Fermi Paradox?

[ydoaPs]

As most of you know, the Fermi paradox is the tension between the predicted abundance of life in the universe and that none of it has contacted us.

 

Space is big. It's really big. It's no surprise that they haven't visited us in person, but what about radio? We have radios. Surely some of them have radios. Yet we listen (to a small frequency band in an amazingly small section of sky) and hear no one.

 

I wonder, even if they are sending signals, why should we expect to hear them?

 

Let's assume, for a moment, that we're not near a sun. We'll neglect all radiation other than our signal.

 

[math]{P_r}={G_t}{G_r}(\frac{\lambda}{4{\pi}R})^2{P_t}[/math]

 

With ideal antennae,

 

[math]{P_r}=(\frac{\lambda}{4{\pi}R})^2{P_t}[/math]

 

So, we see that the power drops off with the square of the distance. Note that, as pointed out above, space is BIG. For instance, it takes like about 4 and a quarter years to get to the nearest star. So, let's say we're talking to them. And, since received power is proportional to the square of the signal wavelength, let's use the lowest frequency radio signal that we currently use (ELF is 3HZ-30HZ, so we'll go with 100,000km for our wavelength).

 

According to wikipedia, the maximum power of a radio antenna in the US is 100,000watts.

 

So, plugging in our numbers, if someone at the nearest star is listening to us and there is no interference, the highest legal power radio signal sent here is only 4.3x10^-15W when it gets there.

 

And that's for a singular coherent signal. Then there's the other signals piled on making it look like white noise. All of which will be masked by the sun's radio emissions as well.

 

Why should we expect them to hear us? Why should we expect to hear them even if we know they're transmitting?

[dimreepr]

That’s exactly the point I was trying to make, here +1 (post #4), although it’s an order of magnitude superior in explanation.

Edited June 23, 2014 by dimreepr

[MonDie]

And that's for a singular coherent signal. Then there's the other signals piled on making it look like white noise. All of which will be masked by the sun's radio emissions as well.

 

Why does their signal have to be radio waves? Wouldn't it be easier to distinguish the source of (and isolate) a high frequency signal?

 

High frequency waves require more energy, thus the aliens might turn the signal on and off intermittently to save energy. Wouldn't their recipients find an intermittent signal extra conspicuous anyway?

Edited June 23, 2014 by MonDie

[ydoaPs]

Why does their signal have to be radio waves?

A fortiori argument. ELF gives the best case for successful transmission on long distances. If you look at the formula, you see that the received power is proportional to the square of the wavelength. This means the higher the frequency, the lower the power on the receiving end. So, the low frequency/long wavelength is to prove the point even more.

 

Higher frequencies have more loss.

[MonDie]

If you look at the formula

 

I didn't know what those constants stood for.

[ydoaPs]

I didn't know what those constants stood for.

Generally, when talking about waves, lambda is the wavelength. Though, you raise a good point; I should have explained what the symbols are for. The Ps are the power transmitted and received. The Gs are the antenna gains of the transmitter and receiver. Lambda is the wavelength, and R is the distance from the source.

[MonDie]

I'll have to look up the concepts.

 

 

 

A fortiori argument. ELF gives the best case for successful transmission on long distances. If you look at the formula, you see that the received power is proportional to the square of the wavelength. This means the higher the frequency, the lower the power on the receiving end. So, the low frequency/long wavelength is to prove the point even more.

Higher frequencies have more loss.

Have you considered the raltionship between wavelength and resolution? A finely resolved signal will have less interference.
Flourescent microscopes, which use UV to flouresce visible light from the dyes, somehow get finer resolution through the use of UV. Electron wavelengths as small as 0.01nm make electron microscopes the finest.

 

They could reflect a signal rather than emit it, making wavelength-related energy costs a non-issue.

[ydoaPs]

I'll have to look up the concepts.

 

 

 

 

 

Have you considered the raltionship between wavelength and resolution? A finely resolved signal will have less interference.

Flourescent microscopes, which use UV to flouresce visible light from the dyes, somehow get finer resolution through the use of UV. Electron wavelengths as small as 0.01nm make electron microscopes the finest.

 

They could reflect a signal rather than emit it, making wavelength-related energy costs a non-issue.

Directional signals still have losses proportional to the square of distance. AND they have the added detriment to visibility of being directional. They're less likely to be seen because they're 'talking' in fewer directions. Remember, space is BIG; there's lots of room to miss.

[MonDie]

What unit of distance is used for R?

 

I had no idea we were talking about antennae. I just took a crash course on them.

http://www.antenna-theory.com/basics/radPattern.html

http://www.antenna-theory.com/basics/directivity.php

http://www.antenna-theory.com/basics/gain.php

 

How do you know that a signal of 4.3x10^-15W wouldn't be detected? Isn't this a question of antenna precision, and a question of standard deviation in the other coinciding sources of radiation or any noise produced by the instrument itself?

 

Anyway, my coutnerargument might not hold water. I found a equation relating angular resolution to wavelength in the context of microscpy. https://en.wikipedia.org/wiki/Angular_resolution#Explanation

The drop in signal strength is proportional to the drop in wavelength squared.

It looks like minimum resolvable distance is proportional to wavelength. Perhaps this could rephrased as an "area of interference", using minimum resolvable distance as the radius, but the usefulness of this conversion would depend on what the signal needs to be resolved from: the star that this inhabited planet orbits, some background radiation, what?

Edited June 24, 2014 by MonDie

[Mordred]

As most of you know, the Fermi paradox is the tension between the predicted abundance of life in the universe and that none of it has contacted us.

 

Space is big. It's really big. It's no surprise that they haven't visited us in person, but what about radio? We have radios. Surely some of them have radios. Yet we listen (to a small frequency band in an amazingly small section of sky) and hear no one.

 

I wonder, even if they are sending signals, why should we expect to hear them?

 

Let's assume, for a moment, that we're not near a sun. We'll neglect all radiation other than our signal.

 

[math]{P_r}={G_t}{G_r}(\frac{\lambda}{4{\pi}R})^2{P_t}[/math]

 

With ideal antennae,

 

[math]{P_r}=(\frac{\lambda}{4{\pi}R})^2{P_t}[/math]

 

So, we see that the power drops off with the square of the distance. Note that, as pointed out above, space is BIG. For instance, it takes like about 4 and a quarter years to get to the nearest star. So, let's say we're talking to them. And, since received power is proportional to the square of the signal wavelength, let's use the lowest frequency radio signal that we currently use (ELF is 3HZ-30HZ, so we'll go with 100,000km for our wavelength).

 

According to wikipedia, the maximum power of a radio antenna in the US is 100,000watts.

 

So, plugging in our numbers, if someone at the nearest star is listening to us and there is no interference, the highest legal power radio signal sent here is only 4.3x10^-15W when it gets there.

 

And that's for a singular coherent signal. Then there's the other signals piled on making it look like white noise. All of which will be masked by the sun's radio emissions as well.

 

Why should we expect them to hear us? Why should we expect to hear them even if we know they're transmitting?

 

 

I've often wondered why we would expect to hear radio waves, considering I used to work for a communication company. You've nailed the problem. Glad to see someone else correlate these details. You've already mentioned the signals piled on each other. To expand on that how many different radio waves on Earth would use the same frequency? Any frequency that is viable in radio waves is used a multitude number times, regulations restrict multiple frequencies in the same regions however we also plant what is called privacy tones onto a primary frequency to allow the same primary frequency to be used more often. Some privacy tones are analog or digital

Edited June 24, 2014 by Mordred

[MonDie]

Apparently aperture synthesis (use of multiple telescopes) is used to get good resolution from radio telescopes.

 

Apparently radio telescopes do get electromagnetic interference, which is why they're often placed in valleys. I would imagine we have at least one radio telecope in space, but I wouldn't imagine it has the advantage of aperture synthesis.

 

If I were the aliens, I would make the signal by reflecting sunlight with a satellite mirror. It would reflect a wide array of wavelengths and avoid the energy costs of generating EM waves.

Edited June 24, 2014 by MonDie

[Airbrush]

A possible answer to the Fermi paradox is that shortly after a civilization achieves the ability to trasmit radio signals, they realize that they have nothing to gain by announcing their position in the galaxy to a predatory and more technologically advanced ETI. Therefore intelligent civilizations learn to mask their radio and TV broadcasts so broadcasts cannot be detected from other stars. They may even learn to mask their atmosphere so it will not give away signs of life, or even to appear like a hellish environment.

 

We better hope that predatory-and-more-advanced ETs never finds us.

Edited June 25, 2014 by Airbrush

[PrinceOfDragons]

A possible answer to the Fermi paradox is that shortly after a civilization achieves the ability to trasmit radio signals, they realize that they have nothing to gain by announcing their position in the galaxy to a predatory and more technologically advanced ETI. Therefore intelligent civilizations learn to mask their radio and TV broadcasts so broadcasts cannot be detected from other stars. They may even learn to mask their atmosphere so it will not give away signs of life, or even to appear like a hellish environment.

 

We better hope that predatory-and-more-advanced ETs never finds us.

 

That is true or like us humans we switch from analog to digital and only broadcast that certain type of radio waves for a short period of time and the more advance civilizations have already pass this up and we just don’t know how to detect their type of transmissions.

Edited June 29, 2014 by PrinceOfDragons

[ydoaPs]

Therefore intelligent civilizations learn to mask their radio and TV broadcasts so broadcasts cannot be detected from other stars.

Per the reasoning in the OP, they wouldn't have to.

[Spyman]

This is what the famous SETI guy Seth Shostak said about our signal strength, back in 2003:

 

Radio was invented in the 19th century, and large-scale broadcasting began in the 1920s. Alas, these early broadcasts were of low power, and at low frequency. The difficulty with low frequency transmissions, such as AM radio, is that they are refracted by Earths ionosphere, and have difficulty making it into space. However, beginning in the 1950s, we started to construct high-power, high frequency transmitters for radar, for FM radio, and for television. These signals leaked off the planet, and headed for the stars.

 

A modern TV transmitter can put out as much as a megawatt of power. Its not very tightly focused, so even though much of the broadcast energy spills into space, its fairly weak by the time it reaches another star system. Consider one of our early TV programs just washing over a planet thats 50 light-years away. To detect the "carrier" signal from this broadcast in a few minutes time would require about 3,000 acres of rooftop antennas connected to a sensitive receiver. Thats a lot of antennas, and an unsightly concept. But its not hard to build, and the aliens could conceivably do it. If the extraterrestrials were unwise enough to actually want to see the program, then theyd need an antenna about 30,000 times greater in area (roughly the size of Colorado). Ambitious, but possible.

 

A rather easier task would be to detect our military radars. The bigger ones typically boast a megawatt of power, and are focused into beams that are a degree or two across. There are enough such radars that, at any given time, they cover a percent of the sky or so. The signal from the most powerful of these could be found at 50 light-years distance in a few minutes time with a receiving antenna 1,000 feet in diameter. Indeed, these military radars are the only signals routinely transmitted from Earth that are intense enough to be detectable at interstellar distances with setups equivalent to our own SETI experiments.

 

Bottom line? With radio technology slightly more advanced than our own, Homo sapiens is detectable out to a distance of roughly 50 light-years. Within that distance are about 5,000 stars, all of which have had the enviable pleasure of receiving terrestrial television. And each day, a fresh stellar system is exposed to signals from Earth.

http://archive.seti.org/news/features/can-aliens-find-us.php

[Airbrush]

"....Bottom line? With radio technology slightly more advanced than our own, Homo sapiens is detectable out to a distance of roughly 50 light-years. Within that distance are about 5,000 stars, all of which have had the enviable pleasure of receiving terrestrial television. And each day, a fresh stellar system is exposed to signals from Earth.

 

"But even if you believe in highly optimistic estimates regarding the prevalence of cosmic intelligence, its unlikely that another civilization exists within 50 light-years. Thats too small a distance. Were no doubt listed in some alien grad students data tables as a world with life, but without the footnote indicating intelligent life. We are the new kids on the block, and so far its a safe bet that none of the other kids know were here."

 

Then why is SETI even bothering to look?

 

Could there be a way for ETs to detect our technology from techno-waste gasses in our atmosphere?

 

My pet peeve about astonomers is when they say that there can be so many Earth-like planets in our galaxy and other galaxies. Who cares about life in other galaxies? Who cares about life beyond a thousand light years? That seems irrelevant for a long time. All that really matters is how many Earth-like plants there are within about a hundred light years.

Edited July 1, 2014 by Airbrush

[MonDie]

This is what the famous SETI guy Seth Shostak said about our signal strength, back in 2003:

 

Although the OP considers whether we would recieve their signal, your response is relevant. If the ETs could pick up our signals, they could have responded to them. We can take this even further. If there's not limit on telescope resolution, then a sufficiently advanced civilization might have witnessed the beginnings of agriculture on Earth, and it might have sent a sufficiently strong signal to Earth.

[Spyman]

Then why is SETI even bothering to look?

 

Could there be a way for ETs to detect our technology from techno-waste gasses in our atmosphere?

 

My pet peeve about astonomers is when they say that there can be so many Earth-like planets in our galaxy and other galaxies. Who cares about life in other galaxies? Who cares about life beyond a thousand light years? That seems irrelevant for a long time. All that really matters is how many Earth-like plants there are within about a hundred light years.

AFAIK SETI is looking because even if our signals haven't reached far yet, IF there exists another civilization then they could be much older and more advanced than us, thus their signals could be stronger and from much further far away.

 

I care about life in other galaxies, while I agree that it would be much much more exciting to find sentient life close enough for us to communicate, an very very old but clearly intelligent signal from Andromeda would at least tell us that we are not alone.

 

Also Earth like planets are what we currently want to colonise but aliens may not find them equally attractive.

 

 

Although the OP considers whether we would recieve their signal, your response is relevant. If the ETs could pick up our signals, they could have responded to them. We can take this even further. If there's not limit on telescope resolution, then a sufficiently advanced civilization might have witnessed the beginnings of agriculture on Earth, and it might have sent a sufficiently strong signal to Earth.

My point is that if we can be detectable, then certainly we can detect a very powerful signal from a more advanced civilization.

 

In 1974 we sent a very powerful message to the stars as an demonstration of human technological achievement, the Arecibo Message, that was strong enough to be detectable by an advanced civilization anywhere in our galaxy. However by purpose we sent it towards the edge of Milky Way, at the globular star cluster Messier 13, which will not be there when the signal arrives 25 000 years from now.

 

The broadcast was particularly powerful because it used Arecibo's megawatt transmitter attached to its 305 meter antenna. The latter concentrates the transmitter energy by beaming it into a very small patch of sky. The emission was equivalent to a 20 trillion watt omnidirectional broadcast, and would be detectable by a SETI experiment just about anywhere in the galaxy, assuming a receiving antenna similar in size to Arecibo's.

http://www.seti.org/seti-institute/project/details/arecibo-message

 

As per the OP they are not likely to find our regular radio and TV signals, as we will not likely find their domestic signals. But an advanced civilization might want to broadcast to others that they exists and therefore builds a powerful beacon that we and others can detect. Someone has to go first, maybe we are alone or first, but IF there are others then they can be older and already transmitting.

 

Thus we don't have to wait for them to find us first, we are more likely to find an old signal addressed to anyone who can listen.

Edited July 2, 2014 by Spyman