60 years of silence - so far

[SLD]

What Does 60 Years of Silence Tell Us About the Search for Extraterrestrials?

Astronomers have only been scanning the skies for a signal from extraterrestrial civilizations for about 60 years. Although no signal has been found, a new study suggests that this lack of detection can help us predict what the future holds for SETI. According to their analysis, this means...

www.universetoday.com

So for 60 years we’ve been trying to detect alien signals and have found nothing. But I think this search makes very unrealistic assumptions.

1) Wouldn’t the aliens have to be beaming the signal in our general direction? The probability of such would be very low, unless they have already detected our signals, which only the closest of stars would have.

2) Wouldn’t aliens most likely direct their signals towards the center of the galaxy, not just all around? Thus at best we should be looking away from the center.

3) How far away can we detect a signal? The galaxy is 100,000 light years in diameter. We are 30,000 light years from the center. 10,000 light years is close. But still very, very far away.

4) Why are we even looking for such a signal? We don’t broadcast such signals, aside from one rogue scientist who did it from Arecibo in the 70’s.

5) Why not look for other signals, such as tv, radio, or military radars, or other similar emissions? Maybe that’s too hard. But I once read that our overall emissions could be detected within a few hundred light years (once they reach that far).

IAE, this article doesn’t answer all those questions, but does indicate that it would take 1800 years of searching before we had a 50-50 chance of detection. Not sure if taxpayers will support a project that long.

 

[rousseau]

Another factor - planet earth is 4 billion years old. We've had the ability to transmit a signal for 0.0000025% of that time. Our civilization may very well collapse due to either climate change or catastrophic weapons in the next 500 - 1000 years. So another planet with life that could transmit a signal would need that ability at the same time as us, which lowers the odds by quite a bit.

If intelligent life has existed elsewhere, it's very likely that their society has already collapsed or doesn't have the ability to transmit a signal yet. But don't ask Astronomers to think about history.

 

[pood]

Radio and TV leakage would be very hard to detect too far out in space. It’s not the same as intentionally directed radio beacons.

Could be there are very few technologically advanced alien species out there. Even on our own planet, if you condensed the entire history of the earth into a single calendar year, with the earth forming on Jan. 1, modern humans didn’t appear until about one-tenth of one second before midnight on the last day of the year, Dec. 31. And then in our brief history it took a long time to develop radio technology, which even now is only a little over one hundred years old.

 

[bilby]

Radio and TV leakage would be very hard to detect too far out in space. It’s not the same as intentionally directed radio beacons.

Indeed.

The people who built our TV and radio broadcast stations, of all kinds from ship-to-shore radio through to wide area TV broadcasts, put vast effort into preventing any significant amount of the signal from going into space (although they weren't always successful, particularly in the earliest days of these technologies). Not because they wanted to hide their signal from aliens, but because they were paying the electricity bills, and wanted to get as much of their energy directed at the receivers here on Earth as possible.

The only really powerful radio broadcasts we have ever aimed deliberately at space were the ballistic missile early warning systems in the Cold War; And these powerful search radars had largely been replaced by more efficient systems by the end of the Cold War era - they essentially existed from the late 1950s until the early '90s, so of the entire time that our planet has existed, only around thirty five years of such broadcasts were made.

The entire history of radio broadcasting has only spanned 126 years so far, and the signal strengths sent into space at the beginning and end of that range are tiny. Today, most of the powerful signals have been replaced - military radars use more efficient designs than the high power systems of the '50s and '60s, and television and telephony are now largely carried by fibre optics rather than by radio signals.

If aliens were listening in, they would be very lucky indeed to detect such a "wow signal" from us even once.

 

[Tigers!]

60 years of silence sounds like a wonderful marriage .......

 

[steve_bank]

In all of time there wouLd have been a first scientific critter. Maybe we are the first.

Everything is in motion. A distant rasmitter on a plnet in anoter solar system has to libne up with the receiving antenna.

I would think information entropy would apply. Eventually a modulated signal over interstate and galactic distances would degrade into noise.

There are practical limits to radio detecion, principally equipment noise.

Maybe the CMBR is all those ET cell phones and TV signals ....

Wow! signal - Wikipedia

en.wikipedia.org

The Wow! signal was a strong narrowband radio signal detected on August 15, 1977, by Ohio State University's Big Ear radio telescope in the United States, then used to support the search for extraterrestrial intelligence. The signal appeared to come from the direction of the constellation Sagittarius and bore the expected hallmarks of extraterrestrial origin.

Astronomer Jerry R. Ehman discovered the anomaly a few days later while reviewing the recorded data. He was so impressed by the result that he circled on the computer printout the reading of the signal's intensity, "6EQUJ5", and wrote the comment "Wow!" beside it, leading to the event's widely used name.[2]

The entire signal sequence lasted for the full 72-second window during which Big Ear was able to observe it, but has not been detected since, despite several subsequent attempts by Ehman and others. Many hypotheses have been advanced on the origin of the emission, including natural and human-made sources, but none of them adequately explain the signal.

Although the Wow! signal had no detectable modulation—a technique used to transmit information over radio waves—it remains the strongest candidate for an extraterrestrial radio transmission ever detected.[3]

The analysis is probably online somewhere.

We know the detection limit on Earth for the ectronics and size of antenna.


All antennas have a beam that diverges. Given an ET distance the transmit power and max divergence of the antenna can be catapulted for us to detect it.

Imagine a parabolic ET transmit antenna. The antenna radiates into a cone(solid angle) that expands with distance. Ignoring absorption and reflection in the path the total energy in the cone at a cross section is constant, but the watts/m^2 at the circular base of the cone goes down.
So the total energy into our receiving antenna is watts/m^2 of the cone base hitting our antenna times our antenna area. It has to be above the receiver noise floor.

Energy density in the cone or solid angle goes down by 1.r^2.

It is like a circular flashlight.

 

[pood]

I was thinking that the Wow signal might have been a one-time only thing sent to announce the senders’ presence, in much the same way as the Aricebo signal was. But the latter was modulated and full of information, whereas Wow was not, so who knows?

 

[Shadowy Man]

Maybe the CMBR is all those ET cell phones and TV signals ....

hehe… no.

 

[steve_bank]

I think designing an antenna and transmitter at the required power would be difficult if not impossible.

You can do a search on highest power trasmitters on Earth.

 

[bilby]

I was thinking that the Wow signal might have been a one-time only thing sent to announce the senders’ presence, in much the same way as the Aricebo signal was. But the latter was modulated and full of information, whereas Wow was not, so who knows?

Seems unlikely; That we would just happen to be listening when such a one time signal went past.

Not impossible; But smart people don't expect to win the lottery.

I don't think we know whether the Wow signal contained any information, I thought the apparatus was just looking at signal strength.

 

[Jimmy Higgins]

60 years? That'd be 60 Light years. That is nothing compared to the size of the Milky Way!

 

[Elixir]

60 years? That'd be 60 Light years. That is nothing compared to the size of the Milky Way!

… which is nothing compared to the galactic cluster, which is nothing compared to …
We should refrain from talking about “the observable universe” as if we can examine it for life. 60 light years radius contains more stuff than we’ll ever be able to examine adequately.

 

[Jimmy Higgins]

All that stuff also needs to align with our existence timeline and be "local". 60 light years such a tiny miniscule speck on a speck of dust.

 

[SLD]

60 years? That'd be 60 Light years. That is nothing compared to the size of the Milky Way!

No! There could be signals emanating from a long time ago from a civilization that have been reaching us for thousands of years. you're Thinking that they’re responding to something we’ve been sending out. That’s not the point of a SETI search.

 

[Elixir]

There could be signals emanating from a long time ago from a civilization that have been reaching us for thousands of years.

There were. Starting 712.85 million years ago, for 22,396 years, a civilization only 49 million light years from here broadcast a greeting in our direction. But eventually they gave up. And it has happened again several thousand times since, with signals arriving here every few hundred thousand years. Gee, what are the odds that we missed them all? 1:1 for all practical purposes.

 

[SLD]

There could be signals emanating from a long time ago from a civilization that have been reaching us for thousands of years.

There were. Starting 712.85 million years ago, for 22,396 years, a civilization only 49 million light years from here broadcast a greeting in our direction. But eventually they gave up. And it has happened again several thousand times since, with signals arriving here every few hundred thousand years. Gee, what are the odds that we missed them all? 1:1 for all practical purposes.

That’s the point the article was making. They said to get a 50-50 chance we’d have to run the project for 1800 years. But I’m not sure how we could make such an estimate.

Again, why would we expect to find such a signal in the first place? Do we think that aliens are truly interested in simply broadcasting to the galaxy that they’re around?

Are there other ways to detect advanced civilizations?

 

[steve_bank]

A way to do it is put a shutter up in space to block the sun. Fash on off keying sending 314159.

 

[Elixir]

A way to do it is put a shutter up in space to block the sun. Fash on off keying sending 314159.

That should fix global warming too!

 

[Jimmy Higgins]

60 years? That'd be 60 Light years. That is nothing compared to the size of the Milky Way!

No! There could be signals emanating from a long time ago from a civilization that have been reaching us for thousands of years. you're Thinking that they’re responding to something we’ve been sending out. That’s not the point of a SETI search.

Again 60 ly isn't a long distance still for signals approaching us. And for signals that already passed.

 

[Politesse]

Are there other ways to detect advanced civilizations?

I know there's been some discussion of how a Dyson-like structure might appear to us, spectroscopically speaking.