Monster gravitational waves spotted for first time

I recommend looking into LIGO and other similar experiments. They use laser interferometry to accurately measure the distance between two points, to an extreme degree. From LIGO's website:

Gravitational waves cause space itself to stretch in one direction and simultaneously compress in a perpendicular direction. In LIGO, this causes one arm of the interferometer to get longer while the other gets shorter, then vice versa, back and forth as long as the wave is passing. The technical term for this motion is "Differential Arm" motion, or differential displacement, since the arms are simultaneously changing lengths in opposing ways.

As described above, as the lengths of the arms change, so too does the distance traveled by each laser beam. A beam in a shorter arm will return to the beam splitter before a beam in a longer arm--as the wave passes, each arm oscillates between being the shorter arm and the longer arm. When they arrive back at the beamsplitter (where they re-merge), the light waves no longer meet up nicely; they are out of phase. Instead, they shift in and out of alignment for as long as the wave is passing.

https://www.ligo.caltech.edu/page/what-is-interferometer

https://en.wikipedia.org/wiki/LIGO

Also, The second data release from the European Pulsar Timing Array I - The dataset and timing analysis [1] was published just today.

[1]: https://arxiv.org/abs/2306.16224

Could one devise similar detectors for time dilation and expansion?

Are there theories? Yes!

Do they require a long list of impossible things to work? Also yes!

There are entirely valid solutions in general relativity which allow for an object in a pocket of spacetime with other spacetime warped around it in such a way that, more or less, space is moving but not the object.

However achieving the arrangement of spacetime to make this happens requires many things which are impossible, aren't known to exist, or require something like all the energy in the Universe to achieve.

Also there are no valid known solutions that transition from normal space to this special spacetime arrangement, so it could only exist if it always existed.

So it comes down to: we're pretty sure such things are not actually possible but we know where to look and what problems to solve if it were. Occasionally we see a paper which removes some of the impossible things from the list.

It's one of those "unlikely but maybe someday" kinds of things.

I understand the gist of this research: pulsars emit radio waves at regular frequencies, so by monitoring radio waves received from pulsars in the sphere surrounding us, we can measure correlated anomalies in their frequencies and infer that they were caused by large gravitational waves that effectively changed the shape of the transmission medium. That makes sense.

But this is not measuring the gravitational wave itself. It's measuring the change in trajectory of the radio signals that are "riding" the wave. In the ocean analogy, it would be as if we were surrounded by a circle of floating turrets that each emitted floating darts at regular intervals in all directions. Then we would measure the time it took the darts to reach us, and from that we could infer the size of the waves the darts encountered along the way. But we never actually see the waves, only the darts.

So my question is: how can we tell the difference between one really big wave, and many really small waves that would sum to the same effect? In other words, we know there is some waveform(s) that changed the velocity vector of the radio signal. But if there are multiple arrangements of waves that would produce the same change in signal, how do we pick the right arrangement?