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Gravitational-wave detector LIGO is back

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Posted on Friday, May 26th 2023 by gmays



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@ Friday, May 26th 2023 by epberry

I absolutely love LIGO. YC actually did a great interview with one of the lead physicists on the project where he described some of the technical hardware and software challenges - https://www.youtube.com/watch?v=1D2j8nTjOZ4


@ Friday, May 26th 2023 by jackmott42 | Parent

That's the guy who when someone told him they got their first detection was incredulous "I don't have time for this" or something to that affect, because he assumed it was a false positive to have gotten something so quickly, but it was real!


@ Friday, May 26th 2023 by throwawaymaths | Parent

How do we know it was real and not overinterpreting noise again?


@ Friday, May 26th 2023 by kmote00 | Parent

It's my understanding that it was correlated by data from the twin facility on the other side of the country.


@ Saturday, May 27th 2023 by throwawaymaths | Parent

The problem is that we are seeing a lot of correlations. I'd be worried.


@ Saturday, May 27th 2023 by eaq | Parent

One technique that is straightforward to understand is that folks have done "time-slide" analyses for many years - i.e. offsetting one detector's data in time - to understand what the "baseline" rate of correlations is in data streams where there is no possible physical origin.

Naturally, much more statistical analysis has been done to ground the claims of "detection"; beyond detailed academic publications, LIGO and others have been producing layperson-accessible science summaries for years/decades that address these and other questions.


@ Saturday, May 27th 2023 by throwawaymaths | Parent

>have been producing layperson-accessible science summaries for years/decades that address these and other questions.

Citation please. Every layperson accessible summary has said "we use advanced statistics and machine learning" and I haven't found a simple high school statistics accessible explanation yet. Unlike say the higgs boson, I think for this experiment a simple statistical treatment is not an unreasonable request.

Please show me and correct me. I would love to be able to believe we have detected gravitational waves.


@ Saturday, May 27th 2023 by ssl232 | Parent


Probably not high school statistics accessible but Bayesian statistics and Gaussian noise and GR isn't.


@ Saturday, May 27th 2023 by throwawaymaths | Parent

thank you, that is helpful.


@ Friday, May 26th 2023 by Gare | Parent



@ Friday, May 26th 2023 by bsder | Parent

That event is amazing.

I had no idea how much cross correlation they produced (see the "Scientific Importance" sections). I love the fact that measurements got like 10 orders of magnitude or more better--that's just absolutely absurd.


@ Saturday, May 27th 2023 by throwawaymaths | Parent

You can't take two detectors and just say "they correlate" without presenting the blinded correlation data. If you apply the crazy filters that they do, how many of those signals happen in both detectors by chance?

In fact, you can't do a proper statistical analysis without three identical detectors, or at least two pairs of detectors, which we will have once ligo1,2 and Virgo are all online at the same time, and watch how the filtered signals drop in count based on how many detectors you look at.

>It is the first GW observation that has been confirmed by non-gravitational means.

This is a pretty weak claim. Worryingly, iirc it is the only multi-messenger observation we've made to date, so, again. How many supernovae do we see at any time in any given segment of the sky, and what is the likelihood that it would have happened to be in whatever ~1/20 spatial angle the Gw detectors happen to have resolutions over, and out of the other n candidate neutron-neutron mergers we've "detected" what are the odds that we wouldn't have seen a supernova in the EM by chance in all of the others?

If almost every single event came with a mm detection, I'd be convinced, but honestly, IMO it's looking kinda grim right now. We will find out if after we jack up the sensitivity we continue to fail to make multi messenger observations at the same rate as we have so far


@ Saturday, May 27th 2023 by pharrington | Parent

>How many supernovae do we see at any time in any given segment of the sky, and what is the likelihood that it would have happened to be in whatever ~1/20 spatial angle the Gw detectors happen to have resolutions over, and out of the other n candidate neutron-neutron mergers we've "detected" what are the odds that we wouldn't have seen a supernova in the EM by chance in all of the others?

The data's all publicly available. You go ahead, do the general relativity, and tell us.


@ Saturday, May 27th 2023 by throwawaymaths | Parent

GR is unnecessary. This is a question of statistics.

>The data's all publicly available

No, they are not. Even if it were, the software and algorithms to do the statistical analysis on the raw data are not open source.


@ Saturday, May 27th 2023 by seanhunter | Parent

I think the data is all publicly available, complete with software, sample python notebooks, tutorials etc.

https://gwosc.org/ and https://www.ligo.caltech.edu/page/ligo-data

@ Friday, May 26th 2023 by _Microft

Beside the mentioned laboratories in the US, Italy and Japan, there is another one in Germany albeit of much smaller size [0]. The length of its arms are only 600m (1/3mi) each but it serves as testbed for technologies [1] that might later be used for other observatories.

[0] https://www.geo600.org/ , https://en.wikipedia.org/wiki/GEO600

[1] https://en.wikipedia.org/wiki/GEO600#Advanced

@ Friday, May 26th 2023 by borissk

Since learning about gravitational waves I was always curious if a type III civilization could potentially use them as a weapon.


@ Friday, May 26th 2023 by cde-v | Parent

Anything can be used as a weapon.


@ Friday, May 26th 2023 by HansHamster | Parent

First thing that comes to mind is a lethal dose of neutrino radiation: https://what-if.xkcd.com/73/

Now the tricky part is probably to build a neutrino / gravitational wave / whatever source that is intense enough to be useful as a weapon without just evaporating everything in a supernova scale explosion before...


@ Friday, May 26th 2023 by causality0 | Parent

One of the most interesting yet sadly least rigorous What-Ifs. He relies on simply scaling up a calculation in absorbed dose at the distance of one parsec. Neutrinos do not interact with nuclei the same way gamma rays do, and the effects of a particular amount of neutrino radiation on living tissue is unstudied and unknown. The paper he cites explicitly points this out but he ignored it.


@ Friday, May 26th 2023 by raverbashing | Parent

Of course it doesn't, I think that was given by the relationship between neutrino count and sieverts

But make no mistake, there is such a thing as a fatal amount of neutrinos. It's just that's a supernova mind boggling amount, but it exists. They do interact due to the weak force, which is more than neutrons do (and a lethal dose of those is well known)


@ Friday, May 26th 2023 by mhh__ | Parent

See also neutrino HFT


@ Saturday, May 27th 2023 by ridgeguy | Parent

In the sci-fi novel Iron Sunrise by Charles Stross, death by neutrino flux from a weaponized star figures into the plot.

(edit - spelling)


@ Friday, May 26th 2023 by bsder | Parent

Any sufficiently advanced propulsion also qualifies as a weapon.


@ Friday, May 26th 2023 by sparker72678 | Parent

Maybe? There would be far less energy-intensive ways to wipe other civilizations out of the universe, though.


@ Friday, May 26th 2023 by dekhn | Parent

step 1: arrange two black holes near your enemies
step 2: wait 2 billion years


@ Friday, May 26th 2023 by groestl | Parent

Depending on your preferred solution to the Fermi Paradox, you might be able to just skip Step 1.


@ Friday, May 26th 2023 by saiya-jin | Parent

I dont think so, they go in all directions as a shockwave, pass through everything including black holes (even though it would warp it a bit), so its dark forest signalling basically to whole universe.

Since we came to exist so early in the overall age of universe, there is absolutely no chance we are the only sentient civ across hundreds of billions x hundreds of billions/trillions x nr of planets realm.

Super focused super dense ray of very hard gamma rays/cosmic rays should do any trick required for anything made out of matter. Or just swipe left with a black hole or two.


@ Friday, May 26th 2023 by MaxikCZ | Parent

Correct me if I am wrong, but since not even spacetime can escape blackholes, even gravitational wave would get swallowed, wouldn't it? Of course, since we can't "point" gravitational waves in a certain direction, because of the rest of the wave traveling around the hole would basically propagate it even directly behind blackhole (from perspective of source), but that's because the wave goes around, not trough.

Or do grav waves really pass ~trough~ black holes?


@ Friday, May 26th 2023 by borissk | Parent

Hmmm, gravity does escape black holes, so maybe gravitational waves do too.


@ Saturday, May 27th 2023 by jakear | Parent

>there is absolutely no chance

If you hold Faith in the axiom "the universe was initialized homogeneously and propagated untampered thereafter", perhaps.

Plenty of axioms for which "there is absolutely no chance we are not the only sentient civ across..." is just as true though. And everything in between.


@ Friday, May 26th 2023 by kadoban | Parent

Seems orders of magnitude too difficult to be worth it. If you can approach that level of energy, pointing a gamma ray burst sounds more fun. Or just throw some rocks at fractions of c.


@ Friday, May 26th 2023 by abecedarius | Parent

Pretty hard to direct.

If you've set up a close-orbiting neutron star binary and you're in a military frame of mind, one thing you could do is accelerate missiles to a good fraction of lightspeed. (Same principle as the gravity assists used by planetary probes like Voyager.) The tides would limit the practical size of the missile, though I haven't tried to compute this limit.

(I don't consider this comment to be aiding the interstellar enemy, it's too obvious.)

@ Friday, May 26th 2023 by fjfaase

For observatory status see [0]. It also gives the estimated detector range in megaparsecs (Mpc). Initial LIGO's "range" (the radius out to which LIGO could detect at least a binary neutron star (BNS) merger) was 15 Mpc. With the latest improvements is more in the 140 Mpc range. Meaning that it can see more than 9 times as far and that the area of space is increased by a factor of more than 800. This will greatly increase the number of gravitational waves being detected.

[0] https://online.ligo.org/


@ Friday, May 26th 2023 by acqq | Parent

Do you know if even longer ranges logged there, like 600, are practically useful or are they too short (in time) for that?



@ Saturday, May 27th 2023 by fjfaase | Parent

Yes, I have seen those as well. It looks like these spikes always appear at the start when the status goes to 'Observing'. For that reason, they may just be start-up artifact, where you have too less data point to arrive at an accurate estimate, resulting in a far too optimistic value.


@ Friday, May 26th 2023 by dr_dshiv | Parent

>Meaning that it can see more than 9 times as far

Hear 9 times as far?


@ Saturday, May 27th 2023 by nrdgrrrl | Parent

Feel 9 times as far?


@ Saturday, May 27th 2023 by onionisafruit | Parent

Smell 9 times as far with the smell-o-scope


@ Saturday, May 27th 2023 by p5a0u9l | Parent

Not hear. The signal was converted to audio to help people grasp the phenomenon. Gravity waves are the same as electro magnetic


@ Saturday, May 27th 2023 by nimish | Parent

No they aren't. Gravitational waves are ripples in the spacetime metric, not EM waves.


@ Saturday, May 27th 2023 by zarzavat | Parent

If gravitons exist, are gravitational waves to gravitons as electromagnetic waves are to photons? Or something different?


@ Saturday, May 27th 2023 by raattgift | Parent

This is a good question.

First, one needs to make a choice of 3+1 dimensional quantum field theory in which a massless spin-2 graviton obeying the Bose-Einstein statistics appears as the carrier of the gravitational radiation from (classical) General Relativity. There are gravitons in higher-dimensional theories, string-theoretical and otherwise, and there are also 3+1 theories with massive gravitons and/or different spin statistics.

So below I'll be talking about gravitons in perturbative quantum gravity and canonical quantum gravity, two specific quantum gravity theories (i.e., "two QGs").

If we make such a limitation, then there are some noteworthy differences between massless spin-1 photon and massless spin-2 bosons: universality, coupling, spin, and background. While these make these boson fields not the same, it does make them pretty comparable, and allows for successful analogizing.

We'll start with universality: not everything feels electromagnetism, so the photon couples non-universally. In particular it does not couple to neutrinos, and does not self-interact (in the absence of charged matter that feels electromagnetism). Gravitons interact with everything, including neutrinos and photons, and including other gravitons (even in the absence of any other matter).

"Matter" in the paragraph above is tricky - the photon does not typically appear in isolation in a quantum theory. In quantum electrodynamics and the Standard Model it has at least an electron/positron field where its partner charged particles can be found. Since in the two QGs a lone graviton is one of a large number found in gravitational waves in classical General Relativity, and since the latter admits vacuum solutions with gravitational waves, we conceptually 'promote' gravitons into matter, even though the stress-energy tensor of the (classical) Einstein Field Equations is set to zero in a vacuum solution. As such, gravitons themselves are gravitationally charged. Universality of free fall means that all other particles -- photons, neutrinos, ... -- also are gravitationally charged.

When a boson interacts with its appropriately charged matter there is a "coupling", which can be constant, or which can depend on energies ("running coupling" or "effective coupling"). As energies increase, both photons and gravitons depart from their default couplings, the fine structure constant \alpha and Newton's constant G. Perturbative methods for the running coupling of photons are better known (the so-called "beta" function of QED for instance <https://en.wikipedia.org/wiki/Beta_function_(physics)#Quantum_electrodynamics>), but the failures of a perturbative running coupling for gravitons is more famous: that's where the "gravity is non-renormalizable" comes from.

For these two types of bosons, the spin determines what happens to charged matter exchanging them. For spin-1 photons, two similarly-charged particles repel and two oppositely-charged particles attract. For spin-2 bosons either (i) there is only one charge, and it's always attractive or (ii) there are two charges that differ by sign, and similar charges attract but opposite charges repel. There is no evidence for the (ii) option, although there are plausible reasons why we might not have found any. The coupling strengths of gravitons are very weak compared to photons, but as with photons masslessness means infinite range. It could be that all matter with the opposite gravitational charge (gravitons, electrons, ..., maybe with some supersymmetry-like partners, or other weirder particles that aren't much like the Standard Model ones) have been pushed out of our observable universe through "anti-gravitation", and there is no decay path to such matter that exists in our region. Who knows. Option (i) is completely consistent with evidence and simpler.

Finally, background: General Relativity has a "no prior geometry" principle. This means that moving matter generates spacetime curvature. If matter moves differently, it generates different curvature. Following the slogan, "matter tells spacetime how to curve, curvature tells matter how to move", there is one collection of moving matter for each different dynamical ("unfixed") spacetime. Perturbative methods like the two QGs above use a fixed background curvature; the Standard Model and QED effectively do this too. There are then arbitrary numbers of distributions of matter that are associated with the background spacetime, and we then have to add extra gravitational information to account for the energy-momentum of those distributions. That information is typically a gravitational backreaction. In other words, when we chop a dynamical spacetime into static flat spacetime and a dynamical component, we have more bookkeeping to do.

One can turn this around a bit. In General Relativity nobody's timeline is precluded from calculating the whole spacetime. It's easier for some timelines, it's harder for others. But one always has choice. In relativistic QFTs one picks out a universal, absolute timeline against which eigenvalues evolve. One thus wants to chop up a General Relativistic spacetime into spaces organized on one timeline (arbitrarily chosen out of infinite options), and then introduce e.g. the "lapse" and "shift" functions from canonical quantum gravity. These functions do not represent anything physical. These functions are needed to deal with the fact that we have approximated a dynamical spacetime with a fixed time along which we arrange successive spaces. Especially when one also carves each space into a fixed-background and dynamical part as in the previous paragraph, one introduces approximation artifacts and loses high-frequency information. For the same spacetime with the same matter moving identically, we can get quite different lapse/shift functions (or bookkeeping fields) if we switch from spaces arranged on one timeline to spaces arranged on a different timeline.

QED and the Standard Model basically ignore these issues: they are theories defined against static flat spacetime. The bright side is that General Relativity guarantees that at every point in any spacetime (no matter how strong the curvature is) there is always a small (and that may mean microscopic or ultramicroscopic or ...) patch which is flat. It's like zooming in on a picture of a circle: zoom in enough and you lose sight of the curvature. It's also like blowing up a circle's radius larger and larger. Humans see this all the time standing on the ground and not noticing the curvature of the Earth's surface.
Make gravity strong and the radius of curvature shrinks; make the moving matter gravitationally relevant and the "circle surface" gets all jagged and bubbly. Relativistic quantum field theories like QED and the Standard Model can be adapted reasonably well when gravitation is weak and when appreciable sources of gravitation move very slowly compared to the speed of light.

Finally, in spite of these differences one can draw some analogies with different levels of formality and with different domains of applicability between photons (and electromagnetism) and gravitons (and gravitation). One generally does this classically though, because of the large masses (and thus particle numbers) involved, so the analogy is really between the Maxwell-Einstein equations and e.g. Gravitoelectromagnetism (GEM). See <https://en.wikipedia.org/wiki/Gravitoelectromagnetism> if you're curious.


@ Sunday, May 28th 2023 by dr_dshiv | Parent

Amazing post, thanks!


@ Saturday, May 27th 2023 by tlarkworthy | Parent

equilibriocepted 9 times as far

[1] https://nautil.us/we-should-count-balance-as-one-of-the-sens...


@ Saturday, May 27th 2023 by Jabbles | Parent

The article says that the number of detected waves will only increase 2x though?

The improvements should allow the facility to pick up signals from colliding black holes every two to three days, compared with once a week or so during its previous run in 2019-20.

@ Friday, May 26th 2023 by galizar

Nice. There's even a citizen scientist initiative for LIGO [0]. I wonder what's the status on LISA though.

[0] https://www.zooniverse.org/projects/zooniverse/gravity-spy


@ Friday, May 26th 2023 by kataklasm | Parent

One of my professors worked on LISA Pathfinder, the demo satellite used to proof-of-concept LISA technology until the financial shortcomings in the early 2010s were overcome and he recently said that everyone in the project is hard at work getting ready for the program review, after which either a contract is made or the program is reformulated. But no one will put in the gigantic work needed to prepare such a review if it is not almost certain the program will pass the review and become a contract, so things are looking quite good for LISA and its early 2030s launch!


@ Friday, May 26th 2023 by wefarrell | Parent

I think it would be really neat to have a space based telescope in close proximity to LISA so that when gravitational waves are detected the telescope can point in the direction of the source and capture the light from it.


@ Friday, May 26th 2023 by cubefox | Parent

I remember reading about LISA as a kid, around 25 years ago. Currently it is planned to launch in 2037. I somewhat doubt it will ever become a reality.

@ Friday, May 26th 2023 by whoisthis4chan

>Typical gravitational-wave events change the length of the arms by only a fraction of the width of a proton. Sensing such minute changes requires painstaking isolation from noise coming from the environment and from the lasers themselves.

i find it utterly fascinating that we're able to detect such a minuscule deviation


@ Friday, May 26th 2023 by pfdietz | Parent

Lasers beams are bounced back and forth many times, so the deviation builds up. The beams have to be very powerful (100s of kW) to reduce photon counting noise sufficiently.


@ Friday, May 26th 2023 by wwarner | Parent

Kip Thorne explains it pretty clearly in this 2002 lecture



@ Friday, May 26th 2023 by dekhn | Parent

interferometry is indeed amazing. When the ultra-important Michelson-Morley experiment was run some ~100 years ago, they were doing interferometry but in those days there wasn't really good vibration isolation technology. They had to float their whole experiment on a pool of mercury (!) in the sub-sub basement of an idle building, and even then, deliveries nearby (by horse) would cause problems.

Nowadays, physics students do the MM experiment in a lab on a benchtop in a day.


@ Friday, May 26th 2023 by acqq | Parent

I'd like to read how these problems are solved "in a lab on a benchtop" today!


@ Friday, May 26th 2023 by funac | Parent

you can build very good hydrostatic vibration isolators in a home machine shop nowadays; commerical optical tables are /very/ steady


@ Friday, May 26th 2023 by dekhn | Parent

the original experiment is pictured here: https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper...

what makes it possible to do in a desktop lab course combination of a large number of different innovations. The first is that we know how to make extremely stiff/rigid/strong/flat/thermally stable tables (https://www.thorlabs.com/navigation.cfm?guide_id=41) which can optionally be placed on active vibration-cancelling struts (https://www.thorlabs.com/newgrouppage9.cfm...). The second is using cage systems for mounting things with everything lined up parallel and centered (https://www.thorlabs.com/navigation.cfm?guide_id=2255). The third is precise kinematic mounts which make real-time angle tuning a lot easier/more reliable (https://www.thorlabs.com/thorproduct.cfm...). The fourth is now we have powerful lasers and LEDs that make generating lots of light all pointing in the right direection easier (https://www.thorlabs.com/thorproduct.cfm...). The fifth is that high quality standardized optical parts (mirrors, lenses, etc) are easily available from a wide range of vendors (https://www.thorlabs.com/newgrouppage9.cfm...).

There are a number of other innovations in material science.
but I'd recommend taking a look at Thorlab's Michelson-Morley educational kit.  For $3K you get
basically everything you need to carry out the experiment:
https://www.thorlabs.com/thorproduct.cfm?partnumber=EDU-MINT2 plus a nice manual
that walks you through physical setup and theory behind the experiment (which among other things
helped lead to special relativity).
if you want more like this, see https://www.thorlabs.com/newgrouppage9.cfm... which is a hardware kit that accompanies an actual optical lab class. The course is online: https://www.thorlabs.com/drawings/5d9e11209b7d4536-820A3379-... and gives a fairly straightforward introduction to optics. With this, you can easily build a microscope from components or any number of other nifty optical systems.

Non-optics people (IE, programmers, etc) with enough time and money can learn how to do real-world optical experiments in their garage (this applies to astronomy too). For example after a significant time/money investment, have started building my own microscopes which use real-time object detection to track tardigrades to do behavior analysis (lest anybody feel imposter syndrome, trust me it took a ton of time and money and even then I'm not quite at the level of a good grad student).

It's not my favorite but you can also read https://www.amazon.com/Perfectionists-Precision-Engineers-Cr...

If you want to truly go down the rabbit hole, https://pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/M...


@ Friday, May 26th 2023 by acqq | Parent

Wonderful answer, thanks!

Do you know if the "Michelson-Morley educational kit" is really enough to achieve the accuracy of the original experiment or is it just to make "any" functioning interferometer?


@ Friday, May 26th 2023 by dekhn | Parent

I'm pretty sure it exceeds the accuracy of the original experiment. I think not being based on a trough of mercury is pretty important as well. But the manual shows several types of interferometers that can be built in lab courses.


@ Friday, May 26th 2023 by acqq | Parent

Still, I see it is actually called "Michelson Interferometer Educational Kit", not "Michelson-Morley" and the user guide I'm reading (your link gives "The resource you are looking for has been removed", so I've clicked on the "User Guide" on the page instead) also takes care to never directly mention Morley or to suggest that the same experiment can be reproduced with that kit.


@ Saturday, May 27th 2023 by replygirl | Parent

maybe this is their gnu/linux


@ Saturday, May 27th 2023 by seanhunter | Parent

Not my field, but my understanding is it's called the Michelson Interferometer because he designed and built the first version a few years before the famous MM experiment for the purpose of measuring the speed of light. See the diagram on page 3 of his 1881 paper "The relative motion of the Earth and of the luminiferous ether" https://zenodo.org/record/1450060 (note this is before the collab with Morley)

Edit to add: I just noticed a fun thing in the conclusion of that paper "In conclusion, I take this opportunity to thank Mr. A. Graham Bell who has provided the means for carrying out this work..."


@ Saturday, May 27th 2023 by moh_maya | Parent

/off topic

Have you documented & written about your microscope experiments (both building them and the experiments themselves) on the web?

If you have, and are comfortable, can you please share the link?

Just curious. Ex-biologist, and among other things, I helped build / assemble an optical trap and other equipment including confocal / TIRF microscopes about a decade+ ago, so curious what a serious (amateur) student with time, passion and resources is able to do.

Thank you :)


@ Sunday, May 28th 2023 by dekhn | Parent

No, I don't document and write about my experiments. I've shared them with a tardigrade biologist, but we didn't decide to continue. Also, I'm not a student, I'm a professional computer guy with a hobby and a budget. That said, smart undergrads coudl definitely do this.

Nothing I'm doing is remarkable or complicated, and compared to a research microscope, what I've done is very trivial. It's just a 10X scope with a grbl-controlled XYZ stage and an object detector that finds the center of a tardigrade in realtime, and sends commands to center the tardigrade. Now my interesting is in high speed scanning- instead of taking photos, you literally take a video while actively moving the head around and then stitching it all together. Not stopping to take pictures increases the rate of acquistion 10X or more.

The reality is that I could have done everything I wanted to do by spending about $10K for a kit from Thorlabs, but I was interested in learning more about building precision stages from inexpensive components, so that if/when I ever do get to play with the expensive toys, I know why they are better, anbd why they cost so much money.

The interesting area now is SPIM see https://openspim.org/Step_by_step_assembly which is definitely something a well-funded hobbyist could do.


@ Monday, May 29th 2023 by moh_maya | Parent

Thank you!


@ Saturday, May 27th 2023 by fsh | Parent

There is no way you could repeat the Michelson-Morley experiment with that small and floppy Thorlabs EDU kit. The experiment from 1887 had an arm length of 11 m and was interferometrically stable (typical length fluctuations much smaller than the wavelength) while rotating. That would still be a considerable engineering challenge today.

Modern Michelson-Morley experiments [1, 2] don't use Michelson interferometers anymore. Instead, they compare the lengths of crossed ultrastable high-finesse cavities (in vacuum, of course). The big innovation is that, with lasers and electronics, we can measure the cavity resonance frequencies (and therefore also the cavity lengths) to something like 15 digits of accuracy. This corresponds to less than a tenth of the diameter of a Proton, and is something like 100 million times more accurate than you can achieve with a simple Michelson interferometer.

[1] https://doi.org/10.1103/PhysRevLett.103.090401

[2] https://doi.org/10.1103/PhysRevD.80.105011


@ Sunday, May 28th 2023 by dekhn | Parent

Thanks. I'm not an optics expert, although my friend (the one who said they built a michelson interferometer in a day in their physics lab) is. Since I don't want to mislead anybody, could you explain what the Thorlabs kit can do, and is it technically a Michelson interferometer? The labs that feature the kit all seem to measure the wavelength of light.

It would seem odd that Thorlabs (generally well respected) would sell something that is not what it really is, or misrepresented its capabilities. my guess is that you're sayting the kit itself couldn't reproduce the original experiments, but that it still is a Michelson interferometer in design, which can be used to carry out less demanding experiments, but not demonstrate the (non) existence of aether?


@ Monday, May 29th 2023 by fsh | Parent

The Thorlabs kit looks like a very decent Michelson interferometer that can be used for a lot of demonstrations such as measuring wavelengths and studying the coherence properties of light sources.

However, repeating the Michelson-Morley experiment is not easy since the expected signal is very small. If there was a stationary aether, the relative length difference for the optical path along the earth's motion compared to the path perpendicular would be (v/c)^2 ? 1E-8, where v is the orbital velocity of earth (3E4 m/s), and c is the speed of light (3E8 m/s). The arm length of the Thorlabs kit is just a few cm, so the shift would be on the order of one nm, or one five-hundreth of a (green) wavelength. Thermal drifts and vibrations of optics on a typical optical table are much larger than that, especially when trying to rotate the setup. Michelson and Morley achieved the necessary stability by constructing their interferometer on a solid stone slab, and made a near-frictionless bearing by floating it on mercury. The resulting stability is still impressive by modern standards, but the construction technique is not very practical. Nowadays, large and passively stable optics setups (for example telescope mirrors or laser gyros) are usually made from massive pieces of Zerodur which has near-zero thermal expansion.


@ Tuesday, May 30th 2023 by acqq | Parent

"the one who said they built a Michelson interferometer in a day in their physics lab"

Maybe you should check with them too: it's possible that they have also built "a" Michelson interferometer (just like Thorlabs kit features one) but maybe their setup was in spite of that insufficient to perform the needed measurements in the way needed for the valid execution of a Michelson-Morley experiment?

Historically, Michelson constructed his first interferometer in 1881 in Potsdam, Germany:


Inventing it was obviously necessary but not sufficient for a valid Michelson-Morley experiment, which was correctly finished only during 1887 in Cleveland, Ohio.


@ Tuesday, May 30th 2023 by dekhn | Parent

I think they must have said they built a michelson interferometer and not that they ran an MM experiment. Basically same as the thorlabs kit (in fact it probably was the equivalent of that kit, but cobbled from edmunds, since thorlabs wasn't really big at the time).

@ Friday, May 26th 2023 by waynecochran

The image at the top of the page is not a real image is it? We don't have real photos of black holes yet right? (except the one at https://www.nasa.gov/mission_pages/chandra/news/black-hole-i...)


@ Friday, May 26th 2023 by sp332 | Parent

The image is credited to the SXS Project, which does black hole simulation.

@ Friday, May 26th 2023 by quercusa

Michelson and Morley smile


@ Friday, May 26th 2023 by causality0

The LIGO song is required background music for this article: https://youtu.be/degD69wnZcY


@ Friday, May 26th 2023 by wwarner | Parent

haha catchy love it


@ Friday, May 26th 2023 by groestl | Parent

Thank you for this (channel), I was one of the lucky 10k today!

@ Friday, May 26th 2023 by captainkrtek

Big LIGO nerd here. If interested, you can get public alerts of LIGO detected activity (on mobile and online):




@ Friday, May 26th 2023 by Simon_O_Rourke | Parent

Good, this is a necessary thing to get back up and running again. Thanks for the update alerts, will subscribe and see what's stirring in the galactic 'hood.

Question though - do gravitational waves diminish significantly as a function of distance or intervening mass?


@ Friday, May 26th 2023 by empyrrhicist | Parent

At distance it's an inverse square law I believe


@ Friday, May 26th 2023 by dataflow | Parent

I think their amplitude falls as 1/r, their power falls as 1/r^2. Just a layman but I imagine the amplitude is the relevant part here?


@ Saturday, May 27th 2023 by dmbche | Parent


This should answer any questions!


@ Saturday, May 27th 2023 by koheripbal | Parent

The Android app is so outdated that my Pixel 3a is too new to run it!

@ Friday, May 26th 2023 by fire

are gravitational waves like, universal? could we use them as a timing reference, I wonder?


@ Friday, May 26th 2023 by gus_massa | Parent

[I'm not sure I understood your question. I hope this helps.]

The gravitational waves travel also at the speed of light. They will reach first the point of the Earth that is closest to the event first. And then travel and reach the oposite point like 40ms later. The Earth is almost almost almost transparent, so the signal reaches all the Earth, but with a different small delay.


@ Saturday, May 27th 2023 by im3w1l | Parent

This might be a stupid question but to me it's hard to grasp that they travel at a "speed" when they are themselves distortions of time/space. Does it always make sense to say they move at the speed of light or only for say small amplitude waves where we can do some quasi-special-relativity trick?


@ Saturday, May 27th 2023 by gus_massa | Parent

It's an interesting question. I must take that General Relativity course some day. Meanwhile, I have to guess ...

I guess you are correct and with small amplitudes the apparent speed is equal to the speed of light. For big amplitudes, I'm not sure.


@ Saturday, May 27th 2023 by raattgift | Parent

Pp-wave spacetimes (pp = plane-fronted and parallel) can with suitable separations can have arbitrary constant wave amplitudes. Such spacetimes admit a Killing vector field letting us have a sensible way of measuring the propagation speed of the wave. At any point where it is measured, the propagation is lightlike.

Parts of a spacetime around an equal-mass circular-orbit binary will be reasonably approximated by a pp-wave spacetime (edge-on, not too close to the sources, and over a duration where their orbit is negligibly contracting).


@ Saturday, May 27th 2023 by raattgift | Parent

It's definitely not a stupid question!

Gravitational waves are usually studied in the context of linearized gravity <https://en.wikipedia.org/wiki/Linearized_gravity> rather than the full theory of General Relativity.

Essentially one fixes some background metric that does not have the dynamical aspects of the inspiralling binary. Those dynamical aspects are then applied as perturbations of the background metric. When one then slices the 4-dimensional static background into 3 spacelike dimensions along some timeline, the departures from the background (the perturbations) then propagate like massless waves.

Masslessness (and no refraction, birefringence, etc.) is why the wave propagates at "c".

Light propagates as massless waves too, which is why the speed of light is "c". The constant is geometrical in origin (it's because our spacetime is 4-dimensional, with one dimension of time: gory details at <https://en.wikipedia.org/wiki/Causal_structure>, particularly the "Curves" subsection of the Introduction), although "c" was discovered by studying the speed of light.

Linearized gravity is a good approximation but not fully general. It breaks down in extremes of compactness, and so one resorts to numerical relativity (on supercomputers) for understanding the final parts of inspirals of merging black hole and neutron star binaries (both species are compact, and in the final inspiral each binary partner orbits within the "compactnes-really-matters" region of the other).

@ Saturday, May 27th 2023 by rainbowzootsuit

I was lucky to take a tour of the Livingston, LA facility. It's within about an hour's drive north of New Orleans. Very much worth a visit.


As someone who's worked on laboratory ultra-high vacuum systems the ~2m diameter by 4km long arms that are vacuum chambers are quite impressive.

A "fun fact" from the tour was that they had a road grading company construct the underlayer for the support of the arms. They asked the company to set the angle of the grading to equal the negative curvature of the earth as the arms have to be straight in space while a surface that's graded "flat" would end up following the curvature of the earth.


@ Saturday, May 27th 2023 by ramraj07 | Parent

The irony of one part of humanity: led mainly by People in the most prosperous country, measuring ever vanishing amounts of interference at the length of a proton to detect exploding stars, and they have to account for among other things the curvature of this planet to the millimetre.

Just a few miles away (and likely everywhere) in the same country there are many people growing in number who insist for some reason that the earth is flat. I am genuinely flummoxed why that's still a thing.


@ Saturday, May 27th 2023 by mlindner | Parent

Flat earth conspiracy is not widespread and is not growing. It's a thing that's primarily on the internet. On the grand scheme of things there's bigger things to worry about than what a tiny fraction of people think.


@ Saturday, May 27th 2023 by shakezula | Parent

I think it's more widespread than you might think. Flat earth and similar conspiracies play on distrust in academia, which is absolutely flourishing right now.


@ Saturday, May 27th 2023 by ttoinou | Parent

And they're right to distrust Academia. Anti flat -earthers play on trust of academia.


@ Saturday, May 27th 2023 by namaria | Parent

It's way easier, and more common, to be wrong than to be right.

@ Saturday, May 27th 2023 by SergeAx

Heard an anecdote about LIGO, when they were experimenting with quarz pendulum without a string(s). They statically charged the pendulum and its base, so the pendulum was repulsed from the base, eliminating even slightest possible friction while oscillating. A journalist from science magazine was visiting and decided to take an impressive picture of vacuum-floating piece of quartz. No one had a chance to say a word when journalist popped a flashlight from their camera and pressed the shutter. Flashlight removed the static charge and the pendulum fell to the base.

@ Sunday, May 28th 2023 by peter303

The new run has six candidate GW events the first week. Three are high confidence.

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