You might have heard of vacuum fluctuations before. For electromagnetic fields, these are what cause inherent uncertainty in measuring e.g. phase or amplitude. That is, if you measure e.g. the intensity of a laser many times, the variance of these measurements is connected with these fluctuations of the electromagnetic field ("photons spontaneously appearing and disappearing"). A good laser is what we call shot-noise limited - i.e. AT this fundamental limit. If they are not, for example electronic noise might be the culprit. Squeezed light lowers this variance in either phase or amplitude (or even something in-between, although the interpretation here is a bit more involved.) Remember that QM is a statistical theory, so it always needs to be seen in this context. Squeezing is generated in practice by creating states of light in which photon pairs are created at the same time. This happens when e.g. some (nonlinear) medium is capable of coupling the frequency of a high-energy photon to the frequency of two, lower-energy photons (with half the energy). The intuition for why this causes squeezing is likewise a bit more involved, but suffice to say that the intensity of a light source is related to the number of photons, and IF you create two photons simultaneously then that must necessarily have some influence on the statistical properties of the measured intensity. You can think of it in the sense that for certain (again, nonlinear) processes, you can deamplify or amplify the intensity of laser light. Given the inherent quantum noise of the light, this amplification or deamplification will also amplify (antisqueeze) or deamplify (squeeze) the noise. Although I do not know if anyone has done a proper analysis of this, it is actually possible to observe "classical" squeezing: a spring-pendulum where the spring oscillation frequency is double or half that of the pendulum oscillation frequency will, assuming you let the initial conditions become some random variable, ALSO "squeeze/antisqueeze" the uncertainty in the position ("amplitude") or velocity ("phase") at some time greater than zero (the governing equations for the nonlinear optical process and the equations of motion for the spring/pendulum system are almost the same). So to sum up: light squeezing has nothing to do inherently with the interferometer here. It is a property of the light itself, generated by a nonlinear quantum optical process. If you measured the intensity of an amplitude-squeezed state of light many times and compared it with many measurements of laser intensity of equivalent power, the variance of your measurements would be lowered for the squeezed light.

The freq you heard representing the gravity wave is just a representation of the wave. The light is measuring this change in distance, which is converted to sound instead of a graph. Think of the sound your voice makes. We hear the sound of your voice that is converted by a speaker that is following the electronic signal. It is not the signal, it is just the representation of it.

@@garylcamp They are squeezing the light (between amplitude and phase) differently according to frequency. Frequency of what? The video either didn't say or I missed it. The light's frequency (color) is very nearly constant, determined by the energy difference of the laser medium's ground and excited states. I'd say that's not the frequency of interest. Instead, to have the desired effect of avoiding moving the mirror, they would IIUC modulate the squeezing smoothly so the laser light gets squeezed to the precise phase for hundreds of microseconds, and then squeezed to constant amplitude for the next interval, with a transition period of no squeezing. I'd expect this process to vary controlled by a white noise audio signal.

@@edwardhuff4727 It's the /modulation/ of the laser freq we're interested in, not the laser's freq. Phase and amplitude manipulation are two ways of increasing desired resolution. That said, I'm not convinced LIGO is actually measuring gravity events. By my calcs, there's still far too much noise in the LIGO design.

Think of it like radio. When you listen to a radio station at 101.3MHz, the sound you're hearing isn't at 101.3MHz. What's happening is that the sound waves are used to *change* the 101.3MHz radio waves, and because the changes happen at audio frequencies, radio receivers can just tune into the fast radio waves, look for the changes, and convert the changes to sound. That's exactly what's happening here. The frequency of the light is the "carrier" for the changes made by gravity waves. Those changes happen to be happening at the same frequencies that we hear sound waves at, so you can convert them.