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How Far Have Our Radio Signals Traveled From Earth?

How far have radio signals traveled in space?


How Far Have Our Radio Signals Traveled From Earth?

The expanding sphere of radio signals traveling outward from the earth is often portrayed in TV shows and Hollywood movies to be like a time machine. The further you get from earth, the further back you go in the history of radio and TV broadcasts. Today we explore how far these signals have traveled, and more importantly, if radio signals actually work that way.

Earth’s Expanding Radio Bubble

how far radio signals have traveledAs depicted in the beginning of the movie ‘Contact’, the earth has an expanding ‘bubble’ of man-made radio signals expanding outward at the speed of light. The first of these early radio transmissions were short range experiments that used simple clicks and interrupts to show transmission of information in the 1890s. In 1900, Reginald Fessenden made the first — though incredibly weak — voice transmission over the airwaves. The next year saw a step up in power as Guglielmo Marconi made the first ever transatlantic radio broadcast.

This means that at 110 light-years away from earth — the edge of a radio ‘sphere’ which contains many star systems — our very first radio broadcasts are beginning to arrive. At 74 light-years away, television signals are being introduced. Star systems at a distance of 50 light-years are now entering the ‘Twilight Zone’.

Will Any Extraterrestrial Life Within That Radio Sphere Detect Us?

While it’s interesting to imagine how far our radio signals have traveled into space, it’s extremely unlikely that an alien civilization will be able to catch the latest episode of ‘I Love Lucy’. This is thanks to the inverse square law. In Layman’s term, it’s a form of signal degradation.

As radio signals leave earth, they propagate out in a wave form. Just like dropping a stone in a lake, the waves diffuse or “spread out” over distance thanks to the exponentially larger area they must encompass. The area can be calculated by multiplying length times width which is why we measure it in square units – square centimeters, square miles, etc. This means that the further away from the source, the more square units of area a signal has to ‘illuminate’.inverse square law

Another way to think of it, is that the strength of a radio signal will be only 1/4 as great once you are twice the distance from the source. At ten times the distance, the strength of the signal would only be one hundredth as great.

Because of this inverse square law, all of our terrestrial radio signals become indistinguishable from background noise at around a few light-years from earth. For a civilization only a couple hundred light-years away, trying to listen to our broadcasts would be like trying to detect the small ripple from a pebble dropped in the pacific ocean off the coast of California – from Japan.

So Why Does SETI Bother To Listen To Radio Signals In Space?


Our Radio Bubble In The Milky Way

While no alien civilization is likely to pick up our television or radio broadcasts unless they’re within a few light-years, radio signals can be focused and amplified. Most of our broadcasts were not intended for detection in space. Radio signals can be aimed, focused and amplified to mitigate signal degradation for interstellar communication. These signals would also eventually degrade but are able to travel much, much further before degradation occurs. Hundreds of light-years or more depending on how much power is used.

It’s now becoming possible to detect the atmospheric composition of extrasolar planets. This breakthrough has allowed researchers to narrow down our hunt for earth-like worlds. It’s quite possible that an advanced alien culture can also do this, and detected an abundance of water in our atmosphere. If they have, they may have sent a focused radio message in our direction. If we’re not listening though, we may just miss it.

Radiowave propagation, edited by M.Hall and L.Barclay, page 2, published by Peter Peregrinus Ltd., (1989)
Ismail Bullialdus, Astronomia Philolaica … (Paris, France: Piget, 1645), page 23
Dictionary of Electronics By Rudolf F. Graf (1974). Page 467.
Demetrius T Paris and F. Kenneth Hurd, Basic Electromagnetic Theory, McGraw Hill, New York



  1. illeglalien

    February 25, 2012 at 2:39 am

    If an alien ship was to hit our airspace they would probably shoot it down.
    Yet, we expect to go to other planets and not be attacked.

    • Jared

      February 3, 2013 at 3:25 pm

      You would have to estimate that an alien race with technology sufficient enough to visit us would be incredibly more advanced. I would be incredibly impressed if we could shoot it down.

      A) they are visiting peacefully to preserve and protect us like we preserve endangered species (not likely) or B) they are vising to exterminate and harvest our natural resources (VERY likely!)

      • Fred Fnord

        February 5, 2013 at 6:27 pm

        …to ‘harvest our natural resources’?

        Good god. Why would they want to do that? There are dozens of other bodies in the solar system that would provide much more ready access to any resource that they could possibly want, without the difficulties of Earth’s large gravity well or its damaging and dangerous (to space ships) atmosphere. Let alone their own solar system, or any of the probably hundreds of other uninhabited ones that are closer to their home than ours.

        Seems like sheer paranoia to me.

        • Ben

          February 28, 2013 at 5:00 pm

          You’re right. Seems much more likely they’d just eradicate us in order to prevent us from ever becoming a threat (considering our history of… you know – being threatening.)

          • Anonymous

            March 7, 2013 at 5:07 pm

            I agree with what the previous poster said. If other civilizations had that kind of technology where they could travel to any corner of the galaxy they would have all the resources they need. If they were to come to earth they would most likely send an ambassador of some sort, because they would be just as curious as we are about them. You watch way too many movies.

        • John Austin

          July 1, 2013 at 5:15 pm

          Well, if you think about it, life is responsible for many resources, like oil and some complex minerals, that would not be available on planets devoid of life. So it’s not to paranoid to theorize that there might be a civilization sufficiently advanced to get to earth efficiently but is still reliant on some resources that life provides.

          • Jacqueline L

            August 14, 2013 at 5:20 am

            Right. An advanced life-form transverses light years of space and yet you theorize they would be reliant on oil or any other minute quantities of natural resources we possess? Perhaps you might want to rethink that one.

          • Garrett

            November 6, 2017 at 11:41 am

            Jacqueline is spot on. What need would an advanced extra terrestrial species POSSIBLY have with oil? Heck, we’re even moving towards better utilization of solar energy & pure electric vehicles, and away from oil usage. If a species has the power to travel across the universe, they sure as heck aren’t going to dredge up old decayed bones in the form of oil.

            ET’s needing oil. That’s hilarious.

  2. Ted

    January 16, 2013 at 10:50 pm

    Maths like the inverse square law was never a strong point of mine. So how far in light years would Marconi’s 25,000 watts of power travel (when he sent radio telegraphy signals across the Atlantic), before they would drop below background noise?

    • Overmars

      April 15, 2014 at 9:50 pm

      It depends on the frequency used. The common frequencies in use today in the civilian sector are VHF and above- which degrade very rapidly after a certain distance. All frequencies do not posses equal capabilities. Our atmosphere is the greatest obstruction to radio communications worldwide, and the sun is the greatest amplifier. If ULF (ultra-low frequency), of ELF (extremely-low frequency) were used, great interplanetary distances could be reached easily. The only problem is that the antenna for the latter one would have to be about 3000 miles long since a wavelength is over 3000 miles long. NASA also claims what the latter frequency is non-ionizing, meaning almost no signal degradation since the molecules wont react… The bottom line is that given the complexity of our universe anything and everything is possible, there are infinite numbers in existence and if you can imagine or think of something it probably exists somewhere out there.

      • Clayton Moore

        August 9, 2016 at 5:07 pm

        You could use a tuner and trick the radio into thinking the antenna is long enough. Amateur radio operators do it all the time, it allows for crazy frequency and massive transmission boosts.

  3. SGTRandyB

    February 18, 2013 at 1:37 pm

    IF a species had the technology to travel from their home planet, to another like ours; I feel like it’s safe to assume a few things:

    1. They would be able to harvest bodies that had no intelligent life, we represent real nuclear opposition, and a streamlined mining operation on such a scale would likely just avoid such problems. There is no telling what even a simi advanced species might have hidden up their sleave when there are an infinite number of mineral bodies in space with nothing to oppose a species from taking them.

    2. The energy required to traverse space on such a scale has to be greater than the cost for that same species to simply breed or even lab engineer workers from just a few samples of a species. So enslaving manking is also unlikely, despite the great SciFi stories based on such alien goals.

    3. Seeking a new habitable planet, this feels more likely than the previous two but even minior viral or bacterial differences could spell disaster. It might actually be simpler to become a permanently space fairing species or to revive a dead planet on the cusp like Mars, since the vast majority of microbial life would be dead or dormant, giving a species time to identify and take measures against the local blend. Not to mention they could implant their own fauna blend far easier, there would be no competing local plants.

    4. Technology could drive a species to be hostile, but a species traveling space from habited planet to habited planet likely wouldn’t be interested in multiple prolonged wars to gain small bits from civilizations less advanced, some still being dangerous. They would likely rather trade and move on.

    I’m not really qualified to say anything of weight of this subject, don’t get me wrong, I’m just some guy. But I can only envision one senario where it’s “more likely” an alien species would be hostile towards us over not…..

    One where we attacked first.

    • JB

      May 31, 2013 at 7:10 pm

      “The energy required to traverse space on such a scale has to be greater than the cost for that same species to simply breed or even lab engineer workers from just a few samples of a species.”

      This is actually not true at all. Remember that space is a vacuum, so once you escape your own gravity wells, you don’t require propulsion to keep traveling in the direction you set out. AND you can use other gravity wells to increase your speed without wasting much in the way of propulsion. Deep space travel doesn’t require all that much in propulsive energy. Now the energy to support life, etc, would be far more…

      “It might actually be simpler to become a permanently space fairing species or to revive a dead planet on the cusp like Mars” Except that “reviving” mars might be impossible. The core of the planet is dead. Which means no volcanic activity, no magnetic activity….so you would have to find a way of sustaining the atmosphere from sheering off into space (like it already has) once you replace it.

      • emailforpublicuseonly

        November 6, 2015 at 9:56 pm

        From our perspective Mars may not be a viable option to be “revived”, but I would imagine a more advanced species than ours could be very much capable of melting the core of a ” dead” planet in a Goldilocks zone, engage the hot core and make it spin thus creating a magnetic field which in itself would start to shield the planet from deadly solar winds causing life to spring up quickly. Being an advanced species they can change the length of the day, manipulate a moon to help regulate/create seasons, etc.

        If this process takes 100 years that isn’t a very long time from their perspective. An advanced race no doubt already has nano-medical technology that enables them to wipe out every disease they have and any disease that arises. They’ve also been able to program nano-medical bots to repair dead or dying cells and replicate healthy cells effectively allowing them to live forever. They would amass knowledge throughout their thousands, millions or billions of years of living.

        From their perspective we are nothing remarkable or unworthy of even bothering to make contact with. Rats, elephants, crows and dolphins are highly intelligent and share our world, and yet we look down on these creatures as lower life forms and we allow them to be destroyed. High civilizations would not be the slightest bit interested in us.

  4. Todd

    March 11, 2013 at 3:56 pm

    I think that the prospect of an (other) species being cohesive, while having the ability to defy time and space as we humans know it, is a scary but beautiful thought.

    In the spirit of the subject, I would imagine that with the power to travel light years or even sustain life in space for a longer time than imagined, suggests a power that our mind can not comprehend.

    I imagine that having such a technology would impair a man’s judgement. And thinking like an alien, I would prefer not to be conflicted with ignorance of a top down species like humans can be. I would have already calculated the possibility to be peaceful and united. I would have already evaluated your worth to the solar system, and I would have already anticipated your abilities to create flying objects to defend, and conquer.

    But if I were a alien, I would not know your intentions because as being advanced “so to speak”, I would not underestimate the abilities of a species, one which could destroy me. By numbers alone, I’ve already estimated that beings of your kind can’t take the thought of my existence.

    We aliens have cameras too. I think you all look like pooh, Aliens take poops too,

    Mr. Alien

  5. Chris

    March 12, 2013 at 1:32 pm

    I think we tend to think of interstellar travelers as being representatives of their people and I think this is probably a mistake. We’ve seen a great deal in our own history traders (usually come later once a land has been identified), explorers (perhaps most likely) or raiders (who pose the greatest risk to us).

    Explorers – usually financed by governments due to the great expense of traveling those distances. Initially present as peaceful, but generally devolve into conflicts over trade or desire for more lands and expansion of power. While I found what was said about slavery above well-thought out, I do think one needs to keep in mind that if a species decided to take up residence on our planet (perhaps because habitable planets are so rare and making them habitable so difficult) that the most effective means of converting the planet to their purpose would be using species already there. They may not look at any differently than we do the use of horses, dogs, and other species in the advancement of our civilization. Also, this would likely be preceded by an event that would wipe out a large portion of the civilization so as to neutralize any hidden threat by the people and to leave us disorganized… a frontal assault makes little sense when even something as simple as a well placed asteroid or virus could cause enough chaos for society to breakdown and leave us disorganized and easy to corral. Still, I think there is also the opportunity that much the way we try to respect (relatively speaking) indigenous tribes around our globe due to pressure from larger nations like us we may find we are allowed to exist and run our own course if there is a society of planets and mixed civilizations.

    Raiders – far more disconcerting would be raiders who would not attack on behalf of a nation, but individuals or individual tribes who branch out stealing what they can. They would likely be merciless. Imagine pirates long ago, Vikings, there are Indian tribes who would plunder and attack other tribes across North America, barbarians … while this is usually preceded by explorer discovery, it is not always the case. If you were to believe in abductions, which I don’t, wouldn’t it make sense that there would be ‘collectors’ if you will of other species who would love to have a specimens showcase them like we do other life on this planet. Just pieces of our society may be valuable, resources we have already mined and require no further work on there part other than forcibly take, nuclear weapons, there are items that we possess that would be attractive to a rogue ship of raiders who made their livings the way similar people have throughout our history.

    Whether they initially come in peace or not, the existence of other lifeforms more advanced than us would be troubling and we would certainly face a grave risk the way lesser developed civilizations have across our globe, particularly as Western Civilization has expanded.

  6. Ally

    April 6, 2013 at 1:03 pm

    If scientists working on SETI projects have picked up many radio signals from space, why are they not claiming to have discovered extra-terrestrial life?

    • Mark K

      January 11, 2014 at 5:39 pm

      Because radio signals are not just created by intelligent life. Stars and black holes can create radio signals. Even Jupiter’s magnetic field creates a radio source we can pick up on AM radio. There would have to be coherence behind the signal, or data embedded in it if its analogue.

    • Clayton Moore

      August 9, 2016 at 5:13 pm

      Because radio signals are hard to understand. Most of what they pick up are either interference from our planet (I went to the GBT, a radio telescope, and they wont even let you use gas powered cars because they require spark plugs) or radio waves from other planets, i.e. the sun. They mostly use them to find the composition of distant nebulas. Currently, they do not take on searches for extra terrestrial life unless it is a private researcher who is willing to shell out some coin.

  7. Adam Anderson

    June 2, 2013 at 7:46 pm

    I’m no expert. I’m only 15. But as explained earlier there is actually trillions of so called ‘radio signals’ literally all around the universe, but this is normal and not signs of ET. It is called something ALONG the lines of either CBR or CMBR, both mean Cosmic Background Radiation. Basically static and weird sounds left behind after the Big Bang I think.

  8. Gordon Cooper

    August 26, 2013 at 11:42 pm

    Misleading timeline. The earliest transmissions didn’t make it to the moon, let alone the nearest star. Look at the power and frequencies involved. Radar technologies, anywhere near the frequencies needed to span interstellar space only came along in WWII. The more powerful of the radars like the 780 KW “space fence” “chirp” systems are only a few decades old. Throw in beam angle and one comes up with a less than in a blue moon chance of getting a statistically significant set of signals.

  9. Truthspew

    August 31, 2013 at 11:10 am

    You have to remember – radio as it exists is only possible because we’ve figured out how to select just the signal and ignore the noise. This wasn’t feasible with amplitude modulation, but with frequency modulation you get higher selectivity and sensitivity on the receiver.

    All goes back to the invention of the MASER aka Microwave Amplification by Stimulated Emission of Radiation. And the MASER begat the LASER.

    So who’s to say an alien culture hasn’t figure out a good quantum method of suppressing noise and has nice sensitive receivers?

    And back to the LASER thing – even NASA is now playing with light based communication.

  10. Baruch Atta

    September 3, 2013 at 11:05 pm

    I imagine that any species more advanced than us would not be using radio to communicate interstellar. We have had radio for about 100 years, and we are close to the next generation. What would be the next generation, you ask? I imagine that quantum physics will provide the answer, either entangled electrons (“spooky action at a distance”) where the connection is instantaneous, or some other type of hyper-dimensional transmissions. These may be 50 to 100 years away.
    So, in the 100,000 years that MAN has been a species, and the 100,000 years that he might remain (hopefully) we have a span of 200 years, or 1/1000 of the time of existence, where RADIO is considered useful for interstellar communication. Let’s put it this way – it would be like a message in a bottle or smoke signals to us, today. Really totally useless, relative to the possibilities.

  11. God's Carbon Based Probe

    September 21, 2013 at 5:20 pm

    You are all such fools. We’ve given you plenty of time. We have been here all along studying you and your vile behavior. Resources are scarce and we can’t risk you escaping Earth, only to kill us one day, so we will just preemptively erase you. We’re vegetarians, so don’t worry about us eating you. You’ve had centuries, but as your technology improves, so does the body count. It seems like it will end up at a point where you try to eradicate us, so we’re just gonna beat you to the punch.

    Anonymous Alien Guy

  12. Nandhu

    September 30, 2013 at 8:02 am

    Ok, first of all why are you people over thinking about aliens and all. Even if aliens do exist and if they are way smarter than us, they would have also made some radio waves and would have started to send it. If that happened, we could have watched alien TV shows. This means that aliens do not exist or they could be so far away that even our signals haven’t gone that far or THEY ARE NOT SMARTER THAN US. This means that we have to simply stop believing in alien attacks. Aliens might exist because there are trillions of stars and life can be possible on another planet. They just might not be an advanced species.

    Everything in this world made for a purpose. Lions were made to maintain the population of herbivores animals. We may have been made for destruction as it is happening now, or we may be the one to expand our ecosystem. If we don’t expand ourselves to other planets or something like that, humans will be endangered. I am sure that humans will soon find a way to artificially make everything – like food and everything and expand ourselves in the universe.

    But I stil belive that evertyhing that is created will have an end so I guess we may be the reson for the destruction for our own planet.If there was no humans.This planet would have survived until our sun gets destroyed or any meteriods strikes the earth.

    • Mark K

      January 11, 2014 at 5:41 pm

      If that happened, we could have watched alien TV shows.

      Or perhaps they don’t use radio signals to send their signals. Maybe they use light, lasers or something we haven’t discovered yet.

      • The watcher

        March 21, 2014 at 3:24 pm

        Consider this; We live around a 3rd generation star. Earlier generations did not yet possess the elements that make up much of our planet (stellar debris is the waste product of a stars life-cycle) Out solar system is about 4.5 Billion years old, the universe is 12 Billion – any sufficiently advanced planet based life form would have to have evolved at about the same time as we did. There have also been some very fortuitous random events that allowed life to exit on this ball of rock. We could very well be alone despite the Universe hold un-number trillions of stars & planets.
        Go forth & multiply. The entire Universe may be a gift for mankind. I believe that even if we manage to kill 99.99% of all life on this planet, enough of us will survive.

  13. Star Trek Reality

    May 25, 2014 at 6:10 am

    One thought that I have had for quite awhile is that WE on Earth are the first civilization in the universe. And, in a billion years, (or less), our planet Earth will populate other planets, and one day the galaxy will have planets that contain Humans (our descendents). So, when people wonder why in Star Trek, most life is humanoid, I think that’s because it really will be like that in some way. You, me, and most of us in America will be the ancestors of a new race of Star Trek Style Romulans, Cardasians etc. Thoughts anyone?

    • Garrett

      November 6, 2017 at 11:49 am

      If we’re the first… that’s a terrifying thought. Equally as terrifying as the thought that we are not the first.

      And Star Trek is based off humanoid thoughts. Thus it will have a humanoid bias.

  14. BVincent

    November 10, 2014 at 1:32 pm

    The odds of two species achieving similar levels of technology at around a similar time in which information could be exchanged, or even detected between two separate planets seems to me to be incredibly small.

    Most likely, our degraded radio waves are washing over either dead civilizations that rose and fell a millennia ago, or another planets version of ‘prehistoric’ life. Add in the likelihood of extinction events, especially once averaged over the life of the universe, and you can probably safely assume that we are effectively alone in our neighborhood of the galaxy at this point in time. At least for technologically advanced life, which is how we measure intelligence.

  15. David

    September 3, 2017 at 3:26 am

    Whales and dolphins are intelligent species and inhabit planet Earth, yet humans kill them nearly to extinction for food and/or by neglect (pollution, etc.). Why would we expect a species traveling from an exoplanet and landing on Earth to treat the tasty humans (and the few remaining whales and dolphins) any differently?

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