Now, maybe I’m just a jaded cynic with a stale physics masters degree, but isn’t there something depressing about this? Like, faced with an interesting anomaly out there in the world, we have to resort to tweaking the model of dark matter that is already an invention to fit observational anomalies. We’ve no way to detect dark matter directly, and so no way to prove or disprove this hypothesis, so is this really progress in physics at all? I mean, I know, I know, modified theories of gravity have a lot of problems, but what are the other possibilities here? Any current physicists care to weigh in?
I actually spent quite a while in grad school thinking about the last parsec problem, and although I'm not in the field anymore I still think about it from time to time. (My thesis was on gravitational dynamics.)
My perception of the field (which is now about a decade out of date, so take it with a grain of salt), is that there is quite a bit of skepticism about invoking exotic physics to solve the last parsec problem. Galaxies are generally pretty messy places, and the centers of galaxies are especially messy, so it's hard to know if you've correctly modeled all the relevant physics. A lot of astronomers aren't convinced that there really is a last parsec problem.
The main "standard" approach to solve the last parsec problem is from scattering stars (which the article mentions). Basically, every now and then stars from the galaxy wander close to the orbit of the black hole binary and then get slingshotted out of the system. This removes energy from the orbit, and causes the black hole binary to shrink. The problem with this approach if you do a naive calculation is that the stars have to come from a particular set of directions, called the "loss cone" in the jargon. And since the orbits of stars in galaxies are probably fairly static, once a star gets kicked out of the loss cone, it doesn't come back. So over time the loss cone empties and the black hole orbit stops shrinking. The question is, does the orbit shrink far enough before the loss cone empties, and the answer to this question has generally been "no."
The way around this is to question how static the orbits of stars in galaxies really are. One of the more important papers on the topic found that if an elliptical galaxy is sufficiently triaxial (that is, sufficiently non-spherical), then interactions between stars in the galaxy can repopulate the loss cone and cause the orbit to keep shrinking. But as I vaguely recall, not everyone was convinced by that result.
I personally have had some ideas that galactic tides might contribute, especially right after the merger before all the orbits have had time to thermally relax. But I'm not in the field anymore and haven't really had time to really model this idea and see if it would work.
A galaxy is a pretty degenerate instance of the n-body problem, though.
A system of one or two giant bodies orbited by a collection of tiny ones. Each tiny one spends most of its time in a very weakly-perturbed two-body problem, quasi-stably orbiting the giant one(s) with tiny deviations caused by all the other ones. So you have to do some statistics to see how often an assumed distribution of quasi-stable orbits results in the tiny bodies approaching each other closely enough to kick one another into meaningfully different orbits of the giant ones.
This gives you a better idea of how long is "long" and how it compares to the age of the universe
> Considering the three body problem and extrapolating I would expect star orbits in a galaxy to be completely chaotic on long time scales?
That's more or less captured by the loss cone calculation. On a very "long time scale" the loss cone will probably be occasionally replenished by stars coming in from scattering on larger scales, but the timescale for this to become significant for galaxies is substantially longer than the age of galaxies (and also longer than the Hubble time, which is the ~age of the Universe). So, at least from this back of the envelope calculation, that doesn't solve the final parsec problem.
> We’ve no way to detect dark matter directly, and so no way to prove or disprove this hypothesis
We can and we are already searching for dark matter directly [1] and indirectly [2]. The phase space is being closed every now and then and this is progress. This gives us information about where to look next.
One way to think of non-detection is as evidence that we're looking in the wrong place. Another way to think of non-detection is as evidence that it doesn't exist.
Remember the principle of conservation of expected probability. P[X] = P[X|Y] * P[Y] + P[X|!Y] * P[!Y] .
If P[X|Y] > P[X] and P[Y] is strictly between 0 and 1, then P[X|!Y] < P[X] .
So, if X is “dark matter is some kind of matter that can be detected directly” and Y is “Dark matter is/would-be detected directly by this experiment (when/if the experiment is/would-be done)” then, if we do the experiment and it does not directly detect dark matter, then, if we previously assigned some positive probability that it would, our probability that we can detect it in some way, should go down (though not necessarily by a non-negligible amount).
How many places do I need to look before I start to have evidence my lost wallet isn’t in my house?
The normal spot on the shelf? …checking my pockets? …searching the bags I used to go shopping?
At each step, another of my theories about where my wallet is gets disproven — but none are direct evidence it’s not in my house.
We are playing the same game with dark matter: they keep checking spots and it keeps not being there. At what point does checking wrong theories start to suggest that the entire idea is flawed?
When you exhausted the parameter space (assuming you can with our current technical abilities) or a competing idea with actual evidence comes along. It's not like people aren't trying.
That’s not the statistical definition of “evidence” — which is an occurrence that makes something more (or less) likely.
I’d argue that each failure in a strictly statistical sense makes it more likely the whole conception is flawed — if only a little. So every failure is statistical evidence that dark matter theories are wrong.
Just like each place I look for my wallet and don’t find it makes it slightly more likely that it’s not in my house.
The parameter space is usually (not only) a two dimensional plot of mass and the coupling force parameter. Those can give you a space that you would want to explore. There will be no one experiment that would give you sensitivity in all the parameter space. So you design experiments and collect data. If you don't find in the region your experiment were sensitive too (and you did your analysis carefully) then you establish a limit that says basically it cannot be in this region, look in other regions, then repeat.
The analogy with your wallet is misleading because we know your habits, we don't know about nature habits.
As I explained, when we exhaust the parameter space regions and can't find anything then this would tell us to give a shot to something else. Not that people are not doing this now already anyways.
The statements "We are attempting to directly detect dark matter, so far without success" and "We have no way to detect dark matter directly" can both be true. The first does not disprove the second.
> but isn’t there something depressing about this? Like, faced with an interesting anomaly out there in the world, we have to resort to tweaking the model of dark matter that is already an invention to fit observational anomalies
This is unnecessarily negative. Physics progresses in fits and starts, with plenty of blind alleys and red herrings. Maybe we're in a phase akin to the aftermath of the Michelson–Morley experiment. Or maybe it really is that complicated out there.
I find dark matter seems to fit into the same pattern as epicycles. We can add additional complexity to the theory to make our models better match observational data, and that's useful but also strongly hints that something more basic about the model is fundamentally incorrect. That's depressing.
Which additional complexity has been added to dark matter? I'm not aware of anything. A WIMP would still explain everything, no? I don't see the comparison to epicycles at all. Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does. Curious how that works.
Dark matter models have different amounts and distributions of it for each galaxy we care to look at, so there is no predictive power, similar to how they had to add new epicycles to explain the motion of every new planet they observed.
Just today I heard a talk about people looking at stellar motions in ultra faint dwarf galaxies, because standard dark-model theories predict they should have centrally cusped dark-matter density profiles -- and fuzzy dark-matter models predict different profiles, which they could potentially discriminate between.
I exaggerated a lot when saying that it has no predictive power, you're right.
However, it is very much true that different galaxies have different amounts and distributions of dark matter. There are many broad categories, and certain models about galaxy formation try to predict these, but not all galaxies fit these models. This has two effects: for one, for any galaxy with unexpected dynamics, you can fit a dark matter distribution to explain the dynamics (but! This is not unbounded, it still has to match certain other observations). The other effect is actually in favor of a dark matter approach: it means modifying parameters of our theories in a general way has little chance to reproduce the same properties, as each different galaxy is different.
See - now this... this I find needlessly negative.
> Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does.
This is also just wrong. I think we're seeing a number of established voices starting to question the underpinnings of dark matter as a theory.
As for
>Which additional complexity has been added to dark matter?
Let's start with the fucking article we're discussing, where we now need to have "self-interacting" dark matter, as a new spin on the theory to account for the forces needed to explain what we're observing...
Which established voices do you mean? All I see is the usual suspects, i.e. McGaugh, Kroupa, etc.
The article is also not adding complexity. We didn't get new evidence that required modification of DM. We got a phenomenon which has many potential explanations as the "fucking" article points out, one of which could be a subset of all DM theories. The final parsec problem isn't challenging DM at all, instead one particular flavor of DM could help explain the observation. If anything, it could constrain DM. Why wouldn't you explore this possibility?
> Any path—periodic or not, closed or open—can be represented with an infinite number of epicycles. This is because epicycles can be represented as a complex Fourier series; therefore, with a large number of epicycles, very complex paths can be represented in the complex plane.
We didn't get any new evidence that required modification of epicycles! We got a phenomenon which constrains the epicycle theory by adding more circles! <-- you.
> If anything, it could constrain DM. Why wouldn't you explore this possibility?
You're the only one suggesting that we throw out DM as a model right now... at no point have I even hinted at not exploring it. You are bashing someone who is simply suggesting that this model might not be correct (how fucking dare I...).
But the thing about models... no model is correct, but some models are useful.
Again - I'm not contesting that DM is currently a useful model for explaining some of things we're seeing. I'm suggesting that it might also be worth considering alternatives (how fucking dare I...).
Go back to trying to burn Galileo alive. It's the same energy.
The two are quite unrelated. Michelson-Morley was looking for a difference in the speed of light reaching the Earth from the Sun depending on the direction of motion of the Earh. Gravitational waves are not related to that in any way.
They’re not unrelated: they’re the same experiment, looking for an aether — because spacetime is an aether theory.
When the original aether theory failed, they pointed out that you squish when moving in the aether, and so they detected aether waves rather than aether motion.
Only in a very vague sense. The aether theories were looking to explain two things: 1, how a transverse wave, like light, can propagate in a vacuum; and 2, why does the speed of light appear to not respect Galilean relativity?
The solution for both was posited to be a physical medium permeating space that had certain special properties, including very low interactions with other matter (explaining why we didn't directly detect it). Light would only propagate in this medium, and since this medium interacted very little with other matter, it explained why it wouldn't be dragged along by, say, a moving train. The speed of light would be mostly determined by the speed of this universal medium, explaining the constancy of the speed of light.
Current physics has nothing to do with these theories. The medium for light's propagation is the electric field, which is not a material medium at all. And Galilean relativity doesn't agree with Maxwell's equation because it is wrong - we have Einstein's special relativity as the successor explanation, and this doesn't require any kind of medium, and doesn't say anything about space-time waves.
General relativity does see space-time as something that looks more like a medium, except that, like the electric field and unlike aether, it is still not a material medium. And gravity waves have absolutely nothing to do with the aether wind. Gravity waves are specific phenomena triggered by specific events. They have a source and a direction of propagation like any other wave, and differenet gravity waves move in different directions. The aether wind was a fixed universal thing: all of the aether in the entire universe moved in a single direction, and had no source.
If you want to look at something that's actually closer to a modern aether, the quantum models of the void are actually much closer. Per quantum mechanics, all of space-time is permeated by fields and random fluctuations in those fields ensure that there exist virtual particles getting created and destroyed at every point in spacetime. So, in QM, there actually exists a physical medium that does permeate all of spacetime (but it doesn't have any of the other properties of the aether).
> The solution for both was posited to be a physical medium permeating space that had certain special properties,
Light is an excitation of the EM aether — or if you prefer, “field”. (Which you then explain later, yourself.)
We failed to detect the aether with MM because we’re made of aether-stuff and so we squish in a way that cancels out what they were trying to detect with the original experiment.
> Current physics has nothing to do with these theories.
> [explanation of how fields are aethers]
Wilczek also points out modern theories are aether theories.
LIGO works because it detects waves in the aether — which do not suffer from the same squishing problem as in the original experiment. But that’s why a scaled up version of the original experiment worked: we’re fundamentally detecting the same thing.
Aether theories are more legitimate than dark matter theories: the MM test for the aether failed, but then they reformulated the theory and successfully observed an effect; dark matter has yet to find a working reformulation that is observable.
MM failed because there is no moving aether. Even if you want to call the EM field an aether, it is not a moving aether, and the explanation for the constancy of the speed of light has nothing to do with this aether. Other massless particles/waves also move at this constant speed, and they are not tied to the EM field(or luminiferous aether).
Also, LIGO is detecting something fundamentally different from what MM was looking for.
MM was trying to prove that light speed measured from the frame of reference of the Earth varies between times when the Earth is moving in the same direction as the Aether vs times the Earth is moving perpendicular to the Aether (since the Earth is orbiting the sun, and the Aether moves in a single direction, at some point in the year a beam of light sent in the same direction must pass between moving along a 0° angle and a 90° angle versus the Aether's direction).
This really has nothing to do with what LIGO observed. LIGO observed waves in space-time. The equivalent waves in the EM field/luminiferous aether are called "light" and we didn't need the MM experiment to discover that they exist.
So again, I agree that we can call fields "aethers", and that the EM field is in some ways similar specifically to the luminiferous aether. But it still doesn't have the property that MM were looking for: the EM field doesn't move globally, even if it fluctuates locally.
Yes — we have a unified approach that describes light as just an aspect of the aether, which is why I used that specific term. But much of that wasn’t known at the time the original aether theory was posited.
Both MM and LIGO were looking for evidence of “the stuff waves move through” — and LIGO found it by changing the experiment, because you can’t measure our own motion because we squish, as we’re also made of aether stuff.
Per wiki:
> The experiment compared the speed of light in perpendicular directions in an attempt to detect the relative motion of matter, including their laboratory, through the luminiferous aether, or "aether wind" as it was sometimes called. The result was negative, in that Michelson and Morley found no significant difference between the speed of light in the direction of movement through the presumed aether, and the speed at right angles.
You also have the MM experiment wrong — they weren’t looking for a pervasive wind, but for signal from our motion through a static aether, by looking for a difference in speed of light aligned with that motion versus perpendicular.
Which is why it’s cancelled by relativity.
The mistake of MM was thinking that the aether only pertained to light and matter was something else that could move through it — but it turns out everything is aether, including us.
What makes it "stuff"? Stuff/substances, as I think of it, has/have a velocity, etc. . In this sense, spacetime isn't a "substance"/isn't "stuff". Stuff/substances is where you can talk about a chunk of it, and say where it is at some later time.
Say you have flat (Minkowski) spacetime. Suppose you could talk about a chunk of space/spacetime in such a way. Take some frame, and in that reference, pick a region of space. Where is it a bit later? Ok. So, you consider it to have not moved? Now consider the system again but in a different reference frame, where things at rest in the first frame are in motion in the second. So, where is the chunk a bit later in the second frame?
Things that are Lorentz invariant are unlike fluids.
Your example fails at Galilean relativity: the water you claim is static from your frame on an anchored boat is moving from my frame on an airplane above it — and so isn’t a counter example. We don’t need Lorentz invariance at all. (Though, your example of non-moving water fails Einstein relativity too.)
So you’ve failed to show a difference between spacetime and a fluid.
And to address your point directly:
Gravitational waves leave a wake that perturbs the medium.
Sorry, I was unclear. The contradiction was that if you could take chunks like that it would be moving in the other frame, but Lorentz invariance says that flat spacetime has the same structure in different frames, and so wouldn’t be moving.
You say that “if the spacetime is moving, there’s local curvature”?
Then, in flat spacetime, as there is no curvature, then (by the contrapositive) there is no “movement of spacetime”?
But if some stuff is not moving in one inertial frame, then in another inertial frame it is moving.
But that implies that, if spacetime is stuff and such that, if it is moving then there is local curvature, it follows that in a different inertial frame, the spacetime is not flat.
But, whether spacetime is flat doesn’t depend on the choice of an inertial reference frame.
As for how to describe those things without talking about spacetime moving,
How spacetime works has the general coordinate invariance or covariance or whatever.
You have a manifold with a pseudo-Riemannian metric tensor. You can talk about like, geodesics and such.
For a warp bubble (suppose a sub-luminal one) you could talk about a non-accelerating time-like path that remains within the bubble, and talk about how the spacetime interval along it relates to the spacetime interval of some time-like path that starts in the bubble, leaves it, “travels besides it”, and finally re-joins the original path.
(Of course, in some reference frame, the bubble would not move.)
I don’t think it really makes sense to talk about spacetime moving, even in the case of a warp bubble. A position doesn’t have a position, or a time a time, but rather a position is a position, and a time is a time.
You can talk about how, in some particular coordinate system, with some particular foliation into space-like slices, some aspects/pattern of the metric tensor at some position and time coordinate values, correspond to those at a later time coordinate at a different position coordinate value, but this is relative to a choice of coordinate system.
We can talk about how a region moves in relation to other regions.
I’m still not following your objection based on relativity:
We have one frame where I’m on Earth; we have another where I’m in a rocket traveling away at a fixed velocity. In one case we see the rocket traveling away; in the other, we see Earth traveling away. But neither one tells us that the spacetime itself is moving — they’re both just local coordinates.
In the case we see the spacetime itself move, eg with a gravitational wave, both the frame from the rocket and the earth agree that the manifold has been changed in a way that left a wake — moving relative to its local, original shape.
As a side note, “inertial reference frames” are a useful fiction: we’re always being accelerated, eg, by the galactic center, fellow galaxies, dark matter filaments, etc. Conclusions based on such a fiction may be practical, but cannot be used to derive conclusions about physical reality — as they don’t exist.
I don't have a physics masters degree, or anything comparable, but when I read "Astrophysicists have a new suggestion: Dark matter could sap angular momentum from the two black holes and nudge them closer" I also thought "Really? Is dark matter now the stand-in explanation for anything unexplainable?"
But who knows, when one day someone will come up with a better model that does away with the "dark matter kludge", it will turn out that these phenomena actually have a common root cause?
More positive spin: every "kludge" you add puts more constraints on what dark matter could be. If we can verify that the angular momentum transfer under this theory is consistent with other dark matter models, we can build a better framework to test a whole suite of theories with one experiment.
If it exists and is ubiquitous then it makes sense to contribute to all sorts of disparate phenomena; are we surprised that "mass" pops up in all sorts of places?
Besides, epistemologically it is very good to suspect and investigate the effects of DM on all kinds of phenomena. Then, if it does not exist, we will have many more points of reference from which to derive experimental or theoretical contradictions.
I mean... isn't that the point of dark matter and dark energy? To be a stand-in name for stuff we have to account for but can't detect properly? The dark here means dark as in "we are in the dark about that".
It does not really matter if those exist or are an artifact of current theories, replacing relativity is not that easy and there are already a ton of physicists working on that anyway.
In the meanwhile of either having instruments capable of detecting these or a new theory that demostrates that those are artifact emerge... they have to exist. That's the point of them.
Dark matter does not mean "we are in the dark about this," at all.
It's astronomy. It means "anything that is not shining like a star but we can detect it via inference." Light matter is stars. Dark matter can include MACHOS: neutron stars, black holes, brown dwarfs, rogue planets. No new physics required for MACHOs, as opposed to WIMPs.
They did mention how they're hoping to find evidence from the pulsar timing array. It's not exactly easy to find the evidence, and it won't be tomorrow, but the idea at least seems falsifiable.
When you get a more complex blank to fill, the normal thing to do is to improve all of your blank-filling models to adapt for it.
There's nothing wrong with that part.
I do have a problem with the single-minded insistence on disqualifying any exploratory study on alternatives to a model that gets more and more partially falsified all the time¹. To the point that only iffy personalities that don't care for their careers decide to work on them. But on a situation like this, those other models would probably be tweaked too.
1 - I've never noticed it before, but that phrase gives me great "Superstring Theory" vibes.
It is interesting though.. if you think about the stage of physics before relativity. The made up compensations accumulating, but these made up formulas and compensation strategies had a signature- a sort of kinematic model connection, from which one could have guessed with a unconventional thinker at the encompassing more correct model. Like a formula, whos convex hull you can trace by swinging it wildly.
The article did mention some other possibilities: stars swung out of the surrounding galaxy into the orbiting black holes, our friction due to surrounding gas disks. Both of these events would take out angular energy as well but it's not sure if it would be enough though.
I don't think you're a cynic at all. I object to the entire "dark matter" hypothesis, because there is zero evidence for it. What we have, are observations that indicate errors in our models. We need to look for those errors, not invent ever more abstruse version of "ether" to explain them away.
The potential to detect Supermassive Black Hole mergers is one of the reasons I'm really excited about the LISA project [1], and hope it actually gets funded and doesn't delay too much.
I am reminded of a quote from Mass Effect 2. Eventually that black hole could hit something.
Damn straight! I dare to assume you ignorant jackasses know that space is empty. Once you fire this hunk of metal, it keeps going till it hits something. That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years. If you pull the trigger on this, you are ruining someone's day, somewhere and sometime. That is why you check your damn targets! That is why you wait for the computer to give you a damn firing solution! That is why, Serviceman Chung, we do not "eyeball it!" This is a weapon of mass destruction. You are not a cowboy shooting from the hip!
Not doing anything yet. If galaxies can collide, the rogue black hole can collide with your galaxy, and you won't get much warning either. (I mean, realistically you're right, in the same way that our galaxy colliding with Andromeda is scary but has negligible chance of affecting us. But, imagine.)
That's likely a mistake in the article. "Small" black holes (i.e. smaller than supermassive) have star/stellar mass, but not size.
Micro black holes are only hypothesized so far, but they could get very small - e.g. a black hole with Earth mass would have less than 1 centimeter in diameter.
The size itself is not that important for spotting black holes, though. Even if it's as large as a star, all you see staring at the black hole "object" is nothing. What's important are the gravitational effects on the environment, and there the differences are stark. At a distance of 1000 light years, it will be difficult to spot a stellar-mass black hole floating through empty space, because its pull is strong enough only at stellar distances and won't produce enough disturbance in interstellar space for us to notice. OTOH supermassive blackholes will deform whole surrounding star systems because of its immense mass and gravitational pull. A micro black hole (e.g. Earth mass) passing through the solar systems would likely go undetected unless it collides with something (which is improbable). There could be a measurable disturbance, but it would be one-off and difficult to attribute to a black hole.
> A micro black hole (e.g. Earth mass) passing through the solar systems would likely go undetected unless it collides with something (which is improbable).
Solar wind? Would it generate some interesting effects when coming too close to a black hole? All these protons accelerated to a near light speed, probably hitting each other and running away into a black hole.
An Earth-mass black hole will have a similar gravitational effect on solar winds as Earth. Can we detect these effects on solar winds from distance? Maybe if we watch for them in that particular direction of empty space, but will we? The effects would be localized and short-lived at each particular place on the trajectory.
There are likely several > earth-mass objects in our solar system that are as-yet uncatalogued, and almost certainly many hundreds of dwarf planet-mass objects too. Granted, they are further out, but their influence is almost undetectable even over millions of years. It's pretty unlikely that the presence of an earth-mass black hole for just a few years (since it would likely be moving many times solar escape trajectory) would have much in the way of a measurable impact.
Galaxies are big, including our own. Unless the rogue black hole is traveling near light speed, you will get at least tens of thousands of years of advance warning. (What you could do with that warning is a different matter, though...)
There are lots of easily visible stars on the other side of any supermassive black hole that's nearer to you than other galaxies are. When those stars start lensing in ways visible to the naked eye, you're going to know something weird is going on.
You would absolutely notice the gravitational distortions a very long time in advance. That's a lot of mass. You can't miss the way it's distorting space for a very long way around it.
OT but, is there any good reason why "parsecs" are used as distance units rather than "light-[time units]" ? The latter enjoys broad familiarity in the general community.
It's customary in astronomy because it correlates more directly with observation. An object 10 parsecs away exhibits a parallax shift of 1/10 arcsecond.
Not a physicist but it seems to be part of astrophysics culture, and it's a good "base measure" for interstellar space (i.e., if something is < 1pc away it's likely within your same star system). A parsec is ~3.26 lys, for context.
Is there an equivalent of Tidal Heating [1] taking place between the two black holes? It would extract kinetic energy and put it into.. heating the black holes.. whatever that may mean. Assuming there is movement and friction in the core of a black hole.
Gravitational waves have a similar effect of sapping the kinetic energy of orbiting black holes. Instead of heating the black holes, the energy is emitted as gravitational radiation. Having said that, now that we have actual observations of the gravitational radiation emmited from merging black holes, we can be pretty confident in our understanding of the magnitude of this energy loss. And that understanding is that it does not become significant until well into the last parsec.
But however insignificant it is, wouldn’t it always be there sapping angular momentum & over a long period of time drag any two objects closer and closer together? What would be the repulsive force counteracting this?
It would be. Which is why any pair of orbiting bodies will eventually collide.
It’s just that for black holes this effect is insignificant (a merger would take much longer than the age of the Universe) until they get close to each other, much closer than 1 parsec.
Yes, dynamical friction can "quickly" (on the order of 100 Myr) bring the two black holes to a distance of ~1 pc to each other. Gravitational wave radiation can "quickly" cause a merger if they are within a bit less than ~0.1 pc of each other. But how they go from 1 to 0.1 pc on a timescale of ~100 Myr is unknown.
A black hole only has mass, angular momentum, and charge. So you can only deal with those three... not deform and knead a physical mass of planet as in tidal heating.
This is often stated but it literally can't be true. It's true for stationary solutions to the relevant equations, but we're very much not living in a stationary world.
Here's a simple thought experiment disproving your claim. A person hovers just above the origin of a supermassive black hole. They chuck a massively charged object into the black hole. If what you said is true they should observe the charge instantly being transported to the singularity, since a black hole can't have any attributes such as where charge is distributed within the horizon.
Where it gets impossible is that someone very far away around the same supermassive black hole could observe a small charge increment. They in turn could chuck charged stuff into the black hole and now you've got faster than lightspeed communication.
I spent couple of minutes trying to understand your thought experiment and was puzzled why can't I understand it. It seems that it is probably because I don't understand what you mean by "just above the origin of supermassive black hole"?
I feel that you have something interesting to say but not clear.
The no-hair theorem (scientific term for what you are descriping) essentially states that black holes can be completely described by only three externally observable classical properties: mass, charge, and angular momentum. The internal distribution of matter or charge within the black hole is irrelevant from the outside once it crosses the event horizon.
The charge of an object thrown into a black hole doesn't need to "instantaneously" reach the singularity. In fact, it doesn't even "travel" in the sense that the observer would see a time delay based on where the charge is inside the horizon. The EM field generated by this charged object can still be observed outside the horizon before the object crosses the horizon. In classical GR, the exterior of a charged black hole is governed by the Reissner-Nordström metric [1], and it already includes the influence of charge. Once the object enters the horizon, the outside observer will perceive the black hole's external field to have changed. The field adjustment is instant for the outside observer because, from their point of view, the object never actually crosses the event horizon (due to infinite time dilation at the horizon from their perspective). The charge appears to have been "absorbed" by the black hole when the object is still outside the horizon.
So no-hair theorem doesn't imply that properties such as charge or angular momentum must be "smeared" instantaneously to the singularity inside the black hole. The theorem only describes how these properties manifest externally, not their behavior inside the horizon. Once a charge object passes the event horizon, the information about its charge is not causally connected to any observer inside the black hole (except in QM contexts, where the information paradox becomes relevant). However, the charges affects the external metric of the black hole immediately and completely as seen by external observers. Also it doesn't say that the charge or mass needs to be uniformly distributed or behave in any specific way inside the event horizon. It only states that from the outside, black holes look like point-like objects characterized by mass, charge, and angular momentum. The specifics of how charge is distributed inside the black hole’s event horizon aren't visible to outside observers and therefore don't affect the validity of the theorem.
I think time would pass incredibly slowly for an observer that close to the black hole? Since the event horizon provides a singularity where time slows to zero, it makes sense that any non-zero speed along it would go to infinity for an outside observer.
I doubt you could usefully exploit this behavior, because any charges would still need to travel to and away from the black hole at no more than light speed, and because time slows down the further you get to the event horizon the shortest path "around" a black hole would probably not go through it.
The point I'm trying to make is that everyone always cites the no hair theorem but completely leaves out the "stationary solution" part, leading to misconceptions. My thought experiment highlights that one can't treat a black hole as a particle that only has mass, charge and angular momentum in a dynamic world, because such a treatment must inherently lead to instantaneous updates to large regions of spacetime in response to 'stimuli' (in the form of mass, charge or angular momentum) for super massive black holes, which is impossible.
I understand that this is valid for stationary solution which is what an astronomical black hole would be most of the time. What I don't get is why do you assume that this would violate causality. In GR the idea that changes must “instantaneously update” large regions of spacetime due to supermassive black holes ignores relativistic effects like time dilation. From the perspective of an external observer, as an object falls into a black hole, time appears to slow down near the event horizon. The infalling object’s influence on the black hole’s external field (e.g., its charge or mass) is perceived gradually by distant observers.
For supermassive black holes, this process might seem slow due to the massive scale of spacetime curvature near the event horizon, but there is no requirement for instantaneous changes. In fact, relativistic causality ensures that no information can propagate faster than light, so updates to the black hole’s charge, mass, or angular momentum are constrained by the speed at which signals (gravitational or electromagnetic) can travel.
The object will be observed asymptotically decelerating as it approaches the event horizon, never gets observed crossing by a distant observer.
IIRC the event horizon expands to encompass the object just before it gets to the original horizon, due to the Schwarzschild formula.
Also IIRC, the electric field is blurred out by the geodesics by this point, as if it were from the interior. But that's based on what I've heard, I have yet to derive useful results from the Einstein field equations, even though I think I should give it a go and I can follow them well enough to code a simple simulation…
You can’t actually observe objects going into a black hole. They just become redshifted and fall slower and slower from your perspective towards the event horizon.
Now, maybe I’m just a jaded cynic with a stale physics masters degree, but isn’t there something depressing about this? Like, faced with an interesting anomaly out there in the world, we have to resort to tweaking the model of dark matter that is already an invention to fit observational anomalies. We’ve no way to detect dark matter directly, and so no way to prove or disprove this hypothesis, so is this really progress in physics at all? I mean, I know, I know, modified theories of gravity have a lot of problems, but what are the other possibilities here? Any current physicists care to weigh in?
I actually spent quite a while in grad school thinking about the last parsec problem, and although I'm not in the field anymore I still think about it from time to time. (My thesis was on gravitational dynamics.)
My perception of the field (which is now about a decade out of date, so take it with a grain of salt), is that there is quite a bit of skepticism about invoking exotic physics to solve the last parsec problem. Galaxies are generally pretty messy places, and the centers of galaxies are especially messy, so it's hard to know if you've correctly modeled all the relevant physics. A lot of astronomers aren't convinced that there really is a last parsec problem.
The main "standard" approach to solve the last parsec problem is from scattering stars (which the article mentions). Basically, every now and then stars from the galaxy wander close to the orbit of the black hole binary and then get slingshotted out of the system. This removes energy from the orbit, and causes the black hole binary to shrink. The problem with this approach if you do a naive calculation is that the stars have to come from a particular set of directions, called the "loss cone" in the jargon. And since the orbits of stars in galaxies are probably fairly static, once a star gets kicked out of the loss cone, it doesn't come back. So over time the loss cone empties and the black hole orbit stops shrinking. The question is, does the orbit shrink far enough before the loss cone empties, and the answer to this question has generally been "no."
The way around this is to question how static the orbits of stars in galaxies really are. One of the more important papers on the topic found that if an elliptical galaxy is sufficiently triaxial (that is, sufficiently non-spherical), then interactions between stars in the galaxy can repopulate the loss cone and cause the orbit to keep shrinking. But as I vaguely recall, not everyone was convinced by that result.
I personally have had some ideas that galactic tides might contribute, especially right after the merger before all the orbits have had time to thermally relax. But I'm not in the field anymore and haven't really had time to really model this idea and see if it would work.
Considering the three body problem and extrapolating I would expect star orbits in a galaxy to be completely chaotic on long time scales?
A galaxy is a pretty degenerate instance of the n-body problem, though.
A system of one or two giant bodies orbited by a collection of tiny ones. Each tiny one spends most of its time in a very weakly-perturbed two-body problem, quasi-stably orbiting the giant one(s) with tiny deviations caused by all the other ones. So you have to do some statistics to see how often an assumed distribution of quasi-stable orbits results in the tiny bodies approaching each other closely enough to kick one another into meaningfully different orbits of the giant ones.
This gives you a better idea of how long is "long" and how it compares to the age of the universe
> Considering the three body problem and extrapolating I would expect star orbits in a galaxy to be completely chaotic on long time scales?
That's more or less captured by the loss cone calculation. On a very "long time scale" the loss cone will probably be occasionally replenished by stars coming in from scattering on larger scales, but the timescale for this to become significant for galaxies is substantially longer than the age of galaxies (and also longer than the Hubble time, which is the ~age of the Universe). So, at least from this back of the envelope calculation, that doesn't solve the final parsec problem.
See, e.g., https://www.astro.umd.edu/~richard/ASTRO620/Dynamics_Lec3.pd...
> We’ve no way to detect dark matter directly, and so no way to prove or disprove this hypothesis
We can and we are already searching for dark matter directly [1] and indirectly [2]. The phase space is being closed every now and then and this is progress. This gives us information about where to look next.
[1] https://en.wikipedia.org/wiki/Direct_detection_of_dark_matte...
[2] https://en.wikipedia.org/wiki/Indirect_detection_of_dark_mat...
One way to think of non-detection is as evidence that we're looking in the wrong place. Another way to think of non-detection is as evidence that it doesn't exist.
> One way to think of non-detection is as evidence that we're looking in the wrong place
We aren't looking in only one place
> Another way to think of non-detection is as evidence that it doesn't exist.
Absence of evidence is not evidence of absence. Exhausting the parameter space until evidence is found is simply the normal scientific progress.
Remember the principle of conservation of expected probability. P[X] = P[X|Y] * P[Y] + P[X|!Y] * P[!Y] .
If P[X|Y] > P[X] and P[Y] is strictly between 0 and 1, then P[X|!Y] < P[X] .
So, if X is “dark matter is some kind of matter that can be detected directly” and Y is “Dark matter is/would-be detected directly by this experiment (when/if the experiment is/would-be done)” then, if we do the experiment and it does not directly detect dark matter, then, if we previously assigned some positive probability that it would, our probability that we can detect it in some way, should go down (though not necessarily by a non-negligible amount).
How many places do I need to look before I start to have evidence my lost wallet isn’t in my house?
The normal spot on the shelf? …checking my pockets? …searching the bags I used to go shopping?
At each step, another of my theories about where my wallet is gets disproven — but none are direct evidence it’s not in my house.
We are playing the same game with dark matter: they keep checking spots and it keeps not being there. At what point does checking wrong theories start to suggest that the entire idea is flawed?
When you exhausted the parameter space (assuming you can with our current technical abilities) or a competing idea with actual evidence comes along. It's not like people aren't trying.
That’s not the statistical definition of “evidence” — which is an occurrence that makes something more (or less) likely.
I’d argue that each failure in a strictly statistical sense makes it more likely the whole conception is flawed — if only a little. So every failure is statistical evidence that dark matter theories are wrong.
Just like each place I look for my wallet and don’t find it makes it slightly more likely that it’s not in my house.
The parameter space is usually (not only) a two dimensional plot of mass and the coupling force parameter. Those can give you a space that you would want to explore. There will be no one experiment that would give you sensitivity in all the parameter space. So you design experiments and collect data. If you don't find in the region your experiment were sensitive too (and you did your analysis carefully) then you establish a limit that says basically it cannot be in this region, look in other regions, then repeat.
The analogy with your wallet is misleading because we know your habits, we don't know about nature habits.
As I explained, when we exhaust the parameter space regions and can't find anything then this would tell us to give a shot to something else. Not that people are not doing this now already anyways.
"Absence of evidence is not evidence of absence"
William Wright [1]
[1] https://quoteinvestigator.com/2019/09/17/absence/
That's only true if you don't look.
Except you don’t know which it is until you’ve exhausted the search space.
The statements "We are attempting to directly detect dark matter, so far without success" and "We have no way to detect dark matter directly" can both be true. The first does not disprove the second.
> but isn’t there something depressing about this? Like, faced with an interesting anomaly out there in the world, we have to resort to tweaking the model of dark matter that is already an invention to fit observational anomalies
This is unnecessarily negative. Physics progresses in fits and starts, with plenty of blind alleys and red herrings. Maybe we're in a phase akin to the aftermath of the Michelson–Morley experiment. Or maybe it really is that complicated out there.
I don't really find it unnecessarily negative.
I find dark matter seems to fit into the same pattern as epicycles. We can add additional complexity to the theory to make our models better match observational data, and that's useful but also strongly hints that something more basic about the model is fundamentally incorrect. That's depressing.
Detecting objects by first detecting their gravitational influence has been successful in the past, see the discovery of Neptune for example.
Which additional complexity has been added to dark matter? I'm not aware of anything. A WIMP would still explain everything, no? I don't see the comparison to epicycles at all. Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does. Curious how that works.
Dark matter models have different amounts and distributions of it for each galaxy we care to look at, so there is no predictive power, similar to how they had to add new epicycles to explain the motion of every new planet they observed.
That's not even remotely correct.
Just today I heard a talk about people looking at stellar motions in ultra faint dwarf galaxies, because standard dark-model theories predict they should have centrally cusped dark-matter density profiles -- and fuzzy dark-matter models predict different profiles, which they could potentially discriminate between.
I exaggerated a lot when saying that it has no predictive power, you're right.
However, it is very much true that different galaxies have different amounts and distributions of dark matter. There are many broad categories, and certain models about galaxy formation try to predict these, but not all galaxies fit these models. This has two effects: for one, for any galaxy with unexpected dynamics, you can fit a dark matter distribution to explain the dynamics (but! This is not unbounded, it still has to match certain other observations). The other effect is actually in favor of a dark matter approach: it means modifying parameters of our theories in a general way has little chance to reproduce the same properties, as each different galaxy is different.
See - now this... this I find needlessly negative.
> Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does.
This is also just wrong. I think we're seeing a number of established voices starting to question the underpinnings of dark matter as a theory.
As for
>Which additional complexity has been added to dark matter?
Let's start with the fucking article we're discussing, where we now need to have "self-interacting" dark matter, as a new spin on the theory to account for the forces needed to explain what we're observing...
Which established voices do you mean? All I see is the usual suspects, i.e. McGaugh, Kroupa, etc.
The article is also not adding complexity. We didn't get new evidence that required modification of DM. We got a phenomenon which has many potential explanations as the "fucking" article points out, one of which could be a subset of all DM theories. The final parsec problem isn't challenging DM at all, instead one particular flavor of DM could help explain the observation. If anything, it could constrain DM. Why wouldn't you explore this possibility?
> Any path—periodic or not, closed or open—can be represented with an infinite number of epicycles. This is because epicycles can be represented as a complex Fourier series; therefore, with a large number of epicycles, very complex paths can be represented in the complex plane.
We didn't get any new evidence that required modification of epicycles! We got a phenomenon which constrains the epicycle theory by adding more circles! <-- you.
> If anything, it could constrain DM. Why wouldn't you explore this possibility?
You're the only one suggesting that we throw out DM as a model right now... at no point have I even hinted at not exploring it. You are bashing someone who is simply suggesting that this model might not be correct (how fucking dare I...).
But the thing about models... no model is correct, but some models are useful.
Again - I'm not contesting that DM is currently a useful model for explaining some of things we're seeing. I'm suggesting that it might also be worth considering alternatives (how fucking dare I...).
Go back to trying to burn Galileo alive. It's the same energy.
> Maybe we're in a phase akin to the aftermath of the Michelson–Morley experiment.
Loving this comparison, and I hope this is the case.
Or, maybe we are not.
The problem with cosmology is that it's based almost purely on theory and not data, to the point of speculation.
Michelson-Morley worked; we call it LIGO.
The period after the apparent failure to the present success has been incredibly fruitful.
The two are quite unrelated. Michelson-Morley was looking for a difference in the speed of light reaching the Earth from the Sun depending on the direction of motion of the Earh. Gravitational waves are not related to that in any way.
They’re not unrelated: they’re the same experiment, looking for an aether — because spacetime is an aether theory.
When the original aether theory failed, they pointed out that you squish when moving in the aether, and so they detected aether waves rather than aether motion.
With an upgraded version of the same device.
Only in a very vague sense. The aether theories were looking to explain two things: 1, how a transverse wave, like light, can propagate in a vacuum; and 2, why does the speed of light appear to not respect Galilean relativity?
The solution for both was posited to be a physical medium permeating space that had certain special properties, including very low interactions with other matter (explaining why we didn't directly detect it). Light would only propagate in this medium, and since this medium interacted very little with other matter, it explained why it wouldn't be dragged along by, say, a moving train. The speed of light would be mostly determined by the speed of this universal medium, explaining the constancy of the speed of light.
Current physics has nothing to do with these theories. The medium for light's propagation is the electric field, which is not a material medium at all. And Galilean relativity doesn't agree with Maxwell's equation because it is wrong - we have Einstein's special relativity as the successor explanation, and this doesn't require any kind of medium, and doesn't say anything about space-time waves.
General relativity does see space-time as something that looks more like a medium, except that, like the electric field and unlike aether, it is still not a material medium. And gravity waves have absolutely nothing to do with the aether wind. Gravity waves are specific phenomena triggered by specific events. They have a source and a direction of propagation like any other wave, and differenet gravity waves move in different directions. The aether wind was a fixed universal thing: all of the aether in the entire universe moved in a single direction, and had no source.
If you want to look at something that's actually closer to a modern aether, the quantum models of the void are actually much closer. Per quantum mechanics, all of space-time is permeated by fields and random fluctuations in those fields ensure that there exist virtual particles getting created and destroyed at every point in spacetime. So, in QM, there actually exists a physical medium that does permeate all of spacetime (but it doesn't have any of the other properties of the aether).
> The solution for both was posited to be a physical medium permeating space that had certain special properties,
Light is an excitation of the EM aether — or if you prefer, “field”. (Which you then explain later, yourself.)
We failed to detect the aether with MM because we’re made of aether-stuff and so we squish in a way that cancels out what they were trying to detect with the original experiment.
> Current physics has nothing to do with these theories.
> [explanation of how fields are aethers]
Wilczek also points out modern theories are aether theories.
LIGO works because it detects waves in the aether — which do not suffer from the same squishing problem as in the original experiment. But that’s why a scaled up version of the original experiment worked: we’re fundamentally detecting the same thing.
Aether theories are more legitimate than dark matter theories: the MM test for the aether failed, but then they reformulated the theory and successfully observed an effect; dark matter has yet to find a working reformulation that is observable.
MM failed because there is no moving aether. Even if you want to call the EM field an aether, it is not a moving aether, and the explanation for the constancy of the speed of light has nothing to do with this aether. Other massless particles/waves also move at this constant speed, and they are not tied to the EM field(or luminiferous aether).
Also, LIGO is detecting something fundamentally different from what MM was looking for.
MM was trying to prove that light speed measured from the frame of reference of the Earth varies between times when the Earth is moving in the same direction as the Aether vs times the Earth is moving perpendicular to the Aether (since the Earth is orbiting the sun, and the Aether moves in a single direction, at some point in the year a beam of light sent in the same direction must pass between moving along a 0° angle and a 90° angle versus the Aether's direction).
This really has nothing to do with what LIGO observed. LIGO observed waves in space-time. The equivalent waves in the EM field/luminiferous aether are called "light" and we didn't need the MM experiment to discover that they exist.
So again, I agree that we can call fields "aethers", and that the EM field is in some ways similar specifically to the luminiferous aether. But it still doesn't have the property that MM were looking for: the EM field doesn't move globally, even if it fluctuates locally.
Yes — we have a unified approach that describes light as just an aspect of the aether, which is why I used that specific term. But much of that wasn’t known at the time the original aether theory was posited.
Both MM and LIGO were looking for evidence of “the stuff waves move through” — and LIGO found it by changing the experiment, because you can’t measure our own motion because we squish, as we’re also made of aether stuff.
Per wiki:
> The experiment compared the speed of light in perpendicular directions in an attempt to detect the relative motion of matter, including their laboratory, through the luminiferous aether, or "aether wind" as it was sometimes called. The result was negative, in that Michelson and Morley found no significant difference between the speed of light in the direction of movement through the presumed aether, and the speed at right angles.
You also have the MM experiment wrong — they weren’t looking for a pervasive wind, but for signal from our motion through a static aether, by looking for a difference in speed of light aligned with that motion versus perpendicular.
Which is why it’s cancelled by relativity.
The mistake of MM was thinking that the aether only pertained to light and matter was something else that could move through it — but it turns out everything is aether, including us.
I don’t see any good reason to be attached to using the word “aether” to describe [actual things that have been found].
Well, because they’re aethers - pervasive “stuff” which is the substance of the traveling wave, eg those detected by LIGO.
Wilczek for instance points out that modern theories are aether theories.
What makes it "stuff"? Stuff/substances, as I think of it, has/have a velocity, etc. . In this sense, spacetime isn't a "substance"/isn't "stuff". Stuff/substances is where you can talk about a chunk of it, and say where it is at some later time.
There’s waves in it (gravitational waves) and objects could be made from it (blackholes).
That sounds like “stuff” is there.
> Stuff/substances is where you can talk about a chunk of it, and say where it is at some later time.
In what sense can you do this with a liquid but not spacetime?
Say you have flat (Minkowski) spacetime. Suppose you could talk about a chunk of space/spacetime in such a way. Take some frame, and in that reference, pick a region of space. Where is it a bit later? Ok. So, you consider it to have not moved? Now consider the system again but in a different reference frame, where things at rest in the first frame are in motion in the second. So, where is the chunk a bit later in the second frame?
Things that are Lorentz invariant are unlike fluids.
Your example fails at Galilean relativity: the water you claim is static from your frame on an anchored boat is moving from my frame on an airplane above it — and so isn’t a counter example. We don’t need Lorentz invariance at all. (Though, your example of non-moving water fails Einstein relativity too.)
So you’ve failed to show a difference between spacetime and a fluid.
And to address your point directly:
Gravitational waves leave a wake that perturbs the medium.
https://www.sciencealert.com/gravitational-waves-could-be-le...
We can also coherently talk about a chunk moving, eg subluminal warp drives.
https://phys.org/news/2024-05-subluminal-warp.html
So we have two ways “where you can talk about a chunk of it, and say where it is at some later time”.
Sorry, I was unclear. The contradiction was that if you could take chunks like that it would be moving in the other frame, but Lorentz invariance says that flat spacetime has the same structure in different frames, and so wouldn’t be moving.
Fluids aren’t Lorentz invariant.
But if the spacetime is moving, there’s local curvature. As in the two examples I gave.
So I’m not following.
Perhaps you could explain how either of those operates without being able to identify chunks of spacetime as moving.
You say that “if the spacetime is moving, there’s local curvature”?
Then, in flat spacetime, as there is no curvature, then (by the contrapositive) there is no “movement of spacetime”?
But if some stuff is not moving in one inertial frame, then in another inertial frame it is moving.
But that implies that, if spacetime is stuff and such that, if it is moving then there is local curvature, it follows that in a different inertial frame, the spacetime is not flat.
But, whether spacetime is flat doesn’t depend on the choice of an inertial reference frame.
As for how to describe those things without talking about spacetime moving,
How spacetime works has the general coordinate invariance or covariance or whatever.
You have a manifold with a pseudo-Riemannian metric tensor. You can talk about like, geodesics and such.
For a warp bubble (suppose a sub-luminal one) you could talk about a non-accelerating time-like path that remains within the bubble, and talk about how the spacetime interval along it relates to the spacetime interval of some time-like path that starts in the bubble, leaves it, “travels besides it”, and finally re-joins the original path.
(Of course, in some reference frame, the bubble would not move.)
I don’t think it really makes sense to talk about spacetime moving, even in the case of a warp bubble. A position doesn’t have a position, or a time a time, but rather a position is a position, and a time is a time.
You can talk about how, in some particular coordinate system, with some particular foliation into space-like slices, some aspects/pattern of the metric tensor at some position and time coordinate values, correspond to those at a later time coordinate at a different position coordinate value, but this is relative to a choice of coordinate system.
We can talk about how a region moves in relation to other regions.
I’m still not following your objection based on relativity:
We have one frame where I’m on Earth; we have another where I’m in a rocket traveling away at a fixed velocity. In one case we see the rocket traveling away; in the other, we see Earth traveling away. But neither one tells us that the spacetime itself is moving — they’re both just local coordinates.
In the case we see the spacetime itself move, eg with a gravitational wave, both the frame from the rocket and the earth agree that the manifold has been changed in a way that left a wake — moving relative to its local, original shape.
As a side note, “inertial reference frames” are a useful fiction: we’re always being accelerated, eg, by the galactic center, fellow galaxies, dark matter filaments, etc. Conclusions based on such a fiction may be practical, but cannot be used to derive conclusions about physical reality — as they don’t exist.
I don't have a physics masters degree, or anything comparable, but when I read "Astrophysicists have a new suggestion: Dark matter could sap angular momentum from the two black holes and nudge them closer" I also thought "Really? Is dark matter now the stand-in explanation for anything unexplainable?"
But who knows, when one day someone will come up with a better model that does away with the "dark matter kludge", it will turn out that these phenomena actually have a common root cause?
More positive spin: every "kludge" you add puts more constraints on what dark matter could be. If we can verify that the angular momentum transfer under this theory is consistent with other dark matter models, we can build a better framework to test a whole suite of theories with one experiment.
If it exists and is ubiquitous then it makes sense to contribute to all sorts of disparate phenomena; are we surprised that "mass" pops up in all sorts of places?
Besides, epistemologically it is very good to suspect and investigate the effects of DM on all kinds of phenomena. Then, if it does not exist, we will have many more points of reference from which to derive experimental or theoretical contradictions.
I mean... isn't that the point of dark matter and dark energy? To be a stand-in name for stuff we have to account for but can't detect properly? The dark here means dark as in "we are in the dark about that".
It does not really matter if those exist or are an artifact of current theories, replacing relativity is not that easy and there are already a ton of physicists working on that anyway.
In the meanwhile of either having instruments capable of detecting these or a new theory that demostrates that those are artifact emerge... they have to exist. That's the point of them.
Dark matter does not mean "we are in the dark about this," at all.
It's astronomy. It means "anything that is not shining like a star but we can detect it via inference." Light matter is stars. Dark matter can include MACHOS: neutron stars, black holes, brown dwarfs, rogue planets. No new physics required for MACHOs, as opposed to WIMPs.
or conversely, I wonder how many different phenomena are "explained" by dark matter, since it's infinitely parametrisable.
They did mention how they're hoping to find evidence from the pulsar timing array. It's not exactly easy to find the evidence, and it won't be tomorrow, but the idea at least seems falsifiable.
When you get a more complex blank to fill, the normal thing to do is to improve all of your blank-filling models to adapt for it.
There's nothing wrong with that part.
I do have a problem with the single-minded insistence on disqualifying any exploratory study on alternatives to a model that gets more and more partially falsified all the time¹. To the point that only iffy personalities that don't care for their careers decide to work on them. But on a situation like this, those other models would probably be tweaked too.
1 - I've never noticed it before, but that phrase gives me great "Superstring Theory" vibes.
It is interesting though.. if you think about the stage of physics before relativity. The made up compensations accumulating, but these made up formulas and compensation strategies had a signature- a sort of kinematic model connection, from which one could have guessed with a unconventional thinker at the encompassing more correct model. Like a formula, whos convex hull you can trace by swinging it wildly.
The article did mention some other possibilities: stars swung out of the surrounding galaxy into the orbiting black holes, our friction due to surrounding gas disks. Both of these events would take out angular energy as well but it's not sure if it would be enough though.
I don't think you're a cynic at all. I object to the entire "dark matter" hypothesis, because there is zero evidence for it. What we have, are observations that indicate errors in our models. We need to look for those errors, not invent ever more abstruse version of "ether" to explain them away.
The potential to detect Supermassive Black Hole mergers is one of the reasons I'm really excited about the LISA project [1], and hope it actually gets funded and doesn't delay too much.
[1] https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...
> In some scenarios, the lightest of the three holes is ejected
That's terrifying. Imagine a rogue supermassive black hole floating in intergalactic space.
But I mostly want to know how badly self-interacting dark matter messes up the existing LCDM simulations that most astrophysicists sort of rely on?
> That's terrifying. Imagine a rogue supermassive black hole floating in intergalactic space.
Why terrifying? It's literally doing nothing, far away from anything. Seems like the safest place for it to be.
I am reminded of a quote from Mass Effect 2. Eventually that black hole could hit something.
Not doing anything yet. If galaxies can collide, the rogue black hole can collide with your galaxy, and you won't get much warning either. (I mean, realistically you're right, in the same way that our galaxy colliding with Andromeda is scary but has negligible chance of affecting us. But, imagine.)
> and you won't get much warning either
A supermassive blackhole floating towards you would have very visible effects, it would be impossible to miss for millenias before it gets to you.
It's the micro black holes which can hit you without a warning.
The article state that small black holes have the size of stars. Micro black holes should be quite big too I guess?
That's likely a mistake in the article. "Small" black holes (i.e. smaller than supermassive) have star/stellar mass, but not size.
Micro black holes are only hypothesized so far, but they could get very small - e.g. a black hole with Earth mass would have less than 1 centimeter in diameter.
The size itself is not that important for spotting black holes, though. Even if it's as large as a star, all you see staring at the black hole "object" is nothing. What's important are the gravitational effects on the environment, and there the differences are stark. At a distance of 1000 light years, it will be difficult to spot a stellar-mass black hole floating through empty space, because its pull is strong enough only at stellar distances and won't produce enough disturbance in interstellar space for us to notice. OTOH supermassive blackholes will deform whole surrounding star systems because of its immense mass and gravitational pull. A micro black hole (e.g. Earth mass) passing through the solar systems would likely go undetected unless it collides with something (which is improbable). There could be a measurable disturbance, but it would be one-off and difficult to attribute to a black hole.
> A micro black hole (e.g. Earth mass) passing through the solar systems would likely go undetected unless it collides with something (which is improbable).
Solar wind? Would it generate some interesting effects when coming too close to a black hole? All these protons accelerated to a near light speed, probably hitting each other and running away into a black hole.
An Earth-mass black hole will have a similar gravitational effect on solar winds as Earth. Can we detect these effects on solar winds from distance? Maybe if we watch for them in that particular direction of empty space, but will we? The effects would be localized and short-lived at each particular place on the trajectory.
An earth mass black hole would surely peturb various orbits of planets in a solar system though?
There are likely several > earth-mass objects in our solar system that are as-yet uncatalogued, and almost certainly many hundreds of dwarf planet-mass objects too. Granted, they are further out, but their influence is almost undetectable even over millions of years. It's pretty unlikely that the presence of an earth-mass black hole for just a few years (since it would likely be moving many times solar escape trajectory) would have much in the way of a measurable impact.
Galaxies are big, including our own. Unless the rogue black hole is traveling near light speed, you will get at least tens of thousands of years of advance warning. (What you could do with that warning is a different matter, though...)
If you were lucky enough to see the ejection happen, yes. But if one was already on the way?
There are lots of easily visible stars on the other side of any supermassive black hole that's nearer to you than other galaxies are. When those stars start lensing in ways visible to the naked eye, you're going to know something weird is going on.
You would absolutely notice the gravitational distortions a very long time in advance. That's a lot of mass. You can't miss the way it's distorting space for a very long way around it.
You'll see bent light and similar effects.
In Neal Stephenson’s Seveneves (read immediately) the moon explodes for an unknown reason.
I always imagined it as being caused by a rogue mini black hole zipping through.
A rogue star having a close interaction with our solar system would be catastrophic enough, and we know there's lots of those in our galaxy.
OT but, is there any good reason why "parsecs" are used as distance units rather than "light-[time units]" ? The latter enjoys broad familiarity in the general community.
It's customary in astronomy because it correlates more directly with observation. An object 10 parsecs away exhibits a parallax shift of 1/10 arcsecond.
Not a physicist but it seems to be part of astrophysics culture, and it's a good "base measure" for interstellar space (i.e., if something is < 1pc away it's likely within your same star system). A parsec is ~3.26 lys, for context.
At the stellar density in our region of the galaxy, sure. The density is hundreds of times higher closer to the center.
Yeah it is obviously informed by our local environment, the very definition of parsec is tied to the structure of our solar system.
supposedly not besides being a good shibboleth
Is there an equivalent of Tidal Heating [1] taking place between the two black holes? It would extract kinetic energy and put it into.. heating the black holes.. whatever that may mean. Assuming there is movement and friction in the core of a black hole.
[1] https://en.wikipedia.org/wiki/Tidal_heating
Gravitational waves have a similar effect of sapping the kinetic energy of orbiting black holes. Instead of heating the black holes, the energy is emitted as gravitational radiation. Having said that, now that we have actual observations of the gravitational radiation emmited from merging black holes, we can be pretty confident in our understanding of the magnitude of this energy loss. And that understanding is that it does not become significant until well into the last parsec.
But however insignificant it is, wouldn’t it always be there sapping angular momentum & over a long period of time drag any two objects closer and closer together? What would be the repulsive force counteracting this?
It would be. Which is why any pair of orbiting bodies will eventually collide.
It’s just that for black holes this effect is insignificant (a merger would take much longer than the age of the Universe) until they get close to each other, much closer than 1 parsec.
So the question isn't how they merge, but how they do it so quickly.
Yes, dynamical friction can "quickly" (on the order of 100 Myr) bring the two black holes to a distance of ~1 pc to each other. Gravitational wave radiation can "quickly" cause a merger if they are within a bit less than ~0.1 pc of each other. But how they go from 1 to 0.1 pc on a timescale of ~100 Myr is unknown.
It was observed long time ago, Nobel 1993
A black hole only has mass, angular momentum, and charge. So you can only deal with those three... not deform and knead a physical mass of planet as in tidal heating.
This is often stated but it literally can't be true. It's true for stationary solutions to the relevant equations, but we're very much not living in a stationary world.
Here's a simple thought experiment disproving your claim. A person hovers just above the origin of a supermassive black hole. They chuck a massively charged object into the black hole. If what you said is true they should observe the charge instantly being transported to the singularity, since a black hole can't have any attributes such as where charge is distributed within the horizon.
Where it gets impossible is that someone very far away around the same supermassive black hole could observe a small charge increment. They in turn could chuck charged stuff into the black hole and now you've got faster than lightspeed communication.
> above the origin of a supermassive black hole
I spent couple of minutes trying to understand your thought experiment and was puzzled why can't I understand it. It seems that it is probably because I don't understand what you mean by "just above the origin of supermassive black hole"?
I feel that you have something interesting to say but not clear.
Mental typo, I meant horizon, not origin.
The no-hair theorem (scientific term for what you are descriping) essentially states that black holes can be completely described by only three externally observable classical properties: mass, charge, and angular momentum. The internal distribution of matter or charge within the black hole is irrelevant from the outside once it crosses the event horizon.
The charge of an object thrown into a black hole doesn't need to "instantaneously" reach the singularity. In fact, it doesn't even "travel" in the sense that the observer would see a time delay based on where the charge is inside the horizon. The EM field generated by this charged object can still be observed outside the horizon before the object crosses the horizon. In classical GR, the exterior of a charged black hole is governed by the Reissner-Nordström metric [1], and it already includes the influence of charge. Once the object enters the horizon, the outside observer will perceive the black hole's external field to have changed. The field adjustment is instant for the outside observer because, from their point of view, the object never actually crosses the event horizon (due to infinite time dilation at the horizon from their perspective). The charge appears to have been "absorbed" by the black hole when the object is still outside the horizon.
So no-hair theorem doesn't imply that properties such as charge or angular momentum must be "smeared" instantaneously to the singularity inside the black hole. The theorem only describes how these properties manifest externally, not their behavior inside the horizon. Once a charge object passes the event horizon, the information about its charge is not causally connected to any observer inside the black hole (except in QM contexts, where the information paradox becomes relevant). However, the charges affects the external metric of the black hole immediately and completely as seen by external observers. Also it doesn't say that the charge or mass needs to be uniformly distributed or behave in any specific way inside the event horizon. It only states that from the outside, black holes look like point-like objects characterized by mass, charge, and angular momentum. The specifics of how charge is distributed inside the black hole’s event horizon aren't visible to outside observers and therefore don't affect the validity of the theorem.
[1] https://en.wikipedia.org/wiki/Reissner%E2%80%93Nordstr%C3%B6...
Again, the no-hair theorem only talks about stationary solutions:
> The no-hair theorem (which is a hypothesis) states that all *stationary* black hole solutions of...
https://en.m.wikipedia.org/wiki/No-hair_theorem
> The field adjustment is instant for the outside observer
And as my above thought experiment shows, any instant changes like such could be used for FTL communication.
I think time would pass incredibly slowly for an observer that close to the black hole? Since the event horizon provides a singularity where time slows to zero, it makes sense that any non-zero speed along it would go to infinity for an outside observer.
I doubt you could usefully exploit this behavior, because any charges would still need to travel to and away from the black hole at no more than light speed, and because time slows down the further you get to the event horizon the shortest path "around" a black hole would probably not go through it.
What is unique about your thoughts experiment that makes it applies only to dynamical phases?
I am really not getting what you are trying to say?
The point I'm trying to make is that everyone always cites the no hair theorem but completely leaves out the "stationary solution" part, leading to misconceptions. My thought experiment highlights that one can't treat a black hole as a particle that only has mass, charge and angular momentum in a dynamic world, because such a treatment must inherently lead to instantaneous updates to large regions of spacetime in response to 'stimuli' (in the form of mass, charge or angular momentum) for super massive black holes, which is impossible.
I understand that this is valid for stationary solution which is what an astronomical black hole would be most of the time. What I don't get is why do you assume that this would violate causality. In GR the idea that changes must “instantaneously update” large regions of spacetime due to supermassive black holes ignores relativistic effects like time dilation. From the perspective of an external observer, as an object falls into a black hole, time appears to slow down near the event horizon. The infalling object’s influence on the black hole’s external field (e.g., its charge or mass) is perceived gradually by distant observers.
For supermassive black holes, this process might seem slow due to the massive scale of spacetime curvature near the event horizon, but there is no requirement for instantaneous changes. In fact, relativistic causality ensures that no information can propagate faster than light, so updates to the black hole’s charge, mass, or angular momentum are constrained by the speed at which signals (gravitational or electromagnetic) can travel.
The object will be observed asymptotically decelerating as it approaches the event horizon, never gets observed crossing by a distant observer.
IIRC the event horizon expands to encompass the object just before it gets to the original horizon, due to the Schwarzschild formula.
Also IIRC, the electric field is blurred out by the geodesics by this point, as if it were from the interior. But that's based on what I've heard, I have yet to derive useful results from the Einstein field equations, even though I think I should give it a go and I can follow them well enough to code a simple simulation…
You can’t actually observe objects going into a black hole. They just become redshifted and fall slower and slower from your perspective towards the event horizon.
The singularity is not the same thing as its event horizon.
That said, it's the no hair theorem. It could of course still be wrong...
link to papers:
- https://arxiv.org/abs/2401.14450
- https://arxiv.org/abs/2311.03412
- https://arxiv.org/abs/2311.18228v1
Every time the math doesn't check out throw in some dark matter :)
Diaspora, by Greg Egan
"heh it's probably dark matter"
Saved you a click.