The 'Backyard' Tourists Don't Know: Tokyo Skytree Was a 'Giant Scientific Laboratory'! (Former Professor, 868th Day of Retirement)
From Five-Story Pagodas to the Skytree
About 10 days ago, I wrote about the 'central pillar' of the five-story pagoda at Hokekyoji Temple as one of the 'pillars' unique to Japan. The central pillar at the heart of the pagoda uses an astonishing construction method called 'suspension' that was perfected by early modern master carpenters; it is not fixed to the outer perimeter of the pagoda, but rather has a gap of several tens of centimeters. This prevents the pagoda body and the central pillar from resonating by swaying together during an earthquake, allowing them to sway differently and disperse the seismic energy. What surprised me even more was that this concept has been inherited by the 'central pillar vibration control' of the modern Tokyo Skytree. This time, the NHK program 'The Backyard' featured the Tokyo Skytree, and I was able to see its central pillar structure on video for the first time. (Please see the photo below as well.)
<Postscript 1> I have a fear of heights, so I don't particularly like high places. However, I once had to guide guests from overseas and was made to climb the Tokyo Skytree. I remember that while I was eloquent at the 'Solamachi Shopping Street' on the first floor, I became silent the moment I climbed the Skytree, and they gave me a puzzled look (laughs). By the way, even this time, just watching the video gave me the 'chills'.
The 'Central Pillar' and Miracle Vibration Control Technology Breathing in the Modern Age
Standing upright through the center of the Skytree for a height of 375 meters is a hollow cylindrical structure made of reinforced concrete called the 'central pillar,' which is about 8 meters in diameter and has a total weight of about 10,000 tons. This central pillar is designed in two parts: a 'fixed zone' from the ground to a height of 125 meters, and a 'movable zone' from 125 meters to 375 meters. At the base of the foundation on the ground, large laminated rubber bearings are installed in six locations to create a structure that suppresses damage to the central pillar during an earthquake. When hit by a major earthquake or strong wind, the steel tower body and the highly rigid concrete central pillar have different natural vibration periods. Therefore, the central pillar, which sways slowly with a delay, acts like a giant 'pendulum' with an opposite phase, canceling out each other's swaying. This is said to reduce the swaying of the entire tower by up to 50% and by about 30% during strong winds. The Skytree was supported by such cutting-edge technology so that it could continue to deliver information to people as a broadcast tower even in the event of a disaster. (Please see the photo below as well.)
<Postscript 2> The effectiveness of this central pillar vibration control was unintentionally proven by the 2011 Great East Japan Earthquake. At the time, the Skytree had reached a height of about 620 meters, and the lifting work of the gain tower was in full swing. Moreover, the central pillar was still under construction, and the vibration-damping oil dampers had not yet been installed, making it an extremely difficult timing in terms of structure. Even so, no structural damage or permanent deformation occurred due to the earthquake. It was truly an extreme verification in an incomplete state, but it was a result that demonstrated the reliability of the design technology and safety management.
Does Time Move Faster at the Skytree?
Furthermore, the program introduced several scientific experiments that were only possible because of the 'Tokyo Skytree.' Among them, the one I was personally most interested in was the experiment to verify Einstein's general theory of relativity. According to the general theory of relativity, time moves slower in places with stronger gravity—that is, on the surface of the earth closer to the center—and time moves slightly faster at higher altitudes where gravity is weaker. Professor Hidetoshi Katori of the University of Tokyo and his colleagues actually measured this phenomenon using an ultra-high-precision 'optical lattice clock' that is said to lose only about one second every 10 billion years. (Please see the title photo and the photo below as well: Note 1)
Einstein's Theory of Relativity Verified at Tokyo Skytree
In the experiment, optical lattice clocks were installed on the first floor and at the Tembo Galleria 450 meters above ground, and their frequencies were compared by connecting them with optical fibers. It was measured that the clock at the higher altitude was running about 4 nanoseconds faster per day. With this, we were able to actually observe the difference in the passage of time according to the general theory of relativity, even within the range of everyday elevation differences. At first, I didn't really understand what this could be applied to, but when I looked into it, I found that it is expected to be applied as a 'relativistic sensor' that directly measures slight changes in gravity and altitude at the installation site. In other words, it has the potential to lead to next-generation disaster prevention infrastructure that constantly monitors vertical crustal movements such as ground expansion, uplift, and the accumulation of strain at plate boundaries. In the future, it may become possible to constantly monitor even the minute deformations of large structures and the expansion and contraction of the ground.
Turning an Average of 10 Lightning Strikes Per Year into 'Data'
Furthermore, as research that can only be done at the Skytree, observations of lightning, clouds, and urban greenhouse gases were introduced. For example, because the Skytree is significantly taller than the surrounding buildings, it is said to be struck by lightning an average of 10 times a year. In addition to the common 'downward lightning' that heads from clouds to the ground, there are also many 'upward lightning' strikes where discharges extend from the top of the Skytree toward thunderclouds. In fact, lightning research has long relied on data obtained about 50 years ago at the summit of Mount San Salvatore in Switzerland. Therefore, a 'Rogowski coil' that directly measures lightning current was installed at a height of 497 meters above the ground on the Skytree. The current data captured here is immediately converted into optical signals and sent to the measurement room via optical fiber, where it is stored. Furthermore, by combining this with 3D visualization of discharge paths using high-speed cameras and waveform analysis of lightning current flowing inside the building, it is said that applications for more rational and robust lightning protection designs for urban infrastructure and high-rise buildings are being advanced. (Please see the photo below as well.)
The backyard was a 'fortress of science'
At an altitude of around 458 meters, even higher than the observation deck, observation equipment such as cloud particle counters and aerosol samplers are installed. The moment the Skytree is enveloped in clouds, we can directly sample the interior of the clouds while remaining on the ground, continuously measuring the particle size distribution and number concentration of cloud droplets. These observations have revealed that aerosols originating from large cities form smaller and more densely packed cloud droplets than usual. Data obtained from direct observation inside clouds is also said to be useful for improving the prediction simulation accuracy of 'cloud radar,' which is a further development of conventional rain cloud radar. (Please also see the photo below.)
In addition, a research team from the National Institute for Environmental Studies and others has set up an atmospheric observation measurement room at an altitude of 250 meters, where they are continuously monitoring carbon dioxide, methane, carbon monoxide, and other substances in real-time 24 hours a day. Furthermore, by analyzing the collected air in detail, they are verifying the effectiveness of the system as a way to monitor urban air in detail by distinguishing between carbon dioxide derived from fossil fuel combustion and carbon dioxide derived from biological respiration. (Please also see the photo above.)
Tokyo Skytree has made the most of its overwhelming 'height' and 'cutting-edge architectural technology' to produce numerous groundbreaking research results. In the 'backyard' of its glamorous tourist attraction facade, it has played the role of a giant laboratory opening new doors to science. Personally, however, just looking at its magnificence from the ground is enough for me. I would appreciate it if you would spare me from having to climb it again... (laughs).
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Note 1: From the NHK 'The Backyard' special feature 'Tokyo Skytree'