Neil deGrasse Tyson explains how gravity affects the flow of time

[n2doc]

By David Ferguson
Thursday, September 19, 2013 10:10 EDT

On Wednesday’s edition of his popular “Star Talk” radio show, astrophysicist Neil deGrasse Tyson explained how gravity affects the flow of time and how time passes on different planets with different gravitational fields.

A listener wrote in to “Star Talk” to ask if it’s the same time everywhere in the universe. Space flights and the International Space Station are all on Houston time because “that’s who they talk to,” but, the reader asked, does time move at the same rate everywhere?

“Different parts of the universe are moving at different rates,” deGrasse Tyson said, “and time has some relative aspects to it. For example, the GPS satellites, the clocks on them tick at a different rate than clocks on Earth’s surface because when you move far away from the source of gravity, your time speeds up.”

“So,” he continued, “the clocks on the GPS satellites are not ticking at the rate of the clocks that they are informing down here on Earth. The military puts a correction into the clock time of a GPS satellite so that it matches the time we need it to have here on the Earth’s surface.

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http://www.rawstory.com/rs/2013/09/19/neil-degrasse-tyson-explains-how-gravity-affects-the-flow-of-time/

[Surya Gayatri]

1. Another gem from n2doc. Thanks for posting.

Neil deGrasse Tyson is one cool brainiac.

[jakeXT]

2. I think there are two main effects, time dilation and gravitational frequency shift

According to the theory of relativity, due to their constant movement and height relative to the Earth-centered, non-rotating approximately inertial reference frame, the clocks on the satellites are affected by their speed. Special relativity predicts that the frequency of the atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by.. about 7 ?s/day

The effect of gravitational frequency shift on the GPS due to general relativity is that a clock closer to a massive object will be slower than a clock farther away. Applied to the GPS, the receivers are much closer to Earth than the satellites, causing the GPS clocks to be faster by a factor of 5×10^(?10), or about 45.9 ?s/day. This gravitational frequency shift is noticeable.

When combining the time dilation and gravitational frequency shift, the discrepancy is about 38 microseconds per day, a difference of 4.465 parts in 1010.[11] Without correction, errors in the initial pseudorange of roughly 10 km/day would accumulate. This initial pseudorange error is corrected in the process of solving the navigation equations. In addition the elliptical, rather than perfectly circular, satellite orbits cause the time dilation and gravitational frequency shift effects to vary with time. This eccentricity effect causes the clock rate difference between a GPS satellite and a receiver to increase or decrease depending on the altitude of the satellite.

http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Calculation_of_time_dilation

[Fantastic Anarchist]

4. I totally forgot about velocity (speed) ...

So, according to your excerpt, they "sort of" cancel each other out, right? Since, the GPS satellite is faster due to relative distance from earth's gravitational field, but also relatively slower due to its speed, wouldn't that minimize the difference between earth stationed clocks?

[jakeXT]

5. Yes it does, instead of ~45 it's ~45-7=38 microseconds

The gravitation is still the main part, but I wanted to mention the other effect.

[Fantastic Anarchist]

6. I very much appreciate your answer.

So, you are saying, though, that the gravitational field does have more of an impact rather than the velocity of the satellite?

[Fantastic Anarchist]

3. Michio Kaku and him are national treasures.

Too bad they can't run for office.

[Mojorabbit]

7. Thanks! nt