Do radio & TV signals travel forever?

I've seen TV shows where they say that radio signals are about 80
light-years out from Earth.

TV signals, about 50 or so light-years out there.

Do they just keep moving at ligt speed forever?

Thanks.




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eferGuy=99
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ReeferGuy asks:

I've seen TV shows where they say that radio signals are about 80
light-years out from Earth.

TV signals, about 50 or so light-years out there.

Do they just keep moving at light speed forever?

Within a reasonable definition of "forever," the answer's yes.

Wirt Atmar

"R" == ReeferGuy writes:

R I've seen TV shows where they say that radio signals are about 80
R light-years out from Earth.

R TV signals, about 50 or so light-years out there.

R Do they just keep moving at ligt speed forever?

Yes, though as one gets farther and farther away from the Earth they
become weaker and weaker. It is highly unlikely that there are aliens
out there enjoying the first run of "I Love Lucy."

--
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"ReeferGuy" bravely wrote to "All" (19 Aug 04 17:46:21)
--- on the heady topic of "Do radio & TV signals travel forever?"

Re From: (ReeferGuy)

Re I've seen TV shows where they say that radio signals are about 80
Re light-years out from Earth.

Re TV signals, about 50 or so light-years out there.

Re Do they just keep moving at ligt speed forever?

Re Thanks.

No, the signals spread out and eventually reception gets too faint to be
heard above the normal radio noise background. This is probably the
reason seti has failed so far. They should instead search for lasers.

A+s+i+m+o+v

.... Isaac Asimov : 1920-1992 : Gone to the stars!

(ReeferGuy) wrote in message news:14033-412549DD-
:





I've seen TV shows where they say that radio signals are about 80
light-years out from Earth.

TV signals, about 50 or so light-years out there.

Do they just keep moving at ligt speed forever?

Well, they will spread for a very, very long time, that's sure.

But I'm not sure that it's possible to detect those signals in another
solar system. One lightyear is 'roughly' 9,450,000,000,000 kilometres -
compare this to the approx. 36,000 km-s orbital distance of the
geostationary TV satellites. I think detecting those signals is comparable
to photographing Earth-like planets in other solar systems, if not even
more difficult.

Thanks.




************************ReeferGuy™
***********************USMC•FDNY






******************§*************************§

In article ,
"Mr. 4X" writes:
But I'm not sure that it's possible to detect those signals in another
solar system.

It's not easy, but it's not hopeless either.

I have some notes from 1998 suggesting that a search carried out then
was sensitive enough to detect radio leakage from an "earth" orbiting
alpha Centauri. (The actual search did not extend far enough south
to cover alpha Cen; the statement refers only to its sensitivity.)
An advanced civilization might have considerably better detection
capability than we do now. A web search, perhaps starting with the
FAQ, ought to turn up better sensitivity estimates.

Of course detecting a signal is very far from demodulating it into
watchable video. I'm guessing the signal to noise ratio needed for
watching is something like 10^4 times greater than for detection.
(That's a lot! But if Joe or another expert has a different number,
believe him, not me.) And of course the nearest technical
civilization may be a lot farther away than alpha Cen.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

In article ,
(Steve Willner) writes:
Of course detecting a signal is very far from demodulating it into
watchable video. I'm guessing the signal to noise ratio needed for
watching is something like 10^4 times greater than for detection.

On thinking about this a bit more, the situation is worse than I
described. The detection sensitivity of a radio SETI experiment
refers to the entire radio output from a putative advanced
civilization: broadcast stations, search radars, whatever. My guess
at 10^4 was for the increase in S/N needed to convert "detecting" to
"watching" for a single station. There's an additional large factor
(10^3 ??) needed to go from detection of an entire planet to
detection of a single station.

Bottom line: radio SETI to _detect_ a putative advanced civilization
is within present or modestly improved capabilities, but "watching"
a "leakage" signal is many orders of magnitude harder.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Steve writes:

On thinking about this a bit more, the situation is worse than I
described. The detection sensitivity of a radio SETI experiment
refers to the entire radio output from a putative advanced
civilization: broadcast stations, search radars, whatever. My guess
at 10^4 was for the increase in S/N needed to convert "detecting" to
"watching" for a single station. There's an additional large factor
(10^3 ??) needed to go from detection of an entire planet to
detection of a single station.

Bottom line: radio SETI to _detect_ a putative advanced civilization
is within present or modestly improved capabilities, but "watching"
a "leakage" signal is many orders of magnitude harder.

I'm not sure that there's reason to be quite so pessimistic. If we use Earth as
a model of a modestly technological civilization on a distant planet, radars
are likely to be the items we would first detect, in great part because their
energy is emitted in a fairly narrow, high-gain beam on a highly specific
frequency.

While there is little or no "information" (in the sense of the profound
information implicit to the content of "I Love Lucy," "Mr. Ed" or "Dr. Phil"
transmissions) in a radar signal, the geometry of its broadcast signal could
quite likely mean that the detection of one radar station would represent the
entire technological civilization of that that distant Earth.

Radar stations on the limb of the planet are likely to be the ones most visible
at any given time, given the horizontal orientations of their sweeps. Moreover,
they tend to be very narrow band pulsed carrier waves, but otherwise
unmodulated, with each station's transmissions focussed on just one narrow
frequency, but placed within a reasonably large variety of frequencies.

If something like the just completed Project Phoenix had observed this distant
Earth, it would likely have only seen one, two or three radars at any one time,
each quite likely on a different frequency, even though tens of thousands of
radars exist on the planetary surface.

Wirt Atmar