
Where are the missing gravitational waves? - versteegen
https://theconversation.com/where-are-the-missing-gravitational-waves-47940
======
wcoenen
Advanced LIGO has come online sometime in the last 10 days[1] and there is a
rumour that it has already detected a gravitational wave[2].

[1] [http://news.mit.edu/2015/advanced-ligo-begins-
operations-091...](http://news.mit.edu/2015/advanced-ligo-begins-
operations-0917)

[2]
[https://twitter.com/LKrauss1/status/647510799678750720](https://twitter.com/LKrauss1/status/647510799678750720)

~~~
cabinpark
I work in numerical relativity and our group produces many, many gravitational
waveforms. I even visited LIGO this past summer. If Lawrence Krauss isn't
bullshitting (and I have no reason to doubt he would be) then this is
absolutely huge news. Nobel Prize worthy discovery no question.

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biswaroop
You'd think a binary pulsar would be ideal for measuring gravitational waves:
they're precise clocks swimming in rapidly changing gravitational fields.

Interestingly, binary systems of pulsars are the only source of evidence for
gravitational waves. Astronomers use the pulsar timing to detect the slowing
of orbital motion due to gravitational wave emission. I wonder if it's
possible to detect a 'local gravitational wave background' modulating the
pulsar timing on top of the usual modulation from the slowing binary.

~~~
raverbashing
> Astronomers use the pulsar timing to detect the slowing of orbital motion
> due to gravitational wave emission.

Correct, at least that's the theory

But that's not evidence of gravitational waves _per se_ , rather an evidence
that a spinning binary system will have a velocity decay (and gravitational
waves fit the description of what might be happening)

So maybe there's an issue with how we measure it, how it actually affects
matter or "it's complicated"/"it doesn't exist"

~~~
versteegen
But the rate of orbital decay of pulsars in binary systems is very precisely
matched by general relativity, which is going to be very hard to explain with
a different cause.

Given that GR has been verified so thoroughly, while on the other hand models
of the gravitational background depend on so many assumptions and simulations
and approximately measured empirical relationships about supermassive black
hole (SMBH) sizes, occurrences, time for SMBH mergers, etc, as detailed in the
article, clearly this is incredibly weak evidence that gravitational waves
don't exist. But something is wrong somewhere.

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idlewords
I don't understand this sentence: "During this time, we have accounted for
every single one of the neutron star’s 116 billion rotations"

Isn't this pulsar just being watched by one radio telescope in Australia,
which presumably can't point at it 24 hours a day?

~~~
arethuza
If it's a circumpolar star then it won't set - e.g. Polaris doesn't set here
in the UK.

~~~
hugh4
True but I think lisper's answer is correct since I'm sure they haven't kept
the Parkes radio telescope trained on one target for eleven years.

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ibizaman
I like how they state clearly that the result they obtained, even though
unexpected, does mean that there are things that we still do not know, not
that all we know must be burned to the ground.

~~~
snorrah
I also like how their possible suggestions for why theories might not be
panning out points to not enough understanding about something. So now we have
a new scientific understanding to pursue. And on the way to figuring out
/that/ new thing, we'll probably run into even more questions that need
answering!

In some ways, this is exciting - deepening mysteries always provoking further
study. But on the flip side, it also makes me ponder the thought that maybe we
will never gain true knowledge of the universe. At least, probably not before
a very long time passes.

~~~
amelius
> But on the flip side, it also makes me ponder the thought that maybe we will
> never gain true knowledge of the universe.

Therefore, we might need to address the problem from the other end. This
requires a totally different mindset. See for example the work done here: [1],
or see the video lectures starting here: [2]

[1] [http://www.liv.ac.uk/physical-
sciences/events/fpl/](http://www.liv.ac.uk/physical-sciences/events/fpl/)

[2]
[https://www.youtube.com/watch?v=W2XdhzCORbo](https://www.youtube.com/watch?v=W2XdhzCORbo)

~~~
imglorp
It's been 472 since humanity realized the Earth moves around a star, 328 years
since we knew the equations of gravitation, only 100 since we learned about
GR, and only 15 years since we were certain the universe is expanding.

I think we might be excused to take a few more years to nail down the rest of
gravitation. There's an embarrassment of riches of data; it just needs
assembling into a picture.

~~~
JackFr
And 412 since someone noticed "[T]here are more things in heaven and earth,
Horatio, than are dreamt of in your philosophy."

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elorant
Relevant [http://www.dailygalaxy.com/my_weblog/2015/09/-missing-
gravit...](http://www.dailygalaxy.com/my_weblog/2015/09/-missing-
gravitational-waves-in-the-fabric-of-the-universe-leads-to-black-hole-
rethink.html)

------
ars
Gravitational waves contain (are?) energy
[https://en.wikipedia.org/wiki/Sticky_bead_argument](https://en.wikipedia.org/wiki/Sticky_bead_argument)

Since they contain energy, they in turn have mass, and a gravitational field
of their own.

Which is quite an interesting implication, because since they travel at the
speed of light, they can't slow down when interacting with mass, instead they
must do some sort of "blue shift".

It also means that when the waves are bent by traveling near a large mass,
those waves emit daughter waves of their own.

And one part of the gravitational wave is attracted by another part of it.

I suspect that a combination of those processes adds so much noise that there
is no detectable signal left.

~~~
drjesusphd
> Since they contain energy, they in turn have mass,

That is not true...

> and a gravitational field of their own.

... although this is.

It's energy that creates a gravitational field, of which mass is a (usually
dominant) subset. It does not mean anything with energy has mass. See: light.

Edit: also, the basic theory of gravitional waves is "linearized", which means
that this kind of self-interaction is weak. That doesn't mean it doesn't
happen, but it wouldn't resemble radiation if the effect is strong.

~~~
ars
> It does not mean anything with energy has mass

We are using different definitions of mass. I'm calling invariant energy mass,
you are calling rest energy mass.

> but it wouldn't resemble radiation if the effect is strong

The wave does have a frequency though, which must blueshift as it interacts
with matter.

~~~
tfgg
> We are using different definitions of mass. I'm calling invariant energy
> mass, you are calling rest energy mass.

Modern physics only refers to rest mass [1] as 'mass', otherwise people tend
to get confused about how 'relativistic mass' (γm) affects gravity.

[1] E/c^2 in a centre of momentum frame, which doesn't exist for an object
travelling at c

~~~
ars
You misunderstand the difference between rest mass, invariant mass, and
relativistic mass.

Invariant mass is the same in all reference frames, so light can possess it
(if there are multiple photons, or they are in a box).

That is my preferred mass.

Rest mass excludes photons, but there is no reason to do so most of the time,
since the mass of the photon acts exactly like mass in most things except when
dealing with change in velocity.

Relativistic mass is special and depends on the relative motion between the
objects.

In any case gravitational waves posses invariant mass, so essentially they do
have mass, and if you could hold them you could weigh them on a scale.

~~~
drjesusphd
Nothing you're saying makes any sense.

> since the mass of the photon acts exactly like mass in most things except
> when dealing with change in velocity

Right: inertia. Funny that.

The fact that photons are affected by gravitational fields, even though
they're massless, was a major victory for the theory of general relativity. It
wouldn't be a big deal if you could just say the photon has mass. Because it
doesn't.

A photon is massless. Fullstop. To say otherwise is to use a nonstandard
definition of "mass". At best, people won't know what you're talking about. At
worst, they will assume you don't. You should be more specific next time.

~~~
jeff_tyrrill
The parent comment's explanations and terms match the explanations throughout
[https://en.wikipedia.org/wiki/Mass–energy_equivalence](https://en.wikipedia.org/wiki/Mass–energy_equivalence).
Is the Wikipedia article all wrong?

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lbenes
I’ve always felt that theoretical physicists should give more thought to the
implication that gravitational waves don’t exist. Just maybe it would require
reworking relativity in a way that’s more compatible with quantum mechanics.
Sadly, it seems that everyone seems to think the flaw is with the standard
model not the other way around.

~~~
biswaroop
A lack of gravitational waves would mean information can travel infinitely
fast, a Lorentz violation. There's no evidence in QM (or anywhere, really)
that this is true.

~~~
cdelsolar
But why do they have to be "waves"?

~~~
hugh4
Ok, assume gravitational influence travels at the speed of light.

Now imagine you pick up a heavy object (the Earth, say) and wave it back and
forth. Can you see how the spreading gravitational influence manifests itself
as a wave?

~~~
cdelsolar
Thanks, this helped.

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PaulHoule
Maybe they went the same place that the missing mass went.

