### Research News

# Heavy Conversation at Gravity Conference

Still searching for that unifying theory of physics, scientists gather in Tucson, Ariz., to talk about gravity. Dimitrios Psaltis, assistant professor of physics and astronomy at the University of Arizona, talks about what researchers hope to learn.

IRA FLATOW, host:

For the rest of this hour, a sneak peek at a weighty issue on the minds of physicists next week. And that is issue is gravity. A whole bunch of physicists are going to meet in Tucson to talk about gravity. Why gravity? So joining me now to talk about it is Dimitrios Psaltis, assistant professor of physics and astronomy at the University of Arizona in Tucson. Thanks for talking with us today, Dr. Psaltis.

Dr. DIMITRIOS PSALTIS (University of Arizona): Nice to be on the program.

FLATOW: Why do scientists need to get together to talk about gravity?

Dr. PSALTIS: Why do they need to talk together? Well, first of all, as scientists, we really like to meet and talk to each other about things. And what is really interesting about this conference that we're organizing is the fact that we're bringing scientists together from various different disciplines. We're bringing together people that do (unintelligible) astrophysics, cosmologists, string theorists, people that do experiments with their labs.

And the important point here is that gravity is such a force that affects phenomena in all those areas. And I think this is going to be one of the very first and very few conferences where all of us are going to be sitting in the same room discussing this very important subject.

FLATOW: And what makes gravity so important?

Dr. PSALTIS: Well, it is, first of all, the last force in our framework of physics that we cannot really understand. I mean we understand it by itself but we cannot understand it within the context of all the other forces. We do not have yet the satisfying unifying theory that unifies gravity to, you know, other forces of (unintelligible) like for example, electromagnetism, the strong force, the weak force. This is point number one.

Point number two is that over the last 10 years or so, we have seen an enormous explosion of ideas and new observations that give us some interesting clues about gravity. Now we're talking about universes with many dimensions that only gravity can see. And cosmological observations tell us that most of the gravitational fields in the universe is determined by things like dark energy and dark matter that we don't understand. So these are the two main reasons, I think, why we need to keep on talking about gravity.

FLATOW: 1-800-989-8255, if you would like to talk about gravity. We're talking about it. Of course Einstein said that gravity was a distortion of the fabric of space, right, by massive objects?

Dr. PSALTIS: Correct.

FLATOW: So one of the problems is, how do you take a geometrical concept and move that into the quantum world which deals with gravity being carried by particles, right?

Dr. PSALTIS: Indeed. This is actually something - this is one of the main reasons why we've been having such a hard time unifying the forces, that our description of the gravitational field and our description of all the other forces are very, very different.

There have been ways - there have been ideas trying to introduce ideas of the quantum world into gravity, ideas that go under the name of quantum gravity. Or trying to reproduce the quantum world using ideas of relativity. So it's like (unintelligible) warping of space-time in more than our normal dimensions. And this is the idea that has to do with string theory.

We're not sure if any of the two are, you know, at all if there's any way of unifying those ideas together. But as physicists our hope is that we will be able to find a theory that unifies all of them, and that's what drives physics to begin with.

FLATOW: Do you think that by getting a whole bunch of scientists in a room and locking the door, you can come to some of these answers?

Dr. PSALTIS: That is a very interesting question. And that is actually the biggest (unintelligible) of this conference. As I said, it's wonderful that all of us from various different communities will be talking to each other. However, it's going to be difficult as well, because first of all, we are using - if you can be able to say the different terminology, talking about the same phenomenon. But even more importantly, we come with different prejudices. So for example, when we all go to the same conference in (unintelligible) astrophysics and we read each other's publications, then we get to take something for granted and be willing to give up some other things. And there's something different that a cosmologist would do, and there's something different that people in the string theory would do.

So here we'll be fitting all together and trying to convince each other about each other's prejudices or what needs to be, you know, given up.

FLATOW: I see. You have the world of the very big. Your gravity (unintelligible), right?

Dr. PSALTIS: Yes.

FLATOW: And people who study that talking to people of the very small, of the subatomic world.

Dr. PSALTIS: Correct.

FLATOW: Trying to find common ground someplace.

Dr. PSALTIS: Correct.

FLATOW: Are there any experiments out there that we could do to solve some of this ambiguity about just what gravity is?

Dr. PSALTIS: There are actually many different experiments and many different ideas. And to give you an idea of how you can actually use experiments from the micro-world - even though not exactly the atomic world - and the universe to study the exact same thing. Let me bring up two new results that will be published this month, actually, in the Physical Review Letter, which is the premier publication of the American Physical Society.

One is from the University of Washington, where they're testing ideas of universes with large extra dimensions by putting together masses at miniscule distances, about two-thousandths of an inch, and they measure the gravitational force between them.

And the other paper is from the University of Arizona, from our group here, when we're testing the exact same theories - theories with (unintelligible) dimensions - by looking how fast black holes evaporate, or actually how fast black holes do not evaporate, in our galaxy.

What is amazing to me, is that both of these experiments, both of these studies, produced almost identical, to within 50-percent constraints, on the properties of the theories with large extra dimensions. And this is exactly the point, that you can do very similar physics looking at the cosmos, looking at black holes in the galaxy, and looking at an experiment on the tabletop in the lab.

FLATOW: Could it be that the particles of gravity that the quantum-gravity people are looking for - could they be - excuse me, but could they be the dark-energy part that we don't know where - you know, could gravity actually exist in the dark-energy side and peeks over here and being - so we don't know what the particles are because it's in the dark energy that we have no idea what that is?

Dr. PSALTIS: I will try to say this question a slightly different way. The point is that Einstein's theory of general relativity really has one main equation, and one side of that equation tells you how strong the gravitational field is, and the other side of that equation tells you how much matter you have that produces that gravitational field.

What we do know now is that in many situations, as you said for example in the universe, those two… When we count as much as we can the matter, and when we count as much as we can the strength of the gravitational field, and we put them together, they don't sum up. They're not equal.

So there's clearly a mistake somewhere. And that mistake can be either on the side that determines matter and energy - for example, we say we might have dark energy - or that mistake can be on the side that determines the force of gravity, in which case we have to change our theory of gravity and maybe talk about ideas in quantum gravity or in string theory to make that equality work out.

We don't know yet, which side of the equation the mistake is, which side of the equation needs to be changed, but that's exactly what we're looking to find out. All the options are available at this point.

FLATOW: 1-800-989-8255 is our number. When you talk to scientists, sub-atomic-particle scientists, you hear them saying oh, you know this Large Hadron Collider is going to be opening at CERN in Switzerland this - any day now.

Dr. PSALTIS: Correct.

FLATOW: And that should answer some of these questions about gravity. Can you tell us what they mean by that?

Dr. PSALTIS: Yes, actually this is a rather (unintelligible) but extremely exciting possibility, that there is a non-zero chance if this idea of universes with large extra dimensions are correct, that in the Large Hadron Collider, these enormous (unintelligible) collisions that they're producing are going to create microscopic black holes - microscopic on the size of, you know, a subatomic particle.

Now these black holes will not stay there, but they will evaporate very quickly. That evaporation is a quantum-gravity phenomenon, a quantum-gravity effect, something that the only way to understand it is by our current theories of trying to put together quantum field theories in gravity. And that evaporation will show up as a very, very unmistakable bright burst of energy.

So if we understand - if we observe something like that, first of all we will observe for the first time a phenomenon that is not described by Einstein's theory of general relativity, but is predicted by some ideas of quantum gravity. And more importantly, we will have all the tools that we will need in order to try to make up, you know, theories to change the Einstein's theory of gravity, unify the series in order to produce those experimental results. It's going to be really one of the biggest revolutions in science in the last 100 years.

FLATOW: So a negative result would be important, also, if you don't see…

Dr. PSALTIS: A negative results will be important also in the sense that you can use that in order to place the other strong constraints on theories of large extra dimensions. One of our main hopes is that these theories are able to tell us why the force of gravity is so much weaker than all the other forces in the universe. This is, you know, the biggest hope that the theories will be right.

If it turns out that we do not observe - I mean, not probably in the beginning, but later on when the experiments become better and more accurate - if we don't observe any of those black holes, then we will start constraining those theories a lot, and that will have some important effects, important implications, for how we can understand why gravity is so much weaker than all the other forces.

FLATOW: Because when you think about it, your kitchen magnet is stronger than gravity.

Dr. PSALTIS: Exactly. The point here is that you can, you know, you can have a fridge magnet and put it on your fridge, and it will stay there, and you defy the force of gravity that is acted on the magnet by the entire Earth. So here you have a tiny little magnet that can overcome the gravitational force of the entire Earth. So gravity is indeed the weakest of all forces.

On the other hand, when we go to really large systems like astrophysical systems, then all the other forces are either - those scales are much bigger than where the other forces act, like the strong and the weak forces, or like electromagnetism, you have the positive and the negative particles that cancel each other, and therefore you don't see any large-scale electromagnetic force. So it's really only gravity that stays. That's why cosmology and astrophysics are the main ways of testing ideas about gravity.

FLATOW: Now the meeting in Tucson next week, is any of it open to the public so that the public might hear you guys debate?

Dr. PSALTIS: Yes. First of all, for everybody who is here and is really interested, too, and you know, can be happy listening to talks with a lot of mathematics and a lot of equations, of course they're welcome to come. But there's also going to be a public aspect of it.

Professor James Peebles from Princeton University is going to give a public talk in the evening on Monday evening at the observatory here, where he's going to discuss the expanding universe and current ideas about dark energy and dark matter. And of course, that is open to the entire public.

FLATOW: When you guys are meeting in closed session, does - how shall I put it? Do the opinions heat up at all? I mean, do you have a frank exchange of views, as they say in Washington?

(Soundbite of laughter)

Dr. PSALTIS: I guess we are people as much as the politicians are, in the sense that in the beginning, the conference always starts happy and polite, and if we are lucky, it will end up the same way. But there are some situations, of course, where you know, as I said, prejudices are very hard to overcome sometimes, and you know, it comes for an interesting conference. It comes for an interesting discussion when things get heated up a little bit. But that's part of, you know, of human interaction, I guess.

FLATOW: Sure. It's passion. Passion.

Dr. PSALTIS: Exactly. We're all very passionate about that. We're not here for the money, we're here for the passion, I guess.

FLATOW: I guess we're all in the same business.

Dr. PSALTIS: Yeah, exactly.

FLATOW: 1-800-989-8255 is our number. We're talking with Dimitrios Psaltis about the upcoming gravity conference next week in Tucson on TALK OF THE NATION Science Friday from NPR News.

Let's see if we can go to the phones and get a call in or two about gravity. Ken(ph) in Cleveland. Hi, Ken.

KEN (Caller): Hi, great show. Hey, quick question. I'll take my comment off the air, or your comments off the air, rather. I heard that theoretically, graviton particles should exist, but they don't. And I also heard a theory that gravity could be possibly the result of an effect caused by a parallel universe. Do either of those hold any credence?

FLATOW: Dr. Psaltis?

KEN: Okay, thanks.

Dr. PSALTIS: The first aspects of it is what we discussed earlier, about whether you can put the theory of general relativity in the same context as the other quantum field theories.

When gravity is seen as purely as the warping of space-time, then there is no graviton involved with that. You know, things move on straight lines in non-straight space-time. That's why they feel - we assign to them a gravitational force. However, there's no exchange of gravitons between them.

On the other hand, if we're talking about gravity as a quantum-field theory, then the only reason that particles interact with each other with gravitational force is because of the exchange of the gravitons. Do they exist; do they not? If you believe that we will find at some point a unified field theory to describe that, then they should exist. But again, this is the kind of thing that we will be discussing this coming week.

The second question - wait, I forgot already.

FLATOW: We'll move on to Eli.

Dr. PSALTIS: Okay, I'm sorry.

FLATOW: Eli in Vermillion, South Dakota. Hi, Eli. Go ahead.

ELI (Caller): Hi. I would like to know if any mathematicians are invited, and to what extent topology and multivariable calculus play a role in your studies. I'll take my comments off the air.

Dr. PSALTIS: Any mathematicians invited? Well, that's very interesting because the theory of general relativity is extremely mathematical and, in many ways, everybody who's working on it has to be very up-to-speed with mathematics.

There are indeed a couple of people that are part of the conference that are formally affiliated with math departments, and you know, as you said, (unintelligible)geometry and (unintelligible) topology and all those things are, you know, tools that we're all using every day in describing gravitational phenomena.

FLATOW: If you have to ask - are you in the camp? Where you do stand in the camp about the recent critiques and criticisms of string theory? Are you still a believer in string theory?

(Soundbite of laughter)

Dr. PSALTIS: I want to say that it is a very exciting possibility, and it's a very exciting - it's probably, from my point of view, one of the best exciting possibilities for how we can unify the forces. Myself, I spend a lot of time on the East coast, you know, the Institute for Advance Study and at Harvard and places where, you know, I've been completely indoctrinated by string theory.

(Soundbite of laughter)

FLATOW: You've drunken from the Kool-Aid.

Dr. PSALTIS: Exactly. But yes, I really think that it's one of our best hopes. And after all, if it turns out to be right, it's going to be totally, totally remarkable, and you know, we'll all be here to see. When string theory first started, it was impossible to - it seemed it was impossible that we will ever be able to produce any experiments or any observations to test it.

What we're doing now with the cosmos, with the forces between particles at sub-inch distances, sub-millimeter distances and with the black holes, it seems that we might be able to test things that string theory predicts and, therefore, this might be a testable theory after all.

FLATOW: Well, I wish you good luck in your conference and hope you come out with some great results.

Dr. PSALTIS: Thank you very much. I really hope so, too.

FLATOW: Good luck to you. Dr. Dimitrios Psaltis is assistant professor of physics and astronomy at the University of Arizona in Tucson, talking about a conference of physicists in Tucson next week.

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