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I If I traveled towards a strobe light, would the flash rate increase?

  1. Mar 15, 2017 #1
    If I travel toward a strobe light, will the flash rate appear to increase? If it does, would that then mean that the light entering your eyes was entering faster than the speed of light? If the strobe flash rate does not increase then this universe is stranger than I thought.
    I have been thinking about this all day but can't think of any way to test it.
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  3. Mar 15, 2017 #2


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    The rate of flashes entering your eye will increase but they will all enter at c. If you travel fast enough they will be blue shifted but that happens whether the light is "flashes" or a continuous stream
  4. Mar 15, 2017 #3


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    Yes, the flash rate will increase. You didn't ask, but the frequency of the light will increase by the same factor. This factor is called the "relativistic doppler shift factor". If you imagine there was some number - say a million - cycles of the EM radiation in the light pulse observed at rest, there would be the same number of cycles when you observed the same pulse while moving.. But the pulses would occur faster. So you would observe the same number of cycles in a pulse, each cycle would be shorter, and the length and separation of the pulses would be shorter.

    Velocities don't add the way one would expect according to special relativity, so the speed of light would be equal to "c" in your frame. It would be travelling at the same speed as any other light, but the frequencies of the strobe and the light itself would be changed in the manner previously mentioned (shorter pulses of higher frequency).

    The detailed mathematical description of what is happening is called the Lorentz transform, I've tried to explain what happens in common language.
  5. Mar 15, 2017 #4


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    Here's a spacetime diagram [time running upwards] that summarizes what is going on with the Doppler Effect.
    Each diamond represents equal units of time on a standard [light-]clock carried by that worldline.

    The source at rest broadcasts [in all directions] at the speed of light....periodically at 4-tick intervals, at events... A, B, meeting-event O, D, E, F...
    You (traveling at (3/5)c) receive the broadcasts at the speed of light... on your worldline at A', B', meeting-event O, D', ...

    As you approach, you receive broadcasts more frequently at 2-tick intervals ("blue shifted" light similar to "higher-pitched" for sound)
    As you depart after the momentary meeting, you receive broadcasts less frequently at 8-tick intervals ("red shifted" light similar to "lower-pitched" for sound).

  6. Mar 16, 2017 #5
    Imagine that source of waves approaches the observer with velocity c. This source oscillates with frequency f. Velocity of wave fronts is c also.

    What would happen in „classical“ case? In classical case all wavefrons would gather straight in the front (on the nose) of the source and would instantly hit the observer. Observer would see frequency as increased to infinitely large value (blueshift of frequency).

    In relativistic case velocity of source will never reach c.

    Due to dilation of source‘s clock source will oscillate slower and slower as it‘s relative velocity approaches that of light. Thus, blueshift will always be „less blue“ than in classical case at time dilation factor (Lorentz factor). Measured frequency will tend to infinitely large value as velocity of source is closer to c.

    Since motion is relative, observer may consider himself being „in motion“ but the source is „at rest“. In this case in classical case maximum observed frequency would be 2f. But, for observer it still tends to infinitely large value as velocity increase. Why? Because observer may consider that his own clock dilates gamma times (at Lorentz factor). Thus, he sees blueshift of frequency because he turns into „dawdler“ and perceives hitting him wavefrons as very frequent.
  7. Mar 16, 2017 #6
    It might help if you consider yourself stationary and the strobe light traveling toward you. You would still measure the flashes from the source to be traveling at c but each flash starts closer to you than the previous one, so the length (duration) of a flash, the spacing between successive flashes, and the wavelength of the light itself all get squeezed.
  8. Mar 17, 2017 #7


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    The strobe's emission rate actually decreases in your reference frame, if you start moving relative to the strobe (regardless of direction). But when that movement is towards the strobe, then you still receive the flashes more frequently (according to your clock).
    Last edited: Mar 17, 2017
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