# PRI at 50 Lightyears OutJanuary 24, 2012 12:14 PM   Subscribe

Suppose an astronomer was sitting on a mirror of Earth less than 50 light years away and it was about 50 years in the future. On this mirror earth technology is similar to Earth's modern technology. What is the likelihood that their astronomers would have discovered Sol given its size and distance. How difficult would it be for a project the scale of the SETI array to listen to a 10K watt analog Earth radio station? Other details welcome.

For purposes of a detailed simulation. Could I hear a whole program such as, RadioLab, as broadcast from WNYC from 50 lightyears away on parallel Earth using my massive radio telescope (assuming i found the signal and figured out how to hook it up to a speaker). Would it be especially static filled, or would I actually be able to make out the words from its hosts?
posted by humanfont to Science & Nature (7 answers total) 4 users marked this as a favorite

Our star would certainly be visible to astronomers 50 light years away.

But you would not be able to hear an undirected radio station broadcasting at 1000 watts-- those 1000 joules per second would be spread out over a spherical shell 30,000 sq light years in area.

Here is a calculator you can use, but I think it assumes a directed signal (i.e., the aliens are aiming at us):

http://www.satsig.net/seticalc.htm
posted by justkevin at 12:26 PM on January 24, 2012

I don't think radio transmissions would be decipherable at 50LY at our technology level.

This seems somewhat accurate as near as I can tell:

You may recall that the Huygens probe transmitted at circa 5Watts and it's signal was detectable on Earth about 1AU away. The Voyagers transmit at circa 20 Watts each and their signals can be read 100AU away. Using this as a cutoff we find that it would take around 10,000MW of broadcast power to send a decodable signal 1LY...

Now, I think you could tell there was some sort of signal
posted by edgeways at 12:29 PM on January 24, 2012

Best answer: I just did a quick back-of-the-envelope calculation and the number was not as far off our current capabilities as I thought it would be.

I assumed a 100,000 Watt transmitter, since that seems to be the highest on the Earth today. That gives us an irradiance at 50 LYr of 2.8 x 10^-31 W/m^2.

Arecibo (the biggest radio telescope on the planet) is apparently able to get to a sensitivity of 10^-24 W/m^2 currently. Although, the plot shown here suggests an even-more-sensitive value of 5x10^-26 W/m^2. It should be noted that the second number (I believe) is for a broad-band targeted search, which probably mean the number for a specific frequency would be much lower. So, we're still a ways off, but 5-7 orders of magnitude were not as bad as I thought it would be.

Thanks; this is the most on-topic procrastination I've done all day in preparing to teach my physics and astronomy classes!
posted by Betelgeuse at 12:40 PM on January 24, 2012 [5 favorites]

There are lots of stations broadcasting on any given frequency. In New York, WNYC's broadcast at 93.9 FM is from only a few miles away, and the broadcast signal of another 93.9 FM, say WKYS 93.9 ("DC's Home Of Hip-Hop") is from 225 miles away, so the signal has dissipated to the point it's too weak to hear. I believe that the signal would decay as the square of the distance (not exactly my field) so being 10 miles away from WNYC and 225 miles away from WKYS means that the WNYC signal is 500 times stronger.

But if the hypothetical astronomer had gear that was sufficiently sensitive to pick up a signal from WNYC, they would also be getting hip hop from Washington, and 93.9 The River ("Real Rock Variety") from Detroit, and 93.9 Lite FM from Chicago, and for that matter, 93.9 DWKC iFM from Manila. Neglecting any difference in the transmitter strengths for the stations, they are all broadcasting from essentially the same point, when viewed from 50 light years away.

So the hypothetical astronomer would be getting noise that would be next to impossible to filter out one particular signal; like if you were standing in the middle of a football stadium trying to hear one particular conversation -- amplifying the sound wouldn't help you pick out one signal versus the others.
posted by Homeboy Trouble at 12:42 PM on January 24, 2012 [1 favorite]

There's a pretty good chance they'd have discovered the sun, because at that distance it would be just barely visible to the naked eye on a dark night, let alone to any telescope.

Also, assuming they lived in a similar solar system, they'd have targeted the sun to look for planets almost as soon as they were able to.

As far as listening to radio. Highly doubtful, because of the inverse square law.
posted by empath at 12:47 PM on January 24, 2012

probably the best explanation can be found at seti@home... particularly this section, on page 4 where they explain in detail what kind of signals they are looking for.
posted by sexyrobot at 2:05 PM on January 24, 2012

No, the opening and premise of Contact, while wonderful, is big on artistic license. Signals fade out rapidly with the inverse square law and become very difficult to distinguish from noise, as far more mathematically-astute Mefites above have stated. (As Homeboy Trouble said, the presence of that noise above the baseline of 4K radiation and the activity of stars might be interpreted as a source of intelligent life, but we / they likely wouldn't be able to hear an individual transmission).

This is a common misunderstanding of the purpose of SETI: while it's very unlikely we'll pick up everyday signals from other civilizations, detecting purposeful, powerful, targeted communication is a possibility, given that we've made such transmissions outselves: for example, the Arcebio message and NASA's transmission of "Across The Universe" being two examples. We've also - possibly - received one, in the form of the famous Wow signal.

As for detection of us from 50 light years away... they'd certainly see the Sun, and likely be able to detect Jupiter, if the gas giant was between them and Sol... i.e. if they were in a position to look at the solar system "edge-on", through the plane of the elliptic. If they were looking at us in the right orientation, they would be right on the cusp of the ability to detect Earth. Add another one or two decades, and they / we will likely have the ability to lens the atmosphere of small, rocky worlds... and if we can do that, spectral analysis would tell us / them the chemical composition of the air.

In short, they wouldn't be able to hear "Blue Suede Shoes", but they'd be able to tell that Earth has polyvinyl chlorides and other pollutants in the atmosphere, in excess of anything that could be produced through natural processes... and thus an intelligent civilization.
posted by Bora Horza Gobuchul at 2:15 PM on January 24, 2012 [3 favorites]

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