Professor Michio Kaku: We are speaking to Professor Stuart Samuel, the spokesperson of a new book called The Bible According to Einstein that will be appear in 1998. The first question is "What is dark matter?"
Professor Samuel: Well, dark matter is one of the greatest puzzles of modern astronomy. Everybody loves a mystery such as a Sherlock Holmes story or an Agatha Christie novel. And mysteries on TV are extremely popular, for example. And when you have a real-life incident such as the Jon Benet Ramsey murder, the whole nation is captivated. But mysteries also arise in science and have an equally captivating effect on scientists. [ah] Albert Einstein, as you well know Professor Kaku, received the Nobel prize not for his relativity theories but for his work on solving the puzzle of the photoelectric effect. Now during the last several decades, a mystery has emerged which not only affects our life on Earth but also affects the future of the ENTIRE universe. Astronomers had once assumed that the gravity of galaxies was almost entirely generated by visible matter, that is, the matter in the stars and gas which give off or interact with light. This is NOT the case. There is a HIDDEN substance lurking in the galaxies which enhances the gravity of a galaxy. This is a great mystery -- it is the mystery of dark matter.
Professor Kaku: Now in [um] high school, we learned that there are about 100 chemicals that make up the entire universe. And now you are telling me that perhaps there's a possibility that the 100 elements that we had to learn in high school do not comprise most of the universe. Is that correct?
Professor Samuel: [Ah, well, ah] That's correct. Well, it's unclear because we don't know exactly what dark matter is. [ah] Dark matter is non-luminous material. It's material that doesn't give off light. Perhaps [ah] to clarify things we probably should discuss in more detail what luminous matter is. Luminous matter is that which gives off light or interacts with light. Stars are an example. When we look up in the heavens at night and see stars, we do not see the stars themselves -- we actually see the light that the stars give off. Stars are luminous bodies. Astronomers can also see clouds of gas and dust that form nebulae. Light passing through such clouds is partially absorbed. Sometimes the absorption effect is so strong that a dark patch in the night sky is created. It was once thought that the bulk of the mass of the universe was in stars and nebulae, that is in the stars and in the gas and dust. But during the last several decades, astronomers have come to realize that most of the mass of the universe is some unknown material, which they've named dark matter. Dark matter, by definition, does not give off or absorb light.
Professor Kaku: So, in other words, if you had dark matter, it would be invisible, but if you dropped it on your foot, you'd feel it?
Professor Samuel: [ah] Probably not. [ah] Again, we don't know exactly the make up of dark matter. It's very possible that it's made up of material that doesn't interact by the electric force or by the magnetic force, in which case, if we dropped it on our foot, it would go right through our foot. It would just go to the center of the earth. It would only interact gravitationally. So there are two possibilities here. One is that it could be made of ordinary material like planets that you can't see, or it could be made up of subatomic invisible particles that only [ah] interact through their gravity.
Professor Kaku: Now the idea of dark matter is pretty outrageous, if you think about it, because scientists say that 90% of [ah] of the galaxy could be made out of this. So the first question is how do we know for sure that dark matter is out there? I mean what about experimental hard data that proves the existence of this rather fantastic theory?
Professor Samuel: Well, that's a very good question. Since we can't see dark matter, then people may be wondering how do we know that it exists? Well, astronomers have sensed the existence of dark matter through its pull of gravity. And let me give you a simpler example of how this works. The Moon circles the Earth in about 30 days, that's what we call a month. For the sake of argument, suppose suddenly the mass of the Earth was quadrupled. How long would it take the Moon to go around the Earth? The answer is about 15 days. If we were living on a planet the size of Earth but four times its mass, a month -- as measured roughly by the time it takes the Moon to go once through its phases -- would be only 15 days. We, of course, would also weigh four times as much. So by looking at the Moon and seeing how long it takes to go around the Earth, or how fast it goes around the Earth, we can determine the weight, or mass, of the Earth.
Now coming back to what I was saying. Astronomers discovered dark matter by studying the speeds of stars orbiting the outer reaches of a galaxy. And the situation is similar to this Earth-Moon setup. Since astronomers can see all the stars in the galaxy, they know how much mass there is in the stars of a galaxy. So they were quite surprised to find that the stars in the outer reaches of a galaxy were traveling faster than they expected. The speeds of these stars tell astronomers that about 80% of a galaxy's mass is invisible. So there must be a mysterious halo of dark matter that engulfs each galaxy. This is really quite embarrassing for us -- we, humans, pride ourselves on being knowledgeable about the world. Yet, here we are, living in a galaxy, called the Milky Way, in which 80% of its material is some unknown substance.
Now there is another way of assessing the importance of the dark matter. You see, some of the dark matter clumps around galaxies, like our own galaxy the Milky Way. If there were no dark matter in a galaxy, then the force of gravity would be less and the stars in the outer reaches of a galaxy would fly off into intergalactic space. So without dark matter, much of the material in a galaxy would fly apart. In other words, the dark matter is playing a crucial role in holding a galaxy together.
Professor Kaku: So in other words, galaxies would simply disintegrate, unless there was dark matter surrounding it to sort of keep it together.
Professor Samuel: That is basically correct. It would be that the outer part of the galaxy would fly off into intergalactic space. The core of a galaxy would probably remain as a unit. So galaxies would be much smaller.
Professor Kaku: Well our next question is why is dark matter important? Why should we care? I understand that perhaps the fate of the universe itself may lie with dark matter.
Professor Samuel: [Well, ah.] It's very important for the universe. [ah] And it's very important for us. I think I mentioned that if we didn't have enough of this dark matter, then [ah] what would happen is that fewer galaxies would form. And galaxies like the Milky Way perhaps wouldn't have formed in the evolution of the universe. And if we don't have our galaxy the Milky Way, then we also wouldn't have the Sun, the Earth and life on Earth. So it's very important to us.
But it not only determines the fate of our planet and galaxy but of the entire universe. Einstein's general theory of relativity predicts that there are two possible fates for the universe. If the mass of the universe is less than a critical value then the universe will expand forever. While expanding, the universe will cool. Eventually, everything including our Earth will exist in a cold frozen state. Your listeners don't need to worry however because, if this happens, it will happen tens of billions of years from now. Now the other possible fate for the universe occurs if the mass of the universe is greater than the critical value. Then, at some point, again far in the future, the universe will cease its expansion and begin to collapse. Distant galaxies, instead of flying apart like they do now, will come together. All the material in the universe will join and the universe will heat up. This is called the Big Crunch -- it's like the Big Bang but it's in reverse. Thus the poet Robert Frost got it right. He said that the world will end in fire, or the world will end in ice. Now how does dark matter fit into the cosmic scheme of things? Well, if there were no dark matter, then the mass in the universe would be less than this critical value and the universe would expand forever, slowly getting colder and colder. But the universe DOES contain dark matter. So if the universe contains a lot of dark matter then the universe will eventually undergo a Big Crunch, while if it contains less dark matter then the universe will expand forever. Unfortunately, our method for weighing the dark matter is sufficiently crude that we do not know the fate of the universe. That's because sensing dark matter through its gravitational effects is not very accurate. So, obtaining a better understanding of dark matter has cosmic conclusions -- it will determine the fate of our universe.
Professor Kaku: So, in other words, whether or not the universe dies in fire or ice, whether or not the universe dies with a fiery Big Crunch or the big chill ultimately depends on how much dark matter there is in the universe. Now unfortunately at this time, physicists don't have a clue as to what dark matter is all about.
So once again, you have been listening to an interview with Dr. Stuart Samuel, a professor of theoretical physics and he's also an editor of a new book coming out in 1998 called The Bible According to Einstein, a rather ingenious attempt to explain the Big Bang theory, the theory of the cosmos, in terms of chapters and verses like the Bible.