker sum-up than delving into the abstract technical details which would be "symbol and jargon babble" to the public, anyway.)

From ScienceAlert:

  • Clare Watson: These sets of concentric shells contain a thin layer of positive mass tucked inside an outer layer of negative mass. The two masses cancel each other out, so the total mass of the two layers is exactly zero. But when a star lies on this shell, it experiences a large gravitational force dragging it towards the center of the shell.

From the press release:

  • “Topological effects are very compact regions of space with a very high density of matter, usually in the form of linear structures known as cosmic strings, although 2-D structures such as spherical shells are also possible. The shells in my paper consist of a thin inner layer of positive mass and a thin outer layer of negative mass; the total mass of both layers — which is all one could measure, mass-wise — is exactly zero, but when a star lies on this shell it experiences a large gravitational force pulling it towards the center of the shell.”
_
 
C C said:
Apparently these spherical topological defects (related to cosmic strings) rotate or something. So that he's maybe appealing to centrifugal force. These structures thereby replacing the proposed effects of dark matter spherical halos. From the paper:

  • Thus, the specific topological defect proposed in this paper is reasonably corroborated by the unusually large number of observations of organized shell-like or (as a projection effect) ring-like manifestations of stars and galaxies on kpc scales and beyond. Such defects can in principle provide the necessary centrifugal force without enlisting ‘missing’ mass [of dark matter].

    [...] we shall argue that the properties of the massless singular shell sources capable of driving an attractive central force field are specified by several parameters having values to be determined observationally; moreover, the increasing frequency of sightings of ring and shell like formation of galaxies in the Universe lends evidence to the type of source being proposed here. Beyond that, we will also show in Section 5 that, in respect of (c), the proposed model can reproduce the bending of light, hence the gravitational lensing by a DM isothermal sphere without necessarily enlisting DM.

And again... although one-dimensional topological defects like cosmic strings have mass, the organization of these shell-like structures "cancel" that out. (Figuratively? I.e., that's surely not quite right, but a quicker sum-up than delving into the abstract technical details which would be "symbol and jargon babble" to the public, anyway.)

From ScienceAlert:

  • Clare Watson: These sets of concentric shells contain a thin layer of positive mass tucked inside an outer layer of negative mass. The two masses cancel each other out, so the total mass of the two layers is exactly zero. But when a star lies on this shell, it experiences a large gravitational force dragging it towards the center of the shell.

From the press release:

  • “Topological effects are very compact regions of space with a very high density of matter, usually in the form of linear structures known as cosmic strings, although 2-D structures such as spherical shells are also possible. The shells in my paper consist of a thin inner layer of positive mass and a thin outer layer of negative mass; the total mass of both layers — which is all one could measure, mass-wise — is exactly zero, but when a star lies on this shell it experiences a large gravitational force pulling it towards the center of the shell.”
_
Click to expand...
So there is mass, but both +ve and -ve. I presume by centrifugal the author means centripetal.
 
here: VIDEO LINK

Sabine Hossenfelder: I recently read that you can have gravity without mass. Ha, no way, I thought and had a look at the paper. It's wild, people. The author says that there are hollow spheres scattered across the universe that have no gravitational attraction and that these spheres explain dark matter. I will try to sort it out for you.

 
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