### The equivalence principle in the archive

This exercise is a part of Educator Guide: Galileo Experiment Re-created in Space / View Guide

1. “Galileo experiment re-created in space” considered the equivalence principle only for cylinders that behaved like classical objects, which aren’t subject to the rules of quantum mechanics. Can you find an article testing the equivalence principle for particles in quantum particles? Summarize the article below.

Possible student response: The Science News article “Key Einstein principle survives quantum test,” published 5/27/2017, describes how the equivalence principle was tested with atoms in quantum states. Scientists used lasers to give clouds of rubidium atoms an upward kick and observed how gravity pulled the atoms down. They compared the acceleration rates of atom clouds in superposition — a kind of limbo in which an atom does not have a definite energy but occupies a combination of two energy levels — with the rates of atom clouds in a normal energy state. Gravity pulled on the atom clouds in a superposition at the same rate as it did the atom clouds in a normal energy state, at least to the level of sensitivity the scientists were able to probe — within 5 parts in 100 million.

2. Adjust the search filters to include articles from the last 30 years. Find and explain the content in an article about measuring the equivalence principle using the moon?

Possible student response: The Science News article “Stronger support for equivalence principle,” published 9/22/1990, discusses how scientists measured the gravitational acceleration of the moon in response to the Earth and sun. Scientists found that the moon’s inertial mass and gravitational mass were equal within 14 parts in 1012. Within the limits of precision of the experiment, there was no sign of any violations of the equivalence principle or of a fifth fundamental force other than gravity, electromagnetism and the strong and weak nuclear forces.

3. Find and summarize an article about measuring the equivalence principle using stars.

Possible student response: The Science News article “Unusual three-star system promises new test of gravity,” published 2/22/2014, describes how astronomers found a pulsar and two white dwarfs that orbit each other very closely. All three objects are dense and have strong gravitational fields, and as the pulsar rotates 366 times per second, its electromagnetic waves sweep through space like a lighthouse beam. By closely monitoring the timing of the pulsar’s beam and making other measurements of the three-star system, scientists hope to test the equivalence principle. Alternative theories of gravity other than general relativity predict that there could be small deviations from the equivalence principle.