Is this possible?

doubtful. that's like asking if something can have negative speed. I guess it's possible that something could interact with teh higgs field in such a way that it produces negative mass. It hasn't been observed yet.

Tachyons are believed to have a negative m^2 value, but that is because their mass is imaginary (includes "i")

I don't think we know of any, but that would be an interesting dark energy argument. Negative mass = negative gravitational field... that's kind of cool.
Originally posted by curioucity
Could there be a matter with negative mass?

An object of negative mass will have less mass than the empty space of a vacuum. It would be a rip in space. Applying forward force to it will make it move backwards. Applying backward force to it and it will move forward. If it were superconductive, dense negative mass would be a useful munition in a rail or coil gun. To send objects in space, electormagnetically shoot a negative mass bubble under it: the pressure from the bubble hitting the object to be sent into space will send the object into space.

Negative mass has its applications. Negative mass would lead to free energy. Hmm.

Negative mass can only exist if space itself may have a void in it.

I doubt it is possible; however, when you build the device that splits space and can then move the void, let me know.

Originally posted by Redrover
Would matter with negative mass be repulsed by gravity instead of being attracted?
According to the gravity equation, negative mass repulses mass while attracting negative mass just as mass attracts mass.

Negative matter would not contradict relatively, think about it.
And besides, there already is proof of particles with properties of anti-gravity, however, they do have positive mass.
*engage handwaving*
*engage jumbling of poorly understood concepts*

Negative matter means negative mass/energy density. I have vague recollections that mass/energy density is not necessarily an absolute value, but rather relative to the 'ground state' of the vaccuum. The existence of negative mass/energy density would imply that the vaccuum ground state is perhaps not zero, but some positive value, or that there is no absolute zero mass/energy density state at all.

And besides, there already is proof of particles with properties of anti-gravity, however, they do have positive mass.
Really? To what particles do you refer?
Exotic matter.
Particles under great stress with a great deal of tensile strength, they were theorized as possible methods of stabilizing MT wormholes, and later in the Alclubierre method of FTL travel.
Ofcourse, it's all theoretical, and scant few particles of exotic matter have ever been found, and vast amounts would be needed to move anything.
And ofcourse, they have mass, so it's all off topic.
No, a few have been found in special reactions.
Would not negative matter affect relativity negatively i.e. when in the presence of negative matter, the faster you travelled the faster time would travel for you?
You're all acting like I'm crazy, I didn't pull this stuff out of a hat.
I came across negative mass once. But then I remembered to multiply by negative acceleration, and the universe got better. Good thing, too - my physics professor probably would have marked me down for that.
Good thing it was not asked if it is possible to have negative weight.... duh! Helium balloons!!! :D

Negative mass though... hmm... I am sure it would have to do with antigravity...

f = (-m)*a where m is the magnitude

If we have a negative mass, wouldn't that instead of pull things in, it would push things away?

-k*m_1*m/r^2 = m*a_1 (where a_1 is the unknown acceleration)
-k*m_1/r^2 = a_1

m_1 is the magnitude of the mass... so it is not negative.
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It's impossible to have a negative weight, even in space there is merely zero have negative weight would imply anti-gravity, which requires exotic matter. Helium is merely lighter than the surrounding atmosphere.
Weight is a force though... the net force would be negative. Well, I guess it depends on one's definition of weight. But to have negative weight, there has to be a difference in signs of mass... once mass negative, the other positive.
Well, but it's not as though helium, or other lighter-than-air gases have no mass, or weight, they still have 'weight' as in positive gravity attraction.