> [...] For this type of star to exist, there must be a stable type of boson with self-repulsive interaction; one possible candidate particle is the still-hypothetical "axion" (which is also a candidate for the not-yet-detected "non-baryonic dark matter" particles, which appear to compose roughly 25% of the mass of the Universe). It is theorized that unlike normal stars (which emit radiation due to gravitational pressure and nuclear fusion), boson stars would be transparent and invisible. [...]
"Boson stars have also been proposed as candidate dark matter objects, and it has been hypothesized that the dark matter haloes surrounding most galaxies might be viewed as enormous [toroidal] boson stars."
I've watched more than enough Star Trek to know that if I take control of the bridge and fly through that despite Kirk's orders to stop, that I'll gain super-ESP powers. You best get off at the next starbase.
A non-interacting boson star would be transparent, but it still has gravity, so it would probably pick up baryonic matter. Should look like an ordinary star that has too much mass for its diameter and burns more brightly due to the additional gravity compression.
Dropping straight through a clean solar-mass boson star, though, would be uneventful. A little bit of blueshift on starlight, but probably not enough to detect by eye.
Avoid a slingshot orbit. If it has the mass of the Sun and the same density, then it would have a "surface" gravity of 28g one million kilometers out. With that sharp of a gradient, a close orbit will want to pull your spacecraft apart as each atom tries to take a different trajectory, tidal forces. https://en.wikipedia.org/wiki/Roche_limit (A plot point in many a Larry Niven story)
Depends very much on the boson and its interaction with the (extended) Standard Model that includes it.
Axions, if they exist, take part in nuclear interactions. You can expect to have a bad day when encountering them at high density (e.g. expect fissions from neutron disruptions).
In the preprint at the top, the Action (eqn (1)) and Lagrangian densities (eqn (2)) are non-interacting (except for self-interaction and gravitation) for tractability reasons, so the scalar boson version is only sorta like the axion. The paper's setting only considers each type of boson in isolation from any other type of matter in the universe. However any infalling body which has any nonzero distribution of these proposed vector or scalar bosons in them would expect the self-interaction term to be important, so you could still have a bad day for non-gravitational reasons.
I don't think a non-relativistic analysis of a manifestly relativistic compact object is particularly enlightening. Boson stars can be more compact than neutron stars (see Figure 2 on p. 7 of the preprint). Qualitatively though, you'd have a bad day if you got too close; you are right that tidal stresses are probably what will get you in the case of only-self-interacting bosons.
The Roche limit, developed well before even special relativity, matters for slowly-orbiting (hyperbolic orbits count) self-gravitating bodies (so big asteroids and comets count). Your parent commentator's flight would presumably be in a spacecraft (or suit) that is held together by intermolecular forces instead of its own gravity. Those forces are typically much stronger than gravity; gravity only overcomes them in relativistic systems.
There are dozens, scores, and likely hundreds of different (hypothetical) bosons described as the sole (or at least dominant) matter in a boson star (cf. link at the end of this comment).
In the preprint linked at the top I count at least sixteen different specific (hypothetical) bosons in their References section, and the preprint discusses two very broad families (in §2).
The authors' claim is that a wide variety of (massive) boson theories can in bulk behaviour produce the important characteristics of a boson star constrained by what they know of this particular binary (see end of 1st paragraph of §6).
> assumed it was just light on details because it's wikipedia
An encyclopedic overview should probably not point to every paper one would get from a search of the literature, let alone to a search of preprints.
Wikipedia is likely light on details because the topic is niche because it's an idea which is nowhere near as convincing as perturbations of black hole solutions of the Einstein Field Equations ("standard black holes"), and not even more persuasive than e.g. fuzzballs, gravastars, and other relativistic dark star hypotheses also aiming to overthrow standard black holes for various reasons.
Even the list of reasons for studying just boson stars gets long: because the compact object may be dominated by a particle that might solve problems in particle physics (but which would be hard to prove/disprove in an Earthbound lab soon); or that might also describe dark matter and/or dark energy; or that (if observed) would support a broader theory like M-theory; or that might put to bed any questioning of unitarity; ... And because some types of boson stars (if found in nature) might support one of these broader aims to the exclusion of the other aims, it's a somewhat busy field with papers and anti-papers. Here's a couple hundred: <https://arxiv.org/search/?query=%22boson+star%22&searchtype=...>
Crushed by what? Weakly-interacting bosons can't crush you with pressure.
As far as gravity goes, as long as you're on a direct trajectory towards the center, you'll be free-falling so won't experience a gravitational "force" in your own reference frame.
The comment you originally replied to was talking about "a non-interacting boson star," i.e. a boson star made up one of the hypothetical types of boson that makes up dark matter.
In that case, the only significant interaction between those bosons and you would be gravity which, when in free-fall, affects the path you follow but won't in itself crush you. This assumes that (a) that there's not much normal matter in the star to crash into, and (b) that the mass is not large enough to generate deadly tidal forces. That's why the OP specified "a clean solar-mass boson star".
But it's worth noting that a "clean" non-interacting boson star can't really form in the normal way, since the bosons in question don't interact with each other in any way that would allow them to form dense clumps. You'd need some normal matter to provide a kind of anchor. In that case you'd see a more normal-looking object that just happens to be heavier than it should be given its visible composition, and you wouldn't be able to "drop through it."
If you know the mass and radius of the object, you can calculate the tidal force. (You can map this with a gravity sensor on a probe or just throwing a bunch of rocks through the system and watching how it bends their trajectories)
Then it depends on the spacecraft. If your ship is 1 km long and made of cotton candy, then it won't take much force at all to destroy it. If it's 10 m long (dramatically reducing the tidal effect) and made of solid diamond then it can fly much closer. Not that there would be anything to see.
Read Neutron Star by Larry Niven (1966) for a dramatic description of a close flyby of a compact stellar mass object.
> If you know the mass and radius of the object, you can calculate the tidal force.
The issue is more the position of the object. Good luck doing trajectory correction for an object you can't see.
The "throw a bunch of rocks" technique might help, but since they'd be unpowered they're not going to be following a trajectory that minimizes tidal force. It's still going to be enormously difficult to thread the needle through an invisible object that's going to tear you apart if you're not on the right trajectory.
Perhaps the technique Beowulf Shaeffer used could work if it were done with instruments - measure the tidal forces aboard the ship and navigate accordingly to minimize them.
Wow, just hopping wikipedias to understand the title one is pushed to Dark Matter land. Boson stars? Pressure accumulates and need emerges to justify how the whole thing does not collapse. Weirdier matter forms arises, but there is some relief that, being fermionic, the Pauli exclusion principle is the last anti-collapse defense. Except that now the guys hypothesize bosonic matter! Weren't bosons supposed to implement interactions?
> the scenario of a central black
hole requires unreasonable amount of fine-tuning within the usual evolutionary channels. In particular, if the system is expected to have formed as a binary in isolation, a common envelope formation
scenario is rather unlikely, given the system’s arrangement. This requires an extreme, and possibly
unphysical, tuning of the relevant parameters of the evolutionary channel under consideration. Moreover, formation within a globular cluster is also improbable given the geometrical characteristics of
the observed orbit. Other evolutionary channels such as formation without a common envelope or via
a hierarchical triple also seem unlikely for similar reasons.
What makes you think their proposal isn't more likely than a black hole?
>At a conference in New York in 1967, Dr. Wheeler, seizing on a suggestion shouted from the audience, hit on the name “black hole” to dramatize this dire possibility for a star and for physics.
Conversely, anything we do manage to observe is more likely to lay within most common probability space simply because we’re similarly observing such a tiny portion of what’s happening in the universe. Think about if you sample uniformly 0.01% of 1B requests / second coming into your system - you’re going to see all the common scenarios and none of the uncommon ones. Sure space isn’t quite the same thing since we don’t observe uniformly, but similarly we’re observing an infinitely smaller part of the overall space anyway.
That doesn't really have any relevance, though. Every single possibility is as likely as the others, it's just there are many more situations in which a system will look "normal", but we still shouldn't be overly surprised by the occasional outlier, we're bound to stumble on them even if rarely.
Is a "boson star" even a confirmed physical construct? I can see confirmed neutron stars like RXJ1856 that they think might be a strange star, but... Just based on what is a confirmed physical construct in the universe, a black hole seems far likelier. Obviously they address this in a way I'm not knowledgeable enough to argue against.
Regardless, it will be an interesting scenario to watch evolve!
The paper rests on the MNRAS published version of <https://arxiv.org/abs/2209.06833> (ref. [EBRQ+22] in the preprint at the top, cited at the end of the paragraph you quote upthread). [EBRQ+22] itself proposes a triple as a possibility:
"The system's evolution may be better-explained in models in which the G star was initially a wide tertiary companion to a close binary containing two massive stars. In this case, interactions between the two stars could have prevented either one from expanding to become a red supergiant, such that the G star could have formed in an orbit similar to its current orbit (but somewhat tighter) and remained there ever since. High-precision RV follow-up offers the tantalizing possibility of testing this scenario."
RV there is "radial velocity measurements".
Moreover, the part you quote and in particular "Moreover, formation within a globular cluster is also improbable" does not touch on their reference's "... dynamical formation in an open cluster that has since dissolved is more plausible."
First see footnote 1 of the first page of the preprint linked at the top. The authors are jumping the gun. That's fine for theorists, but bear in mind that they are literally in the dark on a number of telescopic observables, any of which would shoot down the idea in the preprint. Some of these are achievable by EHT. See e.g. Olivares et al. "How to tell an accreting boson star from a black hole" (2020) <https://discovery.ucl.ac.uk/id/eprint/10112389/1/staa1878.pd...> [pdf].
The exotic bosons available in a 3+1 spacetime have never been detected at energy levels low enough that one might expect them to be reasonably bound by the weak (yes, weak) gravitation required by the boson star's internal repulsive forces. If the internal repulsive forces are too weak (or their self-gravitation isn't weak enough), the bosons cannot support the star against ultimate gravitational collapse (into a black hole). If the internal repulsive forces are too strong, the bosons all escape rather than stick around in the neighbourhood of the star. The latter is especially acute since the boson masses considered by the paper may be smaller than that of neutrinos.
Until a suitable slightly self-repelling exotic boson is discovered, achieving and maintaining equilibrium between it and gravitation is pretty fatal to the boson star idea.
Cold massive bosons are an idea for particle dark matter, incidentally. An example dark matter candidate is the axion. Axions have not been convincingly observed, and might not exist. A compact object made of essentially only axions could plausibly meet the criteria in the preprint linked at the top (cf their axion-motivated scalar field equation at eqn (9)), depending on details of heating during gravitational compaction. Also see e.g. Mohapatra et al., "Dense Axion Stars" (2016) <https://arxiv.org/abs/1512.00108> which is the preprint version of the PRL letter <https://doi.org/10.1103/PhysRevLett.117.121801>. There is as far as I know no clear mechanism to have anything like the axion density required for self-gravitation except in the very early universe, a long time before the first galaxies. So then why would small primordial axion stars be in galaxies, as the binary in the article at the top appears to be? How do you keep them small, rather than coalescing into supermassive compact objects?
The preprint at the top discusses other Beyond The Standard Model (of particle physics; BTSM) possibilities, but they are less convincing than the axion (for starters, such BTSM extensions need to have some non-physical degrees of freedom strongly suppressed, since such BTSM theories are almost always "haunted" by <https://en.wikipedia.org/wiki/Ghost_(physics)>s.). Additionally, the axion is motivated by the strong CP problem in the Standard Model, and if there is a lot of them they become gravitationally relevant. The non-axion options in the preprint at the top are afaik only motivated by a lack of knowledge about the microscopic details of gravitational physics. (See Koberlein's 2021 blog entry on Proca Stars <https://briankoberlein.com/blog/proca-stars/> for a brief intro to (light) massive vector boson stars).
Do we know the distance to the event horizon for Sgr A*? I'm no astrophysicist, but 1.4AU sounds really _incredibly_ close. In my sci-fi thoughts, something at 1.4AU would be attempting to go "in, through, and beyond" and not orbiting. So excuse me while I try to re-evaluate whatever pre-existing notions I might have
I'm no astrophysicist, but the paper and some web data show
- Sag A* has a mass of ~4,000,000 times the sun, and an event horizon of approximately 12,000,000 km (0.08 AU). This is just the event horizon; Sag A*'s accretion disk has a diameter of about 150,000,000,000 km (1000 AU). So your intuition would certainly be right about Sag A* - there is a huge disk of gas and other junk you'd be flying through.
- This object has a mass of at most ~12 times the sun, and (if it's a black hole) an event horizon of approximately 35 km. The paper doesn't say anything about an accretion disk given that it's exploring the idea that the object is not a black hole. Regardless, a star orbiting at 1.4 AU would probably clear out everything in the immediate neighborhood.
> [...] For this type of star to exist, there must be a stable type of boson with self-repulsive interaction; one possible candidate particle is the still-hypothetical "axion" (which is also a candidate for the not-yet-detected "non-baryonic dark matter" particles, which appear to compose roughly 25% of the mass of the Universe). It is theorized that unlike normal stars (which emit radiation due to gravitational pressure and nuclear fusion), boson stars would be transparent and invisible. [...]