"Artist’s impression of strontium emerging from a neutron star merger" by ESO/L. Calçada/M. Kornmesser / CC BY 4.0

Einstein’s core idea about gravity just passed an extreme, whirling test in deep space

Jun 15, 2020

By Rafi Letzter

Once again, physicists have confirmed one of Albert Einstein’s core ideas about gravity — this time with the help of a neutron star flashing across space.

The new work makes an old idea even more certain: that heavy and light objects fall at the same rate. Einstein wasn’t the first person to realize this; there are contested accounts of Galileo Galilei demonstrating the principle by dropping weights off the Tower of Pisa in the 16th century. And suggestions of the idea appear in the work of the 12th-century philosopher Abu’l-Barakāt al-Baghdādī. This concept eventually made its way into Isaac Newton‘s model of physics, and then Einstein’s theory of general relativity as the gravitational “strong equivalence principle” (SEP). This new experiment demonstrates the truth of the SEP, using a falling neutron star, with more precision than ever.

The SEP has appeared to be true for a long time. You might have seen this video of Apollo astronauts dropping a feather and a hammer in the vacuum of the moon, showing that they fall at the same rate in lunar gravity.

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