60 years of silence - so far

[steve_bank]

One limit or reference point is the thermal noise in a 50 ohm resistor which is a typical reciever input impedance. The input impedance of the antenna and receiver are in parallel so the noise resistance is half.

The receiver bandwidth determines the noise power which for the resistor is is generally flat spectrum. Receiver input stages are commonly cooled to reduce thermal noise.

The ARRL has good basic books on receiver design and signal to noise ratio.

There are techniques for picking out sinals below noise. A simple way is averging. The avergae of the noise tends to zero.

Haven't looked at the PDF

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Thermal Noise in Electronic Circuits Explained »Electronics Notes

Understand how thermal noise occurs in electronic circuits, how it is related to resistance and how to calculate it for various resistor values and temperatures.

www.electronics-notes.com

ohm--5f4da8e54a13b90d091ef33d

Johnson–Nyquist noise - Wikipedia

en.wikipedia.org

Noise figure - Wikipedia

en.wikipedia.org

 

[Loren Pechtel]

Unless they can throw out physics

Didn't Einstein do that?
Threw out Newtonian Physics because the best information available in Sir Isaac's day was a bit primitive.
Tom

Almost everything we do is still Newtonian in nature. Relativity intrudes into our daily lives in only one area: GPS. You'll get pretty lost if you try to build a GPS that uses Newtonian math.

 

[Loren Pechtel]

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

You always do. It's just that in almost all situations the mass of the orbiting body is less than the errors in your measurement and thus is irrelevant.

 

[Loren Pechtel]

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.

They're discussing what information about Earth could be derived from examining the doppler shift of the signal. While it's not my domain I can follow the discussion but not actually verify what they're saying.

I do wonder how accurate the transmitters are at staying on frequency, though--how much doppler information can they actually get?

 

[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.

They're discussing what information about Earth could be derived from examining the doppler shift of the signal. While it's not my domain I can follow the discussion but not actually verify what they're saying.

I do wonder how accurate the transmitters are at staying on frequency, though--how much doppler information can they actually get?

I’m not a radio astronomer so I’m a bit out of my lane. it’s stuff that I’ve thought about recreationally rather than professionally. so I would have to do some research and learning to get a better feel for what’s happening at these frequencies.

 

[bilby]

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.

And you are confident that an entirely alien race, with whom you have had no contact whatsoever, and with whom you cannot discuss a suitable code or set of communications protocols, will simply understand this?

Bear in mind, you are still sending a random string of small numbers, with no good reason to assume that the aliens understand the concept of place representing magnitude, no way to be confident that you are using a number base that they are familiar with, and no particular reason to expect them to consider Pi to be significant or important, even if they do work out that you are sending a mathematical constant. You are also assuming that the (absent) decimal point will be inferred by them, despite all of these obstacles to understanding.

If they miss the first few digits, what are the odds of them being able to determine that what they're seeing isn't completely random? (Trick question - as these are a subset of the digits of an irrational number starting from an indeterminate point in the infinite sequence, what they are seeing in that scenario IS completely random).

 

[DBT]

Just to confirm that there are other civilizations out there is no small thing.

 

[TomC]

Unless they can throw out physics

Didn't Einstein do that?
Threw out Newtonian Physics because the best information available in Sir Isaac's day was a bit primitive.
Tom

Almost everything we do is still Newtonian in nature. Relativity intrudes into our daily lives in only one area: GPS. You'll get pretty lost if you try to build a GPS that uses Newtonian math.

Believe it or not, I'm capable of snarkiness.

Honestly, I live in a flat earth creation for all practical purposes.
I don't really think about the reality most days.
Tom

 

[steve_bank]

Maybe we need a new forum, 'The Snark Tank'.

 

[Elixir]

Grasping organs evolved many times in our planet's biota, and what makes them grasping organs is the ability to press on something from opposite sides.

Even insects get in on it.

 

[lpetrich]

The other thing I don’t hear people mentioning (so maybe I’m just wrong about this) is that there’d be multiple signals from different parts of the Earth that get mashed together on the same frequencies so it would turn to noise unless they were all transmitting the same signal. And even if they were they’d not be all in phase. I’d even have to calculate whether Doppler shifts from a rotating source could smear the signal (maybe it wouldn’t significantly but in principle it could). Any ET viewing Earth’s signals would see a point source and thus the total accumulated signal.

That's a problem for full signals, but it's not a problem for carrier waves -- carrier waves have *very* low bandwidth.

I'll now calculate whether the Earth's rotation would smear the carrier signal very much.

As I'd calculated earlier, it will make a Doppler shift of about 10^-6. The Earth's rotation has an angular velocity of about 7.3*10^-5 s^-1 Integrating over the reciprocal of the carrier-signal bandwidth, 10 seconds, gives about 10^-3, and a drift of 10^-9, about the bandwidth itself.

But it should be easy to correct for rotational Doppler shift by adjusting one's frequency reference to follow it.

 

[lpetrich]

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

Mars is typically 1.5 AU away, so it should not be difficult. One may need a big radiotelescope, but one will be able to receive the full signal, and not just the carrier waves.

AU = "astronomical units", the average Earth-Sun distance.

 

[lpetrich]

Unless they can throw out physics

Didn't Einstein do that?
Threw out Newtonian Physics because the best information available in Sir Isaac's day was a bit primitive.
Tom

Now he didn't. Newtonian physics is a low-velocity approximation of Einsteinian physics. Here is how much it is erroneous:

~ (1/2) * (v/c)²

v = typical speed
c = speed of light in a vacuum

• Human walking speed: 1.42 metres per second (5.1 km/h; 3.2 mph; 4.7 ft/s)  Preferred walking speed -- 1.1*10^-17
• Car on highway: 100 km/h (28 m/s, 60 mph) -- 4.4*10^-15
• Airliner cruising speed: 900 km/h (250 m/s, 560 mph) -- 3.5*10^-13
• Low Earth orbit satellite speed: 7.8 km/s (28,000 km/h, 17,000 mph) -- 3.4*10^-10

These are all very small, but the satellite-speed one is large enough for GPS satellites to have to take it into account. Not only that, but also differences in gravitational potential, and that's why their clocks are run a teeny tiny bit slower than their Earth-based counterparts.

 

[steve_bank]

Bandwidth on the rciever, like a spectrum analyzer, determines how well you can resolve two frequencies close to each other.

Bandwidth also affects sensitivity. Narrow bandwidth reduces noise power and raises signal to noise ratio. But narrower bandwidths increase the time it takes to scan a frequency spectrum.

So if you are scanning for an ET signal there is a tradeoff between sensivity and time to scan a set of frequencies.

Convert red shift wavelengths for distant stars to frequency and you will get an idea of the signal frequency shift.

When scanning for ET you would not be looking for a single narrow band signal you woud be sweeping a receiver.

If you are interested look up heterodyne receiver.

Heterodyne - Wikipedia

en.wikipedia.org

Scroll down to the block diagram, bandwidth is set by the Intermediate Frequency(IF) filter.

Heterodyne Receivers and its Components - Rahsoft

Heterodyne Receiver In a heterodyne receiver the received signal from an antenna is down converted from RF signal to intermediate frequency. Win is not equal to WLO. The basic structure of heterodyne receiver is shown below. There is an antenna …

rahsoft.com

Doppler shift is used in Doppler radar to find the relative speed of a moving target. The Doppler sht of a radar signal bouncing off a commecial jet speed is detectable.

 

[lpetrich]

Grasping organs evolved many times in our planet's biota, and what makes them grasping organs is the ability to press on something from opposite sides.

Even insects get in on it.

Yes, mantises use their front limbs for grasping their prey, one limb on each side.

But ants use their jaws to carry stuff, much like dogs.

 

[Loren Pechtel]

• Low Earth orbit satellite speed: 7.8 km/s (28,000 km/h, 17,000 mph) -- 3.4*10^-10

These are all very small, but the satellite-speed one is large enough for GPS satellites to have to take it into account. Not only that, but also differences in gravitational potential, and that's why their clocks are run a teeny tiny bit slower than their Earth-based counterparts.

The GPS birds are pretty far out there,, they're only moving 3.9 km/s.

The main reason we care about Einstein is that GPS requires phenomenal accuracy. Your time fix (for standard navigation use) needs to be accurate to a few dozen nanoseconds and some of the specialized uses require sub-nanosecond accuracy.

 

[steve_bank]

An historical note, LORAN which was replaced by GPS. Gives a little insight into GPS and timing.

LORAN - Wikipedia

en.wikipedia.org

 

[lpetrich]

This issue is a case of the  Fermi paradox - "Where are they?" - and I've seen numerous solutions. These can be divided into categories:

• We are the first or close to the first in our Galaxy
• Advanced ET's are rare
• Advanced ET's know about us, but they have decided not to contact us - the zoo hypothesis

Here, "advanced" means capable of interstellar communication and/or spaceflight

We can use the  Drake equation to assess the likelihood of the first two possibilities.

N = Rs * fp * ne * fl * fi * fc * L

where

• N = number of communicative civilizations in our Galaxy: us and ET's
• Rs = rate of star formation in our Galaxy
• fp = fraction of stars with planets
• ne = number of Earthlike planets per planetary system
• fl = fraction of Earthlike planets where life emerges
• fi = fraction of biotas where intelligence emerges
• fc = fraction of intelligent species that develop interstellar communication
• L = average lifetime of a communicative civilization

This also applies for other locales and for interstellar travel.

The first Fermi-paradox resolution does not constrain N very much, though if N is potentially large, it means that we are very unlucky.

The second one requires a small N, maybe much less than 1 but with us being very lucky.

The third one does not constrain N very much, though it does require some discipline among our zookeepers.

 

[lpetrich]

When Frank Drake proposed his equation back in 1961, we only had a good handle on Rs. But the numerous discoveries of exoplanets strongly suggest that fp is near 1, though ne is still far from well-constrained. That's because the easiest-to-observe exoplanets are weird ones by Solar-System standards, like Jovian planets where Earthlike planets ought to be, and planets very close to their stars.

We find some variations of planet populations by star mass, suggesting that we must sum over Rs*fp*ne for different ranges of star masses. That is likely also true of being in a multiple-star system -- such systems likely clear out planets at distances comparable to the stars' separations, though they allow planets relatively close to each star and relatively far from both stars, by roughly a factor of 3 in each case.

An Earthlike planet ought to be at a distance where it can have liquid water on its surface: not too close and not too far. Too close and the water boils, too far and the water freezes.

It ought to be large enough to retain an atmosphere but not large enough to hold onto hydrogen and helium, because doing so would give it a very thick atmosphere. The boundary between the two types is roughly at 1.6 to 2 Earth radii -  Super-Earth -  Mini-Neptune

It ought to have enough water to cover much of its surface, but not enough to make a planetary super ocean.

Being larger is good for having more surface area, but is not good for growing very large. That is because organisms' sizes are limited by factors inversely proportional to the surface gravity.

I once saw a diagram of how high a kangaroo rat and a horse can jump -- the same absolute height, roughly a meter, but much larger relative to the rat than to the horse. Looking even smaller, I checked on high a grasshopper can jump, and it's roughly the same height.

What makes them so similar? To reach a height h, one must get a kinetic energy equal to m*g*h for acceleration of gravity g and mass m. The amount of energy a muscle can supply is proportional to its mass, the proportionality coming from its biomechanics: E = k*m. Equating the two gives

k*m = m*g*h

and canceling out gives h = k/g -- the same from a grasshopper to a horse.

One finds a similar limit on the height of a tree, because the tree's roots must push their water up to the top of the tree. They must supply a pressure equal to den*g*h for water density den. For pressure P one finds h = P/(den*g).

So the Earth could be the best-sized Earthlike world, large enough for a lot of area, but small enough to allow organisms to grow very large.

 

[lpetrich]

That aside, I now get into fl - the fraction of Earthlike worlds where an organism gets started.

This is still an unsolved problem, but there has been plenty of research into it from both directions.

In prebiotic-chemistry experiments one can make a variety of organism building blocks, though some of them continue to be difficult. It's hard to go much farther than that, however.

Research into the evolution of life has been greatly helped by protein sequencing and then gene sequencing. We now have a good idea of the phylogeny of all well-studied cellular organisms, and even a good idea of the last universal common ancestor. It was much like present-day methanogens, getting its energy from inorganic compounds, making all its biological molecules, much like a plant, and neither making nor using oxygen.

But this is a full-scale cellular organism, and it likely had a lot of evolution behind it. A phase in that evolution is likely the RNA world, where RNA served as both information carrier and as enzyme: "ribozyme". DNA originated as a modification of RNA, and proteins were originally ribozyme cofactors that became the entire enzyme in many cases.

The main criticism of the RNA world that I've seen is that it's hard to make RNA prebiotically. One can make the nucleobases, but not the ribose. So I've seen speculations about ribose alternatives like amino acids, an alternative which would make peptide nucleic acids.

Though one can make all PNA components prebiotically, it's not clear why one might want to go from PNA's to RNA's.