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This question was originally posted in the Sandbox. It's set in the same world as this question, but is not related to it.


I have a speedster character whose top speed is Mach 2. She holds back a lot of the time, but at full tilt she can accelerate to that speed - or slow to a stop from that speed - in exactly one second. Some quick Googling and back-of-the-envelope calculations tell me that this equates to an acceleration of 2,250 ft/s2 (686 m/s2), or about 70G, which is far more than an ordinary human can withstand.

I've therefore decided that my speedster will be passively immune to any physical effects that she would otherwise experience when accelerating or running at those speeds. These immunities would apply even when she isn't actually running. It's not a magical protection like the Speed Force, but a set of latent physical attributes.

I already know that she's going to be immune to G-forces of up to about 75G (735.5 m/s2) - anything below that will feel like 1G to her. She also has superhuman reflexes, to avoid faceplanting into the nearest wall as soon as she starts running.

My question is: What other physical attributes does she need to have in order to be able to survive such intense speed/acceleration? Is there anything else I've overlooked that she needs to be resistant/immune to, and is her existing G-force tolerance sufficient? Answers should ideally mention the implications of these increased tolerances.

Other notes:

  • She wears goggles to protect her eyes while running, so that's not a concern.
  • She's not immune to psychological effects, so a sudden deceleration might not hurt her but it would still disorient her.
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  • $\begingroup$ Would you mind using also SI units? $\endgroup$ – L.Dutch Jul 19 '18 at 15:01
  • $\begingroup$ Sure, gimme a couple of minutes to do the conversions. $\endgroup$ – F1Krazy Jul 19 '18 at 15:01
  • $\begingroup$ 70G is about 690 m/s^2, and 75G is about 740 m/s^2, rounded to two significant digits. $\endgroup$ – a CVn Jul 19 '18 at 15:02
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    $\begingroup$ Rocket boosters, since she will never get enough traction to move or stop at those speeds. $\endgroup$ – John Jul 19 '18 at 16:56
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    $\begingroup$ Turning would be an interesting proposition. $\endgroup$ – Brizzy Jul 20 '18 at 7:05
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The additional pressure she'd receive at that speed would be immense, and the pressure differential between leading surface and trailing surfaces would be twice immense, and the insanely strong low pressure zones which will occur wherever there are vortices generated (any surface discontinuities, wrinkles in clothes, tips of ears, point of jaw, elbows, knees, let alone the stresses on hair - sheesh) will be highly variable, moving across her surfaces, and sometimes truly exponential. So somehow she needs to be utterly unaffected by immense pressure, both positive and negative.

Moreover, as pointed out above, she will pass through the Mach barrier as she accelerates to her top speed, and this will cause not only massive pressure on her leading surfaces, but also shockwaves which would leave vapour bursts and acoustic shockwaves in her wake, stunning anyone close by - effectively being an intrinsic weapon. If you read up on the technical challenges we had breaking the Mach barrier, and the huge resonances, vibration, and shockwaves generated, which literally tore airframes apart, stripped aerodynamic plating right off the airframes, and in many cases killed pilots outright, I think she'd need to be almost as invulnerable as Superman to simply survive one single acceleration / deceleration sequence. Good news is, if she, and all her cells, living and dead, are invulnerable, she won't instantly go bald...

She will need a high energy density energy source other than just food, as her metabolic burn rate would be far higher than even a hummingbird or shrew, necessitating her eating many time her own bodyweight in food to keep out of energy debt and simply dying.

The old Superman comics pseudo-science dodge was that he was made of far denser matter than humans, and that our yellow sun somehow gave him energy - of course, with the level of invulerability he routinely demonstrated, he'd have had to be so dense that he'd have weighed tens of tonnes, and yet somehow we never see Superman leaving crushing footprints in steel or rock, or wading through soil...

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    $\begingroup$ Wow, that's a whole load of stuff I hadn't thought of. Speedsters truly are frightening when you take the time to do the physics calculations. $\endgroup$ – F1Krazy Jul 19 '18 at 20:01
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    $\begingroup$ Superman always floats never letting his full weight touch the ground. How he floats does violate all laws of physics $\endgroup$ – Andrey Jul 19 '18 at 21:10
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    $\begingroup$ Wonderful answer! I had suspected the forces acting on a speedster's body would be destructive. My guess was a form of protection was needed, but your figures exceed anything I had in mind. She will generate more than enough heat to cook herself to death too. Loved invulnerability protects against instant baldness. :) Gave me a good laugh. Plus one. Hope to see more good answers from you. $\endgroup$ – a4android Jul 20 '18 at 2:27
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She'll need some kind of adaptation in her inner ear to avoid getting insanely dizzy with her first step.

Depending on her top speed, accelerating at 690m/s^2 will put her above the speed of sound in just under half a second. She will be unable to hear anything behind her, and anything in front of her would be drowned out by the sound of the wind. Further, at these speeds, air compression starts to cause noticeable heating. She may need some kind of adaptation to avoid cooking herself.

She'll also need improved durability. If she tries to punch anyone or touch pretty much anything while going that fast, it's going to hurt. Normal people standing still occasionally break their own fingers when throwing punches. Imagine how badly mangled your hand would be if you threw a punch at Mach 1.

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  • $\begingroup$ And friction. How can you extert the neccessary force otherwise? $\endgroup$ – Gustavo Jul 19 '18 at 17:09
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As others have said, increased durability would be helpful here. Especially on her feet. Going at that speed is going to be more than a little rough on her shoes (if she wears any, hopefully she does.) The impacts with the ground and the friction are also likely to kill her feet if she doesn't have some sort of ability or technology keeping her feet safe. (I'm not sure if this falls under your "passively immune to any physical effects" thing.) I'm with Ryan_L when he says that everywhere else needs durability too. If something small that you can't avoid with super reflexes (like, I don't know, a piece of gravel?) hits you going at Mach 2, it might kill you. Or if it doesn't, it's gonna give you a bruise and put you in a lot of pain, which you might know if you've been hit by a piece of gravel going only about 30 mph. So yeah, invulnerability is good. (The goggles will also need to be reinforced with something that's strong on a massive scale, or else they're not going to last too long, either.) I might add that invulnerability should only be active when she's using her superspeed, or else she won't be just a speedster anymore. She'd be, well, invulnerable, and that might defeat the purpose to some degree.

She'll also need to have some sort of way to breathe. At high speeds, it might be hard to breathe because of the pressure difference of the air in the lungs and the outside air. (I'm not good at fluid physics, so research this to make sure it's correct, though I'm nearly definite it is, based on theory and personal experience.) I'm not sure how you'll adapt to this- I'd suggest some sort of technology in the form of an artificial oxygenator, though this might be inconvenient when she's not using superspeed.

Other minor things: does she make really fast steps, or are her steps just really long? If her steps are really long, increased reaction time might not be enough, because you can't really control how you're moving if you're in the air. It'd be weird for her to have to shorten her strides if she needs to turn or something, so I'd therefore suggest she makes fast steps.

How does she stop? I feel like that's going to do quite a bit of environmental damage unless she stops gradually. If she decelerates at the rate of a normal car, say, she's going to go really far before she stops. If she decelerates such that she takes the same amount of time to stop, that's going to do work on whatever surface she's standing on, I'd think.

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  • $\begingroup$ 1) She takes fast steps. If you filmed her running and then slowed it right down, it'd look like she's just running normally. 2) She can decelerate at the same rate that she can accelerate (though that doesn't mean she has to). $\endgroup$ – F1Krazy Jul 20 '18 at 6:52
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    $\begingroup$ To have her gait look "like she's just running normally" when slowed down, then in fact she is taking enormous raking strides... her stride appears normal, but her entire personal reference frame is in very high speed motion, then the distance between footfalls is driven by the relative speed of her reference frame vs the environment's reference frame... ∴ huuuuge step length! $\endgroup$ – GerardFalla Jul 20 '18 at 16:06
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When she accelerates like that she will compress air very fast. This will cause flames without a combustion, just like what meteors and satellite debris do on atmospheric entry. She'll have to be immune to that heat.

Notice that the air will also be compressed inside her lungs, whether she holds her breath or not. This will inject a poisonous amount of oxygen in her system.

She might survive that much acceleration, but her intestinal flora will not. If she runs often, she will have a constant case of the runs (pun intended).

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She'll need far greater gee tolerance to survive in the event that she does hit a wall, such as if she's distracted or deflected, or someone trips her. Perfect reflexes only count for so much if you're up against others with super powers; as it stands the deceleration of a forced stop will still be lethal to her and many another standing too close to ground zero.

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