• neumast@lemmy.world
    10·
    1 year ago

    Sooooo, wavelengths (λ) become longer when something moves away (redshift) and become shorter when something moves towards you (blueshift).

    For a red flag (λ0=610nm) to become a green flag (λ1=549nm), it has to move towards you quite fast. But how fast is ‘quite fast’?

    Using the formula

    flag_velocity / speed of light © = difference in wavelengths / starting wavelength

    we get

    flag_velocity = (610-549) / 610 * c = 61 / 610 * c = 1/10 * c

    This means: the flag has to move with about c/10 = 30 000 000 m/s = 108 000 000 km/h = 67 108 100 mph. Yeah, that’s quite fast.

    (Disclaimer:

    1. use info on own risk

    2. values for λ were chosen in a way to make calculations easy. There is no info on what shade of red or green the flag is. The final result will be about the same.

    3. With speeds at around 10% of c, I should use the formula considering the relativistic doppler effect… However, i wont. Thanks.)

    • Jojo@lemm.ee
      5·
      1 year ago

      Well, instead of moving the flag, we could collapse the space in between us rapidly. Would that be easier? I think I have some dark energy around here somewhere…

    • Dave.@aussie.zone
      5·
      1 year ago

      pssst hey you missed the conversion from seconds to hours, your answers are in kilometres/miles per second and need to be another 3600 times larger.

      • neumast@lemmy.world
        2·
        1 year ago

        Are you sure about that? From m/s to km/h you multiply by 3600 (for the time) and divide by 1000 (for the distance) which leads to a factor of 3.6.

        Personally i always remember 25 m/s = 90 km/h = 56 mph because of the somewhat round numbers.