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The standard high school textbooks mention that light takes 499 seconds on an average to travel from the Sun to the Earth. Here, I have three questions:

  1. In which frame is this duration of 499 seconds measured? Is it from Earth's frame of reference or from photon's frame of reference? According to the special relativity, as measured from Earth, time in photon's frame is frozen. So, the 499 seconds must be in photon's frame. Correct?

  2. Further, the statement that "photon does not experience time" is valid only when the photon is viewed from a stationary frame of reference, example, from earth. Correct?

  3. If the one-way speed of light has not been measured yet, then how can one assert that light takes 499 seconds to travel from the Sun to the earth?

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    $\begingroup$ A photon does not experience time. It has no rest frame. $\endgroup$
    – Jon Custer
    Sep 20 at 15:00
  • $\begingroup$ @JonCuster Could you please answer the three questions I have asked above. $\endgroup$
    – Vikash
    Sep 20 at 15:02
  • $\begingroup$ @NiharKarve Not really. The response there is quite ambiguous. $\endgroup$
    – Vikash
    Sep 20 at 15:25
  • $\begingroup$ What's ambiguous? The photon does not have a rest frame. Therefore, what other rest frames could we be talking about. Maybe the frame of the Earth (on the assumption that it isn't accelerating). If not that, how about the frame of the Sun? Calculate how much difference it makes which of these two frames you choose? Is this error more or less than the error you made when you estimated the time we're measuring as 499 s? $\endgroup$
    – The Photon
    Sep 20 at 16:07
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    $\begingroup$ On Earth or in some frame that isn't moving very fast relative to the Earth. $\endgroup$
    – The Photon
    Sep 20 at 16:22
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According to the special relativity, as measured from Earth, time in photon's frame is frozen. So, the 499 seconds must be in photon's frame.

If we measured in a frame where time were frozen, then the measured elapsed time would be 0 s, not 499 s.

So we cannot be talking about the photon's frame (which, as has been pointed out in comments and in answers to previous questions, doesn't exist).

We can't even be measuring in some frame that follows the path of the photon, taking the limit as its velocity relative to Earth approaches c. Because, again, in that case we'd measure an elapsed time close to 0, and not 499 s.

Further, the statement that "photon does not experience time" is valid only when the photon is viewed from a stationary frame of reference,

Every frame of reference is stationary when measured in its own frame. There is no way to distinguish a "stationary" frame from a "non-stationary" frame (except with reference to some other frame).

If the one-way speed of light has not been measured yet, then how can one assert that light takes 499 seconds to travel from the Sun to the earth?

We don't really measure the time it takes for light to travel from the Sun to the Earth, since we can't send an experimenter to the Sun to shine a laser at us. We measure the distance from the Sun to the Earth, and divide by the speed of light.

We are assuming the speed of light is the same in all directions, but in any case sometimes the Earth is on one side of the sun and sometimes it is on the other, so even if the speed of light weren't the same in different directions, ultimately the time for the light to travel from the Sun to Earth would average out to pretty close to the same 499 s.

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  • $\begingroup$ I think it is the observer in photon's frame measures time elapsed is 499 seconds. $\endgroup$
    – Vikash
    Sep 20 at 16:27
  • $\begingroup$ @Vikash, please calculate, what is the distance from the Sun to the Earth in that frame? $\endgroup$
    – The Photon
    Sep 20 at 16:31
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    $\begingroup$ @Vikash As others have stated, a photon does not have a frame. That figure of 499 seconds is relative to an Earth observer. An appropriate time scale for that measurement is Terrestrial Time. However, it actually takes less time, on average, for light to reach the Earth from the Sun. The semimajor axis of the Earth's orbit has a length of 499.005292 light-seconds, i.e., that's the mean distance between the Earth-Sun barycentre and the Earth. (That barycentre is quite close to the centre of the Sun). $\endgroup$
    – PM 2Ring
    Sep 20 at 16:52
  • $\begingroup$ @PM2Ring Tell me if you agree with my two statements. First, time measured on earth = time measured while moving with photon = 499 seconds. Second, time observed from earth's frame in photon's frame = time observed from photon's frame in earth's frame = 0 $\endgroup$
    – Vikash
    Sep 20 at 17:02
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    $\begingroup$ @Vikash You can't move with a photon. And as we keep trying to tell you, a photon doesn't have a frame. It doesn't have a pet cat, or a favourite cricket team, either. ;) $\endgroup$
    – PM 2Ring
    Sep 20 at 17:22

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