Quantisation of light and Doppler effect

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Discussion Overview

The discussion revolves around the quantization of light and its relationship with the Doppler effect, particularly how the photon model can explain the frequency changes observed in light due to relative motion between source and observer. The scope includes conceptual clarifications and technical explanations related to the behavior of light as quanta.

Discussion Character

  • Conceptual clarification, Technical explanation, Debate/contested

Main Points Raised

  • Some participants assert that the frequency of a photon, and consequently its energy, is frame-dependent.
  • Others question whether the change in energy due to motion of the source or observer violates conservation of energy.
  • One participant provides an analogy involving cars to illustrate that energy can appear different depending on the frame of reference without violating conservation principles.
  • There is a reiteration of the idea that energy is conserved but is dependent on the observer's frame of reference.

Areas of Agreement / Disagreement

Participants express differing views on the implications of frame dependence for energy conservation, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

The discussion highlights the complexity of energy measurements in different frames and the need for careful consideration of definitions and assumptions related to energy and motion.

Vibin Narayanan
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Light consists of quanta(small packets of energy). Then how do we explain doppler effect of light the same way we do for sound? What is the valid explanation of light doppler effect which is consistent with the photon picture?
 
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Vibin Narayanan said:
Light consists of quanta(small packets of energy). Then how do we explain doppler effect of light the same way we do for sound? What is the valid explanation of light doppler effect which is consistent with the photon picture?
The frequency of the photon (and hence its energy) is frame-dependent.
 
DrClaude said:
The frequency of the photon (and hence its energy) is frame-dependent.
That's it? I simply didn't get it. Energy changing with the motion of source or observer or both, doesn't violate conservation of energy?
 
Energy is conserved, but it is dependent on the frame. For example, I will measure a ball moving relative to me as having kinetic energy, but in a frame co-moving with the ball, it will have none.
 
Vibin Narayanan said:
That's it? I simply didn't get it. Energy changing with the motion of source or observer or both, doesn't violate conservation of energy?

Nope. Consider yourself in a car moving down the road. If another car comes up behind you 1 mph faster than you're going and taps your bumper, your car is not totaled, despite the fact that you both may be traveling 70+ mph.
 

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