Detecting ET

[steve_bank]

A simple approximation to detecting ET. Good enough to do what if analysis. It can be done in a spreadsheet.

Assumptions

1. ET directional antenna like a dish with a beam divergence angle and no side lobes.
2. No losses in space.
3. Detection limit set by retriever input resistance noise, cooled to 77K.
4. Transmit and receive antennas stationary and and on the same axis.


The ET transmitter radiates into a cone or solid angle. By the time the wavefront searches the Earth it is essentially a flat plane. The power density in the base of the cone is the transmit power per unit area of the base in the pane of the Earth.

Closest galaxy Canis Major 20,000 LY
Closest star Proxima Centauri 4.64 LY

From the net one of the most powerful surface transmitters is 2MW. The main lobes would not be directed into space.

Calculate the base o a cone with the peak at the ET transmitter and assume tx power is distributed across the wavefront as it it expands.

An SNR > 0db is considered detectable fo r the approximation..

You can vary TX power, TX beam divergence, distance, reciever bandwidth, and RX antenna area.

Scliab script

clear
r_earth = 637800/2 //meters

rx_antenna_radius = 150 // meters Arecibo
rx_antenna_area = %pi*(rx_antenna_radius^2) //m^2
r_reciever = 50 //recieer input resistance

light_year = 9.461e15 // meters
et_distance_ly = 10
et_distance_m = et_distance_ly * light_year // meters


et_tx_power = 2e6 // watts higer power earth trasmitters
tx_angle_d = 1 // tx beam diverhence degrees half angle
tx_angle_r = (%pi/2) * (tx_angle_d/90) // radians

Tk = 77 // temperature deg kelvin liquid nitrogen
bw =10000 // reciever bandwidth hertz
k = 1.38*(10^-23) // boltzman constant
noise_power = 4*k*Tk* r_reciever*k*bw // watts rx input restance noise

r_earth_plane = tan(tx_angle_r)*et_distance_m //radius earth plane
a_earth_plane = %pi* (r_earth_plane^2) // cone base area in earth plane
p_density = et_tx_power/a_earth_plane // power desity earth plane
rx_power = p_density * rx_antenna_area

mprintf("Cone Diamter KM %e\n",2*r_earth_plane/1000)
mprintf("Cone Area %e\n",a_earth_plane)
mprintf("Earth Diameters %f\n",r_earth_plane/r_earth)
mprintf("Power Density %e\n",p_density)
mprintf("RX Power %e\n",rx_power)

snr_db = 10*log10(rx_power/noise_power) // signal to noise ratio
mprintf("Noise Power %e\n",noise_power)
mprintf("SNR db %4.2f\n",snr_db)

 

[Loren Pechtel]

That's assuming you're trying to send a signal.

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

 

[Jimmy Higgins]

That's assuming you're trying to send a signal.

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

The more advanced, the more efficient. I think one of the unresolved (and potentially underlooked) issue about detecting intelligent life is how long of a period of a time is a civilization detectable?

 

[lpetrich]

A circular aperture makes an  Airy disk diffraction pattern. The math involves something called a Bessel function, so one can calculate how much it fades off.

 

[Loren Pechtel]

That's assuming you're trying to send a signal.

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

The more advanced, the more efficient. I think one of the unresolved (and potentially underlooked) issue about detecting intelligent life is how long of a period of a time is a civilization detectable?

Once a civilization goes interstellar why should there be a limit on how long it's detectable? Note that what I'm describing about a K2 simply assumes they have to follow the laws of physics, but says nothing about how they do it.

 

[barbos]

That's assuming you're trying to send a signal.

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

The more advanced, the more efficient. I think one of the unresolved (and potentially underlooked) issue about detecting intelligent life is how long of a period of a time is a civilization detectable?

More efficient at what? Energy conserves. You have to re-radiate all the energy you get from the star one way or another.

 

[atrib]

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

And if the civilization is part of a star system that barely occupies a couple of pixels on our sensors, then we won't see anything. I don't think you comprehend the vast distances involved.

 

[atrib]

Once a civilization goes interstellar why should there be a limit on how long it's detectable?

Assuming practical interstellar travel is even possible. Which is not a sure thing.

 

[Elixir]

Once a civilization goes interstellar why should there be a limit on how long it's detectable?

Assuming practical interstellar travel is even possible. Which is not a sure thing.

If you have hundreds of millions, or billions of years to get "there" it's doable. If so called technological civilizations (an oxymoron imo) are/have been abundant in the universe (FAPP universe means means galaxy here), our best chance to detect them might be by visiting Kuyper belt or Oort cloud objects. Especially if intelligence is a lethal mutation, and they regularly blow themselves to smithereens.
Right now, meteorites and comets might be the only accessible telltales.

 

[barbos]

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

And if the civilization is part of a star system that barely occupies a couple of pixels on our sensors, then we won't see anything. I don't think you comprehend the vast distances involved.

He is talking about Dyson Sphere. Complete structure has to be large.
For our Sun it would have 330mil km radius (twice the distance to the Sun).
And it does not really matter. Becauase it can be detected with relative ease even if it just a one pixel - it is very bright black body with temperature around 300-400K.

 

[Loren Pechtel]

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

And if the civilization is part of a star system that barely occupies a couple of pixels on our sensors, then we won't see anything. I don't think you comprehend the vast distances involved.

A K2 will appear as a huge (as in total energy emission, it's still going to be one pixel), warm object. There's nothing remotely like it in the star charts.

Note how much of the brightness/temperature diagram is totally empty? A K2 will be somewhere around the 1 on the Y axis, but well off the right side of this diagram--I'm having no luck finding a formula to figure how far off. Find anything out there and the astronomy community is going to go nuts.

 

[Loren Pechtel]

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

And if the civilization is part of a star system that barely occupies a couple of pixels on our sensors, then we won't see anything. I don't think you comprehend the vast distances involved.

He is talking about Dyson Sphere. Complete structure has to be large.
For our Sun it would have 330mil km radius (twice the distance to the Sun).
And it does not really matter. Becauase it can be detected with relative ease even if it just a one pixel - it is very bright black body with temperature around 300-400K.

I question whether a Dyson Sphere can actually be constructed and I'm trying to avoid putting limits on a K2. However, it need not be solid, a bunch of moving objects could accomplish basically the same thing without requiring the super materials needed to make a solid object. You have the basic idea of what I'm talking about but your math is wrong--you're doubling to get a diameter, not a radius. And the 330Mkm diameter body would have a temperature of about 280K, you're looking at the greenhouse effect which will not be relevant. (For those of you not following our math: Earth blackbody temperature for an object Earth orbit across.)

 

[barbos]

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

And if the civilization is part of a star system that barely occupies a couple of pixels on our sensors, then we won't see anything. I don't think you comprehend the vast distances involved.

He is talking about Dyson Sphere. Complete structure has to be large.
For our Sun it would have 330mil km radius (twice the distance to the Sun).
And it does not really matter. Becauase it can be detected with relative ease even if it just a one pixel - it is very bright black body with temperature around 300-400K.

I question whether a Dyson Sphere can actually be constructed and I'm trying to avoid putting limits on a K2. However, it need not be solid, a bunch of moving objects could accomplish basically the same thing without requiring the super materials needed to make a solid object. You have the basic idea of what I'm talking about but your math is wrong--you're doubling to get a diameter, not a radius. And the 330Mkm diameter body would have a temperature of about 280K, you're looking at the greenhouse effect which will not be relevant. (For those of you not following our math: Earth blackbody temperature for an object Earth orbit across.)

Dude, I have PhD in physics, my math and all the rest is right.

 

[bilby]

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

And if the civilization is part of a star system that barely occupies a couple of pixels on our sensors, then we won't see anything. I don't think you comprehend the vast distances involved.

He is talking about Dyson Sphere. Complete structure has to be large.
For our Sun it would have 330mil km radius (twice the distance to the Sun).
And it does not really matter. Becauase it can be detected with relative ease even if it just a one pixel - it is very bright black body with temperature around 300-400K.

I question whether a Dyson Sphere can actually be constructed and I'm trying to avoid putting limits on a K2. However, it need not be solid, a bunch of moving objects could accomplish basically the same thing without requiring the super materials needed to make a solid object. You have the basic idea of what I'm talking about but your math is wrong--you're doubling to get a diameter, not a radius. And the 330Mkm diameter body would have a temperature of about 280K, you're looking at the greenhouse effect which will not be relevant. (For those of you not following our math: Earth blackbody temperature for an object Earth orbit across.)

Dude, I have PhD in physics, my math and all the rest is right.

Dude, from our perspective you're an Internet random who claims to have a qualification. Which is something that literally anyone could claim.

If you want people to agree with you, you need to demonstrate that you're right, not just get all huffy about being questioned, or throw a "don't you know how incredibly brilliant I am" out there as though it could possibly impress your audience.

A smart person would at the very least understand that your response here is utterly futile - even if it's true, it isn't going to be believed, and even if it's true, it wouldn't render you immune from errors.

Smart people sometimes make mistakes. Dumb people often claim to be both smart and incapable of error, on anonymous forums.

 

[barbos]

If you want people to agree with you, you need to demonstrate that you're right,

I need to demonstrate it?
It was LP who made the claim, not I.

 

[Loren Pechtel]

Instead, look for their actions. A K2 civilization will be catching the energy of their star. It will get reradiated as waste heat at whatever the temperature of their radiators is. Nothing in cosmology will look even remotely like this. Brightness + temperature gives you size and that will be off the charts. Yes, absolute brightness will be hard to determine but if you can get the range to even a few orders of magnitude there won't be any natural explanation.

And if the civilization is part of a star system that barely occupies a couple of pixels on our sensors, then we won't see anything. I don't think you comprehend the vast distances involved.

He is talking about Dyson Sphere. Complete structure has to be large.
For our Sun it would have 330mil km radius (twice the distance to the Sun).
And it does not really matter. Becauase it can be detected with relative ease even if it just a one pixel - it is very bright black body with temperature around 300-400K.

I question whether a Dyson Sphere can actually be constructed and I'm trying to avoid putting limits on a K2. However, it need not be solid, a bunch of moving objects could accomplish basically the same thing without requiring the super materials needed to make a solid object. You have the basic idea of what I'm talking about but your math is wrong--you're doubling to get a diameter, not a radius. And the 330Mkm diameter body would have a temperature of about 280K, you're looking at the greenhouse effect which will not be relevant. (For those of you not following our math: Earth blackbody temperature for an object Earth orbit across.)

Dude, I have PhD in physics, my math and all the rest is right.

This isn't a matter of having a PhD, I'm agreeing with your basic science here. I'm saying you made two oopses:

1) You're looking at Earth's orbit--but you double it and then call it a radius. No, that's a diameter. If you actually use that as a radius you're going to get an even cooler object.

2) You used Earth's temperature, but the greenhouse effect raises it. A blackbody at Earth's orbit (the best possible radiator for that size) is 280K.

 

[Loren Pechtel]

If you want people to agree with you, you need to demonstrate that you're right,

I need to demonstrate it?
It was LP who made the claim, not I.

You asserted certain numbers. I can see where those numbers come from and I agree with your thought process, I'm just saying you made a couple of oopses in coming up with the final answer.

 

[lpetrich]

Interstellar travel is VERY difficult. To see why, let us look at Konstantin Tsiolkovsky's rocket equation:

\( \displaystyle{ \Delta v = v_e \log \frac{m_i}{m_f} } \)

\( \displaystyle{ \frac{m_i}{m_f} = e^{(\Delta v)/(v_e)} } \)

Where Dv is the velocity change, ve is the effective exhaust velocity: dp/dm (momentum change / mass change), and mi and mf are the rocket's initial and final masses, including its propellant(s).

The exhaust velocity is often given as a specific impulse, Isp: ve = Isp * g where g is the acceleration of the Earth's gravity at its surface, with the reference value used here of 9.81 m/s^2.

For some energy E applied to some mass m, the kinetic-energy equation gives us

\( \displaystyle{ v = \sqrt{ \frac{E}{m} } } \)

So one needs as much energy per unit mass as possible. The best chemical rocket fuel would be hydrogen and fluorine, but fluorine is (1) expensive, (2) toxic, and (3) corrosive. So the best one in practice is hydrogen and oxygen.

From  Comparison of orbital rocket engines the champion is H2-O2, with the best that was flown being the Aerojet Rockedyne RL-10, at 465.5 seconds or 4.565 kilometers per second. However, that is very cryogenic, since H2 has a boiling point of 20 K and oxygen 90 K (melting point of water is 273 K).

The best less-cryogenic one is kerosene-O2 (RD-0124), at 359 s or 3.52 km/s, and the best non-cryogenic one is nitrogen-tetroxide-monomethylhydrazine (Aestus) at 324 s or 3.18 km/s. The best solid-fuel one (Zefiro 9A) is at 295.2 s or 2.90 km/s.

So one is *not* going to go very fast with such rocket engines.

 

[steve_bank]

Energy and energy density is the limiting factor in space travel.

Unless there is fundamental science we can find and have not yet found then I'd say tripping around the galaxy is unlikely.

Same with a Dyson Sphere.

The total mass of the structure has to be moved in space and positioned.

At a radius you can estimate the mass of a thin shell of steel. Say 1 meter.

Each kg of finished steel has to accelerated and then decelerated where it will be placed. Energy = work = force * distance and force = mass * acceleration(dv/dt).

There is also the mechanical stability of such a large sphere. A small asteroid hits the sphere and it can resonate. A wave can porpagate around the sphere.

 

[lpetrich]

Spacecraft escaping the Solar System

The five spacecraft leaving our Solar System have these terminal velocities, velocities in the interstellar limit. Pioneer 10: 11.3 km/s, Pioneer 11:10.4 km/s, Voyager 2: 14.8 km/s, Voyager 1: 16.6 km/s, New Horizons: 12.5 km/s.

Let's now look at where they will be going.

lecture11.pdf
The Sun's motion relative to the "local standard of rest", the average motion of the stars near the Sun. In galactic coordinates,

• U = -10 km/s - inward
• V = 5 km/s - faster in its orbit
• W = 7 km/s - northward

By spectral type, velocity dispersions (km/s), scale heights (parsecs: pc) out of the galactic plane:

• B -- 6 -- 60
• A -- 9 -- 120
• gK -- 17 -- 270
• dM -- 18 -- 350
• WD -- 25 -- 500

g = giant, d = dwarf, WD = white dwarf

So to have a good chance of reaching most of the nearby stars, one would have to depart from the Solar System at 30 km/s at least.