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robly18 wrote:

A Random Player wrote: Well, the negative mass wouldn't chase the positive mass - They would both repel each other.
Normal matter--> <--Normal matter
Assume the masses are both 1, and they are 1 unit away from each other. The force would be G*m1*m2/r^2, or 1, toward each other. But if m2 is -1, the force on each particle would still be G*m1*m2/r^; but this is instead -1. This means the particles would each "attract" each other with a force of -1, or repel each other with a force of 1. This is indeed like an opposite of electromagnetism.

No, I see where he's coming from. He has a point.

p = mv, therefor, v = p/m. Therefor, if we took G as 1, r as 1, m1 as 1 and m2 as -1, then p = G*m1*m2*r^-2. This means p = 1*1*-1*1^-2 which would equal -1. However, for the positive mass, as v = p/m, v = -1/1 which would be -1, or a speed of 1 going away from the other mass. However, for the negative mass, v = -1/-1 which equals 1. Therefor, it would chase the positive mass with a speed of one.

I suspect this is the basis of the alcubierre drive.


Anyways, in this case, it's... Wow, gravity is weird. This would have to mean, that if P was a positive mass and N was a negative mass:
P-> <-P
N-> P->
<-N N->

Good grief, this is weird.

Oh, yeah, I forgot to divide by mass. F=ma after all.
This is weird. Really weird.

So, Andy. Mind letting us play a bit with negative mass? :D Like you did for the powers?

Done and done! I did this pretty quick and dirty -- so just like for the extended range of exponents, the only way to add 'negative mass' objects is to tweak their masses in the code. So to get you started:

Gravity Fun at TestTubeGames.com: [ForceG: -2,Qual: 1,Zoom: 1,xSet: -2,ySet: 1], [x0: -10,y0: -8,vx: 0,vy: 0,t0: 0,who: 1,m: 1000], [x0: -87,y0: 54,vx: -1.1,vy: -2.7,t0: 0,who: 2,m: -100], [x0: -99,y0: 64,vx: -0.9,vy: -2.35,t0: 0,who: 3,m: 0]

Positive mass star, negative mass planet... asteroid of (as always) negligible mass.

In the real update, I'll incorporate this much more nicely (and add in Black Holes, then, too).

testtubegames wrote:Done and done! I did this pretty quick and dirty -- so just like for the extended range of exponents, the only way to add 'negative mass' objects is to tweak their masses in the code. So to get you started:

Gravity Fun at TestTubeGames.com: [ForceG: -2,Qual: 1,Zoom: 1,xSet: -2,ySet: 1], [x0: -10,y0: -8,vx: 0,vy: 0,t0: 0,who: 1,m: 1000], [x0: -87,y0: 54,vx: -1.1,vy: -2.7,t0: 0,who: 2,m: -100], [x0: -99,y0: 64,vx: -0.9,vy: -2.35,t0: 0,who: 3,m: 0]

Positive mass star, negative mass planet... asteroid of (as always) negligible mass.

In the real update, I'll incorporate this much more nicely (and add in Black Holes, then, too).

It doesn't appear to work right now (after clearing cache). It still says "...ERROR, one or more values is unacceptable".
Edit: Nevermind, it works now.

testtubegames wrote:Done and done! I did this pretty quick and dirty -- so just like for the extended range of exponents, the only way to add 'negative mass' objects is to tweak their masses in the code. So to get you started:

Gravity Fun at TestTubeGames.com: [ForceG: -2,Qual: 1,Zoom: 1,xSet: -2,ySet: 1], [x0: -10,y0: -8,vx: 0,vy: 0,t0: 0,who: 1,m: 1000], [x0: -87,y0: 54,vx: -1.1,vy: -2.7,t0: 0,who: 2,m: -100], [x0: -99,y0: 64,vx: -0.9,vy: -2.35,t0: 0,who: 3,m: 0]

Positive mass star, negative mass planet... asteroid of (as always) negligible mass.

In the real update, I'll incorporate this much more nicely (and add in Black Holes, then, too).

Hooray, thanks!

Also, just as I predicted:

Positive mass -- Positive mass => you already know
Gravity Fun at TestTubeGames.com: [ForceG: -2,Qual: 1,Zoom: 1,xSet: -2,ySet: 1], [x0: -100,y0: 0,vx: 0,vy: 0,t0: 0,who: 2,m: 100], [x0: 100,y0: 0,vx: 0,vy: 0,t0: 0,who: 2,m: 100]

Negative mass -- Negative mass => easy enough
Gravity Fun at TestTubeGames.com: [ForceG: -2,Qual: 1,Zoom: 1,xSet: -2,ySet: 1], [x0: -100,y0: 0,vx: 0,vy: 0,t0: 0,who: 2,m: -100], [x0: 100,y0: 0,vx: 0,vy: 0,t0: 0,who: 2,m: -100]

Positive mass -- Negative mass => Mind = Blown
Gravity Fun at TestTubeGames.com: [ForceG: -2,Qual: 1,Zoom: 1,xSet: -2,ySet: 1], [x0: -100,y0: 0,vx: 0,vy: 0,t0: 0,who: 2,m: 100], [x0: 100,y0: 0,vx: 0,vy: 0,t0: 0,who: 2,m: -100]

I have no qualification to even ASK this question intelligently, but...

In the context of the above, wouldn't that be at least coincidental that our universe is accelerating apart AND there is more dark/anti matter than regular matter? (btw, is dark matter and anti-matter the same thing?)

If anti-matter repulsed against itself, then matter, which we might consider being caught in the "field" of anti-matter, would be repulsed with it -- similar to two swimmers in a river trying to swim to each other -- they are moving closer to each other within the repulsive current of the river. I guess then there would be some other evidence in our gravity constant or something.

Sorry if this is a simplistic postulation, but I love analogies, so my mind went there

DavidAllyn68 wrote:I have no qualification to even ASK this question intelligently, but...

In the context of the above, wouldn't that be at least coincidental that our universe is accelerating apart AND there is more dark/anti matter than regular matter? (btw, is dark matter and anti-matter the same thing?)

If anti-matter repulsed against itself, then matter, which we might consider being caught in the "field" of anti-matter, would be repulsed with it -- similar to two swimmers in a river trying to swim to each other -- they are moving closer to each other within the repulsive current of the river. I guess then there would be some other evidence in our gravity constant or something.

Sorry if this is a simplistic postulation, but I love analogies, so my mind went there

That's a great connection -- the fact that negative masses push other objects away makes me think of the accelerating expansion of the universe, too.

To start off with a bit of housekeeping, though:

-Antimatter is well understood -- it occurs around us fairly frequently (if in small quantities), and an example of an antimatter particle is the positron (anti-electron). An antiparticle has the opposite charge to its matter counterpart, but it still has positive mass. A planet made out of it would attract us just like a planet made from matter.

-Dark matter is not very well understood. We're pretty sure these are particles that barely interact, except through gravitational attraction. Dark matter has a positive mass, as well.

-Matter with negative mass, as far as we know, doesn't really exist. There are some complex phenomenon (like the Casimir effect) that give us, in some way, negative energy (and thus mass). But you won't find any particles that have a negative mass.

The accelerating expansion of our universe is thought to (possibly) come from Dark Energy. Just like Dark Matter, it is barely understood -- but that is where the similarities end. Dark Energy is some 'vacuum energy' that extends through all of space... and it has a couple of neat properties. It has a positive energy -- and a negative pressure. And because of those properties, it drives the expansion of the universe. It's confusing (and neat, and awesome!), so I'll leave it at that.

Most importantly for us, though, Dark Energy does not have negative mass. So that's not the reason it leads to a bunch of expansion. The fact that negative masses repel, too, seems to be just a coincidence.

I do wonder if negative masses were ever floated as a possible explanation for the expansion, though (before Dark Energy came along as an idea).

Thanks for the lesson! So all matter (that we know of) has positive mass and therefore "positive" gravity. However, there is something "pushing" the universe apart, yes? I've read that the universe's expansion is increasing? Is that true? If so, then wouldn't the very property of gravity contradict our observation? Wouldn't there have to be some form of "negative" gravity in order for that acceleration to occur?

Yeah, and that is what we call dark energy.
It goes like this:
So the big bang caused a bunch of outward expansion of space. We can think of this in terms of speed. It started with a set speed, which decreased do to the forces of gravity. However, it was a slow process, and the more the universe expanded, the more dark energy occupied space, and more was the distance between stuff, making gravity weaker. Eventually, a tipping point was hit, where the force of gravity was inferior to the force of the dark energy, and so the universe's expansion started accelerating, as opposed to decelerating.

DavidAllyn68 wrote:Thanks for the lesson! So all matter (that we know of) has positive mass and therefore "positive" gravity. However, there is something "pushing" the universe apart, yes? I've read that the universe's expansion is increasing? Is that true? If so, then wouldn't the very property of gravity contradict our observation? Wouldn't there have to be some form of "negative" gravity in order for that acceleration to occur?

As Robly said, yes indeed there is.

The 'repulsive' force stems from Dark Energy -- which (again, completely in theory) permeates all of space. In order to fully understand why it accelerates the expansion, you need to dip a bit into general relativity. Basically, it stems from its negative pressure.

In looking up more on the subject, I came across an interesting (if old) thread in the physics forums, where someone asked about this very topic: 'if dark energy has a repulsive effect on gravity, then what is its mass?' Positive? Negative? Zero? What?!? A bit of confusion ensues in the thread, since, it turns out, mass can be kinda hard to define in that complex situation. In general relativity, the terms you end up using are things like 'energy density' and 'pressure' instead. Alas.

So the moral of the story is that yes, dark energy drives expansion... and negative mass (in my simulator) drive repulsion. Quite similar, indeed. But they aren't quite based on the same physics. Dark energy isn't quite so straightforward as being something with 'negative mass'.