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.
Hear 9 times as far?
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)#Quantu...>), 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.
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.
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.
 https://www.geo600.org/ , https://en.wikipedia.org/wiki/GEO600
Question though - do gravitational waves diminish significantly as a function of distance or intervening mass?
This should answer any questions!
i find it utterly fascinating that we're able to detect such a minuscule deviation
Nowadays, physics students do the MM experiment in a lab on a benchtop in a day.
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?objectgroup_id=10...). 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?partnumber=KM100#ad...). 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?partnumber=CPS532-C...). 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?objectgroup_id=10...).
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-MINT... plus a nice manual https://www.thorlabs.com/drawings/5d9e11209b7d4536-820A3379-... 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?objectgroup_id=11... 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...
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?
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..."
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.
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?
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.
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.
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 :)
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.
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.
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.
Probably not high school statistics accessible but Bayesian statistics and Gaussian noise and GR isn't.
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
The data's all publicly available. You go ahead, do the general relativity, and tell us.
>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.
https://gwosc.org/ and https://www.ligo.caltech.edu/page/ligo-data
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.
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.
Or do grav waves really pass ~trough~ black holes?
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.
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.)
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...
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)
(edit - spelling)
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.
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).
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.
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).