How Far Has Our Earthly Influence Spread Through Space?

[Mountain]

I have been reading different articles on the voyager space probe with fascination, I can remember when it was first launched at 8 years old. The idea that craft reached interstellar space in that time period is a little exciting. I was thinking about the information contained within it were it to ever encounter intelligent life and just how far it could potentially get within my life time.

In reading the latest article that includes a radio telescope picture of the probes 22 watt signal, I realized that our radio transmissions are traveling at light speed and have already reached great distances. I was kind of curious as to just how far our influence may have spread at this point.

Marconi developed and used his first commercial radio in 1895, I don't know how powerful that signal was, but signal is still traveling out there at light speed. One hundred and eighteen years have gone by since that radio, that would mean a radio signal extending in a sphere 236 light years in diameter. How many planets and stars are within that sphere of influence I wonder.

Just thought that was kind of a cool thought on http://www.physicsforums.com

[moonguy]

Yes, it is a cool thought. Though the signal would have been blocked by the mass of the Earth from going anywhere but straight up or to the 'four winds' as it were. It would only propagate towards whatever half the sky was directly in view. Still, I would not be too surprised if aliens arrive speaking italian. . .

[CraigD]

One hundred and eighteen years have gone by since that radio, that would mean a radio signal extending in a sphere 236 light years in diameter. How many planets and stars are within that sphere of influence I wonder.

The Gliese Catalog is a good reference for nearby stars, cataloging those within 81.5 ly of Earth. It lists 3803 stars. Because it’s a catalog of stars that have actually been seen with telescopes, and we know that many of the nearest stars (with 5 ly) would not have been seen were they further away, this number is expected to be about 4 times too small. From all this, astronomers estimate that there is about 1 stars per 280 ly³. A 236 ly diameter sphere has a volume of about 6,900,000 ly³, and is enclosed with room to spare in our arm of the galaxy, which is about 1,000 ly thick, us being within 100 ly of its center plane. So there should be about 24,000 stars within range of Marconi’s first radio transmissions, ignoring moonguy’s point about much of the sky being blocked. Marconi’s first radios were in southern England, at about 51 N latitude, which means about 37% of the sky wasn’t visible, so accounting for this, the signal should have reached about 15,000 stars.

 

Recent astronomy tells us that planets are the rule rather than the exception for stars. A common rule of thumb is 5 planets per star, so Marconi’s early transmission should have reached about 75,000 planets.

 

If the question is not “how many stars and planets?” but “how many places where something who might be able to recognize an artificial radio signal are there?”, this number might be much too low. Because we live on one, we tend to assume that planets orbiting bright stars are the only likely homes for radio astronomers. Planets around brown dwarfs and other big, warm sub-stellar objects, or even these bodies themselves (the coolest brown dwarf yet observed has a surface temp of only about 25 C), might be good homes, too.

 

Until a few years ago, some theorists thought there might be many – perhaps on the order of 10 times – more such bodies than there are stars. Data from NASA’s WISE infrared space telescopes, though, cast doubt on this hypothesis, though, suggesting that fewer than 1 in 5 stars are brown dwarfs (see this space.com article).

 

Even if brown dwarfs and similar sub-stellar objects aren’t as common as once hypothesized, though, they might still be good homes for radio astronomers.

 

The killer issue for detecting artificial radio signals, be them from Earth or us detecting ones from elsewhere, is how very faint they are, and how noisy stars and other sky objects are in the radio bands. Telling the difference between an analog FM or AM signal (which were pretty much all there we were sending ‘til about 1970) and the many naturally occurring radio sources that sound similar, but are much more powerful, is the stuff of SETI nightmares (or wet dreams, depending on the SETI person). Digital signals, which have checksums and other clear artifacts in them, seem to me to much improve the chances of detection.

 

Still, I would not be too surprised if aliens arrive speaking italian. . .

Or they could arrive singing Beatles songs. :)

[moonguy]

Or Elvis!

[Mountain]

Not to argue or anything, but, the earth does turn. I am pretty certain that Marconi likely sent out many different radio signals with the earth at different positions each time, over time that would send signals out in pretty much all directions.

 

Yes I would agree that Marconi's signal would have been terribly weak, but power ratings continually increased with time. I was blown away with how easily and brightly the Voyager signal was detected, it is only 22 watts. We have signals of megawatts going out for decades now. Our radio detection systems are becoming incredibly advanced at this point, we are able to detect and discriminate pretty minor radio signals at this point.

 

It was just a thought that fascinated me thinking about it. I am fairly certain that at some point we will eventually manage interstellar travel, but at this point we "have" managed interstellar communication. If we ever meet another form of life it is likely that our radio signals will have reached them long, long before we ever could.

[CraigD]

Marconi’s first radios were in southern England, at about 51 N latitude, which means about 37% of the sky wasn’t visible, so accounting for this, the signal should have reached about 15,000 stars.

Not to argue or anything, but, the earth does turn. I am pretty certain that Marconi likely sent out many different radio signals with the earth at different positions each time, over time that would send signals out in pretty much all directions.

The Earth rotates, yes, but it does so around its axis with only a very small and slow precession (wobble). So unless Marconi had sent his 1897 transmissions from a point on the equator, there’d be an area of the sky covering either the North or South celestial pole they didn’t reach.

 

At the North or South pole, only 50% of the sky is visible. At the equator, 100% is.

 

Marconi send his first signals from Wales, about 51 N latitude, so the missed patch of sky included the South pole, and by my calculations, is about 37% of the total sky. This is the same patch of sky that contains the stars and constellations that can’t be seen from that location, such as the Southern Cross, which can’t be seen from any point north of 15 N latitude.

 

I was lazy with my quick calculation, however, ignoring the Earth’s about 23 axial tilt, so assuming Marconi transmitted for an entire year, not just 24 hours, his “celestial latitude” was as small as 28 N, so the invisible part of the sky was only about 23% of the total sky, not 37.

 

Here’s a quick sketch illustrating what I mean:

 

Yes I would agree that Marconi's signal would have been terribly weak, but power ratings continually increased with time. I was blown away with how easily and brightly the Voyager signal was detected, it is only 22 watts. We have signals of megawatts going out for decades now. Our radio detection systems are becoming incredibly advanced at this point, we are able to detect and discriminate pretty minor radio signals at this point.

The problem is not so much one of sensitivity as signal discrimination (also known as “signal to noise”). Consider this analogy. If we were standing 10 m apart in a silent room, you could hear me speak in a normal voice, tell that it was speech, and likely what I was saying. If we were 10 m apart on a dance club floor with the music playing at full volume, you’d not be able to tell I was speaking, only that loud music was playing. In this analogy, my voice is an artificial radio signal, and the music is the “music of the spheres” from the many powerful radio sources. Though less than 1% of most star’s EM output are in radio frequencies, and only a fraction of that in a given narrow band, there are a lot of stars, and a lot of variability in their collective signals.

 

To have a chance of finding an artificial signal in all that noise, the receiver either must have very sophisticated antennae or equivalent arrangements and systems (we use electronic computers for this) to analyze the raw input, or make some assumptions to allow them to concentrate their analysis on some special frequency they reason a sender wanting to be heard would use. Our SETI follows this second strategy, looking for signals at frequencies such as the “waterhole” (1,420,000,000 to 1,666,000,000 hz). Marconi and other early radio makers, however, weren’t choosing their frequencies with the intention of making sense to SETI astronomers on other planets, but on what their hardware could do. Marconi’s early transmissions were believed to be in the vicinity of 1,000 hz, in the “longwave” radio band. Not only is this frequency not one a clever ET SETI astronomer would be looking in, it’s one at which the Earth’s atmosphere is nearly 100% opaque, so would have been barely detectable even from low Earth orbit. “Shortwave” frequencies to which the atmosphere wasn’t transparent didn’t come into common use until the 1920s.

 

So we should likely knock about 30 years off our calculations for how far one of our radio signals has reached, unless we assuming a hypothetical ET radio SETI astronomer has vastly better antennae and signal analysis systems than we do.

 

If our own astronomy is an indication of how ET’s may progress or have progressed, it might not be radio signals that are detected, but subtle spectral signatures in our atmosphere from due to the release of gases unusual from other than artificial sources. ETs only a bit more advanced than us might have very large aperture (and thus very high resolution) optical telescopes. So they might detect us not by our radios, but by our streetlights.

[JMJones0424]

So we should likely knock about 30 years off our calculations for how far one of our radio signals has reached, unless we assuming a hypothetical ET radio SETI astronomer has vastly better antennae and signal analysis systems than we do.

 

I disagree. With a very few exceptions, our clock hasn't even started yet. None of our broadband radio, television, or microwave transmissions would be detectable by an Arecibo size radio telescope beyond our solar system. I've long lost an excellent technical article from the Seti@Home forum that explains why this is, but quick googling led me to an appropriate sci.astro faq that spells out why our typical RF communications are not at all detectable outside of our solar system.

 

http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html

Table 1 Detection ranges of various EM emissions from Earth and the

Pioneer spacecraft assuming a 305 meter diameter circular

aperture receive antenna, similar to the Arecibo radio

telescope. Assuming snr = 25, twp = Br * Tr = 1, <eta>r =

0.5, and dr = 305 meters.

-------------+--------------+-----------+--------+--------+-----------+
Source       | Frequency    | Bandwidth | Tsys   | EIRP   | Detection |
            | Range        |    (Br)   |(Kelvin)|        | Range (R) |
-------------+--------------+-----------+--------+--------+-----------+
AM Radio     | 530-1605 kHz |  10   kHz | 68E6   | 100 KW |  0.007 AU |
-------------+--------------+-----------+--------+--------+-----------+
FM Radio     |  88-108  MHz | 150   kHz |  430   |   5 MW |    5.4 AU |
-------------+--------------+-----------+--------+--------+-----------+
UHF TV       | 470-806  MHz |   6   MHz |  50  ? |   5 MW |    2.5 AU |
Picture      |              |           |        |        |           |
-------------+--------------+-----------+--------+--------+-----------+
UHF TV       | 470-806  MHz |   0.1  Hz |  50  ? |   5 MW |    0.3 LY |
Carrier      |              |           |        |        |           |
-------------+--------------+-----------+--------+--------+-----------+
WSR-88D      |   2.8    GHz |  0.63 MHz |  40    |  32 GW |   0.01 LY |
Weather Radar|              |           |        |        |           |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo      |   2.380  GHz |  0.1   Hz |  40    |  22 TW |    720 LY |
S-Band (CW)  |              |           |        |        |           |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo      |   2.380  GHz |  0.1   Hz |  40    |   1 TW |    150 LY |
S-Band (CW)  |              |           |        |        |           |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo      |   2.380  GHz |  0.1   Hz |  40    |   1 GW |      5 LY |
S-Band (CW)  |              |           |        |        |           |
-------------+--------------+-----------+--------+--------+-----------+
Pioneer 10   |   2.295  GHz |  1.0   Hz |  40    | 1.6 kW |    120 AU |
Carrier      |              |           |        |        |           |
-------------+--------------+-----------+--------+--------+-----------+

It should be apparent then from these results that the detection of AM

radio, FM radio, or TV pictures much beyond the orbit of Pluto will be

extremely difficult even for an Arecibo-like 305 meter diameter radio

telescope! Even a 3000 meter diameter radio telescope could not

detect the "I Love Lucy" TV show (re-runs) at a distance of 0.01

Light-Years!

[JMJones0424]

I'd like to expand on my previous post with a little bit of introduction to my understanding of how RF communication works in respect to SETI.

 

Signal Detection

Unless you are trying to jam transmitters such as remote detonators, it is insufficient to merely measure an EM emission at a given frequency. The universe is a very noisy place in the RF spectrum. While, due to physics, there are certain frequencies more promosing than others, if you are searching for intelligence, you need to find signals that are modulated in a way that is not likely to occur naturally in order to convey information. The most basic way of modulating a signal is to turn a carrier wave on or off, this is known as CW and is the modulation used in Morse code. One step up from that is to modulate the carrier wave to various frequencies, giving frequency shift keying. The advantage being that you can convey more information per unit of time, at the sacrifice of increased bandwidth (range of frequency) necessary to transmit the message. You can also modulate the carrier wave in more complicated ways such as amplitude modulation, phase shift keying, or combinations of amplitude, frequency, and phase shift keying.

 

Bandwidth

The range of the RF spectrum used to convey information is important when searching for an unknown transmission. While simple modulations like morse code could conceivably use a very narrow bandwidth, and therefore focus the radio's emitted energy in a very narrow range, real time voice transmission typically requires at least a bandwidth of 3 kHz. Higher quality voice transmission requires much more bandwidth. Timely transmission of pictures or other data intensive transmissions require even more than that. The problem from a SETI perspective is that the higher the bandwidth, the greater the distortion while the signal is moving through interplanetary and interspace mediums. The more distortion, the more the signal looks like random background noise and is therefore less likely to be detected by an observer that is ignorant of the transmission.

 

Focus

If you are in the path of a laser emitting 1W of energy, you will receive far more flux than if you were receiving an omnidirectional beam from say an incandescent lightbulb of the same wattage at the same distance. The chart I gave above has a column labeled EIRP which stands for equivalent isotropically (all directions the same) radiated power. Basically, this measures what the strength of an omnidirectional transmission would need to be to be received at the same strength as a focused beam. This is why radars and radio telescope transmissions such as those emitted by Arecibo have far higher EIRPs than their power emitted. The downfall of this method of intentionally transmitting information is that the antenna must be precisely focused towards the recipient.

 

Conclusion

For technical reasons, none of our communications would be detectable by us outside of our solar system. However, some of our focused, unmodulated emissions could easily be detected, if the receiver was lucky enough to find themselves in the path of that emission. Unfortunately, as they aren't modulated, it would be nearly impossible to definitively show that the emission wasn't a natural occurring phenomenon. SETI is a search for alien civilizations that are intentionally broadcasting their existence, not a search for the accidental leakage of RF transmissions of a civilization such as ours.

[CraigD]

For technical reasons, none of our communications would be detectable by us outside of our solar system.

Assuming we mean “communication not intended to be received far from our solar system”, I agree

 

However, some of our focused, unmodulated emissions could easily be detected, if the receiver was lucky enough to find themselves in the path of that emission. Unfortunately, as they aren't modulated, it would be nearly impossible to definitively show that the emission wasn't a natural occurring phenomenon.

To the best of my knowledge, most of the powerful focused radio transmissions we’ve made have been for radar mapping of the planets, moons, asteroids, comets, and planetary rings, and were unmodulated, there have been some that were modulated with the intention of making them recognizable as artificial to listener lightyears away. Such efforts are commonly called Message to Extraterrestrial Intelligences (METI) to distinguishing them from SETI, the Search for Extraterrestrial Intelligences via listening with very sensitive radio receivers (telescopes).

 

According to the wikipedia article List of interstellar radio messages, the first such message was in 1962. (I found it interesting that this message, “the Morse Message”, which contained "MIR, LENIN, SSSR" in Morse code. was intended for possible listeners on Venus. In 1961, the idea that some intelligence might receive a radio message was more plausible than it was by the end of the 1960s, when a series of Russian attempted and successful probe landings revealed that Venus had a much greater surface atmospheric temperature and pressure than previously believed.)

 

Perhaps the most famous METI message was the 1679 bit “Arecibo Message” sent in 1974 in the direction of M13, 22,200 ly away. Even if this message was not recognized, recorded or understood, its brief brightness would certainly raise strong suspicions of being artificial to radio astronomers like ours in the 1970s through today, being brighter and briefer in a narrow EM band than any known natural phenomena.

 

To folk impatient for ET contact, a shortcoming of the Arecibo Message it is that it hits no stars until the 300,000 or so about 22,200 to 22,284 ly away in M13, so the only reasonable scenario for ETIs having already detected it involves them having radio observatories scattered between stars. The demise of the brown dwarf hypothesis I mentioned in post #3 makes it much less plausible there are many natural ET homes between stars, so such observatories would require ETs to have tremendous spaceflight and engineering capabilities. If this is the case, Fermi’s famous “where is everybody?” question comes to mind, as it’s unreasonable to assume that ETs that could fly radio telescopes between the stars also fly spacecraft to interesting places, like the planet that sent the Arecibo Message, our truly, Earth.

 

SETI is a search for alien civilizations that are intentionally broadcasting their existence, not a search for the accidental leakage of RF transmissions of a civilization such as ours.

I agree this is how SETI has been done by us Earthfolk since we started about a century ago, and is likely to be how we do it for at least a few decades more. However, I’m not certain it’s the only way SETI can be done – that is, I’m not sure it’s not possible in principle to search for unintended signs of folk someone would like to know about and possibly communicate with. As I hinted at earlier, I suspect such a search might rely mostly on optical, rather than radio, astronomy.

 

At this point, I find my mind getting tangled into a Gordian knot of sociological assumptions and contradictory logical conclusions.

 

If such SETI technology is in principle possible, and not incredibly big (for example, doesn’t require consuming all the resources of many start in the manner of a Kardashev scale type 3 civilization), and civilizations like ours not exceedingly rare, it stands to reason that there are listeners within our about 100 ly radius neighborhood who are doing it. However, if they’re doing it, it’s a stretch for me to imagine any motivation other than curiosity, and a desire to communicate with whoever they find by it – that is, folk like us. If this is the case, it’s a stretch for me to imagine why they wouldn’t have done the much easier METI activity of regularly blasting narrow beam messages like the Aricibo message and the half dozen other “serious” METIs toward every likely intelligence-supporting star in their neighborhood. But if any of our neighbors have been doing this for at least a century, our present-day SETI would have surely and unambiguously detected it, which hasn’t happened.

 

To cut this knot, I’ve only 2 swordstroke-like hypotheses:

• Civilizations as smart and technological as ours are very rare, or very short-lived.
  - SETI isn’t and won’t find any ETIs, because there aren’t any to be found.
  
• Some advanced technological something doesn’t like communication. Sometimes called the “babes in the woods”, or my preferred “evil pan-galactic civilization crushing, Ming the merciless-like superpredator thing out there” hypothesis.
  - Just as some folk’s SETI/METI project is about to succeed, a technologically advanced (perhaps god-like) something swoops in and makes sure they stop
  

These 2 positions demark, I think, two fundamentally opposite schools of thought on issues SETI/METI: those who support doing everything we can to make contact with ETIs, and those who oppose it on the grounds that doing so will bring a superpredator down on us. Nikolai Kardashev is in the first camp, believing it the inherent nature of intelligent being to want to communicate, and be civilized, with one another. Stephen Hawking is vaguely in the second, opining "if aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn't turn out very well for the American Indians."

[Moontanman]

Craig, there is always a third option... We simply haven't discovered the technology other civilizations use to communicate. With the theoretical possibility of FTL becoming more accepted what could a civilization 10,000 years ahead of us be able to do, can we even speculate intelligently on the possibilities?

 

Then there is the possibility of intelligent life and civilizations being quite wide spread but having little or no common ground to communicate on or even a desire to do so...

 

An intelligent being would almost certainly be radically different than us, the Star Trek/Star

Wars idea that many if not all intelligence's would be humanoid is almost certainly bogus, the idea that others would be interested in us in any but a technical fashion is hubris. Does a scientist who studies ants attempt to communicate with them?

[Sparky49]

One factor that is not mentioned are the effects of the rotation of the earth, our orbit around the sun, and the sun's orbit around the milky galaxy on a directional signal (or even on a omnidirectional signal given the blocking of the mass of the earth) . The beam of such a signal would effectively be a lever sweeping across the universe. The further from the earth the lever is, the faster it moves. The faster it moves, the less time an antenna can be capture the data, the less data, the less it is able to recognize that the signal contains information.

 

What would be the effects of just the rotation of the earth be on a (focused) METI at Proxima Centauri? How long would the signal last on a Arecibo sized antenna? How many bits of data would be received during that period?

 

A brief search revealed this interesting document: ""http://www.ieti.org/articles/metiinst.pdf"" But, it to does not discuss the lever effect. It does however, mention the military radars of the USA and Soviet Union which have been transmitting out of the solar system since the 1960s.