Let's suppose "antigravity" does exist: how could we "detect" it or demonstrate it exists?!?

I was thinking about two well known formulas:
F= KqQ/r^2
F= GmM/r^2

First one is electrical attraction, second one is gravitational force.
You of course see two formulas are identical, apart from K and Q.
But what they represent appears not to be identical: we know positive and negative electrical charges do exist. We know positive "gravitational charges" do exist: they're commonly named... bodies!
Ok, where are "negative gavitational charges"???
If they exist, how could we detect them?

As same-sign gravitational charges, unlike electrical ones, cause attraction, it comes to mind that probably different-sign ones would reject one from the other; so, during solar system formation, all positive masses got "grouped" into planets and satellites... but they "rejected" negative masses away: no negative mass can stay close to a positive mass due to "gravitational repulsion".
So, where did they end?!?
How do "negative planets" react to light? Can they be seen?
If a black hole attracts light... does a "negative black hole" reject it?!? Does this make it visible to telescopes?!?

I think that "negative masses" could explain universe expansion quite better than imagining "dark matter", "dark energy": just calling "dark" something you are not able to see or to demonstrate that is existing is not a good scientific approach, IMHO.

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-- Jumpjack --

I submit that antigravity doesn't exist, on the basis that if gravity comes from "bodies", then the "antibodies" in my bloodstream should have launched me into space.

Ok, ok, bad joke :P
It's boring where I am right now

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This signature has been compressed due to rising photon costs.

Actually, it is a perfectly good, and really necessary scientific approach to do just that. It would not be a good scientific approach to stop there, and pretend you know what something is just because you gave it a name. But neither "science" nor scientists do that.

I think "negative masses" are an inferior explanation to "dark energy". By simply adding a repulsive term into the appropriate equations from general relativity, we can easily model dark energy, either as a consequence of a cosmological constant, or as an additional scalar field. When we do that we are able to generate a theory which agrees with observation, and we do not at the same time create any serious physical problems or conflicts. Dark energy is a perfectly reasonable, practical explanation for accelerated expansion, which is in turn a perfectly reasonable, practical interpretation of observation.

Another reasonable alternative is modification of general relativity to slightly change the law of gravity. We know from experience that this does not work as well as the usual dark energy explanation, so it is not as popular an idea. Still, there are quite a few scientists working along those lines.

There is no direct, close to hand, laboratory verification for either dark energy or modified gravity, or negative mass. In fact, negative mass is the one choice most strongly disfavored by observation. Everywhere we look we see only positive masses, and no sign of even the tiniest piece of negative mass. Now if you assume that the negative mass comes only as a sparse background of atomic or sub-atomic size particles spread evenly through space, than you are describing dark energy. And you have the disadvantage of postulating a new kind of matter that is remarkably different from ordinary matter, in more than simply being "negative". That is a much bigger deal then simple dark energy, and so rather less likely to be a good idea.

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Don't try this at home - We're what you call "professionals" - MythBusters.

Ditto Tim, plus this:

One obvious problem with negative gravitational charges is this:

Consider Newtonian dynamics with Newtonian gravitation. Assume (as physicists usually do) that inertial mass (the "mass" which appears in Newton's laws of motion) and gravitational mass (the "mass" which appears in his gravitational force law) are the same thing. Now consider a gravitating system consisting of a pair of positive and negative mass objects with masses m,-m. Show that the midpoint of a line segment drawn between the two objects accelerates without bound wrt any inertial observer, along the line drawn between the two objects, with the negative mass object chasing its positive mass partner. (Hint: if you push leftward on an object with negative inertial mass, it accelerates rightward.) This is generally agreed to physically be implausible.

Similar remarks hold for relativistic gravition theories, except that now constant acceleration of the midpoint of the system behaves like a Rindler observer in Minkowski space. If there were pairs of subatomic particles with m,-m accelerating like Rindler observers zipping about our universe, we'd almost certainly have noticed them by now!

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

my english is too poor to understand this sentence...
Should I explain/show how the midpoint among two bodies accelerates.... what..?

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-- Jumpjack --

Negatives masses indeed violate the equivalence principle .
The acceleration of a body is in general given by : m*a = GM*m/r² , where m can be eliminated . So regardless of the sign of the mass any mass is attracted by a positive mass and any mass is driven away by a negative mass . Also negative masses are repelling each other .
Applying a force to a negative mass in order to make it move would generate energy ! this is implausible ias is quoted .
Another point of view may be considering repelling masses instead of negative masses. This means considering masses which have the hypothetical property to repell each other instead of attracting in a gravitional field .
The resulting action may be the following :
Two Rmasses attract , two normal masses attract , a Rmass and a normal mass may repell . This seems to be what has been meant in the head of this thread . The repelling feature of such a mass is then not anymore linked to the mass itself but becomes then a feature of this mass , the feature being repelling or attracting . Then some analogy with the "charge" of ions is given .

Why?!?
Same "sign" mass, as we know, does attract, not repell! (unlike electrical charges, which have opposite behaviour).

This is interesting. How do we "applyia force to a negative mass"? Using another negative mass to physically push (not "repell") the negative mass? (i.e., touch a nm with a nm) Or using gravity field of a negative mass to attract the negative mass?

What is the formula which "states" that applying force to negative mass results in producing energy?
That's what just what I was supposing from the beginning: same-sign masses attract, different-sign masses repell. Else, why would have I quoted the Gravitational and Elctric force formulas?!?

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-- Jumpjack --

Well , you can write down the formulas you wrote : F = GmM/r² and : F=m*a , where a is the acceleration due to gravity in this case.
If you apply the formulae for the 4 possible cases : positive massae, negative masses and positive+negative masses , you'll see what direction ( acceleration ) the masses get .
Work done by a force is F.ds , where ds is the displacement which is function of the acceleration .
ds turns out to be negative for negative masses as the acceleration will be negative and therfore the energy will be negative .

Sorry, I can't get the point: how can negative displacement lead to negative energy? Which formula are you referring to to calculate energy?
E = 0.5 mv^2 (classical)
E = p^2*c^2 +m0^2*c^4 (high speeds)

E = F*ds requires an absolute value for ds, else my car would produce energy when moving backward!

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-- Jumpjack --

In the thought experiment I mentioned, we have two objects with masses m,-m in an otherwise empty universe. Newton's gravitational law says they repell each other. But Newton's law of motion says that when you push rightwards on an object with negative mass, it moves leftwards. For this reason, the object with mass m moves away from the object with mass -m (since it is being repelled), but the object with mass -m moves in the same direction with the same acceleration. Thus, the pair maintain constant distance and accelerate as a system wrt any inertial observer, with constant acceleration, indefinitely. Thus after finite time they achieve any given desired velocity wrt any given inertial observer!

That is a Newtonian thought experiment, so something similar must happen in any gravitation theory which has a Newtonian limit.

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Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

Weird.

So I have two bodies which repell each other but which can't get away one from the other!

Attached Thumbnails

 

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-- Jumpjack --

If you were to build a small platform and fly around the room, down the hall, out the front door, around the house, and in through the back door, and hover over a single spot of my choosing for a while as I check some things out...

...I think that would be a sufficient demonstration.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that.

I am human. Fully human.

No.

You're confusing the rightwards physical push (which would move it leftwards) with a rightwards gravitational repulsion (which isn't a push at all, but a warping of space-time).

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that.

I am human. Fully human.

Just so. In your diagram, let's say that the red body has mass m and the cyan body has mass -m, where m > 0. The gravitational force on both is repulsive (from Newton's law of gravitation), so the red body moves left. But the cyan body also moves left because (by Newton's law of motion) a rightward force is acting on it. Indeed, the cyan and and red bodies move left with the same (constant) acceleration, so the midpoint of this two-body system accelerates with constant leftward acceleration.

Remember, this argument depends upon the assumption that inertial and gravitational mass are always the same. (Exercise: rework things assuming that an object with negative gravitational mass -m still has positive inertial mass m > 0.) Also recall that it is nonrelativistic. But by what I said, you should expect something similar to happen in gtr (and it does).

You didn't read what I wrote with sufficient care; try again.

__________________
Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!

I guess you are referring to the term "negative mass" which is entirely a hypothetical concept, and is deemed "non-physical"
Yet, at the basis of GR we can see as negative masses as a possible solution to the GR equations.

How would negative mass interact with normal positive mass:

• A negative mass in a gravity field of a (larger) positive mass would not be repelled (as might be thought) but will be attracked by the positive mass. This is because a negative mass will accelerate in the opposite direction as the direction of the force applied, due to F = m a.
• A negative mass in a gravity field of a (larger) negative mass would be repelled by the negative mass.

The weirdest combination is a negative mass and positive mass of the same absolute amount. They will accelerate continously, the negative mass chasing the positive mass.

You might wanna look up more on this, for example Bondi was the first researcher that has looked into this.

Though generally this is regarded as a non-physical solution to the Einstein equations, and disregarded.

However, to my knowledge, the theory of cosmological inflation also has a concept of a repelling gravity force, that causes the very rapid expansion of a false vacuum bubble in the very early universe.
So I am not exactly sure why in that case a gravity force that is opposite to the normal direction of gravity is allowed but nowhere else.

I saw recently the work of some other (pseudo?) researcher, that came up with the idea that any region of space with a nett-outward force of gravity (so for example the intergalactic voids) can be seen as containing a negative energy, and thus negative mass, where normally (such as around a massive body) the gravitational field has an inward direction (the force of gravity increases when going inward).
This is in fact an (invalid?) analogy of solving the Einstein equations, since in those equations there is no mass, the mass turns only up after integration of the equations.

I read it again.

Carefully.

My answer stands.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that.

I am human. Fully human.