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My story involves people with supernatural abilities. I want to create a character that has classic superspeed like the comic book superhero The Flash, but I want my character to only have realistic applications of such a power.

Some applications I have already debunked:

  • Saving people: Grabbing people at lightning speed and then transporting them out of harm's way would just turn their insides into jelly, due to the sudden changes in speed. (Law of Conversation of Momentum) I do not accept mystical reasons like "the speed force protects them while in transit". In my story there is no "speed force".
  • Running up completely vertical walls indefinitely: Friction is needed to go up the wall. But to get friction, one needs to press their feet against the wall, which pushes them away from it. And then eventually they're not close enough to keep running up the wall.
  • Super speed in general: At higher speeds, the user would superheat the area around themselves, incinerating objects and people around themselves. This is caused by the friction of the air molecules; the same reason most objects burn up during atmospheric entry.
  • Time travel: This is just stupid, to be honest.
  • The list goes on

It seems that most of the applications used in comics are actually impossible. There is one that has caught my attention, though: the ability to create illusions.

According to the theory, if a person moves fast enough, they can move back and forth at a speed great enough to cast the illusion of duplicates.

Is it possible to cause completely opaque illusions by moving back and forth at incredible speeds?

Let's say that a person spends 49.9999% of their time in one pose, and 49.9999% of their time in another pose. The remaining 0.0002% is spent transitioning between the two different poses.

My theory: An outside observer would see two poses that are roughly 50% transparent. After all, each pose can't be in each location 100% of the time.

A good example is that when you look at a ceiling fan turned on high, you can see everything that is behind the fan at the same time. A helicopter's propellers show this phenomenon, as well.

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    $\begingroup$ Going up the wall is not completely impossible. You just have to flap your arms fast enough to give you enough push to stay in contact with the wall. Of course, then you could fly... $\endgroup$ – puppetsock Oct 7 '19 at 17:26
  • $\begingroup$ I mean, I said running which means you use your feet. $\endgroup$ – overlord Oct 7 '19 at 17:27
  • $\begingroup$ Scientifically speaking, even horizontal running is problematic for speedsters. As the speed increases, the hero would go airborne and the locomotion would resemble hopping on the Moon. $\endgroup$ – Alexander Oct 7 '19 at 17:35
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    $\begingroup$ How do you compensate for air friction and make sure your person doesn't burst into flames like a shuttle upon re-entry? $\endgroup$ – kleer001 Oct 7 '19 at 18:07
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    $\begingroup$ @kleer001 stay well hydrated. $\endgroup$ – Starfish Prime Oct 7 '19 at 18:10
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I don't think it's possible while remaining within the bounds relative plausibility. This character has to be so fast that even The Flash might raise an eyebrow. I'll call the character UberFlash.

So, let's say UberFlash tries to create two "illusions". He stands in one spot, then quickly goes to another and back. Visual diagram:

A <---> B

This has to be done multiple times a second. About the lowest frames per second you can get nowadays in video games is 30FPS. That means that an image is shown 30 times on the screen each second. This makes the transition between images seem like it's (mostly) non-existent and it's actually a continuous video. Some people can easily distinguish 30 FPS, as it "flickers" or "looks choppy" and otherwise doesn't maintains good illusion for a smooth stream of visual data. Still, I'm going to use 30 FPS as the baseline for UberFlash.

Running to each spot 30 times.

So, UberFlash has to show up in two places and stand at each 30 times over the course of one second. So he has to move between A and B 29 times in a second.

Taking the proposed 0.0002% of time spend travelling, then UberFlash will have to move the ENTIRE distance 29 times in 0.0002% of a second, or 2 microseconds. For further reference, that's 0.002 milliseconds. That is the TOTAL time to make 29 trips. Let's say the two "illusions" are fairly close - 1m apart. If both "illusions held their hands outstretched, they would be touching or even overlapping.

(29 * 1m) / 0.002ms = ???

UberFlash has to be moving at about 14 500 m/s or 52 200 km/h. UberFlash can run a marathon in just under 3 seconds.

Mach 1 speed (speed of sound) is about 1 235 km/h and going over it produces a sonic boom. UberFlash would be moving at roughly 42 times that speed. I don't think it would be very subtle or very safe for...anybody and anything.

Running to each spot 10 times

But perhaps 30FPS is too high. The "illusions" don't need to be perfect. Let's cut it down to 10 FPS - it will produce noticeable flickering of the images. Let's also make the distance smaller - 50cm, the two are almost shoulder to shoulder. Oh, and let's give UberFlash a bit more leniency - instead of 0.002 milliseconds, let's make it few orders of magnitude more and give him 1 millisecond. Just 1/1000th of a second to travel between the two locations.

(9*0.5m) / 1ms = ???

That means that UberFlash will move at 4 500 m/s or 16 200 km/h. Still more than 10 times the sonic boom speeds.

I think at these speeds, aside from the images of UberFlash, there would be some high winds, loud noises, and maybe even flames around.

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    $\begingroup$ Mach 10 at sea level will cause enough aerodynamic heating to incinerate pretty much everything. The Sprint missile needed a super aerodynamic shape with no control fins and an ablative heat shield and generated a plasma sheath. $\endgroup$ – Starfish Prime Oct 7 '19 at 18:29
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    $\begingroup$ Oh, and also consider the bone-liquifying accelerations required to reach mach 10 in under a millisecond and slow down to a stop in less than a millisecond afterwards. $\endgroup$ – Starfish Prime Oct 7 '19 at 18:30
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    $\begingroup$ @StarfishPrime I guess any onlookers would be too busy being dead to notice that the illusions are not perfect. $\endgroup$ – VLAZ Oct 7 '19 at 18:31
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    $\begingroup$ I don't think it would be fatal... extremely unpleasant and probably resulting in permanent deafness, if you were close enough. You probably wouldn't have to be too far away to avoid that, too. It would rather spoil the illusion though. $\endgroup$ – Starfish Prime Oct 7 '19 at 18:33
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    $\begingroup$ Uberflash says, "Stand further back! I promise, this is cool... at a distance!" $\endgroup$ – SRM Oct 7 '19 at 22:11
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As long as you are above 60 frames (transitions from one location to the other) per second, most humans will see two people. This is how films work. At 120 FPS, it’ll be indistinguishable from two people standing there to all biological observers and most cameras.

Note that the continuous peals of thunder as you shift back and forth may give you away. :-) Also will have problem in fog — a clear channel between your two positions will open.

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    $\begingroup$ Human eyes don’t actually have a ‘frame rate’ like film. It’s a handy lie, but it’s nowhere near as simple as FPS, so I’m not sure this wouldn’t just cause a blur that’s oddly solid at either end. $\endgroup$ – Joe Bloggs Oct 7 '19 at 17:31
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    $\begingroup$ I'm pretty sure some people will notice the two shapes "flickering". Maybe not all but people can and do notice frame transitions. Also, this requires moving between the two locations in such high speed that you'll even give The Flash a run for his money. $\endgroup$ – VLAZ Oct 7 '19 at 17:40
  • $\begingroup$ That's not how human perception works, that's not how light works. It is how cameras work, so your answer would be fine for security cameras. But that wasn't really OPs question. $\endgroup$ – kleer001 Oct 7 '19 at 18:04
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    $\begingroup$ Much research has been done on this... the flicker fusion threshold is about 60hz but fast eye movements can result in much higher frequency flickering becoming apparent, even up as high as 500hz. $\endgroup$ – Starfish Prime Oct 7 '19 at 18:18

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