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<jats:title>Testing physics using the hydrogen atom</jats:title> <jats:p> Discrepancy between the proton radius determined from hydrogen and muonic hydrogen spectroscopy data, the so-called “proton radius puzzle,” has been a focus of the physics community for more than a decade now. Using two-photon ultraviolet frequency comb spectroscopy below 1 kilohertz, Grinin <jats:italic>et al.</jats:italic> report a high-precision measurement of the 1S-3S transition frequency in atomic hydrogen (see the Perspective by Ubachs). Combining this measurement with the data for the 1S-2S transition, the authors obtained the Rydberg constant with improved accuracy and an independent value for the proton charge radius that favors the data from muonic hydrogen. However, the present frequency value differs from the one obtained previously using a different spectroscopic technique, leaving the puzzle still unresolved. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6520" page="1061" related-article-type="in-this-issue" vol="370" xlink:href="10.1126/science.abc7776">1061</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6520" page="1033" related-article-type="in-this-issue" vol="370" xlink:href="10.1126/science.abf0589">1033</jats:related-article> </jats:p>

<jats:title>Limits to the growing season</jats:title> <jats:p> The length of the growing season in temperate forests has been increasing under recent climate change because of earlier leaf emergence and later leaf senescence. However, Zani <jats:italic>et al.</jats:italic> show that this trend might be reversed as increasing photosynthetic productivity begins to drive earlier autumn leaf senescence (see the Perspective by Rollinson). Using a combination of experimental, observational, and modeling studies based on European forest trees, the researchers conclude that leaf senescence will advance by 3 to 6 days by the end of the 21st century rather than lengthening by 1 to 3 weeks as current phenological models have predicted. In turn, this predicted phenological pattern will limit the capacity of temperate forests to mitigate climate change through carbon uptake. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6520" page="1066" related-article-type="in-this-issue" vol="370" xlink:href="10.1126/science.abd8911">1066</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6520" page="1030" related-article-type="in-this-issue" vol="370" xlink:href="10.1126/science.abf4481">1030</jats:related-article> </jats:p>

<jats:title>Roaming dynamics in real time</jats:title> <jats:p> Roaming is distinct from conventional reaction channels because of the unusual geometries that chemical systems use to bypass the minimum energy pathway. It is a relatively new phenomenon that is usually determined in experiments through spectroscopic characterization of the roaming products. Using a combination of time-resolved Coulomb explosion imaging and quasiclassical trajectory analysis, Endo <jats:italic>et al.</jats:italic> report real-time observation of individual fragments of the prototypical reaction of deuterated formaldehyde (D <jats:sub>2</jats:sub> CO) dissociation as they roam on ultrafast time scales. They show that roaming not only occurs several orders of magnitude earlier than previously expected but also that it can terminate in a radical (D + DCO) rather than the well-known molecular (D <jats:sub>2</jats:sub> + CO) product channel. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article issue="6520" page="1072" related-article-type="in-this-issue" vol="370">1072</jats:related-article> </jats:p>

<jats:title>When all routes are exceptional</jats:title> <jats:p> The exploration of non-Hermitian physics and parity-time symmetry have provided a route to develop a wealth of exotic physical effects. In such dissipative systems, the balance of gain and loss of the system lead to what are called exceptional points, or “sweet spots,” which relate to optimal device operation or material properties. The ability to tune the gain and loss over a range of system properties leads to exceptional arcs. Tang <jats:italic>et al.</jats:italic> show that systems can be designed in which the tuning of multiple parameters leads to a crossroads, or nexus, of exceptional arcs. Illustrating the effect in an acoustic system, the same properties should be attainable over various types of dissipative systems and thus provide a versatile route to fine-tune optimal performance of materials and devices. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article issue="6520" page="1077" related-article-type="in-this-issue" vol="370">1077</jats:related-article> </jats:p>

<jats:title>Driving the pores away</jats:title> <jats:p> The formation of “keyholes” (vapor-filled depressions) during additive manufacturing leads to porosity, which degrades alloy performance, especially fracture properties, and remains a big challenge for the 3D printing of metals. Zhao <jats:italic>et al.</jats:italic> used high-speed x-ray imaging to take a detailed look at how keyhole formation connects to porosity in a titanium alloy. They found that instability at the keyhole tip drives pores away to get trapped in the solidification front. Understanding this process and the operating parameters under which it occurs provides a roadmap for avoiding porosity and building high-quality metal parts. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article issue="6520" page="1080" related-article-type="in-this-issue" vol="370">1080</jats:related-article> </jats:p>

<jats:title>Survival of the most variable</jats:title> <jats:p> As more species become highly threatened because of human activity, there has been an increasing push to understand how best to reintroduce or translocate individuals from wild or captive populations. Suggestions have varied from choosing individuals from the most environmentally similar regions to choosing those that might have the best ability to adapt to new environments. Scott <jats:italic>et al.</jats:italic> used long-term data collected during translocations of Mojave Desert tortoises, including animals formerly kept as pets, to test these questions. Although the overall rates of survival for all tortoises at the site (both reintroduced and native) were extremely low, translocated individuals with the highest heterozygosity survived at much higher rates than those determined to be similar to the target population. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article issue="6520" page="1086" related-article-type="in-this-issue" vol="370">1086</jats:related-article> </jats:p>

<jats:title>Structure of a vaccine candidate</jats:title> <jats:p> Much effort is being targeted at developing vaccines that will provide protection against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A trimeric spike protein that decorates the virus is a primary target of the host immune system and the focus of vaccine development. Bangaru <jats:italic>et al.</jats:italic> present the structure of a leading vaccine candidate: a full-length spike protein with some modifications aimed at enhancing stability that is formulated in polysorbate 80 detergent. The study confirms that the full-length immunogen is in a stable prefusion conformation and provides a basis for understanding immune responses to the vaccine. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6520" page="1089" related-article-type="in-this-issue" vol="370" xlink:href="10.1126/science.abe1502">1089</jats:related-article> </jats:p>