60 years of silence - so far

[lpetrich]

Eavesdropping Mode and Radio Leakage from Earth - WOODRUFF T. SULLIVAN III
 Television channel frequencies

TV broadcasts are separated in frequency by 6 Mhz, meaning that that is the upper limit of their bandwith. However, their carrier signals have bandwidths of around 0.1 Hz, some 60 million times less. Since the interstellar background has an intensity proportional to the bandwidth, as is typical of "noisy" signals, that means that it's MUCH easier to detect carrier signals than TV signals in general. One needs to look at lots of *very* narrow frequency bands, and that's typically done in SETI efforts.

So WTS worked out what one would find if one only looked at the carrier signals.

 Television channel frequencies - VHF: 41 to 224 MHz - UHF: 471 to 951 MHz - it varies from place to place, so I was trying to be inclusive. For instance, the US has 54 to 82 MHz (channels 2 to 6), 174 - 210 MHz (channels 7 to 13), 470 - 884 MHz (channels 14 - 83).

That means that TV carrier waves can be observed with a frequency resolution of 10^-9 and in velocity terms, that is 30 cm/s, 1 foot per second. But for that high resolution, one needs to integrate over at least 10 seconds.

WTS then noted that TV broadcasts tend to be concentrated at around a few degrees around horizontal, so as not to waste transmission energy broadcasting in directions where there are no receivers. That means that as each TV station will be most visible around when it rises and sets. At station dawn, its signal jumps to its maximum, then more slowly declines to near zero. Then at station dusk, the signal rises to its maximum, then declines to zero.

From station rise and set times, one can find the location on the Earth of each one, and one will soon discover that many stations are clustered, with nearly the same location.

The Doppler shift over the year should be readily observable, 30 km/s or 10^-4 when edge-on. The observers should find it easy to observe the Sun, and they can work out that it's a 4.5-billion-year-old main sequence star with a certain mass. With that mass value, they can then find out how far away the Earth is from the Sun, and from that, how much sunlight it received. Enough to make water liquid.

 

[Shadowy Man]

Sure, but then it would see nothing whatsoever of significance* in the number sequence 31415, and would not even recognise 9 as a digit at all.

they would understand that when something pulsed 10 times for them it would be the same as our 9. How they write down the number doesn’t matter. They can still count.

We can count in multiple languages and though we have different words for “ten” we agree on how many objects we are counting.

Maybe I’m not explaining it well.

 

[Shadowy Man]

Eavesdropping Mode and Radio Leakage from Earth - WOODRUFF T. SULLIVAN III
 Television channel frequencies

TV broadcasts are separated in frequency by 6 Mhz, meaning that that is the upper limit of their bandwith. However, their carrier signals have bandwidths of around 0.1 Hz, some 60 million times less. Since the interstellar background has an intensity proportional to the bandwidth, as is typical of "noisy" signals, that means that it's MUCH easier to detect carrier signals than TV signals in general. One needs to look at lots of *very* narrow frequency bands, and that's typically done in SETI efforts.

So WTS worked out what one would find if one only looked at the carrier signals.

 Television channel frequencies - VHF: 41 to 224 MHz - UHF: 471 to 951 MHz - it varies from place to place, so I was trying to be inclusive. For instance, the US has 54 to 82 MHz (channels 2 to 6), 174 - 210 MHz (channels 7 to 13), 470 - 884 MHz (channels 14 - 83).

That means that TV carrier waves can be observed with a frequency resolution of 10^-9 and in velocity terms, that is 30 cm/s, 1 foot per second. But for that high resolution, one needs to integrate over at least 10 seconds.

WTS then noted that TV broadcasts tend to be concentrated at around a few degrees around horizontal, so as not to waste transmission energy broadcasting in directions where there are no receivers. That means that as each TV station will be most visible around when it rises and sets. At station dawn, its signal jumps to its maximum, then more slowly declines to near zero. Then at station dusk, the signal rises to its maximum, then declines to zero.

From station rise and set times, one can find the location on the Earth of each one, and one will soon discover that many stations are clustered, with nearly the same location.

The Doppler shift over the year should be readily observable, 30 km/s or 10^-4 when edge-on. The observers should find it easy to observe the Sun, and they can work out that it's a 4.5-billion-year-old main sequence star with a certain mass. With that mass value, they can then find out how far away the Earth is from the Sun, and from that, how much sunlight it received. Enough to make water liquid.

And if those television stations are showing different programs at the same time? It’s not like every station in the world has a unique frequency, right? Channel 2 in Seattle isn’t showing the same programming as Channel 2 in San Francisco even though they’d both be coming over the horizon at about the same time.

ETA: sorry, perhaps the point in the phrase “carrier signal” was just to point out that a signal could be detected though not necessarily decoded. The presence of strong signals at these frequencies could be considered unnatural by ET observers and hence give enhanced probability that intelligence is at play on this planet.

 

[lpetrich]

The Earth's equator has a rotation velocity of 460 m/s, giving a Doppler shift of 1.5*10^-6. If one can detect the rotational Doppler shift between rising and setting, then one can find the inclination of the Earth's spin axis to the line of sight, and also the latitudes of the TV stations. Even without that, one can find their longitudes relative to some reference point.

One also has the Earth's rotational period, and combined with the rotational velocity, one finds its size.

The Earth's Moon makes the Earth move at about 13 m/s, giving a Doppler shift of 4.2*10^-8 So with enough precision, one will discover that celestial body, and with the Moon's period and the Earth's mass, one will observe that the Moon's mass is about 1% of the Earth's mass. A complication is that its observed mass has a projection factor from the tilt of its orbit axis to the line of sight.

One will find the Earth's spin and orbit tilt angles with respect to the line of sight, and that will give an estimate of the Earth's spin-orbit tilt angle. For the Moon, one finds that it orbits far enough to be perturbed mainly by the Sun, and one may even observe some interesting orbital effects. The Moon's orbit has an eccentricity of about 0.055, and it's inclined to the Earth's orbit by about 5.1 degrees. These effects have a size of about 0.6 m/s or a Doppler shift 2*10^-9 making them borderline observable. But if they can be observed, one can observe the orbit's perigee-apogee line precessing forward with a period of about 8.55 years, and one can observe the orbit's line of nodes, where it crosses the plane of the Earth's orbit, precessing backward with a period of about 18.6 years. Both periods can be calculated with celestial mechanics, and discovering them would be a valuable check on the reliability of one's observations.

 

[steve_bank]

Could Aliens pick up the Arecibo transmission?

IIRC, it was aimed at a globular cluster, but I don't know which one.

But could others pick it up too? Maybe only slightly off the path?

You would have to look at the trasnmit power and what the power density is at the other end, and aht we take o be a min signal to noise ratio required for recption given what we know.

 

[Shadowy Man]

The Earth's equator has a rotation velocity of 460 m/s, giving a Doppler shift of 1.5*10^-6. If one can detect the rotational Doppler shift between rising and setting, then one can find the inclination of the Earth's spin axis to the line of sight,

Typically in astronomy you get “v sin i” which means you cannot find the inclination (i) from the velocity (v) alone, unless you have an independent measure of rotational frequency.

 

[lpetrich]

The Earth's equator has a rotation velocity of 460 m/s, giving a Doppler shift of 1.5*10^-6. If one can detect the rotational Doppler shift between rising and setting, then one can find the inclination of the Earth's spin axis to the line of sight,

Typically in astronomy you get “v sin i” which means you cannot find the inclination (i) from the velocity (v) alone, unless you have an independent measure of rotational frequency.

One can get the frequency from one's observations, and the Sun's mass from observing the Sun -- those are enough to find the v in v*sin(i).

 

[steve_bank]

Broadcast TV has migrated to digital.

With either digital or analog TV transmission way off in the distance on a spectrum analyzer I;d say it wouldl all look like background noise. Especially digital like cell phones.

For a large antenna on Mars could TV and commercial radio be recieved?

 

[Shadowy Man]

The Earth's equator has a rotation velocity of 460 m/s, giving a Doppler shift of 1.5*10^-6. If one can detect the rotational Doppler shift between rising and setting, then one can find the inclination of the Earth's spin axis to the line of sight,

Typically in astronomy you get “v sin i” which means you cannot find the inclination (i) from the velocity (v) alone, unless you have an independent measure of rotational frequency.

One can get the frequency from one's observations, and the Sun's mass from observing the Sun -- those are enough to find the v in v*sin(i).

Sorry. I was assuming per your text that the measurement was simply the Doppler shift. If there are more observations you are referring to you didn’t mention those. And i don’t see how the Sun’s mass is playing in to the Earth rotational velocity measurement.

 

[Shadowy Man]

My point is that if the Earth’s axis were in the plane of the orbit and the ET were looking along that plane then there would be no Doppler shift regardless of the rotational velocity. As the axis were tipped upward you’d start to get some shift but the velocity and the inclination are not directly distinguishable in that measurement alone.

 

[lpetrich]

The Earth's equator has a rotation velocity of 460 m/s, giving a Doppler shift of 1.5*10^-6. If one can detect the rotational Doppler shift between rising and setting, then one can find the inclination of the Earth's spin axis to the line of sight,

Typically in astronomy you get “v sin i” which means you cannot find the inclination (i) from the velocity (v) alone, unless you have an independent measure of rotational frequency.

One can get the frequency from one's observations, and the Sun's mass from observing the Sun -- those are enough to find the v in v*sin(i).

Sorry. I was assuming per your text that the measurement was simply the Doppler shift. If there are more observations you are referring to you didn’t mention those. And i don’t see how the Sun’s mass is playing in to the Earth rotational velocity measurement.

Sorry, I got mixed up. I thought that you were talking about the Earth going around the Sun, not the Moon going around the Earth.

For the Moon going around the Earth, what one sees is

m/M * v * sin(i)

If one knows what inclination i is, from the Moon's orbit precession, or if one decides to use the Earth's orbit as an estimate, then one gets

m/M * v ~ m/M * sqrt(M/a)

The period one can observe, and it's sqrt(a³/M) Thus,

a ~ M^1/3
v ~ M^1/3
m/M * v ~ m / M^2/3

So one will get a sort of combined Earth-Moon mass estimate.

 

[Shadowy Man]

The Earth's equator has a rotation velocity of 460 m/s, giving a Doppler shift of 1.5*10^-6. If one can detect the rotational Doppler shift between rising and setting, then one can find the inclination of the Earth's spin axis to the line of sight,

Typically in astronomy you get “v sin i” which means you cannot find the inclination (i) from the velocity (v) alone, unless you have an independent measure of rotational frequency.

One can get the frequency from one's observations, and the Sun's mass from observing the Sun -- those are enough to find the v in v*sin(i).

Sorry. I was assuming per your text that the measurement was simply the Doppler shift. If there are more observations you are referring to you didn’t mention those. And i don’t see how the Sun’s mass is playing in to the Earth rotational velocity measurement.

Sorry, I got mixed up. I thought that you were talking about the Earth going around the Sun, not the Moon going around the Earth.

For the Moon going around the Earth, what one sees is

m/M * v * sin(i)

If one knows what inclination i is, from the Moon's orbit precession, or if one decides to use the Earth's orbit as an estimate, then one gets

m/M * v ~ m/M * sqrt(M/a)

The period one can observe, and it's sqrt(a³/M) Thus,

a ~ M^1/3
v ~ M^1/3
m/M * v ~ m / M^2/3

So one will get a sort of combined Earth-Moon mass estimate.

I wasn’t talking about either. I was talking about the Earth going around its axis. Is that not what the discussion was? Sorry.

You mentioned the Earths equatorial velocity and the Doppler shift resulting thereof

 

[Gospel]

I always find it challenging to follow when things get too technical. To simplify, are you two discussing extraterrestrials searching for us and the challenges they'd face in detecting our planet? Not that I'd ever grasp in detail what's being talk about but at least I can feel good about myself knowing I got the gist.

 

[Shadowy Man]

I always find it challenging to follow when things get too technical. To simplify, are you two discussing extraterrestrials searching for us and the challenges they'd face in detecting our planet? Not that I'd ever grasp in detail what's being talk about but at least I can feel good about myself knowing I got the gist.

There’s probably a few discussions going on but I’m interested in what an ET might be able to detect of Earth as seen from another planetary system. The earth would be a point source unresolvable on its own and possibly unresolved from the Sun.

If they observed Earth in the frequencies that we use for communications (like television) what would that look like?. Could they discern signal from noise?

 

[Gospel]

So I can feel good about myself then. Thanks

 

[Shadowy Man]

So I can feel good about myself then. Thanks

Yes. You got the gist. And if you ever have questions about the technical aspects I’m sure I could explain it in terms you could understand. Better in person with a white board but not impossible over the internet.

 

[Loren Pechtel]

What sort of stuff? Solaris? The point here isn‘t to assume that there are sentient worldwide oceans, though for all we know maybe there could be. It’s to point out that while aliens may be intelligent, their perceptual/cognitive structure may differ from ours radically, leading to entirely different maths. The maths that describe relativity and quantum physics would have been gobbledeygook to people even 150 years ago. What other physics may aliens know about that we don’t, such that their maths may be incomprehensible to us? We used to think that Euclidean geometry described the real world but it does not. Consider the example of intelligent aliens who do not construct or perceive circles, while perhaps perceiving and constructing other geometrical abstractions to which we are cognitively closed. Pi would mean nothing to them, and what they measure and describe would mean nothing to us. Are you so sure these are impossibilities? I am not. Maybe you think our sort of maths are necessary to build high technology, but I think that’s an assumption that may not be true.

While the person from 150 years ago isn't going to understand relativity it's irrelevant. Newtonian answers still work for anything they were doing 150 years ago and even now almost everything is Newtonian. Relativity and quantum mechanics peek around a few corners and actually influence some hidden things--gold is golden because of special relativity (the inner electrons move at a sufficient fraction of lightspeed to matter--by Newtonian math the color should be outside the visual spectrum and thus it would be a silvery metal like most things near it on the periodic table) but someone from 150 years ago wouldn't have a detailed enough knowledge about the atom to realize it was relativity at work.

 

[Loren Pechtel]

I keep constantly reading about how our TV programs from the fifties are already some 70 light years out, and therefore aliens within that range could right now be tuning into I Love Lucy and wondering what the hell Ricky and Lucy are on about, why they are wearing such funny constumes, and why they have two arms and two legs instead of good old 20 tentacles. Whenever I read this stuff I wonder what the writers are thinking (or drinking), because as you say the signal that far out would be far too weak to pick up except with a ginormous antenna as you describe.

Figuring it out would be very difficult but I would expect it to happen.

At first all the ETs would notice is the carrier signal--but that's enough to make them realize it's the product of intelligence. At that point they'll be building the equipment they need to get the whole signal.

Note that they inherently can't figure out color--nothing about the signal indicates what frequencies the image represents.

 

[Loren Pechtel]

In structural engineer, the shear, moment, angular rotation, and deflection are all intimately related, with the later being equal to the integration of the prior related equation. Calculus is very very real and very very physical. While numerical systems can vary, can Calculus? Two people invented generally the same thing, somewhat independently on this planet. Can science exist with an alternative to calculus? What would it look like?

Exactly. There's a lot of things where calculus simply provides the why for what we learned long before. Newton saw the pattern reality created, he didn't invent it. Any ETs we are actually able to communicate with will likewise have seen the pattern. What's hard about calculus is figuring out the formulas for doing it, the concept itself is one of these things that's obvious once you see it.

The trouble with alien signals is that we need to be intersecting the signal in a sense of space and time. A civilization from 50,000 years ago and 40,000 light years away, could have been broadcasting for 1000 years, up until the 1850s, and then stopped. One possible problem is intelligent races aren't communicating out there with others, because the limits on travel are quite real, and the inability of travel faster than light (for anything), makes the entire thing pointless. Even when signals reach another, it isn't actually possible to respond due to the distance involved. Sure, we could reply, but if they are thousands of light years away, we are less responding and more just saying "hi" to even more random strangers if they are still there. The reality is that intelligent life could be all over the universe, but spaced apart at such distances that tens of thousands, hundreds of thousands, millions of light years separate us.

As quoted in the sci-fi classic film Spaceballs, "Lightspeed is too slow."

I do agree we will never have a discussion over ranges like that. One civilization around one star could easily miss another civilization around another star. Will they stay around only one star, though?

 

[Loren Pechtel]

An octopus may use base 8 but it can tell one from two.

Sure, but then it would see nothing whatsoever of significance* in the number sequence 31415, and would not even recognise 9 as a digit at all.

Pi in base 8 starts 3.11037, which presents another problem; How do you send zero flashes? Assuming, of course, that you even have a concept of zero and of place dependent magnitude for digits?

What is Pi expressed in Roman numerals?

The ancient Romans were human, and highly skilled and advanced engineers**; But if they saw something flashing:
--- - ---- - ----- --------- -- ------
why would they think it was anything other than a random sequence of flashes?
III, I, IV, I, V, IX, II, VI, V, III, V, VIII, IX, VII, IX...
This is just a patternless bunch of small numbers.

If we can't even communicate with human beings from a couple of millennia ago, what hope is there for communication with other species, particularly ones from possibly very different environments?

A couple of millennia ago poses big problems because of their lack of science. However, such civilizations will not be able to detect things at interstellar range anyway, they're irrelevant. Nobody's pulling radio signals out of the sky without knowing Pi.

As for how to transmit zero flashes:

.***.*.*..***.***.*******.

If you truly want to work without a secondary symbol you simply leave the transmitter off for twice as long as your normal spacing to send a zero. In practice we do not because that causes errors. (In reality your hard drive is not capable of actually writing 8 bits of 0 or 8 bits of 1. All bytes are encoded into longer patterns that have no long repeated strings of 1 or 0.)