"Gravitational telescopes" let scientists observe fluctuations in spacetime itself. They are, in a word, crazystupidamazing.
Telescopes have been observing waves for a long time. The ones set up by brilliant past astronomers (or by modern people looking to bore their kids) are designed to call attention to visible light waves. Over time, people came to understand that there was more to the universe than what us yokels can gawp at, and started measuring radio, ultraviolet and x-ray waves. Lately, scientists have turned their attention to measuring gravitational waves.
A good way of picturing gravitational waves is imagining the universe as a stretched-out piece of fabric. Planets and stars sitting on the fabric pull it out of shape, and anything placed close to them will fall towards them. If heavier objects, like stars and black holes, are still, then the fabric is as well. If — on the other hand — things like neutron stars or black holes are romping around like happy, infinitely-massive puppies, the fabric will ripple around them. Those ripples in the universe are what scientists will be measuring.
The major facility attempting to measure gravitational waves is called LIGO – Laser Interferometer Gravitational-Wave Observatory. The process starts with – what else? - a laser. The laser is put through a beam splitter, which is a divides up the laser beam exactly, sending each half into a 2.5 mile tube. The tubes are equipped with two mirrors, which play not-itsies with the laser, tossing it back and forth between them. At last the dazed laser beam is sent back to the beam splitter, to rejoin the twin laser beam it was so callously torn away from in the splitter at the beginning of its journey.
But what's this? The two beams are out of phase with each other! Although they were produced from the same source, and the two arms that they were sent out to were identical, the peak of one of the beams is slightly behind the peak of the other. It seems that one of the beams of light had a longer, rougher journey than the other. It's seen some things, some things it can never un-see, not unlike the end of The Sound of Music when Rolfe joins the Nazi party and turns on Liesl. In short, it isn't the same beam of light that it used to be. The source of its new, jaded worldview? Gravitational waves. As they pass through the universe they stretch or compress the things they travel through. This means that they will have compressed, or stretched, one of the arms more than the other.
The more out of phase the two beams are, the more light gets kicked over to a photodetector. That photodetector makes a record of how much, or how little light it's seeing, and that record shows the amount, wavelength, and intensity of gravitational waves that pass through our neck of the woods.
Gravitational waves are incredibly faint. Precautions have to be taken to make sure that LIGO only measures gravitational waves. This is where things get ridiculous. When reading about LIGO seems a little bit like reading "The Emperor's New Clothes," only no one ever calls anyone's bluff. The laser beams can't risk scattering willy-nilly through air, so the tubes through which they are shot are at an air pressure of one-trillionth of an atmosphere. The tubes, as mentioned before, are two and a half miles long. Oh, and the mirrors which reflect the laser are protected from vibration. It turns out that they vibrate so little that the motion of the atoms in the mirrors can be measured. There's a reason for all this care. The differences in length that they are measuring, over the space of many miles, are one one-thousandth of the diameter of a proton. Methinks that some scientists out there are wearing no clothes.
But it doesn't look like they'll be covering up anytime soon. LISA – Laser Interferometer Space Antenna – is a plan to — by the year 2020 — do pretty much this exact same thing. In space. Only each of the arms of this interferometer will be 5,000,000 kilometers long.