
What Will It Mean If LIGO Detects Gravitational Waves? - oneworld
https://www.theguardian.com/science/2016/feb/09/watch-this-spacetime-gravitational-wave-discovery-expected
======
antognini
Not only will the discovery of gravitational waves confirm an important
prediction of general relativity, but it will be the first test of GR in the
strong gravity regime. So far, all the tests of GR have been in the weak
gravity regime, where Newton's law of gravity is approximately correct. The
precession of Mercury's perihelion, gravitational redshift, the gravitational
deflection of light, and the measurement of frame dragging by Gravity Probe B
have all been probing similar regions of parameter space --- slow velocities
(compared to the speed of light) and weak gravity (i.e. far from the event
horizon of a black hole).

If the rumors are true that LIGO has detected the merger of two black holes,
the observed waveform will be the first test of the predictions of general
relativity in the strong gravity limit. This is the most interesting limit
because this is where physics diverges the most from Newtonian gravity.
Moreover, it's much easier to detect deviations from general relativity in
this limit.

LIGO's detection won't be the only test of GR in the strong gravity limit for
too long, however. The Event Horizon Telescope has been trying to directly
observe the event horizon of the supermassive black hole at the center of the
Milky Way (or its shadow, anyway) and should have its first images within a
few years.

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madaxe_again
I'm curious as to why ligo is the only project ever mentioned, particularly
given that they _have_ to work with virgo and geo600 to be able to triangulate
their observations - a two axis interferometer only gives you partial
information.

Better PR team?

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camillomiller
In Italy there's a lot of buzz around Virgo (located near Pisa), but I had the
impression that it's mostly national talk, too.

~~~
madaxe_again
Yeah - I visited virgo years ago when they were about to start calibration -
it's super cool, but then again, I think michelson-morely interferometers are
cool as a category. They were invented for a not dissimilar purpose - testing
the properties of the luminiferous aether seeing if light and matter moved at
different rates depending on the earth's motion through the aether. The result
was negative, and a patent clerk got thinking.

Now his ideas are being put to the test with the same apparatus.

 _So_ cool.

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oneworld
Great explanation of what it might mean:
[http://www.forbes.com/sites/startswithabang/2016/02/09/what-...](http://www.forbes.com/sites/startswithabang/2016/02/09/what-
will-it-mean-if-ligo-detects-gravitational-waves/#34ef8e7e4726)

Also, how effing cool is it that we are able to detect changes in spacetime
itself!

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dang
An announcement of an announcement is always off-topic here, so we changed the
URL from [http://www.ligo.org/news/media-
advisory.php](http://www.ligo.org/news/media-advisory.php) to that article,
which is much more substantive. Thanks!

The announcement itself, of course, will be on topic when it happens, but we
can be patient.

~~~
porsupah
FWIW, there's also [https://www.theguardian.com/science/2016/feb/09/watch-
this-s...](https://www.theguardian.com/science/2016/feb/09/watch-this-
spacetime-gravitational-wave-discovery-expected), with no adblock issues.

~~~
dang
Ok, we changed to that from
[http://www.forbes.com/sites/startswithabang/2016/02/09/what-...](http://www.forbes.com/sites/startswithabang/2016/02/09/what-
will-it-mean-if-ligo-detects-gravitational-waves/#34ef8e7e4726).

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mrfusion
And conversely what would it mean if we never detect gravity waves? Are there
any reasonable theories that posit that gravity waves don't exist?

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madaxe_again
That general relativity is either incomplete or wrong. It would mean that
gravity either propagates far more rapidly or far more slowly than we think.

Given however that c is considered and so far demonstrably the maximum
velocity of information propagation in this spacetime, instantaneous
propagation seems unlikely, particularly given the Mercury orbital
confirmation.

That all said, we may be looking at things naively.

~~~
ars
It could also mean that there are no objects large enough to produce gravity
waves we can detect.

Black holes for example dilate time, and may either not exist (since they take
infinite time to form), or may produce gravity waves that are too time dilated
(i.e. slow) to detect.

~~~
semi-extrinsic
> Black holes (...) take infinite time to form

This is wrong, and based on not taking into account that in GR, simultaneity
is not only a relative thing, but also a local thing. There is much more
detail in several great answers to this Stack Exchange question:

[http://physics.stackexchange.com/questions/5031/can-black-
ho...](http://physics.stackexchange.com/questions/5031/can-black-holes-form-
in-a-finite-amount-of-time)

~~~
ars
Did you actually read the answers you linked to? They say exactly the same
thing: A distant observer never sees a blackhole form, but infalling matter
does. Because GR has no such concept of simultaneity the two observers can
never agree on when the black hole formed.

But since we are here on Earth we care about how it looks to a distant
observer, and from our point of view black holes never form. And what the
infalling matter sees is irrelevant to us.

NOTE: You do not need black holes to make gravity waves, neutron stars will
also make them. But time is still enormously dilated by neutron stars, and I'm
wondering how that affects the gravity waves they make.

~~~
semi-extrinsic
In the original comment I replied to, the point was "black holes cannot form
because we cannot observe it, so black holes don't exist". This is a
misunderstanding, and is what I was pointing out.

Saying that something objectively "doesn't happen" because an observer
infinitely far away can't see it is ridiculous.

~~~
ars
> This is a misunderstanding, and is what I was pointing out.

No. You are misunderstanding things.

> because an observer infinitely far away can't see it

No, it's not that the observer can't see it (too hard to see or something like
that). It never _occurs_ in the timeline of the observer.

From the point of view of the observer it never actually exists. That it
exists from the point of view of someone else is irrelevant. It's not a
semantic game about I can see it you can't.

It literally and actually simply does not exist.

(Not to mention the context is gravity waves, so if it never exists from the
point of view of the observer it also can't make gravity waves.)

~~~
semi-extrinsic
Pardon me if I am being obtuse, but the point of the original comment was
"black holes do not exist in our universe, since time dilation at the event
horizon is infinite", yes? This is what I am arguing against.

The event horizon is within the future null infinity of your observer; i.e.
the observer may choose to travel towards to black hole, and will
asymptotically fall into it and thus undeniably observe it. Hence it does
actually exist even for this observer. Moreover, the observer can observe the
shadow, effect on nearby masses and the gravitational lensing produced by the
black hole, thus indirectly observing the black hole, even though he cannot
see the actual event horizon.

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polemic
Am I right that this is basically this is a super high tech Michelson–Morley
experiment, to detect waves instead of aether?

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InclinedPlane
That's exactly what it is.

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cornholio
I hope Forbes realizes that blocking access to users with add blockers is not
only a way to get rid of freeloaders, but also a way to ensure those people
avoid Forbes altogether in the future and also avoid passing around Forbes
links, since not everybody can see them. Alienating your audience is a very
expensive mistake and the cost savings related to page serving are negligible.

While I can turn off my add blocker, I won't, my mother can't because she's
not aware I have installed one for her and has no idea what it is and what it
does.

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robhu
They do not detect uBlock Origins.

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e40
With the 3rd-party filter "Anti-Adblock Killer | Reek‎" enabled.

