Saturday


A few days ago in Why the Fermi paradox must be taken seriously I attempted to demonstrate that the technology of any peer civilizations extant in the Milky Way would have singled out the earth as an interesting place to visit and thus would likely have made us the target of alien exploration if advanced peer civilizations existed in the Milky Way.

I neglected to mention that, to a certain extent, this applies even to nearby galaxies, although the farther away the galaxy we reference, the more difficult it would be to obtain the scientific knowledge of the earth at a distance, and the more difficult it would be to travel. But difficulty is not impossibility, and if we contemplate the possibility of very old peer civilizations in the universe, their technology would be so advanced that the difficulties would be reduced.

It is one of my dissatisfactions with most books on astrobiology, exobiology, SETI, and space travel that they implicitly confine their scope to the Milky Way galaxy without explicitly acknowledging this restriction. Of course, the Milky Way galaxy is a very big place, but in the several posts in which I have referenced the Hubble Ultra Deep Field Image (which has been called “the most important image you will ever see”), when we consider the universe on a very large scale, galaxies fill the sky like the familiar stars filling our night sky. The Milky Way is a very big place, but the universe is a much bigger place, and we must understand the Milky Way in the context of the universe.

The nearest large galaxy to us (excepting the Magellanic Clouds) is the Andromeda Galaxy, which is an elegant spiral galaxy larger than the Milky Way. In the fullness of time, the Andromeda spiral galaxy and the Milky way galaxy will collide, the supermassive black holes at the center of each galaxy will eventually merge, and a new and even larger galaxy will be born from the collision. But that will be a very long time from now.

In the meantime, the Andromeda galaxy is about two and half million light years from us. That means that any observation of the earth from Andromeda would be two and a half million years old. While this is a long time ago for us, in geologic terms it is not all that long ago. While a peer civilization in the Milky Way would experience a lookback time of not more than 100,000 years, bringing observations to the time of the emergence of homo sapiens, the lookback time from the Andromeda galaxy would bring the observer back to a time when several hominid species were ranging around Africa. This corresponds roughly to the time of the emergence of homo habilis and the beginning of tool use among hominids. While this time scale means a lot to us, the biosphere then and now is almost identical, and to an advanced peer civilization then and now on the earth would look pretty much the same. The earth would still be positively brimming with life and therefore a very interesting place to visit.

Assuming only advanced technology and no exceptions to the laws of physics, a starship launched from the Andromeda galaxy would take at least two and a half million years to arrive, but due to time dilation at relativistic velocities, hardy explorers could make the trip in a single lifetime. Somewhere I read (I can’t recall exactly where) that a starship accelerating at the relatively modest rate of 32 feet per second (which has the added value of providing artificial gravity onboard) would only experience about 24 years of elapsed time on the ship during a voyage between Andromeda and the Milky Way. If we were to combine this sort of feasible travel technology with induced hibernation, it is entirely plausible that a group of explorers could travel between galaxies. And the closer one approximates the speed of light, the greater the time dilation, so for explorers there would be a strong incentive to “push the envelope” as it were.

Again, this involves some very advanced engineering, but it doesn’t violate any known laws of physics, and the technology involved is at least comprehensible to us, even if we aren’t in a position to build it ourselves any time soon.

Now, you might ask why anyone would leave behind their world by two and a half million years in order to go to another galaxy. In the books I have been reading lately I have found that several authors are remarkably sanguine about this, and confidently predict that robotic exploration would be so much more preferable to actual exploration by conscious agents that the latter possibly is simply set aside. For example, I have found this more or less to be the implicit viewpoint of Timothy Ferris in Coming of Age in the Milky Way, of Michio Kaku in The Physics of the Future, and of Paul Davies in The Eerie Silence.

I don’t buy this at all. Just as there are, in our contemporary civilization, many people who enjoy the comforts of home, there are always a few people who climb mountains. And, similarly, when the technology is available, many people will continue to enjoy the comforts of home, but there will always be those who are so driven by the need to explore that they will leave behind home and family and indeed the entire world that they know in order see to what lies beyond the horizon. It is perfectly reasonable to me that a group of explorers might choose to leave behind the Andromeda galaxy merely for the purpose of investigating an interesting planet in the Milky Way. In fact, I might choose to do this myself, were it a viable option.

As we consider galaxies and possible peer civilizations at a further reach, beyond the local group and the local cluster of galaxies, the possibilities of relativistic time dilation continue to make exploration possible on an inter-galactic scale, but it would become much more difficult to find interesting planets at this distance, even with techniques like gravitational lensing. However, as we have seen, difficulty is not the same thing as impossibility.

However, another factor comes into play as we move further away from the Milky Way. While those on board a very fast intergalactic starship (approximating while never achieving the speed of light) would experience very little time, time outside this starship would elapse at the accustomed rate, and that means that the more distant the galaxy, the longer ago in time a ship would have to have been launched.

The problem with this, and the problem with much SETI research, is a failure to engage with the anthropic cosmological principle, which seems to be concerned with human existence, but is equally valid (in its valid forms, that is) for any organic conscious agents that emerge according to the laws of nature and natural selection. The farther away we consider, the further back we go in time, and the further back we go in time, the less the universe has evolved toward its present state. At much earlier states of cosmic evolution the elements requisite for peer life, and most especially for peer industrial-technological civilizations, simply do not exist.

A solar system that could support peer industrial-technological civilization would have to have formed after the heavier elements had been formed inside stars from earlier stellar populations, since the only way you can get elements like iron and uranium from an initial stage of hydrogen is, over the course of galactic evolution, for these elements to be cooked up inside successive generations to stars, and then ejected into the universe by way of supernovas. These elements then go on to form solar systems that include the kind of metals that are required for industrial-technological civilization. This takes many generations of stars. As a result, if you have far enough back in time, you arrive at a time before these generations of stars have elapsed, and therefore the conditions for peer civilizations do not exist.

There is a cosmological window in the natural history of the universe for industrial-technological civilizations to emerge. We cannot yet state with any precision how long this window persists, or when it starts. Almost certainly there could be peer civilizations a million or more years old in the universe, but somewhere there is a limit older than which a civilization in our universe could not be. Thus when SETI researchers confidently speak of civilizations millions years old, I am immediately skeptical. It is not impossible, but the further back in time you go, the less possible it becomes.

It is worthwhile to think about this in more detail, as it also has consequences for the Fermi paradox. If we regard it as a mere matter of chance when an industrial-technological civilization emerges from its organic origins — which, it seems to me, is something we must acknowledge in the spirit of methodological naturalism — then it is just as likely that our civilization just happens to be to first such to emerge in the Milky Way, on perhaps even in the local group of galaxies, as it is that we are not the first. Of course, this is not a function or mere chance — it is chance constrained by the anthropic cosmological principle, as well as chance constrained by natural selection. But this is only a rough formulation. An adequate formulation would take more time and more thought.

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Thursday


The world is usually more complicated than we realize; there are almost always further layers of reality to discover. A widely reported discovery of a particular exoplanet made me aware of another layer of complexity in the world. An article in Science, subsequently reported on Science Daily and the BBC, described a particular exoplanet, i.e., a planet outside our solar system. Exoplanets are not news anymore, since hundreds have been discovered. We know that planets are plentiful in our galaxy. Now, it seems, we can even get a glimpse of planets in other galaxies.

I have been aware for some time that, in the long term history of the universe, galaxies collide, and the larger galaxies swallow up smaller galaxies. I have mentioned this in relation to the supermassive blackholes that reside in the center of spiral galaxies (in Appearance and Reality in Cosmology). It is pretty well certain that, in the distant future, the Milky Way and the Andromeda galaxies will meet in a slow motion collision, and at some time the supermassive black hole in the center of the Milky Way will be swallowed up by the even more massive black hole at the center of the Andromeda galaxy (or maybe they will end up orbiting each other). What I learned today, and what I hadn’t thought of previously, is that our galaxy has already swallowed up smaller galaxies in the distant past.

As it turns out, the Milky Way is surrounded by stellar “streams” that are the remnants of galaxies that have had the misfortune to run into the Milky Way galaxy, and were torn up and largely absorbed by the Milky Way. There is a list of stellar streams on Wikipedia. Not all galaxies have supermassive black holes at their center, and it would seem that the galaxies absorbed by the Milky Way in the “recent” past of the universe, and which have left traces in the form of stellar streams strung out by tidal forces, were stellar clusters or dwarf galaxies something like the Magellanic clouds.

Astronomers have managed to detect an exoplanet around a star in the Helmi Stream, which is a stellar stream likely the result of another galaxy absorbed by the Milky Way. The star and the planet are sufficiently old that they likely originated in their formerly independent galaxy, before it was absorbed by the Milky Way. And so it is that we can “see,” after a fashion, an extragalactic planet right here as part of the Milky Way. From this we can infer that exoplanets are not only to be found elsewhere in the Milky Way, but also in other galaxies, and indeed in galaxies of a very different construction than ours.

Almost a year ago, in Other Worlds, I discussed our increasing knowledge of extrasolar planets. At one time, all of this was sheer speculation. Now we have a growing body of scientific knowledge about extrasolar planets, what other solar systems are like, how plentiful they are, where we are likely to find them, and the like. This growing body of exoplanetary science has been inductively confirming the Copernican Principle, also called the Principle of Mediocrity, which holds that there are no privileged observers in the universe, which is equivalent to the statement that we are not unique. Now we know, and can demonstrate, that planetary systems are not unique to the Milky Way. From this stronger inductive position, we can with greater confidence extrapolate our existing knowledge to the furthest reaches of the universe.

The Copernican Principle tutors us in metaphysical modesty, but the growing evidence for the Copernican Principle, and the paucity of counter-examples, inspires us to metaphysical ambition (perhaps driven by metaphysical pride). Scientific knowledge is the expression of this metaphysical ambition as much or more than it is an expression of metaphysical modesty.

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Copernicus continues to shape not only how we see the universe, but also our understanding of our place within it.

Copernicus continues to shape not only how we see the universe, but also our understanding of our place within it.

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Sunday


monster of the milky way

Today I was watching an excellent PBS NOVA episode, Monster of the Milky Way, about recent cosmological theories that all, or almost all, galaxies have supermassive black holes at their center, including the Milky Way galaxy, our own little home in the universe. I don’t consider the theory problematic; in fact, I think it makes perfect sense. Just as our solar system revolves around the sun, so too the much larger structure of galaxies, especially spiral galaxies, would seem to be revolving around something much more massive than your typical, run-of-the-mill main sequence star. So I’m not going to address supermassive black holes in this post.

In the NOVA episode, astrophysicist Andrew Hamilton makes the following statement:

“Albert Einstein had this crazy idea that space and time were curved, and it was the curvature of space that gave the appearance of gravity.”

I found this to be of great interest. To speak of “the appearance of gravity” suggests a contrast to the reality of gravity, and the appearance/reality distinction is deeply embedded in Western metaphysics since Parmenides and Plato (and given life again in the recent recrudescence of metaphysics). Physicists often speak loosely, and I doubt that Andrew Hamilton intended to propound a philosophical thesis within cosmology, but it is a thesis worth exploring, even if unintended.

Is gravity the mere appearance by which a deeper reality manifests itself? And is that deeper reality the structure of spacetime? Is gravity less real than spacetime? Can gravity be reduced to the structure of spacetime? All of these questions can be reformulated as their opposite number: Is the structure of spacetime a mere appearance manifesting the deeper reality of gravity? Is spacetime less real than gravity? Can spacetime structure be reduced to gravity?

It would be strange indeed if gravity were epiphenomenal to the cosmos. Contemporary physical theory distinguishes four physical forces at work in the nature of things: gravity, electromagnetism, the strong force, and the weak force. Unified field theories have done a passable job of providing a common framework for electromagnetism, the strong force, and the weak force, but gravity has proved resistant to these unified field theories, not least because of the difficulty of giving a quantum account of gravitation. There are plenty of quantum gravity theories, but none of them are yet considered definitive, and their connection to the other forces and a unified theoretical framework is more speculation than physics.

The problem of appearance and reality is an old one in the philosophy of science. Russell caricatured F. H. Bradley as the “classical” tradition in philosophy (in his Our Knowledge of the External World, and elsewhere as well I think), and Bradley is remembered for his treatise Appearance and Reality. But Russell himself opens his The Problems of Philosophy with a chapter on appearance and reality, where, writing about a table, he says:

With the naked eye one can see the grain, but otherwise the table looks smooth and even. If we looked at it through a microscope, we should see roughnesses and hills and valleys, and all sorts of differences that are imperceptible to the naked eye. Which of these is the ‘real’ table? We are naturally tempted to say that what we see through the microscope is more real, but that in turn would be changed by a still more powerful microscope. If, then, we cannot trust what we see with the naked eye, why should we trust what we see through a microscope? Thus, again, the confidence in our senses with which we began deserts us.

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Similar difficulties arise when we consider the sense of touch. It is true that the table always gives us a sensation of hardness, and we feel that it resists pressure. But the sensation we obtain depends upon how hard we press the table and also upon what part of the body we press with; thus the various sensations due to various pressures or various parts of the body cannot be supposed to reveal directly any definite property of the table, but at most to be signs of some property which perhaps causes all the sensations, but is not actually apparent in any of them. And the same applies still more obviously to the sounds which can be elicited by rapping the table.

Thus it becomes evident that the real table, if there is one, is not the same as what we immediately experience by sight or touch or hearing. The real table, if there is one, is not immediately known to us at all, but must be an inference from what is immediately known. Hence, two very difficult questions at once arise; namely, (1) Is there a real table at all? (2) If so, what sort of object can it be?

Russell often expresses himself in the language of contemporary science, but the distinctions he makes in this passage (which I have greatly shortened) are not dependent upon science. But science does add another layer to the distinction between appearance and reality. The instruments of scientific research give us unprecedented ways to extend our senses, and with each novel perspective on things that science opens up, there is another way to describe these things. Moreover, scientific theory appeals to non-observable entities to explain the way the world is and naïve scientific realism assures us that the elementary particles are truly real and calls into question the manifest realities of macroscopic experience.

The problem of gravity, however, can’t even be settled by naïve scientific realism. Scientific realism would hold that the elementary particles that make up the objects studied by cosmology and astrophysics are real, and it would not deny the collections of elementary particles into atoms, molecules, stars, and galaxies to possess a certain reality. But whether gravity is epiphenomenal to spacetime structure, or whether spacetime structure is epiphenomenal to gravity is not readily settled by an appeal to scientific realism. Asking “What comes first, the gravity or the structure?” is a lot like asking, “What comes first, the chicken or the egg?”

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