Neutron Stars Hint at Another Dimension | Hacker News Reader

Neutron Stars Hint at Another Dimension

nautil.us

192 pointsdnetesn87 comments

> Gravity, the thinking goes, can escape our brane and extend into the bulk. That explains why it’s so weak. All the other forces must play in only three spatial dimensions, while gravity can extend itself out to four, spreading itself much too thin in the process.

Wouldn't this cause gravitational force to fall off with distance using something other than an inverse-square law? I think this explanation would be a better fit for the weak force than gravity for this reason. Thoughts?

More broadly: inverse-square behavior (Gravity, EM etc) strikes me as an intrinsic property of 3D geometry; more so of a tell of dimensionality than the magnitude of the force. (I believe the article is inferring higher dimensionality from relative magnitude, vice distance falloff)

Yes, exactly. That is why we think the extra dimensions might be small, und the inverse square law is only violated at and below the size of the extra dimensions. This is also why we are using the Yukawa Potential to constrain that possibility, because it has a length scale and a strength of a potential deviation from the inverse square law. See also: https://en.wikipedia.org/wiki/Fifth_force

Why does the extra dimension need to be small?

Because gravity will be observed to decay with distance cubed for distances on the scale of the extra dimension, and distance squared beyond that; and we have not found a scale where we see gravity decay faster than distance squared (but it gets harder and harder to measure at small scale, so the error bars grow).

If it was big, you could see it.

IIRC experimental gravity data rules out any compactified dimension bigger than 50μm, but a question I keep coming back to is "surely the pictures of atomic bonds taken by electron microscopes rules compactified dimensions larger than 1Å?"

interesting question. my (somewhat naive) thought about it is that bonds are maintained by the EM force, which is so strong that it swamps out any contribution from gravity.

Not necessarily, 2D cannot easily see 3D, etc...

Yes but you would sure as heck bump into it if it was big.

Like literally in the middle of your sitting room. Isn’t it a known meme horror thing - monster slices from another dimension splicing across into ours as they move through their planes .

Basically it doesn’t happen but the dimensions do exist so they must be small.

Hence why we don’t bump into them.

If a compactified spatial dimension exists in our universe, and was big enough to fit an atom, why couldn't we see two atoms that seem like they're in the same 3-dimensional coordinates?

Sometimes compactified dimensions are analogised to a straw: seen from a distance it seems one dimensional, up close (an ant's perspective) it's got one long dimension and one short dimension.

I don't know how far to take the analogy. It sounds like surely photons with wavelengths smaller than the compactified dimension would be likely to take a spiral path, looping around compact dimension n times for every m units of 3-space travelled, which would seem like they were mysteriously slow if you weren't expecting the compact dimension to exist.

I vaguely remember the idea of wavelength-dependent speed of light is a thing that's been ruled out by tests with supernova data, but not to what wavelength or sigma.

The same reason why flatlanders don’t see two circles in the same 2D coordinates, even if a 3D tube was penetrating through their world.

Because they can’t see above or below to the rest of the tube. They can only see a single infinitely thin slice of the tube.

I think you're describing a completely different geometry than I'm describing.

An ℝ²-brane such as flatland existing in a ℝ³ bulk is different to an ℝ²⨯S¹.

If the S¹ part* is present in our universe to the degree that it can explain anything about gravity, it should also have an impact on everything else in the universe larger than the radius of the S¹ dimension's circumference.

* well, S^n ⨯ T^m, the version of string theory I hear most about has n+m = 6, but there are others, and this thread is a toy model where n=1, m=0

Edit: Apparently the U+1D54A character is stripped, so put a plain ASCII "S" back in.

I’m describing why the flatlanders wouldn’t see multiple circles even though a 3D tube is composed of infinitely many 2D circles.

I noticed you were doing so, yes.

The "tube" (compactified dimension) isn't a higher dimensional object going through our space, in string theory it is an actual part of our space.

To put it another way: for compactified dimensions, we're not in flatland.

(For brane theory, we are in flatland, but they're two different ideas about how stuff might work).

How can a dimension be smaller compared to other dimensions?

It could be a compact[0] dimension, i. e. of finite length. In the simplest case you might imagine it as a circle attached to every point in our 3-dimensional Euclidean space. The aforementioned length scale would be the circumference of that circle.

[0]: https://en.m.wikipedia.org/wiki/Compact_space

Trying to wrap my head around this explanation and I’m picturing a looping gif. You have your normal x and y dimensions and then time through the gif. If the loop length is very short then distance between any two pixels will mostly only depend on x and y. Is that right?

Yes, that sounds right.

In the simplest case, yes. Though, once curvature (gravity) enters the picture, it could (in theory) become more complicated, as the additional dimension could get stretched or compressed.

The classic example is a garden hose seen from afar looks like a line, but up close it is a cylinder that can be walked “around” by an ant.

Interesting case if we are the “ants” and it is our 3 dims happen to be compact looping somewhere beyond our event horizon. Multitude of Universes in that garden hose in which gravity can be falling as cube or more while at small scale if our compact Universe we’ll see square, and only very precise measurements may notice a bit larger than square.

Another possibility is if our brane has a lot of folds coming close/touching - that would make gravity there stronger like say that dark matter idea inducing rotation speed curve of the disk stars.

Would also be nice for possibly bridging gaps

> Interesting case if we are the “ants” and it is our 3 dims happen to be compact looping somewhere beyond our event horizon. Multitude of Universes […]

I think you're mixing up two different cases here: 1) Our established 3 dimensions are actually compact, i.e. loop around or hit a boundary somewhere. No multiverse here. 2) There are extra dimensions, meaning that for every point in that extra dimension there's another 3-dimensional universe as we know it.

> Our established 3 dimensions are actually compact, i.e. loop around

Do they not loop? What other option is there? I assume you can't sail off the edge of the disk, so to speak.

Option 1: They loop.

Option 2: They go on forever without looping.

Option 3: They end - there is some kind of boundary to spacetime.

The expansion of the space is the feature which prevents any physical process inside to distinguish between those options. Kind of a hack - make compact Universe, add expansion and it would inside look and feel indistinguishable from non-compact.

How does option 2 fit with the big bang? The obvious issue (at minimum) being accounting for the CMB.

the 1. makes 2. "easier", i.e. having a multitude of compact Universes is "cheaper" than having a multitude of non-compact ones

Another visual that may be useful is imagine being stuck between two portals squeezed close together.

And yet that circle has as many "points" as any other 1-dim independent axis, so ...

The "number" of points is irrelevant, topologically these are very different spaces (one is compact, one isn't).

Imagine if Flatland were a very long string in a big circle. In one direction you go around the big circle and it's a long distance. At a right angle to that, you go around a tiny little circle.

I wonder if a higher dimension could also be the explanation for extra mass in the universe instead of dark matter. It's outside our perceptible space, but it still exists as mass, poking through into black holes or gently resting on the skin of our 3d volume.

The weird thing about it though is that whatever the dark matter is it has to be spread out. It couldn’t be little planets or brown dwarfs or burned out stars (in a hidden dimension or not) because we’d see more gravitational lensing events than we do

https://en.wikipedia.org/wiki/MACHO_Project

After digging a bit into astromy, computationally myself... There are some heavy assumptions used in the functions that maps pixels to mass densities. Outsider's 2c, but I assess a misalignment between CDM confidence in papers, and this mapping.

Interesting. It would be extraordinary if many of the discrepancies dark matter is required to explain are actually caused by some flaw in the data analysis. It seems unlikely, but not impossible.

I'm not familiar with the topic. Did you have any particularly suspect assumptions in mind?

I am overall suspicious of the degree of confidence used in papers in conjunction with the sheer number of assumptions regarding luminosity, the model of gas and stars in galaxies etc, vs what is discernible in the images (It's a low-resolution set of pixels). Of particular note is inferring mass (or lack thereof) that doesn't correspond to leading-edge luminosity. I.e. gas and stars that are away from the camera, and dim gas.

> The weird thing about it though is that whatever the dark matter is it has to be spread out.

In fact, they'd have to be so spread out that rotation curves remain flat past a million light years [1]. There seems to be no plausible particle dark matter distribution that can satisfy all of the necessary constraints at this point.

[1] https://tritonstation.com/2024/06/18/rotation-curves-still-f...

I thought dark matter was only observed through movements of matter within galaxies. Outer layers of spiral galaxies are observed to move faster than they should, so there has to be additional gravity and therefore mass that binds them on their (fast) orbits around the center.

Perhaps there is a negative gravity outside of galaxies where space seems to bubble out of nowhere anyway and the universe is expanding.

This seems as an attempt to combine gravity with the standard model again, which in my very amateurish understanding comes with multiple extra dimensions anyway. Isn't the higgs field basically a recently discovered additional dimension already? Among the other forms of particles that can be seen as an excitation of fields that compose these dimensions.

But for extreme cases like neutron stars or black holes, we probably do need to combine these theories since gravity is a main reason these objects exist in the first place. And also isn't a curvature of space not already be an additional dimension as well? It would be mathematically as I understand it.

Fun fact: Newton attributed the inverse square law to Pythagoras. It’s esoteric, but it relates to harmony of the spheres and the fact that the weight/tension of a string has an inverse square relation to tone. More here, in this Royal Society article: https://www.researchgate.net/publication/250902005_Newton_an...

I guess it also implies the extra dimensions aren't massive. Unless that's the explanation for unexplained gravitation.

If anyone wants a super approachable lecture on Neutron stars, this was released just a couple of weeks ago - https://youtu.be/I12SQ7YOebY

That was a great watch, thanks!

> The force of gravity is weak. And not just a little bit weak. It’s so much weaker than the other three fundamental forces—electromagnetism and the strong and weak nuclear forces—that it’s almost impossible to provide analogies.

Nothing in nature prevents gravity from just being super weak. Some forces could just be super weak.

The unspoken premise of gravity being weaker than other forces is that all forces were unified at some point. So iff you assume all forces in nature were once one force, then gravity being weak is an anomaly.

if you use planck units instead of anthropocentric ones gravity isnt weak. its the mass of the proton that is much less than its charge. but why should those two values be equivalent to begin with?

Strength is relative. It will always be a rounding error in particle interactions.

Far from expert in the field, but assuming that gravity is acts in a 3+ND and we observe it in our 3D world, shouldn't we observe weird peculiarities with it rather that just its amplitude?

Think that you live on a line, and you see projections of a 2d object doing circles on top of you. You would see the shade moving and changing sizes in a non-explainable manner to you.

We do observe really weird gravitational effects. Dark matter, for instance. Under Newtonian and Einsteinian physics, galaxies shouldn't be able to form in the way we observe. The way galaxies and their contents move makes no sense with our present understanding of gravity-- unless we assume there's a lot more mass. So we invented dark matter as a sort of placeholder variable to make the math make sense.

More anomalies: simply being near a large gravitational field alters the flow of time. Frame dragging around black holes (spacetime itself twists into a rotating spiral). The final parsec problem (co-orbiting black holes bleeding energy as gravitational waves). And don't forget the gravitational singularity of a black hole.

But perhaps the most important thing to know is that we've only just gained the ability to examine gravitational waves. Once we build more detectors (especially LISA), we'll probably discover a lot more is wrong with our understanding of gravity.

> More anomalies: simply being near a large gravitational field alters the flow of time. Frame dragging around black holes (spacetime itself twists into a rotating spiral). The final parsec problem (co-orbiting black holes bleeding energy as gravitational waves). And don't forget the gravitational singularity of a black hole.

These are not really anomalies per se - they are predicted by the relatively well tested theory of GR and (except for the singularity part) also experimentally observable. They are weird from our point of view, but not weird to contemporary physics.

From recent discussion though it seems as though the time dilation effects or that time itself moves differently in different patches of spacetime- that could remove the need altogether for dark energy/dark matter:

https://www.theguardian.com/science/2024/mar/09/controversia...

Yes, but none of that is anomalous. The primary novelty of the timescape model is just a more careful application of boring old General Relativity to cosmological models, which are highly idealized in order to make them tractable. It is still very misleading to characterize effects like frame dragging or time dilation as unresolved mysteries. They are very straightforward, experimentally confirmed, elements of classical General Relativity.

I don't think anyone is stating it is a mystery, just that no one apparently had been willing to comprehensively do the math until now.

> The way galaxies and their contents move makes no sense with our present understanding of gravity-- unless we assume there's a lot more mass

Explain for a layman? I don't know what it means for movement to not make sense.

Being able to precisely calculate movement of planets in Solar system and also calculate their mass was a huge triumph of physics. The problem is the same math doesn't work with visible stars orbiting in galaxies nor galaxy clusters. Simplest explanation is there's much invisible mass - dark matter. Or the laws are different at these scales.

We know that in a vacuum everything falls that ~9.8m/s acceleration on earth.

We get a ball made up of something, and for some reason only it accelerates at 10m/s for no discernible reason.

It’s a bunch of nonsense really.

They greatly simplify models, otherwise they’re too complicated to calculate.

So they simplify the data points, assume point particles, assume no interactions due to electromagnetism, no tidal locks, and Newtonian gravity instead of relativity.

And then it turns out galaxies sometimes rotate too quickly.

Yeah, no shit. If your data is known to be wrong and your model uses the wrong theory of gravity and makes known false simplifications it would be quite strange if it somehow did predict without some discrepancies

Movement doesn't make sense when we can't predict or explain it. Based on our understanding of gravity as confirmed by observations of our solar system we expect to see galaxies do X but instead they do Y, and then we collectively fail to identify a simple explanation like bad math or a bad assumption.

In one sense this is a vindication of our application of the scientific method and the way we make theories: a bad theory wouldn't be able to be checked, whereas a good theory can make precise enough claims that when a limitation is found (such as when our predictions about reality do not match our observations) that the results of the check are clear.

I thought the final parsec problem is that co orbiting black holes can't have accretion discs which makes GW the only way for them to inspiral, and that's way too slow for us to have seen any.

The more distance along your 4th dimension you allow, the more strange geometric effects you will observe. If you let a 4th dimension be very, very, very small (imagine a 2d universe that actually has a third dimension, it's just subatomic in scale) the geometric effects are negligible.A 3d volume can exist in that 2d + 1 tiny dimension, in the technical sense, but not in any macroscopic sense. Your 3rd dimension curls around to where it started nearly immediately.

Extra dimensions are always a desperate measure. If you add them to your theory you open up a huge parameter space, because you have no clue about the number, size and topology of the extra dimensions. What is missing is a real physical symmetry or reason that would enforce the existence of extra dimensions and on the other side restricts its geometry and topology.

Just adding more parameters to your theory will allow you to overfit the data better, but that does not mean you understand more about nature.

To what degree are these Nautilus stories based off of the work of a single researcher or lab that does not have broader consensus amongst the research community?

What's a good way for a layperson to tell if this is a new scientific consensus arrived at after hundreds of researchers come to the same conclusion or a breakthrough result that has shocked the entire research community?

> “In 1999, theoretical physicists Lisa Randall and Raman Sundrum proposed a wild restructuring of the cosmos”

> “The brane-bulk model is a speculative idea for sure, but a fun one.”

I feel like it’s communicated pretty clearly that it isn’t some breakthrough finding that everybody agrees on. You could google the mentioned researchers/theories and find out more information if you still weren’t sure.

Agreed. Yes, a bit roundabout, but it's pretty wild that we live in a spot in the universe where the distance we need to travel to "confirm plausibility" of a "deep truth of the universe we just heard about" is just to type a few glyphs into a magic box and decide if the person speaking the purported truth has a reputation in the relevant human thought-stuffs.

The world we live in is crazy. To know such a thing so easily at an earlier time, would be unfathomable :)

This is not consensus. There are lots of anomalies in what we observe in the cosmos. Here someone links two of those to a speculation about extra dimensions. It would get interesting if they have predictions that can be checked.

A promising new theory should fit known observations, explain previously unexplained phenomenon, and predict something testable. That will be difficult to judge as a layperson.

Virtually all reasonable alternatives to GR have been strongly ruled out, including theories with large extra dimensions in them. In general, these theories have a some kind of parameter which measures their deviation from GR, which is being squashed to zero. There's also just generally no reason to think there are extra dimensions at all

This article brings up neutron stars being slightly larger than expected, but the reality is there's no real expected maximum mass for a neutron star - because the equation of state and physics for neutron star interiors is unknown. The spin, and magnetic field of a neutron star can also serve to increase the maximum mass of a neutron star, which are very hard to model as there are no analytic solutions to a spinning body (nor an oblate body)

There are too many approximations in the paper to even come close to saying that the brane model explains this better than standard physics, and there's no reason to think that this event isn't explainable by standard physics

I'm ignorant when it comes to physics, admittedly, so please forgive me if my question has an obvious answer... But when I read articles like this, in particular when they mention branes, I want to ask: How do we know that dark matter is not just some interaction coming from the "bulk"?

We don't know! But we also can't feasibly test for something like that.

God, so many words to cover only one phrase with a "possible hint" at an extra mass coming from imaginary source ...

If you could orient something along a higher dimension in the correct way could we conceivably create some kind of anti gravity or artificial gravity?

Where are the flaggers? Come on, you're slacking off.

Neutron Stars? Other Dimensions? This has GOT to be woke, right?

The level of flagging on HN lately is totally out of control.

And they complained that the woke were cancelling people...

No they don't

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