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 Aug 23rd 2017, 09:51 PM #1 Junior Member   Join Date: Jul 2017 Location: China Posts: 3 one question According to the principle of the speed of light remains the same, imagine a spacecraft fly away from the earth at speed of 0.9 c [c is the speed of light] , after a period of time, a beam of light is emitted from the earth to the spacecraft, after 1 hour , another beam of light is emit from the earth to the spacecraft. The question is: how long is the time intervals of two receiving beams of light at spacecraft ?(the time is in spacecraft)
 Aug 23rd 2017, 10:47 PM #2 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 552 Time runs slower for distant inertial (constant velocity) objects. You can use the Lorentz factor to determine how much it changes. https://en.wikipedia.org/wiki/Lorentz_factor
Aug 24th 2017, 03:46 AM   #3
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 Originally Posted by linxf According to the principle of the speed of light remains the same, imagine a spacecraft fly away from the earth at speed of 0.9 c [c is the speed of light] , after a period of time, a beam of light is emitted from the earth to the spacecraft, after 1 hour , another beam of light is emit from the earth to the spacecraft. The question is: how long is the time intervals of two receiving beams of light at spacecraft ?(the time is in spacecraft)
Let t = time interval on spacecraft and t' = time interval on Earth. Then

t' = t/sqrt[1 - (v/c)^2]

or

t = t'*sqrt[1 - (v/c)^2]

v/c = 0.9

t = 0.43 hours = 26 minutes

 Originally Posted by kiwiheretic Time runs slower for distant inertial (constant velocity) objects.
The object doesn't need to be distant or have a constant velocity.

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