47

The most obvious — though possibly least convincing — way is by noting the "mass gap": the heaviest neutron stars we know of (by other means) are lighter than 3 solar masses, while the lightest black holes we know of (by other means) are heavier than 5 solar masses. So if the constituents of a binary that LIGO detects have masses in one group or the other, ...


35

Summary Yes they can. False positives arising from the acoustic sources you name are ruled out by seismological analysis and the examination of correlation between the separate gravitational wave detection stations. Pretty obviously, the LIGO detectors are probably the most sensitive microphones ever built. So how do we know they are detecting gravitational ...


22

This is an excellent question! The LIGO arm cavities are about 4 km, which takes a photon roughly $10^{-5} s$ to traverse. On top of that, as you mentioned a typical photon will bounce around a few hundred times (maybe 500 times) inside the cavity. So a typical photon may spend, say, a few milliseconds in the cavity. Another way to say this, is that we ...


19

A measurement of the frequency of the gravitational waves from a binary system tells you the orbital period of the binary. The rate of change of that frequency tells you how fast that orbital period is changing. The combination of the two uniquely determines the "chirp mass". Basically, a lower mass binary system exhibits a much slower rate of increase in ...


11

I want to address your main question, rather than the various subsidiary ones. You asked if it's possible that LIGO's results are anomalies. No. There's no way that LIGO's results are anomalies. The most vivid proof is the detection by LIGO of GW170817/GRB170817A, a binary neutron star merger that emitted a gamma ray burst. (Paper here.) Binary ...


9

LIGO's arms are 4km long, which makes the problem even worse. Besides, you can just move a quarter of the way around the planet and build another 2-arm facility there, and it'll automatically be at right angles to your first facility (and as long as you do know their relative positions and orientations precisely, you don't have to put them at exactly right ...


8

There are a couple things about the gravitational wave signal from a kilonova that distinguish it from the gravitational wave signal from merging binary black holes, some of which are discussed in the discovery paper for GW170817: The signal provides information on the chirp mass of the system, a quantity that can be used to infer the actual masses of the ...


8

Gravitational waves were actually just about as difficult to detect as expected. Nobody expected any detections before the first one because of the sensitivity required (detecting mirror motion less than the diameter or a proton). The upgrades were always planned; they were not in response to "failures." All of what you call failures previously were trial ...


6

I'll break the question up into two parts: First, whether any type of real-time observation of a merger is possible. (The answer is no, until we get space-based detectors.) Second, whether LIGO is good enough to point detectors to where a merger has happened. (The answer is no, but with Virgo's help, it can be done for very bright sources.) First part: ...


6

Shortly after the first detection, the LIGO/Virgo collaborations published a paper in Annalen der Physik about The basic physics of the binary black hole merger GW150914. This is aimed at the general public, and shows with some back-of-the-envelope calculations why they think that the signal was due to two colliding black holes. It discusses the chirp mass ...


6

All three direct detectors, pulsar timing, space based interferometers, and terrestrial interferometers, all use the same principle to detect gravitational waves (GW). Measure the change in distance between two objects due to a passing GW. The amplitude of a GW is proportional to the strain $h = \Delta L / L$, the change in length divided by the total ...


5

General relativity tells us that gravitational waves move at the speed of light, and there is lots of experimental evidence to support general relativity. So before 2017 there was already a lot of indirect evidence that gravitational waves travelled at the speed of light. So much so that no physicist I know thought otherwise. But what changed with GW170817 ...


5

I will try to answer specifically to your last question because I am more knowledgable on the experimental aspects of gravitational physics. As you may know, the CMB is the oldest radiation that we can see in the universe. It can be referred to as the surface of last scattering for photons and it coincides with the moment the universe stopped to be an ...


5

To expand on Ernie's answer, there are in fact qualitative changes in the posterior distribution of the sky location as you add more detectors to a network, beyond just shrinking the contours. With a single detector, you can't localise the source at all; with two detectors, timing triangulation across detector sites allows you to localise the source to a ...


5

It is in principle possible that two different gravitational waves from independent sources could arrive simultaneously at the LIGO detectors, which would then probably pose problems in identifying the precise origin of the signal. However, the probability of this is vanishingly small, because LIGO consists of two detectors, not just one, and the signal ...


4

We will start getting more data before long from gravitational detectors. Virgo comes online soon, but LIGO is to be taken down for a while with updates. When fully functioning, and a couple others added in, we should start detecting maybe 10 a year, and after 10 years we'd have about 100 black hole (BH) mergers detected. And measured. The statistics as to ...


4

LIGO has the potential to give insights into the possibility of one component of dark matter: primordial black holes. Key targets of gravitational wave interferometers are merging binary systems (either black holes or neutron stars), and so if enough black hole binaries could be detected over time, scientists might be able to constrain the mass ranges of ...


4

No. LIGO can not measure the expansion of the universe. LIGO only detects a specific class of distortions of the space-time, which at least are of spin-2 and with a frequency of the order 100Hz. Gravitational waves are spin-2 (quadrupolar) modes, and the one that was observed this time has the dominant frequency from 35Hz to 150Hz, which is in the sensitive ...


4

Well, CuriousOne gives the most direct answer, that the universe is not expanding on scales which the gravitational attraction between objects dominates, like on the Earth (indeed, the entire Milky Way). But, let's pretend that we take LIGO, stick it out in space (even away from our local group, to be sure it's in an isolated region), and ask it to measure ...


4

From a philosophical point of view, yes, of course the LIGO results could be an anomaly. So could every scientific measurement ever made. Science admits the possibility that its conclusions are incorrect. If you take a look at the LIGO papers, you’ll find that they present an error analysis to estimate the possibility that the signals were mere artifacts. ...


4

This figure comes from the paper describing LIGO's initial detection of gravitational waves. This GW event was named GW150914. The LIGO Virgo Collaboration writes a "science summary" for each major paper they write. The summary for this paper is here. Observed GW Strain The top panels show LIGO's strain measurement, or fractional change in length $\...


4

In principle, any vibrational disturbance in the detection band of a gravitational wave detector will "cause the arms of the interferometer to move." There are all sorts of vibrations that effect the detector beams, as shown in this theoretical LIGO noise curve for the LIGO detectors. So, to detect gravitational waves is to identify a specific ...


3

As I understand the introduction of sideband via phase modulation is done before the beam enters any cavities/the interferometer. Is this usually correct? Yes. Figure 1 of this paper, for example, shows the key components with the modulator in front of everything else. Which frequency is kept on resonance with the cavities? Carrier or sidebands? How ...


3

The time difference between the detections indicates that the gravitational wave came roughly from the direction of the earlier detection. With more detectors operating in widely dispersed locations on the Earth's surface, re-construction of the source direction will be more precise, as more points on the gravitational wave's vector, from the earlier ...


3

Each LIGO facility is more similar to a microphone than a telescope. Microphones can only measure sound intensity, not the direction of the source. You can figure out the direction of a sound by using multiple microphones in multiple locations and recording the times that each microphone detects the sound. The order in which the microphones hear the signal ...


3

Firstly you have misunderstood the recent article about the black holes at the centre of the galaxy. The finding is that there may be tens of thousands of stellar mass black holes orbiting the central supermassive black hole. The supermassive black hole typically has a mass of several million stellar masses, so the ten thousand or so smaller black holes ...


3

Here is the starting paper, Abstract On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics ...


2

The interferometer arms look roughly like this: There is a housing made up of the vacuum tube and the building around the tube, and this housing is rigid. That means when the gravitational wave passes through the length of the housing, $\ell$, does not change. This is analogous to Feynman's rigid rod. The mirrors are suspended as delicately as possible ...


2

From the LIGO website on their "blind injections" (http://www.ligo.org/news/blind-injection.php), it does indeed look like it is possible to point telescopes in the general direction, but they haven't been able to do it accurately enough to get images yet. Here's what they said about a blind injection event from 2010: The detector network is capable of ...


2

The basic idea is that you have destructive interference. This is hard(-er) to achieve with three photons as well as I don't see a trivial way to do the beam-splitting.


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