How far outward can we rule out intelligent life?

[phinds]

I stumbled across this article, which comes very close to answering my original question. If there were humans on a planet just 4LY away, we likely would not know it and they wouldn't know about us either.

Good article. I think the points are well made. Thanks for posting.

 

[russ_watters]

The article is OK, but asks a limited question, specifically regarding the scenario in Contact. Military radars are much more powerful than TV broadcasts and would be visible and identifiable as being unnatural in origin from much further away. I said "omnidirectional" in my first post, but the best examples are probably phased array radars, which send ridiculously powerful but narrow beams -- something like a megawatt at a degree beamwidth. "Omnidirectional" in a way that is much more powerful than normal continuous omnidirectional.

The article also doesn't actually answer its own question:

For example, the most distant human-made object is https://briankoberlein.com/2014/09/15/pale-blue-dot/, which has a transmission power of about 23 Watts, and is still detectable by radio telescopes 125 AU away. Proxima Centauri, the closest star to the Sun, is about 2,200 times more distant. Since the strength of a light signal decreases with distance following the https://briankoberlein.com/2013/12/22/distant-star/, one would need a transmission power of more than 110 million Watts to transmit a signal to Proxima Centauri with the strength of Voyager to Earth. Current TV broadcasts (at least in the States) is limited to around 5 million Watts for UHF stations, and many stations aren’t nearly that powerful.

One might argue that an advanced alien civilization would surely have more advanced detectors than we currently have, so a weaker signal isn’t a huge problem.

Well ok -- but how much further away could we still detect Voyager's signal? I'm not sure what the answer is, but this suggests the Voyagers are nowhere close to far enough away for us to no longer communicate with them:

Distance isn't such a big problem. Technological advances since the 1977 launch have made our antenna arrays incredibly powerful, Hodder says. For example, the http://deepspace.jpl.nasa.gov/dsn/ —a series of three antenna arrays strategically placed in rural locations around the world—can send and receive messages to and from the areas well outside our solar system.

http://www.popularmechanics.com/spa...longer-will-we-talk-to-the-voyagers-11479518/

 

[russ_watters]

So I found a link and a calc I'd done previously on the subject:
According to this link: http://archive.seti.org/epo/news/features/arecibo-diaries-location.php

If an extraterrestrial broadcast breathes as much as 0.00000000000000000001 watts onto Arecibos 18-acre reflector, we could detect it.

That's a sensitivity of 1.0x10^-25 watts/sq m. So for a 1 MW, omnidirectional transmission, that's a distance of 94 light years, if I did my math correctly...

 

[AgentSmith]

Let's assume there is a planet out there with life equally intelligent as humans that is putting in the exact same amount of effort to detect alien life that we are. Their science and technology also developed in line with human technology. Given that we're looking and they're looking, what is the closest this planet could be to us given that we don't know about them (yet)? Any guesses?

That would depend on the kind of signals they might be putting out, and how strong those signals are. You can play around with signal strength, and our detection limits, and come up with answers rather easily. You have have two situations: a spherically broadcast signal, and a laser-like signal. If I were Director of Planetary Signals, I'd opt for a laser pointed at different stars on some kind of rotating basis. (Haven't heard back on my job application).

 

[Alltimegreat1]

Great discussion here. Have we Earthlings actually been sending these high-powered laser signals directly at nearby stars on a regular basis, such that one would assume any humans there with our technology would have detected us and responded by now?

 

[rootone]

So far we haven't detected an exoplanet which shows a strong potential for being habitable, they mostly have been 'Hot Jupiters'.
There are a small number of discoveries which are more Earth-like, but we don't really know much other than they are orbiting their star at a safe distance and might be rocky rather than gas giants.
This is mainly because our present technologies for studying exoplanets are at a very early stage.
Doubtless this will improve, but until it does improve any directed signalling whold have do be done more or less randomly.
That obviously reduces a lot the chances of being successful.

 

[Jenab2]

This is reminiscent of the Fermi paradox - where are they - If intellligent life is rampant, why have we not already detected their radio signals? The most likely answers are 1] radio communication is rare, 2] intelligent beings are rare. If we choose to dismiss option 2, then we must come up with a reason for the lack of radio detection. Contenders include 1] the universe is too vast for radio signals to be detectable beyond short distances; 2] we are the crown jewel of intelligent beings within radio detection distances; 3] we are too moronic to recognize an intelligent signal; 4] we are the only ones dumb enough to paint a radio bullseye on ourselves in a universe teeming with exploiters and predators. Our ability to generate radio signals detectable across many light years allows us to rule out option 1. Option 2 can be eliminated on anthrocentric grounds. That leaves my personal favorites, options 3 and 4, as the most reasonable explanations

Choice 2 might be correct. Anthrocentrism doesn't mean 'false'; it only means go carefully. Even a biased mind is right once in a while.

 

[newjerseyrunner]

I think the most likely solution to the Fermi Paradox is the "they're made of meat" argument. We're too primitive to be considered civilized life. The universe is probably littered with animals that almost made it, but didn't. If our obsession with medicine is anything to go by, our self preservation instinct extends to our technology, and since that's a driving force of evolution of greater beings, I would expect species to embrace some sort of digital immortality not soon after it's invented. What could a being that's hundreds of thousands of years old have to talk about with an ape that can barely understand it's place in the universe.

An elite American special forces team could spy on an ancient Roman battalion without much effort, I see no reason that an alien species couldn't observe us from afar without us knowing.

 

[DavidLloydJones]

Let's assume there is a planet out there with life equally intelligent as humans that is putting in the exact same amount of effort to detect alien life that we are. Their science and technology also developed in line with human technology. Given that we're looking and they're looking, what is the closest this planet could be to us given that we don't know about them (yet)? Any guesses?

Seems to me they have to be at least thirty light-years away. Any civilization at our level of ability and confusion will spend at least thirty years debating a loud reply once they find out about us. If they'd been on Mars, their thirty years would have been up some time ago, so they're not there at least.

-dlj.

 

[Bernie G]

What could a being that's hundreds of thousands of years old have to talk about with an ape that can barely understand it's place in the universe.

Love.

 

[Alltimegreat1]

Seems to me they have to be at least thirty light-years away.
-dlj.

Thanks for specifically answering my question.

 

[dougfgd]

We are receiving coherent radio signals regularly with modulation that are 4 milliseconds long inconveniently they that turn up at random intervals.
The signals have been traveling for millions of years and we don't know if they contain useful information. Will make a good basis for science fiction story.
My story uses the information to decode frozen entanglement for instantaneous hyperspace communication.

 

[ThomasLLS]

We may or may not be the most intelligent beings in the universe. The point of creating this thread was to determine how far outward from the earth/sun scientists would feel safe about saying there is no human-equivalent intelligent life. I was hoping to get some more specific answers, even if merely based on intuition. The observable universe has a diameter of some 90 billion light years.

What if there were humans just like us living on a planet 100 light years away from Earth (with the same telescopes and radios and satellites) who were also interested in finding alien life? Would we have detected them by now?

Wait.. 90 billion light years? Isn't it estimated that the big bang happened around 14 billion years ago? Therefore we should suppose that 90 billion light years away from us there should be nothing at all, at least for now.
I think that we cannot be the only intelligent beings in the entire universe, there should be another civilization out there and if they are more intelligent than us they most likely know about our existence.
Anyway, this beliefs that there are intelligent beings like us somewhere out there, in the known space, have no proof whatsoever (I believe them though).
But the human race always wants to look out there, and imagine we are not alone.

 

[phinds]

Wait.. 90 billion light years? Isn't it estimated that the big bang happened around 14 billion years ago?

This is a common misconception until you understand the basics of cosmology. The universe is expanding. The objects that are 47 billion light years from us now were MUCH closer when they emitted the light that is getting to us now and they have move very much farther away during that time.

Therefore we should suppose that 90 billion light years away from us there should be nothing at all, at least for now.

This shows an even more fundamental misunderstanding. The observable universe is not expanding into anything. The universe is possibly infinite, possible just many many orders of magnitude larger than the observable universe. Also, for what you say to be true, we would have to be at the center of the total universe which is staggeringly unlikely. Google the Cosmological Principle.

 

[Alltimegreat1]

How can we tell that the most distant objects we can observe are 45 billion LY away when the light we can view from them now was emitted from less than 13.7 billion LY away?

 

[ThomasLLS]

This is a common misconception until you understand the basics of cosmology. The universe is expanding. The objects that are 47 billion light years from us now were MUCH closer when they emitted the light that is getting to us now and they have move very much farther away during that time.

This shows an even more fundamental misunderstanding. The observable universe is not expanding into anything. The universe is possibly infinite, possible just many many orders of magnitude larger than the observable universe. Also, for what you say to be true, we would have to be at the center of the total universe which is staggeringly unlikely. Google the Cosmological Principle.

Understood, so we should consider the observable universe much much smaller than the actual universe, and the observable universe measures 94 billion light years aproximately (considering it a straight line).

 

[phinds]

Understood, so we should consider the observable universe much much smaller than the actual universe

yes, that's what I said.

and the observable universe measures 94 billion light years aproximately (considering it a straight line).

yes, 94 billion light years diameter. Your "straight line" would seem to suggest you are thinking of a radius, as in "straight line from here".

 

[rootone]

How can we tell that the most distant objects we can observe are 45 billion LY away when the light we can view from them now was emitted from less than 13.7 billion LY away?

Because the light of these distant objects is red shifted due to the expansion of space.
From the amount of red shift it is possible to determine how long ago the light was emitted, and from that we can deduce where they would be now if they still exist.

 

[ThomasLLS]

Yes, I expressed myself badly, I meant the diameter.

 

[Alltimegreat1]

Alright, but how far away were these most distant observable objects today from the Earth when they emitted the light we see today?

 

[Jenab2]

I think the most likely solution to the Fermi Paradox is the "they're made of meat" argument. We're too primitive to be considered civilized life. The universe is probably littered with animals that almost made it, but didn't. If our obsession with medicine is anything to go by, our self preservation instinct extends to our technology, and since that's a driving force of evolution of greater beings, I would expect species to embrace some sort of digital immortality not soon after it's invented. What could a being that's hundreds of thousands of years old have to talk about with an ape that can barely understand it's place in the universe.

An elite American special forces team could spy on an ancient Roman battalion without much effort, I see no reason that an alien species couldn't observe us from afar without us knowing.

I hold the opinion that our kind of life is ineluctably biological. If we create computer-mechanical life, we do not transition ourselves to it. Rather, we facilitate our own replacement by it. Our devices were originally conceived as our tools, what we use to extend our perceptions and our powers while we, ourselves, remain biological organisms. We are, by nature, more than our wants and our wills, and although evolution will sooner or later change us, we will remain us only so long as we remain biological.

There is yet room for improvement in the human kind by biological means. The major impediments are ideological, not theoretical or practical. It seems that every time someone says 'eugenics,' someone else says 'Holocaust,' and the discussion rapidly attenuates, dissolves, or mutates. The betterment of unknown billions of people yet to be born is judged to be of less importance than the present condition or the civil rights of a few millions who live now. Those future people don't get to vote on whether they shall be born strong or weak, constitutionally sound or prone to illness, with senses acute or impaired, or of great intelligence or retarded. And so what can be done, and what should be done, is never done.

 

[newjerseyrunner]

Love.

Chimps love each other, think you could have a stimulating conversation with one?

Alright, but how far away were these most distant observable objects today from the Earth when they emitted the light we see today?

45 billion light years, that's the definition of the Hubble Radius.

 

[Rubidium_71]

Humans ruling things out isn't necessarily the final word on a subject, even on our own planet. Just ask a coelacanth. (or see https://en.wikipedia.org/wiki/Living_fossil)
As far as detection goes, how do we know other civilizations would use radio waves to communicate? Especially given how impractical the radio method is when applied to interstellar distances. It could be that there is another, currently unknown method that other life forms use to communicate between the stars and we simply haven't discovered it yet.
It's also possible that some of the subsurface oceans here in the Solar System might contain some fairly intelligent creatures as well. We'll never know until we actually check.

 

[AgentSmith]

Love.

They would not know we were "apes" until communication had commenced, assuming they had not picked up old broadcasts of Mr. Ed and My Mother the Car.

 

[newjerseyrunner]

Humans ruling things out isn't necessarily the final word on a subject, even on our own planet. Just ask a coelacanth. (or see https://en.wikipedia.org/wiki/Living_fossil)
As far as detection goes, how do we know other civilizations would use radio waves to communicate? Especially given how impractical the radio method is when applied to interstellar distances. It could be that there is another, currently unknown method that other life forms use to communicate between the stars and we simply haven't discovered it yet.
It's also possible that some of the subsurface oceans here in the Solar System might contain some fairly intelligent creatures as well. We'll never know until we actually check.

I think the fact that we can't see anyone else using radio waves is evidence towards that. Say the Alcubierre drive idea actually can work, it'd actually be faster and more secure to send a ship with a message than to beam it across the universe with radio waves.

While it's remotely possible that some sort of intelligence lives under subsurface oceans, we can use laws of physics to determine that it's astronomically unlikely. Complex life requires a great deal of energy density and tidal forces alone couldn't generate that.

They would not know we were "apes" until communication had commenced, assuming they had not picked up old broadcasts of Mr. Ed and My Mother the Car.

No, they wouldn't know we were apes until observation had commenced. That may have started thousands of years ago for all we know. If I were immortal and wandering the universe, I'd drop probes at any planet that even had the potential to evolve technological beings. That's the nice thing about immortality, eons are irrelevant. If a curious being or species stumbled across our little planet millions of years ago, they may have been observing us that entire time.

 

[Ralph Dratman]

This is reminiscent of the Fermi paradox - where are they - If intellligent life is rampant, why have we not already detected their radio signals? The most likely answers are 1] radio communication is rare, 2] intelligent beings are rare. If we choose to dismiss option 2, then we must come up with a reason for the lack of radio detection. Contenders include 1] the universe is too vast for radio signals to be detectable beyond short distances; 2] we are the crown jewel of intelligent beings within radio detection distances; 3] we are too moronic to recognize an intelligent signal; 4] we are the only ones dumb enough to paint a radio bullseye on ourselves in a universe teeming with exploiters and predators. Our ability to generate radio signals detectable across many light years allows us to rule out option 1. Option 2 can be eliminated on anthrocentric grounds. That leaves my personal favorites, options 3 and 4, as the most reasonable explanations

Or, successful technological civilizations use radio only for a few decades to a few centuries, because there is a better method of communication we do not know about yet.

 

[Ralph Dratman]

[QUOTE="Would an aquatic species use radio?[/QUOTE]
An aquatic species with tool-making and tool-using capabilities comparable to ours might use radio. The gaseous atmosphere on the surface of our planet would seem like a kind of low-level outer space to such creatures, but their capabilities certainly might grow to allow them to roam pretty freely on land and in the air. Hint to this species: dry off before you start building electronics.

 

[Chronos]

I agree an evolving technological civilization may only utilize radio for a brief time, but, they would surely realize exo civiliations would likely utilize this technology at some point during their development - encouraging them to listen for, and perhaps even broadcast radio signals. The devlopment of electrical technology in an undersea environment would obviously progress in a different way and rate than it did for human civilization, but, it would surely occur for an advanced civilization.

 

[write4u]

I recently watched a very interesting program, which approached the problem of biochemical molecules from a purely chemical viewpoint. The link is to a formal lecture by Robert Hazen, at the Carnegie Institute for Science.

Robert Hazen

• Senior Staff Scientist at Carnegie Institution for Science
• Washington D.C. Metro Area

• This is a recorded video of the lecture itself, but I found it on YouTube. In the past, the fact that something is shown on YouTube automatically disqualified the link as "unreliable". But after receiving several notifications inviting me to return to the Physics Forum, I'll give it one more shot. It might give some new insight into the almost certain occurrence of biochemical molecules on other planets.

Before ruling this link out, I request that a moderator with expertise in chemistry review the clip before rejecting this very interesting lecture, which precisely addresses the OP question.
(if you want to skip the lengthy introduction start @ 25:00)

 

[Promytius]

I'd add that the "Universe" is infinite, as there cannot be a 'container' with nothing beyond it; our local 90 billion chunk is filled, teeming with life and always will be; add the infinite number of other intelligent planets, and contact is always possible. There may be life very close to us, but on the galactic plane and therefore very obscure. Also out tiny little chunk of under 100 solar years of transmissions is a pathetically insignificant "noise" to detect. Perhaps not where, or how close, but rather when, and how recent.
Finally, so far, what we've broadcast may not be of any interest, even to nearby neighbors, especially if they are significantly 'different' than we are. It is perfectly plausible (except to hubristic homo sapiens) that a civilization has no interest in such a 'young' and primitive planet such as ours; maybe we just don't rise to the level of interest.
So, my answer is: how far outward can we rule out intelligent life? We cannot, all the way to forever, they all are theoretically reachable. Stop thinking in only 3 dimensions for an answer.