If you have a perfect mirror, can you store energy in lasers indefinitely?
I guess you'd still lose energy to the air
I guess you'd still lose energy to the air
Aside from tiny losses like tunnelling, scattering from vacuum excitations and thermally-generated fields, etc., a perfect optical cavity will store light indefinitely. No laser required, only light of the appropriate wavelength and phase.
The combination of Fabry-Perot cavities and power-recycling in the optics of the LIGO gravitational wave observatory "gains" a ~100 W laser up to a few megawatts. If a cavity is formed in free space, like LIGO, then careful attention to vacuum is required to prevent loss.
ULE clock reference cavities have quality factors that are much higher, and need no vacuum, as the light propagates entirely within a glass substrate.
Edit: I should add that the "tiny losses" mentioned at the outset are precisely what prevent you from making a "perfect cavity". As the quality of a cavity/oscillator increases, the number and deviousness of loss mechanisms does too. This is especially the case at frequencies that are low compared to those at which an experimenter can iterate.
For HN, consider building a host that can run uninterrupted for 10^12 seconds (30,000 years).
In the sense in which I'd understood the question "Cavity losses are often dominated by reflectivity losses at the mirrors: does an improved mirror make a big difference?", I think my reply is relevant.
However, the article said “about 35 degrees” so the actual number is probably something 34.5 ≤ x ≤ 35.5 but not actually 35.
My geometric intuition says that in 3D you can always construct a cycle but I don't know how to formalize it. Imagine a flat zig-zag chain of alternating bends; you can reshape it to a curve of arbitrary radius (around the axis of the width of the chain) by turning each bend slightly relative to its neighbors (about the axis of the beam), in alternating directions. Then you can choose a radius which makes the ends meet exactly.
Example: my laptop battery has an energy storage of about 100 Watt-hours = 360 kJ. Suppose we want to store the same amount of power in an optical cavity (two mirrors pointed at each other) with a length of 0.5 meter. The time required by light to make a roundtrip in this cavity is 2L/c = 3 nanoseconds. Thus the power incident on the mirror at any given instant is (360 kJ)/(3 nanoseconds) = 10^14 watts!
This power is high enough that I suspect that even the vacuum itself couldn't conduct it without strange effects (production of particles, for instance).
For this to work as a storage medium it would have to work at an angle much closer to 90 degrees to avoid excessively large equipment and there would have to be a near perfect conversion both on the input side and on the output side (which as far as I know we currently don't have).
Aaah, good old "in a vacuum," surely one of physics most convenient simplifications, together with frictionless surface, the point particle and it's big-boned cousin perfect sphere and of course the infinite featureless plane. Except that a vacuum is actually kinda sorta possible in the real world too.