Interferometry What is it? 1. You make a monochromatic beam of light where all the wavelengths are exactly the same (a laser). 2. You make the beam coherent so that all of the waves are in phase (peaks align and troughs align). 3. Polarisation (optional), you polarise the beam to make the orthogonal oscillations of the beam aligned, this is for measuring rotation. 4. You split this beam in two and produce two beams that are monochromatic and coherent. 5. The first beam is the reference beam, the second beam is the target beam. 6. You bounce the target beam off a target and when it returns you compare it to the reference beam and the delta tells you information about spacetime. 7. Because light is fast you can collect an enormous volume of data with a very high sample rate. You can know exactly how far away the target is, how fast it is traveling, how it is vibrating, you can see all of its resonant frequencies and harmonics, right down to molecular lattice level. You can predict what the target is and how it is behaving, you can see its temperature from the Brownian motion of its surface. Because light is so fast you can really inspect it, because the wavelength is so short and precise you can inspect extreme resolution. This is the most powerful sensor family in science. As someone who once built an interferometer tech I find this is very cool, this is a whole domain of science and engineering that is very under discussed. The Fermilab Holometer was completed in 2014 it is a Michelson interferometer that was built to prove the universe was a hologram and that we live in a pixelated simulation. It did the opposite. After 145 hours of data, their specific model of correlated holographic shear noise was excluded at 4.6σ. No evidence of the predicted Planck-scale jitter was seen. As they collected more data they excluded a broader class of quantum-geometrical shear-noise models. The measured correlations were consistent with ordinary classical spacetime. Finally, the interferometers were reconfigured to be sensitive to rotational (twisting) fluctuations instead of shear. Again those models were also constrained; the data again matched a classical spacetime model. Lots of people still believe simulation theory, and that’s OK. People should believe whatever makes them happy. The same dataset also produced the strongest limits known on a stochastic gravitational-wave background in the megahertz band and on nearby primordial black-hole binaries, science that is independent of the quantum-geometry hypothesis. Interferometers in the hands of smart people unlock secrets. You can multiply the sensitivity (sensor power) of your interferometer by using Fabry Perot Cavities and bouncing your laser between two mirrors. This can give you many kilometres of beam length in a device that fits on your desk. Fabry Perot cavities are what LIGO uses (Laser Interferometry Gravitational Observatory), LIGO was built to measure intergalactic gravitational waves from distal black holes and supernova events and works incredibly well. LIGO can detect vanishingly small disturbances in spacetime that are from events that happened billions of years ago before our Sun even formed. We can actually measure and observe past events in extreme detail and understand what happened. We can observe things not just from the other side of the Milky Way, but from entirely distant galaxies that exist on completely different superclusters. We have been able to build phenomenal instruments for a while, we also have phenomenal DAQ, more recently we have the technology to train on this empirical data. There is stuff to find there. The 20th Century was dominated by theorists, I expect the 21st Century is an empirical age, build-test-build. We can ask the questions and the universe will just tell us.