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The Science of the Missing Gravitational Waves (nautil.us)
91 points by pmcpinto on July 21, 2016 | hide | past | web | favorite | 41 comments



There's a great deal that's not understood about supermassive black holes (SMBHs). This article deals with one of these open questions (Why is the GW background from SMBH mergers less than predicted?), but there are a number of other related problems. Here are a few off the top of my head:

* How do SMBHs form? Is it from the merger of a bunch of stellar-mass black holes or do is it from the collapse of a huge gas cloud 1000s of times the mass of the Sun (or more) when the galaxy is first forming?

* Why are the masses of SMBHs so tightly correlated to the masses of their host galaxies? This is known as the M-sigma relation and there's been a lot of research on this topic, in particular. The existence of this correlation implies that SMBHs are somehow involved in some sort of feedback mechanism with the rest of the galaxy, but this is very poorly understood. (There have been lots of different feedback mechanisms invoked, but it's been hard to discriminate between them to see what's really going on.)

* How do SMBHs merge? We know that SMBHs are at the center of nearly all galaxies (if not all of them), and we know that galaxies merge. We don't usually see multiple SMBHs in a galaxy, so somehow the SMBHs must merge as well. But models of these mergers stall out at around a few light-years. At this distance, stars are too far away to influence the orbit, but gravitational waves are too weak to do anything. Somehow real SMBH binaries in the universe manage to overcome this problem, but how? This is known as the last parsec problem.

* We see SMBHs driving huge, extremely powerful jets. What causes the formation of these jets? Why are they so collimated? The Blandford-Znajek process is usually invoked, but (at least in my opinion) while it's plausible, there's a fair amount of hand-waving and not as much observational evidence as one would like.

* Not a question dealing with SMBHs specifically, but do black holes obey the No Hair Theorem? (Or, put another way, is GR correct in the strong gravity regime?) This is being tested with the Event Horizon telescope, which is attempting to observe the event horizon of the SMBH at the center of the Milky Way.

They're fascinating objects, and due to their enormous size and energetics they are relevant to a huge number of problems in astrophysics and fundamental physics.


What if we're looking at this from the wrong perspective?

We say that a black hole forms inside a galaxy, but what if it's more accurate to say that a galaxy takes form around a black hole? So the galaxy extrudes from the black hole; and the larger the black hole, the larger the galaxy extruding from it. The size of the galaxy doesn't determine the size of the black hole within it; the size the black hole determines the size of the galaxy extruding from it.

It starts with the formation of a black hole from empty space, followed by the extrusion of matter from it. Eventually the extrusion fades out, and only the black hole is left for a while, until it, too, disappears into space.

So the black hole isn't really separate from either matter or space; it's the interface between space and matter - where the two converge. It would appear that infinite and zero density are the same, making the nature of the black hole both matter-like (infinite density) and space-like (zero density).


My theory:

Black holes don't exist (time dilation prevents them from forming in a finite time).

Dark matter is gravitational potential energy. (i.e. the potential energy of every star in a solar system falling into every other star, plus the energy of every galaxy falling into every other galaxy.)

Cosmological redshift is actually gravitational redshift. (So dark energy is not necessary as a concept.) There is a lot more gravity out there in my theory. Don't forget that the relative motion of galaxies is inferred from redshift - if the cause of the redshift is not velocity then galaxies are not moving in the way we currently think they are.

I wish I had the math to flesh these ideas out to see if they hold water.


Ok, let me try to address these:

> Black holes don't exist (time dilation prevents them from forming in a finite time)

It's true that distant observers cannot observe anything past the event horizon. But if you doubt that a black hole exists, you can always dive into it; the black hole will then certainly exist for you, and in a finite time.

> Dark matter is gravitational potential energy

Consider the Bullet Cluster, which is two colliding galactic clusters. The gravitational center of mass is offset from the visible center of mass. This offset cannot be explained as potential energy of visible matter. There must be something else, invisible and massive, i.e. dark matter.

> Cosmological redshift is actually gravitational redshift

We see cosmological redshift in all directions. If this redshift were gravitational, it would imply that our galaxy is at the top of a gravitational potential hill. This means that our place in the universe is very privileged, which is unlikely.


>> Black holes don't exist (time dilation prevents them from forming in a finite time)

> It's true that distant observers cannot observe anything past the event horizon. But if you doubt that a black hole exists, you can always dive into it; the black hole will then certainly exist for you, and in a finite time.

But because of time dilation, in the meantime, the entire rest of the universe comes to an end. At the very point at which you pass the event horizon, time dilation is infinite, and therefore the universe outside passes by at infinite speed. Therefore, from our outside observation, no matter actually passes through the event horizon. Therefore, a black hole does not consist of a singularity protected by an event horizon, but rather a sphere of density such that time dilation is almost equivalent to infinity throughout. The sphere is prevented from becoming more dense by time dilation itself.

Now, I know that the above speculation is completely crackpot (but hopefully reasonably informed crackpot), but I would absolutely love to have a proper black hole expert tell me exactly why it is wrong.


Relativity is about the laws of physics being the same for everyone regardless of reference frame.

You are correct that you can never observe the singularity. It will never form within a finite time in any reference frame you can take. Anything falling in will appear stuck as a final ghost/imprint on the surface as time dilation approaches zero.

The problem with your argument is you are not taking into account the reference frame inside the black hole. It's still a valid reference frame in relativity even if you can never observe it, so it must be considered. Also, the same theory that is giving you time dilation to make your argument is what predicts black holes, so it's a bit of a circular reasoning.


> You are correct that you can never observe the singularity. It will never form within a finite time in any reference frame you can take.

So that means there are no black holes in the universe as actual objects - none have formed yet, and none will (in our time). So why does LIGO claim two black holes merged, if black holes can't exist?

> The problem with your argument is you are not taking into account the reference frame inside the black hole. It's still a valid reference frame in relativity even if you can never observe it, so it must be considered.

Considered from a mathematical point of view, definitely. Considered as an observable object? No.

So to consider it: All the other matter that is falling in also time dilated (relative to you) to infinity. So basically you will fall in alone, with nothing accompanying you, and no black hole will form.


Disclaimer: Not a physicist, and very minimal physics knowledge. Someone please correct me if my understanding is wrong.

>So that means there are no black holes in the universe as actual objects - none have formed yet, and none will (in our time). So why does LIGO claim two black holes merged, if black holes can't exist?

A black hole is more than just its singularity. We can't ever actually observe a singularity, or even its event horizon, but we can observe the gravitational collateral damage of the entire black hole as a system.

http://www.physicsoftheuniverse.com/topics_blackholes_event....

We've observed accretion disks of size and speed that could only be feasibly explained if they were spinning around a black hole.

Your understanding regarding time dilation seems correct from the perspective of observing objects actually entering the black hole, but when we deal with things that have not quite entered it, we can infer its existence.

>So to consider it: All the other matter that is falling in also time dilated (relative to you) to infinity. So basically you will fall in alone, with nothing accompanying you, and no black hole will form.

Even as the universe outside you disappears due to the time dilation, why does this mean it can't exist in reality?

Just because an observer can't really experience the event occurring (regardless of whether they're past the event horizon or not) doesn't mean it can't happen. Relativity means you can have crazy events like infinite dilation inside of the black hole, while reference frames outside of it simultaneously have a very different perception of time.

You seem to be arguing from a philosophical perspective that because the singularity effectively singularizes time itself, it can't exist in the universe because the universe doesn't exist for it. But, again, a black hole can be viewed in a layered fashion. It is not just a singularity. The core may be this impossible-ish sort of object, but the levels of "crust" as an entire system create different effects in different frames.

If this were not the case, then the first black hole to ever form would effectively have eliminated the entire universe instantaneously.


> you can always dive into it; the black hole will then certainly exist for you, and in a finite time.

Will it? All the other matter that is falling in also time dilated (relative to you) to infinity. So basically you will fall in alone, with nothing accompanying you, and no black hole will form.

> Consider the Bullet Cluster, which is two colliding galactic clusters.

I'm aware of that, but it's not conclusive. Imagine you never heard of dark matter - would that be the first place you jump to? Or would you imagine some very large object emitting hot gas, and moving, and not being slowed by the incoming gas.

Such an object could not be seen because of the intensely hot gas between it and us, but the gravity would show up in the gravity map. And there's your "dark", but ordinary, matter.

> If this redshift were gravitational, it would imply that our galaxy is at the top of a gravitational potential hill. This means that our place in the universe is very privileged, which is unlikely.

When we see a distant "star", it's actually an entire galaxy, or even galaxy cluster. So the gravitational mass of that entire object is redshifting the light, but only our little sun is blueshifting it on the other end.

The net result is redshift, as we see it.

It's not a coincidence that objects close to us are actually blueshifted.


> Consider the Bullet Cluster, which is two colliding galactic clusters. The gravitational center of mass is offset from the visible center of mass. This offset cannot be explained as potential energy of visible matter. There must be something else, invisible and massive, i.e. dark matter.

i'd argue another possible explanation to add to your i.e. was addressed by the gp

gp> then galaxies are not moving in the way we currently think they are.

the movement of gyroscopes can alter a system's center of mass

https://www.youtube.com/watch?v=n_6p-1J551Y


The rotation is the gyroscope. And the meaning of your last statement (the movement of gyroscopes) is unclear. The video is cool, but well understood by the same physics that is being applied to the understanding of the galaxies.


exactly, but the gp threw in the caveat

gp> then galaxies are not moving in the way we currently think they are.

so if our calculations already take into account the gyroscopic movement of the bodies within the system, still the gp's thought experiment accounted for this by saying the real nature of the motion of the bodies is different than the motion that we are currently using in our calculations

just saying dark matter ignores the potential for other valid explanation for the phenomena

instead of there being dm offsetting the center of mass the gyroscopic motion of the bodies within the system also could explain the offset

'not moving in the way we currently think' is a vague statement and could be a direction of rigor for the gp if thae wished to pursue the idea

some underlying questions that need to be answered by anyone that suggests our current understanding is incorrect in this regard would need to address:

    how are they moving differently? 
    how could we detect the difference?  
    how can our incorrect perception be explained?
my> the movement of gyroscopes can alter a system's center of mass

the cube video was intended to show that yes, this is well understood, and exhibit how changing the nature of the rotation of the gyros: location, direction, speed; in the cube you could move the center of mass to wherever you'd like

add another dimension allowing the gyros to move freely stead being fixed within a cube and even more freedom to alter the center of mass could be achieved

https://www.youtube.com/watch?v=UnpLEnIqj_A

i agree it is well understood.. that is why i was using it as naive evidence to defend the gp's thought experiment from being dismissed poorly


add another dimension allowing the gyros to move freely stead being fixed within a cube and even more freedom to alter the center of mass could be achieved

It's the fact that it is fixed within the cube that allows it to move the cm. That only happens because of the reaction force it can exert against the ground. In the absence of external forces the cm won't move (strictly, won't accelerate). In a sense this is a tautology.

So it's not an uncalculated motion that can alter the motion of the c.m., it's an unaccounted for force.

When "dark matter" arrived on the scene it was a catch-all for matter we couldn't see. It's only when the explanations for "normal matter hidden from view" fell one-by-one that dark matter took on exotic properties. (In other words, super-smart people have already done the math on hidden galaxies of brown dwarfs and neutron stars being the source of the gravity).

I'm betting that if there is a revolutionary simplification to it all, it is because fundamental constants are changing with time. Thus redshift, as ars also speculates, isn't a true measure of velocity. But I understand that super-smart people are doing the measurements on this as well and are finding no changes.


I don't mean to be rude (so please don't be offended if my tone sounds overly negative online), but are you aware that your theories are incredibly crackpot in nature? I'm asking a sincere question, not trying to insult you.

It's clear you have very little understanding of physics. That's totally fine and cool. The vast majority of people don't know anything meaningful about physics. You just make very bold claims that go against standard theoretical and experimental knowledge in the field. If I knew very little knowledge about a topic, be it physics or cooking or dog breeding, I would at least be very aware of that fact and probably not make claims on topics I'm uneducated about.

Again, I'm not trying to be rude, just trying to understand how you feel comfortable making these bold claims when I believe you're aware that you know almost nothing about the topic.


> but are you aware that your theories are incredibly crackpot in nature?

Yup. The difference is I'm looking for info, not assuming I'm right.

> It's clear you have very little understanding of physics.

Now, that, is an insult considering how long I studied it. I wish I did more, but my math wasn't good enough - to my constant sorrow.

> make claims .. bold claims

No claims! It's bad you read it that way. Rather unorthodox ideas, and looking for confirmations or refutations based on explanation, not appeal to authority.

For example: How much energy does the gravitational potential energy of all the stars in a galaxy embody? A calculation of that would confirm of deny me easily. But I don't know how to do it. I was hoping to get a reply from someone who did.


> Yup. The difference is I'm looking for info, not assuming I'm right.

I can respect that.


But certainly belittling. Why not just respond to the points made? Doubtless readers can draw their own conclusions as to the possible crackpot coefficient on a scale of 0 to 10.


What leads you to believe that no experienced astrophysicists have serious considered any of these "obvious" theories?

Because that's basically what you're suggesting.


Maybe they did?

I've asked a question like this and was confirmed that we can't observe black hole formation, only "collapsars", which are almost like black holes but without matter crossing event horizon.

Looks like astrophysicists assume one thing and cosmologists another thing.


This is really interesting, and I believe you should be right to share ideas even if you don't really know if they hold water or not.

> Black holes don't exist (time dilation prevents them from forming in a finite time).

This is recalling me of a hypothesis called Cosmological censorship: https://en.wikipedia.org/wiki/Cosmic_censorship_hypothesis

Basically, the universe is not allowing black holes to be directly observed, which does not mean they don't exist, but as I understand it: it means that we are not allowed to consider they exist.

Because black holes are kind of an absurdity (well, we accept their existence, but… there is so many mysteries around them.)

> I wish I had the math to flesh these ideas out to see if they hold water.

You should not limit or censor yourself because you don't have the maths knowledge to test it and verify it. I believe that you can teach you enough maths if you are passioned enough. There are a lot of MOOCs out there, materials, papers, conferences, trainings to learn you more about what you need.


Matter does not have to cross existing horizon for horizon to grow. Imagine a horizon radius R for a mass M (R=horizon(M)). Despite no matter would be seen touching that, time in sphere between R and R+dR ticks slowly even for external observers. So matter can get into this thin sphere in finite time. As soon as dM of matter gets into this sphere so that horizon(M+dm) > R+dR, the black hole now has increased radius and mass. Everything that was frozen on the surface of BH is now inside the BH.


I can tell you right now, they don't hold water. They do not fit observations, even a little.


Can you elaborate? People are downmodding me for having an idea and putting it out there for comment (is that really so bad?)

But I really do just want to know if my ideas hold water or not, and if not why not.

For example the time dilation on black holes fits every description of black holes I've ever seen.

Gravitational potential energy of stars being significant makes sense to me - after all the farther from the center, the higher the "apparent" mass of the star (as seen in its orbit), which matches well with the potential energy also being higher when it's farther. Is it not enough maybe?


OK, so, go read the wikipedia article on dark matter in its entirety (it's not that long) then come back. One of the reasons you're being downvoted is because it's fairly obvious you haven't done much due diligence. And just throwing out half-baked ideas may seem fun, but it's not particularly helpful.

The biggest problem with your ideas is that we already have numerous lines of evidence that dark matter and regular matter can become separated, and that would seem to be something that directly challenges your hypothesis.


An alternative criticism to the other comment. I actually don't really know what your sentences are trying to say. When you say, 'Black holes don't exist (time dilation prevents them from forming in a finite time)'. Do you mean that you think a singularity would form asymptotically slowly from an outside frame of reference as gravitational time dilation increased? That's the most generous construction I can give your statement and it's still ridiculous because the event horizon would exist anyway so the effect will be the same.

I think the majority of people try to be generous and give the benefit of the doubt, but I'm a physics grad and even after a lot of thought I'm not sure what you're saying. It's hard to give informed criticism in that case but easy to downvote and move on.


> Do you mean that you think a singularity would form asymptotically slowly from an outside frame of reference as gravitational time dilation increased?

Yes.

> it's still ridiculous because the event horizon would exist anyway so the effect will be the same.

How would exist? It would only form after infinite time - time dilates as a function of how close it is to a black hole (i.e. you don't have to already be a black hole to dilate time), reaching infinity as it forms the event horizon.

> I think the majority of people try to be generous and give the benefit of the doubt, but I'm a physics grad and even after a lot of thought I'm not sure what you're saying. It's hard to give informed criticism in that case but easy to downvote and move on.

Thanks for the reply. If I gain nothing else except an understanding of how a black hole can ever form the downvotes would be well worth it. As best as I can understand it can't form, yet everyone just seems to implicitly believe it can.

The rest I just kind of threw out there to see what criticism I would get (none as of this writing, just some insults), but the black hole one really bothers me because that one is grounded in ordinary, orthodox, descriptions of black holes.

Every description describes them as matter caught falling in infinitely slowly, yet simultaneously say matter actually falls in and somehow forms a black hole. It's a contradiction - I can't be the only person in the entire world to see that.


Oh, I see now. Yeah, you're right. From our outside frames of references we'll never see anything cross the event horizon, instead matter approaches the event horizon asymptotically slowly. In our exterior frame of reference/coordinate system you'll not see one form and because they'll never form from the outside we'll never see a black hole because there aren't any.

So as far as that goes you're right. What this doesn't mean is that black holes don't exist in any reference frame. The simple test is to jump into the black hole. Time dilation won't affect you, you're in a free fall after all, following your geodesic. You'll pass a well defined event horizon and pass into what is undeniably a black hole.



While we're throwing around ideas, how about this:

Black holes do exist. They are simply the beginning of the inverse of what we call the "Big Bang". The Big Bang was space and matter appearing from nothingness. A black hole is space and matter disappearing back into nothingness, where it came from. At the current time in our universe, black holes exist only locally, dispersed thinly throughout space. As time progresses (billions of years), distinct black holes will merge, forming larger black holes, eventually usurping all space and matter in existence, leaving behind nothing. And then, a new Big Bang, followed by its inverse, can occur again.

The only somewhat confusing part is that, while the universe may have expanded from a single point (the Big Bang), it will not contract into a single point before disappearing. I'm not entirely sure why, but this is exactly the same we see with all extinct life forms: evolving from a single point/origin, but disappearing as a multitude of separate things spread out over their habitat. Or perhaps it's simpler to just say: from the perspective of the universe space doesn't exist. The universe comprises space. It doesn't move in space, it is space (and all that it contains).


Might as well throw my own ideas out:

1. Near the surface of a black hole, spacial dimensions become time-like, and the time dimension becomes space-like. If our universe started as a singularity, then our time dimension may be the squashed higher spacial dimension of some higher order universe.

2. Black holes supposedly can have electric charge. If the space around a black hole is distorted as described in 1., the electric charge would appear to the rest of the universe as a magnetic monopole. Black holes are magnetic monopoles.


Err, a black hole having an electric charge and spinning would, I think, produce a magnetic field, but it would m not be a monopole. It would be a magnetic dipole, like an electron.


This is the accepted belief.

I'm considering a more basic model of a static, non spinning but electrically charged black hole. It's really an electromagnetic field not two separate fields, and it seems the gravity would warp the electromagnetic field back around so that from our reference frame we see a magnetic not electric charge aka monopole.


Can someone explain why it takes so long to form an image? The article reads like they still don't have enough data to see anything. From my understand of physical measurement, you start with a `blurry` image (or a graph with a wide peak), and it slowly comes into focus with more data. If the image is too faint because of the signal to instrument precision (noise) ratio is too low, you will never get an image, no matter how much data you have. I was bad at analytical chemistry, so my understanding may be completely wrong.


> "If the image is too faint because of the signal to instrument precision (noise) ratio is too low, you will never get an image..."

I'm curious why this makes sense to you? Does it make sense that you could use an ordinary film camera and take a picture with an arbitrarily short exposure, say a picosecond?

With very, very low SNR it can take a long time to build up data. The reason why this is possible is because the real data, the signal, is correlated with itself, it builds up consistently with each addition of measurements. Noise, however, is uncorrelated, it's random, so it tends to evenly cancel (or even) itself out with additional measurements.


If you're talking about making a map of the astrophysical or cosmological backgrounds of gravitational waves -- you're right. As you integrate (i.e. capture more data), the noise will tend to cancel out while the signal will reinforce itself. But the problem is that the signal-to-noise-ratio is so small -- essentially zero -- that, with current detector sensitivity, it would take literally forever to get an image with any features at all.


> The article reads like they still don't have enough data to see anything.

That's because whatever signal is out there is quite a bit fainter than the initial estimates predicted it would be. If the signal were as strong as the initial estimates predicted, it would have been seen by now.


This reminded me that one of the things I felt would be interesting for Apple to do, would be to drape a conductive net over the center of their new campus with supports to hold a receiver above it at the focal point, turning their headquarters building into a 1,000' diameter radio telescope.


Is not being installed inside a city a big problem for interferences?


I started thinking the electric universe guys were a bunch of crackpots, but I'm wondering if they're actually right.


Come on, remember falsifiability, if the theory wins in any likely scenario it's not science but ideology


missing = we are "unable" to detect




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