This question is about speedsters and defending against them (Dr Wells' gun thing doesn't work).

There is a speedster. His 'cruising speed' is about 1000 mph, but can get up to 2000 mph if needed. This speedster happens to be an evil speedster (basically Zoom or Reverse Flash). So the world has combined their remaining smarter-than-average humans to try and find a way to defend against this speedster. After some debating, they decided to post a question online and see what the general population thought.

They need a device that can:

  • defend against a human sized human moving at 1500+ mph
  • stun (for at least 30 seconds) or kill the speedster
  • maybe a carry able version, but that seems useless (by the time you load a round, you'd be dead but still, it'll make you feel safer)
  • probably will be mounted to the inside of a door, so if the speedster comes through they get stunned/zapped

Assume that:

  • they have 3 million dollars available
  • the speedster can't phase through walls, but can smash through it if its weak enough, but rarely does (keep in mind hes moving at 1500 mph, he'd break some bones if it wasn't made out of marshmallows)
  • they need at least 20 of these anti-speedster-things
  • they have two weeks before the speedster finds and kills them, so they need at least one anti-speedster-thing by then
  • technology is the same as it is today
  • they have 1-day shipping from Amazon and other companies

If this group of humans fail in making a device, the world basically ends and then sad music plays in the background as evil speedster man kills them all. So, they could use some help.

QUICK EDIT- Would plasma shields work well?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Sep 5, 2020 at 4:49

20 Answers 20


they have 2 weeks before the speedster finds and kills them

Seems like they are headed for a final showdown. Since they are the defendants, they get to pick the battlefield.

I suggest this:

A speed bump

That evil guy will either have to slow down or he'll smash his toes against this thing. In either case he'll lose his advantage.

You can buy them online at 216 canadian dollars a piece if you buy 5+. Converting 3 million freedom dollars, and considering pricing... you could order 18,500 of those if you didn't have to pay taxes and shipping. So round down to 15,000 that you can order.

With a maze of 15,000 speed bumps, you can actually sell tickets so that people may gather inside and take potshots at the speedster. At 4,000 USD a ticket and with a thousand participants you may actually make some profit out of this. The ticket price might seem steep but shooting down a speedster is something you get to do only once in a generation!

Edit: user253751 commented:

... why wouldn't he just step over the speedbump, at lightning speed?

Yeah, the villain might do that. The solution is scaling up. Use these bumps instead, and have the shooters positioned in towers:

Mother of all speed bumps. Yes, that's the commercial name.

Mother of all speed bumps, from another angle.

  • 5
    $\begingroup$ Remind the ticketholders that hearing protection from the up-close sonic booms is their own responsibility so the organizer doesn't get sued (that would blow the budget). $\endgroup$
    – user535733
    Sep 2, 2020 at 13:57
  • 5
    $\begingroup$ Best laugh I've had all night. +1. what-if.xkcd.com/61. He may also get airborne, and when unexpectedly airborne be unable to change direction. $\endgroup$
    – Ash
    Sep 2, 2020 at 16:28
  • 29
    $\begingroup$ Or worse. Put a LEGO brick. That would make you the evil guy though. $\endgroup$
    – Kepotx
    Sep 3, 2020 at 11:55
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    $\begingroup$ The town I live in already has these defenses in place. $\endgroup$
    – Aganju
    Sep 3, 2020 at 12:28
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    $\begingroup$ Whoever approved those speed bumps deserves jail time. $\endgroup$
    – Nosajimiki
    Sep 3, 2020 at 13:20

1500+ mph is about Mach 2. This is within the operational envelope of the AN/SEQ-3 LaWS.

Unless your speedsters can resist a 30 kW laser bolt, things don't look very good for them.

It is also possible to engineer a speed-triggered electrical discharge (must be less than about 20 kV per meter). This has more chances of just stunning the victim, but losing control at that speed is likely lethal all by itself.

However, simply renting a submarine might be a more cost effective proposition. You won't capture the speedster but he won't be able to get you either.

  • 20
    $\begingroup$ Realy, any modern anti-aircraft weapon would be sufficient in case High Energy Lasers are out of your price range. $\endgroup$
    – Nosajimiki
    Sep 2, 2020 at 16:16
  • $\begingroup$ @Nosajimiki I imagine modern anti-aircraft weapons are also not cheap though :p $\endgroup$
    – Mark
    Sep 3, 2020 at 10:34
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    $\begingroup$ @Mark yes, the basic CIWS system goes for about 5 million USD, the LaWS is probably around double that. I believe a capacitor trap "powered" by a high speed camera (a 330fps costs about 200 USD on Amazon, and the speedster will only travel about two meters in each frame) and a fast CV system could be doable for around USD 5000. Also, a trap-room with mesh-reinforced walls and four high-speed explosive-propelled steel doors would go for around 50K USD. $\endgroup$
    – LSerni
    Sep 3, 2020 at 10:41

Taut wire.

sore neck


Yeah, that should do for him. If he does not hit a junebug at 1500 mph on the way in. Heck the gas mass by itself should flap his flappy bits pretty good.

Speedsters are fun but they are pretty flagrant about violating physical laws, especially as regards the atmosphere.

  • 8
    $\begingroup$ If this kind of thing would hurt the speedster it’s hard not to imagine their own muscles sloughing off the bone shortly after a 1500 mph wind tears off their epidermis... $\endgroup$
    – Joe Bloggs
    Sep 2, 2020 at 14:07
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    $\begingroup$ Considering that a human going 1500mph would need the entirety of the sahara desert to accelerate or decellerate (ignoring that friction will limit his speed) and have a turning circle of 300km or larger I suspect that the speedster, who is somewhat like the Flash as per OP's suggestion, is able to react fast enough to his environment and cheat physics a bit. So a taut wire would simply be ducked under or cut as the speedster approaches. $\endgroup$
    – Demigan
    Sep 2, 2020 at 15:00
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    $\begingroup$ @Demigan How did you arrive at those figures? By my calculation, a normal athletic human can accelerate at about 3m/s^2, meaning the speedster can start at rest and reach 670 m/s (1500 mph) in under 4 minutes. Over those 4 minutes, he has an average speed of 335 m/s, meaning he only covers about 80km (50 miles) while getting up to speed, less than 5% of the shortest distance across the Sahara. $\endgroup$ Sep 2, 2020 at 19:04
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    $\begingroup$ @NuclearWang As you accelerate the windresistance goes up. Eventually that wind resistance, which pushes with a deceleration force against you, is equal to the acceleration force. This is why in a car in the first/reverse gear you dont accelerate forever when you give it a small amount of gas and you start accelerating. This means that the maximum speed is limited by the maximum ground reaction force you can generate without slipping or breaking the shoes/wheels/ground. No matter how strong or fast your body is these properties will limit your speed. Think Usain Bolt's maximum speed on ice. $\endgroup$
    – Demigan
    Sep 2, 2020 at 20:55
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    $\begingroup$ @NuclearWang also that's besides the point: if it takes 4 minutes to speed up, he's not going to be able to take a 90 degree turn in less than 4 minutes or break safely in 4 minutes. His maximum speed would instantly become irrelevant as he'll never achieve it, and the answer to the question would be "just bring a gun". And considering the OP says things like "like the Flash" who can accelerate and decelerate at will it would be weird to assume speed powers that are not like Flash. Like answering a seatbelt safety question with "they are unsafe because the driver didnt wear one". $\endgroup$
    – Demigan
    Sep 2, 2020 at 21:02

Oobleck Moats

world |XXXX/  bunker
_     |XXX|     ____
XX\           /XXXXX
XXX\ oobleck /XXXXXX

If your speeder can't blast through walls, they will have to slow down to manageable speeds if they want to visit, giving you plenty of time to deal with them, while still allowing easy (if messy) access in and out for yourselves.

  • 11
    $\begingroup$ Ordinary water would work just as well at 1500+ mph. $\endgroup$
    – The Daleks
    Sep 3, 2020 at 16:17
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    $\begingroup$ @TheDaleks but where's the fun in that? Ordinary air should work just as well, but this is THE SPEEDSTER vice.com/en_us/article/wnn7ay/… $\endgroup$
    – Cireo
    Sep 3, 2020 at 22:35

Knockout gas

Simple solutions work best. The day before the speedster attacks the lair of the good guys, all they have to do is flood the place with knockout gas, don gas masks, and let nature take its course as the speedster's super-fast metabolism takes care of the rest. Oxygen tanks are, of course, impossible for the speedster as the about of oxygen he would need would greatly exceed any you could carry around in a tank.

  • 6
    $\begingroup$ If the amount of oxygen in a tank is exceeded that easily, then the air around his throat would experience choked flow and he would both rip his own throat apart breathing and he wouldnt be able to draw enough breath to survive high speed. I think we need to assume that the Zoom/Flash abilities that let him move so fast also prevent such problems. So the speedster might experience going 200mph like he's just walking along. $\endgroup$
    – Demigan
    Sep 2, 2020 at 15:05


All you need is a system that creates a slight curtain of sand across the entrances and along corridors and the speedster will have to slow down to a much more reasonable speeds to pass through.

Sand blasting is a thing. A sand blaster running at 100psi outputs sand at around 420mph and will quite happily strip flesh from bone. Your speedster hitting the sand at 1500mph will do the work for you without you having to worry about accelerating the sand or having restrictions on passing through the entrances yourself. He will effectively be bound to movement speeds below 200mph while in the sand.

For a portable version you just need a supply of sand and a leaf blower.

Downsides: Your cleaners will hate you and your laundry will always be full of sand.

  • 4
    $\begingroup$ If there's one thing you can count on with sand, it's that it's coarse and rough and irritating, and that it gets everywhere. $\endgroup$
    – bta
    Sep 4, 2020 at 21:11

Ordinary physics obviously doesn't hold much sway over these people. I have to assume some "brings his own dimension" shenanigans like the Flash does to give the speedster the time to walk, turn, open doors etc without instantly breaking themselves or the thing they operate.

Solution 1: a big heavy door and reinforced walls and an airlock.

They might be fast, but if it would take days or weeks to open a door without the key or actual vehicles designed to wreck the place, they will take long enough that you can deploy countermeasures against them. Let's say you increase the pressure inside until the speedster is knocked out. That way you circumvent the speedster having prepared with gas masks or similar. Without gas masks the fact that they entered an airlock would mean they use the air up so quickly that they'll knock themselves out before they breach the door.

Solution 2: electrify it.

The problem with being a speedster is that all your internal processes (and a few external one's) have to be accelerated as well. This means that electricity will pass through you that more quickly while dealing the same damage as before. A sufficient jolt of electricity can knock the person unconscious, or have him writhing across the floor as the speedster finds out that his rubber boots protected him from the floor until he fell down after grabbing the door handle. If you are less friendly, just kill them with a high enough jolt.

Solution 3: low resistance surface.

If the floor is so slippery that a normal person cant get up anymore, for example because the floor is bowl-shaped and outlets make sure a nice film of ultra-slippery material coats it constantly (or it is slippery in normal circumstances, like a thick layer of ice you've cultivated), then a speedster wouldn't be able to get up either despite his speed. If you are friendly you have softer materials at the edges to make sure the speedster doesn't kill himself with the impact before sliding back to the center and getting stuck.

Solution 4: mines and boobytraps.

The speedster has to accelerate a part of his environment in order to move that fast. Without that the friction between the floor and his shoes would limit his speed to around 50km/h (I did a question about this once), and that is ignoring the fact that his shoe rubber would likely split apart. So his acceleration has to extend to his shoes and the ground he's standing on as well in order to reach his speed... and if he steps on a mine trigger, that will be accelerated and go off as well (not that it needs to! The chemical reaction upon standing on the trigger will be fast enough!). Mine shrapnel like from a claymore travels at 1200m/s, faster than 2000mph (+/-900m/s). 300m/s difference isn't going to give even a speedster much chance to react. Similarly, a door with a bomb strapped to it will kill the person if he opens it.

Solution 5: be somewhere their speed is irrelevant.

Let's say that the moment the evil speedster starts trying to kill you, you are in an aircraft crossing large bodies of water. How is he going to catch you? If he swims he'll die from starvation before reaching the other side, boats are too slow and unless the speedster knows how to operate functioning military aircraft he's not going to be able to catch you in the air. And that is ignoring the fact that he might be able to get to an operational, loaded and fueled aircraft but said aircraft would still need to pilot to the runway and take off, then avoid interception at aircraft speeds rather than your own until he reaches you.

And that is assuming he even knows where you are.

  • 5
    $\begingroup$ Someone running at 1000+ mph will be able to run on top of water, no swimming required. If he runs off a ramp, he can reach an altitude of tens of thousands of feet, so you can't really hide in a plane, either. He can follow you anywhere, and will be there when you land. $\endgroup$ Sep 2, 2020 at 14:55
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    $\begingroup$ @NuclearWang yes he can run on top of water. But how far can he reach? He has to bring his food and water with him, so he'll starve to death before reaching the other end of say the Atlantic ocean even assuming he uses the same calories/water per mile as ordinary humans. If he uses a ramp he needs to A: have a sufficient ramp, B: have a ramp facing the right direction, C: know you are actually on that specific plane, D: be an incredible aim to actually land on top, E: survive the impact (especially if he misses), I suspect he'll die from a fall just as easily as anyone else. $\endgroup$
    – Demigan
    Sep 2, 2020 at 15:12
  • $\begingroup$ Agree the physics of this individual are quite murky and undefined, so there's lots of room for interpretation. I'm assuming his metabolism is more efficient than a normal person, or else he'll need to eat 3 months' worth of food and drink a barrel of water every hour. Surviving acceleration is always a strange topic for these speedsters, but if he can survive going from 0 to 2000 mph very quickly, it stands to reason he can survive going from 2000 mph to 0 in a short time equally well. $\endgroup$ Sep 2, 2020 at 15:25
  • $\begingroup$ @NuclearWang I would assume that the Flash rules apply: if you catch him off-guard he'll still be just as vulnerable to damage as normal human beings. He'll die from a fall from the same height as other humans, or a bullet, or a baseballbat against the face (assuming he doesnt notice and can react). Otherwise the question doesnt make sense, since you would need anti-tank weapons to penetrate his skin if he's really physically strong enough to resist his own forces. Also if he's able to jump that high then he needs to jump rather than walk, a low doorway would require him to stop... $\endgroup$
    – Demigan
    Sep 2, 2020 at 18:50
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    $\begingroup$ @Demigan , with 1500 mph, it takes about two hours to cross the atlantic. Why would you need food and water for that? They do have food on the other side. $\endgroup$
    – Aganju
    Sep 3, 2020 at 12:31

Antipersonnel landmines

For the sake of this, I am assuming that (from the speedster’s standpoint) they have the reaction time and such of someone moving at a slow jog, similar to the flash, even though they move faster in reality. So obviously cheesewire wouldn’t work very well. What you need is the same thing that we use against all pedestrian enemies: a shitload of landmines, specifically, anti-personnel landmines.

Blast waves from high explosives travel considerably faster than 1500 mph, so you should have no issue there. You can get a lot of landmines for 3 million dollars. His high speed isn’t going to make him better at navigating a minefield, even with his advanced reaction speed. Especially if he doesn’t know it’s there.

  • $\begingroup$ What about trigger delay to detonation? Isn't he long gone by the time the bomb goes off? $\endgroup$
    – ThisClark
    Sep 4, 2020 at 18:33
  • $\begingroup$ @thisclark spring loaded fuse would be way too slow, but a piezoelectric should be fine. $\endgroup$
    – Daniel B
    Sep 4, 2020 at 22:45

From a comment on the original question by @Ceramicmrno0b: "they dont work well under pressure". This is unfortunate because otherwise they could move their defense base into an underwater habitat.

The presence of over 100 feet of water in every direction should slow speedster to a more manageable speed, or batter him into a sort of bone-fragment jelly if he declines to slow down.

SEALAB I is currently in storage at the Museum of Man in the Sea, in Panama City Beach, Florida. I'm sure that the U.S. Navy could be persuaded to part with it, once they realize that the end of the world would probably have an impact on congressional defense funding.


The door with the interior you want to attach it to is between two (secretly) reinforced walls. Opening it triggers the closing of two reinforced doors, one before, one after. Then you set off bombs, very quickly. (Multiple ones because at superspeed, it's less likely that he will detect and disarm them all.)

Poison gas might also work. Bombs kill faster and are released more quickly, and you don't have to worry about the gas escaping the room with a bomb, but it would probably be harder to sabotage gas's release.

  • 1
    $\begingroup$ If you go through the trouble of detonating bombs, why wait for reinforced doors to slam shut? Wouldnt you just detonate the bombs on either side of the hallways and spray it with shrapnel? A basic claymore sends shrapnel at 300m/s faster than the speedster is able to move, so the speedster would not be able to disarm or avoid the traps in any case. With a wall or two between you and the hallway you are safe, the speedster is dead. Boobytrap all windows and doors with one bomb, you sit in the bathtub with a flak vest or two over your body. Bring a magazine as you might be there a while. $\endgroup$
    – Demigan
    Sep 2, 2020 at 14:56
  • $\begingroup$ To avoid the speedster getting out. Just in case. $\endgroup$
    – Mary
    Sep 2, 2020 at 22:32

Staying indoors seems like a reasonable defense against something who's only power is being very fast. You could even add a time-locked double door to your main lair forcing anyone who wants to come in to wait 5s in a small lobby/antechamber where their powers would be near useless. Then all you need to do is bait the trap with something they value (kidnap their hamster?) and wait.


(keep in mind hes moving at 1500 mph, he'd break some bones if it wasn't made out of marshmallows)

I feel you answered your own question there.

For fun and giggles, at 670 m/s in real units, a wall of marshmallow is actually made of concrete. Or in more scientific terms, you pack such kinetic energy at that speed that blasting through any surface is going to kill you. So a fortress of marshmallows would be more than enough to force your evil speedster to slow down or become raspberry jam.

This meets the budget requirements, you can buy 1 million dollars of marshmallow, a truck to make it portable, a few tasers and assorted electrical traps, and spend the rest to treat yourself. You've deserved it, having a supervillain chase you must be stressful.

Since nobody in their right mind would build a fortress out of marshmallow, your speedster would more than likely have to stop and assess the situation and wonder what kind of elaborate trickery is this. I mean, there could be deadly spikes behind that marshmallow and you definitely don't want to find out by smashing into it.

Just make sure you add a door to funnel the speedster right where you want them. They're a villain, they're probably over-confident, they think they can handle a trapped door.

Then you can:

  • electrify the door knob;
  • electrify the floor;
  • electrify the marshmallow with a hidden layer of metal sheet;
  • sneak behind them while they're scratching their head wondering what trap you've put on your door;
  • poison the marshmallow so that if they try to eat their way through it they get paralysed;
  • taunt them to eat all of it before killing you and hope diabetes will kill them first.

Then light a fire, carve pieces of your fortress, put them on a stick and celebrate. Unless you've decided to use the poisoned marshmallow fortress technique.

  • 1
    $\begingroup$ The diabetes answer is quite funny, but it would apply if you eat a million dollars worth of marshmallows. $\endgroup$ Sep 3, 2020 at 13:43


enter image description here

Sprinkling caltrops around and he's going to have issues. At 1000mph he's going to lose control and crash and most likely kill himself.

Caltrops have been around since ancient times and can be easily made yourself for next to nothing from scrap metal or even ordered online.

Nothing expensive required.

  • $\begingroup$ I feel though he'd pull some "super-speed cleanup" BS, lol. But if he has a hard max speed, that would slow him down. $\endgroup$ Sep 3, 2020 at 17:00
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    $\begingroup$ Honestly, a whole bunch of d4s would likely be as effective and cheaper. $\endgroup$ Sep 3, 2020 at 18:50
  • $\begingroup$ Lego or marbles in a pinch..... $\endgroup$
    – Thorne
    Sep 3, 2020 at 22:51

A Bright Light

Think a search light suitable for Vegas or a ship at sea.

You put it at the end of a long tunnel, and turn it off.

Put some mild broken terrain in the tunnel - loose debris, stationary hazards, all low to the ground.

The light is on a motion sensor - once the speedster is committed to navigating the broken terrain at speed, the motion sensor detects him, and energizes the light.

The speedster is now blind, moving at 1000+ miles per hour, and surrounded by trip hazards. The situation resolves itself.

Set up this system at several places the speedster is likely to look - it's cheap, effective, and easy to implement.

Backup plan

A belt fed machine gun sits next to the light - the motion sensor also kicks off the gun, which fires off the full belt. Now he's screwed if he enters the tunnel at speed, and screwed if he doesn't.


Weak looking facade overtop of very strong wall

Leave the door open with a clearly placed trap (i.e. some piano wire strung across). He will suspect a trap, see it, and try the wall entrance. Make him splat himself.

  • $\begingroup$ Or for an even greater effect, put spikes in the wall so that he impales himself... oh wait, who's the bad guy in this scenario? $\endgroup$
    – Nosajimiki
    Sep 2, 2020 at 16:13
  • $\begingroup$ All he has to do to avoid the spikes is not try to kill them. Seems reasonable to me. $\endgroup$
    – Mary
    Sep 2, 2020 at 22:34

A ramp!

Make him run over a 30 degree inclined ramp at 1500 mph, he will be projected into the air approximately 700m according to this calculator and when he lands he'll go splat.



Put a glass wall in front of somewhere you think the speedster is going to go like in front of a door and the speedster will die ramming into it.


Strong magnetic field, spatially nonuniform

For example, the defenders could tile a large area with fairly spaced strong magnets. While slow-moving defenders could navigate through such environment safely, taking the same precautions one would take while undergoing a MRI scan, moving at high speed through a strong enough nonuniform magnetic field should be equivalent to standing still inside a induction furnace. Eddy currents inside the body of the speedster, if sustained long enough, could eventually cook him from the inside. Carried metallic objects, like piercings and rings, could heat up even faster and cause burns. Induced currents could mess with any electronics he may be carrying, even causing the battery of his cellphone to explode in his pocket.

There is large amounts of iron scrap lying around, that could be converted in lots of magnets. I didn't do the calculations to see if the required field strenghts are practical to achieve the effect in a reasonable time interval, but if the full induction furnace roast can't be achieved in practice, we can speculate subtler effects could still be exploited. For example, adjusting the spacing of the magnetic field so the eddy currents fall in sync with brainwaves, the defenders could try to give the speedster a epilectic seizure with a smaller amount of power.

As a shortcoming, the defenders should have equipment compatible with operation in strong magnetic fields.


Put the things you need to protect in a hole in the ground.

If you're traveling at 1000 mph and you hit the edge of a 10 ft hole, they will cross that hole in about 7 milliseconds. That's about enough time to fall about 0.2 millimeters into the hole.

If your evil speedster wants to descend into the hole they're going to need to slow down considerably. That makes them an easy target for any weapons trained on the hole from below.


There is a technology called active protection that is used on some military vehicles. Basically its a device that can shoot a missile out of the air as it approaches the vehicle.

Just sit in your tank, turn on the system, and wait for your opponent to approach and get blown up.



Also see...



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