Alex Lopatka

Ganymede, the largest moon in the solar system, was imaged by NASA’s Galileo spacecraft in the 1990s. Galileo’s magnetic field measurements revealed that the moon has an active dynamo. View larger Of the hundreds of moons in the solar system, Jupiter’s satellite Ganymede is the only one that’s known to have an active dynamo. As liquid metal flows in the moon’s core, circulating electric currents form and sustain a magnetic field.

A vacuum isn’t empty space. According to quantum electrodynamics, virtual particles and antiparticles flit in and out of existence across exceedingly small spaces and in short periods of time. One way to study quantum vacuum fluctuations is to use an extremely strong electromagnetic field. For intensities greater than 1029 W/cm2, known as the Schwinger limit, the field is able to spontaneously create virtual electron–positron pairs from the quantum vacuum.

In this image of Earth, yellow indicates areas that have experienced short-term events that primarily increased the brightness of nighttime artificial light from 2014 to 2022. Purple denotes areas where such events led to a sustained brightness decrease, and white means that some events in the area brightened the night sky across the time period and others dimmed it. View larger Satellite-based measurements have shown that the night sky above much of Earth’s population has brightened over time.