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 Mar 11 awarded Commentator Mar 11 comment Why do objects appear smaller when viewed from a distance? Your point about absolute vanishing points isn't really well-founded. Vanishing point does not mean the distance at which objects appear as point sources. You can't ask "what's the vanishing point of the Andromeda galaxy", because that question doesn't make sense. A vanishing point means this: given a particular line of infinite length, if you take a picture of that line from a particular vantage point, the line will appear to "end" somewhere on the image. This is the vanishing point. But if you took a different line, or a different vantage point, the vanishing point would change. Dec 3 awarded Popular Question Aug 3 answered Earth still exists - does this fact tell us anything about LHC safety? Jun 19 awarded Popular Question Jul 4 awarded Yearling Sep 21 awarded Custodian Jul 7 accepted If quarks didn't have mass, could protons (and neutrons) exist? Jul 7 awarded Teacher Jul 7 answered Why are the physical sciences described perfectly by mathematics? Jul 7 awarded Critic Jul 6 asked If quarks didn't have mass, could protons (and neutrons) exist? Jul 4 comment Why did we need relativity to derive $E=mc^2$? @JerrySchirmer ... so ... if the total amount of energy lost is not invariant in different reference frames with a classical Doppler effect, why do we need special relativity to discover the relationship between energy and mass? Jul 4 awarded Scholar Jul 4 accepted Why did we need relativity to derive $E=mc^2$? Jul 4 awarded Supporter Jul 4 reviewed Approve Why did we need relativity to derive $E=mc^2$? Jul 4 comment Why did we need relativity to derive $E=mc^2$? Also, just to clarify, I recognize the need for special relativity in general. I just wasn't sold on why it is needed to discover the mass-energy relation. Jul 4 comment Why did we need relativity to derive $E=mc^2$? Okay, so if I understand correctly: in classical physics, if the bulb emits light in all directions, then the total amount of energy lost is invariant, even in other frames of reference (so, classical Doppler effect has no impact on the total amount of energy lost due to the light pulse). It is only with special relativity, and a relativistic Doppler effect, that the light pulse carries away different amounts of energy in different reference frames. Is that correct? Jul 4 comment Conversion of the units BeV (US) and GeV (UN) @Non-standardmodel Hmmm, that is tricky! The wikipedia article I linked has a list of countries that use each scale, so if you know the nationality of your author, you can make a pretty educated guess. Also, you know that your author is going to consistently use only one system, so if you ever see terms like milliard or billiard, you know (s)he is using the long-scale. Likewise, if (s)he ever says "one billion eV (or 1 GeV)", you know (s)he is using the short scale.