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The Latest Mystery of the Universe
——What do you think is the greatest unanswered question in modern physics?

We think M-theory is the ultimate theory of the universe, but we understand it only at certain limits, where some quantities are zero or very small. It is as if we had fitted together the edge pieces in a jigsaw puzzle, but we still have a gaping hole in the middle. And of course it may be that we find we have the wrong picture. We have learned to expect the unexpected.

* M-theory
M-theory is believed to be the ultimate unification theory, which incorporates and generalizes all the laws of nature. It reveals the relationships of the four forces that exist in the universe: gravity, electromagnetism, weak nuclear force and strong nuclear force.

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——One of the remaining grand mysteries of the universe is the problem of dark matter, which is invisible but exerts a gravitational force, and dark energy, which causes accelerating expansion of the universe. How do you expect this problem will be solved in the time to come?

Ordinary matter, the material of which we and the stars are made, makes up only 5 per cent of the mass of the universe. Another 25 per cent of the mass of the universe is in the form of dark matter, matter we can't see but whose presence we can detect by its gravity. But it seems that 70 per cent of the universe is in the form of what is called dark energy, a mysterious kind of matter which causes the expansion of the universe to speed up, rather than slowing it down like ordinary and dark matter. With luck, we will identify the dark matter as some weak interacting particle. The dark energy is more difficult. It may be a slowly decaying field, or as I think more likely, what is called vacuum energy. More accurate measurements may enable us to distinguish between these possibilities. Dark matter Photo
An image of dark matter observed by ASCA

——How do you think our everyday life will be affected, if string theory, which is discussed in your book A Briefer History of Time, or the more advanced version of string theory, M-theory, is proven to be true?

We already know the laws that govern nature in all but the most extreme conditions, such as the origin of the universe. When we understand string theory, we will know how the universe began. It won't have much effect on how we live, but surely it is important to understand where we come from, and what we can expect to find as we explore the universe.

* String theory
String theory (a.k.a. superstring theory) holds that particles are not points but vibrating strings. The theory was first postulated in the late 1960s, but it was not until 1984 that it finally gained popular support, in anticipation of it potentially being the ultimate unification theory. Today M-theory, which merges five different string theories, is the most likely candidate for a unification theory.

A great number of entities unobservable by electromagnetic waves are thought to exist in the universe. One unidentified entity, which creates powerful gravity, is called dark matter. Another mysterious entity, dark energy, is believed to be the energy source of the accelerating expansion of the universe. illustration/Hisashi Hirabayashi

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