My staff report from 2003 on the speed of gravity is now on the front page again, but there are a few important updates to it. In particular, I mention LIGO and LISA, two instruments which might eventually detect gravitational waves. LISA, unfortunately, has been canceled, and if something like it ever eventually launches, we have no idea when it would be. But LIGO has, in the past few years, made multiple unambiguous detections of gravitational waves, from merging black holes. These detections included at least one where an optical counterpart was also detected: In other words, we “heard” gravitational waves from a far-distant event, and saw light from that same event, at the same time, thus directly demonstrating that the light and gravitational waves were moving at the same speed.
I was going to suggest this update but you beat me to it.
Powers &8^]
Does this give us a more precise measurement of the gravitational constant G? Or is most of the imprecision coming from the mass measurements?
Dumb question time, but what are gravitational waves “made” of? Are they EM waves? And what is the frequency or wavelength of the waves?
Pleonast, any masses determined from any of these observations are based on knowing Newton’s constant to begin with, and there’s no way to determine the masses independently of Newton’s constant. Which is irrelevant, anyway, since although Newton’s constant is the least-precisely known of the fundamental constants, we do still know it to four decimal places. But all of the masses in any of these events are known only to within tens of percents.
Crafter_Man, gravitational waves are most assuredly not EM waves. To the extent that it’s meaningful to ask what they’re “made of”, the best answer that can be given is that they’re made of spacetime itself. Or you could say that they’re made of streams of gravitons in the same way that light is made of streams of photons, but our knowledge of quantum gravity is so rudimentary (basically nonexistent) that that description is not at all useful. I’m speaking literally when I say that we’ll probably never detect an individual graviton.
Since gravitational waves have the same speed as light waves, they also have the same relationship between wavelength and frequency. But the typical frequencies of gravitational waves are much, much lower (or, equivalently, the wavelengths are much, much longer) than for EM waves: In principle, just like with light, there’s no limit on either end, but we don’t know of any source in the Universe that would produce gravitational waves above a few kilohertz (corresponding to wavelengths of hundreds of kilometers).
What would a hypothetical graviton detector even look like?
Is it possible to draw an analogy to low frequency radio waves? I mean, at some frequency it becomes difficult enough to detect and/or emit continuous waves. Single photons would be an even taller order. Yes?
That’s part of the problem: With the relationship between frequency and the energy of individual particles, extremely low-frequency waves have an extremely low energy per particle. The other part is that gravity is a much weaker force than electromagnetism, which means that any gravitational phenomenon is much harder to detect than its electromagnetic equivalent, regardless of energy.
You can, in the classical experiments, block light so that you are getting, at most, one photon every day. Which you can detect.
I think that apart from the difficulty of detecting a single ‘graviton’, you’d have difficulty blocking gravity in the same way that you can block light.
(Thank you for the update Chronos. I came here to look.)
Ligo was turned back on after an extended downtime for upgrades, etc. And it’s already detected another set of waves.
Gravity waves detected, ho hum, amiright?
For something like 10 Hz “light”? How do you do that, and has it been done?
So far as I know, the lowest frequency for which single photons have been detected is somewhere in the microwave range, but that’s not my specialty.
We can detect single photon events. The rod cells of the vertebrate eye are actually sensitive enough to detect single photons in the visible range although humans will not consciously observe it.
Gravitons, on the other hand, are too weak to plausibly be detected by any means, and we have no means to filter gravitons or expect them to appear in small quantities in nature.
Stranger
So all I need is some material that blocks out all incoming gravitons plus a source that produces a single graviton at a time and a device that measures said graviton and I’ll win a Nobel Prize.
Hold my [del]beer[/del] caffeine-free diet cola. 
These guys from Cornell can get you started. I only skimmed, but I think it involves building a detector the mass of Jupiter in close orbit around a neutron star…while solving some other unsolved problems along the way in order to possibly get one detection a year.
Chronos, there’s something that always puzzled me about this staff report.
Gravity is often illustrated in terms of a rubber sheet. Place a bowling ball in the middle of the sheet, and it deforms creating a slope. Put a marble some distance from the bowling ball, it will roll down the slope.
So, asking about the speed of gravity, isn’t that like asking about the speed of slope? The slope doesn’t have a speed, it’s just there. Or Crafter_Man’s question, what gravity waves are made of, isn’t that like asking what slope is made of?
Would it make more sense to phrase the question in terms of how quickly the slope changes if you add or remove mass?
It’s very likely I’m completely misunderstanding something here.
Gravity does act like a deformation of space, but it is an acceleration, not a static “slope” as such.
I know that we can detect single photon events and I understand that gravitons would have very low energy. I was trying to ask if we could draw an analogy to low frequency radio spectrum EM and how hard it would be to produce or detect single photons of such low, but, and I’m guessing here, higher energy than gravitons.
Chronos has come the closest to answering that question with the tidbit that the lowest energy single photons detected might be in the microwave region.
Microwaves have frequencies (and thus photon energies) millions to billions of times higher than those of the longest wavelength radio waves we can detect as waves.
I thought it possible one could use the challenges of detecting a single photon of Ultra Low Frequency EM-radiation to aid in understanding why single gravitons will never be detected, but based on my inability to convey that to people who actually understand physics that seems implausible. 
What’s wrong with “the speed of slope”? The slope “isn’t just there”, it moves with whatever object is making it, and it doesn’t move instantaneously when the object accelerates.
Wiggle that bowling ball back and forth, to stick with the analogy, and you’ll create moving slopes in the sheet. It’s tricky to do it in a way that really mimics gravity waves, since the wave energy will be dampened quickly by the sheet in a way space-time doesn’t, but the speed with which does waves in the sheet spread out would be analogous to the speed of gravity.
I know, hence my question : * “Would it make more sense to phrase the question in terms of how quickly the slope changes if you add or remove mass?”*
Maybe I should have added: “or move the bowling ball”
The point is, that’s the speed of gravity. I thought my analogy might help explain why your question doesn’t make sense to me.
Here’s a different approach, which might fail as well.
Just as there is always gravity around in the real universe, there is also always electromagnetic radiation, so what we observe is just changes in the level of that radiation (or disturbances in the electromagnetic field if you’re into that). How fast those changes propagate is called the speed of light, not the speed of changes in light intensity.