The "Simple is the Best" Experiment

When conducting a physics experiment, there is almost always some discrepancy with the theory. This is common even in very simple experiments (such as those in high school physics), where minor deviations from the theory occur. When considering the reasons for this difference, various factors come to mind, such as temperature fluctuations, slight differences in measurement timing, and the influence of noise. In such cases, there are two approaches. One is to modify the theory that explains the experiment and construct a new theory that takes these factors into account. The other is to eliminate these factors as much as possible, bringing the experiment closer to the theory. Such an experiment, where factors causing deviation from the theory are eliminated as much as possible, can be called a "simple experiment."

What can we learn from conducting such a "simple experiment"? If we can measure discrepancies with the theory caused by more subtle factors, we can uncover new physics. For example, it is known that atoms absorb light of specific wavelengths. A graph with the wavelength of light on the horizontal axis and the intensity of the light transmitted through the atoms on the vertical axis is called an absorption spectrum. An absorption spectrum usually looks like a single line, but when observed microscopically, it spreads out to a certain width. This is due to the influence of the Doppler effect caused by the thermal motion within the atoms. Upon closer examination, multiple spectral lines are observed. This is due to the influence of electron spin and isotopes within the atom. Investigating this even more closely reveals that the spectral lines are split even further. This is due to the influence of the nuclear spin. Thus, by making precise measurements, it is possible to discover more subtle effects (new physics). In short, it can be said that precise measurements have the potential to observe any physical phenomenon.

However, a simple experiment is not as easy as it sounds. For example, much effort is required to create an environment where the temperature does not change or to ensure vibration-free conditions. As an example, let's consider creating theoretically simple light. In physics theory, the most fundamental light is a monochromatic sine wave—in other words, light represented by a simple trigonometric wave. This type of monochromatic sine wave light is theoretically very easy to handle, but in reality, such light does not exist. This is because a perfect sine wave, one that has existed since before the birth of the universe and will continue to exist forever after its end, does not exist. Real-world light that is close to the ideal is light that behaves as a sine wave for a very long time. This duration of the sine wave is called the coherence time. For example, the coherence time of sunlight is about 10 ^-14 seconds, and even for light from a light-emitting diode, it is about 10 ^-12 seconds. In contrast, laser light can have a very long coherence time, ranging from about 10 ^-9 seconds on the short end to 10 ^-6 seconds on the long end. However, this is still far from ideal light. The reasons for such a short coherence time include temperature changes in the laser, external mechanical vibrations and sounds, and power supply noise. By removing these factors or incorporating mechanisms to cancel them, it is possible to generate laser light with a long coherence time of one second or more.