# Which specific law of physics is broken by the “double jump”? [closed]

So, there's a cliche in computer gaming known as a "double jump". This can be described as:

A character jumps from a solid surface, and then is able to extend their jump by carrying out the jump action again in mid-air, starting their ballistic arc again from that position.

I appreciate that, barring some rocket propulsion, this is not physically possible, but which specific law would have to be suspended in game dynamics in order to allow this to happen?

• One could argue it breaks conservation of linear momentum, but then again one could press the air down with such intensity that the jump in mid air actually becomes possible. – Gabriel Golfetti Jan 10 '19 at 12:28
• I'm not sure physics works like that. The "laws" of physics are just understandings of how things work, not restrictions on what is possible. You can't double jump in real life because jumping is pushing off from the ground, and when you're not in contact with the ground, you can't push off of it. I suppose you could make an argument that General Relativity is violated, since a double jump would involve a body in a gravity field would somehow behave in a way that contradicts how we understand gravity to work. – Todd Wilcox Jan 10 '19 at 14:06
• are any laws of physics necessarily outright violated? A jump is just you exterting force against an object - typically the ground. The doublejump just does this again, but instead of pushing against the ground, you're pushing against air. I'd imagine that if you could somehow do the second jump with enough downward force, or otherwise generate enough air-resistance to have a "solid enough" surface to push against. I mean, birds doublejump all over the place like it's nobodies business – Kim André Kjelsberg Jan 10 '19 at 14:29
• "which specific law would have to be suspended in game dynamics in order to allow this to happen" It sounds like there's an assumption that game dynamics are implemented by programming laws of physics; that's not really how it's done except in a few special cases. – JiK Jan 10 '19 at 15:15
• I'm surprised that nobody yet has pointed out that birds continually double jump, except they do it with their wings instead of their feet. If you attached large wings to your feet and had very strong legs, you could probably double jump. – Javier Jan 10 '19 at 17:52

I would argue that this violates Newton's 3rd law, usually stated something like "for every action there is an equal and opposite reaction". To put it in words, there is no reaction to the action that is the second jump. On the first jump, there is a force down on the ground which displaces the ground (i.e. the planet, or whatever) by a usually very small amount; you could call this the reaction to the action of the first jump. On the second jump nothing reacts, unless you argue that some air has been kicked downward at really high speed.

Of course since the laws of physics don't apply, you could attribute the unphysical behavior to some other violations of physical laws as well, but this one seems intuitive, to me, at least.

• "unless you argue that some air has been kicked" A realistic analogy would be like using flippers in water. In order to make a double jump, you need to do the same but in a sufficiently dense atmosphere, with feet that have sufficient drag to actually push off against the atmosphere itself. (not sure of drag is the right word here?) – Flater Jan 10 '19 at 14:23
• Note that the ground the characters jump off is also almost certainly not programmed to react, and so violates the same law. – Vaelus Jan 10 '19 at 15:35
• @Vaelus While true, adding it in would result in negligible change to the actual game world in most cases (games set in space where the bit of "ground" has only 2 or 3 orders of magnitude on a human could be affected), so abstracting it away is no real loss of precision in this regard. – Draco18s no longer trusts SE Jan 10 '19 at 15:58
• As an exercise for the reader, one could compute just how fast you would have to kick your legs to produce a sufficient reaction force against the air to effect a typical "jump". I suspect the integrity of the human body would deteriorate catastrophically before one could practically achieve such a result. – J... Jan 10 '19 at 20:05
• That makes it the first law that is violated, not the third, in that there was a change of velocity without force being applied. – MichaelK Jan 11 '19 at 9:53

This doesn't have a unique answer. As others have said, Newton's Second Law is a reasonable answer, but it also definitely violates conservation of momentum. It depends to some degree in which laws you take to be fundamental and which you derive as a consequence from those.

• Except for birds, as noted in the comments. – Carl Witthoft Jan 10 '19 at 18:49
• @CarlWitthoft Birds couple themselves sufficiently well to the air with their wings, and are sufficiently light, that sufficient reaction force is available from flapping to keep them aloft. Typical videogame characters do not exhibit sufficient aerodynamic coupling in their form or strength in their muscles to support their assumed weight (which is often evidenced by their ability to attack effectively). – Chromatix Jan 10 '19 at 21:54
• Conservation of linear momentum is merely the sum of Newton's three laws. I'm pretty sure angular momentum is too but I can't quite prove it. – Joshua Jan 11 '19 at 2:57

# Newton's 3rd

## Either you can't make the first jump or you can't make the second.

For every action there is an equal and opposite reaction. If the air was dense enough to act as a second floor then your first jump would be like jumping into a ceiling and you wouldn't make it to your second jump. If the air isn't that dense then no, you don't make the second jump.

Edit: This is going off the assumption that:

A character jumps from a solid surface, and then is able to extend their jump by carrying out the jump action again in mid-air, starting their ballistic arc again from that position.

Refers to the case where the ballistic arc of the second jump is equal to that of the first - usually the case in games.

• If a dense enough medium that allows you to push off from forbids you to 'jump' through that medium I suppose swimming is an impossibility in the real world. Glad I'm living in this glitched version then! – Hans Janssen Jan 10 '19 at 15:32
• @HansJanssen I should clarify this is for A character jumps from a solid surface, and then is able to extend their jump by carrying out the jump action again in mid-air, starting their ballistic arc again from that position. - The situation where the character starts an equal ballistic arc (usually the case in games). For swimming this would be the case where you're attempting to get the same increase in momentum by pushing off water as pushing off air. (I've edited to add this information in, cheers for pointing out my omission) – Lio Elbammalf Jan 10 '19 at 15:47
• @LioElbammalf surely you mean "For swimming this would be the case where you're attempting to get the same increase in momentum by pushing off water as pushing off the wall". Comparing pushing off a solid object, vs pushing off air/water. – djsmiley2k in darkness Jan 10 '19 at 17:45
• @djsmiley2k I did - seems I'm full of mistakes recently! Thanks for catching that one :) – Lio Elbammalf Jan 12 '19 at 11:15

I think the problem with the double jump isn't so much that it contradicts physical laws. I's that it contradicts the physical fact that you create almost no force by going through the motions of a jump while airborne.

Absent this fact, I'm not sure the double jump would violate any physical law. But given that the second 'jump' generates no force, all three of Newton's laws of motion prohibit it.

• Newton's First Law states that a body in motion will maintain its motion unless a force is acting on it. And for all practical purposes, there isn't any force.

• Newton's Second law states that the sum of all forces acting on a body with a mass will accelerate according to the relation $$F = m *a$$. Given that $$F$$ is zero and the jumper's mass $$m$$ is not, the acceleration $$a$$ from the jump has to be zero.

• Newton's Third Law states that each action has an equal and opposite reaction. But the action (kicking the air down) and the reaction (kicking the body up) are both zero, given that the second kick exerts practically no force on the air.

So, to repeat, the core of the problem isn't Newton's laws. It's the false impression that a jumping motion in mid-air generates a force on one's body.

• +1 for highlighting that physics is an experimental discipline more so than a normative one. Given that arbitrary motion, Newton's laws would indicate what is missing to make that motion possible. – XavierStuvw Jan 10 '19 at 19:54

I'd say momentum conservation is violated. If you jump in mid-air, you accelerate the air-molecules underneath you. But as their viscosity is not high enough, they easily disperse, without taking enough mass with them. This means that not enough momentum is conserved by the acceleration of these few particles (atoms are very light...). With a higher viscosity, more atoms would be accelerated, i.e. more mass and therefore enough momentum us carried away.

• Hence why we can propel ourselves using our feet when submerged in water. A jumping maneuver isn't the most efficient way to do so but the water here acts just like a sufficiently dense/viscous atmosphere would. – Flater Jan 10 '19 at 14:30

With Newton's 3rd law. "for every action there is an equal and opposite reaction."

in mid air, there is not enough friction/density to give you more lift. When you are in water ,you can push yourself above.For less dense medium like air ,it makes almost impossible for human being to double jump.

Air is a medium with low density so it is very hard to push the air down to jump up as frogs can do in water. Here no physical law is violated just the medium is not suitable.

I am adding some examples. A bird flying in mid air is a similar case of jumping.because of it's low weight and muscles it can push itself up.In a similar case a frog in water can do many jumps in the medium.When bird starts the flight it can jump from the ground with legs but to jump in mid air it will need to use its wings

• But what physical law is violated? – Aaron Stevens Jan 10 '19 at 13:17
• none. only medium is not suitable – user212727 Jan 10 '19 at 13:21
• If we remove the medium ,then newton's third law will be violated I don't think this is true. If there is no medium, then there is no force on a medium. There has to be forces involved to talk about the third law. I think you mean to say the second law is violated, where we have experienced an upwards acceleration without experiencing any sort of upward force. – Aaron Stevens Jan 10 '19 at 13:25
• I mean If we remove medium and can jump again in mid,then 3rd law will be violated @aaron stevens – user212727 Jan 10 '19 at 13:27
• @AaronStevens and TheBroly, you are both correct, you just assume the situation differently. If we assume that there is a force pushing up and causing the acceleration, then the 3rd law is violated (since there is no reaction force). If we assume that acceleration happens without a force, then the 2nd law is violated. It all depends on which parameter that starts it all in this unphysical situation. – Steeven Jan 10 '19 at 13:54

# Newton's first law, i.e. "Conservation of Inertia"

Newton's first law is (boldface mine):

Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.

After leaving the ledge, the body should have continued on its ballistic trajectory, affected only by the forces of gravity and air drag(+). Without another force acting on it, it should have "[persisted] in its state".

But instead the body suddenly changed velocity/inertia(++). In the case of a double-jump the body's velocity/inertia changes without a corresponding force acting upon it.

Hence, it is Newton's first law that is violated by the double-jump.

(+) in case that is modelled by the game

(++) we assume the mass stays constant, hence any change of velocity comes with a corresponding change of inertia

None as long as you have huge wing like feet which allow you to displace enough air to be lifted (and obviously you weight extremely little compared to your feet size).

However the 'laws' are such that they must be followed for an object in our universe, however they only describe how objects act. That is, a object might follow all the laws and still not be able to move in the described way in our universe due to not following the physical constraints of such.

In the real world, and assuming you're not flapping madly to exert sufficient force on the air to instigate a second "jump", then you're breaking:

• Conservation of Energy,
• Conservation of Momentum,
• All of Newton's Laws,
• Probably most of the laws of Thermodynamics
• and likely a few more that are all derived from the first two.

Dealing with the question itself though - which game dynamics would have to be suspended - the answer is strictly "None". Games allow you to arbitrarily change the velocities on objects without explaining it to the dynamics systems.

• The intention of the question is more “how would the game physics differ from known laws of physics?” – AJFaraday Jan 10 '19 at 22:45
• Easier to list the laws that aren't broken. +1 – Mazura Jan 11 '19 at 2:23

If we assume the atmosphere is air with the same properties as in real life and if we assume we have an intuitive sense of the force the character is exerting when performing a second jump, then there would be an apparent violation of momentum conservation. This is because to have a vertical jump mid-air, the air needs to experience an equal amount of momentum downwards. But because air is in the gaseous phase and has low density, when we kick downwards very little momentum is transferred directly below us. Helicopters get away with this because they not only have very big wings to beat more air downwards, but also spin very fast to increase both the velocity and mass flow rate of air going down. We can expect the momentum from a midair jump is in great excess of what the conservation of momentum would suggest from a human foot kicking air down.

## No law is broken

• Imagine that the planet has a magnetic field. Also imagine that the double-jumper's feet are strapped to a lightweight board with a high-temperate superconducting material. The jumper jumps off the board (fixed by contact with the ground), pulls it up with her feet, cools the superconductor below the critical threshold, jumps again off the board (fixed by electromagnetic repulsion, also transferring the momentum to the ground), heats it back up above the threshold and pulls it with her feet.
• Imagine that the double-jumper's shoes have extending lightweight poles on the bottom. The jumper jumps off the ground, extends the poles, and jumps again off the ground.
• Imagine that the double-jumper's shoes fire a massive invisible bouncy ball each, then catch it again. The balls would hit the ground and bounce back really quickly, transferring half the momentum and then half again to make the full momentum transferred between the ground and the double-jumper.

There are many ways of doing this. That's just the first three I thought of. Other magic like I've listed here could be substituted.

Some people are attributing this to conservation of energy or momentum. But really those are derived principles, aside from energy which was partially set up/defined such that it would be conserved. Ultimately this is an issue about forces and what sort of forces are allowed to exist.

Basically there are 4 fundamental forces (that I am aware of)

The strong nuclear force

The weak nuclear force

Gravity

Electromagnetic forces

If we assume that nothing pushes the player upwards (and since some games allow for jumping in space) and that this works even in a vacuum then the violation isn't energy or conservation of momentum directly. The real violation here is that there must be a fifth type of force that acts on the player for the instant they jump. The reason is that the forces given are already well defined in physics. Therefore if one claimed to have done such an experiment in a vacuum (and no flaw was found in the experiment itself) and concluded that it wasn't electromagnetic or nuclear in nature (or involving physical fracturing or expulsion of mass/energy) then it would mean that for conservation of energy to be preserved (which is a bedrock principle and should be left intact at all costs) there has to be something else exhibiting a force on the object. Therefore my assumption here would be that another elementary particle or type of force needs to exist in this system. Of course, whether that system has conservation of energy can be further debated and studied but the experiment itself should not lead to immediate contradiction. I would say that the law here being contradicted is the law of completeness (which is a name I just made up). What that means is that our current model is assumed to be "complete". While we do not know everything in the universe and how it works entirely we assume that we have all of the pieces. Gluons, protons, etc. We have rules saying how all of them work and interact. None of those rules or laws are violated if another particle is added. The interactions between those particles still all continue in the same way. It's just that there's another thing that causes unexpected results such as double jumping. That's my take on this.

Of course in a more realistic scenario my presumption is that the player is somehow giving off a repulsive electrical charge or something involving dark matter/negative mass. However, the latter I only know by name and is probably not going to give the desired result.

The fundamental problem with this question however is that if any rule is specifically contradicted then because physics (as a system rather than an ongoing experimental study) is a logically consistent system and because of the principle of explosion (that any contradiction leads to all facts being simultaneously true and false), the contradiction or law violation can be set up to be anything without specifically drawing upon the presence of a contradiction. Therefore while of course one could try to pin down "obvious" candidates for a violation of physical laws one could probably easier than I am imagining set up a formal proof by contradiction to this being possible in the current defined system of physics where the contradiction is found by showing that in such a scenario $$0 = 1$$. Of course that isn't obvious but under such a scenario of assuming that is possible in a vacuum without charge being involved and with no other particles being newly defined one could show that $$0 = 1$$ and state that therefore the assumption leading to that must be contradictory to the system as a whole.

On another side however which goes back to my first thoughts physics is not a system of formal proofs. One never proves anything except that data is not being lied about or improperly collected. I would say however that you aren't asking about that as you speak of what it would contradict, which means you are asking about a model of physics and the mathematical rules used to construct rather than the actual ideal physics which may already contain such contradictions that we just haven't yet found and therefore we cannot claim that your scenario is impossible outside of reasonable doubt (unless you specify the question very very narrowly such that undiscovered physics is barred in which case see the previous paragraph).

It's a law of Nature that all elementary particles (be it an electron, a quark, a neutrino, or their associated two families which only differ in mass from the first generation) have an associated fixed rest mass. The values of these masses can't change. That is to say, at low temperatures; at the very high temperature of about $$10^{15}(K),$$ the Higgs field, which allegedly gives elementary particles, evaporates so the elementary particles become massless, but that is obviously not the case here, because in that case, our man will cease to exist. Now, if at the moment the computer figure (who in the real world is made up out of electrons and quarks) tries to jump for the second time these values lower drastically (in such a way that his mass becomes much less than the mass off the air he occupies), he should be able to jump. By pushing himself against the air upwards, by the buoyant force of the air (in which case he doesn't have to do any effort), or by a combination of both. Right after his second launch, the rest masses of the electrons and quarks resume their original values.

So it's the law of the constancy of the rest mass of elementary particles that is broken).

## No laws are broken, but Planck's constant is higher than in our world.

I would say that no physical laws are broken, it's just that the double jump is performed in a universe where Planck's constant has a (much) higher value.

Looking at the uncertainty principle for energy: $$\Delta E \Delta t \geq \frac{\hbar}{2}$$ Just like a virtual particle can break energy conservation for a short time, so can your jumper. When he jumps from the wall, he converts $$\Delta E$$ muscle energy into $$\Delta E/2$$ kinetic energy. And a short time later, mid-air, he converts the other $$\Delta E/2$$ into kinetic energy. In the time period between the two jumps, energy conservation was broken, but because Planck's constant is so large, it is ok.

Another way to look at it, using the uncertainty principle for position-momentum: $$\Delta q \Delta p \geq \frac{\hbar}{2}$$

You say that at the second jump, he is mid-air. Are you sure? If you are very sure that he is not close to the wall, you can not be certain about his momentum, so how do you know that his momentum changed upwards? And if you are certain that he jumped upwards, according to the uncertainty principle you don't know much about his position, so he might still be at the wall the second time...

By the way: for this to be true, Planck's constant needs to be at least $$10^{34}$$ times higher than it currently is. The universe would look very different.

• Aside from the dubious interpretation of the uncertainty principle, $\hbar$ is Planck's constant, $k_B$ is Boltzmann's. – ZachMcDargh Jan 10 '19 at 15:25
• @ZachMcDargh: Agreed! The dubious interpretation is on purpose, the wrong name for the constant a stupid mistake that I will fix immediately. – PA71 Jan 10 '19 at 15:36
• Of course, in a world where Planck's constant is that large you have significantly more issues dependably landing on the floors or not going straight through walls, much less the absolutely awful things that happen to you when you go through a doorway! – throx Jan 11 '19 at 5:12

Has no one mentioned non-Newtonian fluids? Double jump would (to some extent) work in a corn starch solution, so all that is “broken” is that air is treated as a strongly non-Newtonian liquid.

• I'm not completely sure that it would be possible to make the initial jump within a non-newtonian fluid, because the upward force would be met with the hardening effect. – AJFaraday Jan 11 '19 at 9:34
• @AJFaraday haha, yeah you’re probably right. Maybe if the fluid were nonlinearly non-Newtonian, one could contrive a way to make it work. – Gilbert Jan 11 '19 at 19:39