Brightness detected based on direction of travel relative to source

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Discussion Overview

The discussion revolves around the perception of brightness from a light source (a match) in a moving train, specifically how the direction of travel affects the brightness detected by observers at both ends of the train car. The scope includes conceptual reasoning and implications of relativistic effects on brightness measurements.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes a scenario where a match is struck in a moving train, suggesting that the rear brightness detector should measure greater brightness than the front detector due to the distances involved (r1 > r2).
  • Another participant emphasizes the need to consider various effects that validate Lorentz Transformations, implying that these transformations play a crucial role in understanding the situation.
  • A different participant notes that energy is frame-variant, indicating that luminosity and brightness are also affected by the motion of the detectors relative to the light source, with Doppler effects influencing the energy detected.
  • One participant expresses gratitude for learning about the "headlight effect," suggesting it may relate to the discussion but does not elaborate further.

Areas of Agreement / Disagreement

Participants express differing views on how brightness is perceived based on the motion of the train and the effects of relativistic physics. There is no consensus on the implications of these effects or the correct interpretation of the brightness measurements.

Contextual Notes

The discussion highlights the complexity of brightness perception in relativistic contexts, with assumptions about distances and the nature of light not fully resolved. The implications of Doppler effects and Lorentz Transformations are acknowledged but not definitively concluded.

wsellers
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I don’t see that this question has been asked before and I am hoping the generous moderators of this forum will be able to answer it.

Suppose you have the “standard” situation with the man M on the embankment and the woman W in the center of the moving train car. (The train has lots of windows enabling M to see in.) W strikes a match (it’s a special kind of match that has a constant luminosity, L). The distance between W (and the match) and the front of the car is D1; the distance between W and the back of the car is D2. Since we assume W is in the exact middle, D1=D2.

At each end of the car is a brightness detector with a digital LCD readout. Brightness is equal to L/4[pi]r, I believe, where L is luminosity and r is the distance from the light source to the detector. The distance that the light travels from the match to the front detector is r1; the distance that the light travels from the match to the rear detector is r2.

Because the train is moving toward the front, r1 > r2. This should mean that the rear detector measures a level of brightness that is greater than that measured by the front detector. Thus the rear detector would display a number that is greater than the number displayed by the front detector.

Wouldn’t M and W see the same numbers? In the same train car going in the opposite direction, wouldn’t the rear detector display a number that is less than the number displayed by the front detector?
 
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Because the train is moving toward the front, r1 > r2. This should mean that the rear detector measures a level of brightness that is greater than that measured by the front detector.
You forgot the http://www.anu.edu.au/Physics/Searle/Obsolete/Seminar.html" (I found this site independently :wink: ).
Quick explanation: There are many effects you have to consider, all of which conspire to make the Lorentz Transformations valid. However, if you look at it differently: The LT are indeed valid (or, at least, self-consistent), and all those effects are simply derived from it. So it is no miracle that all works out self-consistently.
 
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Energy is a frame-variant quantity so luminosity (energy/time) and brightness (luminosity/area) are also frame variant. A detector moving away from a source will generally detect lower energy than one moving towards a source as the Doppler red-shift lowers the energy of the detected photons, but does not change the number of photons.
 
Thanks very much! I wasn't aware of the headlight effect.
 

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