
The Science of the Missing Gravitational Waves - pmcpinto
http://nautil.us/issue/38/noise/the-hidden-science-of-the-missing-gravitational-waves
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antognini
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.

~~~
ars
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.

~~~
millstone
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.

~~~
mnw21cam
>> 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.

~~~
snarfy
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.

~~~
ars
> 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.

~~~
meowface
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....](http://www.physicsoftheuniverse.com/topics_blackholes_event.html)

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.

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dexwiz
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.

~~~
InclinedPlane
> _" 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.

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ChuckMcM
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.

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

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DiabloD3
I started thinking the electric universe guys were a bunch of crackpots, but
I'm wondering if they're actually right.

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

------
known
missing = we are "unable" to detect

