Why We Hum

In the earliest days of electrical power, (the late 1800s) the inventor of the light bulb, Thomas Edison, elected to power his creation with direct current. Direct current 'DC' is fine for short distances, and when no need for changing voltages is required DC works fine, but hums not a bit. It was soon discovered that AC 'Alternating Current' was far superior for transmission over longer distances, could properly pass through transformers, allowed the use of brushless electric motors, and is what powers our world today.

The laws of nature seem to be universal across the universe so this same alternating current discovery will be made by any sentient species bright enough to develop electricity.
Their lights will hum!

In the western hemisphere AC alternates, which produces a 'Sine Wave', with a frequency of 60 times per second. In Europe, and other countries, AC alternates at a rate of 50 times each second. This produces copious quantities of hum.

Twice each cycle the voltage in an alternating circuit goes to zero and reverses polarity. At 60 cycles this produces 120 pulses of light from most lamps each second. At 50 cycles lights pulse 100 times each second.

Click the animation for more about Sign Waves

The western hemisphere hums at 120Hz. (cycles per second), and most of Europe hums at 100Hz.

50 and 60 cycles per second were chosen for a verity of technical reasons. Much below 50 cycles and our visual senses perceive flicker, which would make thirty or forty cycle electric lights troublesome for us to use. As electricity is generated by spinning a coil of wire around a magnetic field, frequencies much above 100 cycles per second would require spinning the generators too fast to insure long trouble free operation.

The laws of physics seem to be the same throughout the universe, and so should we expect animal physiology to be similar to ours. Animals living in a similar environment to ours would be expected to have a similar visual system, and as they became technological, would choose similar electrical frequencies, for the same visual and mechanical reasons.

Classic SETI
and why it's
likely to fail

For over a half century SETI has been listening for radio signals from extraterrestrials. For a verity of technical reasons, the odds of classic SETI ever hearing an extraterrestrial radio signal are worse than one in several hundred billion. Scott Bidstrup has written an excellent paper 'Why The SETI Project Is Doomed To Fail'.

Obviously you can't prove a negative, and Mr. Bidstrup's article weighs heavily to the negative. As much as I'd rather not, I must agree with many of Bidstrup's conclusions.

I believe Hum-SETI avoids many of the pit falls of the previous SETI experiments. And will shine a light on our neighbors.
(please excuse the pun, for I simply could not help myself. :-)

and why it's
likely to succeed

High power output from ET's outdoor lighting, and its, broad pattern of radiation, will sweep over us slowly, over many hours, not as the fleeting glance of a directed radio or laser signal would.

Very narrow signal bandwidth. Alternating current varies by less than a fraction of one cycle per second (Hz), so is relatively easy to sort from the background noise, which will be exceedingly high.

Likelihood that a civilization will radiate AC light for more than a hundred years is very high, where our civilization is now transitioning away from analog radio and TV signals towards digital, which is far harder to receive. We may someday move away from alternating current lighting, but AC lights will outlast high power analog radio transmissions.


For nearly 70 years we have broadcast our television in what's called 'Analog'. This analog signal is nearly as easy to decode as placing grandmother's sewing needle on an old fashioned record to hear the sound recorded in the wiggly, analog groves. Analog line and frame sync pluses, imbedded in the TV signal made decoding such a signal very easy. The bad news, at least for the SETI people, is we are transitioning away from analog broadcasting to digital. It's assumed that ET will follow a similar path, and after a hundred years or so may switch to digital transmission.

Digital TV is a much more difficult signal to decode. Some have said, "Even detecting an extraterrestrial digital television signal may be impossible, as the digital signal looks so much like background noise it may be undetectable". That was until recently. Scientists and engineers, through their work improving our own digital transmission and reception, and through the decryption of encrypted digital data, have discovered a multitude of ways to detect and decode digital signals previously thought impossible.

I have no doubt - should we hear the lighting-hum from a neighboring star system, some country on this planet earth will construct the required equipment to receive and decode their TV transmissions in record time, regardless of their mode of modulation or encryption.

Since the 1960s the Search for Extra-Terrestrial Intelligence, 'SETI', has been limited to listening for radio transmissions from intelligent extra-terrestrial beings. So far we've heard not a peep from neighboring worlds. This is understandable as our radio receivers are too insensitive to receive anything but a very high power transmission intentionally directed at our solar system. A less than likely prospect to my mind.

A project called Optical SETI has been tried, and it too has been fruitless. Optical SETI, as practiced by Harvard and others, is looking for a high energy LASER beam, intentionally directed at us by a world of beings intent on advertising their presence to neighboring worlds like ours. Presuming a race of sentient beings would expend the time and effort to build and operate a multi-gigawatt laser simply to say 'Hello' seems less than sensible in my opinion. We should be looking for something they may be unintentionally sending in a wide swath away from their planet, and that is their artificial lighting.

Earth, for the last hundred years or so, has been radiating manmade light into space. We now emit nearly 100-terawatts of artificial light out to the stars. This manmade optical radiation is far brighter than all of our radio transmissions combined, and unlike earth's myriad of radio transmitters operating on millions of different frequencies, our artificial light is modulated at only two very narrow, and easily detectable, frequencies.

Assuming most extraterrestrial sentient beings utilize the same part of the electromagnetic spectrum for their vision as we, (a near absolute necessity if they're biological), the supposition that they'll eventually invent electric lighting is a pretty safe bet.

The artificial light, generated by earth, blinks 'Hums' at a low note from 100 to just under 400 cycles, or pulses, per second, depending on phases and harmonics. Stars, other than a few pulsars, don't blink or hum at anything near these frequencies, so detecting this attribute of artificial light over a few hundred light years, and even through the glare of bright stars is something we can accomplish with today's technology.

Why does our light, and presumably that produced by other intelligent species, blink or hum? See the sidebar to the left.
I propose a search for extraterrestrial intelligences in the optical band of the electromagnetic spectrum, utilizing existing terrestrial telescopes, and some simple off-the-shelf electronics, not to directly detect the light, but to detect the alternating current attribute of the light radiated from planets inhabited by sentient beings who are at, or near, our level of technical achievement.

Scientists have recently detected the reflected light from 51-Peg-b, a half Jupiter size planet orbiting a star about 50 light-years from earth, and soon ESPRESSO at the VLT, and the European E-ELT, will have the ability to do spectral analysis of the reflected light from earth-size exoplanets around many other stars. The reflected starlight from a planet will be orders of magnitude higher in amplitude than any artificial light that might be generated by the inhabitants of such a planet, but filtering for the AC modulation of that light will make the artificial light much easier to detect. Hum-SETI cares little about the spectrum and all about the modulation.

The following is a simple block diagram of my proposal to detect the hum of extraterrestrial artificial light, which I call Hum-SETI to differentiate it from other forms of SETI:

The above is an oversimplified diagram, to convey the overall Hum-SETI concept. In actuality the star's light will most likely overshadow that of the artificial light from the planet, making detection from earth impossible. A technique called Nulling Interferometry and/or Coronagraphy will most likely need to be employed to eliminate the starlight, and enhance any artificial light radiated from planets surrounding it.

Nulling Interferometry employs two telescopes, such as the Large Binocular Telescope located in southeastern Arizona. The starlight from one telescope is delayed by 180-degrees and combined with the non-delayed light from the second telescope. This effectively eliminates the starlight, while reinforcing the out of phase planetary light that is coming in at a slight angle to the starlight. This scheme should greatly reduce much of the unwanted noise, but of course, will limit the viewing time to about a third of that planet's year.

Obviously as the planet's orbit takes it through the back 180-dgrees of its orbit, the hemisphere facing us will be in daylight and not have their lights on. As the planet passes in front of its star the hemisphere facing us will be in the dark, so will have its lights aglow, but its angle to our telescopes will coincide with the star's angle, thus the planets light will be nulled-out via the nulling interferometry. Only about twenty or thirty percent of its orbit will the planet be far enough from its star to not suffer from our nulling interferometry, but still have some of its night-side exposed to us.

Assuming planets, fit to support sentient beings, will have an orbital period somewhere near ours, plus double or so, or minus half our year or so, we will need to sample each star of interest every few weeks, or months, to insure we catch the planet in the proper part of its orbit, in order to see an appropriate portion of its night-side.

The above are simplified block diagrams, in order to illustrate the Hum-SETI concept. Some R&D will be required. The actual apparatus may include other components such as spectral filters, optical gates, and other optical and electronic hardware, in order to optimize the overall performance of the Hum-SETI system.

Phases & Signal to Noise
The Two Big Challenges

Phase shift, across the face of a target planet, will cause some of the light's A.C. modulation to be at, or near, 180-degrees out of phase, which will negate a percentage of the modulation effects, but an equal and opposite phase shift will reinforce the A.C. modulation, making at least some of the radiated light appear to have an even higher amplitude. Digital signal processors, running the appropriate algorithms, should easily 'see' the several phases of continent-wide alternating current lighting.

The ratio of signal to noise, in a project of this sort, will be extremely low. Without the recent advances in digital signal processing the level of extraterrestrial artificial light entering the Hum-SETI apparatus would be overwhelmed by the background noise and completely undetectable.

Stars, dust, gasses, other interstellar objects, and earth's atmosphere, produce high levels of undesirable optical noise increasing the challenge to 'hear' the relatively weak hum from any possible galactic neighbors. With the advent of modern digital signal processing, extended sampling times, the extremely low modulation rate of the expected signal, and the use of nulling interferometry, I believe these challenges can all be overcome.

With today's highly sophisticated digital signal processing electronics, detecting the optical 'hum' of a nearby civilization, that was near the same level of development as earth, in my opinion, will offer a far better chance of success than do the other forms of SETI.

What the Signal is Likely to Look Like
The incoming signal will be mostly random noise, but if there's a repeating signal, such as a low frequency sine wave, the Digital Signal Processor will over time accumulate small amounts of nonrandom noise. As the nonrandom noise accumulates, it will eventually describe a sine wave, with the frequency, "Hum", of the extraterrestrial's alternating current lighting.

I'm steadfastly in the Carl Sagan camp , as are most other scientists, when it comes to extraterrestrials. The late Carl Sagan believed the universe was probably teaming with intelligent beings, but they have never visited earth in the flesh.

Until some yet unknown law of physics allows us to violate, or sidestep, the speed of light limit, I'll stay with Carl on this matter. But like Dr. Sagan, I so do want to detect an intelligent extraterrestrial species.

Just knowing that we are not alone in the universe will have profound implications for us earthlings. And just maybe we'll learn how an advanced civilization has worked through their social and other challenges, and apply that information for the betterment of all mankind.

What do we do after Hum-SETI receives an unambiguous indication of artificial light emanating from a far-off planet? We watch their television of course!

The Next Step
What do we do after we hear the hum?

It's been postulated that should we ever receive an audio only, or text only, transmission from an extraterrestrial civilization, we'd never be able to translate it to something we could understand.

Until the discovery of the Rosetta Stone in 1799, and its decipherment over twenty years later, we had no idea what the ancient Egyptian hieroglyphs were trying to say. There is no Rosetta Stone in an audio or text transmission.

Once a sentient species develops electricity it follows that they will soon develop radio, followed shortly thereafter by the invention of television. Unless the extraterrestrials are bereft of the sense of hearing, their television, like ours, will include sound. Moving pictures with a running commentary is an instant Rosetta Stone, and would allow us to understand these new neighbors in a very short period of time, but receiving their television signals will be a challenge.

Television signals are far weaker than the light pollution given off by a developed planet, and are much more complicated and wider in bandwidth than simple low frequency alternating current. Receiving and decoding ET-TV will require a very large antenna, sensitive receiver, and much more processing power than that needed to hear the hum of their lights. One of the few radio telescopes capable of possibly receiving ET-TV is the 300 meter Arecibo dish in Puerto Rico, or the Five hundred meter Aperture Spherical Telescope (FAST) located in China, assuming the signal impinges on those hemispheres of earth, and is within their tracking ranges.

A bit like the physical laws dictate the frequency of an extraterrestrial's alternating current lighting, similarly their analog television signals will most likely be broadcast in a range of about 50MHz to around 500MHz. This range is below that which nearly all of today's radio telescopes operate. Other than Arecibo, most radio telescopes operate above 500MHz. Arecibo is capable of operating as low as 50MHz, but may be in the wrong position on earth, or too low in gain to properly receive ET's relatively weak television signals.

The proposed, but not yet constructed, Square Kilometer Array (SKA) may have the gain and ability to receive ET-TV, but like so many other radio telescopes it, as planed will, by necessity, ignore the frequencies below 400MHz. This is an economic consideration, as lower frequencies require larger dish diameters, and far higher costs to construct, than do smaller dishes optimized for the reception of higher frequencies.

An array of large dishes, from two to three hundred-meters each, could be easily excavated into the earth of a high and dry area, such as the Atacama desert in Chile, or even in the ice of the Antarctic continent. The Arecibo dish is 167 feet deep from rim to center. A similar dish, excavated in flat ground, or ice, would require an excavation less than 90 feet below grade at the center. The excavated material would be pilled around the rim to a height of little more than 80 feet to create a dish with similar dimensions to Arecibo. In that 50 to a few hundred MHz, the frequencies we will be interested in, does not require a reflective surface as uniform as do >1GHz dishes, simple screen material could be used to follow the excavation's contour. Sharing simple low cost commercial radio towers for receiver support; these dishes could be constructed fast and at a relatively low cost. Additional dishes would be excavated as needed, until sufficient signal levels were achieved.

Television requires a relatively wide bandwidth, between 3 to 5MHz for 20 to 60 or so frames per second. Unlike current SETI receivers, which sample very narrow frequencies, the receivers for this venture would be specifically designed for high gain, low noise, and wide bandwidth.

An analog TV signal is relatively simple to detect and decode. The horizontal and vertical framing pluses define each frame. By necessity ET's modulation scheme will be something similar to ours but may differ in polarity and other factors. A first pass analysis of the 50 to 500MHz band will scan for the framing pluses only. Detecting the framing pluses will locate the exact transmission frequencies. The receivers would then be optimized for those particular frequencies. The frame content would be analyzed. An algorithm would then be developed to decode and display the video and sound.

Will ET Go Digital?

Like us, in fifty to one hundred years of analog TV transmission an extra terrestrial civilization will likely have the technology, and the will, to switch to a higher resolution digital TV system. If no analog TV is detected in the lower 50 to 500MHz band our dishes and receivers would be modified to scan up through a few GHz for ET's digital signals. Digital TV differs greatly from analog TV but will still have some relatively easily recognizable attributes. Decoding the digital signal to produce viewable television will be a much larger challenge than the simpler analog signal, but with enough computing power it is well within our abilities.

We Want Our ET-TV
Assuming we are eventually successful at receiving ET-TV - the several channels of extraterrestrial television, that we'd most likely tune into, would then be put on the Internet for all to view and learn from.


If you find any errors in the facts or logic of this web page, or simply differ in philosophy, or opinion, I am interested in your feedback. Email JR.

If you personally know someone at NASA, at one of the SETI organizations, or a member of congress, please pass this web site on to them. We need our ET-TV!


Permission granted to use text, and graphics,
on this site, as long as attribution and a link to this site is included.

Translations welcome. If you have a translation of this site please send a link and it will be listed here.

If you reference Hum-SETI in any blog, forum, web site, or article, an email would be appreciated. J.R. Whipple

Power & Phases

Electrical power is commercially generated and distributed as three phases, each phase being 120-degrees from the other. Three is the optimum number of phases for the most efficient distribution of alternating current, so would most likely be chosen by any advanced sentient species as they went through their electricity development cycle.

Separate branches of street and commercial lighting are powered by only one phase of the three, so in earth's western hemisphere a large area of artificial illumination will exhibit flicker twice the rate of all three phases, or hum at 360 cycles per second, and 300 cycles in the countries utilizing 50 cycle A.C.

At the upper practical limit of alternating current, about 100 cycles per second, is of course a relatively long time period of ten milliseconds. Elevation variations, as we have across our planet, will cause but a few microseconds of phase shift to the aggregate radiated light, as will Doppler shift, due to planetary spin and orbital velocities - So these effects can safely be ignored. Phase shift across an entire continent, or hemisphere, will cause noticeable phase smearing, but should be easily corrected in the digital signal processor. (See the center-panel block diagram.)

In that 100 cycles per second is most likely the high end of most extraterrestrial's alternating current, and 30 cycles would likely be the low end, the sampling period would be relatively long, insuring the weakest of signal could be differentiated from the high background noise.

An Overview

Digital Signal Processing is a relatively new technology which is rapidly evolving. DSP is the representation of a signal by a sequence of numbers. The numbers can then be manipulated or changed by a computing process to change or extract information from the original signal. Often this includes the extraction of wanted signal from unwanted noise. Bandpass shaping is another possible change that could be made.

DSP can even originate or create a signal from numbers in sort of a reverse process. One advantage of this is that there is no requirement for tuning as the signal is now just a sequence of numbers in the computer. This makes DSP a very stable and flexible way of dealing with electronic signals, and especially shines at resolving very weak signals from very strong background noise.

An input signal is first passed through a low pass filter and then digitized with an analog to digital converter. This is called sampling. Discreet samples are taken of the input analog signal and represented at that moment by a digital value. The higher the sampling rate verses the frequency of interest the better we can reconstruct the original signal, or pull it out of the noise.

Processing of the digital signal from of the A/D converter's output often consists of addition, multiplication, and delay. Addition and multiplication are very familiar terms and computers are very efficient at handling those operations. Delay refers to the ability of the processor to cause phase shifts, or comparisons of different parts of the signal and causing a change to take place in the output signal such as eliminating background noise, and other undesirable attributes of the original signal.

The DSP process can and often does involve complicated higher order mathematics such as Discrete Fourier Transforms (DFT). This is a mathematical technique to determine the content of a signal mathematically. Other mathematical methods include the Inverse DFT (IDFT), the Fast Fourier Transform (FFT) and the Z-transform.

All of these mathematical tools are employed to manipulate a digital signal in special ways to produce the desired result. For example we may want to eliminate any specific impulse signals that may happen to come along, for example noise, phase shift, signal smear, and more.

With the proper digital signal processor, and appropriate programming, the weakest of signal can be pulled up out of the strongest noise.

Much of this overview from: 101science.com

Time & Space

Some relatively nearby sentient beings may have started their evolution to a technological society a hundred or a thousand years ahead of us - But if they're a hundred or a thousand light years away from earth, we may be just in time to hear them humming today.

Maybe most life starts and follows an evolutionary path similar to ours, but some planets never had an event that killed off their dinosaurs. Are technological beings limited to mammals, or mammal-like creatures with opposable thumbs? If so, some planets, much older than earth, my still have only dumb lizards roaming about. After all, our big lizards had over 150 million years to grow a sentient brain, and apparently failed to do so, while we, in a scant million years or so, grew brains and thumbs.

Time and space are spread out in front of us, displaying all things past. Have some planets hummed in the past, but have outgrown the need?

Is it possible that we're alone in the universe? Sure it's possible, but highly unlikely.

If we don't look
We'll never know!