Say we find extraterrestrial life, how to prove it?

Originally posted by Shallow Blue
Yeah, but that's the point, isn't it? Those moons aren't actively broadcasting anything. They're dark lumps of rock that merely reflect what's around them, which is more rock. If they were broadcasting radio waves, as opposed to reflecting a broad, generic spectrum, we'd a) have detected them a long time ago and b) know that there was something i ...[text shortened]... here. But they don't, which is why we've only just found them, because there isn't anyone there.

RF detection methods have reached very high standards these days. There is an RF telescope recently built that was nothing but an array of sticks basically, just double dipoles and connected together for gain but with extremely sophisticated software connected to super computers. The gist of that is the examination of signals on the order of one hertz or less with extreme sensitivity.

The first test for that system when the system was only about 1/4 finished was to find the Voyager, now billions of miles from Earth. It found the transmission quite handily. That is something like 5 watts or so with a gain antenna on the voyager but still, that signal is so far buried in the mud it takes supercomputers to suss it out but they did. Those same kind of detectors and computer analysis is the new norm for RF telescopes, like Aricibo
which has ACRES of antenna reflector so it's a new ball game for the RF guys.

Originally posted by Shallow Blue
Yeah, but that's the point, isn't it? Those moons aren't actively broadcasting anything. They're dark lumps of rock that merely reflect what's around them, which is more rock. If they were broadcasting radio waves, as opposed to reflecting a broad, generic spectrum, we'd a) have detected them a long time ago and b) know that there was something i ...[text shortened]... here. But they don't, which is why we've only just found them, because there isn't anyone there.

Note that I have not claimed anything about radio waves with regards to Pluto. I said that we would not have noticed human like lights. If we didn't even spot the moons, then I doubt we would have spotted slightly unusual light levels on the surface of Pluto.

Originally posted by sonhouse
RF detection methods have reached very high standards these days. There is an RF telescope recently built that was nothing but an array of sticks basically, just double dipoles and connected together for gain but with extremely sophisticated software connected to super computers. The gist of that is the examination of signals on the order of one hertz or le ...[text shortened]... es, like Aricibo
which has ACRES of antenna reflector so it's a new ball game for the RF guys.

How does Voyagers transmission compare to the Earth's typical output? Are we transmitting anything with significantly more power into space?

How much fainter would the Voyager signal be if it were 10 light years away? How much more power output would a spacecraft have to transmit from 10 light years away in order to have the same power when the signal reached earth?

Originally posted by googlefudge
Dead easy to spot, without any surface lights.

But we would not have spotted surface lights.

My point is that if we cannot see surface lights on Pluto, we aren't going to see them on a planet around another star.
I am no saying that if humans were living on Pluto we wouldn't know.

Originally posted by twhitehead
But we would not have spotted surface lights.

My point is that if we cannot see surface lights on Pluto, we aren't going to see them on a planet around another star.
I am no saying that if humans were living on Pluto we wouldn't know.

But we aren't looking for surface lights, we are looking for radio signals.
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And when you say 'we wouldn't have spotted surface lights' on Pluto [or it's moons] that really depends
on how many we are talking about.

A city with [say] 2 MW visible light and 10 MW IR [just from the lightbulbs'] would absolutely be visible.
That's before we add in the ~20+GW thermal from the nuclear power plants powering and heating the city.

Solar radiation at the distance of Pluto is ~1.55W/m2 https://en.wikipedia.org/wiki/Sunlight
Pluto's radius 1,184,000m
Cross sectional area 7,439,000m2
Overestimate of maximum reflected light energy from Pluto assuming 100% reflectivity 11.5 MW
more realistically we are talking about ~1~2MW total reflected power.

In other words the thermal output from a single commercial nuclear reactor is 100 to 10,000 times
the brightness of Pluto, let alone the much tinier moons.

Originally posted by googlefudge
But we aren't looking for surface lights, we are looking for radio signals.

I know. I was originally responding to sonhouse's comment with regards to artificial light.

And when you say 'we wouldn't have spotted surface lights' on Pluto [or it's moons] that really depends
on how many we are talking about.
The same as are on Earth perhaps? We might see something in spectra, but we would not have noticed a pattern as can be seen in pictures of Earth at night. Even a spectral anomaly would almost certainly be based on knowledge that it is very cold, whereas planets we are interested in in other solar systems would typically be similar to Earth temperature.

A city with [say] 2 MW visible light and 10 MW IR [just from the lightbulbs'] would absolutely be visible.
Evidence of that? visible in what sense? A detectably large amount of light emitted? Or would you see it in the spectra? And as I have said before, infrared is not what was being discussed. Have we even looked at Pluto via infrared prior to the New Horizons mission?

Solar radiation at the distance of Pluto is ~1.55W/m2 https://en.wikipedia.org/wiki/Sunlight
Pluto's radius 1,184,000m
Cross sectional area 7,439,000m2
Overestimate of maximum reflected light energy from Pluto assuming 100% reflectivity 11.5 MW
more realistically we are talking about ~1~2MW total reflected power.
If I read that correctly, Pluto, despite being highly reflective in parts is still much dimmer than Earth at night. Is that correct?

Originally posted by twhitehead
I know. I was originally responding to sonhouse's comment with regards to artificial light.

[b]And when you say 'we wouldn't have spotted surface lights' on Pluto [or it's moons] that really depends
on how many we are talking about.
The same as are on Earth perhaps? We might see something in spectra, but we would not have noticed a pattern as c ...[text shortened]... pite being highly reflective in parts is still much dimmer than Earth at night. Is that correct?[/b]

https://en.wikipedia.org/wiki/Earth%27s_energy_budget

Earth's internal heat and other small effects

The geothermal heat flux from the Earth's interior is estimated to be 47 terawatts.[7] This comes to 0.087 watt/square metre, which represents only 0.027% of Earth's total energy budget at the surface, which is dominated by 173,000 terawatts of incoming solar radiation.[8]

There are other minor sources of energy that are usually ignored in these calculations: accretion of interplanetary dust and solar wind, light from distant stars, the thermal radiation of space. Although these are now known to be negligibly small, this was not always obvious: Joseph Fourier initially thought radiation from deep space was significant when he discussed the Earth's energy budget in a paper often cited as the first on the greenhouse effect

If we just take the geothermal heat flux and ignore the solar completely.
And then assume that only 1% of that heat flux is radiating from Earth's dark/night side in IR.

Then Earth is emitting 470 MW thermal IR on it's night side. Which is ~1~2 orders of magnitude larger than what I
optimistically calculated for Pluto.

Pluto orbits between ~30 and ~49 AU https://en.wikipedia.org/wiki/Pluto with an albedo of 0.49 to 0.66

This means that Pluto receives between ~1/900 and ~1/2,400 times as much sunlight as the Earth and reflects roughly half of it.

Earth receives [on average] 1367W/m2 [in space] from the Sun.
Which means Pluto receives between ~1.51 and ~0.57 W/m2 solar radiation. [almost all in The Visible wave band]

This means that [using the numbers from last time] Pluto will be reflecting roughly 5.6MW to 2.1MW visible light.
[Assuming no geothermal energy remaining, the remaining energy must be re-radiated away thermally.]
Which means that Pluto must be 'glowing' in long wave with roughly 2.1~5.6MW total. Assuming a 80~20 split between the
energy emitted back towards the sun on the day light side and the energy emitted out into deep space on the night
time side. [There is no atmosphere to act as insulation and a 6.39 Earth day day length] then Pluto should be glowing
in longwave radiation with 1.7~4.5 MW. At a temperature of between 33K and 55K https://en.wikipedia.org/wiki/Pluto

If we assume an Earth sized city on Pluto, with 1 million homes in it, each using ~4KW energy [which is not a lot for
people living essentially in space] then those homes are going to use ~4GW energy. Now add in all the manufacturing,
and the huge nuclear powered green houses for food production etc etc. And you are talking about 100's of GW of energy.
All of which eventually gets radiated away as heat.
A 100W incandescent light bulb emits ~5% of its energy as visible light. The remaining ~95% as IR thermal energy.
If we assume a city lit like one on Earth [for whatever reason] with over 1 million homes plus businesses and farms then
we can expect well over 1 million light bulbs. Which gives 5MW of visible light being emitted, and 95MW IR.

A large metropolitan area like New York or Tokyo on the Earth will emit WAY more.
An inhabited hemisphere [basically as long as your not over the middle of the pacific] will emit way way more than that.

So to answer the question, yes, the Earth's night side is way brighter than Pluto's day side at closest approach.
By many orders of magnitude, in both IR and Visible radiation. The visible part is almost entirely down to us.

Originally posted by googlefudge
So to answer the question, yes, the Earth's night side is way brighter than Pluto's day side at closest approach. By many orders of magnitude, in both IR and Visible radiation. The visible part is almost entirely down to us.

As I have said before, I am not interested in infrared. But I see you have made your case with visible light.

I must point out that Pluto does have a thin atmosphere.

Originally posted by twhitehead
As I have said before, I am not interested in infrared. But I see you have made your case with visible light.

I must point out that Pluto does have a thin atmosphere.

Yeah but it's really really thin... estimated at ~20 millionths atmospheric pressure at the surface.

Which for the rough ballparkish figures I was calculating made it irrelevant.

http://www.space.com/29885-pluto-atmosphere-to-be-revealed-by-nasa-new-horizons-spacecraft.html

EDIT: Why are you not interested in IR. Most of our 'optical' telescopes also have IR cameras on them [including Hubble]?

Originally posted by twhitehead
How does Voyagers transmission compare to the Earth's typical output? Are we transmitting anything with significantly more power into space?

How much fainter would the Voyager signal be if it were 10 light years away? How much more power output would a spacecraft have to transmit from 10 light years away in order to have the same power when the signal reached earth?

well, let's look at the numbers. One LY is 5.8E12 miles. So ten LY would be 5.8E 13 miles. Now, lets say Voyager is 5.8 E 9 miles away. 1/000th of a light year. So at one light year, you would need 1 million times the power to get the same signal strength as now. 10 more light years, 100 times more. So total, about 100 million times the power to get the same S/N ratio. 1 million is 60 Db more needed and 100 times more, it needs 80 Db more signal. So if you have a transmitter 100 million times stronger that would do it, or you try for 80 db more from your antenna. For instance, if you can manage 60 Db in the antenna then the power needs only to be 100 times the power of the present day Voyager. For antenna gain, it is a matter of how many square wavelengths it collects.

And that in turn is related to the wavelength itself. If you were using say 1 meter wavelength, 300 mhz, you can see if you used a wavelength of 1 mm you get 1 million times the square wavelengths collected, which is why a relatively small optical telescope can collect so many wavelengths compared to what you would need for a radio telescope to collect and focus 1 meter waves.

If I remember correctly, the frequency of the Voyage is about 5 gigahertz, call it 6 for grins.

.3 ghz (1 meter wavelength) and 3 ghz, 1/10 meter, 10 cm and 6 ghz, 5 cm, 50 mm, about 2 inches wavelength. So 50,000 mm square or 500 meter wide antenna would give you about 60 db of gain. Then with a power of not 5 watts but 500 watts and you now have the same signal at 10 light years as you have at 5 or 6 billion miles from the sun.
Not impossible for sure. You can see, as far as power goes, it is a lot more efficient to build the biggest frigging antenna you can. With the newest materials and robots in space doing the building, they would be incredibly light weight for its size. This also lends to the idea of using light for propulsion, you don't get a lot of thrust but it is 24/7 and free energy. Which reminds me of my own idea in that area: using the focal effect of the sun to find tracks of amplified light from nearby stars and get in that path for free thrust. The only thing wrong with that, assuming it works, is you can't chose the direction of travel. I am talking about energy coming in from stars like Sirius, at 8 LY away. On the opposite side of the sun that energy will be concentrated by the sun's gravity lens effect, starting at about 55 billion miles,or about 90 billion kilometers after which energy starts being focused from Sirius. You can do the same thing in effect if you can reflect a lot of light from the sun with the reflectors at say 100 billion km out, reflecting energy back to the sun to make a focus line of energy, but now in the direction you want to go. Of course this would require a level of technology not available for maybe a couple hundred years but it is within the realm of possibility.

Originally posted by twhitehead
Note that I have not claimed anything about radio waves with regards to Pluto. I said that we would not have noticed human like lights. If we didn't even spot the moons, then I doubt we would have spotted slightly unusual light levels on the surface of Pluto.

Point being: yes. Yes, we would. There's a huge difference in quality, not quantity, between intelligent radio emissions and reflected light.

Originally posted by twhitehead
As I have said before, I am not interested in infrared.

That's... that's like studying wine production and saying you're not interested in all those aromatics and tannins, all you care about is the alcohol content. Yes, it's there, it has an influence... but it's not significant. It doesn't distinguish between a good wine and a bad one, and mere reflected visible light does nto distinguish between an inhabited planet and a barren rock - and you're a fool for focusing only on that.

"Say we find extraterrestrial life, how to prove it?"

If you have a strong oxygen O2 line in the spectrum from a star that cannot be accounted for from the star itself, then this star has probably a oxygen rich planet. There is no way that O2 can have a natural cause, other than a biological life. That's a proof of life.

As far as I know, no spectrum from a star has this O2 line. Other than our solar system.

Originally posted by FabianFnas

If you have a strong oxygen O2 line in the spectrum from a star that cannot be accounted for from the star itself, then this star has probably a oxygen rich planet. There is no way that O2 can have a natural cause, other than a biological life.

But O2 from biological life doesn't go into the star so surely you mean a strong O2 line in the spectrum from the planet's atmosphere, not its star, right?

I am not an expert on this so I could be wrong but I assume that even if there is a planet with a very high O2 content orbiting the star, the light coming directly from the star in your line of sight will so massively outshine the star light reflected off the planet's atmosphere that you will never get a 'strong' O2 line in the spectrum of the total combined light coming both directly from the star and from the reflected star light off the planet combined. I could be wrong but I assume you will need to somehow have a way to greatly narrow down which photons come from reflected star light off the planet and which come directly from the star ( i.e. without reflecting off the planet ) before you can get good evidence of a O2 rich planetary atmosphere.

Anyone:
am I right about that?
Please correct me here if I am wrong.

Originally posted by humy
But O2 from biological life doesn't go into the star so surely you mean a strong O2 line in the spectrum from the planet's atmosphere, not its star, right?

I am not an expert on this so I could be wrong but I assume that even if there is a planet with a very high O2 content orbiting the star, the light coming directly from the star in your line of sight will ...[text shortened]... lanetary atmosphere.

Anyone:
am I right about that?
Please correct me here if I am wrong.

Of course, there is no biological life on a star.

And no, a O2 line from a planet, really outshine the one of a star. You can easily distinguish a O2 line from a planet from that of a star (does stars have O2 lines at all...?) because of the periodic doppler shift.