Open Questions

Physics has answered many questions about space, time, and matter. Thanks to technological advances, we have been able to look deeper and deeper into the large-scale structure of the universe and the small-scale structure of matter. From the invention of the telescope to the time of particle accelerators, insight and understanding have grown. Yet, there are still many unsolved mysteries. The contemporary models of matter, space, and time are incomplete and our picture of the world still has holes. Some of today's most challenging questions in physics are:

What is dark matter?

There seems to be a halo of mysterious invisible material engulfing galaxies, which is commonly referred to as dark matter. Scientists infer the existence of dark (=invisible) matter from the observation of its gravitational pull, which causes the stars in the outer regions of a galaxy to orbit faster than they would if there was only visible matter present. Another indication is that we see galaxies in our own local cluster moving towards each other.

The Andromeda galaxy -about 2.2 million light years away from the Milky Way- is speeding toward us at 200,000 miles per hour. This motion can only be explained by gravitational attraction, even though the mass we observe is not nearly great enough to exert that kind of pull. It follows there must be a large amount of unseen mass causing the gravitational pull -roughly equivalent to ten times the size of the Milky Way- lying between the two galaxies.

Astronomers have no idea what the dark matter is that supposedly makes up 23% of all matter in our universe. Black holes and massive neutrinos are two possible explanations. Dark matter must have played an important role in galaxy formation during the evolution of the cosmos. But, even taking into account all known and suspected black holes, there seems to be much more matter out there than we can presently see or extrapolate.

What is dark energy?

Dark energy is perhaps even more mysterious than dark matter. The discovery of dark energy goes back to 1998 when a 10-year study of supernovae took an astonishing turn. A group of scientists had recorded several dozen supernovae, including some so distant that their light had started to travel towards Earth when the universe was only a fraction of its present age. The group's goal was to measure small changes in the expansion rate of the universe, which in turn would yield clues to the origin, structure, and fate of the cosmos. Contrary to their expectation, the scientists found that the expansion of the universe is not slowing, but accelerating.

The acceleration is supposedly due to the anti-gravitational properties of the so-called dark energy. While the exact nature of this energy is presently unknown, scientists agree that dark energy is the dominant constituent of our universe, which means that it is larger than the sum of visible and dark matter. Einstein already postulated an anti-gravitational force at the beginning of the 20th century. He acknowledged that the observed matter would lead to gravitational collapse, and hence, introduced a cosmological constant to bring Relativity into line with observation. After it was discovered by Hubble that the universe is expanding, Einstein called his cosmological constant the greatest blunder of his life.

Yet, at the beginning of the 21st century it seems that anti-gravity is coming back with vengeance. A possible explanation is that the energy content of a vacuum is non-zero with a negative pressure. This negative pressure of the vacuum would grow in strength as the universe expands and it would cause the expansion to accelerate. If the acceleration does not stop, this will lead to the Big Rip scenario suggested by Caldwell, in which the universe will be literally torn apart by the anti-gravitational force in several billion years.

Home did the universe come into being?

Stephen Hawking says in the foreword of The Cosmos Explained (Cambridge, July 28, 1997): "At the Big Bang, the universe and time itself came into existence, so that this is the first cause. If we could understand the Big Bang, we would know why the universe is the way it is. It used to be thought that it was impossible to apply the laws of science to the beginning of the universe, and indeed that it was sacrilegious to try. But recent developments in unifying the two pillars of twentieth-century science, Einstein's General Theory of Relativity and the Quantum Theory, have encouraged us to believe that it may be possible to find laws that hold even at the creation of the universe. In that case, everything in the universe would be determined by the laws of science. So if we understood those laws, we would in a sense be masters of the universe."

It is uncertain whether mankind is able to develop such a theory in the near future, and it may be even more questionable whether this knowledge would indeed help us to become masters of the universe, as Stephen Hawking connotes. Obviously it is difficult to speculate on a theory that has not been developed yet. The theory might as well have no practical value at all. The great 20th century physical theories showed us that complexity and abstraction are growing, while intelligibility and practical applicability are decreasing. From a unified physical theory we can expect a more complete picture of matter, space, and time and a better understanding of the beginning of the universe. It may satisfy our curiosity in view of some big philosophical questions. Any practical value beyond this is rather uncertain.

Unified theories: How does gravity fit into the big picture?

The theory of gravity as formulated by Einstein is incompatible with the rules of quantum mechanics. Physicists encounter serious difficulties when trying to construct a quantum version of gravity. In the later years of his life, Einstein tried but failed to devise a theory that unifies gravity with quantum theory. In the 1960s, the weak nuclear force was united with electromagnetism to form the electroweak theory, which was subsequently verified in particle accelerator experiments. The next step is to create a model that unites the other fundamental forces.

Theorists are working on such a model, which they call grand unified theory (GUT). It amalgamates electromagnetism with the weak and strong nuclear interaction, but omits gravity. From GUT we expect the answer to why particles have the masses we observe. Although we observe the masses of electrons, protons, and neutrons generated through what is called "electroweak breaking," we don't know how this breaking mechanism works. GUT should be able to interpret the electroweak breaking process and thus provide an explanation for the mass of a particle.

Beyond GUT, there is a theory that accounts for all four fundamental forces in nature, including gravity. The greatest endeavour of physics is to draw hitherto unrelated and incompatible theories together into a single unified theory. The advantage of such a system is obvious: It would account for all currently known phenomena without leaving theoretical holes and it may point towards future areas of study. It is hypothesised that such a theory could create a new fundamental understanding of nature. String theory, supersymmetry, and M-theory are some candidates currently considered.

Are quarks and leptons actually fundamental, or are they made up of even more fundamental particles?

Presently it is not known whether quarks and leptons are elementary or compound particles. It seems that physicists have become more careful with announcing the fundamentality of particles after having learned that atoms, atom cores, and finally protons and neutrons are divisible. What is more, quarks and leptons are so small that they may be thought of as geometrical points in space with no spatial extension at all. This is perhaps not as miraculous as it first sounds, because after having learned from Rutherford's model that the volume of an atom is mostly made of "empty" space, it would not be too surprising to find out that matter is in fact nothing but empty space.

While the commonly accepted standard model of matter provides a very good description of the phenomena observed in experiments, the model is still incomplete. It can explain the behaviour of particles fairly well, but it cannot explain why some particles exist as they do. For example, it has been impossible to predict the mass of the top quark accurately from theoretical inference until it was determined experimentally. As mentioned before, the standard model of matter does not provide any mathematical model that allows us to calculate the observed mass.

Another question concerns the fact that there are three families of quarks and leptons. Of the three families (or generations) of particles, only the first is stable, namely that of up/down quarks, e-neutrinos, and electrons. There seems to be no need for the other two generations in the natural world, yet they exist. Theoretical physics has no explanation for the existence of the two unstable generations. Likewise, the question why there is hardly any antimatter in the observable universe remains unaccounted for. Since there is an almost perfect symmetry between matter and antimatter, one would expect some regions of the universe to be composed of matter and others of antimatter, yet almost all mass we can observe is composed of conventional matter.

Is our universe unique, or are there many universes?

Andrei Linde at Stanford has brought forward the cosmological model of a multiverse, which he calls the "self-reproducing inflationary universe." The theory is based on Alan Guth's inflation model, and it includes multiple universes woven together in some kind of spacetime foam. Each universe exists in a closed volume of space and time. Linde's model, based on advanced principles of quantum physics, defies easy visualisation. Quite simplified, it suggests quantum fluctuations in the universe's inflationary expansion period to have a wavelike character. Linde theorises that these waves can "freeze" atop one another, thus magnifying their effect.

The stacked-up quantum waves can in turn create such intense disruptions in scalar fields -the underlying fields that determine the behaviour of elementary particles- that they exceed a critical mass and start procreating new inflationary domains. The multiverse, Linde contends, is like a growing fractal, sprouting inflationary domains, with each domain spreading and cooling into a new universe.

If Linde is correct, our universe is just one of the sprouts. The theory neatly straddles two ancient ideas about the universe: that it had a definite beginning, and that it had existed forever. In Linde's view, each particular part of the multiverse, including our part, began from a singularity somewhere in the past, but that singularity was just one of an endless series that was spawned before it and will continue after it.

Will a complete physical model of the world help us to understand ultimate reality? Can we understand ultimate reality at all through science?

Some physicists believe that a complete physical model can explain everything we observe. They hold that once the fundamental laws are known and powerful computers allow us to compute models of the world by applying these laws, we can eventually deduce explanations for all phenomena. In other words, physics can lead us to understanding ultimate reality. Is this really possible?

One may doubt it. Even if we give physicists credit for their remarkable discoveries, we have to realise that their research takes place in an isolated field of knowledge. Physics does not concern itself with issues outside its own domain. For example, the subjects of biology, life, and chemistry, as well as the phenomena of mind and consciousness cannot be explained in physical terms. In addition, the following fundamental questions arise:

1. Physics deals only with what can be measured. A complete physical model must therefore necessarily produce a materialistic view of reality. Although materialists usually deny the possibility that phenomena exist which cannot be measured or somehow quantified, they may actually exist.

2. There are limits to what can be measured, as demonstrated by the Uncertainty Principle.

3. The materialist view is generally allied with reductionism. Materialists often claim that high-level phenomena, such as biological or psychological phenomena, can be reduced to physical phenomena. However, this is far from being obvious. For example, there is no generally accepted reductionist theory of consciousness. Reductionism fails in most practical cases. For example, it is practically impossible to describe the process of DNA replication in terms of subatomic properties.

4. Advanced physical models are abstract to the degree of being unintelligible to most people. Modern physics is based on higher mathematics and can hardly be put into common language, much less can it be imagined. The multidimensional worlds of Relativity and string theory, for example, are elusive to plastic imagination. The value of any science depends on how useful its models are for the thoughts and actions of humanity as a whole, hence, its usefulness leans partly on intelligibility.

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