Degrees of Freedom

Degrees of Freedom

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On the Physics Nobels, The Atlantic Gets Dark Energy All Wrong


Kirk and Shahna

Let's keep it within the galaxy, Jim

The piece run by The Atlantic last week on the Nobel Prizes for Physics, sadly, contained a number of misleading or inaccurate statements on physics and cosmology.

Gregg Easterbrook, the journalist who wrote it, has a storied past as a science writer. He was one of the clear-minded people who saw early on the nonsense behind the space shuttle. The full Rube Goldbergian magnificence and absurdity of it is well detailed in a masterful article by Easterbrook that is practically a history of the whole program, including its cost overruns and its deadly failures—and he wrote that in 1981.

That story makes for a chilly read. You can almost see, unfolding before your eyes, the disasters of Columbia (“The tiles are the most important system NASA has ever designed as ‘safe life.’ That means there is no back-up for them. If they fail, the shuttle burns on reentry”) and Challenger (“Here's the plan. Suppose one of the solid-fueled boosters fails. The plan is, you die”).

More recently, his criticism of NASA’s priorities, seen for example in Wired, has been (mostly) spot on.

But Easterbrook has also periodically displayed creationist beliefs that are a bit disturbing for a science writer called on to write about the origins of life (also in Wired).

And occasionally he seems to overestimate his ability to understand science. Several years ago, Carl Zimmer took him to task for some nonsensical statements about string theory and other dimensions. Last week, Easterbrook ventured again into cosmology, on the web site od The Atlantic. Unfortunately his piece contains a number of bizarre or outright wrong statements. I don't mean this blog to engage in character assassination, and certainly not of Easterbrook, as I like the guy, but I would just like to correct his article for the record.

His article’s summary gets it off to a bad start, by talking of “galactic expansion.” But that may be forgivable. Dark energy is often a source of confusion even for physicists. People often ask whether the expansion of space means that galaxies—or perhaps even the solar system, even Brooklyn—are expanding.

S'far as we can tell, they are not. The only thing dark energy seems able to do is make orbits imperceptibly larger than they would otherwise be. Dark energy is so dilute that it only has appreciable effects on larger scales, larger than the millions of light years of galaxy clusters.

But perhaps by “galactic expansion” the editors of The Atlantic meant the motion of galaxies with respect to one another. Let’s move on.

“Some theoreticians,” Easterbrook writes, “predicted the expansion, driven by the momentum of the Big Bang, should be slowing." Ok.

"Eventually the galaxies would either stop their outward travel and the cosmos become static, or gravity would pull everything back together for a Big Crunch.”

Not really. As far as I know, no one thought that the universe could go from dynamic to static. In standard cosmological models that were prevalent until the end of last century, no matter how far the galaxies spread, they will still "feel" each other's gravitational tug. The universe either expands for ever or recollapses.

“Other theoreticians felt cosmic expansion would continue at a steady pace essentially forever.”

Perhaps here Easterbrook was referring to the “steady state” universe model of Fred Hoyle. But Hoyle’s whole point was that he didn’t want to believe in a big bang, so his model was not a variant of the big bang models.

“Since the galaxies must long ago have overcome the gravity of the Big Bang in order to be rushing outward, this reasoning went, they'll simply keep going.”

This is just so wrong I don’t even know how to unwrap it. There is no “gravity of the big bang.” If the galaxies keep going it is because the geometry of spacetime allows them to do so.

“Dark energy appears strong enough to push the entire universe - yet its source is unknown, its location is unknown and its physics are highly speculative.”

Here Easterbrook seems to picture scientists going around with a magnifying lens and saying, “where did I put the dark energy?” In reality, physicists believe that dark energy is everywhere.

“For a century, physicists have elaborately fine-tuned a concept of the natural world in which there are four fundamental forces: gravity, the ‘strong force’ that holds atoms together, the ‘weak force’ that keeps electrons in place around atoms…”

Wait, what?!?

First of all, the “elaborately fine-tuned concept of the natural world,” by which Easterbrook presumably means the Standard Model of particle physics, did not arise until the 1970s. Even wanting to be charitable, the idea that there was a fourth force was proposed 80 years ago at the most. But where Easterbrook goes stupendously wrong is in describing the weak force as holding atoms together. The weak (nuclear, to be precise) force is only involved in nuclear decay, and has nothing to do with keeping electrons in place around atoms. It is the electromagnetic force that does that.

Easterbrook goes on. “Entire academic careers have been made on work refining four-forces thinking.” Fair enough.

“Suddenly there's a fifth force, dark energy. And although it appears to comprise three-quarters of the universe, nobody's noticed it till now.”

Actually, although it is seductive to think that dark energy is a fifth force, it does not fairly represent mainstream physicists’ thinking, which is that dark energy is part of the stuff that fills the universe and that its effects on the expansion of the universe are gravitational--that is, entirely predictable within Albert Einstein theory of gravitation. (See Sean Carroll's Dark Energy FAQ.)

Moreover, physicists had been conjecturing the existence of dark energy (in its incarnation as the "cosmological constant") long before the accelerating expansion of the universe was discovered.

“It does not appear to be possible to make a vacuum in which there is nothing at all, so it is now assumed that intergalactic space is not a true void.”

But nobody ever assumed that. Space is crisscrossed by a multitude of photons—including those of the cosmic microwave background and those emitted by stars—as well as by neutrinos and by wandering cosmic rays particles such as protons.

And the fields of quantum field theory extend everywhere, so physicists did not really expect the vacuum to be totally empty. The point of the existence of dark energy is not about interstellar "true void." Anyway dark energy, assuming it exists, would not just pervade the interstices between elementary particles, it would be truly everywhere.

Finally, Easterbrook's concluding remark personally offended me as a Trekkie. “Intercourse between galaxies will become unimaginable," he writes, "even if Captain Kirk's warpdrive is invented.” But as far as I recall, Kirk never ventured much outside of the galaxy.

For reliable information about dark energy, I recommend Sean Carroll's wonderfully clear and thorough Dark Energy FAQ.

The views expressed are those of the author and are not necessarily those of Scientific American.

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