Wikipedia:Peer review/Speed of light/archive1

Speed of light

This peer review discussion has been closed.
I've listed this article for peer review because, while I think it is close enough to FA standards, I think it would benefit from "fresh" readers, not yet familiar to it, giving it a read.

Thanks, ___A. di M. 15:31, 22 October 2009 (UTC)

Here are my comments:

1. Some of the images are lacking ALT text.
2. In the lead, it seems to go into too much detail in paragraph 3.
3. The lead does not cover sections such as faster than light and practical effects of finite speed.
4. Lead suggests that gravity is speed of light, but not known as fact, but later it is presented as fact: Disturbances of this curvature, including gravitational waves, propagate at the speed of light. Unless this is not exactly the same, the two need to be harmonized.
5. Can you link anti-parallel velocities to give it a definition for those not knowing that term?
6. It says Classically, when an electromagnetic wave meets..., does this mean that this is just an approximation and not what is really happening? Please clarify.
7. Might want to mention that parsecs are generally used in the literature and light-years in popular media.
8. In Terrell rotation the but they will be actually seen as is confusing as the word but seems out of place.
9. The following two sentences seem contradictory: Euclid and Ptolemy advanced the emission theory of vision, where light is emitted from the eye, thus enabling sight. Using that theory, Heron of Alexandria advanced the argument that the speed of light must be infinite, since distant objects such as stars appear immediately upon opening the eyes.

Overall looks very good. WilliamKF (talk) 21:04, 24 October 2009 (UTC)

Here's my take on points 3, 5, and 8. Is this better?

Yes, looks good. WilliamKF (talk) 14:27, 27 October 2009 (UTC)

As for point 6, "what is really happening" depends on one's interpretation of quantum mechanics; anyway, I don't think you get "wrong" answers with classical electrodynamics as far as the phenomenon of diffraction is concerned. Maybe just removing the word "classically" would be an option.

As for point 9, it means that if the light were emitted by the eye and it traveled at a finite speed, when you open your eyes you'd have to wait for the light to reach the stars before being able to see them. So, assuming that the light is emitted by the eye, the fact that you can see stars immediately means that it must travel at infinite speed. How did you misunderstand it? ___A. di M. 10:58, 25 October 2009 (UTC)

Upon rereading, it is clear now. Not sure why I didn't get it before. Perhaps I misread infinite as finite? WilliamKF (talk) 03:31, 27 October 2009 (UTC)

Can we have something short and easy to understand in the lead, or near the beginning of the article, about i) definition of speed of light (and explanation that this is in a vacuum), ii) about 'real' speed of light (linked to refraction maybe) in different mediums, iii) very short mention of trouble with measuring SOL using units that are defined using SOL. NotAnIP83:149:66:11 (talk) 16:46, 29 October 2009 (UTC)

RJHall commants: Overall it looks to be in good shape. I just had a few minor observations:

• The first paragraph of the lead should mention the importance of c to physics, in addition to astronomy and space travel.
• Some lay readers may have a little difficulty with the statement: "equivalent to of the order of 10⁻⁵⁰ g." It may be cleaner to just say "equivalent to roughly 10⁻⁵⁰ g".
• The "Galaxies moving faster than light" section should clarify that it is possible for two locations to recede from each other at a velocity greater than the speed of light because of the expansion of space, and explain why this is not a violation. (I've been asked this question before in person, so it seems pertinent.) You might also mention cosmic inflation as an extreme example.
• "One of the group's discoveries, announced by Cassini in 1675, was that the periods of the moons appeared to be shorter when the Earth was approaching Jupiter than when it was receding from it." Is this statement accurate? It suggests time dilation, rather than a timing difference due to the speed of light. My understanding was that Cassini produced the ephemeris of Jovian moons, and discovered periodic delays in their occultations compared to his calculations.

As I understand it this is just the (nonrelatvistic) Doppler effect for very low frequencies (the periods of the moons) caused by the motion of the reciever, the Earth. (TimothyRias (talk) 10:38, 4 November 2009 (UTC))

I think it should be modified to make clear the role of the finite speed of light's travel time, rather than a frequency shift. The latter only depends on the relative velocity of the two planets; not their separation.—RJH (talk)

But the effect only depends on the relative velocity. The difference in observed period is simply the time it takes light to travel between the position of the earth when it observes the first transition and the position when it observes the second transition. (In the frame that Jupiter is stationary.) The effect thus measures the SoL in terms of the relative speed between earth and Jupiter. The fact that it takes a while for the light from jupiter to reach Earth does not enter the effect.

Does this solve your proble, or am I misunderstanding it?(TimothyRias (talk) 13:28, 5 November 2009 (UTC))

Yeah, that's it, more or less. IIUC: If the Jovian system is 54 light-minutes away, we see it as it was 54 minutes ago, and if it's 66 light-minutes away, we see it as it was 66 minutes ago. (I'm making the numbers up, but that's the point.) So the eclipses in the latter case appear to be 12 minutes "too early" than what would be predicted from observing the former ones. ___A. di M. 16:08, 5 November 2009 (UTC)

Not really, because I don't think the article explains the phenomenon with sufficient clarity. I think it should focus on the time difference between when the occultation was predicted to occur (by Cassini's ephemeris) and when it did occur, rather than the more obtuse observation about the apparent changes in the periods. (Again, I'm trying to think from the perspective of a reader who is new to the subject.) Sorry to be difficult.—RJH (talk) 21:45, 6 November 2009 (UTC)

Thanks.—RJH (talk) 00:39, 4 November 2009 (UTC)