The Fermi Paradox: where are they?

[Loren Pechtel]

If every colonized planet sends out colony spaceships to its uncolonized neighbors, then the colonized region will grow at a steady rate:
(neighbor distance) / (colonization time)

where (colonization time) = (travel time) + (colony-development time) where the latter is the time between arrival of a colony ship and the development of the colony to the point where it can send out colony ships.

From stars' velocities relative to their local average, one would need at least 50 km/s, and that is 50 parsecs per million years, or 1 parsec per 20,000 years.

The nearest stars are a few parsecs away:  List of nearest stars and brown dwarfs (56 stars, all closer than 5 parsecs),  List of nearest bright stars (out to 15 parsecs). So I'll use an average travel distance of about 10 parsecs. That means traveling for about 200,000 years.

This time seems like plenty of time to found a colony and build it up to the point where it can easily send out more colony ships. So I'll take that time as much less.

Thus, there is a wave of advance of colonization at about 50 km/s or 50 parsecs per million years. The distance to our Galaxy's center is about 8 kiloparsecs, and its radius about 15 - 25 kpc. So it would take about 160 million years to spread to the center, and about 500 - 700 million years to spread to the farthest outer rim.

This is still younger than our Galaxy.

I can't see why you are using stellar velocities as how fast a civilization can spread. Fusion Orion can do a lot better than that and almost certainly will work.

Eventually enough people who want to push the horizons will get together and head for a nearby star and colonize it. So long as there is adequate matter around the star it doesn't need planets. The progress will be very slow, but that doesn't matter. Lets say it takes a thousand years of civilization before such a group forms. They spend 500 years in flight (a reasonable value for the closest stars.) That's 5ly in 1500y, or 1/300th of lightspeed. X the size of the galaxy means they'll have crossed it in 30My. Lets figure they didn't go straight and double that, still only 60My. That's not even 1% of the age of the universe.

Now, they might be deliberately choosing to avoid stars that have the potential for indigenous life but we should still see evidence of them about other stars. They don't need to be actively seeking communication for us to notice them.

All I can conclude is that we are alone in this galaxy...

You appear to be assuming that building a starship will eventually be so cheap that a small group of disaffected people looking to start over without their government will be able to afford to pay for one, as if they were Voortrekkers building covered wagons. If we assume instead that a starship is always going to be a massive undertaking that requires the commitment of a national or planetary government, then the calculation needs to take into account whether the government has a good reason for deploying a large fraction of its available resources for many years. It seems to me the only nation-scale problem for which a starship is likely to become a practical solution is extinction insurance. But once a species has expanded to a half a dozen planets spread over ten or twenty light-years, it will as you say be very hard for intelligence to do itself in. Natural disasters, likewise. So what motive is there for further expansion, that would be convincing to rational politicians?

Not that every group of disaffected people can choose that option, but that there will be some that can. Given enough time I see no reason to think this wouldn't be the case. The price of basically everything other than labor and rare materials drops over time. Given enough time it should get down into the realm of the wealthy.

Look at Earth--50 years ago rocketery was in the realm of governments, individuals had no hope of trying. In today's world the brightest star is SpaceX--basically one man's dream. The government will probably never have another manned rocket, space will join the atmosphere as the realm of private enterprise.

And I don't think politicians will colonize the stars for species survival. It wouldn't help their survival.

 

[lpetrich]

I can't see why you are using stellar velocities as how fast a civilization can spread. Fusion Orion can do a lot better than that and almost certainly will work.

I was mentioning 50 km/s as a *lower* limit to how fast one can travel from star to star. With nuclear fusion, the best case is an exhaust velocity of around 0.1 c or 30,000 km/s.

Here's where the star-motion problem comes in.

Let's see that we want to reach a star whose coordinates are {r + vr*t, vt*t} at time t. You want to hit it with a rocket with position v*t*{cos(a),sin(a)}, for speed v and target-leading angle a. So one finds which values of a and t make them equal.

First, a. sin(a) = vt/v.

This means that v > vt, the transverse velocity, or else one will not reach the star.

Substituting in yields for t

t = r/(sqrt(v² - vt²) - vr)

So to hit, vr < 0 or else v > sqrt(vr² + vt²).

 

[Loren Pechtel]

I can't see why you are using stellar velocities as how fast a civilization can spread. Fusion Orion can do a lot better than that and almost certainly will work.

I was mentioning 50 km/s as a *lower* limit to how fast one can travel from star to star. With nuclear fusion, the best case is an exhaust velocity of around 0.1 c or 30,000 km/s.

Here's where the star-motion problem comes in.

Let's see that we want to reach a star whose coordinates are {r + vr*t, vt*t} at time t. You want to hit it with a rocket with position v*t*{cos(a),sin(a)}, for speed v and target-leading angle a. So one finds which values of a and t make them equal.

First, a. sin(a) = vt/v.

This means that v > vt, the transverse velocity, or else one will not reach the star.

Substituting in yields for t

t = r/(sqrt(v² - vt²) - vr)

So to hit, vr < 0 or else v > sqrt(vr² + vt²).

You could launch to a star heading towards you. Thus this is not a minimum speed.

 

[lpetrich]

You could launch to a star heading towards you. Thus this is not a minimum speed.

Its sideways velocity still has to be lower than one's spaceship's cruising speed. So that won't help very much.

Since we want to arrive at our target star, we would then need more delta-V at the star than in the Solar System.

Here is the delta-V budget:
Departing from the Solar System: v
Arriving at the target star: v - vr
Their sum: 2v - vr

So if we have vtot available, then v = (vtot + vr)/2 and (v - vr) = (vtot - vr)/2.

So to arrive, vtot must be greater than |vr|. So a large radial velocity toward us can be a problem.

 

[Speakpigeon]

You could launch to a star heading towards you. Thus this is not a minimum speed.

If the star is going fast it will take less time for your spaceship to get within the confines of the star but then the star shouldn't go so fast that your spaceship wouldn't be able to get into any orbit around the star.
EB

 

[Speakpigeon]

The lack of overt and widely-agreed-on evidence of extraterrestrial intelligent entities is a serious problem, when one considers how large and old our Universe is. This is the Fermi Paradox, after physicist Enrico Fermi allegedly once asking "Where is everybody?" or "Where are they?"

I've come across numerous proposed solutions, like what's listed in  Fermi Paradox, The Fermi Paradox , Expanded | Second Nexus, Possible Answers to the Fermi Paradox, and 11 of the Weirdest Solutions to the Fermi Paradox.

It might be that we are the only technological civilization in all of the Universe. We could be the first to emerge, or else our emergence is an extremely lucky accident that will never be repeated.

That can be the case if it takes too many lucky accidents to produce a civilization capable of interstellar communication or spaceflight. Drake's equation is a simplified version of what is necessary. Here is my estimate: Earthlike planets in dynamically stable orbits around Sunlike stars. Origin of life. Autotrophic metabolism, making all one's biological molecules from inorganic precursors. Photosynthesis. Oxygen release. Multicellularity, both plantlike and animallike. Living on land, both plants and animals. Social groups. Language. Manipulation and tool making. Sentience. Nerdiness, for lack of a better word. Agriculture. Abstract science. Industrialization. Radio. Computers. Avoiding self-destruction.

Another sort of lucky accident would be surviving some big natural disaster like a big asteroid impact or a nearby supernova. If such disasters are common, that could make it difficult to evolve much complexity.

This argument would also work if there are many ET civilizations, but they are all too far away for feasible communication or travel, like intergalactic distances.


If there are lots of ET civilizations, there are plenty of additional possibilities.

• Some of them like to destroy any others that they find, because they are competition or whatever.
• They want to hide from other civilizations.
• They do not have any interest in interstellar communication or travel.
• They are using some advanced technology that we do not know about.
• We are unable to recognize their messages or their artifacts.
• They broadcast only briefly. This can happen from their technology advancing.
• Everybody is listening, and nobody is transmitting.
• We have not searched enough, or with enough sensitivity.
• The Zoo Hypothesis: they know about us, but they avoid letting us know about them.
• "They are made of meat" -- incredulity at our nature.
• They prefer to live away from us, like in the outer parts of our Galaxy to avoid overheating.
• The Simulation Hypothesis: it includes no ET's.

Etc.

A lot to argue over, I'm sure.

They could also be two small to notice.

They may have no use for electromagnetic communication.

Maybe they don't need to communicate at all.

I guess my meal is ready.

Bye.
EB

 

[Loren Pechtel]

You could launch to a star heading towards you. Thus this is not a minimum speed.

Its sideways velocity still has to be lower than one's spaceship's cruising speed. So that won't help very much.

Since we want to arrive at our target star, we would then need more delta-V at the star than in the Solar System.

Here is the delta-V budget:
Departing from the Solar System: v
Arriving at the target star: v - vr
Their sum: 2v - vr

So if we have vtot available, then v = (vtot + vr)/2 and (v - vr) = (vtot - vr)/2.

So to arrive, vtot must be greater than |vr|. So a large radial velocity toward us can be a problem.

At such speeds you don't need to bring enough fuel to stop. Aerobrake at a gas giant.

 

[T.G.G. Moogly]

The lack of overt and widely-agreed-on evidence of extraterrestrial intelligent entities is a serious problem, when one considers how large and old our Universe is. This is the Fermi Paradox, after physicist Enrico Fermi allegedly once asking "Where is everybody?" or "Where are they?"

I've come across numerous proposed solutions, like what's listed in  Fermi Paradox, The Fermi Paradox , Expanded | Second Nexus, Possible Answers to the Fermi Paradox, and 11 of the Weirdest Solutions to the Fermi Paradox.

It might be that we are the only technological civilization in all of the Universe. We could be the first to emerge, or else our emergence is an extremely lucky accident that will never be repeated.

That can be the case if it takes too many lucky accidents to produce a civilization capable of interstellar communication or spaceflight. Drake's equation is a simplified version of what is necessary. Here is my estimate: Earthlike planets in dynamically stable orbits around Sunlike stars. Origin of life. Autotrophic metabolism, making all one's biological molecules from inorganic precursors. Photosynthesis. Oxygen release. Multicellularity, both plantlike and animallike. Living on land, both plants and animals. Social groups. Language. Manipulation and tool making. Sentience. Nerdiness, for lack of a better word. Agriculture. Abstract science. Industrialization. Radio. Computers. Avoiding self-destruction.

Another sort of lucky accident would be surviving some big natural disaster like a big asteroid impact or a nearby supernova. If such disasters are common, that could make it difficult to evolve much complexity.

This argument would also work if there are many ET civilizations, but they are all too far away for feasible communication or travel, like intergalactic distances.


If there are lots of ET civilizations, there are plenty of additional possibilities.

• Some of them like to destroy any others that they find, because they are competition or whatever.
• They want to hide from other civilizations.
• They do not have any interest in interstellar communication or travel.
• They are using some advanced technology that we do not know about.
• We are unable to recognize their messages or their artifacts.
• They broadcast only briefly. This can happen from their technology advancing.
• Everybody is listening, and nobody is transmitting.
• We have not searched enough, or with enough sensitivity.
• The Zoo Hypothesis: they know about us, but they avoid letting us know about them.
• "They are made of meat" -- incredulity at our nature.
• They prefer to live away from us, like in the outer parts of our Galaxy to avoid overheating.
• The Simulation Hypothesis: it includes no ET's.

Etc.

A lot to argue over, I'm sure.

They could also be two small to notice.

They may have no use for electromagnetic communication.

Maybe they don't need to communicate at all.

I guess my meal is ready.

Bye.
EB

People seem to be overlooking the enormity of space and the gargantuan distances involved. It's not like walking to the next door neighbor unless you're talking about a walk that takes 40,000 years. Seriously, this isn't a discussion about economics where you can "imagine" a can opener, this is reality. The Fermi Paradox should be re-titled the Fermi non-paradox, owing to the distances involved.

It makes eminently more sense to image distant planets and plumb their emissions for gasses that signature life as we know it. Forget radio astronomy, the distances are too great.

 

[lpetrich]

[
People seem to be overlooking the enormity of space and the gargantuan distances involved. It's not like walking to the next door neighbor unless you're talking about a walk that takes 40,000 years. Seriously, this isn't a discussion about economics where you can "imagine" a can opener, this is reality. The Fermi Paradox should be re-titled the Fermi non-paradox, owing to the distances involved.

However, with advanced-enough technology, it should be possible to travel across interstellar space.


We have some good estimates of typical relative velocities for nearby stars, out to a kiloparsec or so. From Local stellar kinematics from Hipparcos data ([astro-ph/9710077] Local stellar kinematics from Hipparcos data), is a value of the Sun's velocity relative to the Local Standard of Rest:
U = + 10.00 +- 0.36 +- 0.08 km/s
V = + 5.25 +- 0.62 +- 0.03 km/s
W = + 7.17 +- 0.38 +- 0.09 km/s
Total = 13.38 +- 0.42 +- 0.08 km/s
where
U = radially inward
V = perpendicular to the radius in the Galactic disk along the direction of the Sun's orbit
W = perpendicular to the previous two northward

Now for other stars. Their greatest velocity dispersion is approximately radial, but it is tilted in the (inward-forward) - (outward-backward) direction. It has no such tilt in the out-of-plane direction, however.

The Sun (spectral type G2V) has color index B - V = +0.656, and stars like it have velocity dispersion (standard deviation)
S(U) = 38 km/s
S(V) = 24 km/s
S(W) = 21 km/s
Tilt = 10d
Total = 50 km/s
Avg per axis = 29 km/s

Fomalhaut (spectral type A3V) has B - V = +0.09, and stars like it have
S(U) = 14 km/s
S(V) = 9 km/s
S(W) = 5 km/s
Tilt = 30d
Total = 18 km/s
Avg per axis = 10 km/s

The velocity dispersion increases approximately linearly in (B - V) from Fomalhaut-like to Sunlike, then levels off, an effect called Parenago's discontinuity. It is from younger stars not getting jiggled as much by others stars as older stars have.

 

[DBT]

There may be unknown hazards present in interstellar space that makes it difficult to navigate, comets, asteroids, etc, ejected from solar systems during the turbulent period of planetary formation. Sure interstellar space is vast and empty but there may be just enough bits and pieces of debris to pose a problem at relativistic velocities.

 

[Treedbear]

Apart from extreme difficulty in transporting anything form one star to another there is also the issue of radio transmissions. Most of these we broadcast would be very hard to detect more than a few light years away.

^ This. Broadcasting a signal towards another planetary system seems futile given the low probability they will be listening at that moment in precisely our direction. And then we'd need to be listening for the return message over some "reasonable" window of time after the period of time it takes for a signal to travel both ways. I would think radio broadcasts used for a civilization's local communications are probably just too weak and noisy to identify as from an anatural source. And then there's the possibility that communication and even radio communications becomes less and less broadly-cast as technology evolves until it completely winks out.

 

[lpetrich]

Yes, that's one of the possibilities that I mentioned.

 Search for extraterrestrial intelligence has  File:NASA-SETI-Sensitivity.jpg, showing how far away one can observe transmitters with different Effective Isotropic Radiated Power (EIRP) values. A beamed transmission will have more EIRP than its power, since its broadcasts are concentrated in a small solid angle or angular area of directions. The Arecibo radio transmitter can do 10 terawatts (10¹³ W) of EIRP, but it has a beam width of about 10^-3 radians, giving a beam solid angle of about 10^-6 steradians. So its transmitter's actual power is something like 10 megawatts (10⁷ W).

So to have any chance of being seen by a large number of possible receivers, one's transmitter must broadcast at least a terawatt. By comparison, humanity's electricity consumption is currently around 2.3 terawatts, and total energy consumption 12.3 terawatts. It would be impractical to build a terawatt-scale transmitter on a planet's surface, I think.

 

[Malintent]

No, that was just a case of idiots. Bullets are supersonic. Cracked whips are supersonic.

The issue was whether people could survive such a flight.

It is still impossible to flap our arms and fly. That has not changed. The way we worked around the limitation to use the natural laws to our advantage in that case is not much different than how we WILL break the "light speed barrier"... if not by demonstrating there is no such barrier, then by working around the barrier in a novel way.. like not by accelerating, but by taking a shortcut 'around' space-time..

The problem is that there might not be any such shortcut.

While the laws of physics do not preclude there being some way of FTL, neither do they indicate any way that FTL could work. Note, also, that wormholes and warp drives cause some very nasty time travel issues. (However, time travel issues do not mean all forms of FTL are out of the question. A FTL system that forces a reference frame avoids the problem and is not inherently incompatible with relativity. Einstein showed we do not need a reference frame for evaluating physics in our universe but that does not mean there might not be a reference frame present in a hyperdrive.)

I think the term "idiots" is a bit harsh, no? We all stand on the shoulders of giants that came before us. It may be easy to forget that you had textbooks to learn from.... they did not have those textbooks. They wrote the textbooks for you so you could hit the ground running, so to speak.

Also, I did not get that specific because I didn't think it was necessary to be, but the "impossibility" of supersonic flight came from observation of the aerodynamics of an airfoil (a wing) in transonic conditions... and multiple failed flight tests

I don't know how aware they were that the crack of a whip came from a sonic boom, but it is irrelevant... no one thought that "nothing" can go faster than sound (light does, duh)... it was believed that an aircraft could not be controlled at those velocities. until they started experimenting with novel wing designs... and the short, swept delta wing was invented. But the power to generate sufficient lift from such a wing to even get off the ground was the next hurdle. It was a catch 22... need more lift but without the drag. which is impossible. even today. so need a bigger engine.. but that was so much more weight that even MORE lift was needed... and then that is too much lift for transonic... it really seemed an impossible engineering feat.

 

[Treedbear]

Yes, that's one of the possibilities that I mentioned. ...

Thanks for the question. It made me think about something that has bugged me for quite a while. Now I can go back to the "hard questions" such as who built Puma Punku?