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Are Protons Even Smaller Than We Thought?

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Are Protons Even Smaller Than We Thought?

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Are Protons Even Smaller Than We Thought?

Are Protons Even Smaller Than We Thought?

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  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
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An international team of physicists reexamined the radius of a proton, and found it to be 4 percent smaller than previously thought. Are they mistaken, or is something missing from the long-held theory of quantum electrodynamics? Physicist Brian Odom of Northwestern University discusses.


This is SCIENCE FRIDAY, from NPR. I'm Ira Flatow.

Last week, an international team of physicists reported that the proton - you know, the centerpiece of the atomic nucleus - could be smaller than we had all previously thought, not much smaller by numeric standards, only about four percent, but large enough to have physicists around the world scratching their heads.

Joining me now to talk about the implications is Brian Odom. He's an assistant professor of physics at Northwestern University in Evanston, Illinois. Our number: 1-800-989-8255, if you'd like to talk about it. You can also tweet us @scifri, @-S-C-I-F-R-I.

Welcome to SCIENCE FRIDAY, Dr. Odom.

Dr. BRIAN ODOM (Assistant Professor of Physics, Northwestern University): Thanks, Ira.

FLATOW: Why is this such an important finding?

Dr. ODOM: Well, there is really a surprise here. These folks have been working on this experiment a very long time, and they expected to measure a number which was in agreement with previous measurements, the proton size. And instead, they were very surprised to find strong disagreement.

FLATOW: And what does that mean to the physics world?

Dr. ODOM: Well, it could be very exciting. It could be boring. It could be someone did an experiment wrong, or someone, you know, dropped a minus sign in a calculation, or it could be very exciting. There could be new physics out there that we don't know about yet.

FLATOW: When you say new physics, you mean that we have to change our understanding about what we think the atoms are made out of?

Dr. ODOM: Not exactly the atoms, but it would be - it's possible that there are new particles out there, new heavy particles. There are perhaps super-symmetric particles that people are looking for in the Large Hadron Collider at CERN that are affecting this measurement, this very precise measurement.

FLATOW: So you're saying that let me see if I can parse this a bit that the proton size may be the same that it always has been, but that these other spooky particles we don't know about may be affecting the measurement of how we measure it.

Dr. ODOM: That's exactly right.


Dr. ODOM: Yeah, so this it's a really very interesting story here that people with relatively small experiments, sometimes tabletop-size, can get some of the same physics that we get out by building big, powerful colliders. There are kind of two complimentary approaches to finding new physics.

FLATOW: So what you're saying is that they didn't need to use a big, fancy Hadron collider. They just needed some desktop - more or less - sized physics experiment.

Dr. ODOM: Well, if you can do both, that's much better - build the collider and do the precision experiments, because they both tell you different parts of the story.

FLATOW: Well, if you know this now, can you use those giant colliders, like the new Hadron Collider, the Large Hadron Collider, could you tweak an experiment to see, now, maybe we should look for some of that new physics?

Dr. ODOM: Absolutely, yeah. That's going to be one of the important implications coming out of whatever we find out of LHC.

FLATOW: Mm-hmm. And how do we know that, you know, maybe they made a mistake in measuring here? I imagine other people will be checking or conducting new experiments.

Dr. ODOM: That's right. So the most important thing to check first is that the theory was correct. The experiments have been put well under control, I think, and the first place people are going to look for a boring sort of mistake is in the theory. They have to run through a lot of very complicated calculations to extract the proton size from the laboratory measurement.

FLATOW: Mm-hmm. Tell us what kind of particles they might be looking for that would affect how you measure the protons.

Dr. ODOM: Well, one of the interesting possibilities are the so-called super-symmetric particles. It's an idea that we have in physics that what we we don't really like the standard model. It works great, but there are a lot of problems with it. There - some of them are aesthetic. Some of them are observational, like dark energy and dark matter. We don't know what those things are, and so there must be something besides a standard model of physics.

One of the things that might solve some of these problems is the super-symmetry theory, which says that for every normal particle, like an electron, there's also a much heavier counterpart. And those heavy counterparts, you can make in a collider, or you can see their effects in a precision experiment like this muonic hydrogen result.

FLATOW: Mm-hmm. And so you would be looking for those particles in the collider to see if they exist.

Dr. ODOM: That's right.

FLATOW: Can you actually predict what they might be?

Dr. ODOM: That's right. There are predictions. Yeah, and we hope that the collider will be able to produce some of them, if they exist.

FLATOW: And did you have to come up with that prediction now that the measurement is off for the proton?

Dr. ODOM: No, very interesting question. That prediction has been around for a very long time. And there have been a few hints up till now that that theory might be correct.

Another interesting hint is the measurement of the little magnet, which is embedded inside the muon. The muon is a particle that they use in this experiment to orbit the proton instead of a normal electron orbiting a proton.

So when you replace it with a muon, you can probe for this new physics in a better way. And similarly, when you look at the muon by itself, look at what its magnet is, there's a strong disagreement with theory there, also. So it's another tantalizing hint that there might be some new physics out there, like new particles that we'd like to see.

FLATOW: So you did the experiment by taking away the electron that goes normally around the proton, and you put a very heavy muon in there.

Dr. ODOM: Exactly.

FLATOW: And then when you measured the muon, there was something wrong. And when you measured the proton, there was something wrong.

Dr. ODOM: That's right. Yes.

FLATOW: Two for one.

Dr. ODOM: Two for - well, yeah. Different experiments, but they both involve the muon, and it's possible they're hinting at the same new physics.

FLATOW: And so does this have anything to do with that famous Higgs-boson they're looking for?

Dr. ODOM: Let's see. That's not clear. It possibly could, or it could be something else completely.

FLATOW: Mm-hmm. So it'll be a more exciting event when they start looking through the cloud of what comes out at this collider.

Dr. ODOM: Exactly. That will be very exciting.

FLATOW: And are physicists looking more to try to say the proton - you know, the old measurement is right? Or would they be more excited if this newer measurement is right?

Dr. ODOM: Oh, we would love to find a problem with the standard model. We would love to find new physics. So I think everyone's, everyone's hoping for new physics, but most people would put their money on a silly mistake in a theory.

FLATOW: You think they would?

Dr. ODOM: Right now we're cautious. You know, we don't like to claim new physics for poor reasons.

FLATOW: Yeah. And then how long will it take before we, you know, we find out if this is dependable or not?

Dr. ODOM: Well, I think the theory will take some few months, probably, to crank through again. And then the same collaboration that published this proton radius result is moving on to different types of atoms. They want to do it with a heavy version of helium rather than hydrogen. And the results from that laboratory experiment will also shed light on what's going on here.

FLATOW: All right. We'll stand by and get back to you. Is that okay?

Dr. ODOM: Very good.

FLATOW: All right. Thank you, Dr. Odom.

Dr. ODOM: Thanks, Ira.

FLATOW: Brian Odom is assistant professor of physics at Northwestern University in Evanston, Illinois.

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