In the second episode of the CW Flash TV show, Barry runs around Iris so fast and speaks so quickly that to her it appears that he was standing in front of her the whole time(She doesn't even notice a blur). How fast would Barry be moving and speaking that he would A: have the pitch of his voice be moved into the ultrasonic range and B: be red-shifted out of the visible light specturm. I'm assuming he would have to move very close to or faster than the speed of light and speak nearly as fast. However, I was wondering if someone has a more specific answer as well as possibly another explanation because during this time his maximum speed ever reached is 700 mph in the first episode. Thanks for any help.

1 Answer 1


It's less about "redshifted out of the visible spectrum" (which doesn't really make much sense anyway) and more about the density of the blur.

Suppose that Barry was standing in the same place for 0.08 seconds, then ran a circle in 0.02 seconds (just plausible within the 700 mph limit), then standing in the same place for 0.08 seconds again, and so on. Then what would be visible for normal humans is 4/5 of Barry being stationary (nearly indistinguishable from standing all the time) and 1/5 of Barry stretched all over as a blur; that 1/5 is total over the entire circle, so would have tiny density.
(Actually, due to the way human vision works, it might well be possible for Barry to spend an even bigger part of the time circling and still end up with a well visible standing shape and an invisible blur. But this 4:1 ratio is definitely safe enough.)

As for the voice, assuming that Barry's voice is sped up by the same factor as his running is, we get a factor of about 50-100 (I'm assuming Barry is not a particularly fast runner from his perspective). This means that his voice must have been over 200-400 Hz from his perspective to be shifted into ultrasound; this is around countertenor territory (i.e. a bit high for a male but not too unlikely).

(Note: I have never seen the show in question; I'm just trying to figure out how the situation described could make sense. If the answer is wrong due to that, sorry.)

  • No, that's not right. Check out the limits of human perception of brightness changes, and the solid angle subtended by Barry when he is talking to someone.
    – Rex Kerr
    Commented Feb 7, 2016 at 1:42

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