At the end of 1998 my wife, daughter, and I attended the Renaissance Weekend at Hilton Head Island, South Carolina. That gathering is an invitation-only assemblage of politicians, pundits, writers, academics, media people, and others including the First Family, which has been described as the world's best new years party.
Programming at the Renaissance Weekend is rather like that of an up-scale science fiction convention, except that the panels have far more participants than SF Con panels, with each panelist allowed only a 5-minute "sound bite" to describe his or her take on the subject under discussion. On a panel about science and technology I used my few minutes to describe the big unsolved problems in physics and astrophysics. This strategy worked well and stimulated considerable interest and many questions during and after the panel.
An investment banker told me later that he was surprised to learn that science had so many unsolved problems. He had been under the impression that in science we understood almost everything, with only a few odds and ends to be cleaned up. This opinion, held by many otherwise well informed people, reflects the assertions recently proclaimed by an editor of a prominent science magazine. His book claimed that we were at the end of science, that we had solved all the big problems, and what remained was to work out a few details. The author of that book seemed unaware of most of the problems on my list.
So here's my list of the top seven unsolved problems in physics and astrophysics.
Problem 1 - Vacuum Energy and Dark Matter in Cosmology. Everything we thought we knew about the universe is wrong! In the past year astronomical observations of type Ia supernovas have led to the conclusion that the universe is not only expanding and will expand forever, but also that it is accelerating as it expands, with a long-range "antigravity" repulsive force driving its expansion. (See my column in the 5/99 Analog). The current explanation for this observation is that about 3/4 of the energy from the Big Bang is not in the form of matter but in the vacuum itself and that this vacuum energy is creating a negative pressure that accelerates the expansion of the universe. So the book-keeping seems to be that about 70% of the energy in the universe is in the vacuum, about 5% is in the form of normal matter (protons, planets, stars, galaxies, etc.), and the remaining 25% is "dark matter", mysterious invisible particles that inhabit the haloes of galaxies.
This new understanding raises far more questions then it answers. Why and how does the vacuum store energy? What is the dark matter? Why is the energy from the Big Bang distributed in this particular way? Is the energy in the vacuum constant with time, or is it changing? Could the vacuum suddenly decide to dump its energy and restart the Big Bang? And so on. The universe is a stranger place than we had imagined.
Problem 2 - The Arbitrary Parameters of the Standard Model of Particle Physics. The standard model of particle physics, which is called quantum chromodynamics or QCD, is in many ways an excellent theory. It is in good agreement with essentially all of the data collected by particle physics experiments during the past decades. Yet in another way, it is unsatisfactory. It fails to provide a deep understanding of the inner workings of the universe. Instead, it depends on about two dozen arbitrary parameters, for example the masses of quarks and leptons, the strengths of the fundamental forces, and the strengths of interconnections between particles. Some of these parameters are set to zero, while others have definite values that must be derived from measurements. We have no idea where these parameters come from or how they are related to each other. We are confident that there must be a better, more fundamental theory behind this façade. So far, however, we have been unable to make the Standard Model "break", to find places where its predictions conspicuously fail, to find a crack in the brick wall which might provide some inkling of what lies behind it. (See my column in the 11/97 Analog).
A new higher energy particle accelerator, the large hadronic collider or LHC, will begin operation at the CERN laboratory in Geneva around 2005. It is hoped that this machine will provide clues to the underpinnings of QCD. But there are no guarantees, and it is not clear when or how we will be able to go beyond the present impasse.
Problem 3 - The Origin of Gamma Ray Bursts. Gamma ray bursts were discovered 32 years ago by accident. In 1967 the United States entered a treaty that banned the testing of nuclear weapons in space. To verify that the terms of the treaty were being observed, in the late 1960s Los Alamos Laboratory and the Air Force launched the VELA series of surveillance satellites. The VELA satellites never detected a nuclear explosion, but from the time when the first VELA probe was activated bursts of gamma rays were detected every few days. The intelligence analysts were baffled and suspected satellite malfunctions or some form of deliberate jamming to hide real tests. Finally, when several satellites were in orbit simultaneously it became possible to triangulate on the radiation. It was discovered that the bursts came from outside the solar system. (See my column in the 10/95 Analog).
During the last decade space-based detectors, particularly the BATSE experiment on NASA's Compton Gamma Ray Observatory satellite, have recorded and mapped over 2,200 gamma ray bursts (GRB). Since 1997 astronomers have been able, within about 1 day after the burst, to point their optical, X-ray, and radio instruments toward observed GRB positions and to observe the "afterglow" from the bursts. According to Doppler-shift measurements of spectral lines, the optical afterglow comes from dim galaxies 10 to 12 billion light years from Earth. At such distances, a non-beamed gamma ray burst would require an energy release at the source of about 1044 joules, roughly 47 times the energy you would get by converting the mass of the planet Jupiter completely into energy.
It remains a deep mystery what astrophysical catastrophe could have released so much energy, how the process takes place, and why so much of the energy ends up in the form of gamma rays.
Problem 4 - The Origin of Ultra-High Energy Cosmic Rays The most energetic particles ever observed come not from large particle accelerators but from the cosmos itself. These particles from space, usually protons, have energies up to 10,000,000 times higher than the most energetic protons we can produce with accelerators. The most energetic cosmic ray particle observed up to now, which was recorded at the Fly's Eye detector in Utah, had a measured energy of about 3 ´ 1020 electron volts or 50 joules, roughly the kinetic energy of a baseball thrown at 60 mph.
The existence of such ultra-high energy (UHE) particles creates a problem. They should not be able to reach us. The universe is permeated with cosmic microwave background radiation, low energy photons released about 500,000 years after the Big Bang when the protons and electrons paired off and matter and light went their separate ways. Protons with energies above about 5 ´ 1018 electron volts, on colliding with these photons, should produce pi-mesons and should rapidly lose energy. This process, called the GZK cutoff, should result in a sharp drop in cosmic rays above 5 ´ 1018 electron volts. (See my column in the 01/96 Analog).
Paradoxically, instead of a cutoff, more cosmic rays are observed above this energy than extrapolation from lower energies would predict. The only plausible resolution of this paradox is the assumption that these UHE particles are being produced relatively close to the Earth, within about 160 million light years, which would place their source within the Local Group, i.e., in our galaxy or one of our near galactic neighbors. Yet there are no known objects in the Local Group that might produce such energetic particles.
It has been suggested that there are some very heavy particle-remnants left over from the Big Bang that are now decaying to produce the observed UHE cosmic rays. Clearly more detection data is needed, and experimental astrophysics groups, including my own at the University of Washington, are designing new distributed detectors to gain more information about this mysterious phenomenon.
Problem 5 - The Solar Neutrino Problem. A growing body of evidence indicates that only about 1/3 of the electron neutrinos expected from the best models of the fusion reactions in our Sun actually reach the Earth to be detected. The first evidence of this began to emerge 30 years ago, when the detector in the Homestake gold mine went into operation. The neutrino deficiency was later confirmed by the Kamiokande II detector in Japan and by the SAGE experiment in Russia and the GALLEX experiment in the Mont Blanc Tunnel which connects France and Italy. (See my column in the 09/92 Analog).
The alternatives are that either the Sun is making less neutrinos than the best models predict (perhaps because the Sun has gone out inside and we haven't noticed yet!) or because some physical process is interfering with their detection. The neutrino oscillations recently detected between mu and tau neutrinos by the Super Kamiokande detector cannot explain this problem, which involves a deficit of electron neutrinos.
Physicists hope that the new heavy-water-filled SNO neutrino detector, which will go into operation deep in a mine in Sudbury, Ontario, Canada, should provide new insights into this long-standing problem.
Problem 6 - The Origin of Matter/Antimatter Asymmetry in the Universe. It is now clear from astrophysical evidence that the universe is dominated by matter, even in very distant regions. There are far more electrons than positrons and far more protons than antiprotons in our universe. There is no hint of antimatter stars or galaxies.
From the viewpoint of particle physics, this is a problem because at the particle level energetic processes normally produce matter particles and antimatter particles (e.g., electrons and positrons) together, so that the balance between matter and antimatter is preserved. The only known exception to this is the so called "CP Violation" that has only been observed in the decay of K0 mesons (the matter-antimatter combination of a strange and a down quark). This decay exhibits a matter antimatter asymmetry. (See my column in the 09/88 Analog). However, this process could not have caused the universe's proton excess because the K0 has only about the mass of a proton. Therefore, the matter/antimatter asymmetry remains a major unsolved problem of particle astrophysics.
However, new clues to the puzzle may be coming. The Department of Energy is constructing a new kind of accelerator, a "B Factory" at the SLAC Laboratory in Palo Alto, California. The B0 mesons it will produce, matter-antimatter combinations of a bottom and a down quark, are expected to exhibit CP violations similar to those of the K0 mesons and to provide new insights into Nature's preference for matter over antimatter.
Problem 7 - The Origin of the Arrow of Time. In the everyday world it's clear that the past and the future are not the same. We remember the past but not the future. We can send radio signals to the future (e.g., to a distant star) but not to the past. Isolated systems have lower disorder in the past and become more disordered in the future. The universe was smaller and hotter in the past but will be larger and cooler in the future. The K0 meson has decay probabilities that are larger for the normal decay process than for the equivalent time-reversed process. These different manifestations of an asymmetry in time are called, respectively, the subjective arrow of time, the electromagnetic arrow of time, the thermodynamic arrow of time, the cosmological arrow of time, and the CP-violation arrow of time.
These time arrows are all related to one of the most fundamental and mysterious laws of physics is the principle of causality, the rule that a cause must always precede all of its effects in any reference frame. No violation of causality has ever been observed, but it is not clear where causality comes from or how it is imposed on the universe.
This fundamental question can be approached by asking how the different time arrows are related and which arrows cause the others. It is generally accepted that the CP-violation arrow is probably primary, in the sense that if it did not exist the universe would be time-symmetric and contain equal quantities of matter and antimatter. The CP arrow probably leads to the cosmological arrow, making possible the Big Bang and its aftermath.
Beyond this point, there is no consensus. One view is that the cosmological arrow causes the thermodynamic arrow, which in turn causes the electromagnetic and subjective arrows. The other viewpoint, which I have advocated in several papers, is that the cosmological arrow causes the electromagnetic arrow, which causes the thermodynamic arrow, which in turn causes the subjective arrow. The issue is not resolved, and there are few experiments that can shed light on the dilemma. We simply do not understand the origin of causality and the arrows of time that underlie our thinking and existence.
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Other scientists might add other scientific problems to this list, which represents only my own priorities and interests. I'm confident that in 10 years we will have understood most of the items on my list, but others will replace them. That's the nature of science. It's clear to me that we are not at the pinnacle of science, but only on a ledge well below the summit, and that we must scramble very hard, with all the intellectual tricks and technological tools we can muster, to make it to the next ledge up. It will be fun to watch, and even more fun to do.
And remember, it's what you don't understand that can kill you.
AV Columns On-line: Electronic versions of more than ninety past "The Alternate View" columns by John G. Cramer are available on-line on WorldWideWeb at the URL: http://www.npl.washington.edu/AV.