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Name: Bryan
Status: other
Grade: N/A
Location: AZ
Country: N/A
Date: 7/28/2005


Question:
Dear Physics Professors:

I am a graduate accounting student and astronomy is a big hobby of mine, so I like to occasionally study physics. I have been reading "The Fabric of the Cosmos" by Greene and I rather enjoyed this book's explanation of Einstein's time dilation theory. The book indicates that "stationary" objects (such as a car) have 100% motion through time, and 0% motion through space; therefore time progresses at "normal" speed for the stationary car. Furthermore, when the car is in motion speeding down the highway at 60 mph, some of its motion through time is diverted to motion through space. Therefore time for the car and it's passengers "slows down" relative to the observer on the sidewalk. My question is as follows: The earth is moving along it's orbit AND rotating at extremely high speeds, so no object on earth is really "stationary". How does this fact figure into all of this? Does this impact the entire planet's perception of time? Is the passage of time different for us than it would be for a colony on Mars, since Mars is moving at a different relative speed? I hope this question is clear.


Replies:
Bryan,

The "key" to relativity is that there is no such thing as one real time. As measured by someone riding in the moving car, the car has 100% motion through time. It is the ground that "slows down". For the person on the sidewalk, the ground is going through normal time and the car's clock slows down. Both observers are correct. A man in the car sees a clock on the sidewalk as moving too slowly. A man on the sidewalk sees a clock in the car as moving too slowly. Of course, this time dilation is much too small to even try to measure when the speed is only 60 mph.

What you measure is all that there is. The universe does not have one absolute truth that everyone measures differently. What is actually true about space and time does not just look different for different observers. It IS different.

For those of us on the Earth, things on the Earth happen in normal time. For someone "riding a comet", things on the Earth are dilated in time.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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