Having heard that laser light has bored through steel and is being used in microwelding, some have asked whether the laser will ever be used to weld bridge members and other structural girders. This is missing the whole point of the laser: It would be like washing your floor with a toothbrush (even one with extra stiff bristles)! There would be no advantage to using lasers for large-scale welding; present equipment for this operation is quite satisfactory and far less wasteful of input power. The sensible approach is to use lasers where existing processes leave something to be desired. Until the advent of the laser, for example, there was no good way to weld wires 0.001 inch in diameter. Nor was there a good way to bore the tiny hole in a diamond that is used as a die for drawing such fine wire. It used to take 2 days to drill a single diamond. With laser light the operation takes 2 minutes—and there is no problem with rapid wear of a cutting tool. So much for the first category of application. In the second category, namely use of the laser as a scientific tool, we enter a more theoretical domain. Here we use coherent light as an extension of ourselves, to probe into and to look at the world around us. [Illustration: Figure 20 _A laser beam was used (and continues to be used) for precise alignment of Stanford University’s 2-mile-long linear accelerator. This view shows the aboveground portion during construction._] Much experimental science is a matter of cooling, heating, grinding, squeezing, or otherwise abusing matter to see how it will react. With each new tool—ultrafast centrifuges, high- and low-pressure and extreme-temperature chambers, intense magnetic fields, atomic accelerators and so on—more has been learned about this still-puzzling world. Since coherent light is something new, we can do things to matter that have not been done before, and see how it reacts. The laser is being used to investigate many problem areas in biology, chemistry, and physics. For example, sound waves of extremely high frequency can be generated in matter by subjecting it to laser light. These intense vibrations may have profound effects on materials. [Illustration: Figure 21 _Subterranean view of Stanford accelerator housing. Alignment optics (laser systems) are housed in the large tube, which also acts as support for the smaller accelerator tube above it._] [Illustration: Figure 22 _Laser beam spot as observed at the end of the accelerator._]