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A literal flag colored red, if you or the flag is moving towards the other so fast as to blueshift it to green.
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 (c) = 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:
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use info on own risk
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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.
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With speeds at around 10% of c, I should use the formula considering the relativistic doppler effect... However, i wont. Thanks.)
So you're saying I should date her right?
uhhh maybe, idk
Well, they certainly did the math
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Just have congenital red–green color blindness, way easier.