# Can Superspeed Cause Illusions?

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.

• 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... Commented Oct 7, 2019 at 17:26
• I mean, I said running which means you use your feet. Commented Oct 7, 2019 at 17:27
• 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. Commented Oct 7, 2019 at 17:35
• How do you compensate for air friction and make sure your person doesn't burst into flames like a shuttle upon re-entry? Commented Oct 7, 2019 at 18:07
• @kleer001 stay well hydrated. Commented Oct 7, 2019 at 18:10

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.

• 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. Commented Oct 7, 2019 at 18:29
• 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. Commented Oct 7, 2019 at 18:30
• @StarfishPrime I guess any onlookers would be too busy being dead to notice that the illusions are not perfect.
– VLAZ
Commented Oct 7, 2019 at 18:31
• 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. Commented Oct 7, 2019 at 18:33
• Uberflash says, "Stand further back! I promise, this is cool... at a distance!"
– SRM
Commented Oct 7, 2019 at 22:11

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.

• 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. Commented Oct 7, 2019 at 17:31
• 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.
– VLAZ
Commented Oct 7, 2019 at 17:40
• 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. Commented Oct 7, 2019 at 18:04
• 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. Commented Oct 7, 2019 at 18:18

### Only with proper lighting

Let's have a look at some real life example: https://www.youtube.com/watch?v=nVjWF74shEw (Assassination Classroom season 1 episode 1; superspeed, in that case mach 20, is invoked at about 1:08). The obvious issue is, that while the teacher grabs your attention and therefore looks more solid, the boring stationary background is still visible (33% opaqueness, as Koro-sensei switches between three positions). Note that the limited frame rate of the camera causes temporal aliasing; if you'd watch this through your own eyes, it would look quite a bit different. You ceiling-fan analogy is valid and correct.

Apart from Assassination Classroom, there are also quite a few amusement parks in the more "western culture" which employ an illusion with "ghosts" which appear out of this air and reappear again a few seconds later. Basically, you look at a dimly illuminated scene (like, for example, a room which looks like it's inside a medieval-age caste) through a pane of glass (which you cannot see). There is another room which you cannot see directly, but the pane of glass is angled in such a way that it the reflection will superimpose the hidden room (typically above or below the track of your ride) onto the obvious room. When the hidden room is dark, no light is reflected, so no mystery there. When the hidden room is illuminated, you will see its reflection, but the stuff in the hissen room will appear semi-transparent superimposed over the obvious room. If you change the relative illumination of the two rooms, you can make the hidden room objects look more substantial (best when the hidden room has 100% illumination and the obvious room is absolutely dark) or less substantial.

However, your question is not about an amusement park ride. The only relevant thing is that you need 0% background illumination together with enough illumination of the object/person/sensei you want to appear solid.

Luckily, there are real-life offices where the fluorescent tubes are powered by three-phase electrical power. If you manage to find spots where the fluorescent tubes illuminating the foreground are at 100% voltage (= 100% power) and the ones illuminating the background are at 50% voltage (= 25% power), the illusion will look more substantial (but still far perfect), assuming your character can sync to the three-phase electrical grid. Next, find the alternate spot where the same is true 1/150 second later (assuming a place in Europe, 1/180 second in the US - assuming real three phase is available at the given place).

Regardless, since these "optimal lighting times" do not switch abruptly, but fade from one to the next, the illusion will suffer even a bit further.

Now, what if you could control background and foreground lighting perfectly? High speed stroboscopes would allow you to do that.

However, if you already that much control of the environment and the situation, hiring a stunt double which looks exactly like the hero might be the way to go. Mainly because the supersonic boom caused by the rapid movements would be quite...annoying, to say the least.

# You could if you're Johannes Stoetter

The advantage of Super Speed is that you have plenty of time to look over your environment and plan out your next move. You don't have to run at Mach 10 ... just keep covering yourself in different camouflage from time to time, and hope they don't notice their blurry vision is outside their eyes. You don't have to stand in a certain spot 30 times a second - you just have to put up a little piece of artwork you drew in front of each person's face (carefully suspended from the ceiling, I suppose, to reduce the drafts from unnecessary trips) that matches what each person in the room expects to see, given your intended version of events. Look at the videos on this page to get a notion of the possibilities -- painted illusions can be remarkable! Of course, you could draw on paper instead of people if you wish. And, assuming you have subjective millennia to practice your skills, you might do better than these some day.