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# Einstein's Concept of Gravity

## Updates Newton's Law of Gravity, showing: gravity is a result of the warping of space-time.

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GPS and Einstein

### GPS and Einstein

Credit: HawaiianMama
Source: http://en.wikipedia.org/wiki/File:GPS_on_smartphone_cycling.JPG
License: CC BY-NC 3.0

This device is a GPS, or Global Positioning System, receiver. It uses information from satellites to find the user’s location. Have you ever used GPS? If so, you’ve made use of Einstein’s concept of gravity!

#### Why It Matters

• GPS can tell you your location on Earth, including your altitude, in a matter of seconds. It seems like magic, but it’s really just based on geometry.
• Your GPS receiver calculates your exact distance from at least four GPS satellites. Then it uses the distances to pinpoint your location.
Source: http://www.flickr.com/photos/peterwright/4428342932/
License: CC BY-NC 3.0

Interestingly, the technology for GPS was first developed to track the Russian satellite Sputnik in space [Figure2]

• GPS depends on extremely accurate measurements of time. That’s where Einstein comes in. Einstein argued that gravity is the result of the curvature of space and time around massive objects. This means that time isn’t absolute. It’s relative. It goes faster in some situations than others.
• Watch this video to see how time changes with motion and gravity and how it affects GPS:

#### Can You Apply It?

At the links below, review the connection between Einstein’s ideas and GPS. Then answer the questions that follow.

1. What are some specific uses of GPS? Do you or your family use GPS in any of these ways?
2. Identify the four dimensions that GPS measures.
3. Explain in detail how a GPS receiver calculates its distance from a satellite.
4. According to Einstein, why is time relative?
5. What would happen if the clocks on a GPS satellite were not adjusted for the relativity of time? How would distance calculations be affected?

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