100 Years Later, Einstein’s Theory Tested by Satellites

SCIENCE

Teachers, scroll down for a quick list of key resources in our Teachers’ Toolkit, including today’s 5-Question Quick Quiz on general relativity.

Discussion Ideas

According to the Guardian, the newest tests of general relativity started out as “a total fluke,” because the satellites were launched for an entirely different purpose. What were Galileo 5 and Galileo 6 supposed to do?
- Galileo is the global satellite navigation system the European Space Agency (ESA) is working to establish. Galileo is being developed to “break Europe’s reliance on the American GPS system.” (Most countries rely on GPS. Russia is an exception; it uses its own global satellite navigation system called GLONASS.)

The Galileo satellites will be testing general relativity, a theory that was published 100 years ago by rock-star physicist Albert Einstein. What is general relativity? Read this anniversary article from NASA for some help.
- General relativity is nothing less than the “current description of gravitation in modern physics”—that is, everything you ever wanted to know about gravity but didn’t know to ask. Key takeaways:
  - Space and time are part of a single continuum, conveniently called spacetime.
  - Spacetime is a changeable, “dynamic entity that can be distorted by any matter that is contained in it.” Massive matter (that is, anything with mass) can create a gravitational field around itself, so spacetime is distorted by gravitational fields.

What parts of general relativity will the Galileo satellites be testing?
- Gravitational time dilation. According to general relativity, time passes more slowly in a gravitational field, because spacetime is distorted. Earth has a gravitational field, so time moves more slowly closer to Earth’s center of gravity. In other words, time really does go faster at the top of a skyscraper than the bottom of a mine, and “clocks on Earth ‘tick’ more slowly than clocks in space.”

Why is gravitational time dilation an important concept for navigational satellites? Let Neil deGrasse Tyson tell you about it here, or let NASA tell you about it here.
- Satellite clocks speed up by about 35 microseconds every day. If gravitational time dilation was not taken into account, this would cause navigation errors of around 10 kilometers (6.2 miles) per day.

Is E=MC² part of general relativity or gravitational time dilation?
- No. Einstein’s field equations, a set of 10 equations which mathematically describe gravitation as a result of spacetime being curved by matter and energy, are the most famous equations used to explain general relativity.
- E=MC², the mass-energy equivalence, is the most famous equation used to explain the special theory of relativity. Einstein formulated the special theory years before the general theory, and first introduced the idea of spacetime there. (Let Einstein tell you about E=MC² here.) The special theory is special because it only applies where gravity has little impact on the curvature of spacetime. According to the good folks at the Max Planck Institute for Gravitational Physics, “the prime example of a situation governed by special relativity is a region far, far away in the depths of space, far away from all stars and planets (and their gravitational influence).”