IRA FLATOW: A few weeks ago, on the day the film “Oppenheimer” came out, we talked about the history of the Manhattan Project, the legacy of the Trinity test, where the world’s first nuclear weapon was detonated in the desert of New Mexico, and we heard from a survivor of the Hiroshima bombing.

But our listeners responded with even more questions about what we couldn’t get to, including this one from Randy, in Orlando, who wrote, “I’ve heard Neil deGrasse Tyson say the new bombs aren’t that dirty.” I think he’s referring to the astrophysicist’s interview last November, in which he said, “Modern nukes don’t have the radiation problem. It’s a different kind of weapon than Hiroshima and Nagasaki.” Is that right?

We wanted to answer this question and others about current nuclear weapons technology, an important issue now because of Russia’s implied threats of using nuclear weapons against Ukraine. Returning to discuss is Dr. Zia Mian, physicist and co-director of Princeton’s Program on Science and Global Security. He’s joining me from Princeton, New Jersey. Welcome back to Science Friday.

ZIA MIAN: Hello, Ira.

IRA FLATOW: Nice to have you. OK. Nuclear weapons technology certainly has come a long way since the 1940s. So how do Oppenheimer’s bombs compare to the kinds of nukes we have today.

ZIA MIAN: So Oppenheimer’s and the team at the Manhattan Project’s development of atomic bombs relied on the process of nuclear fission, the splitting of atoms, heavy atoms, to release their nuclear energy. Modern nuclear weapons use those atom bombs just as the trigger for a much larger explosive process that relies on the fusion of light nuclei to produce energy.

So the Hiroshima bomb, for example, was of the 10,000 to 20,000 tons of chemical weapon equivalent. Modern nuclear weapons, like the one that the United States and other countries have, are in the hundreds. And the biggest bomb in the US arsenal today is more than 80 times as powerful as the Hiroshima bomb, because it relies on this thermonuclear process to create most of the energy that is released in the explosion.

IRA FLATOW: And let’s talk about the lasting effects. Let’s get right to the question asked by our listener. A couple of weeks ago, we talked about the effects of radiation on New Mexicans, on people in Hiroshima and Nagasaki, and other places where nukes have been tested. Neil deGrasse Tyson said, the nuclear fallout isn’t as bad. So there is no fallout from an H-bomb?

ZIA MIAN: That’s just not true. I’m sure he meant something else by that statement. Because just from the testing of thermonuclear weapons of H-bombs, we’ve seen enormous amounts of radioactive fallout traveling across the globe and remaining in the atmosphere for a very, very long time. In fact, almost everybody in the whole world has been exposed to radioactive fallout from the thermonuclear weapon testing that took place in the ’50s and the ’60s and the ’70s. And the carbon-14, radioactive carbon-14, released in those thermonuclear explosions is still up there in the atmosphere, and some of it has become incorporated into the food chain and is part of living things.

IRA FLATOW: How much more powerful are H-bombs than atomic weapons? You talked about it a little bit.

ZIA MIAN: So the modern nuclear weapons, the range for example in the US nuclear arsenal, the US has nuclear weapons with the explosive power a fraction of the Hiroshima bomb, as well as weapons that go all the way up to almost 100 times the power of the Hiroshima bomb. And the US had much more powerful weapons in the past, but it slowly retired those. Because as missiles became more accurate, you didn’t need so much explosive power. Presently, there are about 12,500 nuclear weapons in the world. They are enormously more destructive than the weapons that were first developed in the 1940s.

IRA FLATOW: And we have talked about how many times over you can kill all the people on Earth with this amount of weaponry.

ZIA MIAN: There are so many ways to do an assessment of the harm that these weapons would do to the world. But the biggest thing that we’ve learned about nuclear weapons, in terms of their effect on human beings, is that it’s actually the secondary effects that are absolutely catastrophic, not just the people who are immediately blown up or burnt or exposed to radiation. It’s the fact that thermonuclear weapons produce vast fires, much more fire than normal, old-fashioned nuclear weapons used to do.

And these fires will set entire cities ablaze, and the smoke and the soot from those fires will rise into the atmosphere and stay up in the stratosphere above the clouds– so it doesn’t rain back so quickly– for a decade or more, blocking the sunlight. And this stopping of the sunlight causes cold and dark on the surface of the Earth, freezing temperatures, and stops the growth of plants, and it will create mass famine.

IRA FLATOW: That’s Carl Sagan’s nuclear winter you’re talking about.

ZIA MIAN: That’s exactly right, and that was an idea that they first had in the 1980s, that Carl Sagan and others did. But they thought you needed thousands and thousands of nuclear weapons to do this. The most recent science suggests that even a few hundred nuclear weapons, like the numbers held by India and Pakistan, for example, if they fought a war with each other would produce enough smoke from the burning of their cities to actually create a catastrophic collapse of world’s food supply for most of the world for over a decade.

And the most recent estimate published last year says that more than two billion people could die around the world from a nuclear war between India and Pakistan, a war in which the rest of the world would have no involvement. And five billion people would die, five billion, out of the seven billion people in the world from a war between the United States and Russia, because their arsenals are just so much bigger.

IRA FLATOW: Let’s end on some good news, if we can. For decades, people like downwinders in New Mexico and beyond have been fighting to expand the government’s compensation program for victims of radiation exposure. And now, the Senate just voted to expand that program, which would end up including folks in New Mexico and more Navajo people. What is your reaction to this?

ZIA MIAN: This is taking such a long struggle by downwinders and by their supporters. And it’s welcome news, but I think the thing we have to remember is that the US tested nuclear weapons in the Pacific Ocean, as well in the Marshall Islands and elsewhere. And so there are lots and lots of people who were exposed to radiation from these tests, and so we have to accept the enormous humanitarian toll that was exacted by the development and testing of nuclear weapons, to say nothing of the use of nuclear weapons.

The new treaty on the prohibition of nuclear weapons, which is now signed by 100 countries– almost half of all the countries in the world have signed this treaty– bans nuclear weapons. It bans the testing of nuclear weapons, and it calls for assistance to victims and the remediation of the environment harmed by the testing and use of nuclear weapons. All the countries with nuclear weapons should accept that this is a responsibility that they have, because they made these absolutely terrible weapons.

IRA FLATOW: Yeah. I know that that’s one of your aims as co-director of the Program on Science and Global Security, at Princeton. How do you think it’s going? You mentioned this treaty. Is it heading in the right direction, or is it just something that is going to be a little bit too little too late?

ZIA MIAN: I think that it’s a very, very important step, because what the countries of the world have done is to show that they’re not ready to just keep waiting for the nine countries with nuclear weapons to do the right thing. And the fact that 100 countries have signed this treaty, despite objection from the United States and Russia and France and so on– the United States wrote to all these countries saying, you should sign this treaty.

IRA FLATOW: Really?

ZIA MIAN: Can you imagine telling people to sign a treaty that bans nuclear weapons? That’s how bad things have become, but the choice is becoming increasingly clear. On the one hand, the United States and others seem to be intent on modernizing and keeping their nuclear weapons. On the other side, the majority of the world’s countries says, look, this has to stop, and here is how it can stop.

IRA FLATOW: Zia, thank you for taking time to be with us today.

ZIA MIAN: Thank you, Ira

IRA FLATOW: Dr. Zia Mian, physicist and co-director of Princeton’s Program on Science and Global Security, joining me from Princeton, New Jersey.

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