Physics of the Future: How Science Will Shape Human Destiny and Our Daily Lives by the Year 2100

piece together boards to create larger structures like a house according to a plan. You have to carefully guide this process from above at every step of the way.
    In the bottom-up approach, things assemble by themselves. In nature,for example, beautiful snowflakes crystallize all by themselves in a thunderstorm. Trillions upon trillions of atoms rearrange to create novel forms. No one has to design each snowflake. This often occurs in biological systems as well. Bacterial ribosomes, which are complex molecular systems containing at least fifty-five different protein molecules and several RNA molecules, can spontaneously self-assemble in a test tube.
    Self-assembly is also used in the semiconductor industry. Components used in transistors sometimes assemble by themselves. By applying various complex techniques and processes in a precise sequence (such as quenching, crystallization, polymerization, vapor deposition, solidification, etc.) one can produce a variety of commercially valuable computer components. As we saw earlier, a certain type of nanoparticle used against cancer cells can be produced using this method.
    However, most things do not create themselves. In general, only a tiny fraction of nanomaterials have been shown to self-assemble properly. You cannot order a nanomachine using self-assembly like you can order from a menu. So progress in creating nanomachines this way will be steady but slow.
    In sum, molecular assemblers apparently violate no law of physics, but they will be exceedingly difficult to build. Nanobots do not exist now, and will not in the near future, but once (and if) the first nanobot is successfully produced, it might alter society as we know it.
    BUILDING A REPLICATOR
    What might a replicator look like? No one knows exactly, since we are decades to a century away from actually building one, but I got a taste of how a replicator might appear when I had my head examined (literally). For a Science Channel special, they created a realistic 3-D copy of my face out of plastic by scanning a laser beam horizontally across my face. As the beam bounced off my skin, the reflection was recorded by a sensor that fed the image into a computer. Then the beam made the next pass across my face, but slightly lower. Eventually, it scanned my entire face, dividing it up into many horizontal slices. By looking at a computer screen, you could see a 3-D image of the surface of my face emerge, to an accuracy of perhaps a tenth of a millimeter, consisting of these horizontal slices.
    Then this information was fed into a large device, about the size of a refrigerator, that can create a plastic 3-D image of almost anything. The device has a tiny nozzle that moves horizontally, making many passes. On each pass, it sprays out a tiny amount of molten plastic, duplicating the original laser image of my face. After about ten minutes and numerous passes, the mold emerged from this machine, bearing an eerie resemblance to my face.
    The commercial applications of this technology are enormous, since you can create a realistic copy of any 3-D object, such as complicated machine parts, within a matter of a few minutes. However, one can imagine a device that, decades to centuries from now, may be able to create a 3-D copy of a real object, down to the cellular and atomic level.
    At the next level, it is possible to use this 3-D scanner to create living organs of the human body. At Wake Forest University, scientists have pioneered a novel way to create living heart tissue, with an ink-jet printer. First, they have to carefully write a software program that successively sprays out living heart cells as the nozzle makes each pass. For this, they use an ordinary ink-jet printer but one whose ink cartridge is filled with a mixture of fluids containing living heart cells. In this way, they have control over the precise 3-D placement of every cell. After multiple passes, they can actually create the layers of heart tissue.
    There is another instrument that might one day record the location of every atom of our body: the MRI. As we observed earlier, the accuracy of the MRI scan is about a tenth of a millimeter. This means that every pixel of a sensitive MRI scan may contain thousands of cells. But if you examine the physics behind the MRI, you find that the accuracy of the image is related to the uniformity of the magnetic field within the machine. Thus, by making the magnetic field increasingly uniform, one can even go below a