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A photon watching us 
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#1
Apr1510, 01:22 AM

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How fast does time elapse for us from the point of view of a photon watching us?
Thanks, Jake 


#2
Apr1510, 02:21 AM

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See http://www.physicsforums.com/showthread.php?p=2650120.
(Also, your question kind of contradicts itself, since "for us" sounds like reference to our point of view. I'm guessing you meant to ask how fast our clocks are from a photon's point of view). 


#3
Apr1510, 02:34 AM

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Thanks, Jake 


#4
Apr1510, 03:30 AM

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A photon watching us
From the point of view of a particle moving very close to the speed of light, our clocks are moving extremely slow.
Time dilation, not time contraction. ;) 


#5
Apr1510, 03:41 AM

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#6
Apr1510, 03:46 AM

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I know, but the implication that to the particle our clocks moving faster would imply time contraction.



#7
Apr1510, 04:00 AM

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#8
Apr1510, 04:48 AM

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If person A were moving very fast relative to person B then, yes, person B would see Person A's clocks moving more slowly than his and see person A aging more slowly.
But it is also true that, from A's reference system, B is moving very fast relative to A. Person A would see Person B's clocks moving more slowly and see person B aging more slowly. But unless A and B can "get together" in the same reference frame (same speed) there is no paradox. And to get into the same reference frame, one must accelerate. That breaks the symmetry. (Well, of course, they could accelerate symmetrically then, when they got into the same reference frame, they would find their clocks and their aging to be the same.) 


#9
Apr1510, 05:12 AM

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So the astronaut thing you hear so many times is a myth?



#10
Apr1510, 05:26 AM

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This is a spacetime diagram I made for another one of these threads a couple of years ago, which shows the Earth twin's point of view, and explains how the other twin would describe things some of the events on his world line (when we use comoving inertial frames to define the "point of view"). If we don't care about "points of view" and only want to know how SR predicts that the astronaut twin will be younger, the answer is that it follows immediately from an axiom of the theory: A clock measures the proper time of the curve in spacetime that represents its motion. 


#11
Apr1510, 06:03 AM

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Thanks again, Jake 


#12
Apr1510, 09:04 AM

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#13
Apr1510, 09:26 AM

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I am confused. If someone can go into space and return younger than their twin who stayed on earth, then from the astronaut's point of view, events on earth must have progressed faster than from the twin on earth point of view. Right? How else could the astronaut return and find their twin older unless the passage of time was different between them? And what does Fredrik mean by "compensates for light travel time"? Does he mean the astronaut compensates for the time it took the light from earth to reach him at his current, far away location? Since the astronaut is younger when he returns, could you say that he "saw" (with the compensation) the earth's events go in fast forward when he was in space?
Thanks for bearing with me guys, I really want to understand this, Jake 


#14
Apr1510, 10:13 AM

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With compensation: He sees earth eventsin slow motion on the outbound trip, and slow motion on the inbound trip. During turnaround, he has to adjust his compensation procedure, such that directly after turnaround (with the new procedure) the earth is quite a bit older than directly before turnaround. Note: that's a calculated, adjusted "time warp", not something observed. Nothing jumps into the future, the astronaut simply uses a different coordinate system after turnaround. 


#15
Apr1510, 11:09 AM

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Thanks! Jake 


#16
Apr1510, 12:37 PM

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A different (and less exact) way is to imagine lines parallel to the lower dotted line in the diagram, that intersect the t axis at the beginning of each year. Those lines represent light pulses sent from Earth once a year. It should be obvious from the diagram that the spaceship will (literally) see a lot more of them on the return trip than on the outbound trip. If you count the number of pulses that reach the spaceship during the two parts of the trip, you can determine the "speed up" and "slow down" factors as "pulses received"/12. 


#17
Apr1510, 03:26 PM

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#18
Apr1510, 03:54 PM

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Thank you



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