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Comment by tobinfricke | Hacker News Reader

It's special because it's a different field. Radio/light are oscillations of the electromagnetic field, and sound is an displacement in a physical medium. This is an entirely different medium: the metric tensor of spacetime itself. And the universe is, by and large, completely transparent to gravitational waves. So these signals would not be blocked by intervening matter.

A similar thing could be said about the prospect of sending messages using neutrinos.

But it is only just now barely possible to detect the loudest gravitational wave signals with extreme difficulty and machines costing on the order of a billion dollars. It will remain impossible for humans to generate gravitational wave signals of a detectable amplitude for the foreseeable future.

Source: PhD in gravitational wave detectors.

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A lame question from someone who barely remembers college-level physics: isn't that "electromagnetic field" just waves in the spacetime metric-tensor?

The electromagnetic field is a function F(x), where F is the field tensor (which groups together the electric field E and the magnetic field B) and x is the position in space. Maxwell's equations tell you how F (or E and B) evolve in time, and give you wave-like solutions.

In a continuous medium like air, water, or a solid, you could describe a "displacement field" d(x) that tells you how far the piece of material that used to be at position x is displaced from its equilibrium position. This is governed by the equations of physical acoustics, which also have wave-like solutions: sound.

The Riemmann curvature tensor R is a function R(x) of spacetime coordinates x, which tells you the curvature of spacetime at location x. Einstein's equations tell you how R evolves, and it also turns out to have wave-like solutions: gravitational waves.

In radio, it's F (or E and B) that's waving. In gravitational waves, it's R.

What's confusing is that R also tells you "the shape of space," and you need to use it to figure out "where" the coordinate x corresponds to.

If R is the shape of space, is it fair to say that gravity waves from a black holes merger are large waves in R, like large waves in a lake we could surf on; while there are also tiny ripples that we describe with F? In other words, F is really R with large waves subtracted.

> In other words, F is really R with large waves subtracted.

No, they are completely independent.

Here's another analogous example. Suppose you have a two-dimensional rubber sheet. You put a coordinate system on it, so any position on the rubber sheet can be identified by coordinates (x, y). You deform the rubber sheet so it has a curvy shape - anything you want.

You can have a tensor R(x,y) that tells you the curvature of the rubber sheet as a function of position (x,y). And a scalar T(x,y) that tells you the temperature of the rubber sheet at (x,y). These are completely independent quantities. Different fields.

T is indeed independent from R, but how do we know that F and R are also independent? Would theory fall apart if we try to model F as microscopic modulations of R?

I'm trying to look at R and F from the signal processing point of view: if we apply Fourier transform to both, we'd see that R is described by low frequency band (planetary scale distortions), while F would be ideally described by a high frequency band.

I wonder about something different - if R and F are independent, and R also tells you the shape of space, what would happen if EM and gravity waves crossed through each other? My guess is the EM wave would be modulated somehow, within the intersection volume, as if crossing into a different matter medium, while gravity wave would be unaffected.

Both effects can occur; GW <-> EMW. You can search for Gertsenshtein Effect, there is also this old paper by Zel'dovich: http://jetp.ac.ru/cgi-bin/dn/e_038_04_0652.pdf

That's correct. The "shape of space" can act as a lens for light. Even in static situations this can be quite interesting:

https://en.wikipedia.org/wiki/Einstein_ring#:~:text=An%20Ein....

It's possible that you could observe a kind of "shimmer" in the position of background stars as a gravitational wave passes between you and them. In fact there is an attempt to detect gravitational waves this way through pulsar timing:

https://en.wikipedia.org/wiki/Pulsar_timing_array

>You can have a tensor R(x,y) that tells you the curvature of the rubber sheet as a function of position (x,y). And a scalar T(x,y) that tells you the temperature of the rubber sheet at (x,y). These are completely independent quantities. Different fields.

I get that it's an analogy, but I don't think they would be completely independent quantities here. There is likely going to be some correlation with temperature and curvature of the rubber sheet. It would likely pale in comparison to almost everything else though, but on the topic of discussion that might be a large enough correlation.

Also, don't high energy photons curve spacetime?

> There is likely going to be some correlation with temperature and curvature of the rubber sheet

Yeah, that's a failure of my analogy. Deforming a rubber sheet does create heat, but deforming spacetime does not create light.

But R(x) is required to determine x -- whereas F(x) is not. Thus, F and R are fundamentally different.

If we were able to catch neutrinos effectively, could we use neutrinos for the same purpose?

We already had a working point-to-point neutrino beam [0]. But I'm not a physicist and I don't really know if you could modulate it in any meaningful way.

[0] https://en.wikipedia.org/wiki/CERN_Neutrinos_to_Gran_Sasso

Neutrinos have energy (as particles), so they should have a wavelength (as a wave), so frequency modulation works. And you can measure intensity, so AM works, too, and you can build all other modulation schemes from these two.

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