
Gravitational waves detected by LIGO - apsec112
http://www.telegraph.co.uk/news/science/12150944/Gravitational-waves-Einstein-was-right-and-this-announcement-is-the-scientific-highlight-of-the-decade.html
======
kbart
Oh come on, can't you wait few more hours for an official announcement? It's
the third (to my count) "announcement of announcement" today.

~~~
ebbv
I think articles coming out ahead of the announcement, explaining to the
general public what the announcement means is actually really helpful. Then
more people who might be interested can turn in to the announcement itself.

~~~
dwightgunning
I agree. It's just the click-baiting that irks me.

------
madaxe_again
I'm am really confused as to how Martin Rees has written such a poor article.
He's bloody Astronomer Royal. I suspect a ghostwriter.

a) Gravitational waves do NOT "shake the mirrors". They make spacetime
contract in orthogonal directions around and within the beam tube (and of
course elsewhere) thus causing the light to travel a tiny bit further in the
tube, thus causing interference by moving the two beams out of phase.

b) As I posted the other day - what about Virgo? What about GEO600? I feel
really sorry for the scientists who've spent decades working on this as part
of a global collaboration to now have LIGO take _all_ of the credit for this
discovery.

~~~
jessriedel
Physicist here. You're off base.

a) Saying "gravitational waves don't shake the mirrors, it's length
contraction" is as confused as saying "the Sun doesn't pull on the Earth, it
bends spacetime around it". In each case, you're describing the same mechanism
with different words. The whole point of the equivalence principle is that
gravitational forces are equivalent to changing the local inertial frame.
Ultimately, the Einstein field equations are what they are, but both types of
description in words are correct (and inexact).

b) Virgo and GEO600 didn't detect gravitational waves. LIGO did.

~~~
mturmon
Just a bit more background on (b) - from the paper in Phys. Rev. Lett.
([https://dcc.ligo.org/LIGO-P150914/public](https://dcc.ligo.org/LIGO-P150914/public)):

"Only the LIGO detectors were observing at the time of GW150914. The Virgo
detector was being upgraded, and GEO 600, though not sufficiently sensitive to
detect this event, was operating but not in observational mode."

The GW150914 event was the strongest one observed in the period LIGO was
operating:

"Detected with ηc = 20.0, GW150914 is the strongest event of the entire
search."

So, the strongest event happened to occur when the other detectors were not
running.

------
cshimmin
> On Thursday, experimenters will report the first detection of a phenomenon
> that has been long predicted

So... this is news before news? I.e. rumor. LIGO is not scheduled to announce
results for another 3.5 hrs.

~~~
apsec112
The author is Sir Martin Rees, the leading astronomer in the UK (Astronomer
Royal), and former president of the Royal Society. It's not some random
journalist.

[https://en.wikipedia.org/wiki/Martin_Rees](https://en.wikipedia.org/wiki/Martin_Rees)

~~~
lmm
He's The Right Honourable Lord Rees these days.

------
sgrytoyr
How do they measure (not just calculate, if I understand this correctly) "a
distance less than a millionth of the size of a single atom"? That sounds very
difficult to do with equipment that is presumably made of atoms.

~~~
Aeolos
It's pretty much as Velox and bbrazil said.

You can use "low-coherence interferometry" to measure tiny signals that would
be undetectable otherwise. Combine a "reference" beam with a "signal" beam and
you get a measurable interference pattern, even when the magnitude of the
signal beam is miniscule.

This is what a real-life interference pattern looks like (I just acquired this
from an actual interferometer illuminating a painted metal surface):

[https://dl.dropboxusercontent.com/u/30682604/fringe_signal.p...](https://dl.dropboxusercontent.com/u/30682604/fringe_signal.png)

This is now an established medical imaging method (Optical Coherence
Tomography) to create 3d scans of biological tissue. It can also be used to
measure distance or elevation changes on a surface of anything from a
micrometer-level scale to a planetary surface. All you need is to use light
with the right wavelengths and two measurements "arms" of roughly the same
length.

~~~
sgrytoyr
That does make sense, but I guess what I still don’t understand is how light
can be reflected by a mirror (made of atoms) so precicely that it doesn’t hide
sub-atomic differences. Won’t the light hit "different atoms" on the mirror,
so to speak, thus changing the distance travelled by much more than fractions
of the size of a single atom?

~~~
snowwrestler
> Won’t the light hit "different atoms" on the mirror, so to speak, thus
> changing the distance travelled by much more than fractions of the size of a
> single atom?

Yes, it will, but that is already represented in the interference pattern.

They're not measuring the absolute distance to the mirror. If they were, you
are right about how the precision would be limited.

Instead they're using the interference pattern to measure a _change_ in the
distance measurement over time. So even though the distance is somewhat of an
average over many atoms, as long as it is the same mirror, it will be the same
average at the same distance.

Because the interference pattern represents photons interfering with each
other, its precision is limited by the size of photons--which are much smaller
than atoms.

~~~
Aeolos
Exactly. The imprecision of the mirror surface (and various other optical
surfaces in the system) cause their own interference pattern that can be
measured and subtracted from the recorded signal.

Even so, measuring gravity waves requires ridiculous amounts of precision in
the construction of the interferometer. I'm working at the 10^-6 scale, where
optics can still be adjusted by hand. They are working at the 10^-21 scale -
the sheer engineering challenge is awe-inspiring.

------
merraksh
Live stream at
[https://www.youtube.com/watch?v=zyo4DFr4D4I](https://www.youtube.com/watch?v=zyo4DFr4D4I)

------
ascorbic
The live stream has moved, and has now started here:
[https://www.youtube.com/watch?v=c7293kAiPZw](https://www.youtube.com/watch?v=c7293kAiPZw)

~~~
pavanky
Thanks! I was stuck at the other stream wondering when it was going to start.

------
joolze
Post something not the telegraph.

Seriously, the only thing this article is says: Physicists and astronomers are
agog. On Thursday, experimenters will report the first detection of a
phenomenon that has been long predicted: bursts of gravitational waves
generated by cosmic collisions of black holes.

That is NOT a report, that's someone guessing what the report will be. There's
not a single quote from anyone remotely involved in the project. This is a
piece of shit summary of a wikipedia entry on LIGO with a clickbait headline.

SAGE

~~~
apsec112
The second paragraph, immediately afterwards, says:

"Sadly it is not unknown for hyped-up scientific claims to be mistaken or
exaggerated - claims of particles going faster than light, gravitational waves
from the big bang, and so forth. I count myself a hard-to-convince sceptic.
But what is being claimed will be the culmination of literally decades of
effort by scientists and engineers with high credentials, and this time I
expect to be fully convinced."

ie., Sir Martin Rees (the author), one of the most famous astronomers in the
world, is putting his credibility behind this. That matters.

------
feider
"This is why it’s been crucial to have two similar detectors separated by
nearly 2,000 miles - one in Washington State, the other in Louisiana - and to
seek events that show up in both detectors, thereby ruling out effects caused
by local seismic events, passing trucks, and so forth."

Sounds like a good old survival bias ;) Seriously though, what is confidence
interval in LIGO?

~~~
olympus
The standard for physics reporting on things like this is "five sigma" or that
the test statistic in their hypothesis test must be greater than 5. Since this
is a one-tailed test, this corresponds to a p-value smaller than 0.00000029,
so the chances of being wrong are about 1 in 3.5 million. This isn't a
"confidence interval" like you were asking about, but I think that it answers
what you meant to ask. Let me know if you wanted something else.

~~~
feider
Thanks, that pretty much explained what I was after.

------
roddux
Headline should read: Telegraph reporter jumps gun for ad revenue

~~~
apsec112
The author is Sir Martin Rees, one of the most accomplished astronomers in the
world, not a Telegraph reporter.

[https://en.wikipedia.org/wiki/Martin_Rees](https://en.wikipedia.org/wiki/Martin_Rees)

~~~
db48x
These two statements are not mutually exclusive.

~~~
apsec112
Per Googling site:telegraph.co.uk "by sir martin rees", Rees has written one
piece for the Telegraph before, in 2013. He doesn't seem to write for other
news outlets (although he has written a book), and if he has any background as
a journalist, Wikipedia doesn't mention it. Calling him a "reporter" seems
quite silly.

~~~
db48x
If you report on current events, you're a reporter. Regardless, the
substantive portion of the statement is 'jumps the gun for ad revenue'.

~~~
realo
My friends invite me to dinner sometimes. They make the meal. That does not
make them chefs, or cooks by profession. At best we can say they cooked that
particular meal.

Being a journalist or reporter by profession is one thing. Saying that someone
who wrote a single news article is a reporter seems to be an abusive usage of
the word. At best we could say that he reported that particular event.

Just saying...

------
bakhy
i want to take my upvote away from this. seeing the title, i foolishly thought
that the conference already took place, and just clicked before reading and
seeing that the title is a lie. :/

------
amelius
Wouldn't it be much more spectacular if these waves were _not_ detected?

~~~
agentgt
I actually was sort of curious as to the opposite. What does it mean if
gravitational waves (that we can detect) are extremely common and become
fairly easy to detect with future technology given todays news?

The article says many expected LIGO to take much longer but it found it rather
earlier equating it to beginners luck (I know the article is written for the
layperson but it leaves much to be desired with that phrase).

You never know we could have telescopes that use Gravitational waves instead
of light.

~~~
jarin
It would mean that there are a lot more black holes in our local group, which
could be a little concerning.

But yeah that's actually an application of LIGO once it's sensitive enough,
because gravitational waves travel through just about anything including those
annoying dust clouds :)

------
lobster_johnson
As an aside, it's hilarious that the announcement was spoiled by Twitter pic
of the backroom marzipan cake posted 8 minutes before the announcement:
[https://twitter.com/marckuchner/status/697802191873114112](https://twitter.com/marckuchner/status/697802191873114112).

------
ck2
I supposed for now it's impossible to detect the source or even general
direction of such waves?

Are gravitational waves also limited to the same speed as light?

Wait, is it the actual fabric of space that is "waving" ? Whoa.

ps. fun fact "razzmatazz" appears in google less than a million times

~~~
MereInterest
The general direction may be possible, but the source itself would not be. The
compression is smallest parallel to the direction of the wave's travel, and is
largest perpendicular to the direction of the wave's travel. LIGO measures a
change in the difference between the lengths of the two arms, ad so it would
be more sensitive to waves moving along one of the arms. It would not be at
all sensitive to waves that are moving perpendicular to both arms.

Since there are two such facilities, located on different parts of the Earth,
they may be able to compare the relative size measured by each facility, and
narrow down a part of the sky. Since the facilities are relatively close to
each other (only 2000 miles, 30 degrees along Earth's circumference), the
margin of error would be very large.

Ideally, to localize the direction, you would have three facilities, each
located at 90 degrees away from the other two, so that you have one facility
"pointed" in each direction. Even then, it would only be able to narrow it
down to 2 possible origins, as the direction of travel of the wave would not
be measurable.

Anything that carries information is limited to the speed of light.
Gravitational waves carry information about the location of the merging black
holes, and so they are limited to the speed of light. If anything that carried
information were to travel faster than the speed of light, it would break
causality, because you could find some frame of reference in which the effect
happened before the cause.

And yes, it is spacetime itself that is vibrating.

~~~
OscarCunningham
How exactly is "speed of light" defined when the spacetime is moving in the
precisely the region in which we are trying to measure the speed of light?

~~~
solipsism
Same way an ant walks a constant speed even if the balloon he's walking on is
being blown up.

------
kkamperschroer
A useful layman's explanation of gravitational waves and the LIGO experiement:
[https://www.youtube.com/watch?v=1Tstyqz2g7o](https://www.youtube.com/watch?v=1Tstyqz2g7o)

------
anotheryou
wild speculation or a broken press embargo?

press conference is streamed in an hour here:
[https://youtu.be/zyo4DFr4D4I](https://youtu.be/zyo4DFr4D4I)

------
srikar
Link to research paper here:
[https://news.ycombinator.com/item?id=11080678](https://news.ycombinator.com/item?id=11080678)

------
antome
Any idea on what the certainty is for these results? 5 Sigma?

~~~
hcrisp
Rumored to be 5.1.
[https://news.ycombinator.com/item?id=11073020](https://news.ycombinator.com/item?id=11073020)

------
amelius
Shouldn't we have multiple detectors, to determine which direction the waves
are coming from? (Like an antenna array can also be direction-specific using
computational techniques).

Or are these detectors already directionally sensitive?

~~~
Steuard
LIGO is based on two detectors, one in Louisiana and one in Washington. With
those, according to today's talk, we can get a rough idea of the direction of
the source: it was in the southern sky, vaguely in the direction of the
Magellanic Clouds. Once some of the other planned experiments come online over
the next few years, they expect to be able to localize sources to within 5-10
degrees on the sky. (Or so I just heard from the press conference.)

------
ww520
This is huge. The amount of precise measurement went into it is remarkable.

