How fast does a character need to move to be effectively invisible?

Question

One of my players wants his monk to reach blinding speed like Rock Lee from Naruto. He is so fast he's invisible when he moves between attacks, like the character in this video after he takes off his weights.

I worked it out with him that reaching this kind of speed will not increase his damage output. I suggested he could use his ability scores improvements to increase his speed instead of his stats.

If he reaches the speed where he is effectively invisible, I stated that attacks of opportunity on him are no longer a thing.

So my question is:

How high must a character's movement speed roughly be, to be effectively invisible while moving?

Answer 1

RAW, you cannot become invisible via moving fast

As the other answers have stated, there are no rules that imply this. When a monk reaches 18th level, they gain access to the Empty Body class feature (PHB, p. 79):

Empty Body
Beginning at 18th level, you can use your action to spend 4 ki points to become invisible for 1 minute. During that time, you also have resistance to all damage but force damage.

But this requires you to reach level 18, and still isn't related to movement.

Try the Mobile feat

You mentioned in your question:

If he reaches the speed where he is effectively invisible, I stated that attacks of opportunity on him are no longer a thing.

The Mobile feat (PHB, p. 168) both increases your speed and allows you to avoid opportunity attacks (at least against anyone you have attacked that turn). You could reflavour this feat (without changing the mechanical benefits, which would unbalance the feat) to state that, at the point where they take this feat, they can now move so fast that they are invisible when they move, and that's why they don't take opportunity attacks (but again, all from a flavour perspective; in mechanical terms, it's because they have this feat and they don't really go invisible).

I would advise that such a reflavouring be discussed openly with the player so that they know they aren't really invisible from a mechanical perspective, which should hopefully avoid any misaligned expectations around wanting to use this "invisibility" for any other benefits (e.g. "but what if I ran straight past the guard to the other side? He wouldn't see me because I move so fast that I'm invisible, right?" - I mean, you could still allow that scenario if you wanted to, but if you say no to this, at least the player would understand why... so long as you're consistent).

That's just a suggestion anyway, from the "Yes, but..." school of DMing rather than "No".

Answer 2

There is no such speed in D&D 5e. You will have to add it yourself.

There is no speed in D&D 5e that causes spontaneous invisibility.

Since you are deciding that there is such a speed, but D&D can’t tell you what it is, it’s up to you as the DM. You’ll have to pick a speed and create your own house rule that says, “a Speed of [whatever you decide] makes a creature invisible when it moves.”

There is no such speed in reality either.

In case you’re asking what D&D speed equals the real-world velocity that would make a human invisible to an observer, that speed doesn’t exist either, so it can’t be translated into a D&D Speed stat anyway. In reality, very high speeds just make a very visible blur.

In fact, if you move fast enough, the atmospheric heating just causes you to glow, becoming even more visible. Even if somehow (magic?) you exceeded the speed of light (without exploding first) you still might not be invisible—you (or parts of you) might just start emitting visible Cherenkov radiation (the light equivalent of a sonic boom).

Answer 3

The closest RAW example of this is the Quickling

These creatures are described as "so fast their movement is blurry" and their base movement speed is 120ft. You can still make Opportunity Attacks against them, although those are made with Disadvantage. So are normal attacks, unless you manage to reduce their speed to 0.

Perhaps you can use some of this as an example or guideline for what you wish to accomplish. However, Quickling are not a playable race at this time, so they can serve as inspiration only.

Stats for them are in Volo's Guide To Monsters, p. 187.

Answer 4

Moving fast can't make you invisible by RAW

There is no rule that exists where a characters speed can be used to make them invisible while moving. If such a rule existed it would be found in the general rules for movement or the rules for movement and position in combat but there is nothing there.

The best that you will find is having enough speed to start and end your turn in total cover where you are unseen by the target of your attack. The speed required to do this would vary based on the environment you were in and the distance to the nearest source of cover. This still doesn't work though as they will likely be seen if they try to approach.

Answer 5

None of this is covered in the rules

In DnD 5e, movement speed very rarely affects anything except one thing: how much you can move over the course of a single turn. There are no rules for being fast enough to "blink" from one place to another to an untrained eye, so there is no concrete speed we can give you.

Invisibility for being fast enough can have serious balancing considerations so I would not implement it lightly. If you're satisfied with the invisibility between attacks being mainly "fluff" (as opposed to being mechanically significant) there's little cause for the player to not be allowed to describe themselves as "blinking" if they so please. If you want it to be mechanically significant, note that it will slightly nerf the Monastic Tradition Way of Shadow which provides a somewhat similar invisibility power.

Answer 6

Monks can already do this (in short bursts) using Step of the Wind

Starting at 2nd level, a monk can spend 1 ki point to take the disengage action as a bonus action. Then he can move up to his speed without provoking opportunity attacks. When the player does this, you can describe it as a sudden burst of speed that allows them to move so quickly that no one has time to react. The rules for Step of the Wind only describe the mechanics - they don't say anything about what it "looks like", so you are free to flavor it in whatever way makes your player most happy, as long as it fits the mechanics of how it works. This allows you to give your player what he wants without having to modify the rules at all. You can also flavor other abilities in a similar fashion to reinforce the theme of "blinding speed", such as describing Patient Defense as the monk having lightning-quick reflexes to sidestep incoming attacks.

Of course, your player may not be satisfied with just short bursts of speed like this. It sounds like the player wants to be able to attain this "blinding speed" continuously and over longer periods of time. However, as the monk levels up, this becomes possible as well. When the monk reaches his "final form" (i.e. 20th level), he gets 20 ki points every rest, always starts every combat with a minimum of 4 ki points, and can move about 60 feet per round. Given that most fights don't even last longer than 4 rounds, a 20th level monk using Step of the Wind on every turn can spend the entirety of most combats moving 60 feet per round without provoking opportunity attacks.

Short-range teleportation could also be re-flavored as "blinding speed", with some work from the DM

Beyond the bonus-action disengagement offered by Step of the Wind, spells or effects that allow short-range teleportation can also achieve the desired effect. The two that come to mind are the Misty Step spell (30-foot bonus action teleport) and the Shadow Step class feature from the Way of Shadow monastic tradition (60 foot bonus action teleport, but must start and end in shadow). However, both of these have drawbacks that would probably require you to do some home brewing to make them fit into the theme your player is going for, so going this route is going to be a lot more work for you and may risk unbalancing the game. For this reason, I recommend these options only if your player finds the above suggestion completely unsatisfying.

First, Misty Step. Unfortunately, there is no monk class feature that I know of that gives the ability to cast Misty Step, so you would either need to give the player a magic item that allows them to cast Misty Step or home-brew some class feature or racial trait to provide it. For an example of this, see the variant Eladrin subrace presented as an example home-brew in DMG Chapter 9, which includes a racial ability to cast Misty Step once per rest.

As for Shadow Step, it sounds almost perfect: teleport 60 feet as a bonus action, and it even gives advantage on an one attack after teleporting. However, this ability is limited to starting and ending in dim light or darkness. This limitation makes no sense if you're going to treat this "teleport" as a burst of speed, but removing the limitation would obviously make the ability far too powerful. So once again, if you want to give this ability to your monk player, you will need to do some home-brewing to make it work with the desired theme while keeping it balanced.

Lastly, the 5th level spell Steel Wind Strike deserves a mention because it seems almost perfectly designed for what your player wants: it literally says you "vanish to strike like the wind." For this reason, you might want to find an excuse to give your player access to this spell once they reach the epic tier of play. (For reference, consider that martial spellcasters like paladins and rangers get their 5th level spells at 17th level.)

Answer 7

I think people are missing the real question here. The question is:

How high must a character's movement speed roughly be, to be effectively invisible while moving?

The question is not:

How high must a character's movement speed roughly be, to not reflect light?

A better phrasing of the question, then is:

How high must a character's movement speed roughly be, to be quicker than the [human] eye?

So the answer here isn't about movement speed vs the speed of light at all. It's about what people can see. This is a question that relates to the observer's ability to track the target, not the physical capability for light reflecting from the target to reach the observer. So that if an observer were present while the character moved 200 feet from point A to point B, with a plain open view in daylight, the observer would be unable to distinguish between actual teleportation and very-fast-but-otherwise-normal movement.

For example, a bullet fired from a high powered rifle cannot be tracked by the human eye from almost any angle (you can from parallel to the trajectory over long distances). That's part of the reason that bullets can't be dodged in reality. Therefore, a bullet from a high powered rifle is effectively invisible while moving, and they move at about 3000 fps (feet per second). That's partially because the bullet is small, but either way it's still well below the speed of light.

Let's continue to use human eyesight as our baseline. Firstly because many of our observers in game will be humans, but also because we don't have data about fantasy creatures. Let's also assume, for spherical-cow simplicity's sake, that double your movement is your maximum velocity (i.e., from a double move). Let's also assume infinite acceleration when moving because I'm no longer capable of the calculus required.

There are articles which describe this. In that article, they use an example of kicking a soccer ball faster than the human eye can follow. They state:

In order to become invisible then, the ball would have to go so fast that the brain doesn't have time to process any light reflecting off it. In an experiment, air force pilots were able to recognise an image of a plane that was flashed on screen for as little as 1/220th of a second. With this in mind, we can imagine that the ball would have to be in front of you for at most 1/250th before the brain just never picked it up.

Therefore, any movement faster than 1/250th of a second is indiscernible from teleportation. Since a round is 6 seconds, there are 1500 observations in a round (1500 / 250 = 6). Therefore, the distance you move divided by 1500 is the distance your character can move between each observation from a human observer. And remember that this is all assuming that double your movement is your maximum velocity and assuming infinite acceleration when moving.

For a character that moves 60 feet in 6 seconds (move of 30 plus double move action) this is about half an inch: 60 feet per round / 1500 observations per round = 0.04 feet per observation = 0.48 inches per observation. You'd have to have a move speed of about 3750 in order to move 5 feet and have that movement be indiscernible from teleportation. Double move is 7500 feet in 1 round. 7500 feet per round / 1500 observations per round = 5 feet per observation. In order to move 60 feet, we need a movement speed 12 times that: 45000. A double move is 90000 feet per round / 1500 observations per round = 60 feet per observation.

Fortunately (or unfortunately, depending on your point of view) none of that really matters. RAW is that unless you're hiding, concealed, in cover, or actually invisible, if creature A has line of sight to creature B then creature A knows where creature B is. RAW, creatures have an infinite rate of detection.

Answer 8

I agree with everyone else here that this is not allowed in the rules and there are other rules that do exist that would give your player approximately what they're looking for without creating new rules. That being said, I also believe that fun trumps rules as long as we don't get too ridiculous.

According to a quick search on google, the human eye refresh rate is not exactly known but is somewhere from 100 - 1000 Hz, maybe higher but let's not get crazy. Let's assume that one frame is sufficient to notice movement, and we can do some math. In order to move 5 feet without you seeing it, I need to move 5 feet within your eyeball refresh rate. Let's assume your eyeball refreshes at 1000 Hz. That means I need to move 5 feet in 1/1000th of a second, or 5,000 ft/s. Since a D&D turn lasts 6 seconds, my move speed needs to be 30,000 ft/rnd, or about Mach 4.4. The table below shows the required move speed in ft/round in order to move a given distance based on an assumed "refresh rate":

\begin{array}{r|lll} \text{Invisibly moved distance} & \text{@ 100Hz} & \text{@ 500Hz} & \text{@ 1000Hz} \\ \hline 5 & 3,000 & 15,000 & 30,000 \\ 10 & 6,000 & 30,000 & 60,000 \\ 15 & 9,000 & 45,000 & 90,000 \\ 30 & 18,000 & 90,000 & 180,000 \\ 60 & 36,000 & 180,000 & 360,000 \\ 100 & 60,000 & 300,000 & 600,000 \\ \end{array}

So best case, you need a move speed of 3,000 ft just to move 5 feet without being seen. That takes us back to the idea that maybe, you should just take the mobility feat.

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It has been pointed out to me that eyes don't work like monitors with an actual refresh rate. This is true and I'm not trying to give anyone a course on biology. This answer is intended to be used as a rough order of magnitude calculation to let everyone know just how ridiculously fast a character would have to move in order for their movement to be imperceptible. I could also point out that in a world of magic and fantasy creatures an analysis of Human vision might not be very worthwhile.

All that being said, our eyes do have a limit on how much light they need to reflect off of an object in order to perceive it. According to this article, human eyes can perceive a flicker in a 500 Hz display, which means while we might not be able to recognize an object that we see for 1/500th of a second, we can perceive it. I'm not going to go into a dissertation here but there is a limit to what our eyes can see though the studies I've seen haven't settled on a number (hence the table).

After all this analysis, this is still a rough order of magnitude which only serves the purpose of telling us, that you need to move really really fast. I have been playing D&D for years and I have never seen move speeds in the thousands. Someone out there might be able to do it, but there are easier ways to get from A to B without someone seeing you.