[Marxism] The Universe: ‘The Important Stuff Is Invisible’

Louis Proyect lnp3 at panix.com
Fri Feb 19 06:42:00 MST 2016


NY Review of Books, MARCH 10, 2016 ISSUE
by Lawrence M. Krauss

Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the 
Universe
by Lisa Randall
Ecco, 412 pp., $29.99

The notion that there is more to the world than we can see was probably 
hardwired into our ancestors’ consciousness by natural selection. Only 
those in the African savannah who suspected that behind the rustling 
branches might lie a predator likely survived long enough to pass on 
their genetic information. In any case, as our mental facilities evolved 
further the notion of hidden realities became more formalized when human 
tribal groups created religions to help them make sense of the world 
around them, provide solace for the inequities of nature, and ultimately 
deal with their own mortality.

Hidden realities continued to dominate thinking as early religious myth 
gave way to more refined philosophical speculation. Searching for the 
fundamental essence underlying all matter became a common quest in Greek 
and Roman philosophy, while air, earth, fire, and water were 
sequentially dispensed with as providing such an essence. Ultimately 
philosophers from Empedocles to Aristotle decided that this fundamental 
essence must be something distinct; and Plato, through his derivation of 
five perfect solids, focused on a “quinta essentia,” a fifth essence. 
This also became known as “aether” and was imagined to comprise the 
fundamental essence of all space, permeating both heaven and earth. By 
connecting the stars and planets with terrestrial matter, this aether 
also motivated the ancient Alexandrian practice of astrology, which, 
while the aether has disappeared, unfortunately remains prevalent even 
today.

As religious myth and early philosophical speculation then gave way to 
scientific discovery and we developed machines to perceive what our eyes 
and ears could not, the fact that the world of our experience reflected 
merely a small part of a much greater whole became manifest. Light 
itself is just one small piece of a continuous spectrum of invisible 
electromagnetic waves that are filling the space around us and 
bombarding us at all times. When we look up at the night sky, we now 
realize that the seemingly dark emptiness between stars is not in fact 
empty. If one were to create a dime-sized hole between thumb and 
forefinger and hold it out at arm’s length, in that small region the 
largest telescopes today, like those in Chile or Hawaii, could discern 
literally hundreds of thousands of other galaxies like our own Milky Way.

Indeed, given the myriad recent developments in modern astronomy, it is 
sometimes hard to appreciate that less than one hundred years ago the 
entire universe, as conceived by astronomers, consisted of a single 
galaxy, the Milky Way, surrounded by a possibly eternal static void. We 
now know not only that there are at least 100 billion galaxies beyond 
our own in the observable universe, but also that our universe is 
expanding, and most recently we have discovered that the expansion is 
actually speeding up, for reasons we have yet to understand.

So perhaps it is not surprising to find that in the intervening century 
we have discovered a host of other previously invisible entities 
surrounding and in many cases permeating the space we occupy. Consider 
two cases:

1. Every cubic centimeter of space is teaming with three hundred 
microwave photons left over from the Big Bang explosion—particles that 
last interacted with matter when the universe was 300,000 years old. Yet 
while perhaps nothing is easier to detect than electromagnetic 
radiation, this background, now referred to as the cosmic microwave 
background radiation, remained unnoticed until 1964, when it was 
discovered by accident by two Bell Laboratory physicists who were using 
a radio telescope to search for other signals.

2. Every second over 600 billion particles called neutrinos penetrate 
every square centimeter of your body, traversing it, and the earth, 
without interaction. These neutrinos emanate from nuclear reactions deep 
inside the sun, the very reactions that power our star and make our 
lives possible. Only in the 1990s were we able to experimentally confirm 
that such a background of neutrinos existed and to ascertain its 
magnitude, through a set of observations that resulted in the awarding 
of one half of last year’s Nobel Prize in physics. A similar background 
of neutrinos, left over from the Big Bang, is predicted to exist, but to 
date no experiment has been sensitive enough to directly detect it.

In hindsight then, after these discoveries of invisible exotica it does 
not take a great leap of the imagination to wonder if there might be 
other undetected backgrounds out there, or even in the room in which I 
am typing this essay.

Needless to say, science doesn’t proceed by hindsight, but rather by 
insight, and the history of astronomy in the twentieth century involved 
a long struggle in which scientists were dragged, against their a priori 
prejudices, to the realization that the universe of our 
experience—stars, galaxies, planets, and life—is essentially an 
irrelevant sideshow. The important stuff is invisible, quite possibly 
made of some new type of matter.

Writing, as I am now, from the remote shores of frozen Antarctica, it is 
naturally tempting to compare our visible universe to the tip of a vast 
cosmic iceberg, most of which is invisible, dark, and out of sight. 
Tempting or not, the comparison is particularly apt. As the captain of 
the Titanic found out, often what you cannot see is more important than 
what you can.

So too this invisible background of cosmic material, which physicists, 
with great linguistic perspicacity, have come to call “dark matter”—a 
name that Lisa Randall takes issue with in her new book Dark Matter and 
the Dinosaurs—is now understood to have ultimately determined the 
dynamics of the formation of structure in the universe, including all 
the cosmic structures we observe with our telescopes today. In short, 
without this invisible background of cosmic material we would not exist.

The story of how this came about is typical of major revolutions in our 
understanding of nature. It involved a series of baby steps, missteps, 
and hard work, as well as the growing convergence of two fields of 
physics that on the surface couldn’t seem farther apart: particle 
physics, the study of the dynamics of the very small, and cosmology, the 
study of the dynamics of the universe on its largest scales.

The physicist and Nobel laureate Sheldon Glashow first used the example 
of the ancient Egyptian symbol ouroboros, a snake eating its own tail, 
to describe this convergence. Since Edwin Hubble’s groundbreaking 
discovery in 1929 that the universe is expanding, we have recognized 
that the entire observable universe, all 100 billion or so galaxies, 
each containing 100 billion or so stars, was, some 13.8 billion years 
ago, confined to a region that was perhaps smaller than a single atom 
today. If this is the case, then the initial conditions that determined 
the origin, makeup, and nature of the largest cosmic objects today were 
determined on subatomic scales. So to understand the universe on its 
largest scales we ultimately must push forward our understanding of the 
fundamental structure of matter and forces on the smallest scales.

The first suggestion that on the scale of galaxies normal matter—made up 
of atoms, themselves comprised of protons, neutrons, and electrons—is 
not all that there is came indirectly, and largely without fanfare. 
Fritz Zwicky was a brilliant and irascible astronomer at Caltech whose 
relationship with his colleagues was summarized by his description of 
them: “spherical bastards,” because he felt they were bastards any way 
you looked at them. He nevertheless made a series of discoveries in the 
1930s that essentially presaged almost all the developments in cosmology 
over the next half-century. In 1933 he observed the velocities of 
individual galaxies in the distant Coma Cluster of galaxies. 
(Essentially all galaxies, including our own, are bound together in huge 
conglomerations containing hundreds or sometimes thousands of individual 
galaxies.) He found that the relative velocities of the galaxies in the 
cluster were so large that they shouldn’t have remained bound within the 
cluster unless there was perhaps four hundred times more mass than could 
be accounted for by the luminous stars in each galaxy. Although others 
had previously made similar observations, Zwicky was the first to 
suggest that this invisible matter might be some new sort of exotic 
material, which he called “dark matter” (dunkle Materie in German).

This evidence languished for almost forty years until the groundbreaking 
work of Vera Rubin and her colleague Kent Ford in the 1970s. Rubin, only 
the second woman to be awarded the Gold Medal of the Royal Astronomical 
Society, did not come by her discovery easily. She graduated with a 
doctorate from Georgetown (taking night classes while her husband waited 
in the car because she didn’t know how to drive), after being turned 
down by Princeton because they didn’t accept women into their graduate 
program at the time. Careful measurements by Rubin and Ford, and then a 
host of others, of stars and hot gas in our own galaxy ultimately 
established beyond a doubt that the outer parts of our galaxy were 
orbiting around the center too fast.

If the gravitational pull caused by the visible stars in our galaxy were 
governing their motion, their orbital speeds should fall off the farther 
from the center of the galaxy they are, just as the speeds of the outer 
planets in our solar systems do. Instead, the rotation rate appears to 
be constant no matter how far out one probes, well beyond the region 
where most of the stars and gas lie. Unless the nature of gravity 
changes at these distances, the only explanation is that the mass of our 
galaxy (and essentially all others whose rotation curves have been 
measured) continues to increase linearly with distance, even though the 
density of stars and visible matter falls off at these distances.

Fritz Zwicky appears once again in the story. In 1936 Albert Einstein 
published a paper pointing out that the fact that light bends in a 
gravitational field means that massive objects can act like 
“gravitational lenses,” distorting and magnifying images of objects 
located behind them. He felt that this phenomenon would never be 
observable, but within a year Zwicky wrote a paper demonstrating that 
not only should gravitational lensing by galaxies and clusters of 
galaxies be detectable, but doing so would allow us to “weigh” these 
systems, including both visible and dark matter—anything that 
contributes to the gravitational forces. Sure enough, sixty years later 
when astronomy had progressed to the point where this was possible, the 
lensing effect of gravitation was used to establish that the largest 
objects bound by gravitational forces in the universe today, clusters of 
galaxies, contain forty times more matter than can be accounted for by 
stars and gas.

Other techniques, including measuring the radiation from the Big Bang 
itself, the so-called cosmic microwave background radiation, which 
allows very precise measures of the amount of matter in the universe at 
a time when the universe was only about 300,000 years old, as well as 
the overall geometry of the universe, have now confirmed unequivocally 
and precisely that the average density of matter in the universe exceeds 
the density associated with stars and hot gas by the same factor of forty.

On one hand, perhaps it is not surprising that a lot of matter exists in 
the universe that is not visible to telescopes. Planets don’t shine, and 
neither do snowballs or book reviewers. However, over the past fifty 
years or so it has become clear that there simply isn’t enough normal 
matter, made of protons and neutrons, to account for all the dark matter 
in the universe. Careful calculations of nuclear reactions forming light 
elements like helium and lithium in the early universe put an upper 
limit on the abundance of protons and neutrons engaging in these 
interactions, and that limit is only about 20 percent of the total 
inferred density of dark matter today. This value has recently been 
confirmed using observations of cosmic microwave background radiation, 
and in fact is a factor of five to eight times as large as the abundance 
of visible material in the universe. So while we now know that most 
normal matter in the universe doesn’t shine, there is not enough to 
account for all the inferred dark matter.

Moreover, we also know that all or most of the inferred dark matter 
cannot interact as normal matter does, or galaxies wouldn’t have formed. 
By observing the neonatal universe, and comparing it to the universe we 
see today, we can calculate that normal matter simply would not have had 
sufficient time to collapse to form galaxies and stars over the last 
13.8 billion years. What is needed is to find some new form of matter 
that interacts much more weakly than protons and neutrons, and that also 
is moving slowly enough so that it could not escape even the gentle 
gravitational potential present in the early universe. With this form of 
matter, resistance to gravitational collapse would be much smaller early 
on and the formation of structures could proceed apace, with normal 
matter falling into these nascent structures later on, forming stars and 
the visible parts of galaxies.

In short, what is needed is some new kind of weakly interacting 
elementary particle produced in the early universe—something like 
neutrinos, but much heavier. And this is where particle physicists began 
to get involved in the game. Not only do we have lots of possible 
candidates, but it may be possible to detect dark matter directly in the 
laboratory if we are clever, or perhaps produce it directly in particle 
accelerators like the Large Hadron Collider. With these realizations 
over the past thirty years or so, the race has been on to try to 
determine the nature of most of the matter in the universe.

The dark matter saga is sufficiently exciting and mysterious that a host 
of popular books have been devoted to the subject over the past 
twenty-five years. The most recent in this line takes a slightly 
different tack, however. Dark Matter and the Dinosaurs, written by a 
distinguished particle theorist at Harvard University, Lisa Randall, is 
not actually focused on dark matter per se. Nor is it focused on 
dinosaurs. Rather it reflects an effort to explore a possible 
implication for astrophysics of an idea in elementary particle physics 
that Randall and a colleague proposed several years ago.

In her book, Randall argues that exotic types of dark matter could alter 
the structure of the galaxy, and as a result a previous proposal, by 
Michael Rampino and colleagues, that galactic gravitational effects 
might result in periodic impacts of comets on earth could now become 
viable. Specifically, if an exotic type of dark matter forms a disk 
within the Milky Way galaxy, and if the sun crosses the disk every 30 
million years or so in its voyage around the galaxy, the resulting extra 
gravitational forces might nudge comets out of the Oort cloud, the 
sparse cloud of trillions of objects surrounding the solar system. Some 
of these comets might then hit the earth and in so doing might produce 
devastating periodic extinctions of life, including the impact 65 
million years ago that appears to have led to the demise of the dinosaurs.

The book is engaging, and written from an accessible personal 
perspective, which is not surprising, given the personal importance to 
the author of the story being told. Randall’s excitement about the areas 
she has studied in the process of her research is evident. The book 
easily flows over a diverse collection of interesting fields of science. 
There is a relatively brief introduction to the nature of dark matter 
(including an amusing digression on why the term “dark matter” is really 
a misnomer—“invisible matter” would be a better, if less sexy, name), 
followed by general discussions ranging from comets and asteroids to the 
nature of the galaxy, astrophysical impacts, and ultimately extinctions.

The problem, however, is whether the proposal itself warrants packaging 
these individual pieces together into an entire book. When scientists 
write popular books about science, there is an implicit mandate to 
present a balanced perspective of the most exciting recent developments. 
Because the general public does not as a whole possess the critical 
scientific knowledge adequate to the task of distinguishing which new 
scientific claims are widely supported and which are not, it is easy for 
a book to either knowingly or unknowingly mislead. The danger of doing 
this, often seen when dubious preliminary results are instead reported 
as exciting discoveries in the popular press, is that when they are 
later retracted or shown to be false the public’s trust in the 
scientific process, and in the dependability of results that have stood 
the test of time and experiment, diminishes.

Randall is not guilty of such hyperbole in this regard. She is clear 
about the fact that the particle physics model she has proposed is 
speculative, and even that the premise that extinctions of life on earth 
have been periodic is not necessarily generally accepted. It should also 
be added that there is no evidence that all major extinctions have been 
due to impacts from outside the earth. For example, evidence has mounted 
recently that the greatest mass extinction in recorded history, the 
so-called Permian-Triassic event about 250 million years ago, which 
killed more than 90 percent of species on earth, was due not to an 
impact but to unprecedented continuous volcanic activity in what is now 
Siberia over tens of thousands of years, which generated perhaps a 
million cubic kilometers of lava. This covered a region as large as the 
US, and produced unprecedented worldwide climate change and 
acidification of the oceans.

Yet turning what originally was a four-page paper published by Randall 
and her associates in a scientific journal into a four-hundred-page book 
for the general public suggests some significance for the claim. A quick 
scan of the citation record for that paper, however, reveals a total of 
only six citations in the year and a half or so since it was published, 
a very small number by standards in the field. The idea has thus far 
generated little scientific interest, whether or not it may be 
intriguing a priori, or whether it has excited interest among science 
journalists.

This is in stark contrast to Randall’s first book, Warped Passages 
(2005), which was also based on speculative work by her and 
collaborators—in this case the proposal that there may be large 
otherwise invisible extra dimensions in nature. The difference there was 
that the motivation for the proposal involved an attempt to solve one of 
the central outstanding puzzles in particle physics, namely why gravity 
is so weak compared to other forces in nature. Moreover, the work itself 
sparked considerable interest in the physics community, becoming one of 
the most frequently cited articles in recent years. It made sense to 
explain this excitement to the public, even without direct empirical 
evidence thus far.

Dark Matter and the Dinosaurs is reminiscent in this sense of The Life 
of the Cosmos (1997), by another well-established physicist, Lee Smolin, 
which also promoted into a book a proposal from an article that didn’t 
gain much traction in the scientific community—that there is a kind of 
cosmic natural selection process for universes, similar in operation to 
biological evolution. It is easy to understand how well-respected 
scientists like Smolin and Randall can be sufficiently excited by their 
ideas to want to write about them more broadly. It is just such 
excitement that drives theoretical physicists to devote so much energy 
to their work. But that doesn’t guarantee that the proposals are really 
ready for wide-scale publication.

Geoplanetary concerns aside, from a particle physics perspective there 
are reasons why this new proposal for dark matter hasn’t excited broad 
interest. As physicists have examined myriad ways in which exotic new 
microphysics might solve cosmological problems, it has become common to 
suggest that it is acceptable to incorporate a single “tooth 
fairy”—namely a single new relatively unconstrained speculation—to form 
the basis of a proposal. But two tooth fairies seem less convincing. In 
this case, it is well established that dark matter exists in a roughly 
spherical halo around our disk-like Milky Way sea of stars. To enable 
the process proposed in Randall’s book, some dark matter would have to 
collapse into an additional compact galactic disk. For this to happen 
Randall writes that there must be at least one as yet unknown additional 
component of dark matter. Randall also argues that this component must 
be able to interact in new ways that would allow it to radiate energy 
while still remaining undetectable to telescopes. As described near the 
end of the book, which is where Randall gets to the meat of her 
proposal, such models can be developed by creative theorists like her. 
But being possible and being likely are two different things.

One should always be skeptical in physics, but nevertheless it is also 
worth stressing that we currently have no clear understanding—merely 
well-motivated preconceptions—about the nature of dark matter. A priori 
proposals about what seems likely could easily be wrong. Skilled 
theoretical physicists will continue to explore new ideas, as they 
should, and no one can be sure where the evidence may lead us. 
Independent of the likelihood of Randall’s recent proposal, for over 
half a century the story of dark matter has established remarkable new 
connections between the very large and the very small that have been 
worth celebrating.

The story of dark matter, as it has evolved over the past fifty years, 
has surprised us at many turns. I fully expect that it will continue to 
do so in the future. Every time we open a new window on the universe, 
unexpected new connections arise. Like other scientists working in this 
area, each day I am surprised if I am not surprised.



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