Red shift and the train thought experiment

In summary, when using the train thought experiment to teach relativity, there is typically no mention of "redshift" because it is not necessary for the purposes of the experiment. While there is a mention of the doppler effect in relation to the measurement of light and sound from the train, the important difference is that the speed of the train must be added when measuring the speed of sound, but not when measuring the speed of light.
  • #1
Pjpic
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1
It seems there is usually no mention of "redshift" when train thought experiment is used to teach relativity. Is there a reason for not bringing redshift (blueshift) into the experiment?
 
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  • #2
It isn't brought in because it isn't needed. Why do you think it is?
 
  • #3
russ_watters said:
It isn't brought in because it isn't needed. Why do you think it is?

Because (I thought?), there's mentioned made in this experiment on how the measurement of light from a train is different than that of the sound from a train. But, it seems (?) they both have the doppler effect.

Is then the important difference that the person on the train has to add the speed of the train when measuring the speed of sound but not when measuring the speed of light?
 

1. What is the train thought experiment?

The train thought experiment is a theoretical scenario that helps explain the concept of red shift, which is the observation that light from distant galaxies appears to have longer wavelengths (shifted towards the red end of the spectrum) when compared to light from nearby galaxies.

2. How does the train thought experiment relate to red shift?

In the train thought experiment, imagine a train moving away from you at a high speed. As the train moves away, the sound waves from its whistle will appear to have a longer wavelength, making them sound lower in pitch. This is similar to how light waves from distant galaxies appear to have longer wavelengths, or be shifted towards the red end of the spectrum, due to their relative motion away from us.

3. Why is the train thought experiment helpful in understanding red shift?

The train thought experiment provides a relatable and visual example of how motion can affect the wavelength of waves. This helps us understand how the relative motion between Earth and distant galaxies can cause the observed red shift, which is an important piece of evidence for the expanding universe theory.

4. Can the train thought experiment be used to explain blue shift?

Yes, the train thought experiment can also be used to explain blue shift, which is the observation that light from objects moving towards us appears to have shorter wavelengths (shifted towards the blue end of the spectrum). In the train thought experiment, if the train were moving towards you, the sound waves from its whistle would appear to have a shorter wavelength, making them sound higher in pitch.

5. Are there any limitations to the train thought experiment?

While the train thought experiment is a helpful tool for understanding the concept of red shift, it is not an exact analogy. In reality, light behaves differently than sound, and the train thought experiment does not fully capture the complexities of red shift and the expansion of the universe. However, it serves as a useful visualization to aid in understanding the concept.

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