Negative masses

[ummwut]

I decided to try playing around with the config for my stand-in mod part, and ended up sticking a negative mass in for it. But oddly enough, when I launched it, it sat on the launch pad instead of floating away like I would expect something with negative mass to do.

Why did it not float away?

[EndOfTheEarth]

Did you move it?

An object that isn't in motion will remain that way, quoth Newton.

[Scotius]

I'm not sure physics engine can even model something as...exotic as negative mass.

[Jason Patterson]

I won't pretend to know how the physics engine does what it does, but it seems likely that the game calculates the force of gravity and the acceleration due to the net force on an object independently. A negative mass should experience an upward force from gravity, but that would cause a downward acceleration (because the minus sign would cancel out in F=ma.) It's also possible that if you stuck a rocket underneath it that an upward force on the object would cause it to accelerate downward, so it would stick to the pad even more firmly.

[Xionis]

Did you move it?

An object that isn't in motion will remain that way, quoth Newton.

Remember that if you are sitting in you computer chair "not moving" the only reason is the chair/floor/building/earth is pushing UP on you with the same force gravity is pulling DOWN. Take away the chair and you begin moving quite handily towards the floor thanks to gravity.

F = ma is the equation the OP is trying to play with here, by making m negative, it reverses the normal direction of gravity (gravity being -9.8m/s2 assuming UP is positive and DOWN is negative). So an object with negative mass should fly AWAY from the gravity well at whatever appropriate acceleration, no initial movement necessary.

UNLESS:

1) Physics engine is going "what, negative mass...that can't be right" and having it's brain blown/ignoring it for avoiding brain blowing.

or

2) Your total ship mass is still positive. Are you testing only that 1 part with negative mass? Or did you add some regular, physics obeying bits too and they took the ship back into positive?

[TheDarkStar]

It should still stay on the ground, mass is irrelevant in the equation for acceleration due to gravity.

Edit: Try launching a craft (with engines) with negative mass, though. THAT will bug.

[ummwut]

I tried doing negative mass with just an unmanned probe. It did not float away, so there's something weird going on with gravity. I'll try building a negative mass rocket next.

[Xionis]

It should still stay on the ground, mass is irrelevant in the equation for acceleration due to gravity.

If the mass counts as negative inertial mass, than any force vector will cause it to behave the opposite regular mass would. Gravity causes a force vector towards the center of the gravity well (center of the planet in this case) causing normal matter to cling to it's surface. Negative mass would instead fly off away from the well, or, out into space, it's acceleration dropping the further it got from the planet.

[Cesrate]

As far as I know, the available mass number for U3D is from 0 to inf.

[Temstar]

Wouldn't negative mass cause the object to fly away ONLY if it's the planet's mass that's negative? What's keeping you planted on the ground is not (at least mostly not) your own gravity field but the gravity field of the planet under you. If if the mass of the object is negative and is exerting a force that pushes all matter away from it it will be overpowered by the "positive" gravitational field of the planet.

[Xionis]

If both objects were negative mass, they would behave normally with each other (the -'s cancel out when you add 2 of them to the gravity equation).

Making the mass negative means a normal equation of Force = mass * acceleration get's it's sign changed. Normally you get (Negative Force) = regular mass * (negative gravity). Make it negative mass and it becomes (Positive force) = (negative mass) * (negative gravity).

[ummwut]

A negative mass ship flies like a normal one. I think the game just takes the absolute value of the mass. This is very unexpected behavior. Even assuming I could get into orbit, it would be elliptical and not hyperbolic.

I would expect a negative mass to float away, even in KSP which should take into account negative masses, but this seems to not be the case!

[Temstar]

Yes but how would a negative mass interact with a positive gravitational field? How would that interaction differ from interaction between negative mass and "negative" gravitational field that this mass and mass similar to it will generate?

If negative gravitation field work like we imagine - eg negative matter push each other apart then negative mass planet can't exist since planets (and stars, and galaxies, and superclusters) are held together by the force of gravity. But that doesn't mean negative mass spacecraft parts can't exist since ordinary objects are held together by electromagnetic forces instead of gravity. Since we're toying with the idea that this negative mass still has normal electromagnetic properties since we can build spacecraft parts out of it then I feel it's pretty safe to assume it interacts with normal gravity in the usual way. Eg - it still gets pulled by regular objects and their regular gravity, but at the same time its own negative gravitational field repulses both regular and negative matter.

[Xionis]

I'm guessing it's a safeguard in the physics engine against lots of very bad things that can happen w/ negative mass.

Edit in reply to Temstar above me:

The equation for gravitational attraction between two bodies is F = ( G * m1 * m2) / r^2 G is a constant, the m's are the bodies masses and r is the distance between them. There isn't a thing as "negative gravitational fields" except as would result from a very large negative mass.

That force (when the masses are both positive) simply pulls the two objects towards each other. Reverse just 1 of the masses to negative, and the force pushes both object away from each other. Make BOTH masses negative and once again they get pulled together. Negative mass planets could totally exist by this math (ignoring all the other problems...) but would need to orbit negative mass stars and on and on etc etc.

We're just experimenting with a tiny negative mass (relative to a planet) and what it should be doing in a gravity well (i.e. zoom off into space on it's own).

Edited April 24, 2013 by Xionis

[ummwut]

Safeguard, probably. Although, as someone who has studied a substantial amount of physics, I'm sort of disappointed. I suppose if I want negative masses, I should go and make my own game.

[ummwut]

Yes but how would a negative mass interact with a positive gravitational field? How would that interaction differ from interaction between negative mass and "negative" gravitational field that this mass and mass similar to it will generate?

If negative gravitation field work like we imagine - eg negative matter push each other apart then negative mass planet can't exist since planets (and stars, and galaxies, and superclusters) are held together by the force of gravity. But that doesn't mean negative mass spacecraft parts can't exist since ordinary objects are held together by electromagnetic forces instead of gravity. Since we're toying with the idea that this negative mass still has normal electromagnetic properties since we can build spacecraft parts out of it then I feel it's pretty safe to assume it interacts with normal gravity in the usual way. Eg - it still gets pulled by regular objects and their regular gravity, but at the same time its own negative gravitational field repulses both regular and negative matter.

A positive mass and a negative mass would repel each other, simple as that. Settle down.

[falofonos]

If you want antigravity drives, why not just mod an ion engine cfg to be even more powerful and produce its own xenon at the same time?

[Xionis]

@falofonos That point wasn't antigravity drive, it was "How does the game handle negative mass?"

And the conclusion is "It dodges it's calls like a bitchy ex girlfriend and pretends it doesn't exist."

[N_las]

A positive mass and a negative mass would repel each other, simple as that. Settle down.

Now, that is to easy. The gravitational Force for both bodys may point in the outwards direction, but the body with negativ mass woulnd't be accalerated in the direction of the force, but in the opposite direction:

Imagine:

Normal situation:

Mass Earth = big

Mass You = small, but negative

Direction of the Forces: The Forces are equally strong

<-----Earth You----->

Direction of the acceleration:

<-Earth <-----You

Since your mass is negative, it accelerates in the opposite direction of the Force.

Both objects accelerate in the same direction, but you are still bound to earth, since your acceleration towards earth is much greater than earths acceleration away from you

Of course that hasn't necessarily anything to do with reality.

[foamyesque]

KSP clamps mass to a minimum non-zero value to avoid wonkiness. Flat 0 is also impossible, from what I recall of testing it, presumably because you could then impart infinite velocity to a part with any thrust whatsoever, thereby bugging out the game (NaN etc).

However, if you specify an extremely low but non-zero mass, that works. Negative thrust numbers for engines used to produce some odd results, though:

Wonder if they still do?

[Xionis]

You are correct if negative mass is both inertial and gravitational. I was under the impression we were only calculating with inertial mass however.

I don't think KSP models it this way (rockets do not have their own gravity, no matter how slight.) Pretty sure it uses simply inertial mass in it's calculations.

And that would cause the rocket to fly away from the center of gravity that was affecting it.

[ummwut]

Well, foamyesque, I suppose I'll be testing this later.

Now I know it dodges negative mass when it loads the part, but what about mass being changed during flight?

[shand]

Interesting discussion so far!

http://en.wikipedia.org/wiki/Negative_mass

in general its a subject area open to arguement i'd have to go with Forward's aurguement, the positive would be pushed the negative attracted. this generates lots of interesting properties. not least of all would be the use of negative mass fuel as a hyper efficent propellent. use a low mass of propellent and it will literally push you along.as it got closer it would push you faster, withdraw it from your vessel (electromagetic chamber anyone?) and your acceleration would drop. you'd still need energy to manipulate the magnetic field, but its a reactionless drive for sure. now someone just needs to actually come across the damn stuff.

Interestingly the same arguement occurs for antimatter - note antimatter has positive mass, it is the charge that changes. personally i think it relatively intuitive that since the mass is the same the gravity forces are the same. but apparently not. photons are interesting for this topic too, no mass there but still subject to gravitational pull. perhaps mass isnt the be all and end all of gravity!

[Darren9]

Sometimes you just can't have negative, this may be one of those occasions

Photons travel at the speed of light because they have no mass, isn't that the rule? Anything without mass moves at the speed of light, anything with mass will move slower than the speed of light, something with negative mass? Can only move faster than the speed of light? I think that breaks the universe - it doesn't seem surprising that unity can't deal with it

[stijnovic]

Actually, in an atmosphere, you don't even need a negative mass, a density lower than the density of the atmosphere is enough to make you float upwards like a balloon.