Science Talk

Dark Matter; New Daily Scientific American Podcast, 60-Second Science; Steve Irwin, the "Crocodile Hunter"

In this episode, MIT astrophysicist Paul Schechter offers some perspective on recent research about the universe's mysterious "dark matter." Journalist Karen Hopkin and host Steve Mirsky talk about the new daily Scientific American podcast, called "60-Second Science." And Scientific American magazine contributing editor Sarah Simpson shares some thoughts about the late Steve Irwin, the "Crocodile Hunter." Plus we'll test your knowledge about some recent science in the news. Websites mentioned on this episode include,, and Paul Schechter's home page,

Science Talk September 6, 2006 -- Dark Matter; New Daily Scientific American Podcast, 60-Second Science; Steve Irwin, the "Crocodile Hunter"

Welcome to Science Talk, the podcast of Scientific American for the seven days starting September 6th. I am Steve Mirsky. This week on the podcast, we are going to talk about one of the most mysterious objects in the universe--no not the teenage brain, that was last week--this week, the mysterious dark matter with MIT astrophysicist Paul Schechter. Journalist Karen Hopkin will tell us a little about this program's new companion, the daily Scientific American 60-Second Science podcast. We will test your knowledge about some recent science in the news, and Scientific American magazine contributing editor Sarah Simpson shares some thoughts about the late Steve Irwin. First up! Astrophysicist Paul Schechter. There was quite a buzz a couple of weeks ago when a NASA Harvard University study announced striking new evidence for dark matter. I want to check his perspective on the new findings. I got through to him last week while he was doing his own dark matter research at the Las Campanas observatory in Chile.

Steve: Professor Schechter thanks for talking to us today.

Schechter: My pleasure to be with you.

Steve: Can you give us the brief introduction to dark matter?

Schechter: Astronomers measure masses of things by measuring the motion of things, and so we measure the masses of stars by watching them orbit around each other. We measure the masses of galaxies by watching stars orbit in the galaxies, and by watching the motions of massive bodies around the other massive bodies, we can infer masses. In 1930, Fritz Zwicky, an astronomer in[at] Caltech measured the mass of the Coma cluster of galaxies--a cluster of galaxies, but it sounds like it's a cluster of milky ways like our own, hundreds of them--and when he measured the mass of that cluster of galaxy, he measured the mass, which was very much larger than the mass he would have gotten if he just added up the individual galaxies. And so he said, there is something else there.

Steve: And that something else has come to be what's known as dark matter.

Schechter: Yes, first it was people. Zwicky was a difficult person and for many years people just ignored him, but the evidence continued to accumulate. There was an interesting moment of excitement in the early 1970s with the Uhuru satellite. This was one of the first satellites to observe the sky and x-rays and the Uhuru satellite measured x-rays coming from the Coma cluster of galaxies. The x-rays are emitted by a very, very hot gas. It's called plasma. The presence of such a hot gas and the amount of gas employed by that x-ray emission was very much larger than the mass the people had thought was present in the galaxy and people thought they might have in fact found the stuff that Zwicky had been measuring--the dark matter.

Steve: They thought that, but what was the upshot?

Schechter: Well, the upshot was that it still fell short. Zwicky said that somewhere between the factor 10 and 30 more masses had been present than what is there is in the galaxies, and the x-ray-emitting gas is maybe a factor of five more than is present in the galaxies; but it's still not enough and probably fall[s] short by another factor five from Zwicky was seeing.

Steve: Okay, so what that means is that most of the matter in the universe would have to be dark matter.

Schechter: It's[If] the Coma clusters' representative, yes. Now, we have since looked for dark matter in many different ways that in fact that's a pretty good ratio--five times as much dark matter is matter in protons, neutrons, electrons, and stuff we're made of.

Steve: Okay, so we assume that there is always stuff out there that has mass and the influence of that mass is being felt, but we can't see it and we don't really know what it is.

Schechter: I wouldn't say we assume, but we have measured it.

Steve: Okay, right--we assume that based on the measurements, okay. So, we know it.

Schechter: I will compromise--we have inferred.

Steve: Okay, we infer on measurements. So, know let's talk a little bit about this latest research that got so much attention recently.

Schechter: What makes this interesting and [a] lot of fun is that a variety of different techniques have watched[worked] there on the same problem. We think that dark matter is very different from protons, neutrons and electrons and in particular we think that it doesn't interact with protons, neutrons and electrons because protons, neutrons and electrons pass light through it. They don't know they are there. And what you see in this particular cluster of galaxies is two concentrations of galaxies-- two clumps of galaxies--and also two clumps of hot x-ray gas; and the clumps of galaxies are further separated within the clumps of x-ray gas. The clumps of x-ray gas are closer to each other. We think the reason that they are closer to each other is that they pass[ed] through each other and slowed each other down. Galaxies appear mostly empty. There are a few stars and [a] great void so they can pass by through each other, and we think that the two clumps of galaxies pass right through each other, that the two gas clouds will collide with each other. And so we see two clusters of galaxies which pass through each other, and they are separating from each other; but two clumps of galaxies have a distance between them--the two clumps of gas are also separating from each other--but the distance between them is smaller. Thus far, there is nothing in what I said that is incredibly spectacular, but at third, we used optical techniques to see the galaxy and used x-ray techniques and the chances are like to see the x-rays. So far, everything is okay, but our third technique is how we got there in that. That's the technique called weak gravitational lensing and the idea there is that light passing gas mass first gets deflected, but it also gets distorted--you know, an image of an object seen behind a mass will be distorted in a phenomenon that's not unlike a mirage on Earth--and so, by measuring the distortions of objects behind this cluster of galaxies, galaxies that are behind this cluster of galaxies, you can measure the mass in this cluster of galaxies. And what's interesting is that that mass in the cluster of galaxies is not associated with the x-ray gas. There is five times as much mass in the x-ray gas as there is in the galaxy, but there is five times as much mass in something else associated with the galaxies then there is in the x-ray mass. The dark stuff has passed through the two clumps of dark stuff that have passed through each other and are traveling with the galaxies leaving the x-ray gas behind. It is mass actually--not the x-ray gas--that's doing the re-lensing, that's doing the distorting.

Steve: It's something else and that something else is the dark matter.

Schechter: Well, you know, physicists and scientists in general have a wonderful trick that they play when there is something they don't understand: They elevate it to some principle and do discovering; and so you don't say, I don't understand what's going on here, you say, I have discovered dark matter.

Steve: Right. Now, why is it so exciting if we are still inferring the presence of the dark matter based on our observations when that seems to me to be what we have always done about the dark matter.

Schechter: You are correct. What is different and exciting here is that the dark matter and the ordinary matter and particularly x-ray gas are not coexistent, and they are not co-located, you know. Because the x-ray gas interacts as a gas and gets slowed down and one of the two clumps shows this beautiful shock, you know, it looks like a wick of a boat. There is just no question but that one clump of gases pass[es] through the other at high speed--you see this wick from one of them, it's called the bullet cluster. It looks like a Harold Edgerton picture. It's spectacular. There is no question that the gases collided there, but the galaxies have traveled further than the gas. The gases then retarded and the dark matter-- whatever it is--that mass has traveled further and so the hot gas, which might have [been] explained through these observations, is not producing the gravitational distortion. What's producing the gravitational distortion is associated with the galaxy. So, what's special about this is that the dark matter and hot gas have segregated. They are not coexistent. You can look at the picture, and by god, they were in different places.

Steve: Tell me what are you doing in Chile?

Schechter: Well, I am studying dark matter, naturally. I study it by studying the deflection of light around galaxies in my taste rather than clusters of galaxies. It's very interesting, you know, on different scales. There is segregation between the dark matter and the ordinary matter. The solar system is entirely ordinary matter or almost entirely; what that means is that the ordinary matter has condense[d] within the milky way, and formed stars that are 100 percent ordinary matter. The dark matter has stayed much more extended the dark matter. But the thing that what we are seeing in this cluster of galaxies, or double cluster, is that baryonic matter, or ordinary matter, tends to interact with itself, and it gets slowed down in the course of these interactions. It also loses energy in these interactions and it can condense. The dark matter, as far as we know, doesn't interact with ordinary matter and probably doesn't interact with itself either, and so it can't lose energy, it can't condense, it can't collapse and cool, but stays how it is and stays more extended. So, I use gravitational lensing to study--as one goes from the center of a galaxy to the outer part of the galaxy,--what is the road of proportion of dark matter to ordinary matter. At the very center, we think it's mostly ordinary matter; at the edges we think it is mostly dark matter. They are, in this case, probably center the same, so it's not quite so dramatic to see the difference, you are just seeing a difference in proportions rather than a difference in central location. But I am trying to determine the fraction of dark matter as a function of position in these other galaxies.

Steve: How long have you been there?

Schechter: I got here--it's very hard, I counted nights rather than days.

Steve: Sure.

Schechter: I have been on the mountain for three nights. I have been observed[ing] for those three nights, and I am on the mountain for another three nights to work on some instrumentation here.

Steve: And it's very hard. People might not know, but when you get that incredibly precious time with the telescope, you are basically going to be pulling all-nighters for the whole week, right?

Schechter: Well, actually we are working on the instrumentation a little more relaxed, but yes, it's three successful all-nighters, and I went to bed at 7:45 this morning. And of course, my biological clock wouldn't let me sleep much more than about four hours.

Steve: Plus, you're all amped up because you're there in the first place.

Schechter: Yes.

Steve: And, high altitudes. So, there is not a lot of oxygen.

Schechter: It's not so bad. We are at 7,500 feet, you got three-quarters of an atmosphere. You know it. You know, if you know what your resting pulse is, if you're to lying down and resting up here, your resting pulse will be higher.

Steve: Did you say which specific galaxies you are looking at?

Schechter: The galaxies I am looking at are not people's favorites. The galaxies I look at are ones that happen to be in front of very bright objects behind them.

Steve: Right, because that makes sense in terms of what your approach [of]by trying to see the gravitational lensing.

Schechter: That's my topic, any galaxy that does need a favor of drifting in front of another. I say drifting in front; actually that drifting takes hundreds and millions of years.

Steve: Right.

Schechter: It is as instantaneously at this moment in front of another.

Steve: Right, and it actually happened probably millions of years ago.

Schechter: (Laughs) More than that.

Steve: Well, good luck with your particular search for the dark matter. I hope you'll keep us up to date on what you find too.

Schechter: Well, thank you very much. It's been a pleasure talking.

Steve: It's been our pleasure. Thanks a lot.

Schechter: Take care.

Steve: If you are interested in more about dark matter and gravitational lensing, Paul Schechter wrote a really nice accessible article on this subject that's available on the Web. Go to his MIT Web page that's at and then click on the link to the article called "Einstein's Mirage". Also, for more about dark matter and dark energy, which we didn't even discuss today, go to the Scientific American home page,, and hit the link for Ask the Experts; you'll see a nice explanation of the dark stuffs in the astronomy area from Robert Caldwell, a cosmologist at Dartmouth College.

Now it's time to play TOTALL.......Y BOGUS. Here are four science stories, but only three are true. See if you know which story is TOTALL.......Y BOGUS.

Story number 1: It really is tough to lose weight because you're fighting built-in mechanisms that your body has to try to keep the weight on.

Story number 2: An Australian academic's doctoral thesis consists of an analysis of Star Trek based on her study of every episode of the original series and its spin-offs.

Story number 3: Researchers are turning waste straw into something more useful than gold--liquid gas.

Story number 4: Paleontologists say most dinosaur species have already been discovered.

We'll be back with the answer, but first, there's a new member of the Scientific American podcast family. Every weekday we will be bringing you 60-Second Science, a minute packed with "sciencey" goodness. The programs are available free at and over at the iTunes music store. One of the voices you'll hear on 60-Second Science is veteran science journalist, Karen Hopkin.

Steve: Hey Karen. How are you?

Karen: I am great Steve. How's it going?

Steve: It's going okay. So, I think the first time I ever met you, you were operating the audio/visual controls in a lecture room at Einstein Medical School or the Albert Einstein College of Medicine as they prefer to be known.

Karen: I believe I was one of those unfortunate students that was losing the slide[s] down in the slots and begging people in the back row to lend me a pencil to try to pull them out.

Steve: Well, fortunately registering audio; so you did go on from running the projector there to get a doctorate in biochemistry from Einstein.

Karen: That's right. [I] actually finished my PhD in 1992; was working on superoxide dismutases and bacteria.

Steve: That's one of my favorite enzymes. I had to write an actual paper on that in a class I took in school.

Karen: [W]hat's up free radicals--what's not to love?

Steve: And you also have some radio experience with a program that I think a lot of our listeners had probably heard of.

Karen: I was a producer for three years for the Talk of the Nation's Science Friday program with Ira Flatow; and that was a really interesting experience to get to produce an hour-long radio program where we got to talk to guests [on]and all sorts of different topics. You know, I trained as the biochemist, but, you know, I got to interview people about subatomic particle physics or science education and anything that was in the news.

Steve: And also now this is really unusual because you wound up being a science journalist, but you're also the coauthor of a really big major textbook.

Karen: Yeah, I came onboard to work on the basic cell biology textbook called Essential Cell Biology, a sort sophomore/junior-level undergraduate textbook on cell biology; basically you know this is the gold jeep color-me-green kind of thing. The lead author is Bruce Alberts, a guy who was formerly president of the National Academy of Sciences. And this book is the companion book to the larger book that a lot of people know called Molecular Biology of the Cell; that book is usually called the Big Albert's or Fat Albert's, and this other book is known finally as Baby Albert's or in the German, Kleine Albert's. Actually says Kleine Albert's on the cover, you know, a kind of tricky.

Steve: So, you're going to be doing some of these--60-Second Science is the name of the new program.

Karen: Well, I am looking forward to doing it. I think what we are going to try to do is cover some things that are in the news that have to do with science and also some of the cor[quirkier] things that might fall through the cracks and that you wouldn't otherwise hear about. I think it's going to be a lot of fun.

Steve: Thanks for your time, Karen; and get back to work.

Karen: Well, thank you. I know I[’ve] got the following week of 60-Second Science stories to whip up.

Steve: Yes, please go whip.

Karen: Thanks.

Steve: Bye.

Karen: Bye.

Steve: Now, it's time to see which story was TOTALL.......Y BOGUS. Let's review the four stories.

Story number 1: Human body resists attempts to lose weight.

Story number 2: Star Trek doctoral thesis.

Story number 3: Researchers turn straw into liquid gas.

Story number 4: Most dinosaur species are now known to science.

Story number 1 is true. Research presented at a major obesity conference this week shows that the body has strong mechanisms that fight weight loss, but weak ones to defend against weight gain because starvation is really bad. For more, check out the September 4th story "Evolution, Not Just Gluttony, Lead to Obesity Pandemic"; that's in the news section on the Web site,; and click on this Thursday's edition of 60-Second Science at

Story number 2 is true. Dr. Djoymi Baker's doctoral thesis was titled Broadcast Space: TV Culture, Myth and Star Trek. She treated the series as mythology rather than schlock sci-fi and won a chancellor's prize for excellence at Melbourne University. Congratulations from Ee’d Plebnista, Dr. Baker.

Story number 3 is true. Agricultural scientists have a small pilot study going, in which they turned straw into liquid gas. A byproduct of the huge grass-seed industry is millions of tons of straw. Researchers are developing methods to reduce the straw to carbon particles and residue that can then be turned into liquid synthetic gas. If the system pans out, the seven million tons of straw produced each year could become 420 million gallons of liquid fuel, according to the Agricultural Research Service. That's seven million tons of straw produced in the Pacific Northwest.

All of which means that story number 4 about most dinosaur species having been discovered is TOTALL.......Y BOGUS, because researchers writing in the Proceedings of the National Academy of Sciences estimate that over 70 percent of all dino genuses are still out there to be found. You can read more in David Biello's September 4th article in the news section on our Web site [www.] titled "Majority of Dinosaurs May Await Discovery".

Monday morning came the news that Steve Irwin, better known as the Crocodile Hunter, had died after being stabbed by a sting ray. Irwin had a big TV persona, but he was also a committed conservationist who published some scholarly work as was discussed coincidentally in my column in the current issue of Scientific American magazine and back on the June 28th podcast. Scientific American contributing editor Sarah Simpson spent some time with Irwin in 2001 and wrote about him in the April issue that year. I called her Tuesday morning in Kansas City.

Steve: Hi Sarah, how are you?

Sarah: Hi Steve, how are you?

Steve: I am okay. So, you spent some time with Steve Irwin. What was your experience with him like?

Sarah: It was really quite remarkable and somewhat of a surprise. I went to Queensland, Australia and set up an interview with him at his Australia zoo where he grew there near north of Brisbane really wanting to find out if he was really the wildlife passionate, wildlife conservationist I had heard him say he felt he was; or whether he was just sort of the wild-man entertainer that we see on TV, and just get a hand on that. And when we arrived, he and his wife Terri and his producer John Stainton had set up a basically a whole big welcome for us and we sat around the table. I was there with my geochemist husband and students and a colleague and we just all sat around the table and had just an amazing conversation that was obviously incredibly genuine and from the heart.

Steve: So, what was it though that really convinced you that he really was on the level about conservation?

Sarah: I don't even think that if--that he knew really what my purpose was in coming there--to sort of try to really determine how committed he was; but every story he would tell about saving crocodiles in Indonesia or swimming with the sharks, whatever it may be, you know, there were always times when he would talk about the animals in danger and you know, there would be a tear in his eye, he would be choked up just a little bit, you know; not anything that came across as an act, but just as a person who is absolutely everything that you see on TV. That's just who he is; that is his passion, and he was just at every moment determined to get that across to his audiences, whether they were face-to-face or on television.

Steve: I know, I got to spend some time with great evolutionary biologist Ernst Mayr who then passed away. He was a [hundred]100 years old when he died, and you hear the news, you're saddened with--you know, this is a whole different thing. I mean, he was 44 years old.

Sarah: Yes, you know, and I hadn't even realized in retrospect that that he was, you know, just still in his thirties when I met him. He had--at the time we met his daughter, Bindi Sue, and she was probably two maybe three, and it's just heartbreaking to think that she and her younger brother will have to go on only seeing their father on these television programs.

Steve: Sarah, thanks a lot. I appreciate it.

Sarah: Thank you Steve.

Steve: Well that's it for this edition of the Scientific American podcast. Our email address is And also remember, science news is updated daily on the Scientific American Web site,; and don't forget the September single-topic issue of Scientific American is out. The subject is "Energy's Future beyond Carbon." Tune in next week to hear from one of the authors of that issue's article about the future of nuclear energy. For Science Talk, the podcast of Scientific American, I am Steve Mirsky. Thanks for clicking on us.

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