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TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. Yoga was once an esoteric practice, but now it's a global phenomenon. Many claims are made about the physical and mental benefits of the practice. My guest, William Broad, wanted to see what the scientific research has to say. He's a science reporter for the New York Times and has been practicing yoga since 1970, when he was a college freshman.

His new book is called "The Science of Yoga: The Risks and the Rewards." He found plenty of risks, but concluded they are outweighed by the benefits. In addition to examining the scientific research, Broad traces how yoga evolved from an obscure cult steeped in magic and eroticism to what some people now describe as the yoga industrial complex.

Many people who practice yoga see it as a spiritual discipline. In the book, Broad acknowledges that there is much about life that science cannot address, much less answer, and he says many of yoga's truths surely go beyond the truths of science.

William Broad, welcome to FRESH AIR. So you've been practicing yoga since 1970. What were some of the questions that occurred to you that you wanted to investigate as a yoga practitioner?

WILLIAM BROAD: A basic one, a basic question that had set me in a skeptical frame of mind came to me when I was working at the University of Wisconsin on a research physiology project. And while I was doing that, I learned that no matter how fast or slow you breathe, you can't change significantly the amount of oxygen that you take into your body. That's contrary to so many yoga teachings.

The physiological truths and falsities of yoga were the thing that I was most curious about, and I thought it would be a pretty simple path to follow, but that turned out to not be the case.

GROSS: So I just want to say here, you're still practicing yoga. You still find it beneficial.

BROAD: Absolutely. Absolutely.

GROSS: But at the same time, one of the things you write about - and this is probably what's gotten most attention from your book - is injuries that yoga can cause. And you had an injury. What happened to you?

BROAD: I - this was right at the beginning of my book research in 2007. I was in a Pennsylvania studio, and it was an advanced class. About half the people in there were going through teacher training. And I got into what's called a side-angle pose, and we were working as partners.

And my partner was this beautiful young woman, and I was there kind of really strutting my stuff, right. I was leaning way far over, farther than I had been before, and I was chatting with her, right, you know. And boy, everything was great, and I was really, you know, up to - I mean, I'm a cranky old journalist, right, and here are all these young, beautiful, flexible women, right, and I'm, like, feeling pretty good.

And then my back goes out, and it's, you know, you don't think. It's everything is feeling. You know, there's fire in the base of my back. You know, I can't see the room for the tears. My knees collapse. I slam into a wall. I'm basically in a heap on the floor while all these people rush over, you know, what's wrong and are you OK. And, you know, it was huge humiliation.

Now, that wasn't probably an original injury, right. I've got a weak lower back. But I should have been smart. I was distracted, and I produced a bad injury that took, you know, a couple months to recover from.

GROSS: So how did your injury practicing yoga affect how you practice yoga?

BROAD: The injury didn't change me so much, right. I mean, I didn't do that particular pose again. It wasn't a pose that I did as part of my regular routine. But what did change as I advanced in my research was discovering what I call the super-dangerous poses, right, the ones that can kill you or give you a stroke. And those things I've eliminated from my routine.

GROSS: What are some of those?

BROAD: They are ones that really bend your neck a lot: the shoulder stand, the plow, those kind of things. You're putting your neck - you know, if you think of the neck as usually going, you know, up and down in a line, you're rotating it around about 90 degrees. And it turns out that there are these delicate arteries that run through your vertebra called the vertebral arteries, funny enough.

And if they get tweaked around too much, in rare instances - this is not a common thing, but the clinical literature is so clear, and I run through this in the book, and you see it case after case after case - if you crank those arteries around and you're unlucky, you can tear the linings of the arteries. You can have clots, and you get up from that pose, or you're even in the middle of a pose, and all of a sudden those clots move into the brain and you've got brain damage. You know, some small fraction of people die, you know, from that injury.

Many of them have years of therapy that they have to go through to try to regain, you know, some of the lost functions. I mean, once I hit this kind of rough patch of research, I said whoa, not for me. Thank you very much. I can eliminate that easily.

GROSS: So the injuries you were describing come from what sound to me like pretty extreme positions.

BROAD: Yes.

GROSS: And it sounds like the number of people who actually have those injuries are pretty small. But what are some of the typical injuries that people practicing yoga get?

BROAD: So this great guy named Loren Fishman, who is an adjunct professor at Columbia University and is here in New York City, did this wonderful, enormous survey of more than 1,000 yoga teachers, therapists, and they found that the vast majority of the injuries that people suffered focused on the lower back.

And then in declining order of prevalence, the other problems were the shoulder, the knee and the neck. So it's all - it's mostly complex joints, right, that - where people have problems. And you have dislocations and tears, ligaments and all, you know, kinds of nasty stuff. So there - it's a wide range, but generally, you're not just talking about torn muscles, you're talking about joint problems in one way or another.

GROSS: So you say in your book that you think up until recently, that a lot of yoga teachers and yoga organizations were pretty much in denial about the downside of yoga, the possibility of injury. And now has that improved?

BROAD: Yes. There's been a marked improvement. And if there's one thing I felt bad about with the New York Times excerpt of my book is that they didn't have the space to go into the reform movement.

There is a very large, very intelligent group of people out there - I'd say led by the Iyengar people - who are busy making yoga safer. They are...

GROSS: Do you want to spell that?

BROAD: I-Y-E-N-G-A-R, Iyengar. He is one of the founders of one of the great branches of modern yoga. And they are using props to ease the poses. They use blankets with the shoulder stand to decrease that angle, so your neck isn't flexed at 90 degrees. They do all kinds of smart things.

And I couldn't believe the response I got to this New York Times magazine article. You know, dozens and dozens of groups that I knew nothing about are out there refining poses, doing smart things. There's this group in New York City called Smart Bodies, right. They're yogis who are dedicating themselves to making yoga better.

I mean, that's such a radical thought, right. Most people think of yoga as thousands of years old and perfect. Well, it's changing a lot, right as we look at it.

GROSS: Now, of course, you look at the benefits of yoga in the book. So let's go through some of the benefits that yoga practitioners and yoga teachers claim you get from yoga and see what science has to say about it. Let's start with spinal flexibility, helping to prevent spinal disc degeneration.

BROAD: Right. There's all sorts of wonderful science which I point out in the book that shows - that illuminates that these claims have a lot to them, right. Spinal degeneration, they had an excellent study, control group of people who didn't do yoga and people who did. And they came up with a very firm conclusion that they, the yogis, had less spinal degeneration.

And that makes sense. If you're bending it back and forth, you're stimulating blood flow, you're getting things cooking, and you can keep those pads between the vertebrae from drying out, which is a main problem of aging, right. We all shrink a little bit as we get older because those disks become desiccated. They become dry.

So, yeah, getting them - the fluids moving through them, it makes a lot of sense, in a way, right, if you don't take it to extremes.

GROSS: So cardiovascular benefits, what do yoga practitioners claim, and what does science have to say?

BROAD: You know, cardio is a complex issue, because on one hand, yogis - especially aggressive schools - will say we are a great aerobic workout. As the book shows in a lot of detail, that's false. There's been study after study after study saying you do not get your heart pumping the way you do in aerobic sports like running, like spinning, like swimming, all that kind of stuff, right.

On the other hand, yoga has this remarkable quality to relax you, to de-stress you. That means your heart rate goes down. That means if you're prone to hypertension, that lowers. There's all these wonderful cardio effects that come from just the other end of the spectrum, right, the relaxation of the heart muscle, the relaxation of the body rather than the pumping-up phenomena that you get from aerobic sports.

And those things are really, really important, the downside, the relaxation side, because heart disease is a number-one killer in industrialized societies. So the fact that yoga has this powerful benefit, which is not something that's routinely claimed by yogis, the fact that it's there is important.

GROSS: If you're just joining us, my guest is William Broad, science reporter for the New York Times and author of the new book "The Science of Yoga: The Risks and the Rewards." Let's take a short break here, and then we'll talk more about yoga. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: If you're just joining us, my guest is Pulitzer Prize-winning science reporter for the New York Times, William Broad. He's the author of the new book "The Science of Yoga: The Risks and the Rewards." He's been practicing yoga since 1970. On the other hand, he had a serious back injury as a result of one of the poses, so injuries is one of the things he examines in the book.

So in order to get significant health benefits from yoga, is there a kind of general agreement about how often you have to do it, how much time per session you have to do?

BROAD: No, I don't think there is a general agreement. There are lots of different paths and lots of different techniques, right, but the - what my teacher told me back in 1970, and I think this is true, and it's a formula that works: If you ingrain this in your life and do a little bit every day, even if it's five or 10 minutes, that's how you get hooked, and the benefits start to accrue.

It's like, you know, putting a little bit of money in the bank every week or every month, right, a little bit out of the paycheck. The payoff comes as, you know, these things start to multiply. So there is wide consensus on a little bit done with regularity is better than these kind of episodes where you don't do anything for a while and then start tying yourself into knots. That's where you get into problems.

GROSS: Some people exercise because they want to stay trim. Will yoga help them stay trim?

BROAD: It is one of the big myths. Yoga, if it does one thing extremely well, it relaxes you. What that means technically in physiological terms is that it will reduce your overall metabolism.

Everybody knows yoga produces outer flexibility, right. That's the iconic kind of pretzel pose. What people don't realize so much is that yoga also induces a deep inner flexibility, an inner physiological litheness, you know, where you can, you know, kind of move around in lots of different ways.

And what means is that your body can easily drop into a lower metabolic state. Now, if you take up yoga, and you keep eating the same number of calories every day, that means you're going to burn - your metabolism's going to go down. You're going to burn fewer calories. And that means you are guaranteed that you're going to gain weight.

Now, I don't want to overstate this, right, because, you know, the subtlety of the science and the subtlety of the art is that yoga can help you lose weight, but it tends to be psychological. Yoga helps you develop willpower. Yoga de-stresses you. Yoga can help break the cycle of stress-eating, right. You're getting awfully nervous, you just don't feel right, and you run to the refrigerator.

Yoga can break that, right. It helps instill the kind of self-discipline and the kind of relaxation that lets you escape that kind of response. So, overall, yoga can help, but to the extent that it does, it's not because of some kind of magic bullet or magic physiological change. It's despite it.

GROSS: Let's talk a little bit about how yoga quiets the nervous system.

BROAD: Oh, it's a wonderful, amazing part of the story, and I enjoyed researching it so much. I studied with this wonderful guy in Pennsylvania named Mel Robin, who I think of as sort of the Einstein of scientific yoga. He talks a lot about the two sides of the nervous system, the sympathetic side, and the parasympathetic side.

The sympathetic side is kind of the fight-or-flight side, and the parasympathetic is what they call the rest-and-digest side. Yoga presses the rest-and-digest side. It presses the body's brake, right. It slows you down, and it does this on all sorts of levels throughout your body.

And you - they can measure it in the nervous system. They can measure it in the bloodstream. You can see it happening in the brain, all kinds of interesting scientific indicators of this effect. But it's pronounced. It's like we have two main energetic pedals in our body, the accelerator and the brake, and yoga is tremendous for pressing the brake.

GROSS: So the effect of that is similar to, say, the effect of meditation. What is it about yoga that would give the same effect as meditation, or a similar effect?

BROAD: It is - right. It's this - and it might work better than meditation in some ways, right, because you're dealing with so many aspects. I mean, it's not just psychological and mental. You're also doing breathing. You're also doing these postures. If you're in advanced yoga, you're doing concentration exercises. So it's this sort of arsenal of techniques that produce this relaxation response.

Just the simple practice of breathing slowly, of inhaling slowly, holding it, exhaling slowly, holding it out, you do that for about two minutes. And that's a very common yoga technique done, you know, at the beginning of classes. You can just feel - especially after you've been doing it for a while - waves of relaxation.

And it's not because you're holding in more oxygen. You're actually holding in more carbon dioxide. But that does all sorts of interesting physiological things, including helping press this physiological brake.

GROSS: So what does scientific research have to say about how this kind of breathing and relaxation - while going into poses that stretch the muscles - how does that affect mood, and why does it affect mood?

BROAD: You know, the overall picture is extremely complex, and science - as I try to show in the book - we're just on the sort of cutting edge of really getting a scientific grip on this.

But you can start to see little glimmers of understanding coming in on all this. One of the great ones that I end the mood chapter on is this body of research that's been done at Boston University and Harvard, up in Cambridge, where people looked at advanced yogis, you know, from many different styles, and they did brain scans and they tracked neurotransmitters, and one in particular called GABA.

And GABA is central in mood control and acts as - as the levels rise, it acts as a powerful antidepressant. You know, and they watch these levels of these neurotransmitters rising as the yogis would, you know, do their routine, including all these poses and breathing and all that kind of stuff. They went up.

And these people - and it turned out that the more experienced yogis had higher rises in GABA. You know, it's just - it's something that people know, teachers talk about it. It's one of the many claims, you know, yoga, and it's the one thing that I felt overwhelmingly when I started in 1970: Yoga makes you feel really good.

Well, we're just starting to figure out why, you know, and one of the reasons is this surge in neurotransmitters.

GROSS: Now, through the history of yoga, as you point out, there have been a lot of claims about how yoga can be, like, an aphrodisiac, how it can increase sexual feeling and sexual pleasure, and some yoga early on was apparently all about that. So has there been scientific research into the sexual effects of yoga?

BROAD: Yes, there - it turns out that there's a very strong body of evidence - it's kind of hidden down there in the weeds, but you can see over and over, people seeing rises in sex hormones, particularly in testosterone, brain waves getting zipped up in the same way that lovers' brains look when they're in deep pleasure.

There have been these beautiful studies at the University of British Columbia, where they show that even fast breathing, you know, produces very strong states of sexual arousal. And just recently, there have been studies done in India where they looked at more than 100 couples, married couples, you know, who took up yoga and then had surveyed them, you know, before and after: What did this do to your sex life?

And, you know, across the board, it's, you know, improvements in desire, arousal, orgasm, overall satisfaction. Men have better erections. Women feel more emotional closeness with partners. You know, it definitely does lots of good stuff.

GROSS: I am so discreet, I will not ask you...

(SOUNDBITE OF LAUGHTER)

BROAD: Thank you.

GROSS: ...about how yoga has affected that part of your life.

BROAD: You can talk to my wife. You can get her on the show and ask her.

GROSS: I'll bet I can, yeah.

BROAD: But I'm going to dodge that bullet.

(SOUNDBITE OF LAUGHTER)

GROSS: William Broad will be back in the second half of the show. Broad is the author of the new book "The Science of Yoga: The Risks and the Rewards." He's a science reporter for the New York Times. I'm Terry Gross, and this is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. I'm Terry Gross, back with New York Times science reporter William Broad. His new book is called "The Science of Yoga: The Risks and the Rewards." The book is about what the research shows about yoga, and it traces the evolution of yoga from an esoteric practice to a global phenomenon.

On of the things that you write about in the book is how yoga teachers aren't certified. So there is no - I mean you're very concerned that there's no, like, regulation of yoga or the teaching of yoga. Why are you concerned about that?

BROAD: You know, I don't want to sound like I am, you know, advocating all kinds of red tape, right? I mean, it's kind of - yoga's a beautiful thing, in a way, in that it's, you know, so freeform and it - kind of the spirit of yoga is anti-regulation, right. But in that, there is tremendous abuse, and a huge range. Like, in the beginning of my book, I have a styles of yoga page, you know, where you talk about 10 or 11 styles, and you can see it right there. The Iyengar School requires a minimum of two years of study before they will certify a teacher. But hey, it's the Wild West of yoga training out there. You don't even have to go to a studio to get a yoga certification, right. You can do it online.

Now, probably most yoga teachers have had some sort of in-studio certification, but the minimum requirement is 200 hours. You know, it's not a whole lot. It's a very minimal requirement. And in that, in the inexperienced nature of some of these teachers, I think you find the biggest dangers in yoga and the biggest dangers of injury, right? Because, as I discovered, some of these poses can be really extreme, much more stressful than I ever imagined. But the real problem arises when you get a real inexperienced teacher, or one of these teachers who think look, I'm so flexible, all my students should be. You know, they've been practicing with me for six weeks now. They should be able to get into that pose, right. They try to force them, and you do serious, bad damage.

So I don't know how it should happen. And, you know, the states are trying to start to regulate this, and there are other regulatory moves afoot. I don't know what's right and what's wrong. I do know that yoga teachers hate the idea of red tape, and I don't particularly like it, either. On the other hand, people are paying a lot of money for teaching that's sometimes shoddy, and that's a bad thing.

GROSS: Now, in your book you write about the history of yoga. And you say before yoga was emphasizing fitness and health, in its early history, yoga was steeped in magic and eroticism. And I want you to talk about the 40-day burial that...

(SOUNDBITE OF LAUGHTER)

BROAD: Right.

GROSS: ...a yogi in 1837 did to demonstrate his spiritual powers.

BROAD: Yeah. That was so much fun. You know...

GROSS: For you.

(SOUNDBITE OF LAUGHTER)

BROAD: Well, yeah, it was to write about, right? I mean, it's just - it starts chapter one in the book. And it was done at the Court of Runjeet Singh, this Sikh who ruled over Punjab. He was a, you know, a military man and a very - a kind ruler. And he invited this yogi to go demonstrate his powers, and he buried him for 40 days and 40 nights. And it was a big, elaborate deal.

You know, I had such a great time researching this, because I had this opportunity to weave my way through all these old books. And, you know, you'd hear and read about this account from, you know, several different points of view. Books were published about this episode in, I think, London and New York and Vienna and, you know, it was a big deal back then, right? And this guy goes out for - maybe I can just crack open the book, because his line - I just love it. I'm just - so this is just the beginning of the first chapter.

(Reading) The internment lasted 40 days and 40 nights, a period that, from biblical times, has stood for completeness and unbroken cycles. Then the king rode up on an elephant, dismounted before his assembled court and surveyed the results. The linen bag looked mildewed, as if it had lain undisturbed for a long time. The yogi's legs and arms proved to be cold, stiff and shriveled, his skin pale. No pulse could be detected. Then his eyes opened. The yogi's body convulsed violently. His nostrils flared. A faint heartbeat could now be heard. After a few minutes his eyes dilated. His color returned. Seeing the king nearby, the yogi asked in a low, barely audible voice: Do you believe me now?

(SOUNDBITE OF LAUGHTER)

BROAD: I just love that, you know. When I read that line when I was doing my research, I went, yes. Oh, it's - you know, you can't make up that stuff. So here - this became a very - this kind of magical power thing, right, became an important element that drew scientists to yoga. And in that first chapter, I follow this young, ambitious Indian doctor who worked - was trained in Calcutta and ended up in New Delhi and dug into this case and made all kinds of interesting discoveries about yoga physiology, about how the body slows down and the tricks that yogis used back then and today to slow themselves down.

And to me, it was fascinating. His synonym for yoga was human hibernation, right? I mean, that's so contrary to our view to the kind of, you know, magic pill to boost your, you know, metabolism and rev you up and get you going. Back then, it was all seen as this way to slow you down, which is what it does.

GROSS: If you're just joining us, my guest is New York Times science reporter William Broad. He's the author of the new book "The Science of Yoga: The Risks and the Rewards."

Let's take a short break here, and then we'll talk some more about yoga. This is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: If you're just joining us, my guest is William Broad. He's a science reporter for The New York Times and author of the new book "The Science of Yoga: The Risks and the Rewards."

Early yoga books, you said, had an emphasis on sexual stimulation, and even some of the positions were designed to sexually arouse the practitioner. And you said that that actually discredited yoga in a lot of people's eyes.

BROAD: Yes. It was - and this is something that's not well-known, and it sort of blew me away as I stumbled on the research and saw the scholars, good researchers saying this over and over. Everything that we do today - Hatha yoga, right, everything, all the postures and all that stuff is generally a branch of Hatha. Even, you know, Bikram, that's Hatha yoga, Iyengar, that's Hatha. It's all Hatha.

But if you go back, it turns out Hatha is a branch of Tantra. Tantric yoga is sexual yoga. Now, the sexuality, in some respects, is symbolic, right, trying to draw the ying and the yang - or whatever you want to call it - of the universe, the male and female components in these deep states of enlightenment. But Tantra was also very aggressive to get into the physical states, where men and women would get together and they would stimulate themselves and they would try to reach enlightened states of consciousness through sexual arousal. And many of the sacred textbooks of Tantra go into various rites - sexual rites that would be practiced in groups or individually to try to achieve this sexual arousal.

Well, to me, one of the most fascinating chapters in the history that I uncovered was how yoga, in about the 1920s, got picked up by the Indian nationalists and got laundered. They said, look. We're trying to free ourselves from these British overlords who have mistreated us terribly. And we are going to use yoga as an indigenous art, and we are going to clean it up, and it is not going to have all these sort of sexual associations and seamy underside to it. Because many, many, many people were worried that it was just - the sexual side was being used as a pretext for wild orgies, right? There's plenty of documentary evidence on that score.

So they tried to ditch all that. Yoga was going to be sanitized, and fitness and health, and they created this new thing: modern yoga. It arose in the '20s and '30s, and it's what we do today.

GROSS: Now, you think yoga has arrived at a turning point where it can either grow up or remain an infant. What do you mean by that?

BROAD: I am a science journalist, and I'm looking at this through a scientific lens. And I think we are at a wonderful juncture where science is really starting to understand yoga and what it can do by way of benefits, by way of good things, and there are lots of them. And I think that, you know, yoga can continue down this road. Yoga can become, you know, a - like yoga doctors, right, professional accreditation, people who are trained to uniform standards, people who understand the science and what these yoga positions are really doing, what they can do, kind of, you know, tweaking our own internal pharmaceutical complex, right, to produce the hormones that you need and want.

You can see glimmers of this. NIH this last year spent about $7 million on this. That's not a lot of money, as federal financing goes. But even that little bit of money is starting to produce really important research, really clarifying a lot of areas.

If we spent 50 million as a society, or $100 million a year on this, I think it would produce a revolution. I think yoga would go professional. It would become an important part of health care. It would become an even more important part of disease prevention. It would be integrated into - more deeply into our society. Sat Bir Khalsa, who's a wonderful yoga researcher at Harvard, he says it's got to be like brushing your teeth, where you have to, you know, know the value of this through scientific studies. It's going to prevent decay, and you're going to do it every day. It's just part of the routine, right. It becomes socially accepted.

I think there's a chance that that's in the offing, that 30 or 40 or 50 years down the line, yoga will have grown up and have undergone that kind of transformation.

GROSS: You're a science journalist, but you write that you know that science can't address, much less answer many of the most interesting questions in life. And there's probably a lot of questions - maybe there's a lot of questions that it can't answer about yoga?

BROAD: I would say so. Science works wonderfully at the things it can do. And there are lots of things beyond physics, metaphysics and big questions and things. What is enlightenment? What are these states that yogis are going into? You know, if the Punjab yogi really did go out for 40 days and 40 nights, what was going on there? Where was he? What kind of psychological state was he in? Was it some kind of universal consciousness? I don't think science is ever going to get close to producing a rational answer to that question. But it can take us a long way toward it. It can answer a lot of interesting questions that come up along the way.

GROSS: And debunk a lot of things, as well.

BROAD: Yeah. I can tell you it's - you know, that's what the book does, right. It goes through 150 years - more than 150 years of research to show what's real and what's not, what works and what can be dangerous.

GROSS: If you're just joining us, my guest is William Broad, science reporter for The New York Times and author of the new book "The Science of Yoga: The Risks and the Rewards."

So let me move to a completely different subject now. One of the things you've been writing about for the Times is the state of Iran's nuclear program. And with talk that Israel might try to bomb that program, I'm really interested in hearing what you know about how advanced Iran's nuclear program is.

BROAD: Well, you know, one reason I do yoga and enjoy it is because of these de-stressing effects, right? And, as I said, I spend a lot of time for The New York Times, and have done so for decades, on these nuclear matters. Iran, compared to lots of nuclear states, is in the kindergarten stage of advancement. They're not where the United States or Russia or China or France or Britain or a lot of people are; Pakistan, Israel. But they have been making lots of progress across lots of fields - not just nuclear but also with long-range rockets, which they're pushing for. To me, the evidence is overwhelming. There's no question that they are positioning themselves to go for a bomb if they decide to do that. The whole enrichment of uranium that they're doing clearly points in that direction. They're not really interested in making reactor fuel. They'd do it a different way if they were.

They're developing a capability to get bomb fuel if they decide to do that. And they haven't. And guess what? They're not breaking any laws in doing that. The Non-Proliferation Treaty, the nuclear Non-Proliferation Treaty, lets countries do enrichment and do that kind of stuff. It's only if you divert that material, it's only if you divert that enriched uranium to a bomb-making program that you've violated the law. And the atomic inspectors have found no evidence of that.

GROSS: But they suspect it's happening anyways.

BROAD: They suspect that Iran is getting to the point where it could make a bomb if it wants to, but that it hasn't gotten there yet. Right, they're clearly doing all the research that would get them there.

GROSS: So is there an estimate of how far away they are from actually having a bomb?

BROAD: You know, Terry, I can't go there. There's so many wild estimates, right?

GROSS: Mm-hmm. Mm-hmm.

BROAD: And it's everything from, you know, hype and, you know, a lot of it is guesstimates, right? Because this is a complex technology. It's taken them a long time. How long? Maybe there are experts out there who have a good feeling. But I think, you know, it depends mostly on Iran. What are they really trying to do? Is this just brinksmanship and trying to, you know, raise the stakes for a great grand bargain?

A great negotiating ploy? Or do they really want to go for it? And that will determine the timeline to a large extent.

GROSS: I don't know if you can speak to this or not, but several Iranian nuclear scientists were killed, were assassinated, at least it's assumed that it was basically assassinations. And it's unclear to me whether that was meant to be intimidation or whether it was meant to kind of cripple the nuclear program by killing off the biggest brains behind the program.

BROAD: Look, they just had an explosion at a missile base west of Tehran. The Iranians lost the founder of their missile program and a bunch of his top lieutenants. They've lost people right at the top of the nuclear research hierarchy. The guy who now heads the nuclear research program was in a car bombing incident. He walked away from it.

There's no question that whoever is doing this is attempting to do a decapitation strategy and it's also clear that they are hoping it does lots of other things: intimidate, cause people to have second thoughts about whether they want to go in to nuclear research because, oh, my god, I might be risking my life.

You know, maybe they're thinking of defecting, you know, getting out of this rat race and trying something else in another country where I could really use my talents for good rather than for this evil regime or whatever. Right? That kind of activity can have all kinds of implications. But there's no doubt that part of it is decapitation - going for the best and the brightest, the brains, the people who know best how to pull off this extremely difficult job, which is to, A, make a bomb, and, B, find a way to deliver it over long ranges, preferably atop a missile.

GROSS: So, you know, there is the possibility that Israel will try an airstrike on Iran's nuclear program. From what you know, and I know you can't answer this question. Nobody really can probably right now, but from what you know, is the program defeatable by an airstrike?

BROAD: I don't know. I wrote a whole piece for The New York Times on tunneling. Ever since the Iraq-Iran war, these people have been tunneling like crazy. The country is honeycombed with deep tunnels. You see it at their main nuclear site at Isfahan. You see it at their Fordo complex, you see it outside their Natanz complex. The hills are honeycombed with tunnels.

You get a deep tunnel under a granite mountain and people are going to have a very hard time destroying that site. Now, that said, there are clever things out there, right? And they all aren't - have to do with hammers and anvils and dropping big heavy things. Some of them are electronic. We've seen computer viruses do amazing things in Iran and maybe we'll see more of that. I don't know.

But for that big - nobody, I think, in the West can answer that big question. They probably have intuitions. But whether or not an attack can really succeed, however you define that success, I think is an open question.

GROSS: Well, William Broad, I want to thank you so much for talking with us.

BROAD: No, no. It was a pleasure. Thank you, Terry.

GROSS: William Broad is a science reporter for The New York Times. His new book is called "The Science of Yoga: The Risks and the Rewards." You can read an excerpt on our website, freshair.npr.org.

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