A tunable laser excites an extremely narrow-band resonance in an Yb+ ion of an atomic clock. The electron wave function of the ion's excited state is marked in yellow. Two ions with wave functions that are oriented at right angles are interrogated by means of laser light with an adjustable frequency shift to measure a possible frequency difference. The whole experimental setup rotates together with the Earth once a day relative to the fixed stars. Credit: Physikalisch-Technische Bundesanstalt (PTB) In his Special Theory of Relativity, Einstein formulated the hypothesis according to which the speed of light is always the same, no matter what the conditions are. It may, however, be possible that—according to theoretical models of quantum gravitation—this uniformity of space-time does not apply to particles. Physicists have now tested this hypothesis with a first long-term comparison of two optical ytterbium clocks at the Physikalisch-Technische Bundesanstalt (PTB). With these clocks, whose error amounts to only one second in ten billion years, it should be possible to measure even extremely small deviations of the movement of the electrons in ytterbium. But the scientists did not detect any change when the clocks were oriented differently in space. Due to this result, the current limit for testing the space-time symmetry by means of experiments has been drastically improved by a factor of 100. In addition to this, the extremely small systematic measurement uncertainty of the optical ytterbium clocks of less than 4 × 10-18 has been confirmed. The team consisting of physicists from PTB and from the University of Delaware has published its results in the current issue of Nature. It is one of the most famous physics experiments in history: As early as 1887, Michelson and Morley demonstrated what Einstein later expressed in the form of a theory. With the aid of a rotating interferometer, they compared the speed of light along two optical axes running vertically to each other. The result of this experiment became one of the fundamental statements of Einstein's Special Theory of Relativity: The speed of light is the same in all directions of space. Now one could ask: Does this symmetry of space (which was named after Hendrik Antoon Lorentz) also apply to the motion of material particles? Or are there any directions along which these particles move faster or more slowly although the energy remains the same? Especially for high energies of the particles, theoretical models of quantum gravitation predict a violation of the Lorentz symmetry. Now an experiment has been carried out with two atomic clocks in order to investigate this question with high accuracy. The frequencies of these atomic clocks are each controlled by the resonance frequency of a single Yb+ ion that is stored in a trap. While the electrons of the Yb+ ions have a spherically symmetric distribution in the ground state, in the excited state they exhibit a distinctly elongated wave function and therefore move mainly along one spatial direction. The orientation of the wave function is determined by a magnetic field applied inside the clock. The field orientation was chosen to be approximately at right angles in the two clocks. The clocks are firmly mounted in a laboratory and rotate together with the Earth once a day (more exactly: once in 23.9345 hours) relative to the fixed stars. If the electrons' speed depended on the orientation in space, this would thus result in a frequency difference between the two atomic clocks that would occur periodically, together with the Earth's rotation. To be able to differentiate such an effect clearly from any possible technical influences, the frequencies of the Yb+ clocks were compared for more than 1000 hours. During the experiment, no change between the two clocks was observed for the accessible range of period durations from a few minutes up to 80 hours. For the theoretical interpretation and calculations concerning the atomic structure of the Yb+ ion, PTB's team worked in collaboration with theoreticians from the University of Delaware (USA). The results that have now been obtained have improved the limits set in 2015 by researchers from the University of California, Berkeley with Ca+ ions drastically by a factor of 100. Averaged over the total measuring time, both clocks exhibited a relative frequency deviation of less than 3 × 10-18. This confirms the combined uncertainty of the clock that had previously been estimated to be 4 × 10-18. Furthermore, it is an important step in the characterization of optical atomic clocks at this level of accuracy. Only after roughly ten billion years would these clocks potentially deviate from each other by one second. Explore further: NIST atomic clocks now keep time well enough to improve models of Earth More information: Christian Sanner et al. Optical clock comparison for Lorentz symmetry testing. Nature (2019). DOI: 10.1038/s41586-019-0972-2 , https://www.nature.com/articles/s41586-019-0972-2 Was the US justified in dropping atomic bombs on Hiroshima and Nagasaki during the Second World War? You debate Was the US justified in dropping atomic bombs on Hiroshima and Nagasaki? History Extra readers George Evans-Hulme and Roy Ceustermans debate… George: Yes, it was. The US was, like the rest of the world, soldiering on towards the end of a dark period of human history that had seen the single most costly conflict (in terms of life) in history, and they chose to adopt a stance that seemed to limit the amount of casualties in the war, by significantly shortening it with the use of atomic weapons. It was certainly a reasonable view for the USA to take, since they had suffered the loss of more than 418,000 lives, both military and civilian. To the top rank of the US military the 135,000 death toll was worth it to prevent the “many thousands of American troops [that] would be killed in invading Japan” – a view attributed to the president himself. This was a grave consequence taken seriously by the US. Ordering the deployment of the atomic bombs was an abhorrent act, but one they were certainly justified in doing. Roy: No, the US wasn’t justified. Even secretary of war Henry Lewis Stimson was not sure the bombs were needed to reduce the need of an invasion: “Japan had no allies; its navy was almost destroyed; its islands were under a naval blockade; and its cities were undergoing concentrated air attacks.” The United States still had many industrial resources to use against Japan, and thus it was essentially defeated. Rear Admiral Tocshitane Takata concurred that B-29s “were the greatest single factor in forcing Japan’s surrender”, while Prince Konoye already thought Japan was defeated on 14 February 1945 when he met emperor Hirohito. A combination of thoroughly bombing blockading cities that were economically dependent on foreign sources for food and raw materials, and the threat of Soviet entry in the war, would have been enough. The recommendations for the use of the bomb show that the military was more interested in its devastating effect than in preparing the invasion. Therefore the destruction of hospitals and schools etc was acceptable to them. George: The USA was more interested in a quick and easy end to the war than causing untold suffering. They had in their hands a weapon that was capable of bringing the war to a swift end, and so they used it. The atom bombs achieved their desired effects by causing maximum devastation. Just six days after the Nagasaki bombing, the Emperor’s Gyokuon-hōsō speech was broadcast to the nation, detailing the Japanese surrender. The devastation caused by the bombs sped up the Japanese surrender, which was the best solution for all parties. If the atomic bombs had not had the devastating effect they had, they would have been utterly pointless. They replaced thousands of US bombing missions that would have been required to achieve the same effect of the two bombs that, individually, had the explosive power of the payload of 2,000 B-29s. This freed up resources that could be utilised for the war effort elsewhere. Roy: After the bloody battles of Iwo Jima and Okinawa, the death toll on both sides was high, and the countries’ negative view of one other became almost unbridgeable, says J Samuel Walker in Prompt and Utter Destruction: Truman and The Use of Atomic Bombs Against Japan. Therefore, the US created unconditional terms of surrender, knowingly going against the Japanese ethic of honour and against the institute of the emperor, whom most Americans probably wanted dead. Consequently, the use of the atomic bomb became a way to avenge America’s fallen soldiers while also keeping the USSR in check in Europe. The Japanese civilian casualties did not matter in this strategy. Also, it did not prevent the Cold War, as the USSR was just a few years behind on a-bomb research. At the time, revenge, geopolitics and an expensive project that could not be allowed to simply rust away, meant the atomic bomb had to be hastily deployed “in the field” in order to see its power and aftermath – though little was known about radiation and its effects on humans. An atomic bomb survivor cries as he receives a treatment at temporary hospital at Shin Kozen Elementary School in August 1945 in Nagasaki, Japan. (Photo by Yasuo Tomishige/The Asahi Shimbun via Getty Images) George: Admittedly, the US did use the atom bomb to keep the USSR in line, and for that it served its purpose. It may not have stopped the Soviets developing their own nuclear device, but that’s not what it was intended for. It was used as a deterrent to keep the (sometimes uneasy) peace between the US and the USSR, and it achieved that. There are no cases of a direct, all-out war between the US and the Soviets that can be attributed to the potentially devastating effects of atomic weaponry. The atomic bombs certainly established US dominance immediately after the Second World War – the destructive power it possessed meant that it remained uncontested as the world’s greatest power until the Soviets developed their own weapon, four years after the deployment at Nagasaki. It is certainly true that Stalin and the Soviets tried to test US dominance, but even into the 1960s the US generally came out on top. Roy: The price to keep the USSR in check was steep: the use of a weapon of mass destruction that caused around 200,000 deaths (most of them civilians) and massive suffering through radiation. However, it did not stop the USSR from creating the same weapon within four years. It might be argued that, following the explosions, Japan virtually disappeared from the world stage while the USSR viewed the bombing as an incentive to acquire the same weaponry in order to retaliate in equal force if the atomic bomb was ever used again. Considering the tension between the two countries, a similar attack with tens of thousands of civilian casualties would have created a nuclear apocalypse. If the US had organised a demonstration, as they had briefly considered, the USSR would still have responded in the same manner, while Japan – which had made clear overtures for a (un)conditional surrender – could have been spared. Furthermore, by postponing the use of the bomb, scientists would have had time to understand the test results, meaning further anguish, like the Bikini Atoll [a huge US hydrogen bomb test in 1954 that had major consequences for the geology and natural environment, and on the health of those who were exposed to radiation] could have been avoided. George: The large civilian death toll that resulted from the bombings can be seen as a small price to pay by the United States in return for their assertion of dominance on the world stage. The USSR’s development of an atomic weapon had been underway since 1943, and so their quest for nuclear devices cannot be solely attributed to the events of Hiroshima and Nagasaki. It should also be considered that the Soviets’ rapid progress in creating an atom bomb was not exclusively down to their desire to compete with the United States, but from spies passing them US secrets. Postponing the use of the atom bomb would only have prolonged the war and potentially created an even worse fate for the people of Japan, with an estimated five to 10 million Japanese fatalities – a number higher than some estimates for the entire Soviet military in the Second World War. Ultimately, the atomic bombs did what they were designed to do. They created such a high level of devastation that the Japanese felt they had no option but to surrender unconditionally to the United States, hence resulting in US victory and the end of the Second World War. Roy: Of course civilian casualties of another nation would have been acceptable to the USA. Japan had made clear overtures to peace, but cultural differences made this nearly impossible (the shame of unconditional surrender goes against their code of honour). The determination to use an expensive bomb instead of letting it rust away; the desire to find out how devastating it was and the opportunity to use the bomb as a strong showcase of US supremacy, made Japan the ideal target. Obviously, the USSR would eventually succeed in creating the a-bomb. Therefore, making Hiroshima & Nagasaki the example of the tremendous power of the bombs would make it clear to the USSR that they too needed such weapons to defend themselves. Moreover, other countries claimed the right of nuclear weapons to defend their citizens. Consequently, the tragic bombings became the example of an arm’s race instead of peace. Furthermore, since Japan was already on the brink of collapse the bombing was unnecessary, and peace talks would have taken place within a decent time frame (even after the cancelled Hawaii summit). The millions of deaths calculated by Operation Downfall [the codename for the Allied plan for the invasion of Japan near the end of the Second World War, which was abandoned when Japan surrendered following the atomic bombings of Hiroshima and Nagasaki] actually show that only desperation and honour stood between Japan and unconditional surrender. George, from Wolverhampton, is currently studying for his A-levels, which include modern history, at Kenilworth Sixth Form in Warwickshire. He volunteers for the National Trust as a room guide at the Baddesley Clinton Estate. He has a passion for military and political history, and enjoys visiting historical sites across the UK. Roy, 29, from Belgium, has a master’s degree in the history of the Catholic Church; an advanced master’s degree on the historical expansion, exchange and globalisation of the world, and a master’s degree in management. He has worked as a complaint manager for the city of Leuven, and is now an inspector for the Cadastre [tax office]. Last year during his honeymoon Roy visited the Peace Memorial Park in Hiroshima. Photos taken immediately after the 1945 bombing have left a lasting impression on him. This article was first published by History Extra in July 2014 'The Atomic Tree' explores the bonsai that survived a nuclear blast There are an estimated 10 million VR headsets in the wild. Compare that with the number of people who have Netflix (139 million), or Hulu (25 million), or who watch YouTube (1.6 billion), and it's clear that the VR market is inherently limited. However, what the medium lacks in audience it more than makes up for in raw immersive power. The Atomic Tree is an ideal example of a story built for VR, audience size be damned. It follows the history of a 400-year-old Japanese White Pine bonsai that survived the United States' nuclear attack on Hiroshima in 1945, even though it was just two miles from the blast's epicenter. "We're interested in the media of VR not because of the reach it could give but because of the experience it could offer," The Atomic Tree director Adam Loften told Engadget at SXSW. "A story like this, I think, would be a lot drier and historical and one-dimensional if we told it using the same techniques you're seeing here -- you've got a narrator, you've got documentary footage, you've got recreation scenes, and some animations -- those are kind of very standard documentary tools. But in VR you can make them transportive, and meditative, and truly immersive in a way you can't [traditionally]." The Atomic Tree is a 10-minute dive into the life of a single pine tree that blossomed in a secluded Buddhist temple high in the cedar forests of Japan. The bonsai was then lovingly cared for, for five generations, by the Yamaki family. A high wall in the family's home protected the tree from the atomic blast on August 6, 1945, which obliterated the surrounding landscape, poisoning it with radiation and filling the air with death. The tree survived, and its story, nuclear blast and all, was stored in its rings. Eventually, the tree was given to the US as a bicentennial gift from Japan in 1976, though its trial of atomic survival didn't surface until 2004, when Yamaki family members visited and mentioned its history. "It's a very quiet practice," said David Haskell, the Pulitzer-nominated author behind The Forest Unseen and The Songs of the Trees, the latter of which served as the source material for The Atomic Tree. "You are working with this tree, and particularly this particular tree, over 400 years. Quiet, contemplative work, listening to the tree, deciding when to be still, when to clip a little needle -- it comes down to how each little needle is carefully sculpted and so forth. In not seeking out a lot of publicity, I think some of the spirit of bonsai comes out through that." The Atomic Tree, which launches on Within on March 22nd, approaches VR with an incredibly effective eye. It begins with the tree as it stands today, and then reverts to recreations of its initial years in a serene, mossy monastery, before showing it in the Yamaki home. Between these scenes, viewers are thrust directly into the tree, soaring through its rings in Coraline-esque, celestial tunnels that drive home the beauty and importance of the information stored within the spiraling wood. At the Yamaki house, family members trim the tree, the narrator's deep voice ringing over the scene, when suddenly, the screen is consumed by white. The bomb has fallen.