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Comment by tobinfricke | Hacker News Reader

We don't have to involve quantum mechanics here at all. In fact, we don't yet know how to combine general relativity and quantum mechanics.

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Sure, but what would actually happen in the experiment I've described? I admit that perhaps those gravity waves are so tiny that even a nanometer-scale double-slit device would be too large, but if we had some hypothetical material (not atom based with those fat electron orbitals), that we could construct a 0.001nm wide device with two 0.000001nm wide slits, would it produce the interference image?

I think you might be confused about what we mean when we say that gravitational waves are "small." Their amplitude is small. I suppose the wavelength can be anything, but the wavelengths of gravitational waves generated by known physical processes are large. This is because frequency and wavelength are inversely proportional, and things need to be moving wicked fast (high frequency) to get a short wavelength: lambda=c/nu, where lambda is wavelength, nu is frequency, and c is the speed of light in vacuum. If two black holes are spinning around each other 50 times per second (which is already crazy to think about), then they will generate gravitational waves at 100 Hz, which have a wavelength of (3e8 m/s)/(100 1/s) = 3e6 meters. Three million meters.

Next: What do you propose is going through the double slit experiment here? The gravitational wave? Matter is transparent to gravitational waves, so it would be difficult to build such a double slit experiment. As another poster here mused, you could probably make some kind of interesting meta-material analogous to a crystal lattice and get some interesting gravitational-wave diffraction pattern. But I'm not sure if this is what you're getting at. The point of the famous double slit experiment for photons is that it shows the dual wave/particle nature of photons (or other elementary particles like electrons). Since we don't have a quantum theory of gravity I suppose it's an open question what the outcome of such an experiment with gravitational waves would be.

I think the OP is musing about the idea that as it passes through a micro double-slit setup, a gravitational wave would alter the distance between the slits, and thus alter the interference pattern. Basically an interferometer.

That's basically LIGO.

I think there is no slit involved in LIGO. We only cleverly measure phase shift between two light waves (or rather the same wave but bouncing from a different direction).

That is also what the double slit experiment does, in a sense. A double-slit is a kind of primitive beam splitter.

We'd use regular photons or electrons to produce the interference image (assuming they would "fit" in those micro-slits), while the gravitational waves would modulate the distance between the two slits. Your crystal lattice would be a generalization of my idea.

In the two black holes example, the gravitational wave oscillates at 50 Hz. Its spatial speed is irrelevant. This means that the distance between the two micro-slits would be modulated at 50 Hz with the micro-amplitude of that gravitational wave.

Let's say the distance between the two slits is L, it's modulated in the L0.9..L1.1 range at 50 Hz, the screen that captures the interference image is at 100L distance, to magnify the pattern 100x. This won't work if the modulation amplitude is L1e-60 or something on that scale, as we can't magnify the image 10^60 times.

I see. With some modification of this idea, you invent LIGO. Instead of using two masses as a "slit", the light is bounced between the two masses many times, multiplying the tiny change in distance between the masses induced by the gravitational wave.

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