How far can radio signals *actually* travel through space?

Question

It's common knowledge that there is a sphere 80 LY in radius around the solar system containing our radio and TV signals. However like much common knowledge I hear that it's probably all nonsense.

I know that the universe is bathed in background radiation courtesy of the Big Bang and energetic objects dotted through space which forms a natural static. Our remote space probes' radio signals are eaten away near to nothing by hiss and distance without even leaving the solar system.

A photon, given the energy will happily make it's way 13.7 billion LY so I know that distance isn't the issue here but rather signal strength. My questions are:

How far do our TV signals etc. get before becoming too weak to discern above the hiss of the universe and effectively cease to exist?
What have we broadcast from earth that is guaranteed to be able to reach through interstellar distances?
What implications does this have for SETI? Is there an a distance where it's just not worth looking? (in case aliens aren't polite enough to use 1420mhz!)

Answer 1

This is not such an easy question as it looks.



In theory. radio signals will travel indefinitely through space unless they encounter something to absorb them. Photons have a rest mass of zero so from a qm viewpoint they have an infinite extent. But i"m sure you know all that.



It's all very well to have a photon somewhere but as far as SETI is concerned the question is can we detect it and separate it from the random noise. After all, we know that space is a radio-noisy place. With a single photon it would not be possible, however any source that we were searching for would be radiating a torrent of photons, and by integrating over a sufficiently long timeframe it is possible to pick out such a signal from the noise. In fact in principle, sufficient integration time can pick out any arbitrary signal as long as the integrator is up to the task (ie does not generate random noise itself, does not "forget" photons).



Note that such integration will only detect a signal. It will not be able to extract any form of message from the signal unless the message was implausibly slow.



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!



So the 80LY thing is indeed something of a myth. Noone is watching transmissions of the 1936 Olympics unless they have truly spectacular receivers! I would wonder if a sufficiently large dish for such a detector would be visible from Earth as it eclipsed it's star (exoplanet searchers beware)!



However if we are not attempting to extract information from the signal we can happily integrate away as long as we want and there is hence no theoretical limit to how far out we can listen for the presence of a signal.



You might like to refer to these sites, and extract some useful information from those:

http://www.setileague.org/articles/sensitiv.htm

http://en.wikipedia.org/wiki/File:NASA-SETI-Sensitivity.jpg

http://www.ras.ucalgary.ca/SKA/science/node33.html



The frequency range on which we listen is partly determined by the available window through the atmosphere. But by listening on literally millions of channels we can hope to cover as much of that window as possible. Incidently, this search would potentially also catch harmonics of signals on other frequencies, but at a lower intensity. So effectively we are searching a FAR larger frequency range than it seems.



Cheers!

Answer 2

Recent work has rather put a dampener on the idea that our signals have reached that sort of distance. What now seems to be the case is that at a distance of about two light years, those signals attenuate, and get lost in the general background clutter. In order to send a message much further, a much higher bandwidth is required to concentrate a beam on a target. This is why the SETI Institute is concentrating on finding possible civilizations elsewhere in the Galaxy to which signals may be beamed.

Answer 3

these are question for fairly advanced physics



e-m as a wave has energy which decreases as it spreads. thinking as photons each single photon has an energy proportional to Frequency but the photon count per area would decrease



Quantum mechanics sets a lower limit on what can be detected BUT if you keep the detector on for a long enough time it can pull a signal out of noise. partly an engineering problem partly fundamental physical limit



DO NOT expect a valid answer fro Y Q &A I know just enough to be dangerous and most people are "just dangerous"



the problem with the size of the universe is: nobody knows if it is finite or infinite, bounded or unbounded or if there is any meaning to our concept of three dimensional sphere if we apply it to the universe



what IS OBSERVED is distant galaxies appear to be receeding faster and faster the more distant they are. That is best explained by "expansion of (empty) space itself, NOT movement through space like a bullet from a gun



Background radiation, associated with a temperature near absolute zero is consistent with the concept of ultra hot radiation "cooling" as temperature drops with the lowering energy density in an expanding universe



we can only assume tha thte fundamental laws of physics are the same in all times and all places, if not, we have no way of knowing that 2 + 2 = 13 in some other part of the universe



or maybe god just like to play trick on us.



PS if you are the smartest physics major on Y , what are you asking questions for?

PPS a photon has fixed energy, it is not "given enough energy". C, the speed of light is the only speed it can have.

PPPS physicists are not "fearsome" they are highly educated and very skilled in mathematics and have the imagination to propose new theories to answer questions that only appear to be simple



PPPPS "getting a car" IS useful advice for someone who does not have a car or knows what "road trip" involves



PPPPPS lets all be polite to each other

Answer 4

This answer would be dependent on the technology used to receive a signal. Just how sensitive a receiver you have, and your ability to filter out the background noise. So, there is no physical limitation, but more of a technological one.