by Michio Kaku
Of all the alternatives, the Copenhagen school of Bohr is now recognized to have the least attractive answer to the cat problem, although there has been no experimental deviation from Bohr’s original interpretation. The Copenhagen school postulates that there is a “wall” that separates the commonsense, macroscopic world of trees, mountains, and people that we see around us, from the mysterious, nonintuitive microscopic world of the quantum and waves. In the microscopic world, subatomic particles exist in a nether state between existence and nonexistence. However, we live on the other side of the wall, where all wave functions have collapsed, so our macroscopic universe seems definite and well defined. In other words, there is a wall separating the observer from the observed.
Some physicists, including Nobel laureate Eugene Wigner, even went further. The key element of observing, he stressed, is consciousness. It takes a conscious observer to make an observation and determine the reality of the cat. But who observes the observer? The observer must also have another observer (called “Wigner’s friend”) to determine that the observer is alive. But this implies an infinite chain of observers, each one observing the other, each one determining that the previous observer is alive and well. To Wigner, this meant that perhaps there was a cosmic consciousness that determined the nature of the universe itself! As he said, “The very study of the external world led to the conclusion that the content of the consciousness is the ultimate reality.” Some have argued therefore that this proves the existence of God, some sort of cosmic consciousness, or that the universe itself was somehow conscious. As Planck once said, “Science cannot solve the ultimate mystery of Nature. And it is because in the last analysis we ourselves are part of the mystery we are trying to solve.”
Over the decades, other interpretations have been proposed. In 1957, Hugh Everett, then a graduate student of physicist John Wheeler, proposed perhaps the most radical solution to the cat problem, the “many worlds” theory, stating that all possible universes exist simultaneously. The cat could indeed be dead and alive simultaneously, because the universe itself has split into two universes. The implications of this idea are quite unsettling, because it means that the universe is constantly bifurcating at each quantum instant, spinning off into infinite numbers of quantum universes. Wheeler himself, originally enthusiastic about his student’s approach, later abandoned it, stating that it carried too much “metaphysical baggage.” For example, imagine a cosmic ray that penetrates Winston Churchill’s mother’s womb, triggering a miscarriage. Thus, one quantum event separates us from a universe in which Churchill never lived to rally the people of England and the world against the murderous forces of Adolf Hitler. In that parallel universe, perhaps the Nazis won World War II and enslaved much of the world. Or imagine a world where a solar wind, triggered by quantum events, pushed a comet or meteor from its path 65 million years ago, so it never hit the Yucatan Peninsula of Mexico and never wiped out the dinosaurs. In that parallel universe, humans never emerged and Manhattan, where I am now living, is populated by rampaging dinosaurs.
The mind is sent spinning contemplating all possible universes. After decades of futile arguing over various interpretations of the quantum theory, in 1965 John Bell, a physicist at the nuclear laboratory at CERN in Geneva, Switzerland, analyzed an experiment that would decisively prove or disprove Einstein’s criticism of the quantum theory. This would be the acid test. He was sympathetic to the deep philosophical questions raised by Einstein decades earlier, and proposed a theorem that would finally settle the question. (Bell’s theorem is based on re-examining a variation of the old EPR experiment and analyzing the correlation between the two particles moving in opposite directions.) The first credible experiment was performed in 1983 by Alain Aspect at the University of Paris, and the results confirmed the quantum mechanical viewpoint. Einstein was wrong about his criticism of the quantum theory.
But if Einstein’s criticism of the quantum theory could now be ruled out, then which of the various quantum mechanical schools is correct? Most physicists today believe that the Copenhagen school is woefully incomplete. Bohr’s wall separating the microscopic world from the macroscopic world does not seem valid in today’s world, when we can now manipulate individual atoms. “Scanning tunneling microscopes” can in fact displace individual atoms and have been used to spell out “IBM” and create a working abacus made of atoms. In addition, a whole new field of technology, called “nanotechnology,” has been created based on the manipulation of atoms. Experiments like Schrödinger’s cat experiment can now be performed on individual atoms.
Despite this, there is still no solution to the cat problem that is satisfactory to all physicists. Almost eighty years since Bohr and Einstein clashed at the Solvay Conference, however, some leading physicists, including several Nobel laureates, have converged on the idea of “decoherence” to resolve the cat problem. Decoherence starts with the fact that the wave function of a cat is quite complicated because it contains something on the order of 1025 atoms, a truly astronomical number. Hence the interference between the live cat’s wave and the dead cat’s wave is quite intense. This means that the two wave functions can coexist simultaneously in the same space but can never influence each other. The two wave functions have “decohered” from each other and no longer sense each other’s presence. In one version of decoherence, wave functions never “collapse,” as claimed by Bohr. They simply separate and, for all intents and purposes, never interact again.
Nobel laureate Steven Weinberg compares this to listening to a radio. By turning a dial, we can tune successively to many radio stations. Each frequency has decohered from the others, so there is no interference between stations. Our room is simultaneously filled with signals from all radio stations, each one yielding an entire world of information, yet they do not interact with each other. And our radio tunes into only one at a time.
Decoherence sounds attractive, since it means that ordinary wave theory can be used to resolve the problem of the cat without resorting to the “collapse” of the wave function. In this picture, waves never collapse. However, the logical conclusions are disturbing. In the final analysis, decoherence implies a “many worlds” interpretation. But instead of radio stations that do not interfere, now we have entire universes that do not interact. It may seem strange, but this means that sitting in the very room where you are reading this book, there exist the wave function of parallel worlds where the Nazis won World War II, where people speak in strange tongues, where dinosaurs battle in your living room, where alien creatures walk the earth, or where the earth never existed in the first place. Our “radio” is tuned only to the familiar world we live in, but within this room there exist other “radio stations” where insane, bizarre worlds coexist with ours. We cannot interact with these dinosaurs, monsters, and aliens walking in our living rooms because we live on a different “radio” frequency and have decohered from them. As Nobel laureate Richard Feynman has said, “I think I can safely say that nobody understands quantum mechanics.”
While Einstein’s critique of the quantum theory helped to sharpen its development but may not have brought forth a wholly satisfactory solution to its paradoxes, his ideas have been vindicated elsewhere, most spectacularly in general relativity. In an era of atomic clocks, lasers, and supercomputers, scientists are mounting the kind of high-precision tests of general relativity that Einstein could only dream about. In 1959, for example, Robert V. Pound and G. A. Rebka of Harvard finally confirmed Einstein’s prediction of gravitational red shift in the laboratory, that is, that clocks beat at different rates in a gravitational field. They took radioactive cobalt and shot radiation from the basement of Lyman Laboratory at Harvard to the roof, 74 feet above. Using an extremely fine measuring device (which used the Mossbauer effect), they showed that photons lost energy (hence were reduced in frequency) as they made the journey to the top of the laboratory. In 1977, astronomer Jesse Greenstein and his colleagues analyzed the beating of time in a dozen white dwarf stars.
As expected, they confirmed that time slowed down in a large gravitational field.
The solar eclipse experiment has also been redone with extreme precision on a number of occasions. In 1970, astronomers pinpointed the location of two extremely distant quasars, 3C 279 and 3C 273. The light from these quasars bent as predicted by Einstein’s theory.
The introduction of atomic clocks also has revolutionized the way in which precision tests can be performed. In 1971, atomic clocks were placed on a jet plane, which was flown both East to West and West to East. These atomic clocks, in turn, were then compared with atomic clocks that were stationary at the Naval Observatory in Washington, D.C. By analyzing the atomic clocks on the jets traveling at different velocities (but with constant altitude), scientists could verify special relativity. Then, by analyzing jets traveling at the same speed but different altitude, they could test the prediction of general relativity. On both occasions, the results verified Einstein’s predictions, within experimental error.
The launching of space satellites has also revolutionized the way in which general relativity can be tested. The Hipparcos satellite, launched by the European Space Agency in 1989, spent four years calculating the deflection of starlight by the sun, even analyzing stars that are 1,500 times fainter than the stars in the Big Dipper. In deep space, there is no necessity to wait for an eclipse, and experiments can be conducted all the time. Without fail, they found that starlight bent according to Einstein’s prediction. In fact, they found that starlight from halfway across the sky was bent by the sun.
In the twenty-first century, a variety of other precision experiments are planned to test the precision of general relativity, including more experiments on double stars and even bouncing laser signals off the moon. But the most interesting precision tests may come from gravity waves. Einstein predicted gravity waves in 1916. However, he despaired of ever being able to see confirmation of these elusive phenomena in his lifetime. The experimental equipment of the early twentieth century was simply too primitive. But in 1993, the Nobel Prize was awarded to two physicists, Russell Hulse and Joseph Taylor, for indirectly verifying the existence of gravity waves by examining double stars rotating around each other.
They examined PSR 1913+16, a double neutron star about 16,000 light-years from Earth, in which two dead stars orbit each other every seven hours and forty-five minutes, releasing copious quantities of gravity waves in their wake. Imagine, for example, stirring a pot of molasses with two spoons, each spoon rotating around the other. As each spoon moves in the molasses, it leaves a trail of molasses in its wake. Similarly, if we replace the molasses with the fabric of space-time and the spoons by dead stars, we find two stars chasing each other in space, emitting waves of gravity. Since these waves carry energy, the two stars eventually lose energy and gradually spiral together. By analyzing the signals of this double-star system, one can experimentally calculate the precise decay in the orbit of the double star. As expected from Einstein’s general relativity theory, the two stars come closer by a millimeter every revolution. Over a year, the separation of the stars decreases by a yard in an orbit that is 435,000 miles in diameter, which is precisely the number that can be calculated from Einstein’s equations. In fact, the two stars will completely collapse in 240 million years owing to the loss of gravity waves. This precision experiment can be reinterpreted as a way in which to test the accuracy of Einstein’s general relativity. The numbers are so precise that we can conclude that general relativity is 99.7% accurate (well within experimental error).
More recently, there is intense interest in a series of far-reaching experiments to observe gravity waves directly. The LIGO (Laser Interferometer Gravitational Wave Observatory) project may soon be the first to observe gravitational waves, perhaps from black holes colliding in outer space. LIGO is a physicist’s dream come true, the first apparatus powerful enough to measure gravity waves. LIGO consists of three laser facilities in the United States (two in Hanford, Washington, and one in Livingston, Louisiana). It is actually one part of an international consortium, including the French-Italian detector called VIRGO in Pisa, Italy; a Japanese detector called TAMA outside Tokyo; and a British-German detector called GEO600 in Hanover, Germany. Altogether, LIGO’s final construction cost will be $292 million (plus $80 million for commissioning and upgrades), making it the most expensive project ever funded by the National Science Foundation.
The laser detectors used in LIGO look very much like the device used by Michelson-Morley at the turn of the century to detect the aether wind, except that laser beams are used instead of ordinary light beams. A laser beam is split into two separate beams that move perpendicular to each other. After hitting a mirror, these two beams are then reunited. If a gravity wave were to hit the interferometer, there would be a disturbance in the lengths of the paths of the laser beams, which could be seen as an interference pattern between the two beams. To make sure that the signal hitting the laser apparatus is not a spurious one, laser detectors are required to be distributed around the planet Earth. Only a huge gravity wave much bigger than Earth would be able to fire the detectors all at once.
Eventually, a series of these laser detectors will be placed in outer space by NASA and the European Space Agency. Around 2010, NASA will launch three satellites, called LISA (Laser Interferometry Space Antenna). They will orbit around the sun at approximately the same distance as the earth’s orbit. The three laser detectors will form an equilateral triangle in outer space (about 3 million miles on a side). The system will be so delicate it will be able to detect vibrations of one part in a billion trillion (corresponding to a shift that is one-hundredth the width of a single atom), allowing scientists to detect the original shock waves from the big bang itself. If all goes well, LISA should be able to peer to within the first trillionth of a second after the big bang, making it perhaps the most powerful of all cosmological tools to exploring creation. This is essential, because it is believed that LISA may be able to find the first experimental data on the precise nature of the unified field theory, the theory of everything.
Yet another important tool introduced by Einstein was gravity lenses. Back in 1936, he proved that nearby galaxies can act as gigantic lenses that focus the light from distant objects. It would take many decades for these Einstein lenses to be observed. The first breakthrough came in 1979, when astronomers observed the quasar Q0957+561 and found that space was being warped and acting as a lens to concentrate light.
In 1988, the first observation of an Einstein ring was from the radio source MG1131+0456, and about twenty, mostly fragments of rings, have been observed since then. In 1997, the first completely circular Einstein rings were observed with the Hubble Space Telescope and Britain’s MERLIN (Multi-Element Radio Linked Interferometer Network) radio telescope array. By analyzing the distant galaxy 1938+666, they found the characteristic ring that surrounded the galaxy. “At first sight, it looked artificial and we thought it was some sort of defect in the image, but then we realized we were looking at a perfect Einstein ring!” said Dr. Ian Brown of the University of Manchester. Astronomers in Britain were elated by the discovery, declaring, “It’s a bulls-eye!” The ring is tiny. It is only a second of an arc, or roughly the size of a penny viewed from a distance of two miles. However, it is a verification of Einstein’s prediction made decades ago.
One of the greatest explosions in general relativity has been in the area of cosmology. In 1965, two physicists, Robert Wilson and Arno Penzias, detected the faint microwave radiation from outer space with their Bell Laboratory Horn Radio Telescope in New Jersey. The two physicists, unaware of the pioneering work of Gamow and his students, accidentally picked up this cosmic radiation from the big bang without realizing it. (According to legend, they thought they were picking up interference from the bird droppings that littered their radio telescope. Later, Princeton physicist R. H. Dicke correctly identified this radiation as Gamow’s microwave background radiation.) Penzias and Wilson were awarded the Nobel Priz
e for their pioneering work. Since then, the COBE (Cosmic Background Explorer) satellite, launched in 1989, has given us the most detailed picture of this cosmic microwave background radiation, which is remarkably smooth. When physicists led by George Smoot of the University of California at Berkeley carefully analyzed any slight ripples in this smooth background, they produced a remarkable photograph of the background radiation when the universe was only about 400,000 years old. The media mistakenly called this picture the “face of God.” (This photograph is not the face of God, but it is a “baby picture” of the big bang.)
