The Amazing Story of Quantum Mechanics (37 page)

First, the issue of faster-than-light transmission of information holds
only
if the two electrons are described by a single wave function, and that will be true
only
if the “infinitely stretchy” ribbon does not break as I pull the two ends farther and farther apart. As you might expect, the more the ribbon is stretched, the easier it is for some stray perturbation to disturb the overlapping waves between the two ends. Once the connection between the two ends is severed, then a measurement of the spin of one electron will have no bearing at all on the other electron, as they are now described by two distinct ribbons. The fancy way to describe this is that the two electrons’ wave functions must remain “entangled” in order for this process to hold, and any object or input of energy that disturbs this state (breaks the ribbon) causes “decoherence.” Overcoming the enormous challenges involved in avoiding decoherence keeps experimental physicists busy, and whether a functioning quantum computer is ever constructed that can live up to its potential remains to be seen.

Figure 50:
The Atom (who in his secret identity is physics professor Ray Palmer), informs his Justice League of America colleagues about recent experiments involving entangled quantum states of photons in
JLA
# 19. Professor Palmer is referring to the work of Anton Zeilinger and coworkers, published in
Physical Review Letters
(1998). The second panel is an illustration of the overinterpretation of the principles of quantum mechanics to matters of spirituality—we will avoid this trap, as we do all such traps into which the Justice League may fall.

The second point is that whenever one reads in the popular science press about recent experiments in “teleportation,” what they are always referring to is the transmission of information concerning a quantum state, similar to the situation described above. They are not dealing with “beaming” people as in
Star Trek,
or sending atoms or electrons from one point in space to another. You will have to face your daily morning commute for quite some time.

There have recently been experiments that indeed support the notion that information concerning two entangled quantum entities, such as the polarization of photons, can be transmitted even when separated by a great distance. That is, experimental techniques have now advanced such that considerations that previously had been purely theoretical may be put to empirical verification, as shown in Figure 50, from a 1998 issue of the adventures of the Justice League of America. Real science is now inspiring the comic books, and not the other way round!

As mentioned at the very start
of our narrative, science fiction pulp writers expected that the future would bring a new era in energy production and storage. Instead, it was data manipulation that underwent a profound transformation, enabled by the discoveries of quantum mechanics.

Why is a new type of energy-delivery system needed before jet packs and flying cars become commercially viable? Let’s stipulate that we are not invoking any violations of the laws of physics, such as the discovery of “cavorite” or some other miraculous material with antigravity properties. Thus, our jet pack must provide a downward thrust, equal to a person’s weight, in order to lift the person off the ground. Consider how much energy it takes to lift a 180-pound person 330 feet (one-sixteenth of a mile) up in the air. Just to get up there, neglecting any energy needed to jet from place to place, would require an energy expenditure of a little over eighty thousand Joules, which is equivalent to 0.5 trillion trillion electron Volts.

Recall that nearly every chemical reaction involves energy transfers on the order of an electron Volt. Thus, to lift a person over a twenty-story building involves roughly a trillion trillion molecules of fuel. But that’s not actually as much as it seems, for there are approximately that many atoms in twenty cubic centimeters of any solid (recall that a cubic centimeter is about the size of a sugar cube). A gallon of fuel contains nearly four thousand cubic centimeters, capable of producing over a hundred trillion trillion electron Volts of energy. If this is the case, why are we still driving to work?

The problem is—what goes up must come down. As soon as our jet pack stops expending energy to maintain our large potential energy above ground, back to Earth we return. Thus, every second we spend in the air, we must continue to burn through our stored chemical energy. The rate at which we use up fuel will depend on the particular mechanism by which we achieve an upward thrust, but for most energy supplies, our trip will be over in a minute or two. We can indeed take jet packs to work, provided we live no more than a few blocks from our office.

Note that the largest expenditure in energy is overcoming gravity. It takes more than eighty thousand Joules to get us up in the air. Flying at forty miles per hour, in contrast, calls for a kinetic energy of only thirteen thousand Joules (neglecting the work we must do to overcome air resistance).
This
is why we don’t have flying cars. Your gas mileage would be nonexistent if the vast majority of the fuel you carried went toward lifting you up off the ground, with hardly any left over to get you to your destination (sort of defeats the whole purpose of a car, flying or otherwise).

There have been improvements in the energy content of stored fuel, and prototype jet packs have been able to keep test pilots aloft for more than a minute, but ultimately, the longer the flight, the more fuel needed (and the heavier the jet pack will be). Of course, there are alternatives to chemical-fuel reactions to achieve thrust and lift. One could use a nuclear reaction, which, as we saw in Section 3, yields roughly a million times more energy per atom than chemical combustion, but the idea of wearing even a licensed nuclear power plant on your back is less than appealing (except possibly for the Ghostbusters).

In the 2008 film
Iron Man,
Tony Stark designs a suit of armor that contains a host of high-tech gadgets, all of which are within the realm of physical plausibility—with one big exception. The one miracle exception from the laws of nature that the film invokes is the “arc reactor” that powers Stark’s high-tech exoskeleton. This device is a cylinder about the size of a hockey puck and is capable of producing “three GigaWatts of power,”
⁷⁵
sufficient to keep a real-world jet pack aloft and flying for hours. Sadly, we have no way of producing such compact, lightweight, high-energy-content power cells.

Had the revolution in energy anticipated by the science fiction pulp magazines indeed occurred and we employed personal jet packs to get to work or the corner grocery store, powered by some exotic energy source, the need for conventional fossil fuels would of course be dramatically reduced, with a concurrent dramatic shift in geopolitical relations. There is one important use of potential jet-pack technology that does not involve transportation but rather thirst quenching, that would have an immediate beneficial impact.

According to the World Health Organization, as of 2009, 40 percent of the world’s population suffers from a scarcity of potable fresh water. The most straightforward method to convert seawater to fresh water involves boiling the salt water and converting the liquid water to steam, which leaves the salts behind in the residue. This is, after all, what occurs during evaporation from the oceans, which is why rainwater is salt free. The amount of energy needed to boil a considerable amount of water is not easily provided by solar cells, but if one had a power supply for a fully functioning jet pack, the lives of more than two billion people would be profoundly improved, even if everyone’s feet stayed firmly planted on the ground.

Can quantum mechanics help in the production of energy, so that the jet-pack dreams of the 1930s can be at long last realized? Possibly. Global consumption of energy, which in 2005 was estimated to be sixteen trillion Watts, will certainly increase in the future, with many experts projecting that demand will grow by nearly 50 percent in the next twenty years. One strategy to meet this additional need involves the construction of a new power plant, capable of producing a gigaWatt of power, at the rate of one new facility every day for the next two decades. This does not seem likely to happen.

Another approach is to tap the vast amount of energy that is, for the most part, ignored by all nations—sunlight. The surface of the Earth receives well over a hundred thousand trillion Watts of power, more than six thousand times the total global energy usage and more than enough to meet the world’s energy needs for decades to come. As described in Chapter 16, the simple diode, comprised of a junction between one semiconductor with impurities that donate excess electrons and a second semiconductor with impurities that donates holes, can function as a solar cell. When the diode absorbs a photon, an electron is promoted into the upper band, leaving a mobile hole in the lower filled band. These charge carriers feel a force from the strong internal electric field at the pn junction, and a current can be drawn out of the device, simply as a result of exposing it to sunlight. Work is under way to improve the conversion efficiency of these devices—that is, to maximize the current that results for a given intensity of sunlight. But even using current cells, with conversion efficiencies of only 10 percent (that is, 90 percent of the energy that shines on the solar cell does not lead to electrical power), we could provide all the electricity needs of the United States with an array of solar cells of only one hundred miles by one hundred miles.

The problem is, we don’t have enough solar cells on hand to cover a one-hundred-mile-by-one-hundred-mile grid, and at the present production capacity it would take many years to fabricate these devices. Moreover, even if the solar cells existed, we would need to get the electrical power from bright sunny locales to the gloomy cities with large population densities. Here again, quantum mechanics may help.

In Chapter 13 we saw that at low temperatures certain metals become superconductors, when their electrons form bound pairs through a polarization of the positive ions in the metal lattice. Electrons have intrinsic angular momentum of ℏ/2 and individually obey Fermi-Dirac statistics (Chapter 12) that stipulate that no two electrons can be the same quantum state. When the electrons in a metal at low temperature pair up, they create composite charge carriers that have a net total spin of zero. These paired electrons obey Bose-Einstein statistics, and as the temperature is lowered they condense into a low energy state. If the temperature of the solid is low enough, then for moderate currents there is not enough energy to scatter the electrons out of this lowest energy state, and they can thus carry current without resistance. This phenomenon—superconductivity—is an intrinsically quantum mechanical effect and is observed only in metals at extremely low temperatures, below -420 degrees Fahrenheit.

At least—that was the story until 1986. In that year two scientists, Johannes Bednorz and Karl Müller, at the IBM research laboratory in Zurich, Switzerland, reported their discovery of a ceramic that became a superconductor at -400 degrees Fahrenheit. That’s still very cold, but at the time it set a record for the highest temperature at which superconductivity was observed. Once the scientific community knew that this class of materials, containing copper, oxygen, and rare Earth metals, could exhibit superconductivity, the race was on, and research labs around the world tried a wide range of elements in a host of combinations. A year later a group of scientists from the University of Houston and University of Alabama discovered a compound of yttrium, barium, copper, and oxygen that became a full-fledged superconductor at a balmy -300 degrees Fahrenheit. Liquid nitrogen, used in many dermatologists’ offices for the treatment of warts, is 20 degrees colder at -321 Fahrenheit. These materials are referred to as “high-temperature superconductors,” as their transition into a zero resistance state can be induced using a refrigerant found in many walk-in medical clinics. There is no definitive explanation for how these materials are able to become superconductors at such relatively toasty temperatures, and their study remains an active and exciting branch of solid-state physics. The most promising models to account for this effect invoke novel mechanisms that quantum mechanically induce the electrons in these solids to form a collective ground state.

High-temperature superconductors would be ideal to transmit electricity generated from a remote bank of solar cells or windmills to densely populated regions where the power is needed. While it would need to be kept cool, liquid nitrogen is easy to produce, and when purchased for laboratory needs it is cheaper than milk (and certainly cheaper than bottled water). Unfortunately, to date challenging materials-science issues limit the currents that can be carried by these ceramics, such that if we were to use them for transmission lines they would cease to become superconductors and would in fact have resistances higher than those of ordinary metals.