A Sun-like star orbiting a boson star
arxiv.org
arxiv.org
> [...] 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. [...]
That's quite a sci-fi plot.
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)
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.
I'd seen the wikipedia a few years back, and I assumed it was just light on details because it's wikipedia.
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=...>
If you find being crushed into degenerate matter uneventful.
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.
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."
How would you figure out the safe trajectory to avoid tidal forces?
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.
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.
> 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?
Because a boson star is purely theoretical and I have a much stronger observational prior that the object is a black hole.
See, for example:
>The Country Parson Who Conceived of Black Holes [in 1783]
https://www.amnh.org/learn-teach/curriculum-collections/cosm....
>A Brief History of Black Holes
https://www.amnh.org/learn-teach/curriculum-collections/cosm....
>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.
https://www.nytimes.com/2008/04/14/science/14wheeler.html#:~....
The universe is big and has lots of stars. Even “very unlikely” things ought to turn up.
E.g.: one in a billion chance ought to occur hundreds of times… in our galaxy alone!
Regardless, it will be an interesting scenario to watch evolve!
No. We don’t even have evidence of a candidate boson you could use to make one.
looking at the paper they cite as reference for this claim, those authors thought exactly the same...
https://academic.oup.com/mnras/article-abstract/518/1/1057/6...
> Other evolutionary channels such as formation without a common envelope or via a hierarchical triple also seem unlikely for similar reasons.
Maybe it's hard to say without getting into specifics but it seems that the paper has considered the "unusual circumstances" in your statement.
"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."
Disruptions of open clusters are well known. <https://en.wikipedia.org/wiki/Open_cluster#Eventual_fate>.
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).
The distance between objects in this system 1.4 AU, which is _incredibly_ close.
- 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.
Do we think dark matter stars, black holes and supermassive black holes exist?