
Scientists struggle to stay grounded after possible gravitational wave signal - naves
http://www.theguardian.com/science/2016/jan/12/gravitation-waves-signal-rumoured-science
======
KenoFischer
LIGO is very serious about doing blinded test runs, even going as far as
writing the whole paper before they check whether the signal was real. There's
only a couple of people in the entire collaboration who even know when the
blinded test runs occur. In any case, this rumor has been going around for a
couple of months, so if it's true, we should know soon. Between this and the
LHC blip, I'm quite excited about the next couple of months.

~~~
lutusp
Another level of experimental blinding would be to avoid conferring with the
other observatory (there are two components to LIGO, one in Hanford, WA and
one in Livingston, LA) for confirmation/correlation until after one site's
evaluation is complete. I don't know whether this is the current practice.

In any case, if the observatories are actually able to detect gravitational
waves and if there are no theoretical obstacles, eventually two black holes
will merge, which will produce a completely fantastic observation consisting
of a rapidly rising frequency (as the two BHs circle each other closer and
closer, shedding their orbital energy as gravitational waves), followed by a
very loud cosmological pop as the BHs finally collapse into each other.

~~~
ars
> eventually two black holes will merge ... as the two BHs circle each other
> closer and closer

I've heard this so many times, and I still don't see how it's possible!
Because of time dilation this merger will take infinite time.

No one has ever explained this satisfactorily to me. Why do people just seem
to completely ignore relativity when dealing with black holes?

~~~
jessriedel
This is the exact same conceptual issue as the formation of a single black
hole (BH). Consider a spherical shell of matter (i.e. GR dust) collapsing
inwards to form a BH. As the dust particles get closer and closer together,
one can roughly say that time slows down for them and that none of the
particles ever pass through their mutual horizon. Rather, the argument goes,
the particles just becomes exponentially close to where that horizon would
form. From the outside, this is indistinguishable from a BH for all practical
purposes, since the only way to isolate the still-free dust particles is to
zoom exponentially close to the horizon and then drag yourself out (which
requires an exponential amount of energy).

The problem with this argument is that there is no preferred global time
coordinate with respect to which you declare that "the BH has formed". Rather,
the spacetime events corresponding to "each dust particle crosses their mutual
horizon" (i.e., "the formation of the BH"), along with the rest of the
interior of the BH, comprise an _additional_ spacetime region you can choose
to attach to your manifold or not. This is a free choice because causality
flows one-way across the boundary; no experiments outside the BH can be
sensitive to whether the region "really" exists. There are multiple ways to
extend the normal time coordinates (at infinity) into the interior of the BH,
and a natural choice is the proper time of in-falling particles/observers. In
that case, the BH definitely forms at a _finite_ values of that time
coordinate.

Merging two existing BHs doesn't introduce new fundamental issues. Quantum
mechanics _does_ introduce new fundamental issues, but formally the jury is
still out on whether it resolves this question. The majority opinion is that
the interior of the BH exists just as much as regions beyond your cosmological
horizon (in an expanding cosmology) exist, from which you are likewise
causally disconnected

~~~
ars
> This is the exact same conceptual issue as the formation of a single black
> hole (BH).

I know. No one has ever explained how those can form either.

> From the outside, this is indistinguishable from a BH for all practical
> purposes

Except without the physics breaking properties of black holes. So not the same
at all.

> The problem with this argument is that there is no preferred global time
> coordinate with respect to which you declare that "the BH has formed".

I don't need a global time, my own time frame is perfectly fine with me. If a
black hole can never form, as viewed by me, then black holes don't exist.

> This is a free choice because causality flows one-way across the boundary

This presupposes the black hole exists already, but we have not gotten there
yet. First it has to form, and it can not.

> and a natural choice is the proper time of in-falling particles/observers.
> In that case, the BH definitely forms at a finite values of that time
> coordinate.

It gets interesting here - what does happen from the POV of that particle? I
suspect it never sees a black hole. This particle is moving exponentially
close to the speed of light, relative to the center of mass of the forming
black hole.

This means that time is stopped for the center of mass of the black hole from
the POV of the particle, which means the center of mass can never form a black
hole since it can never do anything at all (since time is stopped for it).

~~~
lutusp
> If a black hole can never form, as viewed by me, then black holes don't
> exist.

But they do -- the evidence is copious:

[http://www.galacticcenter.astro.ucla.edu/animations.html](http://www.galacticcenter.astro.ucla.edu/animations.html)

Arguing that, because the event horizon collapses the time dimension (from the
perspective of a distant observer), therefore BHs aren't real, is like saying
that sum(2^-n,(n,1,oo)) can't equal 1 because it's an infinite series and an
infinity of additions takes too long.

Think of black holes as objects of great mass, that have very bright accretion
disks, surrounding an event horizon that is perpetually almost forming a
singularity but never quite completes it (from the perspective of someone far
from the horizon).

> This means that time is stopped for the center of mass of the black hole ...

From the perspective of a distant observer, the time horizon is located at the
event horizon.

~~~
ars
> But they do -- the evidence is copious:

The evidence is non-existent. The only evidence is for supermassive objects,
not event horizons.

> Arguing that ... BHs aren't real, is like saying ... an infinite series and
> an infinity of additions takes too long.

That's a terrible argument. One is an actual physical process, the other is a
mathematical abstraction. Do you not understand infinity? Infinity means you
can't actually do it. It doesn't mean you can't calculate it.

> Think of black holes as objects of great mass, that have very bright
> accretion disks, surrounding an event horizon that is perpetually almost
> forming a singularity but never quite completes it (from the perspective of
> someone far from the horizon).

Eh? If it never forms a singularity, it has no event horizon. i.e. it has none
of the physic breaking properties of black holes.

And accretion disks don't have to be bright BTW, as stuff falls in time
dilation makes them much less bright (they "slow down", and emit photons less
often).

> From the perspective of a distant observer, the time horizon is located at
> the event horizon.

When I say center of mass of the black hole I am naming the object I am
talking about, not describing the location of the time horizon. Sorry if it
was confusing.

~~~
lutusp
>> But they do -- the evidence is copious:

> The evidence is non-existent.

If you're familiar with Occam's razor, you know that, for a given phenomenon,
the simplest explanation is to be preferred. There certainly are massive
objects at the center of most galaxies, and the most economical explanation is
that they're black holes. No one is closing the book on these ideas -- that
would be unscientific -- but there is strong evidence that these things are
black holes.

That they exist is beyond question. That they have super-high masses confined
to a small space is beyond question. General Relativity then fills in the
blanks in our observations.

> That's a terrible argument. One is an actual physical process, the other is
> a mathematical abstraction.

So is General Relativity. But because nature follows the same rules, General
Relativity accurately predicts many aspects of reality (certainly not all).

> And accretion disks don't have to be bright BTW ...

Well, yes, they do -- they're giving up a huge amount of gravitational
potential energy, and that energy must be conserved -- it's converted to
kinetic energy, it becomes heat and light.

Consider a satellite in an elliptical orbit. At the orbital apoapsis,
potential energy is highest and kinetic energy is lowest. Not surprisingly,
this is the point of smallest velocity. Now consider periapsis -- it's the
reverse: lowest potential energy, highest kinetic energy, therefore highest
velocity.

In a normal planetary elliptical orbit with no friction, even though the two
kinds of energy are constantly changing, they sum to a constant at each point
in the orbit, so there's no net dissipation of energy. But as matter falls
into a black hole, in order for it to pass through the event horizon, it must
give up most of its energy (both potential and kinetic). Most of that energy
is converted into heat and light.

Most accretion disks preserve the rotational momentum of the infalling mass,
so they're in the form of disks surrounding the event horizon. In theory, mass
could fall directly into the BH without any rotation -- in that case, there
would still be a huge energy conversion process and much radiated energy. Sp
accretion disks are very bright.

Here's a graphic of the imagined BH from "Interstellar", thought to be fairly
accurate as movie graphics go:

[http://www.wired.com/2014/10/astrophysics-interstellar-
black...](http://www.wired.com/2014/10/astrophysics-interstellar-black-hole/)

> Eh? If it never forms a singularity, it has no event horizon.

Yes, and by the same reasoning, because a Calculus limit expression never gets
to its limit, therefore Calculus is bunk. My point? Just as (1+1/n)^n
_approaches_ e as n _approaches_ infinity
([http://www.wolframalpha.com/input/?i=limit+n+-%3E+inf%2C+%28...](http://www.wolframalpha.com/input/?i=limit+n+-%3E+inf%2C+%281%2B1%2Fn%29%5En)),
a black hole _approaches_ its final form as mass _approaches_ it from the
perspective of a distant observer.

You might as well have argued that, because this classic-physics gravitational
equation:

f = (G m1 m2) / r^2

assumes that all the mass is a dimensionless point at the center of the
central body, therefore if m1 and m2 occupy the same point in space, the
result is infinite, therefore (because infinity isn't a number) Newtonian
gravitation is bunk.

~~~
ars
> There certainly are massive objects at the center of most galaxies, and the
> most economical explanation is that they're black holes

That is just not true. To be a black hole it must first be possible for black
holes to exist, otherwise they are simply supermassive object - to us they
would look EXACTLY the same!

A black hole is NOT the simplest explanation! It's the most complicated one.
You are using your razor very incorrectly.

> That they exist is beyond question.

Absolutely not true. That massive objects exist? Sure. Are they black holes?
Not so clear.

> That they have super-high masses confined to a small space is beyond
> question. General Relativity then fills in the blanks in our observations.

No, actually General Relativity does NOT fill in the blanks - it claims things
are impossible. The math does the equivalent of divide by zero.

> So is General Relativity. [a mathematical abstraction]

Great, so since GR predicts it would take infinite time, I guess case closed.
Since infinite time, by definition, can not actually happen, it means black
holes can not form. Just because you can calculate what something will do
doesn't mean it will actually do it.

> Well, yes, they do -- they're giving up a huge amount of gravitational
> potential energy, and that energy must be conserved -- it's converted to
> kinetic energy, it becomes heat and light.

Kinetic energy does not become heat and light. Kinetic energy is velocity. To
make heat and light you need some other process. Have you never heard of
conservation of momentum?

On top of that, the discs are dim because time is dilated to nothing not
because there is no energy there.

> But as matter falls into a black hole, in order for it to pass through the
> event horizon, it must give up most of its energy (both potential and
> kinetic). .... in that case, there would still be a huge energy conversion
> process .... Most of that energy is converted into heat and light.

How in the world are you writing this stuff? You can't just "give up"
velocity. You have to transfer that velocity to something else. You can't just
wave your hands and say "magic". Friction might cause objects to equalize in
velocity, but can not actually reduce velocity (only share it amongst the
objects).

> a black hole approaches its final form as mass approaches it from the
> perspective of a distant observer.

And takes infinite time to do so, and since right now is not infinitely in the
future, right now, today, black holes do not exist.

Infinite. Do you not understand that word? It means forever, it means it never
actually does it, just gets closer and closer.

> therefore if m1 and m2 occupy the same point in space, the result is
> infinite, therefore (because infinity isn't a number) Newtonian gravitation
> is bunk.

Nice question, wrong conclusion. The actual conclusion is that since infinity
is not actually seen in the real world (as opposed to math), there must be
some method that prevents the two masses from ever occupying the same spot.

Your argument is entirely backward, you think that if infinity shows up in the
math then it must show up in the real world. No. If infinity shows up in the
math then your math is wrong, or incomplete. In this case there is something
else that prevents r from hitting zero.

Maybe you should read this:
[https://en.wikipedia.org/wiki/Domain_of_a_function](https://en.wikipedia.org/wiki/Domain_of_a_function)

~~~
lutusp
>> it's converted to kinetic energy, it becomes heat and light.

> Kinetic energy does not become heat and light.

So have you told the LHC people that they're misguided for thinking they can
produce the Higgs Boson by colliding protons together? I'm sure you can
persuade them how wrong they are.

> Kinetic energy is velocity.

Yes, among many other things:
[http://physics.info/energy/](http://physics.info/energy/) (see the table
"Types of kinetic energy")

Electromagnetic energy (heat and light) is a kind of kinetic energy.

But you know what? I've decided not to try to correct all the errors you've
made. If you actually understood physics, you wouldn't have objected to the
things you have (you might have found a better reason to object).

> Friction might cause objects to equalize in velocity, but can not actually
> reduce velocity ...

Wow. Those fools in Europe are sure misguided to have spent so much money on a
circular 17 mile pipe dream. if only they had consulted with you before
building that monstrous contraption. They actually think their gadget can
convert near-light-speed velocities into many other kinds of energy, including
electromagnetic, and the creation of particles from energy and the reverse. Do
them a favor -- set them straight, as you've set me straight.

~~~
ars
You have GOT to be trolling me. There is no way.

The LHC collides TWO protons! The net velocity between them is zero.

You seem to know at least some physics, how can you possibly not know this?

But in your black hole scenario there are not two opposite velocities adding
up at the horizon.

You can not just directly turn velocity into light, it's not possible, you
need another object.

> Electromagnetic energy (heat and light) is a kind of kinetic energy.

Some people define only two kinds of energy: Potential and Kinetic, in that
definition electromagnetic energy is a type of kinetic energy, and velocity is
a type of kinetic energy.

But it does NOT follow that electromagnetic energy is the same as velocity
energy!

It's like an apple is a fruit and an orange is a fruit, but an apple is not an
orange.

> They actually think their gadget can convert near-light-speed velocities
> into many other kinds of energy, including electromagnetic, and the creation
> of particles from energy and the reverse.

I take it you've never heard of conservation of momentum? You can not just
turn the velocity into other things, you have to react it with something else
(which is what I said - go back and read it). In this case they react it with
a proton going the other way.

They could also react it with the earth, or a slab of metal.

None of these apply to a black hole.

We have also gotten way off topic from time dilation.

~~~
lutusp
> The LHC collides TWO protons! The net velocity between them is zero.

Yes, but only because their original velocity has been converted into other
forms of kinetic energy, like heat and light, among other things like Higgs
bosons. Here is what you said:

> Kinetic energy does not become heat and light.

I proved your statement to be false. End of story. Now stop trolling.

~~~
ars
LOL LOL LOL.

You most certain did not.

Is this like some kind of contest to you to see if you win? Even if you win,
you'll still be wrong.

Instead of focusing on that, why not actually learn something?

"To make heat and light you need some other process." For example, another
proton to crash into. You can't just directly convert the velocity to energy,
you need another object take the velocity from you.

~~~
lutusp
Before:

> Kinetic energy does not become heat and light.

Now:

> "To make heat and light you need some other process."

Translation: "You were right." Apart from the fact that _heat and light are
kinetic energy._

Further reading: [http://www.scientificamerican.com/article/gravitational-
wave...](http://www.scientificamerican.com/article/gravitational-wave-rumors-
in-overdrive/)

Quote from the above article: "LIGO's detectors, at their current sensitivity,
could pick up only gravitational waves of frequencies well above 10 Hertz. So
if they have picked up a signal, they would have captured the final stages of
the drama, when the black holes circle more than 10 times per second, speeding
up to several thousand times a second just before they merge. Simulations show
that the resulting waves, if played as an audible sound, resemble a bird’s
‘chirp’ ([http://www.ligo.org/science/GW-
Inspiral.php](http://www.ligo.org/science/GW-Inspiral.php))
([http://www.ligo.org/science/GW-
Overview/sounds/chirp40-1300H...](http://www.ligo.org/science/GW-
Overview/sounds/chirp40-1300Hz.wav))."

My original remark from yesterday, to which you ignorantly objected: "...
eventually two black holes will merge, which will produce a completely
fantastic observation consisting of a rapidly rising frequency (as the two BHs
circle each other closer and closer, shedding their orbital energy as
gravitational waves), followed by a very loud cosmological pop as the BHs
finally collapse into each other."

~~~
ars
You clearly have problems reading.

I have said exactly the same thing the entire time, only you kept ignoring it.
Or, to be charitable, maybe just not understanding it. Just because you don't
understand the words I said, doesn't mean I didn't say it.

Do you need me to quote myself?

Also, I take it you did not ever study physics? If you did you would know this
on your own without me telling you.

What I said, over and over, is you can not turn velocity into light directly,
you have to transfer that velocity to something else. In an event horizon
there is nothing to transfer the velocity to, since everything is moving more
or less at the same speed.

I don't know why you can't seem to absorb this. I really don't. It's basic
highschool physics, plus I've said it over and over and over and over. In as
many different ways as I can think of.

> Apart from the fact that heat and light are kinetic energy.

Are you totally incapable of absorbing information? I know that sounds
insulting, but I am serious. You keep saying the same wrong thing over and
over despite being told different.

You are wrong. Period.

It's like you have a mental block stopping you from understanding this, I
don't get it.

> ... eventually two black holes will merge, which will produce a completely
> fantastic observation consisting of a rapidly rising frequency (as the two
> BHs circle each other closer and closer, shedding their orbital energy as
> gravitational waves), followed by a very loud cosmological pop as the BHs
> finally collapse into each other."

Back to this, very nice. The problem is that they won't actually do this -
time dilation means it will take infinite time for this to happen, so we'll
never see this.

Not sure what the point of telling you this is though. If you can't understand
Conservation of Momentum there is zero chance you'll understand relativity.

Try this - stop arguing with me. Clearly I'm not the teacher for you.

Go find some other people - show them this thread if you like. Then have them
explain Conservation of Momentum to you. Also ask them to explain the
difference between kinetic and electromagnetic energy.

Please do that before replying, otherwise we'll just be going in a circle.

~~~
lutusp
> I have said exactly the same thing the entire time

This is a lie -- you keep changing your claim to keep up with the facts other
people provide you with. Examples below.

> What I said, over and over, is you can not turn velocity into light
> directly, you have to transfer that velocity to something else.

That's not what you said originally, and you are now lying. You said that
"Kinetic energy does not become heat and light. Kinetic energy is velocity."
Both statements are false (velocity is one of many examples of kinetic energy
but the reverse claim is an error of omission), and I provided the literature
references, which you refused to read.

You earlier said:

> Friction might cause objects to equalize in velocity, but can not actually
> reduce velocity (only share it amongst the objects).

Totally false -- velocity is not conserved, energy is conserved. There's no
conservation of velocity law. If you were a scientist, if you knew anything
about physics and possessed intellectual integrity, you would drop a lump of
clay, watch it abruptly stop at the floor as it efficiently converts velocity
into heat. Or two billiard balls on a collision course, or two protons at the
LHC, etc. etc..

> Clearly I'm not the teacher for you.

"The teacher?" You're utterly ignorant and a narcissist to boot. You're
severely mentally ill, unfortunately there's no cure for malignant narcissism
([https://en.wikipedia.org/wiki/Malignant_narcissism](https://en.wikipedia.org/wiki/Malignant_narcissism)).

We're done -- you have to learn this on your own without embarrassing yourself
further in this public forum. Have a nice day.

~~~
ars
> velocity is not conserved, energy is conserved. There's no conservation of
> velocity law

But there is, it's called Conservation of Momentum. It's velocity times mass.
Go look it up - it will take just a few seconds on google. I'll wait.

> you would drop a lump of clay, watch it abruptly stop at the floor as it
> efficiently converts velocity into heat

No, it transferred its velocity to the entire planet.

> Or two billiard balls on a collision course

One has positive velocity, the other negative. They cancel out.

But go try making two billiard balls stop when they are both going the same
way (as in your example about particles near the event horizon). You can't.
Try it - maybe it will help. You have to transfer the velocity to something
else.

> We're done -- you have to learn this on your own without embarrassing
> yourself further in this public forum. Have a nice day.

Oh, we were done ages ago.

One you do learn some physics you'll recall this conversion and cringe in
embarrassment, not at your ignorance, but at your refusal to learn.

Your attempt at "diagnosing me" doesn't perturb me, because I know this stuff
backward and forwards. Your obstinacy just amuses me. This stuff is so basic
your comments are just laughable.

~~~
dllthomas
_" One you do learn some physics you'll recall this conversion and cringe in
embarrassment, not at your ignorance, but at your refusal to learn."_

Just FYI, your conversational partner's website says:

 _" Paul Lutus has a wide background in science and technology. He designed
spacecraft components for the NASA Space Shuttle and created a mathematical
model of the solar system used by the Jet Propulsion Laboratory during the
Viking Mars mission."_

~~~
ars
That doesn't make any sense at all. How is it possible that someone with a
"wide background in science" has not heard about conservation of momentum?

Are you sure it's the same person?

~~~
dllthomas
Am I sure? I've done no verification. But while I'm occasionally frustrated
with the holder of that account I've seen no particular reason to believe that
they are not who they claim to be. I am sure that the account holder claims to
be Paul Lutus, as the link is on his profile.

 _' How is it possible that someone with a "wide background in science" has
not heard about conservation of momentum?'_

It's probably not; but this comment thread (wading through the jibes, snipes,
and nonsense) doesn't seem to demonstrate with much clarity that he hasn't.

------
bootload
_" In 2014, researchers on another US experiment, called BICEP2, [0] called a
press conference to announce the discovery of gravitational waves, but others
have since pointed out that the signal could be due entirely to space dust."_

The BICEP2 team thought they detected gravitational waves with their Cosmic
Microwave Background (CMB ) measurements from their Antarctic
observatory.[0],[1],[2] By chance the ESA had the Planck observation satellite
measuring a broader spectrum. Matching the two observations the BICEP2
measurements were capturing light interference from space dust in-between the
observatory and the source. [3]

If gravitational waves are found, this will be the biggest thing since Smoot
and COBE [4] and confirmation of the Gurth Inflation theory. [5]

[0] [http://www.theguardian.com/science/2014/mar/17/primordial-
gr...](http://www.theguardian.com/science/2014/mar/17/primordial-
gravitational-wave-discovery-physics-bicep)

[1]
[https://www.cfa.harvard.edu/CMB/bicep2/](https://www.cfa.harvard.edu/CMB/bicep2/)

[2]
[https://www.cfa.harvard.edu/CMB/bicep2/science.html](https://www.cfa.harvard.edu/CMB/bicep2/science.html)

[3]
[http://www.theguardian.com/science/2014/sep/22/gravitational...](http://www.theguardian.com/science/2014/sep/22/gravitational-
waves-space-dust)

[4]
[http://aether.lbl.gov/www/projects/cobe/](http://aether.lbl.gov/www/projects/cobe/)
and
[https://en.wikipedia.org/wiki/George_Smoot](https://en.wikipedia.org/wiki/George_Smoot)

[5] [http://www.nytimes.com/2014/03/18/science/space/detection-
of...](http://www.nytimes.com/2014/03/18/science/space/detection-of-waves-in-
space-buttresses-landmark-theory-of-big-bang.html)

~~~
privong
> The BICEP2 team thought they detected gravitational waves with their Cosmic
> Microwave Background (CMB ) measurements from their Antarctic observatory.
> By chance the ESA had the Planck observation satellite measuring a broader
> spectrum. Matching the two observations the BICEP2 measurements were
> capturing light interference from space dust in-between the observatory and
> the source.

This is a bit different, though. BICEP2 was claiming a detection of a
polarization signal in the cosmic microwave background, which if present,
would point to existence of primorial gravitational waves resulting from
cosmic inflation. But, as you note, the multi-wavelength observations of
Planck (not taken by chance, but part of a carefully planned all-sky survey
with somewhat coarser resolution compared to BICEP2) showed that the
polarization signal measured by BICEP2 was consistent with foreground dust,
rather than primordial gravitational waves.

> If gravitational waves are found, this will be the biggest thing since Smoot
> and COBE and confirmation of the Gurth Inflation theory.

It will be huge, but the gravitational waves LIGO would detect are not
primordial or from inflation, but rather from supernovae and compact binary
mergers (neutron star pairs, stellar-mass black hole pairs, etc.). So a LIGO
gravitational wave detection would not provide any evidence for or against
inflation. But it would tell us a lot about stellar explosions and the
evolution of compact binaries.

EDIT: I should note that LIGO could detect or place constraints on some
"exotic" gravitational wave sources – cosmic strings, for example.

~~~
bootload
_" It will be huge, but the gravitational waves LIGO would detect are not
primordial or from inflation, but rather from supernovae and compact binary
mergers (neutron star pairs, stellar-mass black hole pairs, etc.)."_

I missed this. How do they tell the difference? (Interferomety?)

~~~
privong
Generally, the difference is direct vs indirect detection. LIGO aims to
directly detect gravitational waves as they pass Earth by measuring the change
in length of two parallel arms (using interferometry). In contrast, BICEP2 was
looking for a polarization signal in the cosmic microwave background (CMB)
which would occur if there were primordial gravitational waves (but this
signal could also be mimicked by foreground dust).

So BICEP2's claimed detection wasn't a direct detection of gravitational
waves. BICEP2 detected polarization (specifically B-mode polarization[0]) in
the CMB. The BICEP2 team initially thought the amount of foreground dust was
too low to explain the degree of polarization they observed, so they
attributed the polarization to primordial gravitational waves affecting the
cosmic microwave background. As you noted, Planck had multiple bands and so
could more effectively determine the amount of foreground dust. Using a joint
BICEP2+Planck dataset, the teams determined the signal could be explained by
foreground dust.

In contrast, LIGO is attempting to directly measure gravitational waves from:

1) discrete, individual events, such as a merger of neutron stars. As the
objects inspiral, they emit gravitational waves, with the emission becoming
stronger (and, presumably higher frequency) as the objects near final merger.
At final coalescence, a burst of gravitational waves is emitted. It is
hoped/expected that this burst (plus some GW emission leading up to the
merger) will be detected by LIGO for individual sources.

2) A gravitational wave background from the ensemble of events too far away to
detect individually, but whose sum adds up to enough to be detectable. In
contrast to the above, this will be some linear combination of all the passing
waves from distant individual events, from a variety of directions.

The frequency of the gravitational waves depends on the bodies involved. With
LIGO's 2km arms, it is sensitive to a range of frequencies which correspond to
expectations for stellar-mass compact binaries. The LISA project[1] would have
longer baselines and so would be sensitive to mergers of more massive objects,
such as supermassive black hole pairs. So, the baseline of your gravitational
wave experiment determines what you can possibly measure. This graphic shows
at which frequencies various phenomena emit[2]. Based on that graphic, LIGO
may be sensitive to the frequencies corresponding to primordial gravitational
waves, but they are likely far far below LIGO's detection ability. My
impression is that direct detection of primordial gravitational waves will not
be possible for a while, if ever. Rather, they may be detected indirectly, via
the imprint they leave on the CMB polarization (as BICEP2 tried).

[0]
[https://en.wikipedia.org/wiki/Cosmic_microwave_background#Po...](https://en.wikipedia.org/wiki/Cosmic_microwave_background#Polarization)

[1] [http://lisa.nasa.gov/](http://lisa.nasa.gov/)

[2]
[https://en.wikipedia.org/wiki/File:The_Gravitational_wave_sp...](https://en.wikipedia.org/wiki/File:The_Gravitational_wave_spectrum_Sources_and_Detectors.jpg)

EDIT: Added new first paragraph and reworded the rest for clarity/flow.

~~~
bootload
thanks @privong, this is a first rate explanation of LIGO/BICEP2 and CMB and a
major reason I hang out on HN.

~~~
privong
Sure thing, @bootload.

By the way, I should have mentioned that we already have indirect evidence for
gravitational waves, from the orbital decay of binary pulsars such as PSR
B1913+16 [0]. The observed orbital decay has an excellent match with the
orbital decay predicted using general relativity and gravitational waves. [1]
shows the agreement, with the line being a theoretical prediction and not a
fit.

[0]
[https://en.wikipedia.org/wiki/PSR_B1913%2B16](https://en.wikipedia.org/wiki/PSR_B1913%2B16)

[1]
[https://en.wikipedia.org/wiki/PSR_B1913%2B16#/media/File:PSR...](https://en.wikipedia.org/wiki/PSR_B1913%2B16#/media/File:PSR_B1913%2B16_period_shift_graph.svg)

------
cmpb
Off-topic, but if anyone has a chance to go tour the LIGO facilities,
definitely take it! It is a very interesting experience, and highly
educational, even for those that don't have a physics background. I've been to
the one in Livingston while studying at LSU, and even after several years, I'm
still astounded at the precision of the technology used there.

