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Astrophysicist Mac Low Explains the Invisible, WIMPS

Mordecai-Mark Mac Low
Astrophysicist Mordecai-Mark Mac Low in his office at the American Museum of Natural History's Hayden Planetarium. Mac Low is the curator of "Dark Universe," on view at the planetarium. Photographer: Manuela Hoelterhoff/Bloomberg

Jan. 21 (Bloomberg) -- Just the other day, dark matter was found hovering over the equator.

Or was it? The news made up for the rather gloomy tone of a column in the January issue of Scientific American, which concluded that the search for dark matter was starting to go cold.

The latest attempts to find WIMPS -- weakly interacting massive particles -- had come up empty in an underground research facility in South Dakota’s Black Hills.

WIMPS are the preferred candidates for dark matter, the invisible particles that hold our universe together. The gravity generated by normal matter can’t seem to manage the job.

“Dark Universe” at the American Museum of Natural History’s Hayden Planetarium in Manhattan is a glittering 25-minute trip into the cosmos. A gorgeous simulation shows how the motions of galaxies affirm the presence of dark matter. Then “dark energy” entered the picture, speeding up the universe’s expansion rate, and also upsetting me.

I was no longer interacting positively with the mass of material. Overwhelmed, I arranged to see the curator of “Dark Universe,” astrophysicist Mordecai-Mark Mac Low.

We spoke in his plain office at the Hayden Planetarium, whose couch pillows feature exploding supernovae that reflect his interest in star formation.

Mac Low, 50, is connected to a national supercomputer center with over 100,000 processors per machine. Once he got up and drew lines on his whiteboard, which showed scientists burrowing ever closer into the realm of WIMPS.

Galaxy Cluster

Hoelterhoff: So what is a typical day for you?

Mac Low: There is none. I just got off a teleconference with two colleagues about how supernovae explosions that stir the interstellar gas inhibit star formation. I am thinking about how to fund a postdoctoral fellow who is studying how planets form.

Hoelterhoff: How do you get your information?

Mac Low: I am a modeler. I get most of my information from supercomputer simulations. I use them to model the flow of magnetized gases under the influence of gravity, supernovae explosions and other sources of energy.

Hoelterhoff: And what do they tell you?

Mac Low: How stars or planets form.

Hoelterhoff: When did “dark matter” come into the vocabulary?

Mac Low: It was first used in the 1930s by Fritz Zwicky, a Swiss astronomer at Caltech, who talked about the necessity for there being cold, dark matter in galaxy clusters.

Hoelterhoff: Why both?

Mac Low: Dark matter -- meaning it is not light emitting. And cold in that it is not moving at the speed of light -- meaning that it could clump.

Hoelterhoff: And this stuff really keeps all the galaxies from flying into pieces?

What’s That?

Mac Low: We don’t know that for sure. There are reputable scientists that continue to develop the idea that we don’t understand the effects of gravity on the larger scale.

Hoelterhoff: Were you surprised to hear that a scientist in Texas thinks there’s dark matter over the equator?

Mac Low: Over the Earth’s equator? What? What does it look like?

Hoelterhoff: Disk. Big. Thick.

Mac Low: This is brand new. Let me pull this up. Hmmm. He finds the Earth is a very little bit heavier than thought? And thinks dark matter accounts for the discrepancy? No, I don’t think so. This is a bit naive.


Hoelterhoff: What about WIMPS? The Cosmic Dragnet column in this month’s Scientific American stated: “Dark matter, the invisible stuff thought to make up a quarter of the universe, has yet to show in even the most sophisticated experiment.”

What are the chances of finding dark matter in your lifetime?

Mac Low: If the hypothesis that particle physicists have proposed is correct, that the weakly interacting dark matter particle is what they describe as the lightest supersymmetric particle, we could know within the decade, and I have every intention of living that long.

Hoelterhoff: Is this purely theoretical physics or could there be a practical application at some point? The way nuclear fission ended up as nuclear energy and also weapons?

Manipulating Mass

Mac Low: No one has identified a practical application. I wouldn’t rule it out -- if you gain an understanding of how mass came to be, I suppose it’s possible that you could learn to manipulate mass.

If that were so, it’s likely centuries away, and I am speculating wildly to even consider the possibility.

Hoelterhoff: Your “Dark Universe” is exceptionally beautiful and illuminating. Yet I did find dark energy was hard to take in after wrestling down dark matter.

So help. How do you weigh energy and decide it is 70 percent of the stuff of the universe, while dark matter accounts for almost all the rest, with just a small percentage left over for normal matter.

Mac Low: Like matter, energy can exert gravitational effects. This equivalence is described by Einstein’s famous equation, E = mc2, which relates energy E to mass m, via the speed of light c.

Dark energy is causing the expansion of the universe to accelerate. We can measure the expansion rate rather well at this point –- there are at least six independent ways of doing it, only one of which we describe in the show.

From the expansion rate, we know the amount of energy, and thus its equivalent mass, which is rather large compared to the mass of visible or even dark matter.

Time Travel

Hoelterhoff: Look up to the sky and you look into the past. Is it possible to see the future in some way?

Mac Low: The only way is to go there. If you travel close to the speed of light away from the Earth and then back to it, you will have aged far less than those left behind on the Earth, and will effectively have embarked on a one-way trip into the future.

Hoelterhoff: I haven’t heard much lately about black holes. Why is that?

Mac Low: Black holes turn out not to be cosmologically significant. They aren’t a significant fraction of the total mass.

Hoelterhoff: Any good for time travel?

Mac Low: No. Affects your health. If you could time travel, you might absorb an infinite amount of energy on the way.

(Manuela Hoelterhoff is an executive editor at Bloomberg News. Any opinions expressed are her own. This interview was adapted from a longer conversation.)

To contact the reporter on this story: Manuela Hoelterhoff in New York at

To contact the editor responsible for this story: John Brecher at

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