Clifford V. Johnson is professor of physics at the University of Southern California and the winner of the 2005 Maxwell Medal from the Institute of Physics in the United Kingdom. He is the author of Encarta’s new article on Dark Matter. (You can access this article by entering "Dark Matter" in MSN Search and then clicking on the Encarta tab just above the search box. You'll see a link to "Learn more about Dark Matter" at the top of the search results page. This will also give you a 2-hour free pass to all Encarta article content. To get access to all of Encarta's rich content—including multimedia features, maps, archive articles, primary source documents, and more—subscribe to Encarta Premium.)

We recently asked Professor Johnson a few questions about his field and its latest developments. Here is the interview transcript.

Encarta: You are a theoretical physicist. For our younger readers, could you briefly explain what a theoretical physicist is and how your work differs from that of other physicists?

Johnson: First I’d like to say how they are similar, since there are more similarities than differences. Both as a theorist and an experimenter, the job of the physicist is to try to understand how nature works. What this means in a practical sense is that one tries to observe physical processes or other phenomena and make models of how they work, or how they came to be. Like explaining how the planets orbit the Sun: Why are the orbits elliptical and not some other shape?

The real test of how good the model is comes when one looks at its consequences. Does it tell you things about other physical situations that are not true, or does it fit nicely with other data? Well, the physics of gravity that we use to understand how the planets go around the Sun is the same physics that explains how the moon goes around the Earth, and how planets go around other stars elsewhere in the Milky Way Galaxy, and probably in the whole universe.

Most importantly, you don’t want to have a different model for every physical situation that can arise. You want what is called an “economical” explanation, one that uses as few assumptions as possible to make a model. So in the example of orbits I mentioned, the key assumption is that all the objects involved have mass and that the same force of gravity works in the same way in all those many situations. The best models then make new predictions about nature that can be tested with new experiments or observations. So for example, that same physics of gravity describing the motions of the planets can also enable you to deduce that there might be other planets you did not know about. That was how British astronomer John Couch Adams and the French astronomer Urbain Jean Joseph Leverrier independently discovered the planet Neptune in the 1840s, for example. Knowing about wobbles in the orbit of Uranus, they deduced that the gravitational influence of another planet was causing these wobbles. That’s how new planets orbiting about other stars are being discovered today.

A theoretical physicist is a specialist whose job it is to take the data about nature that experimenters gather and construct those “economical” models of the physics lying behind those data. The next step is then to generate new predictions from those models to guide the experimenters in carrying out new experiments. The cycle then continues again and again, and we learn more about nature at every step. So Adams and Leverrier actually did not look through a telescope and find Neptune. Instead, their predictions were tested by German astronomer Johann Gottfried Galle who found Neptune where the theorists said it should be. By the way, I should mention that some physicists actually do both theoretical and experimental work. It very much depends upon which field of physics they are working in as to whether this is really practically possible or not.

Encarta: How did you first become interested in physics?

Johnson: Actually, I’m not sure exactly how it happened. I was always interested in science since I was a very small child. Not just physics, but everything. So I did everything from looking at bugs and pond-water through magnifying glasses and (later) microscopes and drawing and collecting interesting leaves, etc., all the way to mixing up various household chemicals to see what would happen, and fixing appliances like radios (and later building them), all for fun. I didn't know at the time that there were different fields of science, or that there were theoreticians vs. experimenters, or scientists vs. engineers. I just liked doing it all.

Because I was always offering explanations for things, my nickname at school was The Professor, although they weren't intending to be complimentary, I think! One day as a child (I might have been about 9 or 10), a family friend asked me what I wanted to be when I grew up and I said that I wanted to be a scientist. He then asked me what kind of scientist and this stumped me, as I did not know that there were different sorts. So I went and got the dictionary and went through it page by page finding all the “-ists” and “-ians” “-ologists”, and read the definitions. They all sounded good to me. But when I found the one for “physicist,” it said something really nonspecific like “studies how nature works” or something like that. So I went for that, since it seemed to allow me to keep my options open for a lot longer than any of the others. I still wanted to study everything, you see, and nature was everything, wasn’t it?

Encarta: In your article, you point out the observational evidence that first led astrophysicists to suspect the existence of dark matter. At the time this was a puzzling and interesting finding. Today scientists believe that if dark matter didn’t exist, then we wouldn’t exist. Can you explain what this means?

Johnson: Well, people have to be careful when they say that. What we can say with confidence is that if dark matter did not exist, we (or life as we know it) would not exist in anything like the form we know now. Maybe some other form of life would have formed, made of other stuff, and their scientists would be saying “It’s a good thing that there’s no hidden matter out there that we don’t know about, otherwise we wouldn’t exist.”

But like I said, maybe some sort of hydrogen-helium-lithium life forms might have evolved, sort of floating out there in the almost smooth featureless universe, and their scientists would have asked other questions.

Encarta: Some people say this is the ultimate Copernican revolution. Not only are we not at the center of the universe, the stuff we’re made of is only a small percentage of all the matter and energy in the universe. And now some of the work you’re pursuing entertains the possibility that our universe may be only one of many universes. How did we get to this point?

Johnson: Let me make it clear from the start that we don’t know if we are at that point yet. We’re very far from it, and the stuff about other universes is all wild speculation at this point, and will remain so most likely for a long time. We should be clear about that, since the other matters are firmly experimentally backed up.

We know that Copernicus was right: We are not the center of the universe. We are orbiting an average star in the suburbs of an average galaxy in an average galaxy cluster at a random point in the universe. We can check this all out with telescopes and other instruments. We also know that the stuff we are mostly made of--protons and neutrons--form only about 4 percent of the stuff the universe is made of. Another 23 percent--the rest of the matter--is there, but we’re just not made of it. We don’t know as much as we would like to about the remaining 73 percent, which we call dark energy, but we know for sure that it isn’t matter at all. We know about this breakdown of the contents of our universe because again, all our instruments and the science we’ve developed over centuries tell us that. We can really put it to the test. 

Now some people in my field are talking about the possibility that maybe our universe is just one of many. This has nothing whatsoever to do with any experiments or observations that anyone has done—at least so far. There are some fun and interesting theories of physics that might allow for this possibility, and that’s nice. Theories like string theory which suggest that our universe has extra hidden dimensions (beyond the three dimensions of space and the one dimension of time) seem to allow solutions where we have our four-dimensional universe right alongside one or more other four-dimensional universes which are separate from ours. We don’t see them because you’d have to move “sideways”--that is, along one of the hidden dimensions--in order to get there. Those universes might have properties that are somewhat similar to ours in the rough, but different in the details. Perhaps they did not have so much dark matter and so they did not form galaxies, stars, etc. Perhaps my hydrogen-helium-lithium creatures (from the previous question’s answer) live there.

It’s fun to imagine, but too early to say whether any of that has anything to do with science. For a start, we don’t really understand the theories well enough yet to know if solutions really exist or not. If we ever come to a point in science where our theories say that those universes really exist, then there would be testable predictions for the theory, and we could develop an experiment for it. The experiment would then tell us whether it’s true--whether the other universes are there or not, even if we can’t see them directly. We are a long way from that point.  

However, let's not forget that less than 100 years ago, we thought that the Milky Way Galaxy was the entire universe! It was a big controversy as to whether the curious objects that could be seen in telescopes jects that could be seen in telescopes (“nebulae” they called them) were in our galaxy or outside it. This was not settled as a debate until more basic science was done, for example on Cepheid variable stars by American astronomer Henrietta Leavitt, and then more refined observations were made by American astronomer Edwin Hubble. I don’t think that there is anything out there (analogous to nebulae) that we’ve seen that is even hinting at there being other universes just yet, but maybe I’ll be shown to be wrong.

But anyway, it is an important part of being a theorist to play with ideas, no matter how outlandish, from time to time. That’s where the really good stuff that we can use for real science often comes from. But we must not forget that a lot of the playful stuff will always be exactly that--playful. We just can never know in advance what parts of it are useful and what aren’t.

Encarta: Several experiments are either underway or will soon be underway to detect some of the proposed dark matter candidates. Which of these experiments do you think are most likely to produce interesting results?

Johnson: I think that it has to be the collider (particle accelerator) experiments. We are quite sure that the bulk of dark matter is made of material that we have never detected before. Totally new stuff. Whenever you talk about new matter, new fundamental material, you should look to particle physics to characterize its properties, and that field proceeds by making the new things in the lab and studying their properties directly. All the signs are that there will be new physics of some kind or other--about the origin of mass, or the role (if any) of supersymmetry, or things we have not even thought of--showing up at the next big international collider experiment to switch on in a few years, the Large Hadron Collider (LHC) at CERN in Switzerland.

Whenever we find new physics it usually has consequences beyond the context in which it is initially found. So that new physics is going to be new particles, perhaps, and those particles may have just the right properties to be relevant in cosmology, telling us about what stuff a lot of our universe is made of. Of course, it may not turn out to be so, but that’s my gut feeling about where progress on this matter will be made. It is an exciting time. Imagine being around when we, as a species, learn what as much as 85 percent of the matter in the entire universe is made of, and that we can make it in the lab!