Starts in 2 minutes.
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.
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.
"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.
Giant's shoulders and all that.
It's gradually morphed from being a Times for mildly xenophobic Conservatives (big C) into a Daily Mail for toffs.
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.
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.
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.
"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.
Scientists and engineers, more than anybody, understand that they are standing on the shoulders of giants.
It would be like crediting one section of the beam tube at cern for detecting the higgs.
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.
This was one of the purposes of LIGO. To prove what what previously conjecture. So yeah, goLigo.
So... this is news before news? I.e. rumor. LIGO is not scheduled to announce results for another 3.5 hrs.
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.
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.
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.
Basically fire two lasers at right angles to each other at mirrors, and see the pattern when they bounce back.
For example their electromagnetic fields may be shifting depending on distance to another atom or another field.
PS: I am no physicist.
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.
"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.
Still, back to common, ordinary, reality,
Sir Rees does have a lot of credibility.
Sounds like a good old survival bias ;)
Seriously though, what is confidence interval in LIGO?
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.
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.
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.
: The Hulse–Taylor binary pulsar: https://en.wikipedia.org/wiki/PSR_B1913%2B16
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.
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 :)
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
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.
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.
*: 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?)
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.)
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.
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.
press conference is streamed in an hour here: https://youtu.be/zyo4DFr4D4I
Or are these detectors already directionally sensitive?