The Fermi Paradox: Too Hard to Contact?

I will now consider the hypothesis that interstellar-capable extraterrestrial civilizations are common, but that it is difficult or impossible for us to contact them or for them to contact us.

  1. Interstellar communication is too difficult.
  2. Interstellar travel is too difficult.
  3. We have not been searching long enough.
  4. We have not been searching for the right kind of evidence.
  5. We already have evidence of ET’s, but we don’t recognize it.
  6. We already have evidence of ET’s, but we are unwilling to recognize it.

I will leave off at interstellar-travel difficulty.

Interstellar communication is too difficult

The easiest way to communicate across interstellar distances is with radio, but even that is very difficult. The interstellar background is all over the sky, and it has an approximately continuous spectrum. Thus, to make a signal more prominent, one needs to reduce how much solid angle of sky that one observes, and also to reduce the bandwidth that one’s detection systems work with. To reduce that solid angle, one needs a larger telescope, with (solid angle) ~ 1 / (telescope aperture)^2, from radio waves being waves. For searching with a narrow bandwidth, many detection systems are designed to search in multiple frequency bands, sometimes as much as a billion bands with bandwidths of 1 Hz (1 wave cycle per second).

So the most easily detected signal is a narrowband one, but narrowband ones cannot carry much information. With a 1-Hz bandwidth, one is limited to about 1 bit per second. But such a narrowband signal may nevertheless be good for advertising the presence of a transmitter, since it is hard to think of an astrophysical signal with such low bandwidth. Thermal broadening in the interstellar medium is likely to be around 100 Hz, for instance. Search for extraterrestrial intelligence – Wikipedia has a diagram showing what we are likely to detect. Either beaming in all directions with a super powerful transmitter or else aiming a beam at us.

As for ET’s watching our TV broadcasts over interstellar distances, that will be very difficult. One needs a bandwidth of at least 3 megahertz (MHz), 3 million cycles per second, and one will need a planet-sized radio antenna to do that. And that is for the nearest stars.

As to aiming a beam at us, they’d have to decide that that’s worth doing. The Sun is a very ordinary star, one that does not have much to distinguish it, so it does not give much reason for ET’s to want to aim a beam at us. If they pick up our broadcasts, they will do so several years or decades or even centuries after we sent them, so if they respond, they will respond a long time after we made those broadcasts.

Interstellar travel is too difficult

To see why, let us consider Konstantin Tsiolkovsky’s rocket equation: v = ve*log(mi/mf) where v is the resulting change in velocity, ve is the effective exhaust velocity or specific impulse, mi is the rocket’s initial mass, and mf is the rocket’s final mass. The function “log” is the natural-logarithm function.

So to avoid consuming a lot of rocket fuel, one must shoot one’s rocket exhaust out as fast as possible. But to get one’s desired velocity change as fast as possible, one has to shoot out one’s rocket exhaust with as high mass rate as possible. Spacecraft propulsion – Wikipedia has a big list of existing, under-development, researched, and proposed methods, complete with estimates of their technological readiness.

The most used sorts of rocket engines are chemical ones, which burn or decompose their fuel, and ion ones, which shoot out beams of ions. The best chemical ones are hydrogen-oxygen ones, which make an exhaust velocity of about 4.4 km/s, but they can have have thrust values as high as 10^7 (10 million) newtons of force. That’s the Earth gravitational force of 1000 metric tons. Some good ion ones are Dawn’s NSTAR ones, with an exhaust velocity of 30 km/s and a thrust of 90 millinewtons (0.09 newtons), the Earth gravitational force of 9 grams. Dawn could not lift itself off the ground with its engines, and it had to be sent into space with a typical sort of chemical rocket, one much larger than the spacecraft itself. Dawn then took several years to travel to the asteroids Vesta and Ceres, running its engines for much of that time.

We have engines with (relatively) low ve and high thrust, and engines with (relatively) high ve and low thrust. So can we ever have an engine with very high ve and high thrust? As far as I can tell, the most plausible such engine to date has been the Project Orion nuclear-bomb pulse-drive rocket. The spacecraft shoots nuclear bombs rearward, explodes them, and then captures the force from some of the bombs’ explosion exhaust. That might get up to 1000 – 10,000 km/s of ve. A similar system, but making nuclear fusion in hydrogen-helium pellets, was proposed in Project Daedalus. Its ve was estimated to be around 10,000 km/s.

So one might get to 3000 km/s with the Orion engine or 10,000 km/s with the Daedalus engine. That’s 0.01 – 0.03 c.

An alternative to the Orion and Daedalus engines is an ion or plasma engine powered by electricity generated by a nuclear reactor. But it may be hard to get both high ve and high thrust out of such an engine.

Let’s see what we are up against. The closest star system to the Sun, Alpha Centauri, is about 4.3 light years away, and another very close star, Sirius, is about 8.6 light years way. So with the Orion drive, it would take 430 years or 860 years for those stars, and with the Daedalus drive, 140 years or 290 years. Existing spacecraft are much, much worse. The faster one, Voyager 1, is moving at 17 km/s from the Solar System, or 0.000057 c. That’s 76,000 years and 152,000 years to the two stars — and that ignores their motions relative to the Sun, motions with similar velocities. Voyager 1 and its fellow interstellar spacecraft Voyager 2, Pioneers 1 and 2, and New Horizons, will have stopped functioning long before getting to another star’s distance, and keeping going for that long will be very difficult.

I have not mentioned more speculative possibilities, like Operation Starshot, a proposed system that aims laser beams at spacecraft to propel them. That seems almost impossibly speculative to me — one has to aim the beams *very* precisely, and one has to get good feedback on the spacecraft directions.

I will leave off here, since this post has gotten rather long.


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