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View Full Version : Our radio waves going out into space, will they last forever?



Ross PK81
2008-Jan-31, 09:20 PM
All those radio waves of old radio and tv shows out in space, are they going to exist forever?

idav
2008-Jan-31, 09:22 PM
I'm not sure how accurate it is, but in the recent show on, I think, the history channel about the world without us they claim they will only reach a light year or two.

NEOWatcher
2008-Jan-31, 09:24 PM
Yes; until they hit something...
But; at such weak levels and possibly so distorted from gravity that they will be completely lost in a field of static.

If you use our technology as a benchmark...I've heard that we wouldn't be able to detect them past Neptune.

TheNick
2008-Jan-31, 09:48 PM
I'd be really interested to know where exactly our own technology would no longer be able to detect our signal. Is it really only as close as Neptune?

undidly
2008-Jan-31, 10:05 PM
TheNick

>I'd be really interested to know where exactly our own technology would no longer be able to detect our signal. Is it really only as close as Neptune?>

That cannot be right.There are space probes (from Earth) way past Neptune
that send back signals with a transmitter of only a few watts.
TV stations are hundreds of thousands of watts.RADAR stations can be millions
of watts and use a directional aerial to concentrate the radio beam so it does not spread out and quickly weaken.You must be in front of the beam to get the signal but the aerial turns,just wait a second for a strong signal.

Ross PK81
2008-Jan-31, 10:07 PM
how do digital signals compare to radio ones, as in going out into space and how long they last? Do they even go out into space?

Argos
2008-Jan-31, 10:16 PM
The intensity decay of our radio emissions [as well as all electromagnetic waves] is inversely proportional to the square of the distance. Sirius, twice as far as A-Centauri would receive only 1/4 of the intensity of the signal.

idav
2008-Jan-31, 10:26 PM
Yes; until they hit something...
But; at such weak levels and possibly so distorted from gravity that they will be completely lost in a field of static.

If you use our technology as a benchmark...I've heard that we wouldn't be able to detect them past Neptune.
That is certainly not right. We still communicate with both voyager probes. Voyager one was at a distance of 104.7 AU as of Dec 7th 2007.

idav
2008-Jan-31, 10:28 PM
how do digital signals compare to radio ones, as in going out into space and how long they last? Do they even go out into space?
You are mixing terms. I think you mean to ask how would analog and digital radio waves compare. I would think that Digital waves would have less degradation simply because they don't depend on the spectrum of the wave only it's extremes. I'm no expert though.

Jerry
2008-Feb-01, 12:14 AM
how do digital signals compare to radio ones, as in going out into space and how long they last? Do they even go out into space?
As electromagnetic waves, there is no real distinction between digitial and analog signals. In both cases, information can be imposed upon a wave as either variation in wave amplitude or variation in frequency...or both.

For a very long time, we corresponded with the Pioneer 10&11 probes by sending out a powerful wave; stopping the transmission, and waiting. When the Pioneer probes received the signal, they would ampify it modulate it to a proportional frequency; and transmit a single frequency back: Perhaps the slowest digital messaging system ever.

Electromagnetic waves are attenuated as they pass through space, and it is a miss conception that this attenuation is 1/r^2. Space is not an absolute void, and there is always a loss as an electromagnetic wave passes through any medium. The attenuation is always greater than 1/r^2.

There is also the possibility of relativistic attenuation, which should be 1(1+z)^4; where z is the ratio of the relative speed of the objects divided by the speed of light.

samkent
2008-Feb-01, 12:56 AM
Would it be as simple as this explaination.

You have 1 gallon of water. You throw the water out in a 360 degree circle. The farther the water travels the more it spreads out. The more it spreads out the less water you have per square inch. Now translate that to 3 dimentional space.

Jerry
2008-Feb-01, 03:55 AM
Would it be as simple as this explaination.

You have 1 gallon of water. You throw the water out in a 360 degree circle. The farther the water travels the more it spreads out. The more it spreads out the less water you have per square inch. Now translate that to 3 dimentional space.
That would be a variation on the 1/r^2 rule. It may seem intuitively like it should be 1/r^3, but you really have to visualize the wave as moving in primarily a plane in all directions - an expanding film or bubble, not a solid...unless you are talking about extremely great distances, or something like gamma ray absorption in lead.

NEOWatcher
2008-Feb-01, 12:49 PM
That is certainly not right. We still communicate with both voyager probes. Voyager one was at a distance of 104.7 AU as of Dec 7th 2007.
Well; I did hear that a few years ago, and Idav was right about the 1-2 LY figure THC gave. We've been making great advances in radio communication.
But; to equate it to a probe is certainly different. Our public broadcasts are spreading in 360, but communications to a probe are directed in both directions.
This is why SETI wants to "beam" a message into space to concentrate the message toward a target.

Larry Jacks
2008-Feb-01, 03:18 PM
There's a huge difference between a signal received from a deep space probe and a broadcast radio or TV signal. The space probe has a high gain antenna that focuses the RF energy into a narrow beam towards the Earth. The receiving side has a much larger and higher gain antenna. Gain is the key. Some large dish antennas have gains of over 60 dB (an amplification factor of 1 million) compared to an omnidirectional antenna. The probe's antenna gain is probably on the order of 35-40 dB (~3100 to 10,000 amplification factor). That means if the transmitter power was only 1 watt, it gets concentrated so that it would be equivalent to a 3100-10,000 watt transmitter with an omnidirectional antenna. The term for this is EIPR - effective isotropic radiated power. High gain antennas and extremely sensitive receivers are what make deep space communications possible. Even then, it's challenging because the signals are so weak. There are propagation losses due to the vast distances to take into account.

Normal broadcast signals are transmitted using omnidirectional (or near omni) antennas. The signals are radiated in all directions instead of being focused into a narrow beam. Like the earlier post said, it's quite likely that our best technology couldn't detect an ordinary broadcast signal past Neptune.

There are other types of RF transmissions that might be more detectable. I'm thinking of high power radar signals such as those from BMEWS (Ballistic Missile Early Warning Systems). The signal strength is stronger and more focused than broadcast signals. There are other characteristics of the signal (such as tight frequency control, precise pulse repetition rates, and changes in PRP in different radar modes) that should indicate the signal is artificial. Even these signals would be very weak by the time they reached the nearest star outside of our solar system.

neilzero
2008-Feb-01, 07:13 PM
Yes they exist forever, but they quickly become weaker than the background noise and are thus both undetectable and the the information is lost. Neptune's distance is likely optimistic for even our most powerful commercial radio and TV broadcasts, but very rare and brief exceptions likely occur. Opinions seem to vary widely on how many light years away our most focused beams may be detectable by very advanced technology.
Neptune is about is about 0.0005 lightyears away. Neil

Argos
2008-Feb-07, 02:03 PM
This link (http://www.setileague.org/software/spreadsh.htm) leads to several SETI spreadsheets. This particular one (http://www.setileague.org/software/ranganal.xls) "Performs range analysis of an electromagnetic communications system, assuming identical antennas at both ends".

Assuming a signal of 1420 MHZ frequency, 10Hz bandwitdth, 100 K receive noise temperature, 1,00E+06 W transmission power, and a 100-meter dish with 63 dB gain, the signal would be detectable by an equal antenna at ~9 Parsecs, or ~30 LY.

Not too far.

It suggests our low power, broadband, omnidirectional Radio and TV signals would be barely detectable in A-Centauri [if so], for the relief of the paranoid.

JustAFriend
2008-Feb-07, 03:11 PM
Radio and TV, yes, has a fairly limited INTELLIGIBLE range.

There comes a point where the data gets so weak that it is masked in the fog of the Universe's background radiation.

However the high-power radars we use could probably be detected out to several hundred light years at least. No data in them but they are a sign of intelligence (or at least something curious enough that someone may come calling...) There are a LOT of stars within a 300LY sphere around us.


ByTheWay: heres an interesting site of a guy who sells poster starmaps. His say there are nearly 300 stars within 30 light-years of us:
http://www.projectrho.com/smap12.html
(this 30LY 3D map is on the page)
http://www.projectrho.com/starmapPoster1e.jpg


Here's a link to a map of the 51 so-called habitable stars within 30LY of Sol...
http://www.projectrho.com/HabHYG30lyHab.png

samkent
2008-Feb-07, 04:40 PM
300 years? That would be 600 for the call back.

Wake me when it comes in.

RalofTyr
2008-Feb-08, 01:34 AM
That's probably why SETI is failing. They are expecting to find massive amounts of radio waves and sometimes they find something interesting, but it doesn't repeat, probably because the alien radio wave was beamed and it was only a miricle that it hit Earth, not to be repeat again for decades.

That and there's probably no one close by anyways.

alainprice
2008-Feb-08, 04:47 AM
If we knew where to point, a modulated laser would definitely help to the point of diffraction losses.

Bring it on Polaris, I'm ready for a reply.

SpiffySkipjackESQ
2008-Dec-17, 09:33 PM
So we started in 1936 with Hitler opening the Olympics and we end on Feb 17th 2009 with reruns of Groundhog Day? If I were an Alien who finally got my "TV" reception figured out from Earth, I'd be pretty cheesed off when the signal stops and the snow starts. Damn cheesed off, indeed.

I'd start calculating how long it takes those aliens to startup their Galactic Battlecruisers and come back here to start slapping the side of the set, like my pappy used to do when the rabbit ears slipped out of alignment after mommy insisted on dusting the back of the old 13" B&W set we had in the den. --Spiffy

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formulaterp
2008-Dec-18, 02:54 AM
They could still get our signals if they really wanted. Assuming they got those government vouchers. It's not like they don't know the switchover is coming.