What do light, radio waves, and x-rays have in common? Everything! They are all electromagnetic radiation which differ only in their frequency, wavelength, and energy. All electromagnetic radiation travels at the speed of light, which in a vacuum is 186,282 miles per second or almost 671 million mph.
There are no distinct boundaries between different regions of the electromagnetic spectrum. The spectrum can, however, be conveniently divided into the following seven general categories (in order of decreasing energy and increasing wavelength): gamma rays, x-rays, ultraviolet, visible, infrared, microwave, and radio.
Depending on how it is observed, all electromagnetic radiation has a dual nature, capable of behaving either as a wave or as a particle. We call these particles photons. The highest energy photons are gamma ray photons, and the lowest energy photons are radio wave photons.
The highest-energy, shortest-wavelength electromagnetic radiation (gamma rays and x-rays) usually behaves like a particle rather than a wave in experiments, whereas the lowest-energy, longest-wavelength electromagnetic radiation (radio waves) usually behaves like a wave in experiments.
Is there a limit to how far the electromagnetic spectrum extends? On the high energy end, we think so. On the low energy end, not really. Let’s explore both extremes.
Gamma rays are produced by the most energetic processes in the universe. The highest energy gamma rays yet detected are 2.5 PeV (2.5 peta-electronvolts or 2.5 × 1015 eV). The theoretical limit to the energy of a gamma ray is the Planck energy, 12.2 ReV (12.2 ronna-electronvolts or 1.22 × 1028 eV). Any gamma ray having this much energy would have a wavelength of a Planck length (1.6 × 10-35 meter). At this wavelength, standard quantum physics breaks down and the gamma ray photon would concentrate so much energy into so small a space that it would likely collapse into a microscopic black hole, ending its existence as radiation.
There is another, much lower energy limitation, however. At gamma ray photon energies around 0.1 to 100 PeV (1014 – 1017 eV), the universe becomes almost completely opaque over large distances. A gamma-ray photon at this extreme energy will travel only a short distance before colliding with a photon from the pervasive cosmic microwave background. This collision converts the original photon into an ultra-relativistic electron-positron pair. As these new particles hurtle through space, they smash into additional background photons via inverse Compton scattering, boosting them into high-energy gamma rays. This triggers an energy-diluting electromagnetic cascade: photons create particle pairs, and particle pairs produce new photons. The chain reaction repeats until the individual photon energies drop below the threshold required for pair production. The end result: the original high-energy gamma ray photon never reaches your detector.
Now, let’s talk about the opposite end of the spectrum, the radio waves. What do we know about the least energetic of photons? To put things in perspective, you may be interested to know that national and international standards have been established for the orderly use of the radio spectrum down to a frequency of 3 kHz (3000 Hz), although the military is using 40 – 80 Hz radio waves for global submarine communications, and every electrical wire in your house is generating low levels of 60 Hz radio waves due to alternating current flipping back and forth 120 times each second.
Below 40 Hz exists a strange realm indeed. The Earth and its atmosphere act like a giant, natural radio transmitter. Lightning storms continuously bombard the upper atmosphere and trigger extremely low frequency radio waves (known as Schumann resonances) that ring around the globe at roughly 7.5 Hz.
To generate radio waves with even lower frequencies than 7.5 Hz, the scale of the “antenna” must grow to planetary proportions. In nature, this occurs when exceptionally energetic lightning events or the shockwaves from large meteors dump enormous energy into the atmosphere, creating radio waves with frequencies of 3 Hz or lower. The immense mechanical stress of shifting fault lines before an earthquake can turn miles of quartz-rich bedrock into a colossal subterranean transmitter, broadcasting seismic radio pulses down to a fraction of a hertz via the piezoelectric effect. Humans can replicate this feat artificially, but the physical constraints are staggering. For example, a 1 Hz wave has a wavelength of nearly 300,000 kilometers. Efficient transmission requires an antenna of comparable size.
There is no theoretical lower limit to a radio wave’s frequency. Any accelerating electrical charge can produce one, and if the system that produces it is large enough, natural processes can generate radio waves with frequencies as low as a nanohertz (a billionth of a Hz), or even lower, resulting in wavelengths measured in light-years.
We can only study these ultra low frequency radio waves which are of a cosmic origin from space because the Earth’s ionosphere reflects away everything below about 5 MHz. But we’ll have to travel far from the Sun if we wish to listen to cosmic radio frequencies below about 20 kHz because the solar wind acts as a reflective shield. Only by traveling beyond the heliopause and into interstellar space will we escape the Sun’s plasma shield, though the ambient interstellar medium itself will still block frequencies below 2 kHz. In order to listen to the cosmic din and chatter at sub-Hertzian frequencies, you’d have to find a completely empty intergalactic void entirely cleared of plasma.
Here are the wavelength ranges of our currently operating general purpose space telescopes (1 Å = 1 angstrom = 0.1 nm = 0.1 nanometer = 10-10 m).
Fermi Gamma-ray Space Telescope
0.000000041 → 1.55 Å
Chandra X-Ray Observatory
1.20 → 120 Å
Hubble Space Telescope Visible Spectrum
1150 → 17,000 Å 3800 → 7500 Å (blue → red)
James Webb Space Telescope
6000 → 283000 Å
There are currently no active general purpose space telescopes operating in the 120 Å to 1150 Å range (between Chandra and Hubble). This is known as Extreme Ultraviolet (EUV), the transition zone between the ultraviolet and x-ray parts of the electromagnetic spectrum. However, space telescopes that observe the Sun do observe in the EUV region.
Because neutral hydrogen in the interstellar medium is highly efficient at absorbing EUV radiation, galactic and extragalactic objects are difficult or impossible to observe at these wavelengths, especially between 400 Å and 912 Å.
There are currently no general purpose space telescopes observing at wavelengths longer than 283000 Å (James Webb), in the far-infrared and radio part of the spectrum.
To detect far-infrared and submillimeter (shortest-wavelength radio) radiation requires a space telescope to be cooled to nearly absolute zero. This is challenging but not impossible.
The Earth’s atmosphere is adequately transparent to most radio waves, so observing this part of the electromagnetic spectrum from the Earth’s surface is easier and cheaper than building a radio space telescope. However, the far side of the Moon would be an even better location to observe almost the entire radio spectrum (except for the short-wavelength submillimeter radiation which requires extreme cooling as mentioned above), both because the Moon lacks an atmosphere but also because terrestrial radio interference would be completely blocked.
