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Defying Common Sense –Success in observing transverse acoustic wave in a liquid

Common sense explanation in physics textbooks

Physics textbooks for high school students state that “A transverse wave propagates only in a solid, because no restoring force is induced when a liquid or gaseous medium is slightly deviated in the transverse direction,” in the section on longitudinal and transverse waves.[Note 1]

You may often hear that the P and S waves generated by earthquakes are examples of longitudinal and transverse waves, respectively. A P wave is a longitudinal wave in which changes in density in the direction of wave propagation are transmitted, and an S wave is a transverse wave, the motion of which is perpendicular to the direction of wave propagation. At the surface of the earth, P and S waves are manifested as vertical and horizontal shaking, respectively, because the seismic waves travel upward from the seismic center.

In textbooks, common sense explanations supported by various theories and experimental results are written. Dr. Shinnya Hosokawa, an associate professor of the Faculty of Engineering, Hiroshima Institute of Technology, succeeded in obtaining experimental results that defy this common sense approach to transverse waves using SPring-8. Namely, he found that a transverse wave can propagate in a liquid and that a restoring force is induced when the liquid medium is slightly deviated in the transverse direction.

Transverse waves exist but cannot be observed

Dr. Hosokawa originally studied the atomic structure and electronic state of substances but started to develop an interest in transverse acoustic wave*1 propagation in liquids in 2000. He noticed a phenomenon that could not be explained only by longitudinal waves while examining data obtained through experiments carried out at the Advanced Photon Source (APS) of Argonne National Laboratory in the US.

He considered that the phenomenon was caused by a transverse acoustic wave and asked researchers engaged in theoretical physics for their opinions; however, his hypothesis of a transverse acoustic wave was immediately rejected; he was told that although transverse acoustic waves can theoretically exist, they cannot be observed experimentally, and it was suggested that the phenomenon must have been caused by other factors.

Among researchers engaged in theoretical physics, the existence of transverse acoustic waves has been considered a theoretical certainty for more than 30 years. It has been claimed that a basketlike atomic ensemble of nanometer size (nanometer size means about one-billionth of a meter) has solidlike characteristics (Fig. 1), and therefore transverse acoustic waves can exist. However, their observation has been considered impossible for the following two reasons. First, the intensity of a transverse acoustic wave is extremely low. Second, it is impossible to detect such a transverse wave by X-ray experiments based on a change in electron density, because a transverse wave is not accompanied by changes in electron density.

Fig. 1 Schematic of basketlike atomic ensemble in a liquid.

Fig. 1 Schematic of basketlike atomic ensemble in a liquid.

Circles represent atoms. The yellow atom surrounded by blue atoms shows solidlike characteristics because it cannot be easily moved by the surrounding atoms. The red curve shows an X-ray wave. The structure of the basketlike atomic ensemble can be examined in detail using an X-ray with a wavelength similar to the size of the basket.

Detection of evidence of transverse wave

Despite such a counterargument, Dr. Hosokawa decided to carry out a detailed measurement while trusting his ability to analyze the data obtained. An acoustic wave in liquid gallium (Ga)*2 was measured using the inelastic X-ray scattering spectrometer (Fig. 2) installed at the SPring-8 beamline BL35XU in 2006.

The results are shown in Fig. 3. A slight convexity can be observed in the flanks of the mountain-shaped graph. The convexity is due to the effect of a transverse wave. Dr. Hosokawa even discovered this convexity in the analytical results obtained at APS, which have less accuracy, although it is difficult to see the convexity at a glance. From a detailed analysis, a transverse acoustic wave was found to exist only for 0.5 picosecond (a picosecond is one-trillionth of a second) over a length of 0.5 nm (a nanometer is one-billionth of a meter).

Two pieces of equipment played key roles in this achievement. One was the X-ray beamline at SPring-8, which has the world’s highest X-ray intensity. The amount of X-rays irradiated to a sample determines the amount of reduction in the effect of noise in this experiment in which a large amount of noise is unavoidable. Dr. Hosokawa said that he had noticed the existence of the transverse acoustic wave on the first day of the three-day experiment at SPring-8 and expressed his thanks to the staff and facilities of SPring-8.

The other key factor was the container used to hold the liquid Ga. An artificial sapphire container (Fig. 4) that did not react with Ga was made using a diamond tool and other tools. This fabrication technology is a unique technology developed by and passed down from Professor Kozaburo Tamura of Kyoto University.[Note 2]

Fig. 2 Giant inelastic X-ray scattering spectrometer with total length of 20 m.

Fig. 2 Giant inelastic X-ray scattering spectrometer with total length of 20 m.

This spectrometer boasts the highest energy resolution and highest X-ray intensity among the four similar systems existing in the world. The direction of the 10-m-long arm can be adjusted by up to 50° while it is levitated above the stage by air pressure.

Fig. 3
Fig. 3 Inelastic X-ray scattering spectra () and theoretical curves (red and blue curves) of liquid Ga.
In the upper graph (red), three graphs, i.e., for a longitudinal acoustic wave component (), a transverse acoustic wave component (), and a quasi-elastic scattering component (), are drawn, and a theoretical curve (red solid line) is obtained by superimposing the three graphs. The experimental results (denoted by ) are in good agreement with the red theoretical curve. In the lower graph (blue), the transverse acoustic wave component () is subtracted. The experimental results slightly deviate from the theoretical curve in the blue shaded areas.
Fig. 4 Artificial sapphire container.

Fig. 4 Artificial sapphire container.

The thickness of liquid Ga at the area where the X-ray is irradiated is 50 μm (a micrometer is one-millionth of a meter).

The difficulty of defending the findings

Although Dr. Hosokawa had a difficult time obtaining the necessary data, the real difficulty started after obtaining the data because without a theoretical explanation supporting the experimental results, his new finding would have been considered scientifically unsatisfactory. In addition, the finding contradicted the common sense explanation given in textbooks. Dr. Hosokawa commented on the immense effect involved in trying to refute what people had taken for granted.

His paper was accepted for publication in spring 2009. Since then, many researchers have raised questions about his experimental result and theoretical grounding, and Dr. Hosokawa has been preoccupied with responses to such questions. Six months have passed, and the end of such questions is now in sight. The existence of a transverse acoustic wave in a liquid has gradually become accepted among researchers.

However, the relevant content in the textbooks may not be rewritten in the near future. After the accumulation of facts demonstrating the existence of a transverse acoustic wave in a liquid through various experiments, the new wording of the textbooks is expected.

“Do something unproductive”

As an educator, Dr. Hosokawa is dedicated to giving physics and other classes to first-year students from Monday to Thursday. He said with a wry smile that he can only carry out experiments on Fridays, Saturdays, and Sundays. He did not complain about this situation; rather, he considers that he can obtain the most useful experimental results by skillfully devising experiments under the given circumstances.

He also said “You should do something that seems unproductive at first glance. In this way, you can improve your ability to effectively carry out experiments.” This attitude toward science may have led to his significant discovery that defied common sense. The thrill of unveiling unconventional results will continuously support his research in the future.

Column: Do you like pursuing difficult activities?


“I don't want to say that my hobby is physics,” said Dr. Hosokawa. On days off, he enjoys jogging and working on puzzles. He sometimes rests at home without doing anything. He prepares himself for research by refreshing his body and mind with exercise.

He is currently hooked on mystery novels. He said that he is a fan of books by Yasuo Uchida, particularly because Uchida uses difficult Japanese words, and added that he never buys mystery books in which the perpetrator is revealed in the middle of the book. He appears to be interested in problems that cannot be solved without complications and that fully excite him in his hobbies as well as in his research.

Interview and text by Tomoaki Yoshito (Sci-Tech Communications Incorporated)


*1 Acoustic wave
The acoustic waves familiar to us are those propagating in air, which we perceive as sound; however, here we use acoustic waves as a generic term for waves propagating through gases, liquids, and solids.

*2 Liquid gallium (Ga)
he atomic number of Ga is 31. It has a low melting point of 29.8 °C even though it is a metal. The experiment was carried out at 40 °C.

[Note 1] Quoted from Physics I for high school students published by Suken Shuppan.
[Note 2] Refer to “Observation of mercury at the moment when it changes from a metal to an insulator at SPring-8” in the Research Achievements and Topics section of SPring-8 News No. 35.

This article was written after an interview with Dr. Shinnya Hosokawa, an associate professor in the Faculty of Engineering, Hiroshima Institute of Technology.