# Why haven't we found intelligent life on other planets?

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If there are supposedly 56 billion "habitable" planets in the Milky Way galaxy (our galaxy) alone then why haven't we found any signs of intelligent alien life yet?

Is intelligent life really so rare that we haven't found any, even though there may billions of habitable planets in our galaxy alone?

What is the reason for this?

Edited by seriously disabled
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It's not that surprising that we haven't found intelligent life on any of the habitable planets in th Milky Way, mostly because we haven't been able to check any of the habitable planets in the Milky Way for signs of intelligent life.

The fact that they're all kind of far away has been something of an obstacle in that regard.

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"Space is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is. I mean you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space." - Douglas Adams

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"Space is big. Really big. You just won’t believe how vastly hugely mindbogglingly big it is. I mean you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space." - Douglas Adams

Also, human generated EM waves (Radio) only reach out about 100 light years this far; that's in a circle around us... then it's got to come back.

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Also, human generated EM waves (Radio) only reach out about 100 light years this far; that's in a circle around us... then it's got to come back.

Along with this https://en.wikipedia.org/wiki/Inverse-square_law

Edited by dimreepr
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With only ourselves as an example of a species intelligent enough to leave its own planet of origin, we must assume others will face certain technological hurdles the way we have. The discovery of what uranium can do may well be a watershed moment for every intelligent species. How it handles the sudden exponential increase in destructive power would be critical in whether or not it survives to explore near systems.

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Don't forget the dolphins.

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Imagine that some alien life is searching for "intelligent" life in our solar system. In the 4.6 billion years of existence, it has managed to send detectable signals in a total of about 50 years. The same would probably apply to other solar systems, so why does anybody expect to detect anything?

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I wonder what the maximum distance would be where reaches a point where only one photon would be detectable at the other end?

Edited by StringJunky
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I wonder what the maximum distance would be where reaches a point where only one photon would be detectable at the other end?

One photon per what span of time?

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I wonder what the maximum distance would be where reaches a point where only one photon would be detectable at the other end?

One photon will be detectable at any distance, the question is, at what point will a message fail to be discernible?

Edited by dimreepr
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Is intelligent life really so rare that we haven't found any, even though there may billions of habitable planets in our galaxy alone?

It may truly be that rare. As far as we know the earth only managed to form life one time. An errant boulder rolling down a hill at just the right time, and the earth may have been as lifeless as any of the other planets in our solar system seem to be.

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It may truly be that rare. As far as we know the earth only managed to form life one time. An errant boulder rolling down a hill at just the right time, and the earth may have been as lifeless as any of the other planets in our solar system seem to be.

If we manage to find another example of life in our solar system, then this answer is moot; why else do we look so hard?

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One photon will be detectable at any distance, the question is, at what point will a message fail to be discernible?

Right. It's a how many photons per second (to satisfy Nyquist) per unit area (for signal/noise). If you have a signal modulated at a kHz, you need to be sampling at at least 2000 points per second to make sense of it. Minimum of one photon per point, but higher to discern the signal amongst the noise

The paradox involves them traveling to visit us, so isn't really on point here.

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The paradox involves them traveling to visit us, so isn't really on point here.

I stand corrected.

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One photon will be detectable at any distance, the question is, at what point will a message fail to be discernible?

Yes, that's a better way to put it

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If we manage to find another example of life in our solar system, then this answer is moot; why else do we look so hard?

So if things are different then they won't be the same? I'm not sure what point you are trying to make.

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If we find life, as we know it, on, for example, Titan, then we can exclude rarity as an explanation for not finding intelligent life.

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It may truly be that rare. As far as we know the earth only managed to form life one time. An errant boulder rolling down a hill at just the right time, and the earth may have been as lifeless as any of the other planets in our solar system seem to be.

Well, that ignores the fact that life arising once may preclude, or at least render much less likely, any additional origin points for further life to have a unique origin event.

Once life forms, it may quickly incorporate all or most available raw materials into its own development as well as altering the environment that gave rise to it in the first place.

There's also the question of whether there was only ever a single origin of life, or whether the other "strains" of life were simply out-competed in the relatively early days such that only the descendants of a single origin point still remain (as far as we know).

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Well, that ignores the fact that life arising once may preclude, or at least render much less likely, any additional origin points for further life to have a unique origin event.

Yes, that comment was not intended to be an analysis of all the possible scenarios under which life could arise.

Once life forms, it may quickly incorporate all or most available raw materials into its own development...

Although in the case of earth that doesn't seem to be the case as there appear to by plenty of raw materials available. I cannot think of any substances in life forms that cannot come from readily available raw materials.

There's also the question of whether there was only ever a single origin of life, or whether the other "strains" of life were simply out-competed in the relatively early days such that only the descendants of a single origin point still remain (as far as we know).

Agreed. That is why I specified "as far as we know".
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There is also the problem of interstellar gas and dust absorbing the signal leakage from our planet. It's doubtful that a civilization around the nearest star could hear our leakage. We would have to beam a powerful signal right at them to be detected...

Edited by Moontanman
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We would have to beam a powerful signal right at them to be detected...

It would also need to be constrained with the energy currently available.

Edit. I guess "right at them" implies constraint.

Edited by dimreepr
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I wonder what the maximum distance would be where reaches a point where only one photon would be detectable at the other end?

Well, that's very easy to calculate.

$P{( r )}=\frac{P_0}{4\pi r^2}$

$P=\frac{E}{t}$

$E=\frac{hc}{\lambda}$

Remember that P0 is initial quantity of photons multiplied by their average energy, divided by time 1s.

After dividing by area, unit is W/m^2

Solve r for E=h*c/532nm=2.32 eV (average visible photon energy)

$r=\sqrt{\frac{P_0 \lambda * 1s}{4\pi h*c}}$

$r=\sqrt{\frac{P_0 * 532*10^{-9}m * 1s}{4\pi 4.135667*10^{-15} J*s * 299792458 \frac{m}{s}}}$

Detector with area 1m^2 should detect 1 photon with average energy 532 nm every 1 second at this distance from ideal light point source.

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