What is the longest detectable EM wavelength?

Question

What is the longest detectable (by today's technology) EM wavelength? and is there a limit of the energy that those with longer wavelengths that we cannot detect can carry? can there be a galactic or "Intergalactic space" scale standing EM waves? e.g. a standing wave between the BHs at the centers of the Milky-way and Andromeda?

Answer 1

The fundamental electromagnetic Schumann resonance has a wavelength approximately equal to the Earth's circumference. No longer wavelength can propagate in the waveguide formed by the Earth surface and the ionosphere. Lower frequency radiation from space cannot penetrate the ionosphere or even the less dense solar wind and interstellar medium. The intergalactic medium may allow electromagnetic waves with wavelengths of a few million kilometers, but those cannot penetrate the Galaxy.

The limit in space is the plasma frequency, an effect of the free electrons in plasma. Plasma is ubiquitous in space. The Schumann resonances are only possible because the Earth's atmosphere contains almost no free electrons.

Note that very close to the plasma frequency, the phase velocity of electromagnetic radiation approaches infinity, so the wavelength can, in principle, be arbitrarily long. The group velocity, however, approaches zero, so such radiation cannot, in practice, propagate.

Answer 2

The practical limit for useful radio communications is in the range of between 3 to 30 kilohertz. Limited work has been done all the way down to ~70 Hz, with limited success. (Note here that research work has shown that the 60 Hz AC grid frequency affects atmospheric electrification by propagating off the power grid in the US.) The issues are as follows:

The effectiveness of an antenna at converting the flow of AC current into electromagnetic waves is strongly dependent on the physical dimensions of the antenna, with the optimal condition for conversion occurring when the length of the antenna is equal to half the wavelength.

To broadcast effectively at, say, 30 megahertz then requires an antenna ~5 meters long. 3 megahertz yields 50 meters, 300 kilohertz 500 meters, 30 kilohertz 5000 meters and 3 kilohertz 50 kilometers- and 300Hz needs an antenna 500 kilometers long.

So the real estate requirements for low frequency EM transmission are onerous and burdensome- and rule out their effective use in mobile applications where the transmitter has to be portable.

Amateur radio enthusiasts (also known as ham geeks) have occasionally purchased parcels of power-line right-of-way with abandoned power lines already strung up, and converted them to function as very low frequency antennas for running experiments late at night when their long-suffering wives are already in bed. Physics dictates that the antenna has to be separated from the ground surface by ~one wavelength for best propagation, which means that even the best of these power line antennas is not very good at producing practical amounts of radiated RF power.

In addition, the maximum data transmission rate scales with the transmission frequency, which means to stream hi-def large format color video in real time with high fidelity stereo sound requires a carrier frequency in the gigahertz range. Scaling this down to the very low frequency range means you cannot transmit even the human voice "down there" and must use morse code or some other digital code to share data.