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[dupe] Gravitational waves detected by LIGO (telegraph.co.uk)
454 points by apsec112 on Feb 11, 2016 | hide | past | web | favorite | 97 comments

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.

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.

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

In other news, Perl 6 is done!


you can do everything for sweet sweet internet points.

For publishers it's sweet, sweet internet munies.

For the official announcement, where you'll hear all of the sound bytes you'll be reading later today, you can get to the livestream from Caltech's website here https://www.caltech.edu/

Starts in 2 minutes.

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.

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.

Just a bit more background on (b) - from the paper in Phys. Rev. Lett. (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.

Also a physicist. I may have let my passion for what the guys at virgo have been up to bias my judgment - visited years ago when they were commissioning the place, and had a long, long discussion about the problems they overcame, mostly around optics and noise, which they shared solutions for with the ligo folks. Now the paper is out I've untwisted my knickers as I can see they've cited the teams around the globe. The media up until that point kinda gave the impression it was a sole effort.

Giant's shoulders and all that.

Re b): Virgo will hopefully soon, and I think they did give appropriate credits to GEO600 in the press conference.

Blame the sub-editors at the Telegraph, what's left of them (or more accurately it's CE Murdoch MacLennan and the weirdo Barclay Brothers that own it). That paper has been on a gradual downwards trajectory in quality for a long time now, and it's rare thing that it produces anything resembling actual journalism.

It's gradually morphed from being a Times for mildly xenophobic Conservatives (big C) into a Daily Mail for toffs.

From Yes, Prime Minister in '87:

The Morning Star is read by people who think the country ought to be run by another country. The Daily Telegraph is read by the people who think it is.


> Gravitational waves do NOT "shake the mirrors".

That depends on what gauge you're working in. What you say is true for the TT gauge, but not for other choices of gauge. See, e.g., section 1.3 of Maggiore's Gravitational Waves book.

a) He says just that, three paragraphs before he uses the shorthand "shakes" - which is in quotes, indicating the inaccuracy of the usage.

True, but it's a poor analogy. I've read many of his books - and it feels like this got editorialised or ghostwritten into "wrong" - he is usually a precise and excellent explainer of complex concepts.

Maybe he makes mistakes, or has trouble explaining some concepts, from time to time?

> 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.

You might as well say the same thing about CERN and the Higgs. That's how credit for discoveries works. You can't just give it to "the whole scientific community" so you pick the most specific attribution that makes sense.

You absolutely could though. Here's what you could say:

"Even though this discovery is our success, I would like to emphasize the fact that none of this would have been possible, had it not been for decades of work in this particular field by thousands of people who dedicated lives to pave the way for this final success. Just like Rome wasn't built in one day, such important discoveries are always the visible part of a huge network of dedicated individuals. This work will serve as a testimony to their hard work, we did build on the shoulders of giants and we'll always be grateful. Now's the time for celebration, but also for taking comfort in the knowing that the coming generations of scientists and engineers will build upon our findings for even greater discoveries in the future. To everyone, thank you."

That kind of stuff is both respectful of past discoveries in the field and creates a sense of community in science as a whole. It's much better in my opinion that singling out a couple of people, who despite their obvious hard work and brilliance, didn't single-handedly come up with the whole concept.

I don't know. Sounds a little like giving everybody a participant ribbon.

Scientists and engineers, more than anybody, understand that they are standing on the shoulders of giants.

I watched the press conference, and they pretty much did that. Generally, I feel that in physics, this is well handled. That's what citations are for.

Yes that is respectful, and a gracious way to announce it. But you'll note they still accepted credit and get attributed with credit.

it's understood that research builds upon past research which is documented and cited. i think that accomplishes the majority of what you're saying, albeit more tersely.

Which would be ligo, virgo, and geo600. It is not possible to triangulate the origin of gravitational waves with only two detectors.

It would be like crediting one section of the beam tube at cern for detecting the higgs.

well they do tend to credit the detector at lhc for the discovery. why won't people think of the tubes and all the other wires :(

If you watch the announcements you will notice that the individual detector teams do thank them for providing them with an excellent beam to work with, etc..

Also I would like to mention that unlike in the GR wave case the detectors do make the discoveries independently. Having multiple independent detectors on one accelerator is one cheap way to take care of reproducibility of results.

> LIGO take all of the credit for this discovery.

This was one of the purposes of LIGO. To prove what what previously conjecture. So yeah, goLigo.

> 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.

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.


He's The Right Honourable Lord Rees these days.

It's common for journalists to be told science news before an official announcement if they agree not to publish on it until afterward. This is called an embargo.

If it is a rumor it's probably on very good authority. The author appears to be extremely reputable.

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.

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):


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.

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?

> 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.

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.

Thanks, excellent explanation.

From what I understand, it doesn't really matter because the laser light acts as a coherent wave, so it doesn't really bounce off of individual atoms, per se. As long as the mirrors are flat, all that matters is the relative distance between the mirrors.

Thanks. Btw: "how ... looks like" -> "what ... looks like"

Cheers, fixed. I wrote this in a hurry while calibrating my device ;)

When light interferes with itself it creates a "pattern". When the light beams are out of phase, the interference pattern is different. The gravitational waves basically minutely increase the distance that one of the beams travels and causes it to go out of phase with the other, thus changing the interference pattern.


Basically fire two lasers at right angles to each other at mirrors, and see the pattern when they bounce back.

BTW, you can build your own Michelson interferometer (when it was first constructed, it was a technical marvel, involving a large hunk of metal floating on mercury, deep down in a building, and even then the experiment was affected by subtle vibrations): https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=66...

You can probably use sub-atomic features of atoms to do sub-atomic measurements.

For example their electromagnetic fields may be shifting depending on distance to another atom or another field.

PS: I am no physicist.

The live stream has moved, and has now started here: https://www.youtube.com/watch?v=c7293kAiPZw

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

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.


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.

You are a very skeptical, no BS, tough nut to crack kind of guy. Good! I like that! HN, the Internet, the world needs much more of that. Such high standards make at least some of HN a crown jewel of the Internet place.

Still, back to common, ordinary, reality, Sir Rees does have a lot of credibility.

"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?

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.

Thanks, that pretty much explained what I was after.

Headline should read: Telegraph reporter jumps gun for ad revenue

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


These two statements are not mutually exclusive.

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.

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

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...

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. :/

Wouldn't it be much more spectacular if these waves were not detected?

At the moment, it would tell us more about cosmology than about the underlying physics, were we not to detect the gravitational waves. We expect gravitational waves to be emitted from two black holes as they orbit very close to each other, just before they merge. From general relativity, we know how big of gravitational waves to expect. However, we don't know how many such black hole mergers occur in nearby space.

LIGO can only measure gravitational waves down to a certain strength, and so we can only detect waves that are caused by black hole mergers within some distance of Earth. Predictions of how many such mergers occur vary wildly, depending on the assumptions in the model. (I don't have exact numbers, and would appreciate if anyone does have the numbers.)

If we did not detect gravitational waves, I would use that as a lower bound of the number of black hole mergers, rather than as a breaking of general relativity.

Not observing any gravitational waves wouldn't prove that gravitational waves didn't exist, so there wouldn't be nearly as much fuss and news articles written. Imagine the press release: "Jury still out on existence of gravitational waves: LIGO not sensitive enough/ Is LIGO a gazillion dollar waste?." So while if someone were able to PROVE that gravitational waves didn't exist that would be pretty spectacular because it would upend decades of scientific research and cause us to rethink relativity.

But this is still pretty spectacular in it's own right. The predictions that physicists make are made by examining mathematical models that are only partially vetted. Some parts of the models remain theoretical because we don't have instruments sensitive enough to measure their predictions (we are talking about energies on a ridiculous tiny scale here). Finally we have invested millions on an instrument that is sensitive enough and it confirms (to the best of its ability) that we were on the right track. This is a great relief to the scientists that they don't have to go back to the drawing board and rethink all their models, plus it is a great victory for predictions made before the age of computers.

If they were not detected, that wouldn't necessarily say much unless you could demonstrate certainly that they also should have been. In which case, yes. Though, in actuality, the physics of gravitational waves have been indirectly observed previously [0], so it's not like it's still entirely in the realm of hypothesis.

[0]: The Hulse–Taylor binary pulsar: https://en.wikipedia.org/wiki/PSR_B1913%2B16

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.

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 :)

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.

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

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.

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?

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

It's deforming (stretching) not translation (moving).

Would the rotation of the earth give some clues? Eg, the detectors are rotating, so the strength of the signal will change over time, assuming the signal lasts long enough.

Unfortunately, no. The signal only lasts for a few seconds, not long enough for the Earth to rotate any significant amount.

It's possible, there are three detectors hundreds of miles from each other, so they can use triangulation to get a fairly accurate read of the source.

It's two detectors, each of which has two tunnels at right angles to each other (and generate an interference pattern when a gravity wave distorts the tunnels length).

Oops, I meant two.

I'm no expert, but wouldn't you need an extremely long baseline to triangulate on distant stars? I mean, too long to fit on planet earth?

Only if your interested in the distance, parallax measurements do that by measuring when the Earth in on different sides of the sun.

If you're only after direction you can measure the time skew between signal hitting detector one and two.As you know the speed (same as light), and that the wavefront is parallel, you have a pretty good idea where in the sky the wave came from.

For the first case, you would need a signal that lasts for at least half a year. Does that cover two black holes merging?

Well, technically, they black holes circle each other for billion of years. So with instruments sensitive enough, you could do that. Unfortunately, LIGO and the others are only sensitive enough to see the last few parts of a second, when the signal is strongest.

If at the same time* we were to observe a supernova via other telescopes, that would give a pretty good indication. :)

*: And this would also possibly serve to answer another question -- do gravitational waves travel at precisely the speed of light? (also, I'd be interested in hearing about neutrino detectors -- how closely (if nonzero) to the speed of light do neutrinos travel?)

Yes, gravity waves travel at lightspeed, at least if you buy into general relativity's tensor equations.

Neutrinos have rest mass and move slower than light. We know this because neutrinos change their flavor in transit, meaning they experience the passage of time, something that would not be possible if they had no rest mass and traveled at light speed.


Nobody knows how much slower, but they must be moving almost light speed because we see neutrinos from supernova collapse before we see the light emission. (The light emission from supernovae is delayed by several hours, because it takes that long to heat up the gas around the collapsed core before it can radiate out.)


you can determine direction with multiple experiments, or with 3d mirror arrangements, see LISA

and yes, they can only exist if they travel at the speed of light. it's a major test of the limiting speed of light.

Multiple experiments wouldn't help very much, because it will be a new source each time. When two black holes merge, it is only during the final few seconds that very large gravitational waves are emitted.

I believe gp meant multiple laser interferometers, not multiple gravitational-wave-emitting events.

You can get the direction of such waves by looking at 2 experiments at once, or experiments with 3-d mirror arrangements instead of 2-d ones.

Yes, the existance of grav waves is only possible if they travel at some top speed, which is presumed to be the speed of light.

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

wild speculation or a broken press embargo?

press conference is streamed in an hour here: https://youtu.be/zyo4DFr4D4I

Link to research paper here: https://news.ycombinator.com/item?id=11080678

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

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?

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.)

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

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