How Difficult Would It Be For Advanced Extraterrrestrials To Get To Earth?

In the wake of the recently disclosed Pentagon UFO files, many have inquired regarding the difficulty of actual ETs making it to our planet. In fact, this question has been explored in papers published in peer-reviewed astronomical journals over the past 40 years. One doesn't need a tremendous amount of savvy to dredge these papers up either.

For example, the diffusion of an advanced alien civilization based on applying known diffusion wave front equations to their spread (together with very pragmatic assumptions) was the basis for the paper ‘Galactic Civilizations: Population Dynamics and Interstellar Diffusion’ by William Newman and Carl Sagan, in Icarus, Vol. 46, June 1981, page 293.

The authors began with a standard diffusion equation, treating the spread of any colonizing civilization similarly to any medium that diffuses – for example, viruses, or general infections, or even human populations (say in the early colonization of the New World).. The basic diffusion equation used was (p. 301, eqn.12):

d(r)/dt = DIV (D(x,t,r) · grad r(x,t)

where, r (x,t) describes the population density at time t, and position x, and D is the diffusion coefficient in terms of x, t and r. The preceding equation is then tweaked and used as the basis for future refinements.

Rather than weary the casual reader with the dozens and dozens of equations leading to the Results section (page 314), I will simply commence at that section and then go from there.

The authors' first major computation is of N’, the steady state number of extant advanced civilizations in the Milky Way. This is essential to obtain because it is one of the key variables used to compute the mean distance between advanced civilizations in the Milky Way:

L _M = (2.5 x 10 ¹¹  /N’) ^1/3

Where the numerator refers to the number of  stars estimated in the galaxy, and N the estimated civilization-bearing stars. The result is in parsecs, assuming the mean separation between stars in the galaxy is 1pc = 3.26 light years. (Bear in mind while our region near the outer rim is sparse with stars, the interior third of the Milky Way is teeming with them, very densely packed)

Based on a star –planet formation factor, f * ≈  1, and a mean lifetime for an advanced civilization of 10⁶ years, the authors obtain: N’ = f(10⁶) = 10⁶ , or one million advanced civilizations in the Milky Way alone.

(* Note: my inclination is actually to increase f  to  ≈ 2, based on the discovery of more than 950 actual extra-solar planets, which were unknown at the time Newman and Sagan published their paper. This would yield double the number of advanced civilizations, but we will retain the more conservative estimate)

Then, the mean distance between advanced civilizations in the Galaxy is:

L_M = (2.5 x 10 ¹¹  /10⁶ ) ^1/3  = 63 pc = 205 Light years

Readers may well not appreciate this, but this is literally “next door neighbors” in terms of the galaxy!

The authors’ next task is to obtain the velocity of the colonization wavefront which they give as (Eqn. (79), page 316):

V = (v  _j )(D g) ^1/2

Here, (v _j ) is a dimensionless constant of order unity(1), and g ≈  0.1 (based on the rate of migration of human populations today (Newman and Sagan estimated 0.01 /yr, but that was nearly 30 years ago before the age of globalization). The diffusion coefficient, D, they (very) conservatively estimate at: D ≈ (2 x 10 ^-8 pc ² /yr).

Thus, the colonization wave velocity would be:

V = (1)[ (2 x 10 ^-8 pc ² /yr) (0.1 /yr)] ^1/2 = 6.3 x  10 ^-9 pc/ yr

But the authors adjust this value (with "bias") i.e. toward younger civilizations - using historic Earth colonization rates-   to: 3 x  10 ^- 4 pc/ yr.  I removed some of that bias using a statistical algorithm, leading to the slower  colonization wave rate of : 

V =   4.4 x  10 ^-5 pc/ yr

Which would imply 1.4 x  10⁶   or 1.4 million years  (instead of 210,000 yrs.) before the colonization wave reached Earth, assuming a 63 pc distance to the planet of the nearest advanced colonizers.  (The lifetime of these advanced colonizer civilizations is assumed to be greater than 30 million years. In other words, they all would have had to survive their critical nuclear energy development phase.)

Now, before anyone gets too ecstatic, bear in mind:

1) Sagan and Newman based their diffusion coefficient on relatively low travel speeds (v much less than c) since anything near v ~ c would be enormously expensive in terms of shielding, propulsion (page 312). They opted then for speeds far below relativistic (e.g. ≈ 40,000 km/h).

2)They deliberately assumed a “random walk” diffusion with directional bias “away from population centers".

I personally suspect the first is way too conservative and ignores the sort of ingenuity and enterprise that may well apply to a truly advanced civilization which is also space faring. And again, just because we can’t imagine humans attaining relativistic speeds, doesn’t mean advanced aliens couldn’t. So, just a shift (reduction) of the base travel time to about one ten thousandth of what the authors use enhances the diffusion wave speed, V to 0.004 pc/yr.

This reduces the time to encounter to 1.57 x 10 ⁴ yrs. or just over 15,700 years. A blink of an eye.

Thus, if the alien colonizers commenced a journey in our general direction from about 3ky before the Holocene geological era, then they’d be roughly 700 years away from finding us. (Give or take 1000 years in terms of uncertainties). This means the colonizers may well be “right around the corner”. Or may even have already entered our terrestrial environment.

The conclusion here is that any of the strange craft we've seen in the released Pentagon files could well be craft from an advanced civilization, based on fine tuning the Sagan- Newman diffusion wave equations.  This doesn't prove the unusual craft seen in the released Pentagon video, e.g.  https://youtu.be/Ce6ZevfbIK0

is an actual alien craft, only that this hypothesis can't be excluded. Especially in light of the Sagan-Newman paper, especially with factors tweaked after the emergence of the exoplanet discovery era. 

Aliens Are Dummies? The WSJ Piece That Many Will Still Believe

As humans have continued over decades to search for extraterrestrial signals, a number of different takes have surfaced as well as proposals, advice. In an article I wrote in 1978, 'We Are Not Alone', I warned that our hitherto emitted radio and TV signals could already have betrayed our position to an alien civilization. I suggested that if humans had any residual smarts at all they would take care to conceal their radio presence in future rather than announce it, because we could not be sure who or what might pick the signals up and what they'd do in response.  I then cited physicist Freeman Dyson, who in a 1972 speech, asserted that the extraterrestrials we're most likely to encounter would be “like a malignant, technological cancer spreading across the galaxy” and we’d "do well not to wish for any meeting."  He predicted that it would be the most aggressive species we'd be most likely to meet, and we'd likely regret it.

I also cited British Astronomer Royal, Sir Martin Ryle, who pleaded for extreme caution in the nature of radio band signals we send, and advocated for restricting any and all types of electro-magnetic signals and noise emanating from our planet. He warned that it would be foolhardy to be anything but passive listeners given we don't know anything about alien intentions .

More recently, this has echoed  in new warnings from Elon Musk and Stephen Hawking, comparing the aliens that are most likely to visit Earth (because of stupidity on our part, i.e. in actively beaming messages) to the first colonizers of the New World - who decimated native populations for resources and territory. Musk, Hawking and others have urged that we do not move to "active SETI" which entails designing radio telescopes to actively transmitting messages as opposed to mere passive reception.

The general tenor of discussion then, has been that the aliens  likely to be contacts will be: a) super -intelligent, far in advance of Earthers' best, and b) highly aggressive.

Never mind. In his WSJ piece (July 25-26, p. C5) Joe Queenan assures gullible readers  (of which there are likely to be many) that "scientists don't want to let on they are searching for signs of average intelligence".

As he facetiously notes:

"The logic is infallible. If aliens were really smart, it stands to reason they would already have contacted us."

Adding, to clarify:

"Really, really smart creatures would want to let us know they were there, if only to show off, the way people who went to Harvard can't order a BLT without letting everyone in the room know they once wrote for the Crimson."

Not really!  And again, though to me his angle was tongue in cheek, it definitely could be literally misconstrued by a hidebound human chauvinist - who would have his belief reinforced that either: a) humans are the only advanced species in the cosmos, or b) any other species would be at the level of "incompetents" compared to us (Queenan uses the analogy of the East Germans before being absorbed by W. Germany)

The reality is that we're more likely to make first hand physical contact with extraterrestrials than radio contact. The logic and basis for this were well laid out in the paper ‘Galactic Civilizations: Population Dynamics and Interstellar Diffusion’ by William Newman and Carl Sagan, in Icarus, Vol. 46, June 1981,

The authors' logic was impeccable and simple. Any civilization worth its salt would at some point enter an Atomic age. Weapons of ultimate annihilation would become widely available as well as potential for nuclear power, possibly fusion. A civilization entering this age would either come out much more advanced and in one piece, or destroy itself. It is the latter category that would most likely limit itself to only radio contact.

The former would be able to parlay their awesome new power into fusion energy, and also interstellar space flight. The latter would open the doors to planetary conquests, given, as the authors noted, interstellar colonization would more likely be accomplished by a technologically aggressive species.  Hawking, for example, compared their arrival on Earth to the arrival of Columbus' expedition to the New World, resulting in the decimation of the Arawaks. Thus, the simple disparity in technology would result in the elimination or the absorption of the less advanced species - without even the veneer of a "war" declaration.

How advanced could such aliens be? Russian astrophysicist I.S. Kardashev, put forward a three-class scheme:[1]

I: Able to harness the equivalent power of the planet Earth
II: Able to harness the power equal to a typical star (10^23 kw)
III: Able to harness the power equal to a typical galaxy (10^33 kw)

Plausibly, an encounter with either Types II or III would amount to Earth hominids elimination or absorption - though it is difficult to see how the latter might work. (Perhaps literally, as food, see e.g.
the Twilight Zone episode 'To Serve Man'). On the other hand, an encounter with a Type I may have more potential for genuine exchange, even if at the analogous level of a human mathematician trying to teach a backward child residue calculus.

What we should not do, is buy into Queenan's theme of meeting or contacting "average" aliens. In terms of actual galactic civilizations, there is no such thing - except perhaps for humans.

[1]Shklovskii, I. and Sagan, C.:1966, Intelligent Life in the Universe, Dell Publishing Co., p. 394.

The Extraterrestrials That You Don't Want Finding Earth

In the great science fiction novel, Childhood's End, by Arthur C. Clarke, an armada of benevolent alien Overlords arrives at Earth and cultivates the growth - maturity of humanity. Only very late in the novel do the Overlords finally reveal their physiognomy - looking just like assorted depictions of "Satan" through antiquity.

But one wonders what would have transpired if the visiting aliens  foolishly revealed themselves first. It doesn't take a lot of imagination to project all the human armies of the world would likely have attacked the invaders but the results would have been radically different from 'Independence Day'. The two scenarios provide different cautionary tales on how humans may have to process any future alien landings.

"Aw, that's the stuff of fantasy! Never happen! Not in a billion years!"

Don't be so sure! As Carl Sagan observed in his own Cosmos series (and Neil de Grasse Tyson does at the end of his),  from deep space Earth appears as an un-twinkling pale, blue dot (also the title of another Sagan book). The actual image-photograph of this pale blue dot exists (shown in the last Cosmos episode) and was accomplished at the behest of Sagan when he requested NASA have the Voyager spacecraft turn its cameras back toward Earth as it ventured beyond Jupiter on its way out of the solar system.

Make no mistake that to any transiting alien craft this would provide an inviting target or destination. What would come next depends on the intent and the nature of the aliens at the helm, Would they be beneficent? Possibly. But it would be foolish to make the assumption that all such visitors would have our interests at heart.

Indeed, in an article I wrote in 1978, 'We Are Not Alone', e.g.


 I warned that our hitherto emitted radio and TV signals could already have betrayed our position to an alien civilization. I suggested that if humans had any residual smarts at all they would take care to conceal their radio presence in future rather than announce it, because we could not be sure who or what might pick the signals up and what they'd do in response.  I then cited physicist Freeman Dyson, who in a 1972 speech, asserted that the extraterrestrials we're most likely to encounter would be “like a malignant, technological cancer spreading across the galaxy” and we’d "do well not to wish for any meeting."  He predicted that it would be the most aggressive species we'd be most likely to encounter first.

Tragically for us and our kind, for whatever reason this alien species would have discovered the engineering basis for practical interstellar travel long before their aggressive and conquistadore dispositions were bred out of them. No surprise then, that no less a luminary than Stephen Hawking has also warned about allowing our hubris to get the better of us, e.g.
http://www.nbcnews.com/id/36769422/ns/technology_and_science-space/t/hawking-aliens-may-pose-risks-earth/#.U5xq6JVOXzA

This is not mere bunkum and the odds for meeting such beings should not be under-estimated. In the paper ‘Galactic Civilizations: Population Dynamics and Interstellar Diffusion’ by William Newman and Carl Sagan, in Icarus, Vol. 46, June 1981, we get a wake-up call.

The authors began with a standard diffusion equation, treating the spread of any colonizing civilization similarly to any medium that diffuses – for example, viruses, or general infections, or even human populations (say in the early colonizations of the New World).. The basic diffusion equation used was (p. 301, eqn.12):

d(r )/dt = DIV(D(x,t,r ) x grad r (x,t)

where, r (x,t) describes the population density at time t, and position x, and D is the diffusion coefficient in terms of x, t and r . The preceding equation is then tweaked and used as the basis for future refinements.

Rather than weary the reader with the dozens and dozens of equations leading to the Results section (page 314), I will simply commence at that section and then go from there.

The authors' first major computation is of N’, the steady state number of extant advanced civilizations in the Milky Way. This is essential to obtain because it is one of the key variables used to compute the mean distance between advanced civilizations in the Milky Way:

L_m = (2.5 x 10¹¹/N’)^1/3

Where the numerator refers to the number of stars estimated in the galaxy. The result is in parsecs, assuming the mean separation between stars in the galaxy is 1pc = 3.26 light years. (Bear in mind while our region near the outer rim is sparse with stars, the interior third of the Milky Way is teeming with them, very densely packed)

Based on a star –planet formation factor, f *~ 1, and a mean lifetime for an advanced civilization of 10⁶ years, the authors obtain: N’ = f(10⁶) = 10⁶, or one million advanced civilizations in the Milky Way alone.

Then, the mean distance between advanced civilizations in the Galaxy is:

L_m ={ (2.5 x 10¹¹/ (10⁶)}^1/3 = [2.5 x 10⁵]^1/3 = 63 pc = 205 Light years

Readers may well not appreciate this, but this is literally “next door neighbors” in terms of the galaxy!

The authors’ next task is to obtain the velocity of the colonization wavefront which they give as (Eqn. (79), page 316):

V = (v_k)(D g)^1/2


Here, (v_k) is a dimensionless constant of order unity(1), and g ~ 0.1 (based on the rate of migration of human populations today (Newman and Sagan estimated 0.01 /yr, but that was nearly 30 years ago before the age of globalization). The diffusion coefficient, D, is (very) conservatively estimated at: D ~ (2 x 10^-8 pc²/yr).

Thus, the colonization wave velocity would be:

V = (1)[ (2 x 10^-8 pc²/yr)(0.1 /yr)]^1/2 = 4.4 x 10^-5 pc/ yr

Which would imply 1.4 x 10⁶ or 1.4 million years before the colonization wave reached Earth, assuming a 63 pc distance to the nearest advanced colonizers.

Now, before anyone gets too ecstatic, bear in mind:

1) Sagan and Newman based their diffusion coefficient on relatively low travel speeds (v much less than c) since anything near v ~ c would be enormously expensive in terms of shielding, propulsion (page 312). They opted then for speeds far below relativistic (e.g. ~ 40,000 km/h).

2)They deliberately assumed a “random walk” diffusion with directional bias “away from population centers".

Personally, I believe the first is way too conservative and ignores the sort of ingenuity and enterprise that may well apply to a truly advanced civilization which is also space faring. And again, just because we can’t imagine humans attaining relativistic speeds, doesn’t mean advanced aliens couldn’t. So, just a shift (reduction) of the base travel time to about one ten thousandth of what the authors use enhances the diffusion wave speed, V to 0.004 pc/yr.

This reduces the time to encounter to 1.57 x 10⁴ yrs. or just over 15,700 years. A blink of an eye.

Thus, if the alien colonizers commenced a journey in our general direction (the next thing I will deal with in (2)) from about 3ky before the Holocene geological era, then they’d be roughly 700 years away from finding us. (Give or take 1000 years in terms of uncertainties). This means the colonizers may well be “right around the corner”.

As for the authors’ assumption (2) that the directional bias is away from population centers, or populated planets, I suspect this is based on their beneficent view of colonizing aliens. Their take is that a true spacefaring civilization would have had to oust its aggressive tendencies for the most part, otherwise they'd not be able to have their civilization reach the stage of interstellar travel. They’d have destroyed themselves long before.

We better hope Newman and Sagan are right and any aggressor aliens have done that before they access the technological wherewithal to head for the "pale blue dot'!